AQA A level Biology 2022(Paper 1 RATED A+)

Explain why plasma membranes are said to have a fluid mosaic structure. - ANSWER 1. Fluid because the phospholipids are constantly moving. 2. Mosaic because the proteins are scattered throughout the membrane-like tiles in a mosaic. What effect does cholesterol have on the cell membrane? - ANSWER 1.Cholesterol molecules fit between the phospholipids and bind to the hydrophobic tails of the phospholipids. 2.Causes them to pack together more closely, restricting the movement of other molecules in the membrane. 3.Cholesterol makes the membrane less fluid and more rigid. 4.Cholesterol helps to maintain the shape of animal cells, which is important because animal cells don't have cell walls (especially important for cells which are not supported by other cells e.g. red blood cells). State 3 functions of glycolipids in the cell membrane. - ANSWER 1. Act as recognition sites. 2. Maintain the stability of the membrane. 3. Help cells attach to one another and so form tissues. State 3 functions of glycoproteins in the cell membrane. - ANSWER 1.Act as recognition sites.2.Help cells to attach to one another so form tissues.3.Allow cells to recognise one another e.g. lymphocytes can recognise an organisms's own cells. Describe the functions of membranes within cells. - ANSWER 1.Control the entry and exit of materials in membrane-bound organelles such as mitochondria and chloroplasts.2.Separate organelles from cytoplasm so that specific metabolic reactions can take place.3.Provide an internal transport system e.g. endoplasmic reticulum.4.Isolate enzymes which could damage the cell e.g. lysosomes.5.Provide surfaces on which reactions can occur e.g. protein synthesis on ribosomes on rough ER. Suggest reasons why most molecules do not diffuse freely through the cell membrane. - ANSWER 1.They're not lipid soluble.2.Too large to pass through protein channels.3.Have the same charge as the charge on the protein channels, so they are repelled even if they are small enough to pass through the channel.1 State two factors which affect the permeability of cell membranes - ANSWER .1.Temperature2.Solvent concentration How can substances move across a cell surface membrane? - ANSWER 1.Simple diffusion (passive)2.Facilitated diffusion (passive)3.Osmosis (passive)4.Active transport (active)5.Co-transport (active) Describe how substances can move across a cell surface membrane. - ANSWER 1.Small/non-polar/lipid-soluble substances can cross by simple diffusion down a concentration gradient.2.Large/polar molecules - pass through channel proteins/carrier proteins, by facilitated diffusion, down a concentration gradient.3.Water - moves by osmosis from an area of high water potential to an area of lower water potential.4.Active transport - moves substances against a concentration gradient, involves protein carriers and requires ATP.5.Glucose/amino acids can be co-transported across a cell membrane alongside sodium ions.* DON'T EVER SAY THAT A MOLECULE MOVES 'ACROSS' OR 'ALONG' A CONCENTRATION GRADIENT - MAKE SURE YOU SAY 'UP' OR 'DOWN' OR 'AGAINST' Define diffusion. - ANSWER The net movement of molecules or ions from an area of higher concentration to an area of lower concentration until evenly distributed.Define osmosis.The movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane. Define active transport. - ANSWER The movement of molecules from an area of lower concentration to an area of higher concentration using ATP and carrier proteins. State factors which affect the rate of diffusion. - ANSWER 1.Concentration gradient2.Area over which diffusion takes place3.Thickness of exchange surface4.Temperature5.Size of molecules (smaller the molecule, the faster the rate of diffusion).2 What is meant by the term simple diffusion - ANSWER .The diffusion of molecules directly through a cell membrane.(most molecules don't easily pass across the cell surface membrane - those that do are small, non-polar molecules). What is Ficks Law? - ANSWER The rate of diffusion is proportional to : (Surface Area x Concentration) Thickness1.The larger the SA, the faster the rate of diffusion.2.The higher the concentration gradient, the faster the rate of diffusion.3.The thinner the exchange surface, the faster the rate of diffusion. What is meant by the term passive process. - ANSWER No energy is needed for it to happen. Why are oxygen and carbon dioxide able to diffuse across cell membranes? - ANSWER 1.They're small, so they can pass through the spaces between the phospholipids.2.They're non-polar, so they are lipid-soluble and so can dissolve in the phospholipid bilayer. Describe and explain the effect of an increase in temperature on the rate of diffusion. - ANSWER 1.Rate of diffusion increase as temperature se the molecules have more kinetic energy and so move faster. Describe the process of active transport. - ANSWER 1.Molecules in an area of lower concentration bind to carrier proteins.2.ATP breaks down to ADP, Pi and energy.3.The Pi binds to the carrier protein, causing it to change shape. The energy is used in the transport of the molecules.4.This moves the molecule across the membrane.5.Carrier protein releases the molecule on the opposite side (in an area of higher concentration).6.Carrier protein releases the attached Pi and returns to its original shape. Which factors affect the rate of active transport? - ANSWER 1.The speed of individual carrier proteins.2.The number of carrier proteins present.3.The rate of respiration in the cell and the availability of ATP.