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34 Cards in this Set
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What is a membrane |
The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same: they're composed of lipids, proteins and carbohydrates. They're partially permeable controlling what substances can leave or enter the cell/ organelle. They act as a barrier between the cell and its environment/ the organelles and the cytoplasm. |
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Functions of the plasma membrane |
A plasma membrane forms the barrier between the cell cytoplasm and the extracellular environment. It must be: ● strong to offer mechanical support ● flexible to allow cells to move, grow and divide ● self sealing so the cell can divide without bursting Other functions include: ■ Selectively permeable - Regulating the transport of substances into/out of the cell ■ Antigens/self-recognition-lots of proteins that act as antigens. Some of them can recognise other cells or invading. ■ Cell signalling – contains receptors for hormones and neurotransmitters (glycoproteins and glycolipids)- these can release chemicals ■ insulation of nerves and transmission of nerve impulses |
Permeability is the ability of a structure to allow substances to pass through |
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Fluid mosaic model |
In 1972 Singer and Nicholson proposed the fluid mosaic model in which proteins are scattered through three membrane and move within a fluid phospholipid bilayer . The phospholipids are constantly moving through the bilayer (lateral movement), making it fluid. |
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Structure of the membrane - phospholipid bilayer |
Phospholipid molecules arrange themselves as a continuous bilayer, with the non polar hydrophobic fatty acid tails facing inwards and the polar hydrophilic phosphate heads facing outwards towards the water on either side of the membrane. The middle of the bilayer is hydrophobic so acts as a barrier to water soluble substances like ions, but allows certain lipid soluble substances to pass through. The phospholipid bilayer is oily, giving the membranes flexibility and fluidity. |
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Structure of the membrane - proteins |
Proteins determine membrane’s specific functions. cell membrane & organelle membranes each have unique collections of proteins. Proteins in the membrane have variable structures and functions but all contribute mechanical strength. |
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Membrane proteins - extrinsic/peripheral proteins |
■ loosely bound to the surface of membranes. ■ many combine with carbohydrate chains to form glycoproteins which extend from the cell surface and act, alongside glycolipids, as chemical receptors which signal the cell to respond to certain chemicals - like identity markers, antigens. ■ some proteins on the inner surface of the cell surface membrane attach to the cytoskeleton to anchor the membrane in place. |
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Membrane proteins - intrinsic/ integral proteins |
■ penetrate lipid bilayer, usually across whole membrane. ■ they act as transmembrane protein, transporting molecules and ions across the membrane. These include carrier proteins and channel proteins. ■ some act as enzymes catalysing specific metabolic reactions at a particular part of the cell |
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Structure of the membrane - lipids |
■ cholesterol molecules fit between the phospholipids, binding to the fatty acid tails so they pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid. This helps maintain the shape of animal cells (which have no cell walls) and free floating cells e.g red blood cells. ■ some lipids in the bilayer have carbohydrate chains attached - glycolipids - and act, alongside glycoproteins, as chemical receptors which signal the cell to respond to certain chemicals. |
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Affect of temperature on membrane permeability |
■ At low temperatures, fatty acids have less energy so move less and become compressed. Unsaturated fatty acids have a kink in the tails giving them more room to move and so there's less compression. Cholesterol also sits between the phospholipid tails, stopping them from being compressed together and allowing for more movement, increasing fluidity and permeability. Intrinsic proteins deform so can't control the movement of substance through the cell membrane, increasing permeability. ■ At normal temperature, the phospholipids can move around and aren't so compressed and the intrinsic proteins function properly, so the membrane is partially permeable. ■ At high temperatures, the molecules have more kinetic energy so move around faster, allowing for more lateral movement, increasing membrane fluidity and permeability. Proteins move around faster so cell signalling and phagocytosis occur quicker (can engulf things easier as membranes will fold quicker), but are soon denatured so can't control the movement of substance through the cell membrane, increasing permeability. |
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Affect of solvents on membrane permeability |
Increasing the concentration of solvents around the cell surface membrane increases membrane permeability because the solvents dissolve the lipids in the membrane, causing it to lose structure. |
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Diffusion |
The passive net movement of particles from an area of high concentration to an area of low concentration, down a concentration gradient until equilibrium is achieved (particles are evenly distributed), caused by the random movement of particles. It occurs whenever there is a concentration gradient and there's no barrier to movement. |
Fick's law: rate of diffusion = SA × concentration gradient / thickness of exchange surface |
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Simple diffusion |
When molecules like oxygen and carbon dioxide diffuse directly through the membrane unassisted, because they're small enough or are hydrophobic. Factors affecting the rate of diffusion: ▪ size of the concentration gradient - the greater the difference in concentration across the membrane, the faster the rate. ▪ thickness of the exchange surface - the thinner it is, the shorter the distance the particles must travel, and the faster the rate. ▪ surface area - the larger it is, the faster the rate ▪ temperature - the higher it is, the more kinetic energy the particles have and the faster the rate ■ in general, larger particles and polar molecules diffuse slower |
