UNIT TWO: CELL BIOLOGY 1(Text from Modern Biology, Holt, Rinehart, and Winston) Chapter Five (Homeostasis and Cell Transport)SECTION ONE: PASSIVE TRANSPORT Cell membranes help organisms maintain homeostasis by controlling whatsubstances can leave or enter the cell. In a process known as passive transport transport,some substances can cross a cell membrane without any input of energy by the cell. DIFFUSION Diffusion, Diffusion the movement of molecules from an area of higher concentration toan area of lower concentration, is the simplest type of passive transport. Theconcentration gradient is the difference in the concentration of molecules across adistance. If a sugar cube is added to a beaker of water, the sugar cube will sink to thebottom, so the concentration of sugar molecules at the bottom is greater than atthe top. As the cube dissolves, the sugar molecules will diffuse throughout the water,moving toward the lower concentration at the top. Molecules are in constant motion because they have kinetic energy, which isthe cause of diffusion. Molecules tend to move from areas where they are moreconcentrated to areas where they are less concentrated. In the absence of other influences, diffusion will cause the molecules to be inequilibrium,equilibrium that is, the concentration of molecules will be the same throughout thespace they are occupying. Going back the beaker of water and sugar cube example, ifthe water is left undisturbed, the concentration of sugar molecules will be the samethroughout the beaker, which the concentration will be at equilibrium. However, even at equilibrium, the random movement of molecules continues.It’s just that they are now as likely to move in one direction as in any other. Therandom movements of many molecules in many directions balance one another, and soequilibrium is maintained.Diffusion Across Membranes If a molecule can pass through a cell membrane, it will diffuse from an area ofhigher concentration on one side of the membrane to the area of lower concentrationon the other side. Diffusion across a membrane is also called simple diffusion, andonly allows certain molecules to pass through the membrane. The simple diffusion of a molecule across a cell membrane depends on the sizeand type of molecule and the chemical nature of the membrane. Molecules that candissolve in lipids can pass directly through the membrane by diffusion. Additionally,molecules that are very small but not soluble in lipids may diffuse across themembrane by moving through pores in the membrane.
UNIT TWO: CELL BIOLOGY 2(Text from Modern Biology, Holt, Rinehart, and Winston) OSMOSIS The process by which water molecules diffuse across a cell membrane from anarea of higher concentration to an area of lower concentration is called osmosis osmosis.Because water is moving from a higher to lower concentration, it does not requirecells to expend energy. Therefore, osmosis is the passive transport of water.Direction of Osmosis The net direction of osmosis depends on the relative concentration of soluteson the two sides of the membrane. When the concentration of solute moleculesoutside the cell is lower than the concentration in the cytosol, the solution outside ishypotonic to the cytosol. In this situation, water diffuses into the cell until equilibriumis reached. When the concentration of solute molecules outside the cell is higher than theconcentration in the cytosol, the solution outside is hypertonic to the cytosol. Thewater will diffuse out of the cell until equilibrium is established. When the concentration of solutes outside and inside the cell are equal, theoutside solution is said to be isotonic to the cytosol. The water diffuses into and outof the cell at equal rates, so there is no net movement of water. The prefixes hypo-, hyper-, and iso- refer to the relative solute concentrationsof the two solutions. Water tends to diffuse from hypotonic solutions to hypertonicsolutions.How Cells Deal with Osmosis Many cells function in a hypotonic environment, such as unicellular freshwaterorganisms. They require a relatively lower concentration of water in the cytosol tofunction normally, so must get rid of the extra water that enters during osmosis. Some of them do this with contractile vacuoles organelles that remove water. vacuoles,These organelles collect excess water and then contract to pump the water out of
UNIT TWO: CELL BIOLOGY 3(Text from Modern Biology, Holt, Rinehart, and Winston)the cell. Since this requires the cell to expend energy, the pumping action is not aform of passive transport. Other cells respond to hypotonic environments by pumping solutes out of thecytosol. Most plant cells live in a hypotonic environment. The cells that make upplant roots may be surrounded by water, which moves into the plant cells by osmosis.These cells swell when they fill with water until the membrane is pressed against thecell wall. The cell wall is strong enough to resist the pressure exerted by theexpanding cell. Turgor pressure is the pressure the water molecules exert againstthe cell wall. In a hypertonic environment, water leaves the cells through osmosis and theturgor pressure is lost, since the cells shrink away from the cell wall. This is calledplasmolysisplasmolysis and is the reason that plants wilt when there is not enough water. smolysis, Some cells cannot compensate for changes in the solute concentration oftheir environment. Red blood cells lose their normal shape when they are exposed toan environment that is not isotonic to their cytosol. In a hypotonic environment, waterdiffuses into the cell, causing them to swell and eventually burst. When they burst, itis called cytolysis cytolysis. FACILITATED DIFFUSION Facilitated diffusion is another type of passive transport that is used formolecules that cannot readily diffuse through cell membranes. Such molecules mightnot be soluble in lipids, or could be too large to pass through the pores in themembrane. In facilitated diffusion, carrier proteins assist in the movement of thesekinds of molecules across the cell membrane. A molecule binds to a specific carrier protein that transports it. As soon as itbinds, the carrier protein changes shape, shielding it from the interior of themembrane. Once shielded, the molecule can be transported through the cellmembrane. On the other side of the membrane, the molecule is released from thecarrier protein and the protein returns to its original shape. There are two important properties of facilitated diffusion. First, facilitateddiffusion can help substances move either into or out of a cell, depending on theconcentration gradient. Second, the carrier proteins involved in facilitated diffusionare each specific for one type of molecule.
