Histology and Cell Biology PDF - First Year of Biology

# HISTOLOGY AND CELL BIOLOGY ## **FIRST YEAR OF BIOLOGY** ## **PREPEARD BY** ### **DR/DOAA ABASS ABD ELKAREEM** ### **The living organism body is an incredibly complex and amazing structure.** At best, it is a source of strength, beauty, and wonder. We can compare the healthy body to a well-desi...

# HISTOLOGY AND CELL BIOLOGY ## **FIRST YEAR OF BIOLOGY** ## **PREPEARD BY** ### **DR/DOAA ABASS ABD ELKAREEM** ### **The living organism body is an incredibly complex and amazing structure.** At best, it is a source of strength, beauty, and wonder. We can compare the healthy body to a well-designed machine whose parts work smoothly together. We can also compare it to a symphony orchestra in which each instrument has a different part to play. When all of the musicians play together, they produce beautiful music. From a purely physical standpoint, our bodies are made mainly of water. We are also made of many minerals, including calcium, phosphorous, potassium, sulfur, sodium, chlorine, magnesium, and iron. In order of size, the elements of the body are organized into cells, tissues, and organs. Related organs are combined into systems, including the musculoskeletal, cardiovascular, nervous, respiratory, gastrointestinal, endocrine, and reproductive systems. Our cells and tissues are constantly wearing out and being replaced without our even knowing it. In fact, much of the time, we take the body for granted. When it is working properly, we tend to ignore it. Although the heart beats about 100,000 times per day and we breathe more than 10 million times per year, we do not normally think about these things. When something goes wrong, however, our bodies tell us through pain and other symptoms. In fact, pain is a very effective alarm system that lets us know the body needs attention. If the pain does not go away, we may need to see a doctor. Even without medical help, the body has an amazing ability to heal itself. If we cut ourselves, the blood clotting system works to seal the cut right away, and the immune defense system sends out special blood cells that are programmed to heal the area. During the past 50 years, doctors have gained the ability to repair or replace almost every part of the body. In my own field of cardiovascular surgery, we are able to open the heart and repair its valves, arteries, chambers, and connections. In many cases, these repairs can be done through a tiny "keyhole" incision that speeds up patient recovery and leaves hardly any scar. If the entire heart is diseased, we can replace it altogether, either with a donor heart or with a mechanical device. In the future, the use of mechanical hearts will probably be common in patients who would otherwise die of heart disease. Until the mid-twentieth century, infections and contagious diseases related to viruses and bacteria were the most common causes of death. Even a simple scratch could become infected and lead to death from "blood poisoning." After penicillin and other antibiotics became available in the 1930s and 40s, doctors were able to treat blood poisoning, tuberculosis, pneumonia, and many other bacterial diseases. Also, the introduction of modern vaccines allowed us to prevent childhood illnesses, smallpox, polio, flu, and other contagions that used to kill or cripple thousands. Today, plagues such as the "Spanish flu" epidemic of 1918-19, which killed 20 to 40 million people worldwide, are unknown except in history books. Now that these diseases can be avoided, people are living long enough to have long-term (chronic) conditions such as cancer, heart failure, diabetes, and arthritis. Because chronic diseases tend to involve many organ systems or even the whole body, they cannot always be cured with surgery. These days, researchers are doing a lot of work at the cellular level, trying to find the underlying causes of chronic illnesses. Scientists recently finished mapping the human genome, which is a set of coded "instructions" programmed into our cells. Each cell contains 3 billion "letters" of this code. By showing how the body is made, the human genome will help researchers prevent and treat disease at its source, within the cells themselves. The body's long-term health depends on many factors, called risk factors. Some risk factors, including our age, sex, and family history of certain diseases, are beyond our control. Other important risk factors include our lifestyle, behavior, and environment. Our modern lifestyle offers many advantages but is not always good for our bodies. In Western Europe and the United States, we tend to be stressed, overweight, and out of shape. Many of us have unhealthy habits such as smoking cigarettes, abusing alcohol, or using drugs. Our air, water, and food often contain hazardous chemicals and industrial waste products. Fortunately, we can do something about most of these risk factors. At any age, the most important things we can do for our bodies are to eat right, exercise regularly, get enough sleep, and refuse to smoke, overuse alcohol, or use addictive drugs. We can also help clean up our environment. These simple steps will lower our chances of getting cancer, heart disease, or other serious disorders. These days, thanks to