clude all protists, fungi, plants, and animals. Protists are organisms like

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1 W O R D O R I G I N Prokaryote is from a mixture of Latin and Greek; the Latin Pro, meaning before, and karyote from the Greek karyon, meaning kernal a reference to the appearance of the nucleus through early microscopes. The combination of the two terms indicates that prokaryotes originated before cell structures such as the nucleus evolved. clude all protists, fungi, plants, and animals. Protists are organisms like Amoeba and Paramecium. Eu is a Greek word meaning good. Therefore eukaryotes have a good or real nucleus as well as other cell structures. Eukaryotic cells are divided into compartments by membranes. These different compartments have specific functions and are called organelles. Each type of organelle has its own unique function. Throughout the rest of this chapter you will learn about the structure and function of the various cell organelles. Cell (Plasma) Membrane The cell membrane is the only thing between a cell and its outside environment. It has a crucial role to play in the life of a cell: it must control what enters and leaves the cell. The cell membrane must allow a sufficient number of food molecules, such as glucose, to pass in and must also allow for the prompt removal of waste products, such as carbon dioxide. Without this control the cell will die. The cell (or plasma) membrane is made of a double layer of phospholipid molecules called the phospholipid bilayer. Because it is too small a structure to be seen clearly with a microscope, scientists have developed a model to explain what they think it looks like. This model is known as the fluid mosaic model. The term fluid is used because the phospholipid molecules and proteins that make up the membrane are free to drift around in fluid motion. The term mosaic is used to describe the position of the protein molecules. The molecules are placed randomly and there is no set pattern. THE PLASMA MEMBRANE 1 phospholipids 2 3 cholesterol proteins cytoskeleton peripheral protein a. integral protein b. FIGURE 2.6 The plasma membrane 1 A double or bilayer of phospholipid molecules, with their hydrophilic heads facing outward, toward the watery environment that lies both inside and outside the cell, and their hydrophobic tails pointing inward, toward each other. 2 Cholesterol molecules 3 that act as a patching substance and that help the cell maintain an optimal level of fluidity. Proteins, which are integral, meaning bound to the hydrophobic interior of the membrane, or peripheral, meaning not bound in this way. Membrane proteins serve four main functions: a. Structural support, often when attached to parts of the cell s scaffolding, or cytoskeleton. b. Recognition. Binding sites on some proteins can serve to identify the cell to other cells, such as those of the immune system. 42 U N I T 1 Ce l lul ar Functions

2 W a s t e N o t, W a n t N o t As it turns out, environmentally harmful substances that would kill most organisms such as crude oil, gasoline, diesel fuel, and other organic pollutants serve as a source of food for other organisms. It is the discovery that certain bacteria and fungi thrive upon pollutants that forms the basis for what is known as bioremediation technology. There are about 1000 species of bacteria known to have the ability to break down toxins and/or pollutants for use as their food source. They then release far less damaging waste products themselves. Bacteria produce enzymes that break down waste materials into substances that they can more readily digest. As the bacteria digest these wastes, they produce more enzymes to break down more waste. The cycle continues until all the waste material is gone. Then the bacteria either become inactive and/or die from starvation. Companies that specialize in this form of biotechnology grow and study many types of micro-organisms, so that they know which type of organism can be used to effectively clean up a certain type of industrial waste. The phospholipid bilayer is composed of two rows or layers of phospholipid molecules. The hydrophilic heads of the phospholipids are found on the outside and inside of the membrane!facing the watery environment located both inside and outside a cell. The hydrophobic fatty acid tails from each layer face one another in the middle of the membrane (Figure 2.7). If you disorganize a membrane, the phospholipid molecules will return to their original arrangement because of their reaction to water. The polar heads will 4 glycocalyx sugar chains c. d. c. Communication. Receptor proteins, protruding out from the plasma membrane, can be the point of contact for signals sent to the cell via traveling molecules, such as hormones. d. Transport. Proteins can serve as channels through which materials can pass in and out of the cell. 4 The glycocalyx. Sugar chains that attach to communication or recognition proteins, serving as their binding sites. The glycocalyx can also lubricate cells and act as an adhesion layer for them. C H A P T E R 2 Ce l l S tructure and Function 43

3 M e m bra n e G ly c o protein C h ain s Play a K e y R ole in th e Fig ht A g ain st Dis e a s e Dr. Harry Jennings of the National Research Council has contributed to a medical breakthrough the production of the first fully synthetic (human-made) vaccine. Dr. Jennings has spent 24 years developing a vaccine to prevent a disease known as group B meningitis. Meningitis is a disease caused by bacteria that kills about 40 people a year in Canada about half of whom are infants and often leaves the survivors with brain damage that causes mental retardation and blindness. Dr. Jennings s research resulted in the making of a combination carbohydrate-protein molecule that resembles the cell membrane glycoprotein chains of the meningitis bacteria very closely. As a result, cells from your immune system think that the meningitis bacteria has invaded your body and produce antibodies to fight the bacteria. However, since the carbohydrate-protein vaccine is harmless, you gain protection against meningitis without risk of becoming ill. Human trials for this vaccine are currently under way, and if successful, it should become available for public use soon. orient toward the watery environment while the non-polar lipid tails will mix with other non-polar molecules. The protein molecules embedded in the membrane are called integral or intrinsic proteins. They have different functions. Some serve as special carriers or transport channels for molecules that are either too large or too hydrophilic to pass through the phospholipid bilayer. The transport proteins allow these molecules to enter the cell. Other membrane proteins have sugar chains attached to them. These carbohydrate and protein combinations, known as glycoproteins, act as attachment sites for molecules that need to enter or carry a message to the cell. They are highly specific to each individual and help the cells of your immune system to recognize your body cells while also identifying foreign cells in your body so that they can be destroyed. Cholesterol is also found within cell membranes. Its function is to help keep the membrane fluid. At low temperatures cholesterol keeps the phospholipids apart. This keeps the membrane fluid. At higher temperatures (around 37ºC) it attracts the phospholipids and helps stabilize the membrane. FIGURE 2.7 The phospholipid bilayer. A double layer or bilayer of phospholipids form the plasma membrane. The hydrophobic tails form the interior of the membrane, while the hydrophilic heads point toward the watery environment inside and outside the cell. polar head nonpolar tails N P hydrophobic molecules pass through freely a) Phospholipid molecule b) Phospholipid bilayer + hydrophilic molecules do not pass through freely watery extracellular fluid hydrophilic hydrophobic hydrophilic watery cytosol 44 U N I T 1 Ce l lul ar Functions

