Anatomy and Physiology: Biology of the Cell PDF

Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 1 of 101...

Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 1 of 101 Page 104 chapter 4 Biology of the Cell INTEGRATE CAREER PATH Cytologist A cytologist examines cells under a microscope to detect abnormalities that may indicate cancer or other diseases. Cytologists use specialized equipment to prepare cell samples, and then they apply stains or other techniques to enhance the specimens for detailed observation. Drawing upon an extensive knowledge of cell structure and function, they then analyze cell samples and submit their findings to a pathologist, who ultimately makes a diagnosis. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 2 of 101 ©SCIENCE PHOTO LIBRARY/agefotostock 4.1 Introduction to Cells 4.1a How Cells Are Studied 4.1b Cell Size and Shape 4.1c Common Features and General Functions 4.2 Chemical Structure of the Plasma Membrane 4.2a Lipid Components 4.2b Membrane Proteins 4.3 Membrane Transport 4.3a Passive Processes: Diffusion McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 3 of 101 4.3b Passive Processes: Osmosis 4.3c Active Processes 4.4 Resting Membrane Potential 4.4a Introduction 4.4b Establishing and Maintaining an RMP INTEGRATE: Concept Overview Passive and Active Processes of Membrane Transport 4.5 Cell Communication 4.5a Direct Contact Between Cells 4.5b Ligand-Receptor Signaling 4.6 Cellular Structures 4.6a Membrane-Bound Organelles 4.6b Non-Membrane-Bound Organelles 4.6c Structures of the Cell’s External Surface 4.6d Membrane Junctions INTEGRATE: Concept Overview Cellular Structures and Their Functions 4.7 Structure of the Nucleus McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 4 of 101 4.7a Nuclear Envelope and Nucleolus 4.7b DNA, Chromatin, and Chromosomes 4.8 Function of the Nucleus and Ribosomes 4.8a Transcription: Synthesizing RNA 4.8b Translation: Synthesizing Protein 4.8c DNA as the Control Center of a Cell 4.9 Cell Division 4.9a Cellular Structures 4.9b The Cell Cycle 4.10 Cell Aging and Death Module 2: Cells and Chemistry Heart cells contract to pump blood out of the heart chambers; retinal cells of the eye detect light; phagocytic white blood cells engulf foreign substances (e.g., bacteria, viruses); and select cells in the pancreas synthesize and secrete the hormone insulin. All human body processes are ultimately dependent upon cells and their activities. For this reason, the cell is often referred to as the “functional unit of the body.” Knowledge of cellular structure and function is critical for understanding the concepts of all later chapters. Throughout this chapter, we present a broad discussion of a cell by describing how cells are studied and the general structural components and functions of cells. Subsequent chapters examine specialized cells and provide details of their unique functions. Page 105 McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 5 of 101 4.1 Introduction to Cells Our examination of cells begins with a description of how we study them. We then describe how the cells that compose the human body vary in both size and shape, and that these differences reflect their specific function. This section concludes with a discussion of the common structural features all cells possess and the general functions that all cells must perform. 4.1a How Cells Are Studied LEARNING OBJECTIVE 1. Compare and contrast the methods of light microscopy, transmission electron microscopy, and scanning electron microscopy. The study of cells is called cytology (sī-tol’ō-jē; kytos = a hollow [cell]). The small size of cells is the greatest obstacle to determining their nature. Cells were discovered after microscopes were invented because high-magnification microscopes are required to see the smallest human body cells. Microscopy is the use of a microscope to view small-scale structures, and it is an invaluable asset in anatomic investigations. The most commonly used instruments are the light microscope, the transmission electron microscope, and the scanning electron microscope. One of the challenges with viewing anatomic specimens with a microscope is that microscopy samples prepared from body tissues have no inherent contrast (difference between specimen and background). To provide contrast so microscopic structures can be seen more clearly, colored-dye stains are used with light microscopes, and heavy-metal stains are used with both transmission and scanning electron microscopes. figure 4.1 allows us to compare the images produced when each of these types of microscopes is used to examine the same specimen. Notice the remarkable difference between these images of the same body structure—in this case, hairlike cilia (see section 4.6c) on the inner lining of the respiratory tract. Figure 4.1 Microscopic Techniques for Cellular Studies. Different microscopic techniques are used to investigate cellular anatomy. (a) A light microscope shows hairlike structures, termed cilia, that project from cells lining the respiratory tract. (b) A transmission electron microscope reveals the two-dimensional ultrastructure of the cilia on the same type of McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 6 of 101 cells. (c) A scanning electron microscope shows the three-dimensional image of the cilia of the same type of cells. Note: The dimensional unit often used to measure the specimen when viewing with an LM is the micrometer (µm). Ten thousand (104) µm are equal to 1 centimeter. In comparison, when viewing with an electron microscope, the unit often used is the nanometer (nm). Ten million (107) nm are equal to 1 centimeter. (a) ©McGraw-Hill Education/Al Telser; (b, c) ©Eye of Science/Science Source The light microscope (LM) produces a two-dimensional image by passing visible light through the specimen stained with colored dyes. Glass lenses focus and magnify the image as it is projected toward the eye (figure 4.1a). A typical LM might magnify a specimen 40×, 100×, or 1000× (depending upon the objective lens that is used). The specimen in figure 4.1a is magnified 720× (as indicated on the side of the image). The electron microscope (EM) uses a beam of electrons to “illuminate” the specimen stained with heavy metal. Electron microscopes easily exceed the magnification obtained by light microscopy—but more importantly, they improve the resolution (ability to see details) by more than a thousand-fold over the light microscope. The two common types of electron microscopes are transmission electron microscope and scanning electron microscopes. A transmission electron microscope (TEM) directs an electron beam through a thin-cut section of the specimen. A two-dimensional image of the specimen is focused either onto a screen or onto photographic film. A TEM allows us to visualize the details of the specimen’s internal structures. The specimen in figure 4.1b is magnified 50,000×! For a detailed three-dimensional image of the surface of the specimen, a scanning electron microscope (SEM) is used. Here, the electron beam is moved across the surface of the specimen, and reflected electrons generate a digital image of the surface topography of the specimen. The specimen in figure 4.1c is magnified 3000×. WHAT DID YOU LEARN? 1 What is the advantage of using a TEM instead of an LM to study intracellular structure? 4.1b Cell Size and Shape LEARNING OBJECTIVES 2. Describe the range in size of human cells. 3. Describe some of the shapes cells may exhibit. Page 106 McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 7 of 101 Cells are typically depicted as being of one size and either spherical or cubelike in shape, when in reality the structure of the approximately 75 trillion cells of the adult human shows great variety. Most cells are microscopic in size, but some are large enough to be seen with the naked eye (figure 4.2). For example, red blood cells (erythrocytes) are relatively small, with a diameter of about 7—8 μm, whereas a human oocyte has a diameter of about 120 μm. To help you to relate to how small some cells are, consider that about 5 million erythrocytes would fit on the head of a pin. Cells also vary greatly in shape (figure 4.3). Although some cells are spherical or cubelike, others are columnlike, cylindrical, disc-shaped, or irregular-shaped. Note that a relationship exists between the size and shape of a cell and its function in the body. Figure 4.2 The Range of Cell Sizes. Most cells in the human body are between 1 micrometer (μm) and 100 μm in diameter. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 8 of 101 Figure 4.3 The Variety of Cell Shapes. Cells throughout the body exhibit different shapes that support various functions. WHAT DID YOU LEARN? 2 Which cell is larger, an erythrocyte or a human oocyte? What are their respective sizes? 4.1c Common Features and General Functions LEARNING OBJECTIVES 4. Describe the three main structural features of a cell. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 9 of 101 5. Compare the membrane-bound and non-membrane-bound organelles and distinguish these from cell inclusions. 6. Explain the general functions that cells must perform. Most cells are composed of characteristic parts that work together to allow each cell type in the body to perform certain common functions. Overview of Cellular Components The generalized cell shown in figure 4.4 is not an actual body cell, but rather a representation of a cell that combines features of different types of body cells. The common features include the following: Plasma membrane. The plasma (plaz’mă; plasso = to form) membrane (also known as the cell membrane) forms the outer, limiting barrier that separates the internal contents of the cell from the interstitial fluid (fluid that surrounds the cell). Modified extensions of the plasma membrane include cilia, a flagellum, and microvilli. Nucleus. The nucleus (nū’klē-ūs; nux = the kernel) is the largest structure within the cell and is enclosed by a nuclear envelope. Much of the internal content of the nucleus is the genetic material, deoxyribonucleic acid (DNA). The fluid within the nucleus is called the nucleoplasm. Within the nucleus is a dark-staining body called the nucleolus. Cytoplasm. Cytoplasm (sī’tō-plazm; plasma = a thing formed) is a general term for all cellular contents located between the plasma membrane and the nucleus. The three primary components of the cytoplasm are the cytosol, organelles, and inclusions. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 10 of 101 Figure 4.4 The Structure of a Cell. This generalized cell illustrates most of the common features found in mature human cells, which include the plasma membrane, nucleus, and cytoplasm. Composing the cytoplasm is cytosol and organelles that are either membrane- bound or non-membrane-bound. Some cells also contain inclusions, which are temporary stores of specific molecules.. Page 107 Cytoplasmic Components The cytosol (sī-tō-sol; sol = soluble), also called the intracellular fluid (ICF) or cytoplasmic matrix, is the viscous, syruplike fluid of the cytoplasm. It has a high water content and contains many dissolved macromolecules that include carbohydrates, lipids, proteins, and small molecules such as glucose and amino acids (see section 2.7). Cytosol also contains various types of ions, — such as potassium ion (K+) and phosphate ion (PO43 ) (see section 2.2a). Organelles (or’gă-nel; organon = organ, elle = the diminutive suffix), meaning little organs, are complex, organized structures within cells that have unique characteristic shapes and functions. Two categories of organelles are recognized: membrane-bound organelles and non- membrane-bound organelles. Membrane-bound organelles, or membranous organelles, are enclosed by a membrane similar to the plasma membrane. The membrane separates the organelle’s contents from the cytosol so that the specific activities of the organelle can proceed without disruption from other cellular activities. Membrane-bound organelles include the McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 11 of 101 endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, peroxisomes, and mitochondria (figure 4.4). Vesicles are temporary membrane-bound structures formed from the endoplasmic reticulum, Golgi apparatus, and plasma membrane. The non-membrane-bound organelles, or nonmembranous organelles, are not enclosed within a membrane. These structures are generally composed of protein and include ribosomes (either free within the cytosol or attached [bound] to the external surface of the endoplasmic reticulum), the centrosome, proteasomes, and the cytoskeleton. Each of these organelles is discussed in detail in section 4.6. The cytosol of some cells temporarily stores inclusions. Cell inclusions are not considered organelles, but rather are aggregates (clusters) of a single type of molecule. Molecules are continuously being added to and removed from inclusions. Pigments (e.g., melanin, a stored pigment in some skin, hair, and eye cells) and nutrient stores (e.g., glycogen in liver cells, triglycerides in adipose connective tissue) are examples of cell inclusions. General Cell Functions Cells must perform general functions, including: Maintain integrity and shape of a cell. The integrity and shape of a cell are dependent upon both the plasma membrane, which forms the external boundary of the cell, and the internal contents, which function to support the cell. Obtain nutrients and form chemical building blocks. Each cell must get nutrients and other needed substances from its surrounding fluid. Cells form new chemical structures and harvest the energy necessary for survival through diverse metabolic processes. Dispose of wastes. Cells must dispose of the waste products they produce so they do not accumulate and disrupt normal cellular activities. Page 108 In addition, some cells are capable of undergoing cell division to make more cells of the same type, as described in section 4.9. These new cells help to maintain the tissue or organ to which they belong by providing cells for new growth and replacing cells that die. However, during development, some cells do not retain this ability (e.g., most nerve cells typically do not; see section 12.2a). WHAT DID YOU LEARN? 3 Diagram the three main components of the cell and label the plasma membrane, nucleus, and cytoplasm. 4 What cellular structure is responsible for forming the boundary of a cell and maintaining its integrity? McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 12 of 101 4.2 Chemical Structure of the Plasma Membrane The plasma membrane is not a rigid boundary, but rather is a fluid matrix composed of an approximately equal mixture, by weight, of lipids and proteins. It regulates the movement of most substances both into and out of a cell. 4.2a Lipid Components LEARNING OBJECTIVE 7. List the lipid components of the plasma membrane, and explain the actions of each component. The plasma membrane contains several different types of lipids (see section 2.7b), including phospholipids, cholesterol, and glycolipids (figure 4.5). Figure 4.5 Structure and Functions of the Plasma Membrane. (a) The plasma membrane is a phospholipid bilayer, with cholesterol and proteins scattered throughout. Carbohydrates, which are attached to some phospholipids and proteins, extend from the surface to form the glycocalyx. (b) The plasma membranes (phospholipid bilayers) of two adjacent cells are visible in a TEM. Each plasma membrane has a width of approximately 5–10 nm. (b) ©Don W. Fawcett/Science Source Page 109 McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 13 of 101 Most of the plasma membrane lipids are phospholipids (see section 2.3c). Often these molecules are artistically portrayed in the membrane as an icon that looks similar to a balloon with two tails (see figure 2.13c). The balloonlike “head” is polar and hydrophilic. In contrast, the two “tails” are nonpolar and hydrophobic. Phospholipid molecules readily associate to form two parallel sheets of molecules lying tail-to-tail, with the hydrophobic tails forming the internal environment of the membrane and their hydrophilic polar heads positioned adjacent to either the cell’s cytoplasm or the interstitial fluid. This basic structure of the plasma membrane framework is called the phospholipid bilayer. The phospholipid bilayer ensures that cytosol remains inside the cell, and interstitial fluid remains outside. Cholesterol, a four-ring lipid molecule (see section 2.7b), is scattered within the inner hydrophobic regions of the phospholipid bilayer. It strengthens the membrane and stabilizes it at temperature extremes. Glycolipids (glī′kō-lip′id; glykys = sweet) are lipids with attached carbohydrate groups (see section 2.7b). Each carbohydrate group of a glycolipid is attached to a phospholipid molecule located on the outer phospholipid layer of the plasma membrane. These carbohydrates extend like “sugar antennae” from the cell’s external phospholipid surface, where they are exposed to the interstitial fluid. These molecules contribute to the glycocalyx, which is described at the end of this section. INTEGRATE CONCEPT CONNECTION Cholesterol is a component of plasma membranes found only in animal cells, not in plant cells. Consequently, any animal-derived food, such as eggs, milk, and meat, contains cholesterol. Foods obtained from plants, including carrots, corn, and potato chips cooked in vegetable oil, do not contain cholesterol. The lipid portion of the plasma membrane is insoluble in water, which ensures that the plasma membrane will not simply dissolve when it comes into contact with water. Rather, this boundary is an effective nonpolar physical barrier to most substances. Only small and nonpolar substances can readily penetrate (move through) this barrier without assistance (see section 4.3a ). WHAT DID YOU LEARN? 