the Internet and other forms of media coverage, people are more aware of health-related matters. The average person knows more about the human body than ever before. Patients want to understand their medical conditions and treatment options. They want to play a more active role, along with their doctors, in making medical decisions and in taking care of their own health. I encourage you to learn as much as you can about your body and to treat your body well. These things may not seem too important to you now, while you are young, but the habits and behaviors that you practice today will affect your physical well-being for the rest of your life. The present course, is an excellent introduction to living organism biology and anatomy. I hope that it will awaken within you a lifelong interest in these subjects. # **1- Cells: The Basis of Life** Cells are the basic units of all living organisms. Some living creatures, such as bacteria and protozoans, consist of only a single cell. In contrast, complex organisms like human beings may be composed of over 75 trillion cells! Just one drop of human blood contains about 5 million red blood cells. ## **CELLS VARY WIDELY IN SIZE AND SHAPE** Although most cells are microscopic, they vary widely in size. For instance, sperm cells are only about 2 micrometers (1/12,000th of an inch) big, whereas some nerve cells are over a meter (3 feet) in length (for example, a single nerve cell connects the spinal cord in your lower back to the little toe). Cells also vary in shape, which reflects their particular function. Nerve cells, for example, have long threadlike extensions that are used to transmit impulses from one part of the body to another. Epithelial cells that compose the outer layers of the skin can be flattened and tightly packed like floor tiles, enabling them to protect underlying cells. Muscle cells, designed to generate force by contracting, can be slender, rod-shaped structures. Red blood cells, which carry oxygen from the lungs to virtually every cell in the body, are biconcave and disk-shaped (Figure 1.1). Whereas some kidney cells resemble a cube. All in all, the human body has over 200 different types of cells. ## **THE DISCOVERY OF CELLS** Because of their small size, the discovery of cells and their structure had to wait for the invention of the microscope. During the mid-seventeenth century, the English scientist Robert Hooke looked at thinly sliced cork with a simple microscope. He observed tiny compartments, which he termed "cellulae," the Latin word for small rooms; hence the origin of the biological term cell (technically speaking, he actually observed the walls of dead plant cells, but no one at that time thought of cells as being dead or alive). In the late seventeenth century, the Dutch shopkeeper Anton van Leeuwenhoek constructed lenses that provided clarity and magnification not previously possible. With these new lenses, he observed very small "animalcules" from scrapings of tartar from his own teeth, as well as protozoans from a variety of water samples. In the early nineteenth century, the German botanist Matthias Schleiden, who also studied cells with a microscope, proposed that the nucleus might have something to do with cell development. During the same time period, the German zoologist Theodor Schwann theorized that animals and plants consist of cells, and that cells have an individual life of their own. Rudolf Virchow, a German physiologist who studied cell growth and reproduction, suggested all cells come from pre-existing cells. His proposal was actually revolutionary for the time because it challenged the widely accepted theory of spontaneous generation, which held that living organisms arise spontaneously from nonliving material, such as garbage. By the middle of the nineteenth century, the scientific community developed several generalizations, which today we term the cell theory. The cell theory includes three important principles. First, every living organism is composed of one or more cells. Second, cells are the smallest units that have the properties of life. Third, the continuity of life has a cellular basis. ## **Microscopes** <br/> Modern microscopes have dramatically increased our ability to observe cell structure. Light microscopes use two or more sets of highly polished glass lenses to bend light rays to illuminate a specimen, thereby enlarging its image. Consequently, in order to be seen, a specimen must be thin enough for light to pass through it. Also, cells are 60-80% water, which is colorless and clear. This, in turn, makes it difficult to observe the various unpigmented structures of cells. This problem is overcome by exposing cells to a stain (dye), which colors some cell parts, but not others. Unfortunately, staining usually kills cells. However, there are several types of microscopes designed to use phase contrast or Nomarski optics, which use light refraction to create contrast without staining. For instance, with Nomarski optics, a prism is used to split a beam of polarized light in two and project both beams through a specimen at slightly different angles. When the beams are later combined, they exhibit bright and dark interference patterns that highlight areas in cells that have differing