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Transcript
Transport Across the Cell Membrane • maintains homeostasis of cell • membrane is selectively permeable – some things can pass through but others can’t • 3 types: active transport, passive transport, and bulk transport 1. Passive Transport • passive = no energy req’d • move through membrane due to differences in concentration gradient • 3 different types 1. Diffusion • Movement of molecules from an area of high concentration to an area of low concentration across a concentration gradient • Used transport small molecules like CO2 H2O, and O2 • Animation: How Diffusion Works 2. Osmosis • Diffusion of water from a region of high concentration to a region of low concentration • Water can diffuse into or out of a cell, it depends on the concentration on either side of the cell membrane • Animation: How Osmosis Works Water potentials • Water moves from a place with a LESS NEGATIVE (higher) water potential to a place with a MORE NEGATIVE (lower) water potential • The water potential of pure water is 0 (zero). • Solutions have negative water potentials – the more concentrated the solution, the more negative the water potential. 3 Types of Osmosis 1. Hypotonic • contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). • cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. 2. Hypertonic • contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). • when cell is placed in a hypertonic solution, water diffuses out of the cell, causing the cell to shrivel. 3. Isotonic • contain the same concentration of solute as an another solution (e.g. the cell's cytoplasm). • when cell placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. • fluid that surrounds the body cells is isotonic. http://www.zerobio.com/flashmx/transport.swf 3. Facilitated diffusion Key features • • • • how glucose/charged ions moves into cells passive – does not require energy uses carrier proteins solute molecules “combine” with carrier proteins in the membrane. • carrier molecules speed (or facilitate) the passage of the solute molecules across the membrane. Spot the difference! 2 kinds of proteins involved: 1.CARRIER PROTEINS • bind to a specific type of diffusing molecule. • have a highly specific hydrophilic region to which the solute molecule binds. • binding cause the protein to undergo a change in shape that moves the solute across the bilayer and release it on the other side Carrier proteins Animation: How Facilitated Diffusion Works Carrier proteins 2. ION CHANNELS • formed by proteins with a central pore that is lined with charged groups. • help the diffusion of charged particles such as Ca2+, Na+, K+, HCO3- and Cl ions. • Some channels are gated and allow cells to regulate the flow of ions from one cell to another. Ion channel Factors affecting Rate of Diffusion 1. Concentration Difference • happens ONLY when a concentration gradient is present and solute travels ALONG (down) a concentration gradient 2. Saturation • there are only a limited number of carrier molecules per unit area of membrane. • rate of movement reaches a max. when all carrier molecules are fully loaded with solute molecules 2. Active Transport • the transport of molecules or ions across a membrane by carrier proteins against a concentration gradient. • requires energy • involves carrier proteins in the membrane. • hydrolysis of ATP releases the energy required for active transport. • Cells involved in active transport have a large number of mitochondria to provide the ATP required Spot the difference Sodium-Potassium Pump • ex. of active transport • Exists in most cell membranes. • Actively removes sodium ions from the cell while actively accumulating potassium ions into them from their surroundings • Animation: How the Sodium Potassium Pump Works 3. Bulk Transport • used for materials to large to enter via passive or active transport • vesicles created by folding of cell membrane onto itself to either engulf or expel materials • 2 types: endocytosis & exocytosis Endocytosis • the transport of large particles into the cell in vesicles formed by folding in of the cell surface membrane • 3 types: 1. Pinocytosis (cell drinking) • intake of small droplet of extracellular fluid along with solute particles • occurs in all cells often 2. Phagocytosis (cell eating) • intake of large droplet of extracellular fluid including particulate matter (bacteria or organic matter) • occurs only in specialised cells like amoeba or macrophages (bacteria fighting immune cells) • Animation: Phagocytosis 3. Receptor-assisted endocytosis • involves intake of specific molecules that attach to special protiens in cell membrane that serve as receptors • have a unique shape that fit only to one specific molecule • ex. animal cells use this to bring cholesterol into cell Exocytosis • the reverse process and is used to secrete proteins, e.g digestive enzymes, out of the cells. • vesicle forms inside cell moves to membrane and empties contents outside of cell • ex. pancreas secretes insulin