5 Diagram the lipid components of the plasma membrane and label the phospholipid bilayer, a cholesterol, and a glycolipid. 4.2b Membrane Proteins McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 14 of 101 LEARNING OBJECTIVE 8. Differentiate between the two types of membrane proteins based upon their relative position in the plasma membrane. 9. Explain the six major roles played by membrane proteins. Although lipids form the main component of the plasma membrane, the proteins dispersed within the lipids make up about half of the plasma membrane by weight. Proteins can “float” and move about the phospholipid bilayer, much like a beach ball floating on the water surface in a swimming pool. Most of the membrane’s specific functions are determined by its resident proteins. Membrane proteins are classified as one of two structural types: integral or peripheral. Integral proteins are embedded within, and extend completely across, the phospholipid bilayer ( figure 4.5). Hydrophobic regions within the integral proteins interact with the hydrophobic interior of the membrane. In contrast, the hydrophilic regions of the integral proteins are exposed to the aqueous environments on either side of the membrane. Many integral membrane proteins are glycoproteins, which are proteins with attached carbohydrate groups (see section 2.7e). These carbohydrates (like those of glycolipids) extend like “sugar antennae” from a cell’s external surface. These molecules contribute to the glycocalyx. In contrast, peripheral proteins are not embedded within the lipid bilayer. They are attached loosely to either the external or the internal surfaces of the membrane and are often “anchored” to the exposed parts of an integral protein. Membrane proteins are also categorized functionally based upon the specific role they serve ( figure 4.6). McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 15 of 101 Figure 4.6 Plasma Membrane Proteins. The major functional categories of plasma membrane proteins include the several types of transport proteins (e.g., channels, carrier proteins, pumps), cell surface receptors, identity markers, enzymes, anchoring sites for the cytoskeleton, and cell-adhesion proteins. Page 110 Transport proteins provide a means of regulating the movement of substances across the plasma membrane. Different types of transport proteins include channels, carrier proteins, pumps, symporters, and antiporters (see section 4.3). Cell surface receptors bind specific molecules called ligands. Ligands are molecules that bind to macromolecules (e.g., binding to a receptor). An example of a ligand is a neurotransmitter released from a nerve cell that binds to the cell surface receptor of a muscle cell. This binding will initiate muscle contraction (see section 10.3a). McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 16 of 101 Identity markers communicate to other cells that they belong to the body. Cells of the immune system use identity markers to distinguish normal, healthy cells from foreign, damaged, or infected cells that are to be destroyed (see section 22.4c). Enzymes may be attached to either the internal or the external surface of a cell for catalyzing chemical reactions (see section 3.3b). Anchoring sites secure the cytoskeleton (the internal, protein support of a cell) to the plasma membrane (see section 4.6b). Cell-adhesion proteins are for cell-to-cell attachments. Proteins that form membrane junctions perform a number of functions, including binding cells to one another (see section 22.3e). One additional cellular feature of a cell’s plasma membrane is a “coating of sugar” at the cell’ s external surface called the glycocalyx (glī′kō-kā′liks; kalyx = husk) (figure 4.5). The carbohydrates of both glycolipids and glycoproteins that extend outward from the plasma membrane compose the glycocalyx. The glycocalyx is unique to each cellular type and is important in cell-to-cell recognition (see sections 4.5a, 22.4c, and 29.2a). WHAT DID YOU LEARN? 6 What type of plasma membrane protein provides the means for moving materials across the plasma membrane? What are three subtypes? Page 111 4.3 Membrane Transport The plasma membrane has four primary functions (figure 4.5). It serves as the physical barrier between a cell and the fluid that surrounds it, called the interstitial fluid, as described in section 4.2. In addition, the plasma membrane is a selectively permeable boundary that regulates movement of materials into and out of a cell through membrane transport, establishes and maintains electrochemical gradients across the plasma membrane, and functions in cell communication. In this section, we discuss its role in membrane transport. Its functions in McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 17 of 101 establishing electrochemical gradients and in cell communication are described in sections 4.4 and 4.5, respectively. Refer to figure 4.7—an organizational flowchart of membrane transport processes—as you read through this section. Figure 4.7 Membrane Transport. Membrane transport is organized into passive processes and active processes depending upon the requirement for cellular energy. Passive processes do not require cellular energy, whereas active processes do. Substances (e.g., ions, molecules, water) move or are moved across the plasma membrane through several different processes collectively called membrane transport. These processes are organized into two major categories based upon the requirement for expending cellular energy. Passive processes do not require expenditure of cellular energy. Instead, these processes simply depend upon the kinetic energy (or random movement) of ions and molecules as each moves down its concentration gradient (i.e., from where there is more of it to where there is less). Diffusion (which involves the movement of solutes down their concentration gradient) and osmosis (which involves the movement of water across a semipermeable membrane down its concentration gradient) are the two major types of passive processes. Active processes differ from passive processes in that they require expenditure of cellular energy. The cellular energy could be used for active transport, where a solute (ion or molecule) is moved up its concentration gradient (i.e., from where there is less of it to where there is more). The cellular energy could also be used for vesicular transport, which involves either a vesicle releasing its contents from a cell or a vesicle being formed as material is moved into a cell. 4.3a Passive Processes: Diffusion McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 18 of 101 LEARNING OBJECTIVES 10. Define the general concept of diffusion. 