thicknesses. These specialized optics obviously enhance the usefulness of light microscopes. Two factors need to be considered when discussing microscopy: a microscope's ability to magnify images and its ability to resolve them. Magnification simply means making an image appear larger in size. Resolution refers to the ability to make separate parts look clear and distinguishable from one another, which becomes increasingly more difficult as magnification increases. Consequently, if a microscope magnified an image without providing sufficient resolution, the image would appear large but unclear. Light microscopes have an inherent limitation regarding resolution because of the physical nature of light. Light, a form of electromagnetic radiation, has wave-like properties, where the wavelength refers to the distance between two wave crests (red light, for example, has a longer wavelength than violet light; 750 nanometers versus 400 nanometers, respectively). Therefore, if a cell structure is less than one-half the wavelength of illuminating light, it will not be able to disturb the light rays streaming past it. In other words, it will be invisible. As a result, light microscopes are not useful for observing objects smaller than several hundred nanometers. Electron microscopes have a much greater resolving power because they use a beam of electrons to "illuminate" a specimen instead of light. Although electrons are particles, they also have wave-like properties, and a stream of electrons has a wavelength about 100,000 times shorter than that of visible light. This allows an electron microscope to resolve images down to about 0.5 nanometers in size. Because a beam of electrons cannot pass through glass, its path is focused by a magnetic field. In addition, specimens must be placed in a vacuum; otherwise molecules of air would deflect the electron beam. There are two main kinds of electron microscopes. A transmission electron microscope (Figure 1.2) accelerates a beam of electrons through a specimen, which allows internal structures within a cell to be imaged. In contrast, a scanning electron microscope moves a narrow beam of electrons across a specimen that has been coated with a thin layer of metal. This method is ideally suited for imaging the surface of a specimen (Figure 1.3). ## **CHEMICAL CONSTITUENTS OF CELLS** Chemically, cells are mainly composed of four elements: carbon, hydrogen, oxygen, and nitrogen. Although these four major elements make up over 95% of a cell's structure, the lesser abundant trace elements also are important for certain cell functions (Figure 1.4). Iron, for instance, is needed to make hemoglobin, which carries oxygen in the blood. Blood clotting, and the proper formation of bones and teeth all require calcium. Iodine is necessary to make thyroid hormone, which controls the body's metabolic rate. A lack of iodine in the diet can lead to the formation of a goiter (an enlarged thyroid gland). Although goiters were relatively common in the past, they are less common today because dietary iodine can be obtained through the consumption of iodized salt. Sodium and potassium are also necessary elements, especially for the transmission of nerve impulses and for muscle contraction. It is convenient to divide the chemicals that enter cells or are produced by them into two main groups: organic substances (those that contain carbon and hydrogen atoms), and inorganic substances (all the rest). The most abundant inorganic molecule in cells (and the entire body) is water. In fact, it accounts for about two-thirds of an adult human's weight. This helps explain why water is essential for life. Water is important as a solvent because many substances (solutes) dissolve in it. Also, water helps stabilize body temperature because, compared to most fluids, it can absorb a lot of heat before its temperature rises, and cells release a great amount of heat during normal metabolism (the sum total of all the chemical reactions taking place in the body). In addition to water, other inorganic substances found in cells include oxygen, carbon dioxide, and numerous inorganic salts, such as sodium chloride (ordinary table salt). Organic substances in cells include carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, such as sugars and glycogen, provide much of the energy that cells require. Carbohydrates also provide materials to build certain cell structures. Lipids include compounds such as fats (primarily used to store energy), phospholipids (an important constituent of cell membranes), and cholesterol (used to synthesize steroid hormones, such as testosterone and estrogen). Proteins serve as structural materials and an energy source. In addition, most enzymes and many hormones are composed of protein. Nucleic acids form the genes found in DNA and also take part in protein synthesis. ## **STRUCTURE OF A GENERALIZED CELL** Although cells differ in many respects, they all have certain characteristics and structures in common. Consequently, it is possible to construct a generalized or composite cell (Figure 1.5). For human beings, our cells typically start out with three structures in common. They all have a plasma membrane, the thin outer boundary that separates the intracellular environment from