4 3.1 Cell Membrane: Gateway to the Cell K e y U n d erst a n d i n g s When you have completed this section, you will be able to: relate the fluid mosaic model of membrane structure to the function of membranes explain the importance of permeability to transportation within and between cells W O R D O R I G I N Permeable from the Latin permeare, meaning to pass through. The cell membrane plays an essential role in regulating what enters and leaves the cell. This role depends largely on its structure. Because most membranes, including the cell membrane, allow some substances to pass through them, they are said to be permeable. In addition, because most living membranes are able to control what passes through them, they are described as being selectively permeable. Non-living membranes that prevent some molecules from passing are called semipermeable membranes. Both the phospholipid bilayer and the protein molecules help to control the passage of materials through the cell membrane. The construction of the bilayer is unique. The hydrophilic phosphate heads point toward the liquid environments inside and outside the cell. The hydrophobic fatty acid tails making up the middle of the membrane, prevent some molecules from entering the cell. Because the phospholipids are tightly packed together, molecules that are too large cannot pass through this portion of the membrane. Hydrophilic molecules that are not fat-soluble cannot dissolve and pass through the middle fatty acid portion of the membrane. The protein molecules embedded in the bilayer provide an entryway for certain small molecules that cannot enter through the bilayer portion of the membrane. FIGURE 3.3 The cell membrane. The cell membrane is selectively permeable. It freely allows the passage of fat-soluble substances through the lipid bilayer and small non-fatsoluble molecules through the protein channels. phospholipids proteins cholesterol sugar chains glycocalyx Communication. Receptor proteins, protruding out from the plasma membrane, can be the point of contact for signals sent to the cell via traveling molecules, such as hormones. Transport. Proteins can serve as channels through which materials can pass in and out of the cell. The glycocalyx. Sugar chains that attach to communication or recognition proteins, serving as their binding sites. The glycocalyx can also lubricate cells and act as an adhesion layer for them. 66 U N I T 1 Ce l lul ar Functions

5 P r o t e i n K i n a s e s For many years the nucleus was considered the exclusive control centre of the cell. However, within the last 15 years, scientists such as Dr. Tony Pawson at the University of Toronto, have learned that the cell membrane and molecules within the cell, called protein kinases, have an equally important role in controlling cell function and allowing the communication between cells that is necessary for the proper functioning of the whole organism. This cell-to-cell communication functions as follows: messenger molecules from other cells (often hormones) travel through the bloodstream and then attach to specialized protein molecules on the outside of the membrane of the target cell. The protein receptor molecule, which spans the cell membrane, changes the shape of its tail (which sticks into the cytoplasm). The shape change then triggers a chain reaction that involves protein kinases in the cell. Protein kinases transmit the commands of many hormones that regulate cellular processes such as cell division and specialization. Once activated by the receptor proteins, the kinases join together like Lego blocks to carry the message to the proper location within the cell and allow the cell to respond to the command. The understanding of this method of cell-to-cell (called intercellular) and within-cell (called intracellular) communication has provided new insights into a number of human diseases. For example, scientists have learned that many types of cancers and some types of diabetes are caused by problems with the protein kinase intracellular communication system. New treatments aimed at correcting these problems are currently being tested in clinical trials. Membrane proteins have functions in addition to transporting molecules. Some of the proteins provide structural support to the cell by binding to the protein fibres of the cytoskeleton. Other proteins have a communication function. They receive chemical messengers sent by other cells. Proteins that have carbohydrate chains attached to them are involved in communication and cell recognition. These carbohydrote sugar chains are called the glycocalyx. Other cells, such as those in your immune system, use these carbohydrate chains to recognize a cell or a molecule as being self or being foreign. Section 3.1 Review Understanding Concepts 1. What is the function of the cell membrane? 2. Name and describe the molecules that make up the cell membrane. 3. Describe the different types and functions of the proteins found in the cell membrane. 4. Contrast the terms permeable and selectively permeable. Making Connections 5. Work with a partner to research the role of protein kinases in cell biology. Investigate their involvement in a particular disease. Present an informed opinion on the effectiveness of new treatments based on knowledge about protein kinases. 6. Cholesterol molecules are a normal part of the cell membranes of mammals; however, some people have high levels of cholesterol in their blood that can lead to heart and/or artery disease. Some doctors have suggested that all adults should have their blood cholesterol level tested, and those who have abnormally high cholesterol levels should be given medication or put on a special diet. Research the cost to society if the Canadian government implemented a plan of this nature. Use a PMI chart to organize the results of your research. C H A P T E R 3 Ce l l Transpor t 67