UNIT TWO: CELL BIOLOGY 4(Text from Modern Biology, Holt, Rinehart, and Winston) DIFFUSION THROUGH CHANNELS Another type of transport involves membrane proteins known as ion channels channels.When ion channels transport ions from higher to lower concentrations, they are a formof passive transport. Ion channels transport ions such as sodium, potassium, calcium,and chloride, providing small passageways across the cell membrane through whichions can diffuse. Each type of ion channel is usually specific for one type of ion. Some ion channels are always open, while others have “gates” that open toallow ions to pass, and close to stop their passage. The gates may open or close inresponse to three kinds of stimuli: stretching of the cell membrane, electrical signals,or chemicals in the cytosol or external environment. SECTION 1 REVIEW 1. Toward what condition does diffusion eventually lead, in the absence of other influences? 2. How is osmosis related to diffusion? 3. If the concentration of solute molecules outside a cell is lower than the concentration in the cytosol, is the external solution hypotonic, hypertonic, or isotonic to the cytosol? 4. What role do carrier proteins play in facilitated diffusion? 5. How is facilitated diffusion similar to diffusion through ion channels? CRITICAL THINKING 6. Sea water has a higher concentration of solutes than do human body cells. Why might drinking large amounts of sea water be dangerous for humans? 7. What would happen to a grape placed in a bowl with highly concentrated sugar water? 8. Using what you know about osmosis, explain what would happen to a jellyfish placed in a freshwater lake.SECTION TWO: ACTIVE TRANSPORT In many cases, cells must move materials from an area of lower concentrationto an area of higher concentration, or “up” their concentration gradient. Suchmovement of materials is known as active transport, which requires a cell to expend transportenergy.
UNIT TWO: CELL BIOLOGY 5(Text from Modern Biology, Holt, Rinehart, and Winston) CELL MEMBRANE PUMPS Ion channels and carrier proteins also assist in some types of active transportThey carrier proteins that work with active transport are often called cell membrane“pumps” because they move substances from lower to higher concentrations. The carrier proteins involved in facilitated diffusion and those involved in activetransport are very similar. However, cell membrane pumps require energy, oftensupplied directly or indirectly by ATP.Sodium-Potassium Pump A carrier protein known as the sodium-potassium pump transports sodium ions sodium-and potassium ions up their concentration gradients. To function normally, someanimal cells must have a higher concentration of sodium ions outside the cell and ahigher concentration of potassium ions inside the cell. This pump maintains theseconcentration differences. 1. Three sodium ions bind to the carrier protein on the cytosol side of themembrane. At the same time, the carrier protein removes a phosphate group froman ATP molecule. 2. The phosphate group from the ATP molecule binds to the carrierprotein. 3. The removal of the phosphate group from the ATP supplies the energyneeded to change the shape of the carrier protein. The protein then carries thethree sodium ions through the membrane and forces them outside the cell. 4. Nowthe protein has the correct shape to bind with two potassium ions outside the cell.5. When they bind, the phosphate group is released and the carrier protein restoresits original shape. 6. This change causes it to release the two potassium ions into thecell. It is now ready to start the process again. A complete cycle of the sodium-potassium pump exports three sodium ions out and imports two potassium ions intothe cell.