the extracellular one. The plasma membrane therefore maintains cells as distinct entities. Indoing so, plasma membranes also allow specific chemical reactions to occur inside the cell separate from random events in the environment. Human cells also typically have a nucleus. There is one notable exception, however. Mature red blood cells do not possess a nucleus. The nucleus contains heritable genetic material called deoxyribonucleic acid (DNA) and molecules of ribonucleic acid (RNA) that are able to copy instructions from DNA. In addition, cells contain a semi-fluid cytoplasm. It surrounds the nucleus and is encircled by the plasma membrane. Cytoplasm contains specialized structures suspended in a liquid cytosol called organelles, which perform specific cell functions. Whereas organelles divide the labor of a cell, the nucleus directs overall cell activities. ## **Levels of Structural Organization** Single-celled organisms (protozoans) have the ability to carry out all necessary life functions as individual cells. For example, they can obtain and digest food, eliminate waste products, and respond to a number of different stimuli. However, in multicellular organisms, such as human beings, cells do not generally operate independently. Instead, they display highly specialized functions, and only by living and communicating with other cells, do they allow the entire organisms to survive. Groups of cells that are similar in structure and perform a common or related function are called tissues. There are four main tissue types in the human body (epithelial, connective, muscle, and nervous), and each performs a different role (a further discussion of tissues is presented in part 6). The study of tissues is called histology, and physicians who specialize in this field are called pathologists (histologists). These doctors often remove tissues from a patient during an operation or from a person during a post-mortem examination, and look at the cells with a microscope to help diagnose the presence of specific diseases. Cancer, for instance, is detected in this manner. Tissues can be organized into more complex structures called organs, which perform specific functions for the body. Some examples of organs include the kidneys, lungs, stomach, liver, and skin (the skin will be discussed in later chapters). Many organs, such as the small intestine and skin, are composed of all four tissue types. The small intestine, for instance, is capable of digesting and absorbing food, which requires the cooperation of a number of different kinds of cells and tissue types. A system is considered a group of organs that cooperate to accomplish a common purpose. An example is the digestive system, which contains a number of organs, including the esophagus, stomach, and small intestine. All the organ systems of the body make up the complete organism. # **2- Cell Membranes: Ubiquitous Biological Barriers** A cell membrane called the plasma membrane surrounds every single cell - there are no exceptions. It encircles a cell, thereby forming a barrier containing the cytoplasm within, and separating cellular contents from the surrounding environment. In addition, nearly all types of organelles are enclosed by a similar cell membrane. Regardless of location, cell membranes are much more than simple boundaries. In fact, they are an actively functioning part of living cells, and many important chemical reactions take place on their inner and outer surfaces (Figure 2.1). ## **GENERALIZED CHARACTERISTICS OF CELL MEMBRANES** In spite of their extreme importance, cell membranes are actually quite fragile and thin. They are typically 7-8 nanometers thick (about 10,000 times thinner than a strand of hair), and thus are only visible with the aid of an electron microscope. In addition to maintaining cell integrity, the plasma membrane also controls the movement of most substances that enter and exit a cell. Because cell membranes have the ability to let some items through but not others, they are referred to as selectively permeable (also known as semipermeable). The permeability properties of the plasma membrane depend on a healthy, intact cell. When cells are damaged, their membranes may become leaky to virtually everything, allowing substances to freely flow across them. For instance, when a person has been severely burned, there can be significant loss of fluids, proteins, and ions from dead and damaged cells in the burned areas. ## **Membrane Structure** Cell membranes have a tall order to fill. Not only must they provide a structurally stable boundary, they also need to be flexible and semipermeable. For these reasons, the basic structural framework of all cell membranes is a double layer (called a bilayer) of phospholipid molecules (Figure 2.2), with protein and cholesterol molecules dispersed within the layers. A close inspection of the structural properties of phospholipid molecules is key to understanding how a lipid bilayer forms and how it provides a structurally stable boundary. Each phospholipid molecule has a phosphate group and two fatty acids chains bound to a three-carbon glycerol molecule (a 3-carbon sugar alcohol that contains three hydroxyl groups), making the whole thing look like a lollipop with two sticks. Phosphate