Cell Membrane Function

Transcription

1 Transport Cell Membrane Function Chapter 7 Pg Transport is the movement of substances into or out of a cell. It can be passive or active. Passive: requires no energy (ATP) Includes diffusion, facilitated diffusion, and osmosis. Active: requires energy (ATP) Includes endocytosis, exocytosis, and pumps. Passive Transport Diffusion of a substance across a membrane Net flow of molecules from high low concentration until dynamic equilibrium is reached Movement of molecules down a concentration gradient Rates of passive transport increase with higher concentration gradients, higher temperatures, and smaller particle sizes. Diffusion Simple diffusion: does not involve protein channels. Occurs because of random, constant motion of molecules. Substance diffuses from more crowded area to less crowded area. Present in the kidney and lungs, for example. Facilitated diffusion: involves protein channels. Requires a hydrophilic protein channel that will passively transport specific substances across the membrane. One type of channel transports single ions like Na +, K +, Ca 2+, and Cl - as in muscle and nerve signals in the body. Also includes aquaporins. Simple Diffusion Example Countercurrent exchange: the flow of adjacent fluids in opposite directions. Maximizes the rate of simple diffusion. Seen in fish gills blood flows toward the head in the gills while water flows in the opposite direction over the gills. Maximizes the diffusion of respiratory gases and wastes between the water and the fish. 1

2 Osmosis Diffusion of water molecules across a selectively permeable membrane. When water moves into a body by osmosis, hydrostatic pressure (osmotic pressure) may build up inside the body. Water moves from an area of high water potential to low water potential. Turgor pressure is the hydrostatic pressure that develops when water enters the cells of plants and microorganisms. Water Follows Solutes There are two parts to a solution: Solute: dissolved substances Solvent: doing the dissolving; usually water Water is going to travel in the direction where there are more dissolved particles More dissolved particles = less water If I have 50 ml of 1% sugar solution and 50 ml of 50% sugar solution, there is less water in the second solution. Water Potential Symbolized by the Greek letter psi (ψ) Results from two factors: Solute concentration Pressure Water potential for pure water = 0 Solutes lower water potential to a value less than 0 Water potential inside of a cell is negative Water moves across a membrane from the solution with the higher water potential to the solution with the lower water potential Tonicity Solutions can be described in terms of tonicity a relative term that compares two solutions. Describing the tonicity of a solution that a cell is in Example 1: The cell is in an isotonic solution. Water diffuses in and out, but there is no net change in the size of the cell. Example 2: In a hypotonic solution, the concen-tration of solute in the beaker is less than the concentration of solute in the cell. Water will flow into the cell, causing it will swell or burst. If the cell is a plant cell, the cell wall will prevent the cell from bursting. The cell will swell or become turgid turgid pressure is what keeps plants like celery crisp and why they spray veggies in the grocery store! If a plant loses too much water (dehydrates), it loses its turgor pressure and wilts. 2

3 Example 3: In a hypertonic solution, the concentration of solute in the beaker is greater than the concentration of solute in the cell. Water will flow out of the cell (because water flows from high low concentration of water). As a result, the cell shrinks (plasmolysis). Plasmolysis Movement of water out of a cell by osmosis. Results in the collapse of the cell (especially plant cells with central vacuoles). When water moves into a cell by osmosis, the cell volume increases and the cell expands. Cell lysis occurs when the swelling causes the cell to burst (especially animal cells and those without a cell wall). Aquaporins Special water channel proteins found in certain cells. Facilitate the diffusion of massive amounts of water across a cell membrane. Do not affect water potential gradient or the direction of water flow they affect the rate at which water diffuses down its gradient. May also function as gated channels that open and close in response to things, like cell turgor pressure. Sudden change in a cell in response to changes in tonicity may be the result of aquaporins. Active Transport Movement of molecules against a concentration gradient low high concentration that requires ATP. There are many examples: Pumps or carriers Contractile vacuoles Exocytosis (vesicular transport) Endocytosis (vesicular transport) Pinocytosis and phagocytosis Receptor-mediated endocytosis Pumps and Carriers Carry particles across the membrane by active transport. Sodium-potassium pump: pumps Na+ and K+ ions across a nerve cell membrane to return the nerve to its resting state. The Na + -K + pump moves Na + and K + ions against a gradient, pumping 2 K + ions in for every 3 Na + ions out. ATP provides the energy. Electron transport chain: mitochondria consists of proteins that pump protons across the cristae. 3

4 Electrochemical Gradient Electric potential (voltage) and chemical potential (concentration gradient) combine to form electrochemical gradients across membranes. Ion membranes generate voltage (separation of charge) If one side has more + ions than ions, it will have a + charge. ions flow towards sides with net positive charges. Cotransport Cotransport proteins use the energy of one molecule diffusing DOWN its gradient to move another molecule AGAINST its gradient. The active transport of Na+ (sodium) OUT of the cell is coupled with the passive transport of glucose INTO the cell. Contractile Vacuole Found in freshwater protists (unicellular, eukaryotic organisms like amoeba) Pumps out excess water that has diffused inward because the cell lives in a hypotonic environment Exocytosis (Bulk Transport) Movement of large particles out of the cell Substance is stored in vesicles; vesicles move toward the cell membrane, merge with it, and deposit the substance on the outside of the cell Usually occurs in white blood cells that destroy and digest foreign particles and bacteria In nerve cells, it occurs as vesicles release neurotransmitters into a synapse 4

5 Endocytosis (Bulk Transport) Pinocytosis: cell drinking; uptake of large, dissolved particles. Plasma membrane invaginates around the particles and encloses them in a vesicle. Phagocytosis: cell eating; engulfing of large particles or small cells by pseudopods. Cell membrane wraps around the particle and encloses it into a vacuole common in white blood cells and amoeba. Receptor-Mediated Endocytosis Enables a cell to take up large quantities of very specific substances. Process by which extracellular substances bind to receptors on the cell membrane. Once the ligand (binding molecule) binds to the receptors, endocytosis begins. Receptors carrying the ligand migrate and cluster along the membrane, turn inward, and become a coated vesicle that enters the cell. This is how cholesterol is taken into cells from the blood. Bulk Flow General term for overall movement of fluid in one direction in an organism. In humans, blood moves around the body by bulk flow as a result of blood pressure created by the pumping heart. Sap in trees moves by bulk flow from the leaves to the roots due to active transport in the phloem. Bulk flow movement is always from source (origin) sink (where it is used). 5