Over time, the concentration gradient decreases until equilibrium is reached, so rate of diffusion decreases. |
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Facilitated diffusion |
When large particles and polar molecules like glucose require specific proteins to diffuse through the membrane as they diffuse very slowly through the hydrophobic bilayer. Factors affecting the rate of diffusion: ▪ size of the concentration gradient - the larger it is, the faster the rate ▪number of proteins - the more proteins there are, the faster the rate. Once all the proteins in the membrane are in use, the rate of diffusion can't go any faster. |
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Surface area : volume |
Generally, as a structure or organism becomes larger, it's surface area to volume ratio decreases. Small, single celled organisms can rely on simple diffusion to satisfy their nutrient and oxygen requirements due to their large SA:V ratio, but large, multicellular organisms can't as not enough substance could pass through their surface to supply their entire volume, so large organisms have complex respiratory and digestive systems which are adapted to maximise the rate of diffusion. |
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Polarity of molecules |
In non polar molecules, the electrons are evenly distributed around the atoms so the molecule has no overall charge, making it hydrophobic. In polar molecules, the electrons are unevenly distributed around the atoms so the molecule has a partial negative charge on one end and a partial positive charge on the other end, making it hydrophilic. |
Non polar molecules can dissolve in lipids and easily pass through the phospholipid bilayer, but polar molecules can't. |
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Channel proteins |
■ Channel proteins are hydrophilic passageways filled with water that act as pores in the membrane through which water soluble molecules and ions can diffuse from a high concentration to a low concentration ■ transport through channel proteins is anyways passive, so requires no additional energy ■ specific to certain molecules with the correct shape and charge ■ they don't bind with the molecule, but open to allow the molecule to pass through ■ movement through channel proteins is faster than through carrier proteins ■ many channel proteins contain a gate that controls their permeability to certain molecules |
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Carrier proteins |
■ Carrier proteins are small globular proteins that move across the membrane ■ they transport large molecules across membranes by binding to the molecules and changing shape, releasing the molecule on the opposite side of the membrane ■ transport through carrier proteins can be active or passive ■ specific to certain molecules with the correct shape and charge |
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Osmosis |
The passive net movement of water molecules from a region of high water potential to a region of low water potential down a water potential gradient and through a partially permeable membrane |
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Water potential |
Water potential measures the potential of water to diffuse - the concentration of free water molecules - the pressure exerted by water when it moves. The water potential of pure water is 0kPa. When solutes are added to water, they dissolve as water molecules cluster around them, so there are fewer water molecules that are free to diffuse to other areas, lowering the water potential of the solution. So as solute concentration increases, water potential decreases. Solute potential measures solute concentration - it's always negative and decreases as solute concentration increases. Water potential = solute potential + pressure potential |
If two solutions have the same water potential then they're isotonic. A solution with a higher water potential than another is hypotonic. A solution with a lower water potential than another is hypertonic. |
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Osmosis and animal cells |
■ when an animal cell is placed in an isotonic solution, there is no water potential gradient, so there's no net movement of water molecules and cell volume remains constant. ■ when an animal cell is placed in a hypotonic solution, water diffuses into the cell down a water potential gradient, so cell volume increases, causing the cell to burst (lysis). ■ when an animal cell is placed in a hypertonic solution, water diffuses out of the cell down a water potential gradient, so cell volume decreases, causing the cell to shrink and shrivel. |
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Osmosis and plant cells |
■ when a plant cell is placed in an isotonic solution, there is no water potential gradient, so there's no net movement of water molecules and cell volume remains constant. ■ when a plant cell is placed in a hypotonic solution, water diffuses into the cell down a water potential gradient, so cell volume increases, causing the cell contents to expand. The plant cell becomes turgid but doesn't burst because of the strong cell wall. ■ when a plant cell is placed in a hypertonic solution, water diffuses out of the cell down a water potential gradient, so cell volume decreases, causing the cytoplasm to shrink and become flaccid. If enough water leaves the cell, the cytoplasm pulls away from the cell wall and the cell is plasmolysed. |
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Active transport |
Active transport is the movement of substances from an area of low concentration to an area of high concentration, against a concentration gradient and through a partially permeable membrane using energy from ATP and carrier proteins to transport molecules across the cell membrane. ATP is produced by respiration and is broken down into ADP and Pi during a hydrolysis reaction, releasing the energy needed for active transport. |
Active transport stops if ATP production is prevented by metabolic poisons like cyanide or by lack of oxygen |
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Co transport |
Co transport is the movement of several substances alongside each other across a partially permeable membrane, using a co transporter. Co transporters are a type of carrier protein that bind several molecules at the same time, with one molecule moving down its concentration gradient and being used to move another molecule against its concentration gradient. |
