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Transcript
CH. 5: CELL TRANSPORT Homeostasis: Maintaining a stable, internal environment - cell membranes maintain homeostasis by controlling what enters or leaves the cell. Permeability - Ability to permeate/ get through 1. If a substance can pass through = permeable 2. Only lets some substances through = semi permeable 3. Can’t get through = impermeable How Cell Membranes Determine What Comes In: - Remember, molecules are in constant motion! 1. 2. 3. 4. 5. Structure of the membrane Size of the particles Chemical make-up of particles (alcohol, lipid, etc) Particles are/are not soluble in water (can dissolve) Conditions in or outside the cell (temp, pressure, etc) Passive Transport: - Involves the movement of molecules across the cell membrane without an input of energy by the cell. NO ENERGY is REQUIRED to move substances across membrane -- water, lipids, and other lipid soluble substances Types: – Diffusion – Osmosis – Facilitated Diffusion – Filtration Diffusion: the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the molecules’ kinetic energy until a state of dynamic equilibrium is reached. Concentration gradient: the difference in concentration across space. Dynamic equilibrium – continued movement of molecules with no net change in concentration Diffusion Diffusion… There are factors that affect the rate of diffusion. - The rate of diffusion refers to how fast it reaches equilibrium: a. The concentration of molecules b. The temperature c. The pressure Osmosis Osmosis is the diffusion of water across a membrane. Direction of Osmosis The net direction of osmosis is due to the relative solute concentrations on the two sides of the membrane. Osmosis… • Direction of Osmosis... • When the solute concentration outside the cell is higher than that in the cytosol, the solution outside is hypertonic (more solute, less water) to the cytosol, and water will diffuse out of the cell. • Plasmolysis = cell’s shrinking due to water loss Osmosis… • Direction of Osmosis… • When the solute concentration outside the cell is lower than that in the cytosol, the solution outside is hypotonic (less solute, more water) to the cytosol, and water will diffuse into the cell. • Cytolysis = cell’s bursting due to water gain Osmosis… • Direction of Osmosis… • When the solute concentrations outside and inside the cell are equal, the solution outside is isotonic (same solute, same water), and there will be no net movement of water. Osmosis… • How Cells Deal With Osmosis • To remain alive, cells must compensate for the water that enters the cell in hypotonic environments and leaves the cell in hypertonic environments. • Cells in multicellular organisms respond to hypotonic environments by pumping solutes out of the cytosol (RBCs cannot compensate for changes in solute concentration) • Contractile vacuoles are organelles that regulate water levels in paramecia. Facilitated Diffusion: - Diffusion of molecules across a membrane when they are not soluble in lipids or are too large (e.g. glucose) to pass through pores in membrane • In facilitated diffusion, a molecule binds to a carrier protein on one side of the cell membrane. • The carrier protein (specific for one type of molecule) then changes its shape and transports the molecule down its concentration gradient to • Ex – Insulin carries glucose into cells. the other side of the membrane. Diffusion through Ion channels • Ion channels are proteins, or groups of proteins, that provide small passageways across the cell membrane through which specific ions can diffuse. • Ions important in cell function include sodium, potassium, calcium, and chloride 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 performed by carrier proteins called cell membrane pumps. Active Transport… • Cell Membrane Pumps: • Sodium-Potassium Pump –The sodium-potassium pump moves three Na+ ions into the cell’s external environment for every two K+ ions it moves into the cytosol. • Animal cells must have a higher concentration of Na+ ions outside the cell and a higher concentration of K+ ions inside the cell –ATP supplies the energy that drives the pump. Sodium Potassium Pump… • The exchange of three Na+ ions for two K+ ions creates an electrical gradient across the cell membrane • Outside becomes positively charged relative to the inside, which becomes negative • Difference in electrical charge is important for the conduction of electrical impulses along nerve cells Active Transport… Movement in Vesicles: • Endocytosis - cells ingest external fluid, macromolecules, and large particles, including cells by folding around them and forming a pouch. –The pouch then pinches off and becomes a membrane-bound organelle called a vesicle. • Some vesicles fuse with lysosomes, and their contents are digested by lysosomal enzymes Active Transport… • Endocytosis –Endocytosis includes pinocytosis, in which the vesicle contains solutes or fluids, and phagocytosis, in which the vesicle contains large particles or whole cells. • Bacteria and viruses are ingested in this way –Receptor-mediated endocytosis – molecules are brought into the cell via coated pits (proteins) ActiveTransport… • Exocytosis - vesicles made by the cell fuse with the cell membrane, releasing their contents into the external environment. • Used to release large molecules, such as proteins, waste products, or toxins that would damage the cell if they were released within the cytosol Active Transport… Endocytosis & Exocytosis https://www.youtube.com/watch?v=2icEADP0J4