11. Distinguish between the cellular processes of simple diffusion and facilitated diffusion. Diffusion is the movement of either ions (e.g., Na+) or molecules (e.g., glucose) down their concentration gradient. Diffusion occurs due to the kinetic energy of the ions or molecules. Kinetic energy is the random, constant motion of ions and molecules, which, when they strike other objects, bounce off in a different direction. For example, when dye is placed in a beaker of water, both the dye molecules and the water molecules are in random motion (figure 4.8). Kinetic energy is a function of temperature; as temperature increases, the rate of motion of the molecules increases, and thus its kinetic energy increases. Figure 4.8 Diffusion. When a drop of dye is placed into a beaker of water, a concentration gradient is established (a). The dye molecules move down their concentration gradient, spreading out by diffusion (b) until equilibrium has been reached (c). Diffusion involves several important characteristics. Note the following: Diffusion is dependent upon a concentration gradient (figure 4.8a). A concentration gradient exists where there is more of a specific substance in one area than another. For example, after a drop of dye is placed in a beaker of water, there is more dye where the drop of dye enters the water than in other areas of water in the beaker. Thus, a concentration gradient exists for the dye. The greater the difference in concentration of the substance in one area and the other, the steeper the concentration gradient. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 19 of 101 Diffusion involves the spreading out of ions and molecules (figure 4.8b). The random movement of ions and molecules allows the substance to spread out or diffuse. The substance moves from where there is more of the substance to where there is less (i. e., down its concentration gradient). The warmer the temperature and the steeper the concentration gradient, the more quickly this occurs. If unopposed, diffusion results in reaching equilibrium (figure 4.8c). Ultimately, if the movement of the substance is unopposed, the substance can become equally distributed within an area or areas. When this occurs, the substance is said to have reached equilibrium. Diffusion Involving a Cell Several concepts are specific to understanding diffusion involving a cell. (a) The distribution of most ions and molecules associated with cells is not equal between the inside and the outside of a cell. Some substances generally have a greater concentration inside the cell than outside the cell (e.g., CO2, K+), whereas others have a greater concentration outside the cell than inside (e. g., O2, Na+, Ca2+, H+). The normal distribution of various substances within and outside of cells can be viewed in figure 25.2. (b) The concentration gradient between the inside and the outside of the cell determines whether the solute diffuses into the cell or out of the cell. As noted earlier, solutes always diffuse from an area where they are more concentrated to an area where they are less concentrated. (c) The chemical characteristics of the diffusing solute dictate whether it moves across the plasma membrane unassisted between the phospholipid molecules or it is assisted with a protein embedded in the plasma protein. It is because of this difference in a substance moving across a plasma membrane unassisted or assisted by a protein that the processes of diffusion are organized into two major categories: simple diffusion and facilitated diffusion. Page 112 Simple Diffusion Molecules that are small and nonpolar (e.g., O2; see section 2.3c) move into or out of a cell down their concentration gradient by simple diffusion. These molecules move unassisted across the plasma membrane (i.e., they do not require a transport protein). Their chemical characteristics (i.e., small and nonpolar) allow these molecules to simply pass between the phospholipid molecules of the plasma membrane (figure 4.9). Molecules that move via simple diffusion include respiratory gases (O2 and CO2), small fatty acids, ethanol, and urea (a nitrogenous waste produced from amino acids). McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 20 of 101 Figure 4.9 Simple Diffusion of Solutes. Simple diffusion occurs when small, nonpolar molecules pass unassisted between plasma membrane phospholipid molecules. Each type of molecule moves down its concentration gradient. Here, oxygen diffuses into a cell and carbon dioxide diffuses out of a cell. The plasma membrane cannot regulate simple diffusion—rather, the movement of these molecules is dependent only upon the concentration gradient. Each type of molecule continues to diffuse across the plasma membrane as long as its concentration gradient exists. Impaired respiratory and cardiovascular function can alter the concentration gradients of oxygen and carbon dioxide, resulting in decreased diffusion of these gases (see sections 23.6b and c). Facilitated Diffusion Small solutes that are charged ions or polar molecules are effectively blocked from passing through the plasma membrane by the nonpolar phospholipid bilayer. Their transport either into or out of the cell, down their concentration gradient, must be assisted by plasma membrane proteins in a process called facilitated (fa-sil’i-tā-ted) diffusion. (To facilitate is to help or to assist; thus, a plasma membrane protein is helping or assisting the substance across the plasma membrane.) Two types of facilitated diffusion—channel-mediated diffusion and carrier- mediated diffusion—are distinguished by the type of transport protein used to facilitate the substance across the plasma membrane (see figure 4.7). Channel-mediated diffusion is the movement of small ions (e.g., Na+, K+; see section 2.2a) across the plasma membrane through water-filled protein channels (figure 4.10a). Each channel is typically specific for one type of ion. The channel is either a leak channel, which (as a general rule) is continuously open, or a gated channel. A gated channel is usually closed, opens only in response to a stimulus (e.g., chemical, light, voltage change), and then stays open for just a fraction of a second before it closes. For example, Na+ leak channels allow Na+ to pass through continuously. In contrast, chemically gated Na+ channels open to allow Na+ to move through McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 21 of 101 the channel only when it temporarily opens in response to the presence of a particular chemical (e.g., neurotransmitter). Channel-mediated diffusion of ions is important in the normal function of both muscle cells and nerve cells. Figure 4.10 Facilitated Diffusion of Solutes. Facilitated diffusion occurs when ions or small, polar molecules are transported down their concentration gradient and assisted across the plasma membrane by plasma membrane proteins. (a) Channel-mediated diffusion: Ions (e. g., Na+, K+) move through specific water-filled protein channels. (b) Carrier-mediated diffusion: Small, polar molecules (e.g., glucose) are transported by protein carriers. Carrier-mediated diffusion is the movement of polar molecules (e.g., glucose, amino acids; see section 2.3c) across the plasma membrane. The relatively larger size of polar molecules (in comparison to ions) requires that their movement across the plasma membrane be assisted by a carrier protein. Three primary events take place in carrier-mediated diffusion: (1) The carrier protein within the plasma membrane binds the polar molecule, which (2) induces the carrier protein to change shape and move or carry the polar molecule to the other side of the plasma membrane, where (3) it is released. Like channels, a carrier moves a substance down its McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 22 of 101 gradient; however, note that carrier-mediated diffusion involves a conformational change in the carrier protein for the transport of the molecule across the plasma membrane. figure 4.10b depicts how a carrier protein binds the substance, changes shape, and then releases the substance on the other side of the membrane. A carrier that transports only one substance is called a uniporter. Glucose carriers are uniporters that normally prevent the loss of glucose in the urine (see section 24.6c). Page 113 The number of channels and carriers in a plasma membrane determines the maximum rate at which a substance can be transported (i.e., its transport maximum; see section 24.6b). Thus, a cell can alter the transport rate of a given substance down its concentration gradient by changing either the number of channels or carrier proteins in the plasma membrane. A greater rate occurs with increased numbers of these transport proteins, and a lesser rate with decreased numbers. WHAT DID YOU LEARN? 