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Absorption of glucose into epithelial cells lining the mammalian ileum |
Glucose concentration in ileum is too low for diffusion into cells, so is absorbed by co transport. ■ sodium ions are actively transported out of the epithelial cells into the blood by the sodium potassium pump, creating a concentration gradient from the high concentration in the ileum to the low concentration in the cell. ■ this causes sodium ions to diffuse down the concentration gradient from the ileum into the cell via the cotransporter, allowing glucose to be co transported into the cell. ■ the concentration of glucose inside the cell increases, causing it to diffuse out of the cell and into the blood down its concentration gradient through a carrier protein by facilitated diffusion. |
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Co transporters |
■ in the sodium potassium pump, 3 sodium ions from the cytoplasm bind to the co transporter, which changes shape and releases the sodium ions in the extracellular fluid. 2 potassium ions from the extracellular fluid bind to the co transporter which returns to its original shape and releases the potassium ions into the cytoplasm. This requires energy from ATP. ■ in the sodium glucose co transporter, glucose and sodium ions from the ileum must bind to the co transporter together, causing it to change shape and release the molecules into the cytoplasm of the epithelial cell. This doesn't require energy. |
● co transporters like the sodium potassium pump which transport substances in opposite directions are called antiports ● co transporters like the sodium glucose co transporter which transport substances in the same directions are called synports |
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Factors affecting the rate of active transport |
▪ the speed of individual carrier proteins - the faster they are, the faster the rate ▪ the number of carrier proteins present - the more proteins there are, the faster the rate ▪ the rate of respiration in the cell - this determines the availability of ATP which provides the energy for active transport. If respiration is inhibited, active transport can't take place |
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Cytosis |
Cytosis is the active transport of bulk material into a cell (endocytosis) or out of a cell (exocytosis) which involves the infolding or outfolding of sections of the cell surface membrane. • phagocytosis - solid substances are brought into the cell by the infolding of the cell surface membrane (invagination) to form a vacuole. • pinocytosis - liquid substances or large molecules like proteins are brought into the cell by the infolding of the cell surface membrane to form small vesicles. • receptor mediated endocytosis - receptor molecules on the cell surface membrane bind with specific substances like cholesterol from the extracellular environment, causing the infolding of the cell surface membrane to form vesicles. |
• exocytosis - vesicles and vacuoles bind with the cell surface membrane, causing it to outfold and release substances from the cell. |
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Membranes in neurons |
Function is to carry electrical impulses – this is dependent on the movement of ions in and out of cell. These membranes will therefore be rich in carrier proteins to move molecules. Schwann cells- make up myelin sheath – insulate the axon. These cells will therefore be made up of mostly lipids as it is a better insulator. |
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Membranes in mitochondria |
Mitochondira- cristae- large SA for more aerobic respiration Inner membrane is mostly made up of protein (76%)- this is because they need lots of electron carriers, H+ Channels and ATP synthase all for respiration. |
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Membranes in chloroplasts |
Chloroplast –thylakoids – large SA- where chlorplast are found-more chlorophylll=more photosynthesis |
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Making a serial dilution |
● line up 5 test tubes in a rack ● add 10cm3 of the initial solution to the first test tube and 5cm3 of distilled water to each of the other 4 test tubes ● use pipette to transfer 5cm3 of solution in first test tube to the second test tube and mix thoroughly to give a solution that's half as concentrated as the initial solution ● continue this process, transferring 5cm3 of solution from each test tube to the next to create solutions of decreasing concentration |
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Osmosis practical |
■ use cork borer to cut identically sized potato cores ~ 1cm diameter ■ separate cores into groups of 3 weigh each group ■ place a group into each sucrose solution and leave for an equal amount of time ■ remove cores, gently pat dry with paper towel and reweigh each group ■ calculate percentage change in mass for each group ■ plot calibration curve of % change in mass against sucrose concentration ■ find the concentration at which % change in mass is 0, signifying that water potential of sucrose solution = water potential of potato cell |
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Functions of the organelle membranes |
Intracellular membranes have a structure similar to plasma membranes, however the proportions of the molecular components vary considerably. For example, the outer membranes of chloroplasts contain very little carbohydrates. Functions include: ■ Gives a cell the ability to have different conditions in multiple areas of a cell, isolating different chemical reactions ■ Metabolic pathway – contain enzymes involved in specific pathways. Bring a role in bringing them closer together. ■ act as a reaction pathway - Chemical reactions will occur on them ■ act as an intracellular transport system - creating packages of secretions (VESICLES) |
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Permeability practical |
■ use scalpel to carefully cut 5 equal sized pieces of beetroot on cutting board and rinse to remove any pigment released during cutting. ■ use measuring cylinder to add 5cm3 of water to 5test tubes and place each piece of beetroot into a test tube ■ place each test tube in a water bath of different temperature for an equal amount of time ■ remove beetroot pieces from tubes, leaving coloured solution. ■ use colorimeter to measure absorbance of each solution, making sure to zero it using pure water, and plot graph of absorbance against temperature ● the higher the absorbance, the more pigment that is released, and so the greater the permeability of the membrane |
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