* if respiration is inhibited, active transport can't take place. Describe the process by which energy is released by the cell for active transport. - ANSWER 1.Energy is released by the hydrolysis of ATP ADP and Pi.3 Contrast active transport with facilitated diffusion.* because the word 'contrast' is used - ANSWER you need to give both sides of the difference in your answer.,1.Active transport uses ATP, whereas facilitated diffusion is a passive process.2.Substances are moved against a concentration gradient in active transport, whereas facilitated diffusion moves substances down a concentration gradient.3.Facilitated diffusion involves channel or carrier proteins, whereas active transport only involves carrier proteins.* carrier proteins can transport substances against their concentration gradient, whereas channel proteins cannot. Explain why facilitated diffusion is a passive process. - ANSWER 1.No ATP is used in the process.2.The only energy used is the kinetic energy of the molecules. Discuss the role of protein channels in facilitated diffusion. - ANSWER 1.Protein channels form pores/channels across the membrane, allowing water-soluble ions to diffuse down their concentration gradient.2.Protein channels are selective and only allow the passage of specific ions. Discuss the role of carrier proteins in facilitated diffusion. - ANSWER 1.Carrier proteins move large molecules across membranes, down their concentration gradient.2.When a molecule specific to the carrier protein binds (e.g. glucose), it causes the carrier protein to change shape and release the molecule on the opposite site of the membrane.3.No ATP is required, the molecules move from an area of higher concentration to an area of lower concentration, using the kinetic energy of the molecules themselves. Discuss two factors which affect the rate of facilitated diffusion. - ANSWER 1.The concentration gradient: the higher the concentration gradient, the faster the rate of diffusion, up to a point which is determined by how many protein channels and carriers are present in the membrane.2.Once all the proteins in a membrane are in use, the rate of facilitated diffusion can't increase any further. State the 3 factors which affect the rate of osmosis. - ANSWER 1.Water potential gradient.2.Thickness of exchange surface.3.Surface area of exchange surface.4 What is meant by a selectively permeable membrane? - ANSWER A membrane that is permeable to water molecules (and a few other small molecules), but not to larger molecules. What is the water potential of pure water under standard conditions of pressure and temperature? - ANSWER Zero If two solutions are isotonic, what does this mean?They have the same water potential .Explain why the water potential of distilled water is higher than the water potential of cytoplasm. - ANSWER 1.Distilled water contains no solute and so its water potential is zero.2.Presence of solutes in cytoplasm, reduce the water potential of the cytoplasm. Describe the property of the cell surface membrane which allows osmosis to take place. - ANSWER 1.Partially permeable so cell surface membranes allow water molecules to pass through, but not solute molecules. What happens if you surround an animal cell with pure water? - ANSWER It swells and bursts. What happens if you surround a plant cell with pure water? - ANSWER 1.It swells, but doesn't burst. 2.The cell wall prevents it from bursting because it is made of cellulose, which is a strong material.3.The cell is turgid. What happens if an animal cell is surrounded by a concentrated salt/sugar solution? - ANSWER 1.It shrinks because water leaves the cell by osmosis.2.Water moves from area of higher water potential in the cell to area of lower water potential in the solution .What happens if a plant cell is surrounded by a concentrated salt/sugar solution? - ANSWER 1.Water leaves the cell by the cell wall prevents the cell from shrinking by keeping the cell rigid. Describe two ways in which osmosis and diffusion are similar. - ANSWER 1.Both are passive and so don't require ATP.2.Both involve movement down a concentration gradient. Give two differences between osmosis and facilitated diffusion. - ANSWER 1.Osmosis only involves the movement of