7 How does O2 diffuse into a cell and CO2 diffuse out of a cell? 8 Diagram a flowchart for the passive processes of diffusion and include the important characteristics, and a sketch, for simple diffusion, channel-mediated facilitated diffusion, and carrier-mediated diffusion. 4.3b Passive Processes: Osmosis LEARNING OBJECTIVES 12. Define osmosis and osmotic pressure. 13. Describe the relationship of osmosis and tonicity. Osmosis is unlike the other types of passive membrane transport, because it involves water movement and does not involve the movement of solutes (see figure 4.7). Osmosis (os-mō’sis; osmos = a thrusting) is the passive movement of water through a semipermeable (or selectively permeable) membrane. This movement occurs in response to a difference in relative concentration of water on either side of a membrane. Please refer to figure 4.11 as you read through this section. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 23 of 101 Figure 4.11 Osmosis in Cells. Osmosis occurs in cells across the plasma membrane, which is permeable to water but non-permeable to most solutes. Water always moves across the plasma membrane from an area of high water concentration to an area of low water concentration until equilibrium is reached. INTEGRATE CONCEPT CONNECTION McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 24 of 101 Recall from chapter 2 that water is a solvent, and substances that dissolve in water are solutes (e. g., Na+, glucose; see section 2.4c), and a homogeneous mixture of a solvent and dissolved solutes is a solution (see section 2.6a). Plasma Membrane: A Selectively Permeable Membrane The plasma membrane is a semipermeable membrane that allows the passage of water, but its phospholipid bilayer prevents the movement of most solutes A plasma membrane is also more specifically a selectively permeable membrane because the movement of most solutes is regulated (selectively) by this barrier. Water molecules cross the plasma membrane in one of two ways: Either they “slip between” the molecules of the phospholipid bilayer (limited amounts) or they move through integral protein water channels called aquaporins (ak-kwă-pōr′in; aqua = water, porus = channel). Thus, cells can alter the amount of water that crosses the plasma membrane by changing the number of aquaporins. The phospholipid bilayer of the plasma membrane is non-permeable to most solutes. In the context of osmosis, solutes are classified into two categories based upon whether their passage across the plasma membrane is prevented by the phospholipid bilayer. Permeable solutes (e.g., small and nonpolar solutes such as oxygen, carbon dioxide, and urea) pass through the bilayer, and non-permeable solutes (e.g., charged, polar, or large solutes such as ions, glucose, and proteins) are prevented from crossing the bilayer. (The term solutes in this discussion on osmosis refers to non-permeable solutes.) Concentration Gradients Across the Plasma Membrane A difference in solute concentration can exist between the cytosol (fluid within the cell) and the fluid surrounding the cell because solutes are prevented from moving across the phospholipid bilayer of the plasma membrane. Note that when a solute concentration exists, a water concentration also exists. A solution with a greater concentration of solutes contains a lower concentration of water. For example, a solution containing 3% solutes has a lower water concentration (97% water) than a solution with 1% solutes (99% water). Note that solute percentage reflects the collective percentage of all of the solutes (e.g., glucose, proteins, Na+). Page 114 Movement of Water into or Out of a Cell by Osmosis The net movement of water by osmosis is dependent upon the concentration gradient between the cell’s cytosol and the solution in which the cell is immersed. For example, water moves down its concentration gradient from the solution containing 1% solutes (and 99% water) into the cell containing 3% solutes (and 97% water). Water continues to move until equilibrium is reached (i.e., the concentration of water in the cell equals the concentration of water in the surrounding fluid). Note that water moves toward the solution with the lower water concentration (stated another way, water moves toward the solution with the greater solute concentration). Figure 4.11 shows water moving across the plasma membrane by osmosis from an area of high water concentration to an area of low water concentration. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 25 of 101 INTEGRATE CONCEPT CONNECTION Osmosis is important in several significant physiologic processes that are discussed later in the text, including capillary exchange between the blood and body cells (see section 20.3), formation of urine (see section 24.6), and regulation of fluid balance (see section 25.2). Osmotic Pressure Osmotic pressure is the pressure exerted by the movement of water across a semipermeable membrane due to a difference in water concentration. The steeper the gradient, the greater the amount of water moved by osmosis and the higher the osmotic pressure. Figure 4.12 helps us visualize the movement of water by osmosis. Each U-shaped tube has two areas separated by a semipermeable membrane that allows the passage of water but restricts the passage of solutes. Initially, side A has more solutes and less water than side B. Water moves from side B into side A by osmosis (against the force of gravity) until the two fluids are equal in water concentration. Figure 4.12 Osmotic Pressure. The semipermeable membrane allows the passage of water but restricts the passage of solutes. If a water gradient exists, water moves by osmosis from where it is more concentrated (side B) to where it is less concentrated (side A) until equilibrium is reached. Osmotic pressure is the pressure exerted by this movement of water. Osmotic pressure can be measured indirectly. Imagine placing a stopper on side A in figure 4.12 and exerting force to return the fluid to its original level. The force exerted increases hydrostatic pressure within the U-shaped tube. (Hydrostatic pressure is the pressure exerted by a fluid on the inside wall of its container.) The osmotic pressure exerted in this setup is equal to the hydrostatic pressure produced to return the fluid to its original level. WHAT DO YOU THINK? McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 26 of 101 1 Which setup would exhibit the greater osmotic pressure: a cell with a cytosol concentration of 0.9% immersed in (a) pure water or (b) a 0.2% NaCl solution? Explain. INTEGRATE LEARNING STRATEGY 4.1 You may find it helpful to think of osmotic pressure as the pressure that is exerted by solutes to draw, or “pull,” water into an area where there is a higher concentration of solutes. The higher the relative solute concentration, the greater the osmotic pressure— thus, a greater amount of water is drawn into an area by osmosis. Osmosis and Tonicity When water crosses the plasma membrane of a cell by osmosis, the cell either gains or loses water with an accompanying change in the cell’s volume. The ability of a solution to change the volume or pressure (sometimes called the tone) of the cell by osmosis is called tonicity. Three specific terms are used to describe the relative concentration of water in these solutions: isotonic , hypotonic, and hypertonic (figure 4.13). We describe the relative water concentration and the net movement of water by osmosis when cells are immersed in each of these categories of solutions. For each type of solution, we refer you to the illustrations and photos in figure 4.13 to specifically show the changes to erythrocytes (red blood cells). Figure 4.13 Erythrocytes Immersed in Three Different Solution Concentrations. (a) Erythrocytes are immersed in an isotonic solution (e.g., 0.9% NaCl). As a result, there is no net McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 27 of 101 movement of water between the erythrocytes and the solution, and the cell shape remains unchanged. (b) Erythrocytes immersed in a hypotonic solution (e.g., pure water) will result in water moving from the solution (where there is more water) into the erythrocytes (where there is less water), and the erythrocytes swell. (c) Erythrocytes immersed in a hypertonic solution (e. g., 3% NaCl) will result in water moving from the erythrocytes (where there is more water) into the solution (where there