groups are polar (meaning charged), making the end of the phospholipid molecule hydrophilic (water-soluble). In contrast, the fatty acid regions are nonpolar (that is, uncharged), rendering the other portion of the phospholipid hydrophobic (water insoluble). Because water is a major component of both cytoplasm and extracellular fluid, the polar phosphate groups orient themselves so that they lie on both the inner and outer surfaces of a bilayer. In contrast, the nonpolar fatty acid "tails" avoid water by lining up in the center of the membrane, sandwiched between the polar "heads." The result is a bilayer composed of two parallel sheets of phospholipid molecules arranged as mirror images. In this way, the two layers lie tail-to-tail, exposing the polar heads to water. This self-orienting property of phospholipids in an aqueous environment allows cell membranes to self-assemble and also to repair themselves quickly. About 10% of the outer facing layer of the membrane is composed of glycolipids, lipids with sugar groups attached to them. In addition, about 20% of the lipid in membranes is cholesterol. This molecule stabilizes the overall structure of a membrane by wedging itself between the phospholipid tails. This also makes membranes less fluid. A lipid bilayer structure is well suited to provide a structurally stable, flexible barrier that is relatively impermeable to most water-soluble substances. However, cells also must acquire water-soluble nutrients found in the surrounding environment. In addition, cells need to eliminate water-soluble waste products. These problems are overcome by the presence of proteins scattered in the lipid bilayer. In fact, proteins make up about half of membranes by weight, and are responsible for most of their specialized functions. In other words, the lipid portion of most membranes is essentially the same; and it is the presence of specific proteins that gives each membrane its specific properties. ## **Membrane Proteins** There are two distinct populations of membrane proteins: integral and peripheral. Integral proteins are inserted into the lipid bilayer; most are transmembrane, meaning they span the entire width of the membrane, protruding on both sides. Integral proteins are mainly involved with transport functions (described below). In contrast, peripheral proteins are attached on either the inner or outer surface of the membrane. These proteins often serve as enzymes or in mechanical functions, such as changing cell shape during cell division or in muscle contraction. Based on its overall structure, the fluid mosaic model is used to describe biological membranes because the lipid portion has fluid-like properties, whereas proteins are dispersed within it forming a mosaic-like pattern. Many proteins on the extracellular side of membranes have attached sugar residues and are described as glycoproteins. The term glycocalyx ("cell coat") refers to the fuzzy carbohydrate-rich area on cell surfaces. The glycocalyx is significant because it provides highly specific biological markers, which can be recognized by other cells. For example, white blood cells of our immune system identify "self-cells" of the body from invading bacterial cells by binding to certain membrane glycoproteins. In addition, sperm recognize an ovum by the egg's unique glycocalyx. The glycocalyx on red blood cells is what determines blood type. Unfortunately, continuous changes in the glycocalyx occur when cells become cancerous. This in turn allows cancer cells to evade the immune system and avoid destruction. ## **Functions of Membrane Proteins** Membrane proteins serve a variety of important functions, giving properties to cell membranes that otherwise would not be possible. Most notably, transmembrane proteins mediate the movement of substances into and out of cells (described in further detail in the next section). Membrane proteins also serve as enzymes, molecules that increase the rate of chemical reactions. In addition, membrane proteins exposed to the outside surface of cells may act as receptors. A receptor is a molecule with a binding site that fits the specific shape of a particular chemical messenger, such as a hormone. In this way, chemical messages released by one cell type can communicate with another cell type, thereby influencing its activity. In a similar manner, some glycoproteins on the outer cell surface serve as identification tags that are specifically recognized by other cell proteins in a process called cell-cell recognition. In addition, membrane proteins of adjacent cells may be linked together. These cell adhesion molecules (CAMs) provide temporary binding sites that guide cell migration, or they may provide more permanent attachments between cells. Unfortunately, CAMs often are not expressed in cancer cells. This explains why cells from a tumor may separate and spread to other locations in the body; a process known as metastasis. Finally, some membrane proteins provide attachment sites for the cytoskeleton (an internal support system) and the extracellular matrix (nonliving material secreted by cells). These membrane proteins are important for helping maintain cell shape. They also help anchor and thereby fix the location of certain proteins within the