water.2.Facilitated diffusion involves carrier proteins.5 Why can glucose molecules not pass easily through the phospholipid bilayer? - ANSWER Only lipid-soluble substances diffuse across the phospholipid bilayer easily, Water-soluble substances like glucose, diffuse very slowly. Suggest two changes to the cell surface membrane that would increase the rate at which glucose could be transported in the cell. - ANSWER 1.More proteins with pores which span the phospholipid bilayer.2.Increase the surface area of the cell surface membrane with microvilli .State two properties that a drug should have if it is to enter a cell rapidly. - ANSWER 1.Lipid-soluble2.Small What is a co-transporter? - ANSWER A type of carrier protein that can bind two molecules at a time (the concentration of one of the molecules is used to move the other molecule AGAINST its own concentration gradient). Describe how glucose is absorbed by co-transport in the mammalian ileum (final part of the small intestine). - ANSWER 1.Sodium ions are actively transported out of the ileum epithelial cells by the sodium-potassium pump.2.Creates a concentration gradient, causing sodium ions to diffuse into the epithelial cells down their concentration gradient via sodium-glucose co-transporter proteins.3.The co-transporter carries glucose into the cell with the sodium.4.The concentration of glucose inside the cell increases.5.Glucose diffuses out the cell into the blood, down its concentration gradient through protein channels, via facilitated diffusion. Why is it necessary for glucose to be absorbed from the lumen of the ileum by co-transport? - ANSWER The concentration of glucose in the ileum is too low for glucose to diffuse out through the epithelial cell surface membrane into the blood. Suggest three ways in which the rate of movement across a membranes can be increased. - ANSWER 1.Increasing the surface area.2.Increasing the concentration gradient across the membrane.3.Increasing the density of protein channels and/or carrier proteins. •Describe how you could use a graph of your results to find the sodium chloride solution with the same water potential as the potato tissue. - ANSWER •Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis.•Find the concentration where the curve crosses the x-axis (i.e. where the percentage change is zero).•*if your results are given as a ratio of mass at the start:mass at the end instead of percentage change, find the sodium chloride concentration where the ratio is 1 i.e. there was no change in mass during the experiment. •What is the advantage of giving your results as a ratio/percentage change. - ANSWER Allows a comparison to be made when all the cylinders/discs had a different starting mass. •Explain why taking additional readings at each concentration of sodium chloride would improve the reliability of the results. - ANSWER It allows for anomalies to be identified and enables a mean to be calculated. •Explain why additional readings could improve the graph. - ANSWER The more readings used to plot a graph, the more accurate the curve of best fit will be and therefore the the point where the line crosses the x-axis will be more accurate. Also, it allows you to plot standard deviation values and error bars to show how spread the results are. •State how evaporation would affect the water potential of the solution in the test tubes. - ANSWER It would lower the water potential. •Explain why it is important to use potato cylinders taken from potatoes of the same variety. - ANSWER To ensure the cells have the same water potential / so there are no genetic differences.8 •The potato skin is removed from the cylinders - ANSWER suggest one reason why the potato skin could have affected osmosis. ,Potato skin is impermeable to water / the skin is a different tissue so the cells of the skin will have a different water potential. •Explain why the student blotted dry the outside of each cylinder before reweighing. - ANSWER water will affect the mass of the potato cylinder and the student only wants to measure the water that was taken up (or lost) by the potato cylinder due to osmosis. •Explain why the percentage increase in mass of a small cube would be greater than the percentage increase in mass for a large cube. - ANSWER Due to the larger surface area:volume ratio of the small cubes for osmosis. •Explain why the potato cylinder in distilled water gained mass. - ANSWER Water moved into the potato cells by osmosis, from a less negative water potential outside the cells to a more negative water potential inside the cells. Explain what is meant by the term eukaryotic cell. - ANSWER •Cell that has a membrane-bound nucleus and chromosomes.•Has other membrane-bound organelles, such as mitochondria and endoplasmic reticulum. Give examples of eukaryotic cells. - ANSWER Animal cells / Plant cells / Fungi / Algae Give an example of a prokar