is less water), and the erythrocytes shrivel (i.e., undergo crenation). ©Dennis Kunkel Microscopy, Inc./Medical Images An isotonic (ī’sō-ton’ik; iso = equal, tonus = stretching) solution has the same relative concentration of solutes and water as the cell’s cytosol. Under these conditions, the relative amounts of water outside and inside the cell are equal (i.e., there is no water concentration gradient). Consequently, there is no net movement of water between the cell and the solution. figure 4.13a shows erythrocytes in an isotonic solution. Notice the normal biconcave shape of the erythrocytes. An example of a solution that is isotonic to erythrocytes in blood is an intravenous (IV) solution of normal saline, which has a concentration of 0.9% NaCl (and 99.1% water). Normal saline is used commonly in intravenous [IV] solutions to maintain a patient’s fluid balance because erythrocytes retain their normal shape with no net movement of water either into or out of these cells (see Clinical View 25.1: “Intravenous [IV] Solution”). Page 115 A hypotonic (hī’pō-ton’ik; hypo = under) solution has a lower concentration of solutes and a higher concentration of water than the cell’s cytosol. Pure water contains no solutes, so it is 100% water and is the most extreme example of solution that is hypotonic to the cell’s cytosol. Under these conditions, water moves down its concentration gradient from where there is more water outside the cell to where there is less water inside the cell. The entry of water into the cell increases both the volume and the pressure within the cell. Figure 4.13b shows erythrocytes in a hypotonic solution. Observe that water has entered the erythrocytes causing them to swell. Lysis (or rupture) of cells can occur if the difference in water concentration is large enough. Hemolysis (hē-mol’i-sis; hem = blood, lysis = destruction) is the specific term for rupturing erythrocytes. Hemolysis is why nurses do not administer intravenous (IV) solutions of pure water—an error that results in cell lysis and, depending on the amount administered, can be fatal. A hypertonic (hī’pĕr-ton’ik; hyper = above) solution has a higher concentration of solutes, and thus a lower concentration of water than the cell’s cytosol. For example, a solution that contains 3% NaCl (97% water) is hypertonic to erythrocytes (which are normally 0.9% solutes and 99.1% water). In this case, water moves out of the cell into the surrounding fluid, where the water concentration is lower. Thus, a decrease in cell volume and pressure occurs. Figure 4.13c shows erythrocytes in a hypertonic solution. Observe that water has exited the erythrocytes causing them to become smaller. If the difference in concentration is large, the cell shrinks, a process called crenation (krē-nā’shŭn; crena = notch). This prevents us from being able to replace fluid by drinking seawater, which has an average salt concentration of 3.5%. Keep the following summary in mind relative to tonicity and the movement of water into and out of cells by osmosis when cells are immersed in a solution: An isotonic solution has the same relative concentration of water as the cell’s cytosol and there is no net movement of water between the solution and the cell. In comparison, a hypotonic solution has more water than the McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 28 of 101 cell’s cytosol and there is a net movement of water by osmosis into the cell, whereas a hypertonic solution has less water than a cell’s cytosol and there is a net movement water by osmosis out of the cell. WHAT DID YOU LEARN? 9 What is osmosis? 10 What occurs to the tonicity of a cell when it is placed into an isotonic, a hypotonic, or a hypertonic solution? 11 What general conclusion can you make concerning the movement of water? There is always a net movement of water by osmosis toward (a) an isotonic solution, (b) a hypotonic solution, or (c) a hypertonic solution. INTEGRATE LEARNING STRATEGY 4.2 Tonicity is a measure of the solute concentration of fluid surrounding a cell relative to the fluid within a cell. Using word roots can help you keep the different types of tonicity straight. Iso means same as. The solute concentration of an isotonic solution is the same as that of the cytosol, and so there is no net movement of water. Hypo means low or under. The solute concentration of a hypotonic solution is lower than that of the cytosol, and so there is relatively more water in the solution and water moves into the cell. Hyper means more than. The solute concentration of a hypertonic solution is higher than that of the cytosol, and so there is relatively less water in the solution and water moves out the cell. Page 116 4.3c Active Processes LEARNING OBJECTIVES 14. Compare and contrast primary and secondary active transport. 15. Compare and contrast the types of vesicular transport (i.e., exocytosis and the different types of endocytosis). McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 29 of 101 Active processes of membrane transport are those that require the expenditure of cellular energy and occur only in living organisms. Active processes may be subdivided into active transport and vesicular transport (see figure 4.7). Active Transport Active transport is the movement of a solute against its concentration gradient (i.e., movement from a low concentration to a high concentration) across a cellular membrane. Recall from section 4.3a that most molecules and ions associated with cells are not distributed equally between the inside and the outside of a cell (see figure 25.2). The energy-requiring processes of active transport are essential in maintaining these differences. Thus, all concentration gradients across the plasma membrane are due to active transport processes, and the cell is continuously expending energy to maintain these gradients! The two types of active transport are primary active transport and secondary active transport, which are distinguished by their specific energy source (see figure 4.7). Primary Active Transport Primary active transport uses energy derived directly from the breakdown of ATP (see section 3.2 b). This breakdown also provides the phosphate group that is added to the membrane transport pump, resulting in a change in the protein’s shape and the subsequent movement of a solute across the membrane. The addition of the phosphate to a protein is called phosphorylation (see section 3.3g). Cellular protein pumps that move ions across the membrane are more specifically called ion pumps. Ion pumps are a major factor in a cell’s ability to maintain its internal concentrations of ions. As an example, Ca2+ pumps embedded in the plasma membranes of erythrocytes move calcium out of the erythrocyte to prevent it from becoming rigid due to the accumulation of calcium (figure 4.14). Therefore, the erythrocyte remains flexible enough to move through capillaries (the smallest blood vessels; see section 20.1c). McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 30 of 101 2+ Figure 4.14 Ca 2+ Pump. A Ca pump uses ATP to move calcium ions against the calcium gradient from the inside to the outside of the cell. H+ pumps are another type of transport protein that function in maintaining cellular pH. Recall that H+ pumps are required in establishing a H+ gradient between the outer and inner compartment of mitochondria as a part of the electron transport system (see section 3.4e). The diffusion of H+ down this concentration allows ATP synthase to harness the energy to form ATP from ADP and Pi (see figure 3.19). H+ pumps are also required to establish the low pH in lysosomes, one of the membrane-bound organelles (see section 4.6a). INTEGRATE CONCEPT CONNECTION Ion concentration gradients established by specialized ion pumps that engage in active transport are important in numerous specialized cells. For example, Na+/K+ pumps establish and maintain concentration gradients for Na+ and K+ in skeletal muscle fibers (see section 10.2d ), cardiac muscle cells (see section 19.7a), and nerve cells (see section 12.7b). In comparison Ca2+ pumps establish and maintain concentration gradients for Ca2+ in synaptic knobs of nerve McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 31 of 101 cells (see section 12.7b), the sarcoplasmic reticulum organelle in skeletal muscle cells (see section 10.3b), and pacemaker cells of the heart (see section 19.6a). In fact, ion pumps are present and function to establish concentration gradients in all cells of the body. Page 117 + + The sodium-potassium (Na /K ) pump is a special type of ion pump. It is specifically called an exchange pump because it moves one type of ion into a cell against its concentration gradient, while moving another type of ion out of the cell against its concentration gradient. (You may find it helpful to think of the Na+/K+ pump as a “dual pump” because it moves two different ions against their respective concentration gradients.) The plasma membrane preserves steep concentration gradient differences for these ions by continuously exporting Na+ out of the cell and moving K+ into the cell. Figure 4.15 shows the steps in the process in which 3 Na+ ions are pumped out of a cell for every 2 K+ ions that are pumped into a cell. The cell must expend ATP to maintain the levels of these ions on each side of the membrane. The Na+/K+ pump is also called a sodium-potassium ATPase because this pump is an enzyme that splits ATP to power the pump. There is a 1:2:3 ratio for this pump: 1 ATP is required to pump 2 K+ ions into the cell and 3 Na+ ions out of the cell. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 32 of 101 + + Figure 4.15 + + Na /K Pump. The Na /K pump is a plasma membrane transport protein + + that uses ATP to move both Na and K ions through the plasma membrane in opposite directions from their region of low concentration to their region of high concentration (1 ATP is split for moving 3 Na+ out of the cell and 2 K+ into the cell). Page 118 Secondary Active Transport Secondary active transport is also called cotransport, or coupled transport. As previously described, primary active transport uses ATP to provide the energy to move a substance up its concentration gradient. In contrast, secondary active transport involves the movement of a substance (e.g., Na+) down its concentration gradient to provide the energy to move a different substance (e.g., glucose, H+) up its concentration gradient. Put another way, the kinetic energy of one substance (usually Na+) moving down its concentration gradient across the membrane provides the “power” to pump another substance against its concentration gradient across the membrane (much as water moving over a dam and turning a water wheel can generate McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 33 of 101 electricity; see section 3.1a). The Na+ gradient is often the source of energy because its concentration gradient across the plasma membrane is extremely steep (about 99% of Na+ is in the interstitial fluid, with only 1% in the cytosol). The Na+ concentration gradient has potential energy that is harnessed and converted to kinetic energy when Na+ moves into the cell down its concentration gradient in secondary active transport (see figure 4.7). The two types of secondary active transport include symport and antiport. Figure 4.16 compares the processes of symport (using a symporter protein) and antiport (using an antiport protein) with the movement of Na+ as the energy source. In the symport example, glucose binds to the symporter (or cotransporter) in the plasma membrane (figure 4.16a ). This binding helps alter the shape of the symporter and then both glucose and Na + are transported into the cell. The Na+ moves down its concentration gradient into the cell and provides the energy to move glucose up its concentration gradient into the cell. Notice that Na+ (moving down its concentration gradient) and glucose (being moved up its concentration gradient) are crossing the plasma membrane in the same direction (i.e., both Na+ and glucose move into the cell.) In contrast, in the antiport example, an antiporter (or countertransporter) moves the two substances in the opposite direction (figure 4.16b). In this case, Na+ (moving down its concentration gradient) and H+ (being moved up its concentration gradient) are crossing the plasma membrane in the opposite direction (i.e., Na+ moves into the cell as H+ is moved out of the cell). Thus, the two types of secondary active transport are distinguished as follows: Symport. The two substances are moved in the same direction by a symporter; a process called symport secondary active transport. Antiport. The two substances are moved in opposite directions by an antiporter; a process called antiport secondary active transport. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 34 of 101 Figure 4.16 Secondary Active Transport. Secondary active transport is powered by the movement of a substance (usually Na+) down its concentration gradient to move a different substance up its concentration gradient. (a) A symporter transports both substances in the same direction; a process called symport secondary active transport. (b) An antiporter transports the two substances in opposite directions; a process called antiport secondary active transport. Remember, in both symport and antiport, the kinetic energy of Na+ moving down its concentration gradient is harnessed to move a different substance (e.g., glucose, H+) up its concentration gradient. Note that secondary active transport mechanisms are ultimately dependent upon the primary active transport mechanisms of Na+/K+ pumps (described earlier). The Na+/K+ pumps produce and sustain a distinct concentration gradient difference between Na+ on opposite sides of the plasma membrane, with substantially more Na+ in the interstitial fluid and less Na+ in the cytosol. Page 119 Vesicular Transport Vesicular transport, also called bulk transport, involves a vesicle (ves’i-kl; vesica = bladder), which is a membrane-bound sac filled with materials (see figure 4.4). Vesicular transport allows for the movement of large substances (or large amounts of a substance) across the plasma membrane. The two types of vesicular transport are exocytosis and endocytosis (see figure 4.7). Exocytosis The means by which either large substances or large amounts of substances are secreted from the cell is called exocytosis (ek’sō-sī-tō’sis; exo = outside, osis = condition of) (figure 4.17). Macromolecules, such as large proteins and polysaccharides (see section 2.7), are too big to be moved across the plasma membrane, even with the assistance of transport proteins. The McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 35 of 101 material for secretion typically is packaged within intracellular transport vesicles. When the membrane of the vesicle and plasma membrane come into contact, the phospholipid molecules of the vesicle and plasma membrane bilayers rearrange themselves so that the two membranes fuse. The fusion of these lipid bilayers requires the cell to expend energy by splitting ATP. Following fusion, the vesicle contents are released to the outside of the cell. An example of exocytosis is the release of neurotransmitter molecules from nerve cells (see section 12.8d). Figure 4.17 Exocytosis. In exocytosis, the cell secretes bulk volumes of materials within cellular vesicles into the interstitial fluid as a vesicle fuses with the plasma membrane. Fusion of the vesicle to the plasma membrane is the energy-requiring step. The membrane of the vesicle becomes incorporated into the plasma membrane and only the contents within the vesicle is released. INTEGRATE CLINICAL VIEW 4.1 Familial Hypercholesterolemia Familial hypercholesterolemia is an inherited genetic disorder that involves either defective or absent cellular receptor proteins that bind low-density lipoproteins (LDLs), defects in the proteins of the LDLs, or other possible mutations (see Clinical View 29.11: “Genetics of Familial Hypercholesterolemia”). Defects in either the LDL receptor or the proteins of the LDLs interfere with the normal process of receptor-mediated endocytosis of cholesterol into cells. LDLs that contain cholesterol remain in the blood, resulting in greatly elevated levels of blood cholesterol. Consequently, cholesterol accumulates in the blood vessels, causing plaque buildup and narrowing of the blood vessels (i.e., atherosclerosis; see Clinical View 20.1: McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 36 of 101 “Atherosclerosis”), especially those providing blood to the heart (coronary vessels; see figure 19.11). Individuals with this genetic defect are likely to experience blockage of the coronary