fluid-like membrane. # **DIFFUSION** Diffusion is the process by which particles spread spontaneously from regions of higher concentration towards regions where they are of lower concentration. All atoms and molecules contain kinetic energy obtained from heat in the environment. Consequently, they are in constant motion. As they move about randomly at high speeds, they collide and ricochet off one another, changing direction with each collision (that is why diffusion is referred to as random thermal motion and why diffusion would cease to occur at absolute zero, -273°C). The overall effect of random thermal motion is that particles move away from areas of higher concentration, where collisions are more frequent, to areas of lower concentration (Figure 2.3). In this manner, particles are said to diffuse "down" their concentration gradient. In a closed system, diffusion will eventually produce a uniform distribution of particles, which is called a state of equilibrium. Although particles continue to move and collide after equilibrium is achieved, their concentration gradients no longer change because the particles move equally in all directions (i.e., there is no "net" movement). The plasma membrane is an actively functioning part of living cells. In addition to maintaining cell integrity, it also controls movement of substances that enter and exit cells. Most organelles also are surrounded by a membrane. All cell membranes are composed of a phospholipid bilayer, with protein and cholesterol molecules dispersed within the layers. Membrane proteins serve a variety of diverse functions. For instance, they transport substances into and out of cells and also serve as cell-cell recognition sites. In addition, membrane proteins act as enzymes, receptors, and cell adhesion molecules. Diffusion is the process by which particles spread spontaneously from regions of higher concentration towards regions where they are of lower concentration. In this manner, particles are said to diffuse "down" their concentration gradient. Although individual molecules travel at high velocities, the number of collisions they undergo prevents them from traveling very far in a straight line. Consequently, diffusion can distribute molecules rapidly over short distances (within the cytoplasm or between a few layers of cells), but is extremely slow over distances greater than a few centimeters. # **3- Movement Through Cell Membranes: How to Cross a Barrier** The cell membrane is a selective barrier that controls movement of substances that both enter and leave cells. Many of these movements involve passive transport processes (not requiring cellular energy), such as simple diffusion, facilitated diffusion, osmosis, and filtration. In contrast, active transport mechanisms require cellular energy in the form of ATP. This includes transport by solute pumps and the processes of endocytosis and exocytosis. ## **PASSIVE MECHANISMS** The unassisted diffusion of lipid soluble solutes through the plasma membrane is called simple diffusion. Such substances include oxygen, carbon dioxide, fat-soluble vitamins, and alcohol. These nonpolar substances are capable of passing through the hydrophobic interior of the plasma membrane. Their direction of net flow will depend on the concentration gradient. For example, the concentration of oxygen molecules is always higher in the blood than in cells, so it continuously enters cells by simple diffusion. The opposite is true for carbon dioxide (Figure 3.1). Most water-soluble substances, however, are unable to diffuse through the lipid portion of a membrane. In this case, special transmembrane proteins shaped like hollow cylinders, called channels, are utilized. Because these proteins are filled with water, they create an aqueous pore that traverses the entire thickness of a membrane. Like a highway tunnel through a mountain for automobiles, channels provide a pathway for small polar particles to diffuse through the membrane. Movement through channels is passive because it does not require energy from cells and simply depends on the concentration gradient. Under most circumstances, it would not be useful for a channel to be open all the time. That is why channels are "gated," which means they have the ability to open and close in response to appropriate chemical or electrical signals. Although the sizes of channel pores vary, they are typically on the order of nanometers in diameter. Channel pores also tend to be very selective as to what they will allow to pass through. Most channels are primarily permeable to a specific ion, such as to sodium, potassium, calcium, or chloride. Certain molecules, such as glucose, amino acids, and urea, are too polar to dissolve in the lipid bilayer and they also are too large to pass through channels. However, they do move rapidly through the plasma membrane. This is accomplished by a passive process called facilitated diffusion. In this case, the transported substance moves across the membrane by interacting with a protein carrier molecule. Although movement by facilitated diffusion follows a concentration gradient, the carrier is needed as a transport "vehicle" to allow a substance to cross the lipid bilayer. If you think of an ion channel as a typical door in a