AS OCR biology - cell membranes
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| Explain the basic structure of a cell membrane | A bilayer of phospholipid molecules with hydrophilic phosphate heads facing outwards into the water and hydrophobic fatty acid tails facing inwards away from the water.
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| Why are cell membranes 'partially permeable'? | They allow water and some solutes to penetrate through them. Some have aquaporins which make them much more permeable to water than others.
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| What has the electron microscope revealed about the cell membrane? | 7-10 nm thick
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| What are the roles of a cell membrane? | Regulates the movement of substances in and out of cells, separates cell contents from outside environment / organelle contents from cytoplasm, cell recognition and signalling
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| What is the fluid mosaic? | Model of the cell membrane proposed by Singer and Nicholson: bilayer of phospholipid molecules that aren't fixed in place, some proteins float through bilayer giving a mosaic appearance, extrinsic proteins are partially embedded, intrinsic span bilayer
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| What are some examples of membrane components? | Glycolipids, glycoproteins, enzymes and coenzymes, cholesterol, carrier and channel proteins
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| What is the role of cholesterol in cell membranes? |
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| What is the role of enzymes and coenzymes in cell membranes? | Involved in metabolic processes e.g. enzymes in thylakoids of chloroplasts for photosynthesis, some reactions of respiration occur in inner membrane of mitochondria (cristae mean more enzymes are present)
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| What is the difference between glycoproteins and glycolipids? | Glycoproteins are proteins with a carbohydrate attached. Glycolipids are phospholipids with a carbohydrate attached.
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| What is the role of glycolipids and glycoproteins? | Cell signalling to allow recognition by immune system, glycoproteins bind cells together in tissues, form receptor sites where drugs and hormones can bind (so they affect cell metabolism)
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| What is the role of carrier and channel proteins? | Allow large, charged or hydrophilic molecules to pass through the membrane - facilitated diffusion
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| What is cell signalling and what are the different types? | Cell signalling is communication in or between cells with signal molecules. Intracellular - inside a cell. Extraceullar - between different cells in a mutlicellular organism
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| What are the roles of cell signalling? | Regulates development and organisation, controls growth and cell division, coordinates cell activities
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| What are examples of signal molecules? | Hormones, drugs, neurotransmitters, poisons, viruses
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| What are hormones? | Chemical messengers released by endocrine glands that are transported in the blood to the receptors on the target cell to produce a cell response - extracellular communication
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| How does insulin cause cell communication? | Insulin is secreted by pancreas. Binds with receptors on muscle/liver cells. Causes intracellular response - enzyme cascade reaction. Protein synthesis - new protein channel, transported by vesicle. More channels, glucose influx, lower blood glucose level
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| How do drugs cause cell communication? | Drugs are complementary to receptors so bind to them. Agonists mimic effect of natural signalling molecule e.g. nicotine mimics adrenaline. Antagonists block the effect of the natural signalling molecule e.g. beta blockers prevent heart rate increasing
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| How do viruses exploit receptors? | Viruses are complementary to receptors so bind to them. HIV binds to receptors on immune system cells e.g. T lymphocytes so enters cell, reproduces, and destroys cell.
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| How do poisons affect the body? (Example) | Lethal toxin from Clostridium botulinum binds to receptors on muscle fibres causing paralysis. Used in small quantities in cosmetic surgery botox, paralyses small muscles in face and reduces the wrinkling of skin
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| How do neurotransmitters allow cell communication? | Neurotransmitters are chemical messengers released by nerve ending that diffuse across the synapse, bind to receptors on the post-synaptic cell, and generate an impulse.
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| What is differentiation of the cell membrane? | Where a membrane becomes specialised by containing particular components to suit its function.
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| What are some examples of cell membrane differentiation? | * cholesterol contents in hot / cold climates
* membranes of cells in growing shoot have receptors that detect molecules regulating growth
* chlorophyll in inner membrane of chloroplast
* glucose channels in muscle cells
* white blood cell receptors
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| How does cholesterol content of cell membranes differ in climates and why? | Hot climate - more cholesterol. Otherwise: more kinetic energy, more gaps, leaky, substances can enter/leave cell that normally don't penetrate membrane. Cold climate - more cholesterol. Otherwise: high PUFA content, double bonds, kinks, don't pack well
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| What is pasive transport and some examples? | Passive transport goes down the concentration gradient and doesn't require energy e.g. simple diffusion, facilitated diffusion and osmosis
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| What is diffusion? | Diffusion is the spontaneous net movement of molecules from an area of high concentration to an area of low concentration, down a concentration gradient, due to random movement of particles.
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| What is equilibrium? | When distribution of particles is even, so particles continue to move but there is no net movement.
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| What are some examples of diffusion in/out of cells? | * lipid-based molecules can diffuse through lipid bilayer. * water and carbon dioxide can diffuse in and out as they are small so can penetrate bilayer.
