Villenas-FA-Biochem-Lecture-Module-5-Anything-About-Cells PDF

Module 5: All About Cells TITLE: A TOUR OF THE CELL… WHERE ORGANELLES DWELL OVERVIEW: This module focuses on the fundamental unit of life, the cell. The complexities in the mechanisms of life resulted from the complexities of the structure of the cell. You may be overwhelmed by the t...

Module 5: All About Cells TITLE: A TOUR OF THE CELL… WHERE ORGANELLES DWELL OVERVIEW: This module focuses on the fundamental unit of life, the cell. The complexities in the mechanisms of life resulted from the complexities of the structure of the cell. You may be overwhelmed by the terms for this set of cell organelles and membranes, but you have to bear with them because from them where everything starts, specifically the reactions happening within our body. We will explore the distinguishing structures found in prokaryotic (bacteria) and eukaryotic cells (plant and animal cells) and their functions. LEARNING OBJECTIVES: After putting your heart and mind in studying this module, you will be able to: 1. Determine the role of cells in organisms; 2. Explain the key points of cell theory and the individual contributions of Hooke, Schleiden, Schwann, Remak, and Virchow; 3. Determine the major structures of prokaryotic cells; 4. Compare and contrast the structures of the eukaryotic cell and prokaryotic cell; 5. Determine the structural differences between animal and plant cells; and 6. Determine the functions of the major cell structures. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 1 of 45 LECTURE DISCUSSION: Figure 5.1 (a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope),and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different organisms, yet all share certain characteristics of basic cell structure. (credit a: modification of work by Ed Uthman, MD; credit b: modification of work by Umberto Salvagnin; credit c: modification of work by Anthony D'Onofrio, William H. Fowle, Eric J. Stewart, and Kim Lewis of the Lewis Lab at Northeastern University; scale-bar data from Matt Russell) Close your eyes and picture a brick wall. What is the basic building block of that wall? A single brick, of course. Like a brick wall, your body is composed of basic building blocks called “cells.” Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is constructed from many cell types. For example, epithelial cells protect the surface of the body and cover the organs and body cavities within. Bone cells help to support and protect the body. Immune system cells fight invading pathogens. Additionally, blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide and other waste. Each of these cell types plays a vital role during the growth, development, and ongoing maintenance of the body. In spite of their enormous variety, however, cells from all organisms—even organisms as diverse as bacteria, onion, and human— share certain fundamental characteristics. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 2 of 45  5.1. What are the main functions of cells? Give at least two specific types of cell and their functions. Cell Theory How are we able to observe the very small cells in which most of them, we cannot see with our naked eyes? We were able to do the observation by using the microscope. There are many kinds of microscope nowadays and the most common one being used in the laboratory is the compound microscope. If you want to have a more vivid and detailed observations, an electron microscope is the best. The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of single-celled organisms, which he collectively termed “animalcules.” The English scientist Robert Hooke first used the term “cells” in 1665 to describe the small chambers within cork that he observed under a microscope of his own design. To Hooke, thin sections of cork resembled “Honey-comb,” or “small Boxes or Bladders of Air.” He noted that each “Cavern, Bubble, or Cell” was distinct from the others. At the time, Hooke was not aware that the cork cells were long dead and, therefore, lacked the internal structures found within living cells. Despite Hooke’s early description of cells, their significance as the fundamental unit of life was not yet recognized. Nearly 200 years later, in 1838, Matthias Schleiden (1804–1881), a German botanist who made extensive microscopic observations of plant tissues, described them as being composed of cells. Visualizing plant cells was relatively easy because plant cells are Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 3 of 45 clearly separated by their thick cell walls. Schleiden believed that cells formed through crystallization, rather than cell division. Theodor Schwann (1810–1882), a noted German physiologist, made similar microscopic observations of animal tissue. In 1839, after a conversation with Schleiden, Schwann realized that similarities existed between plant and animal tissues. This laid the foundation for the idea that cells are the fundamental components of plants and animals. In the 1850s, two Polish scientists (Figure 3.1)living in Germany pushed this idea further, culminating in what we recognize today as the modern cell theory. In 1852, Robert Remak (1815– 1865), a prominent neurologist and embryologist, published convincing evidence that cells are derived from other cells as a result of cell division. However, this idea was questioned by many in the scientific community. Three years later, Rudolf Virchow (1821–1902), a well-respected pathologist, published an editorial essay entitled “Cellular Pathology,” which popularized the concept of cell theory using the Latin phrase omnis cellula a cellula (“all cells arise from cells”), which is essentially the second tenet of modern cell theory. Given the similarity of Virchow’s work to Remak’s, there is some controversy as to which scientist should receive credit for articulating cell theory. See the following Eye on Ethics feature for more about this controversy. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 4 of 45  5.2 What is the “Cell Theory”? 5.3 Who are the philosophers /scientists who contributed , and what are their contributions, on the development of this theory? A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms. Several cells of one kind that interconnect with each other and perform a Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 5 of 45 shared function form a tissue; several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells. There are many types of cells, all grouped into one of two broad categories: prokaryotic and eukaryotic. For example, both animal and plant cells are classified as eukaryotic cells, whereas bacterial cells are classified as prokaryotic. Cells fall into one of two broad categories: prokaryotic and eukaryotic. Only the predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro- = “before”; -karyon = “nucleus”). Cells of animals, plants, fungi, and protists are all eukaryotes (eu- = “true”) and have a nucleus. 5.4 What are the two broad/major categories of cells? Give examples of specific groups of organisms for each category. PROKARYOTIC CELLS Components of Prokaryotic Cells All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; 2) cytoplasm, consisting of a jelly- like cytosol within the cell in which other cellular components are found; 3) DNA, the genetic material of the cell; and 4) ribosomes, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 6 of 45  A prokaryote is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid (Figure 4.5). Figure 5.2 This figure shows the generalized structure of a prokaryotic cell. All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some, but not all, bacteria Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule (Figure 5.2; Figure 5.3). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are used by bacteria to attach to a host cell. 5.5 What are some of the accessory parts of a prokaryotic cell wall? Cell Size At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm (Figure 5.4). The small size of prokaryotes Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 7 of 45 allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 8 of 45 Figure 5.3 This is just a representation of a prokaryotic cell for easier viewing of parts and their functions. Most bacterial cells are either spherical (coccus) or rod- shaped (bacillus). This is a scanned copy from Advance Biology Revision Handbook by WR Pickering (1997), Oxford University Press Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 9 of 45 Figure 5.4 This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured). Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let us examine why that is so. First, we will consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr2, while the formula for its volume is 4πr3/3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide; another way is to develop organelles that perform specific tasks. These adaptations lead to the development of more sophisticated cells called eukaryotic cells. Besides Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 10 of 45 the volume of the cell, the size of the cell is also important for survival. As mentioned before, most cells are approximately spherical in shape. This is because a sphere is the shape with the largest surface area-to-volume ratio. As nutrients diffuse into the cell, a sphere is the shape where nutrients would have to travel the least distance to reach the center. This is important because nutrients and wastes are always exchanged at the periphery of the cell. The shorter the distance these nutrients and wastes have to travel, the faster the exchange of these molecules are. EUKARYOTIC CELLS Eukaryotic cells possess many features that prokaryotic cells lack, including a nucleus with a double membrane that encloses DNA. In addition, eukaryotic cells tend to be larger and have a variety of membrane-bound organelles that perform specific, compartmentalized functions. Evidence supports the hypothesis that eukaryotic cells likely evolved from prokaryotic ancestors; for example, mitochondria and chloroplasts feature characteristics of independently- living prokaryotes. Eukaryotic cells come in all shapes, sizes, and types (e.g. animal cells, plant cells, and different types of cells in the body). Like prokaryotes, all eukaryotic cells have a plasma membrane, cytoplasm, ribosomes, and DNA. Many organelles are bound by membranes composed of phospholipid bilayers embedded with proteins to compartmentalize functions such as the storage of hydrolytic enzymes and the synthesis of proteins. The nucleus houses DNA, and the nucleolus within the nucleus is the site of ribosome assembly. Functional ribosomes are found either free in the cytoplasm or attached to the rough endoplasmic reticulum where they perform protein synthesis. The Golgi apparatus receives, modifies, and packages small molecules like lipids and proteins for distribution. Mitochondria and chloroplasts participate in free energy capture and transfer through the processes of cellular respiration and photosynthesis, respectively. Peroxisomes oxidize fatty acids and amino acids, and they are equipped to break down hydrogen peroxide formed from these reactions without letting it into the cytoplasm where it can cause damage. Vesicles and vacuoles store substances, and in plant cells, the central vacuole Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 11 of 45 stores pigments, salts, minerals, nutrients, proteins, and degradation enzymes and helps maintain rigidity. In contrast, animal cells have centrosomes and lysosomes but lack cell walls. Have you ever heard the phrase “form follows function?” It is a philosophy practiced in many industries. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should be built with several elevator banks; a hospital should be built so that its emergency room is easily accessible. Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells (Figure 4.8). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus; 2) numerous membrane- bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria among others; and 3) several, rod-shaped chromosomes. Because a eukaryotic cell’s nucleus is surrounded by a membrane, it is often said to have a “true nucleus.” The word “organelle” means “little organ,” and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions. At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm, then all other structures/parts follow. The Plasma Membrane Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 5.5), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 12 of 45 ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane. Figure 5.5 The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it. The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus); (Figure 5.6). Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 13 of 45 Figure 5.6 Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit "micrograph": modification of work by Louisa Howard) The Cytoplasm The cytoplasm is the entire region of a cell between the plasma membrane and the nuclear envelope. It is made up of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals. Even though the cytoplasm consists of 70 to 80 percent water, it has a semi- solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there, too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm. The Nucleus Typically, the nucleus is the most prominent organelle in a cell (Figure 5.7). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let us look at it in more detail (Figure 4.11). Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 14 of 45 Figure 5.7 The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus. The Nuclear Envelope The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 5.7). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus. Chromatin and Chromosomes To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 15 of 45 phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin (Figure 5.8, left); chromatin describes the material that makes up the chromosomes both when condensed and decondensed. Figure 5.8 (Left) This image shows various levels of the organization of chromatin (DNA and protein). (Right) This image shows paired chromosomes. (credit Left: modification of work by NIH; scale-bar data from Matt Russell) The Nucleolus We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm. Ribosomes Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 16 of 45 of the nuclear envelope (Figure 5.7). Electron microscopy has shown that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small (Figure 5.9). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins. Figure 5.9 Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins. Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 17 of 45 Mitochondria Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct. In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells do not get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid. Mitochondria are oval-shaped, double membrane organelles (Figure 5.10) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 18 of 45 Figure 5.10 This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. (credit: modification of work by Matthew Britton; scale- bar data from Matt Russell) Peroxisomes Peroxisomes are small, round organelles enclosed by single membrane. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Vesicles and Vacuoles Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: The membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 19 of 45 plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components. Animal Cells versus Plant Cells At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex called the centrosome. Animal cells each have a centrosome and lysosomes, whereas most plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not. The Centrosome The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other (Figure 5.11). Each centriole is a cylinder of nine triplets of microtubules. Figure 5.11 The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins (indicated by the green lines) hold the microtubule triplets together. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 20 of 45  The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division isn’t clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division. Lysosomes Animal cells (Figure 5.12) have another set of organelles not found in most plant cells: lysosomes. The lysosomes are the cell’s “garbage disposal”; others call it as “suicide bag” of the cell. In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 21 of 45 Figure 5.12 An animal cell ultrastructure showing its parts and their functions. Animal cells vary in shapes and sizes. This is a scanned copy from Advance Biology Revision Handbook by WR Pickering(1997), Oxford University Press. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 22 of 45 The Cell Wall If you examine the diagram of a plant cell(Figure 5.15), you will see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose (Figure 5.13), a polysaccharide made up of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That is because you are tearing the rigid cell walls of the celery cells with your teeth. Figure 5.13 Cellulose is a long chain of β-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule. Chloroplasts Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals; plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food. Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (Figure 5.14). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 23 of 45 the stroma. Figure 5.14 The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome. The chloroplasts contain a green pigment called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle. The Central Vacuole Previously, we mentioned vacuoles as essential components of plant cells (Figure 5.15). If you look at the figure, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That is because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 24 of 45 vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant. The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 25 of 45 Figure 5.15 A typical plant cell showing its parts and their functions. Plant cells are usually rectangular in shape. This is a scanned copy from Advance Biology Revision Handbook by WR Pickering (1997), Oxford University Press. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 26 of 45 The Endomembrane System and Proteins In addition to the presence of nuclei, eukaryotic cells are distinguished by an endomembrane system that includes the plasma membrane, nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus. These subcellular components work together to modify, tag, package, and transport proteins and lipids. The rough endoplasmic reticulum (RER) with its attached ribosomes is the site of protein synthesis and modification. The smooth endoplasmic reticulum (SER) synthesizes carbohydrates, lipids including phospholipids and cholesterol, and steroid hormones; engages in the detoxification of medications and poisons; and stores calcium ions. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens. Just like your body uses different organs that work together, cells use these organelles interact to perform specific functions. For example, proteins that are synthesized in the RER then travel to the Golgi apparatus for modification and packaging for either storage or transport. If these proteins are hydrolytic enzymes, they can be stored in lysosomes. Mitochondria produce the energy needed for these processes. This functional flow through several organelles, a process which is dependent on energy produced by yet another organelle, serves as a hallmark illustration of the cell’s complex, interconnected dependence on its organelles. The Endoplasmic Reticulum The endomembrane system (endo = “within”) is a group of membranes and organelles (Figure 5.16) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which we have already mentioned, and the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include the membranes of either mitochondria or chloroplasts. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 27 of 45 Figure 5.16 Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum (RER). The RER also sometimes modifies proteins. In this illustration, a (green) integral membrane protein in the ER is modified by attachment of a (purple) carbohydrate. Vesicles with the integral protein bud from the ER and fuse with the cis face of the Golgi apparatus. As the protein passes along the Golgi’s cisternae, it is further modified by the addition of more carbohydrates. After its synthesis is complete, it exits as an integral membrane protein of the vesicles that bud from the Golgi’s trans face. When the vesicle fuses with the cell membrane, the protein becomes an integral portion of that cell membrane. (credit: modification of work by Magnus Manske) The endoplasmic reticulum (ER) (Figure 5.16) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER, respectively. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 28 of 45  The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope. Rough ER The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope (Figure 5.17). Figure 5.17 This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell. (credit: modification of work by Louisa Howard) Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane (Figure 5.16). Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 29 of 45  Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with cells of the liver, for example. Smooth ER The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions. In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells. The Golgi Apparatus We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to be sorted, packaged, and tagged so that they wind up in the right place. Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes (Figure 5.18). Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 30 of 45 Figure 5.18 The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image. Several vesicles can be seen near the Golgi apparatus. (credit: modification of work by Louisa Howard) The receiving side of the Golgi apparatus is called the cis face. The opposite side is called the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is the addition of short chains of sugar molecules. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations. Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell. In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi bodies. In plant cells, the Golgi Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 31 of 45 apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell. Lysosomes In addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes are considered to be parts of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis or endocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen (Figure 5.19). Figure 5.19 A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium which then fuses with a lysosome within the cell to destroy the pathogen. Other organelles are present in the cell but for simplicity are not shown. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 32 of 45 Cytoskeleton All cells, from simple bacteria to complex eukaryotes, possess a cytoskeleton composed of different types of protein elements, including microfilaments, intermediate filaments, and microtubules. The cytoskeleton serves a variety of purposes: provides rigidity and shape to the cell, facilitates cellular movement, anchors the nucleus and other organelles in place, moves vesicles through the cell, and pulls replicated chromosomes to the poles of a dividing cell. These protein elements are also integral to the movement of centrioles, flagella, and cilia. If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the shape of the cell, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, this network of protein fibers is known as the cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules (Figure 5.20). Here, we will examine each. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 33 of 45 Figure 5.20 Microfilaments thicken the cortex around the inner edge of a cell; like rubber bands, they resist tension. Microtubules are found in the interior of the cell where they maintain cell shape by resisting compressive forces. Intermediate filaments are found throughout the cell and hold organelles in place. Microfilaments Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are made of two intertwined strands of a globular protein called actin (Figure 5.21). For this reason, microfilaments are also known as actin filaments. Figure 5.21 Microfilaments are made of two intertwined strands of actin. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 34 of 45  Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the circular movement of the cell cytoplasm in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract. Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to the site of an infection and phagocytize the pathogen. Intermediate Filaments Intermediate filaments are made of several strands of fibrous proteins that are wound together (Figure 5.22). These elements of the cytoskeleton get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules. Figure 5.22 Intermediate filaments consist of several intertwined strands of fibrous proteins. Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles in place. Figure 4.22 shows how intermediate filaments create a supportive scaffolding inside the cell. The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of fibrous proteins are found in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the epidermis of the skin. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 35 of 45 Microtubules As their name implies, microtubules are small hollow tubes. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins (Figure 5.23). With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly. Figure 5.23 Microtubules are hollow. Their walls consist of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube. Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome). In fact, in animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as discussed below. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 36 of 45 Flagella and Cilia To refresh your memory, flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, many of them extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.) Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center (Figure 5.24). Figure 5.24 This transmission electron micrograph of two flagella shows the 9 + 2 array of microtubules: nine microtubule doublets surround a single microtubule doublet. (credit: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College; scale-bar data from Matt Russell) Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 37 of 45 Connections between Cells and Cellular Activities You already know that a group of similar cells working together is called a tissue. As you might expect that, if cells are to work together, they must communicate with one another, just as you need to communicate with others when you work on a group project. Let’s take a look at how cells communicate with one another. You already know that a group of similar cells working together is called a tissue. As you might expect, if cells are to work together, they must communicate with each other, just as you need to communicate with others if you work on a group project. Let us take a look at how cells communicate with each other. Extracellular Matrix of Animal Cells Most animal cells release materials into the extracellular space. The primary components of these materials are proteins, and the most abundant protein is collagen. Collagen fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Collectively, these materials are called the extracellular matrix (Figure 5.25). Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other. How can this happen? Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 38 of 45  Figure 5.25 The extracellular matrix consists of a network of proteins and carbohydrates. Cells have protein receptors on the extracellular surfaces of their plasma membranes. When a molecule within the matrix binds to the receptor, it changes the molecular structure of the receptor. The receptor, in turn, changes the conformation of the microfilaments positioned just inside the plasma membrane. These conformational changes induce chemical signals inside the cell that reach the nucleus and turn “on” or “off” the transcription of specific sections of DNA, which affects the production of associated proteins, thus changing the activities within the cell. Blood clotting provides an example of the role of the extracellular matrix in cell communication. When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When tissue factor binds with another factor in the extracellular matrix, it causes platelets to adhere to the wall of the damaged blood vessel, stimulates the adjacent smooth Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 39 of 45 muscle cells in the blood vessel to contract (thus constricting the blood vessel), and initiates a series of steps that stimulate the platelets to produce clotting factors. Intercellular Junctions Cells can also communicate with each other via direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata are junctions between plant cells, whereas animal cell contacts include tight junctions, gap junctions, and desmosomes. Plasmodesmata In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell wall that surrounds each cell. How then, can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular tissues (xylem and phloem) primarily. There also exist structural modifications called plasmodesmata (singular = plasmodesma), numerous channels that pass between cell walls of adjacent plant cells, connect their cytoplasm, and enable materials to be transported from cell to cell, and thus throughout the plant (Figure 5.26). Figure 5.26 A plasmodesma is a channel between the cell walls of two adjacent plant cells. Plasmodesmata allow materials to pass from the cytoplasm of one plant cell to the cytoplasm of an adjacent cell. Tight Junctions Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 40 of 45  A tight junction is a watertight seal between two adjacent animal cells (Figure 5.27). The cells are held tightly against each other by proteins (predominantly two proteins called claudins and occludins). Figure 5.27 Tight junctions form watertight connections between adjacent animal cells. Proteins create tight junction adherence. (credit: modification of work by Mariana Ruiz Villareal) This tight adherence prevents materials from leaking between the cells; tight junctions are typically found in epithelial tissues that line internal organs and cavities, and comprise most of the skin. For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space. Desmosomes Also found only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells (Figure 5.28). Short proteins called cadherins in the plasma membrane connect to intermediate filaments to create desmosomes. The cadherins join two adjacent cells together and maintain the cells in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 41 of 45 Figure 5.28 A desmosome forms a very strong spot weld between cells. It is created by the linkage of cadherins and intermediate filaments. (credit: modification of work by Mariana Ruiz Villareal) Gap Junctions Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 5.29). Structurally, however, gap junctions and plasmodesmata differ. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 42 of 45 Figure 5.29 A gap junction is a protein-lined pore that allows water and small molecules to pass between adjacent animal cells. (credit: modification of work by Mariana Ruiz Villareal) Gap junctions develop when a set of six proteins (called connexins) in the plasma membrane arrange themselves in an elongated donut-like configuration called a connexon. When the pores (“doughnut holes”) of connexons in adjacent animal cells align, a channel between the two cells forms. Gap junctions are particularly important in cardiac muscle: The electrical signal for the muscle to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in tandem. 5.6 What are the main components common to both prokaryotic and eukaryotic cells? Describe each. 5.7 What are the structures that are present only in prokaryotic cells? Describe and give the functions of each. 5.8 What are the structures that are present only in eukaryotic cells? Describe and give the functions of each. 5.9 Compare and contrast plant and animal cells by making a Venn diagram. 5.10 Based on the constructed Venn diagram, describe and give the functions of the structures found only in plant cells; and those found only in animal cells. Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 43 of 45 For additional information and better understanding of the topic, please check and watch any (better if all) of the following sites: Keep Theory of The Cell: Calm https://www.youtube.com/watch?v=zk3vlhz1b6k its…. Prokaryotic and Eukaryotic Cells: https://www.youtube.com/watch?v=Pxujitlv8wc https://www.youtube.com/watch?v=xTnNv7YplSo https://www.youtube.com/watch?v=9o6huiw7u5o Cell Structures/organelles; https://www.youtube.com/watch?v=8IlzKri08kk https://www.youtube.com/watch?v=1Z9pqST72is https://www.youtube.com/watch?v=aczbMlSMr8U Plant and Animal Cells https://www.youtube.com/results?search_query=plant+and+animal+ce lls+khan+academy+ Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 44 of 45 REFERENCES: Clark, Mary Ann, Jung Choi and Matthew Douglas. Biology 2nd ed. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0) Parker Nina, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, and Brian M. Forster Nov 1, 2016 ©Jan 16, 2020 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License 4.0 license. https://openstax.org/books/microbiology/pages/ Pickering WR. 1997. Advanced Biology Revision Handbook. Oxford University Press. Oxford New York. Zedalis Julianne and John Eggebrecht. Mar 8, 2018. Biology for AP® Courses. © Apr 10, 2020 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License 4.0 license. https://openstax.org/books/biology-ap- courses/pages/ Biochem Lecture Module 5: All About Cells. This part, with modifications, came from OpenStax book which is available for free at https://openstax.org/books/biology-ap-courses/pages/ Consolidated/Prepared by FAVillenas Page 45 of 45