classroom, then a carrier protein could be thought of as a revolving door in a department store. In other words, unlike the channel that has a continuous tunnel traversing a membrane, a carrier appears to have a binding site that is moved from one face of the membrane to the other by conformational changes in the protein. In addition, as with channels, carriers tend to be highly selective as to what they will transport (Figure 3.2). Osmosis is a special case of diffusion. It occurs when water molecules diffuse from a region of higher water concentration to a region of lower concentration across a selectively permeable membrane (Figure 3.3). In solutions, solute takes up space that water molecules would otherwise occupy. Thus, a higher concentration of solute means a lower concentration of water. The extent to which the water concentration is decreased by solute particles depends only on their number and not their size, kind, or charge. For example, if distilled water were on both sides of a selectively permeable membrane, no net osmosis would occur. However, if the solute concentration on two sides of a membrane differed, the water concentration also would differ. Water would then diffuse across the membrane from the region of lower solute concentration towards the region of higher solute concentration. The flow of water across a membrane by osmosis can change the volume on both sides. Consequently, the movement of water into a closed system, such as a cell, will exert pressure against the plasma membrane, which is referred to as osmotic pressure. Osmotic imbalances (differences in the total solute concentration on both sides of a membrane) would therefore cause animal cells to swell or shrink, due to net water gain or loss. In this case, cells will continue to change size until they reach equilibrium; that is, the solute concentration is the same on both sides of the plasma membrane. Alternatively, before equilibrium is reached, a cell could swell until it bursts. The concentration of water and solutes everywhere inside the body must therefore be regulated so it is the same on both sides of cell membranes in order to keep cells from changing their volume. In fact, a major function of the kidneys is to maintain the volume and composition of the extracellular fluid constant by modifying the volume and composition of urine. Solutions that have the same osmotic pressure as cells and body fluids are considered isotonic, and they do not cause cells to change size. In contrast, a solution with a higher osmotic pressure than body fluids is hypertonic. Cells placed in a hypertonic medium will shrink due to the net movement of water out of the cell into the surrounding medium. On the other hand, cells exposed to a hypotonic solution, which has a lower osmotic pressure than body fluids, will gain water by osmosis and swell. In fact, under some hypotonic conditions, cells swell to the point of breaking, analogous to a balloon that is over-inflated with air (Figure 3.4). In some instances, water and solute particles are forced through membranes by hydrostatic pressure. This process is called filtration. The force for this movement usually comes from blood pressure, which is created largely by the pumping action of the heart. Like diffusion, filtration across a membrane is a passive process. However, in this case the driving force is a pressure gradient that actually pushes solute-containing fluid from the higher-pressure area to a lower-pressure area. An example of this is filtration of blood in the kidneys, which is the first step in urine formation. ## **ACTIVE MECHANISMS** Sometimes particles move across cell membranes against their concentration gradients: that is from regions of lower concentration to ones of higher concentration. This type of movement is called active transport, and it requires cells to use energy in the form of ATP. Substances moved across a membrane in this manner are usually unable to pass in the necessary direction by any of the passive processes. For example, they may be too large to traverse channels and carriers, they may not dissolve in the lipid bilayer, or they may have to move "uphill" against their concentration gradients. It is estimated that up to 40% of a cell's energy supply is used for active transport of particles through membranes. There are two major mechanisms of transport that require ATP: solute pumping and vesicular transport. Solute pumping (also called active transport) is similar to facilitated diffusion in that it uses specific carrier molecules in the cell membrane. That is, these protein molecules have binding sites that combine temporarily and specifically with the particles being transported. However, whereas facilitated diffusion is driven by the kinetic energy of the diffusing particles, solute pumps use ATP. Because this type of transport moves substances against their concentration gradients, the carrier proteins are referred to as pumps. The most ubiquitous active transport carrier is the sodium-potassium pump. This protein transports sodium ions out of the cell, while simultaneously moving potassium ions in the other direction. Consequently, it keeps intracellular sodium levels low, while also keeping intracellular levels of potassium relatively