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| What factors increase the rate of diffusion? | Steep concentration gradient, high temperature (more kinetic energy, increased rate of random movement), small molecules, stir liquid / air currents in gas, thin membrane, large surface area
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| What is facilitated diffusion? | Where molecules that are large, charged or hydrophilic pass across the membrane in proteins, down a concentration gradient.
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| What is a channel protein? | A protein pore that allows molecules to pass through, may be gated and require a stimulus to open. E.g. gated sodium ion channel proteins are involved in nervous system.
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| What is a carrier protein? | A specialised molecule is complementary to the carrier protein, binds to it causing a confirmational shape change, carried to other side of membrane e.g. glucose, amino acids
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| What is osmosis? | The movement of water molecules from an area of high water potential to an area of low water potential, down a water potential gradient, across a partially permeable membrane. Reaches equilibrium when both sides have equal water potential
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| What is water potential? | A measure of the tendency for water molecules to move from one place to another, measured in kPa
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| What effect does kinetic energy have on water potential? | Increases - more particles hit walls of cell, increasing pressure
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| What is the water potential of pure water - how and why does this change when solute is added? | 0kPa - decreases when solute is added, because solute restricts the movement of water molecules by forming hydration shells
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| Define: solute, solvent, solution | Solute = solid that dissolves in liquids. Solvent = liquid that dissolves solids. Solvent = liquid containing dissolved solids.
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| What is a solution called if it has a higher water potential than the cell? | Hypotonic - low concentration of solute
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| What happens when a cell is placed in a solution of a higher water potential? | Osmosis into cell because water potential gradient is form outside to inside. Animal cell - bursts (haemolysis). Plant cell - turgid, vacuole and cytoplasm push cell membrane against cell wall.
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| What is a solution called if it has a lower water potential than the cell? | Hypertonic - high concentration of solute
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| What happens when a cell is placed in a solution of a lower water potential? | Osmosis out of cell because water potential gradient is from inside to outside. Animal cell - shrivels, wrinkles, crenation. Plant cell - vacuole and cytoplasm shrink, cell membrane pulls away from cell wall - plasmolysis.
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| Define isotonic | Cell and solution both have same water potential
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| What are active processes across membranes + examples? | When molecules move against the concentration gradient, requires energy in the form of ATP e.g. active transport, bulk transport
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| What is active transport? | Where molecules move from an area of low concentration to an area of high concentration, against the concentration gradient. Protein pumps are complementary to molecule and use ATP to move it across the bilayer.
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| Why is there only one-way movement in active transport? | ATP energy causes the protein to change shape to allow the molecule to fit, and the shape changes again when the molecule is deposited on the other side of the membrane - this ensures that molecules flow in only one direction.
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| What are some examples of active transport? | * when a muscle needs to contract, calcium ions are released from ER where they are in high concentration, and when a muscle needs to relax, they are pumped back by calcium ion pumps
* magnesium ions - root hair cells
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| What is bulk transport? | The movement of large quantities of substances against the concentration gradient in vesicles, which are moved using ATP energy
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| What is endocytosis? | The movement of material into a cell
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| What is exocytosis + example? | The movement of material out of a cell. Insulin secreted by pancreatic cells - proteins made by ribosomes, golgi modifies and packages, vesicles bud off golgi, secretory vesicles fuse with cell membrane and contents leave cell
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| What is phagocytosis + example? | The movement of solid material e.g. phagocytes engulf pathogens.
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| How does a phagocyte digest pathogens? | Cell cytoplasm flows around the pathogen forming a vesicle. Lysosomes fuse with vesicle so hydrolytic, digestive enzymes digest pathogen. Soluble products are absorbed by the cell, undigested remains leave the cell by exocytosis.
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| What is pinocytosis + example? | The movement of liquid material e.g. the human ovum takes up nutrients from follicle cell by pinocytosis.
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| What is the role of cell memrbanes inside cells? | Compartmentalise organelles, control the movement of substances in and out of organelles, form vesicles, isolate potentially harmful enzymes in lysosomes
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| What is the role of phospholipids in the cell membrane? | They form a bilayer that acts as a barrier, they are fluid so components of the membrane move around, they are selectively permeable so they are soluble to non-polar and small polar substances, but large non-polar substances and ions can't diffuse througj
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| Give an example of local hormones in cell signalling | Cytokines are local hormones that cause B cells to differentiate and secrete antibodeis during the imune reposne
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| Explain how lipid-soluble molecules are used in cell signalling | Lipid soluble molecules (e.g. steroid hormones: oestrogen, testosterone) are transported in the blood stream attached to a carrier, penetrate the lipid bilayer, activate a cytoplasmic receptor, have a high affinity for DNA so cause protein synthesis
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| What happens when beetroot is soaked in hot water and why? | Betalain (red pigment) soaks out because membrane proteins are denatured disrupting the bilayer structure, and the phospholipids have more kinetic energy increasing the fluidity of the membrane (leaky)
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