Membranes

• Roles of cell membranes - compartmentalisation, partially permeable barrier between: the cell and its environment; organelles and the cytoplasm; different parts of the organelles especially chloroplasts and mitochondria.
• Membranes are the site of chemical reactions, e.g. the light dependant reactions of photosynthesis.
• Membranes are the site of cell communication, e.g.contain receptors for signal molecules like hormones and neurotransmitters, contain antigens used for cell recognition.
• Signal molecule outside the membrane.
• Phospholipid with head and tail on either 'side' of the protein.
• Cholesterol molecules in the membrane, almost little chunks of cholesterol.
• Glycoprotein a protein in a membrane with the carbohydrate on of stick out the top of the membrane.
• A channel protein is a protein that goes through the whole of the membrane and can move substances from one side of the membrane to the other.
• A lipid base is the base of the lipid attached to carbohydrates that stick out the top of the membrane with the whole thing being known as a glycolipid.
• Extrinsic protein in on one half of the membrane and not throughout.
• A intrinsic or integral protein is a protein that goes throughout the membrane for example a channel or carrier protein.
• A carrier protein changes shape to allow molecules through the membrane from one side to the other.
• A receptor protein has a space to a signal molecule to be able to bind to the receptor and then the response is triggered inside the cell.
• Factors that affect membrane structure - temperature and solvents.
• Temperature that affects membrane structure - as temp increases the KE of the phospholipid increases, therfore increasing the fluidity of the membrane. Causes temporary gaps between permeability of the membrane, at higher temperatures intrinsic protein molecules will start to denature and membrane will lose its sturcutre, dramatically increasing its permeability.
• Solvents that affect membrane structure - water is a polar solvent, essential to orientate the phospholipid molecules in membrane, phosphate head outwards and fatty acid tails inwards. Organic solvents like alcohols, less polar can get into hydrophobic core of the membrane dissolving it and destroying the membrane, this is why alcohol based sanitiser gel and antiseptic wipes work as they destroy membranes of bacterial cells.
• Precise is values being close together.
• Accurate is closeness to the teafet.
• Cell signalling - cells communicate wit each other by signals. Many different molecules can act as signals. Occurs when one cell produces a chemical that binds to the surface of another cell (the target cell).
• Recognition and communication - glycoproteins and glycolipids, layer around the cell formed by these molecules called glycocalyx.
• Receptor sites - for hormones and drugs to bind to.
• Cell recognition - antigens that identify the cell.
• Hormone receptors - hormones, chemical messangers in multicellular organisms, produced in specific tisues - released into organism (e.g. blood stream) by exocytosis. Hormones bind to specific complementary receptor molecules on cell surface, can then affect function in the cell, a cell with receptors for a specific hormone is known as a target cell.
• Substances crossing membranes - depend on the type of substance and the concentration gradient across the membranes.
• 5 Ways substances can cross membranes - 1 - simple diffusion, across phospholipid bilayers. Hydrophobic core of phospholipid layer provides a barrier for polar charged water soluble substances like ions, glucose and amino acids. Non-polar uncharged lipid soluble substances can diffuse across the bilayer, oxygen carbon dioxide and ethanol.
• Ways substances can cross membranes - 1 - one exception is water, small and numerous of them so can move across the membrane by simple diffusion (osmosis).
• Ways substances can cross membranes - 2 - facilitated diffusion - channel proteins act as hydrophobic pores only allow the passage of one specific molecule/particle. Carrier proteins undergo a conformational change as a particle is carried across the membrane.
• Ways substances can cross membranes - 2 - some channel proteins can be gated, open/closed, allowing facilitated diffusion or not.
• Facilitated diffusion - down a concentration gradient, passive, no ATP required, polar charged, water soluble substances move down concentration gradient by facilitated diffusion, through channel/carrier proteins.
• Ways substances can cross membranes - 3 - active transport, energy from hydrolysis of ATP is used to 'pump' particles against their concentration gradient.
• Carrier proteins undergo change (conformational) as they do 3.
• Channel proteins - intrinsic proteins, span the whole membrane, act as specific pores but do not undergo conformational changes, as a particle moves across the membrane only by facilitated diffusion.
• Carrier proteins - intrinsic proteins, span the whole membrane, do undergo conformational change as a particle is carried across the membrane, passive if particles are moving down the concentration gradient, facilitated diffusion or active transport if it is against the concentration gradient.
• Ways of substances crossing membranes - 4 - endocytosis, the way the particles go in.
• Ways of substances crossing membranes - 5 - exocytosis, also known as bulk transport larger molecules/particles in and out of the cell, waste out by exocytosis.
• Simple diffusion - down a concentration gradient, no proteins required, no ATP required, for example oxygen, carbon dioxide and ethanol.
• Facilitated diffusion - down a concentration gradient, channel and carrier proteins required, no ATP required, for example ions, amino acids, glucose and polar molecules.
• Active transport - against the concentration gradient, carrier proteins needed, ATP required, for example ions, amino acids, glucose and polar molecules.
• Osmosis - form of simple diffusion.
• Plant cell in water - water moves into the cell as higher water concentration outside than inside the cell, becomes turgid (only with plant cells and water).