high (about 10-20 times greater than what is found in the extracellular fluid). These artificial concentration gradients maintained by the pump are necessary for nerve and muscle cells to function normally, and also for body cells to maintain their normal fluid volumes. Because there is a continual "leak" of sodium into cells by diffusion, as well as a leak of potassium in the other direction, this pump operates more or less continuously. The pump also can change its rate of transport, depending on the level of sodium and potassium leak (which, for example, temporarily increases during a nerve impulse or muscle contraction when there is a transient increase in membrane leak for sodium and potassium). Another example of active transport includes a potassium-hydrogen pump found in stomach cells, which is used to form hydrochloric acid. Some substances that cannot move across the plasma membrane by any other means are transported by vesicular transport. Endocytosis ("into a cell") describes vesicular transport where particles are brought into a cell by engulfing or enclosing them within small membranous vesicles. Once a vesicle is formed, it detaches from the plasma membrane and moves into the cytoplasm, where it often fuses with a cellular organelle that contains digestive enzymes (Figure 3.5). This mechanism is well suited for the transport of relatively large substances, such as bacteria or dead body cells and is called phagocytosis (Figure 3.6), a term that literally means "cell eating." Phagocytosis is routinely conducted by certain white blood cells called phagocytes. On the other hand, pinocytosis ("cell drinking") is commonly used by cells to take in liquids that contain dissolved proteins or fats. Exocytosis ("out of a cell") refers to vesicular transport where particles are eliminated from cells (Figure 3.5). Products to be secreted are first packaged into small membrane sacs (vesicles). The sacs migrate to the plasma membrane and fuse with it. This mechanism is often used to secrete hormones, mucus, and other cell products, or to eject certain cellular wastes.. # **Cell Junctions** There are four main types of cell-cell junctions: * Three are different types of connecting junctions, that bind the cells together. * occluding junctions (zonula occludens or tight junctions) * adhering junctions (zonula adherens). * desmosomes (macula adherens). There are also 'hemidesmosomes' that lie on the basal membrane, to help stick the cells to the underlying basal lamina. * Gap junctions. These are communicating junctions. (also known as nexus, septate junction) These types of cell junctions are found between epithelial cells, but can also between other types of cells. ## **1- Occluding (tight) junctions** The borders of two cells are fused together, often around the whole perimeter of each cell, forming a continuous belt like junction known as a tight junction or zonula occludens (zonula = latin for belt). These regions of the cells are very tightly connected together, such that the adjacent plasma membranes are sealed together. Proteins in the membrane of adjacent cells called occludin interact with each other to produce this tight seal. In the cytoplasm of the cell, occludin interacts with the actin cytoskeleton via another proteins called ZO-1. Many pathogens act on the proteins that form this tight junction, making it permeable. This type of junction greatly restricts the passage of water, electrolytes and other small molecules across the epithelium. Transmembrane proteins from each cell membrane interlock across the intercellular space, all around the cell, in this belt (black lines in the diagram). The permeability of tight junctions varies from site to site, and are often can be selectively leaky. For example, these junctions are important in the gut, in acting as a selective diffusion barrier, preventing diffusion of water-soluble molecules. They also act to restrict the localization of membrane bound proteins. ## **2-Adhering Junctions** Epithelial cells are held together by strong anchoring (zonula adherens) junctions. The adherens junction lies below the tight junction (occluding junction). In the gap (about 15-20nm) between the two cells, there is a protein called cadherin - a cell membrane glycoprotein. (The type of cadherin found here is E-cadherin). The cadherins from adjacent cells interact to 'zipper' up the two cells together. Inside the cell, actin filaments (microfilaments, shown here in red) join up the adhesion junctions. These filaments tend to be arranged circumferentially around the cell, as a 'marginal' band. This marginal band can contract, and deform the shape of cells held together in this way this is thought to be key in the morphogenesis of epithelial cells, in forming ducts for example. ## **3-Desmosomes and Hemidesmosomes** Desmosomes connect two cells together. A desmosome is also known as a spot desmosome or macula adherens (macula = latin for spot), because it is circular or spot like in outline, and not belt- or band shaped like adherens junctions. Desmosomes are particularly common in epithelia that need to withstand abrasion. Desmosomes are also found in cardiac cells, but the intermediate filament in this case is desmin, not keratin (which is found in