2.5 Biological Membranes

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  • Created by: elbungay1
  • Created on: 26-04-19 14:32
Why are biological membranes described as partially permeable?
They do not let all types of substances through. The properties of the component molecules of the cell's membrane determines its permeability
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What are the roles of membranes at the surface of cells?
Separates cell from external environment. Regulates transport in and out of cell. May contain enzymes for metabolic pathways. Has antigens so immune system recognisies it as 'self'. Release signalling molecules. Contain receptors for signalling.
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What are the roles of membranes within cells?
The mitochondria folded inner membrane, cristae, has a large surface area for aerobic respiration to occur. Inner membranes of chloroplasts, called thykaloid membranes, house chlorophyll and is where photosynthesis occurs
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What is the fluid mosaic membrane structure?
Model consisting of a phospholipid bilayer with proteins floating within. The lipid molecules can change places with one another, giving fluidity.
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What are the roles of proteins within phospholipid bilayer?
Some have pores and act as channels to allow ions, polar molecules across. Some proteins are carriers and change their shape carrying specific molecules. Some proteins are bound to carrier proteins and act as enzymes, antigens or receptor sites
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How are plasma membranes in neurones different to a typical membrane?
Have many protein channels for ions to allow entry and exit of ions to bring about conduction of impulses. Have myelin sheath creating several layers of membrane.
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How are phagocytic plasma membranes specialised for their function?
Contain special protein receptors that enable them to recognise the antigens on foreign cells, usually from invading pathogens or transplanted organs
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What is diffusion?
Movement of molecules from an area of high concentration to an area of low concentration, may or may not be across a membrane and does not involve ATP
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What is facilitated diffusion?
Movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels/pores, not involving ATP
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What molecules can and cannot move through the plasma membrane by simple diffusion?
Small non-polar molecules such as oxygen and carbon dioxide can diffuse through. Lipid soluble molecules also can. Larger/charged/water soluble molecules have to pass through a channel
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What happens when there are equal concentrations of molecules either side of the plasma membrane?
There will be no net movement of molecules across the membrane, equal concentrations move in and out, system is in equilibrium.
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How are concentration gradients maintained for oxygen for aerobic respiration?
Oxygen diffuses into the cytoplasm of respiring cells then diffuses to mitochondria where is then used in aerobic respiration?
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How are concentration gradients maintained for carbon dioxide for photosynthesis?
Carbon dioxide diffusing into the palisade mesophyll cells of a plant leaf will then diffuse into chloroplasts and used in photosynthesis. (CO2 fixation in calvin cycle)
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What factors affect the rate of simple diffusion?
Temperature: higher means more kinetic energy so faster rate. Diffusion distance: smaller means faster rate. Surface Area: more diffusion can occur with larger SA. Size of molecules: smaller the faster. Conc Gradient: the steeper the faster
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What is Osmosis?
Passage of water molecules down their water potential gradient, across a partially permeable membrane.
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What is water potential?
Measure of tendency of water moleucles to diffuse from one area to another. The higher the concentration of solutes, the lower the water potential, the more likely water will enter
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Describe what happens to plant and animal cells in pure water
Water moves into the cells by osmosis down a water potential gradient. The animal cells bursts - it is cytolysed. The plant cell wall prevents bursting and membrane pushes against wall - it is turgid
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Describe what happens to plant and animal cells in concentrated sugar solution with a very low water potential
Water moves out of the cells down a water potential gradient. Animal cells shirts and appears wrinkled - it is crenated. Plant cell membrane pulls away from cell wall - the cell is plasmolysed.
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What is active transport?
The movement of substances against their concentration gradient (low to high concentration) across a cell membrane, using ATP and carrier proteins
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What is endocytosis?
Process in which large particles may be brought into cell. Membrane surrounds molecule and brings them into cell, enclosed in a vesicle
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What are the two types of endocytosis?
Phagocytosis - phagocytic cell ingests solid mater e.g. bacterium and encloses them into a vesicle called a phagosome. Pino(endo)cytosis - ingesting of liquids by cells.
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What is exocytosis?
How large molecules may be exported out of cells. Vesicles fuse with plasma membrane using ATP and the fused sites open, releasing contents of secretory vesicle
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How does a decrease in temperature affect membrane structure and permeability?
Fatty acids become compressed but unsaturated fatty acids have kinks which push molecules apart maintaining fluidity. Proportion of (un)saturated FA determines permeability. Cholesterol also buffers effect of low temperature by stopping FA packing
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How does an increase in temperature affect membrane structure and permeability?
Phopholipids acquire kinetic energy and move more, increasing fluidity and permeability. May move embedded proteins affecting their function. May affect phagocytosis and exocytosis, due to increased fluidity. Cholesterol buffers
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What is the effect of high temperatures affect proteins within the membrane?
High temperatures cause vibrations of atoms, breaks hydrogen and ionic bonds in proteins which hold structure together. Tertiary structure changes and cannot change back again, they are denatured.
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What is the effect of solvents on phospholipids?
Organic solvents such as acetone and ethanol will damage cell membranes as they dissolve lipids, increasing permeability.
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