Paper 1 AQA A level Biology 2023 with complete solution

Explain why plasma membranes are said to have a fluid mosaic structure. 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? 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. 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. 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. 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. 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 .1.Temperature2.Solvent concentration How can substances move across a cell surface membrane? 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. 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. 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. 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. 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 .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? 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. No energy is needed for it to happen. Why are oxygen and carbon dioxide able to diffuse across cell membranes? 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. 1.Rate of diffusion increase as temperature se the molecules have more kinetic energy and so move faster. Describe the process of active transport. 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? 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. 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 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. 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. 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. 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.