arteries, resulting in a heart attack (see Clinical View 19.5: “Coronary Heart Disease, Angina Pectoris, and Myocardial Infarction”). The age of occurrence of a heart attack depends upon the severity of the protein defect. In severe cases, an individual may experience a heart attack during the teen years. Page 120 Endocytosis The cellular uptake of either large substances or large amounts of substances from the external environment into the cell is called endocytosis (en’dō-sī-tō’sis; endon = within). Endocytosis is used for the uptake of nutrients and extracellular debris for digestion, retrieval of membrane regions added to the plasma membrane during exocytosis, and regulation of composition of membrane proteins to alter cellular processes (e.g., cell communication by altering the number of receptors within the plasma membrane; see section 17.6). The steps of endocytosis are similar to the exocytosis steps, only in reverse. Endocytosis occurs when substances within the interstitial fluid are packaged into a vesicle that forms at the cell surface for internalization into the cell (figure 4.18). A small area of plasma membrane folds inward into the cytosol to form a pocket, or invagination (in-vaj′i-nā-zhun; vagina = a sheath). The pocket deepens as endocytosis proceeds and then it pinches off when the lipid bilayer fuses. Severing of the newly forming vesicle from the plasma membrane requires specialized proteins and is the energy-expending step. The new intracellular vesicle now present contains material that was formerly outside the cell. McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 37 of 101 Figure 4.18 Three Forms of Endocytosis. Endocytosis is the process whereby a vesicle is formed as the cell acquires materials from the interstitial fluid. (a) Phagocytosis occurs when plasma membrane extensions called pseudopodia engulf a relatively large particle and internalize it into a vesicle. (b) Pinocytosis is the incorporation of numerous droplets of interstitial fluid into the cell in small vesicles as many regions of the plasma membrane invaginate. (c) Receptor-mediated endocytosis occurs when specific molecules bind to receptors in the plasma membrane. These receptors with bound molecules then aggregate within the membrane at special regions that contain clathrin on the internal surface of the plasma membrane. The membrane invaginates, forming a vesicle and the contents is internalized. The formation of the vesicle in each of the processes of endocytosis is the energy- requiring step. The three types of endocytosis include phagocytosis, pinocytosis, and receptor-mediated endocytosis. They are differentiated based upon the specific material being transported and the mechanism involved (see figure 4.7). Phagocytosis (fag’ō-sī-tō’sis; phago = to eat) means cellular eating. It is a nonspecific process that occurs when a cell engulfs or captures a large particle external to the cell by forming membrane extensions that are called pseudopodia (sū-dō-pō’dē-ă; pseudes = false, pous = foot) or false feet, to surround the particle (figure 4.18a). Once the particle is engulfed by the pseudopodia, it is enclosed with what was previously part of the plasma membrane. This newly formed vesicle typically fuses with a lysosome (a cellular organelle containing digestive enzymes, described in section 4.6a). The molecules composing the ingested material are broken McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 38 of 101 down or digested by the enzymes within the lysosome. Only a few types of cells are able to perform phagocytosis. For example, it occurs regularly when a white blood cell engulfs and digests a microbe (e.g., bacterium; see section 22.3c). Pinocytosis (pin’ō-sī-tō’sis or pī’nō-; pineo = to drink) is also known as cellular drinking. This process occurs when multiple, small regions of the plasma membrane invaginate and multiple, small vesicles are formed as the cell internalizes interstitial fluid that contains dissolved solutes (figure 4.18b). This process is considered nonspecific because all solutes dissolved within the interstitial fluid are taken into the cell. Most cells perform this type of membrane transport. Receptor-mediated endocytosis uses receptors on the plasma membrane to bind specific molecules within the interstitial fluid and bring the molecules into the cell. This enables the cell to obtain bulk quantities of certain substances, even though those substances may not be very concentrated in the interstitial fluid. Receptor-mediated endocytosis begins when specific molecules called ligands (see section 4.2b ) that are within the interstitial fluid attach to their distinct integral membrane protein receptors in the plasma membrane to form a ligand-receptor complex (figure 4.18c). Following the binding of the ligand, the ligand-receptor complexes move laterally in the plane of the plasma membrane and accumulate at special membrane regions that contain clathrin protein on the internal surface of the membrane. The clathrin-coated regions of the plasma membrane housing the ligand-receptor complex folds inward to form an invagination called a clathrin- coated pit. This invagination deepens and pinches off, and the lipid bilayer of the plasma membrane fuses to form a clathrin-coated vesicle, which then moves into the cytosol. Following the formation of the clathrin-coated vesicles, the clathrin coat must be enzymatically removed before the vesicle may proceed to its intracellular destination. Again, it is the fusion of these lipid bilayers that requires the cell to expend energy. Following entry, receptors and ligands are uncoupled. Ligands may be stored, modified, or destroyed, and receptors (unless damaged) are returned to the plasma membrane. Page 121 The transport of cholesterol from the blood to a cell is an example of receptor-mediated endocytosis. When cholesterol is transported in the blood, it is bound to protein molecules in structures called low-density lipoproteins (or LDLs). LDLs move from the blood into the interstitial fluid and then bind to LDL receptors in the cell’s plasma membrane. LDLs are then internalized by the process of receptor-mediated endocytosis just described (see section 27.6b). The various types of membrane transport mechanisms are illustrated in figure 4.19. Passive processes are depicted on the left and active processes on the right in this two-page summary figure. Note as you observe this figure that the passive processes of diffusion and osmosis (which allow molecules and ions to move down their concentration gradient) facilitate the reaching of equilibrium (equal distribution of ions and molecules) across the plasma membrane. In comparison, active transport (which moves molecules and ions up their concentration gradient) opposes the reaching of equilibrium across the plasma membrane. It is the active transport processes that maintain cellular concentration gradients at the plasma membrane, which are necessary for normal cellular function. Page 122 McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 39 of 101 INTEGRATE CONCEPT OVERVIEW McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 40 of 101 McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 41 of 101 Figure 4.19 Passive and Active Processes of Membrane Transport. Transport processes are separated into two major categories. (a) Passive processes, which do not require expenditure of cellular energy, include simple diffusion, facilitated diffusion (channel-mediated and carrier- mediated), and osmosis. (b) Active processes, which require cellular energy, include active transport (primary and secondary) and vesicular transport (exocytosis and various forms of endocytosis). Page 123 McKinley, Michael, et al. Anatomy and Physiology : An Integrative Approach, McGraw-Hill US Higher Ed ISE, 2021. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/griffith/detail.action?docID=6461450. Created from griffith on 2025-05-30 04:06:18. Copyright © 2021. McGraw-Hill US Higher Ed ISE. All rights reserved. Ebook pages 387-493 | Printed page 42 of 101 WHAT DID YOU LEARN? 12 What transport process involved in the movement of Na+ down its gradient is used to power another substance up its gradient? 13 Diagram a flowchart for the active processes of active transport and vesicular transport. Include the important characteristics and a sketch f