UNIT TWO: CELL BIOLOGY 6(Text from Modern Biology, Holt, Rinehart, and Winston) The exchange of ions creates an electrical gradient across the cellmembrane. This means that the outside of the membrane becomes positivelycharged relative to the inside of the membrane, which becomes negatively charged.This difference in charge is important for the conduction of electrical impulses alongnerve cells. Other pumps work in similar ways to transport materials across cellmembranes. MOVEMENT IN VESICLES Some substances are too large to pass through the cell membrane by thetransport processes discussed so far. Cells employ endocytosis and exocytosis tomove such substances in and out of cells. They are both types of active transportbecause they require cells to expend energy.Endocytosis Endocytosis is the process by which cells ingest external fluid,macromolecules, and large particles, including other cells. These cells are enclosedby a portion of the cell membrane, which folds into itself, forming a pouch. This pouchthen pinches off from the membrane becoming a vesicle Some of them fuse with vesicle.lysosomes and their contents are digested by lysosomal enzymes. Other vesiclesthat form during endocytosis fuse with other membrane-bound organelles. Pinocytosis involves the transport of solutes or fluids, and phagocytosis is themovement of large particles or whole cells. Many unicellular organisms feed byphagocytosis and certain cells in animals use phagocytosis to ingest bacteria andviruses that invade the body. These cells, known as phagocytes allow lysosomes to phagocytes,fuse with the vesicles that contain the ingested bacteria and viruses. The enzymes inthe lysosomes then destroy the bacteria and viruses.
UNIT TWO: CELL BIOLOGY 7(Text from Modern Biology, Holt, Rinehart, and Winston)Exocytosis Exocytosis is the process by which a substance is released from the cellthrough a vesicle that transports the substance to the cell surface and then fuseswith the membrane to let the substance during the cell. They may use exocytosis torelease large molecules such as proteins, waste products, or toxins. Cells in thenervous and endocrine systems also use exocytosis to release small molecules thatcontrol the activities of other cells. SECTION 2 REVIEW 1. Explain the difference between passive transport and active transport. 2. What functions do carrier proteins perform in active transport? 3. What provides the energy that drives the sodium-potassium pump? 4. Explain the difference between pinocytosis and phagocytosis. 5. Describe the steps involved in exocytosis. 6. How do endocytosis and exocytosis differ? How can that difference be seen? CRITICAL THINKING 7. During intense exercise, potassium tends to accumulate in the fluid surrounding muscle cells. What membrane protein helps muscle cells counteract this tendency? Explain your answer. 8. How does the sodium-potassium pump differ from facilitated diffusion? 9. The vesicles formed during pinocytosis are much smaller than those formed during phagocytosis. Explain.
UNIT TWO: CELL BIOLOGY 8(Text from Modern Biology, Holt, Rinehart, and Winston) CHAPTER HIGHLIGHTSSECTION 1: Passive Transport Passive transport involves the movement of molecules across the cell membrane without an input of energy by the cell. Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the molecule’s kinetic energy until equilibrium is reached. Molecules can diffuse across a cell membrane by dissolving in the phospholipid bilayer or by passing through pores in the membrane. Osmosis is the diffusion of water across a membrane. The net direction of osmosis is determined by the relative solute concentrations on the two sides of the membrane. When the solute concentration outside the cell is lower than that of the cytosol, the solution outside is hypotonic to the cytosol, and water will diffuse into the cell. When the solute concentration outside the cell is higher than that in the cytosol, the solution outside is hypertonic to the cytosol, and water will diffuse out of the cell. When the solute concentrations outside and inside the cell are equal, the solution outside is isotonic, and there will be no net movement of water. To remain alive, cells must compensate for the water that enters the cell in hypotonic environments and leaves the cell in hypertonic environments. In facilitated diffusion, a molecule binds to a carrier protein on one side of the cell membrane. The carrier protein then changes its shape and transports the molecule down its concentration gradient to the other side of the membrane. Ion channels are proteins, or groups of proteins, that provide small passageways across the cell membrane through which specific ions can diffuse.SECTION 2: Active Transport Active transport moves molecules across the cell membrane from an area of lower concentration to an area of higher concentration. Unlike passive transport, active transport requires cells to expend energy. Some types of active transport are preformed by carrier proteins called cell membrane pumps. One example of a cell membrane pump is the sodium-potassium pump. It moves three Na+ ions into the cell’s external environment for every two K+ ions it moves into the cytosol. ATP supplies the energy that drives the pump. Endocytosis and exocytosis are active transport mechanisms in which large substances enter or leave cells inside vesicles. In endocytosis, the cell membrane folds around something in the external environment and forms a pouch. The pouch then pinches off and becomes a vesicle in the cytoplasm. Endocytosis includes pinocytosis, in which the vesicle contains solutes or fluids, and phagocytosis, in which the vesicle contains large particles or cells. In exocytosis, vesicles made by the cell fuse with the cell membrane, releasing their contents into the external environment.