AQA A Level Biology 4/5/6 mark questions Paper 1

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane. (5) - ANSWER-1 (Simple / facilitated) diffusion from high to low concentration / down concentration gradient; 2 Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer; OR Large / polar / water-soluble molecules go through proteins; 3 Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential; 4 Active transport is movement from low to high concentration / against concentration gradient; 5 Active transport / facilitated diffusion involves proteins/carriers; 6 Active transport requires energy / ATP; 7 Ref. to Na+ / glucose co-transport; Describe and explain how the lungs are adapted to allow rapid exchange of oxygen between air in the alveoli and blood in the capillaries around them. (5) - ANSWER-1 Many alveoli / alveoli walls folded provide a large surface area; 2 Many capillaries provide a large surface area; 3 (So) fast diffusion; ________________________________________________ 4 Alveoli or capillary walls / epithelium / lining are thin / short distance between alveoli and blood; 5 Flattened / squamous epithelium; 6 (So) short diffusion distance / pathway; 7 (So) fast diffusion; ________________________________________________ 8 Ventilation / circulation; 9 Maintains a diffusion / concentration gradient; 10 (So) fast diffusion; Scientists believe that it may be possible to develop vaccines that make use of microfold cells. Explain how this sort of vaccine would lead to a person developing immunity to the pathogen (5) - ANSWER-1.Vaccine contains antigen/ dead pathogen 2.Microfold cells take up/bind and present/transport antigen (to immune system) 3.T-cells activate B-cells 4.B cells divide / undergo mitosis 5.B cells produce antibodies 6.Memory cells produced 7.More antibodies produced faster in secondary response Explain the role of B-lymphocytes and T-lymphocytes in the defence of the body against a virus infection. (6) - ANSWER-1.B lymphocytes produce antibodies/involved in humoral response; 2.T lymphocytes involved in cell mediated immunity; 3.Macrophages present antigens; 4.(specific) B lymphocytes recognise/bind to antigen; ase in numbers by mitosis; ce plasma cells (which make antibodies); odies bind to and clump/ agglutinate virus; y cells produced by 1st exposure/cloned on 2nd exposure; 9.T lymphocytes(helpers) produce okines/chemicals; aid B lymphocyte cloning; rages phagocytes to engulf clumped virus; r T cells kill virus infected cells; Explain how water enters a plant root from the soil and travels through to the endodermis. (5) - ANSWER-1. Water enters root hair cells; 2. By osmosis; 3. Because active uptake of mineral ions has crated a water potential gradient; 4. Water moves through cortex; 5. Down water potential gradient; 6. Through cell vacuoles and cytoplasms (symplastic pathway); 7. And through apoplastic pathway (cell walls); Root pressure is a force that is partly responsible for the movement of water through xylem in stems. Explain how the active transport of mineral ions into the xylem vessels in the roots results in water entering these vessels and then being moved up the xylem tissue. (5) - ANSWER-1. Entry of ions leads to a reduced water potential; 2. Water potential established between xylem and surrounding cells; 3. Plasma membranes of surroudning cells are partially permeable; 4. Water enters xylem by osmosis; 5. Volume of water in xylem increases; 6. Cannot move back due to gradient; 7. Pressure in xylem increases and forces water upwards. The presence of an air bubble in a xylem vessel in the stem blocks the movement through that vessel. Use the cohesion-tension theory to explain why. (4) - ANSWER-1. Evaporation from leaves (transpiration); 2. Water in xylem under tension (pulled up); 3. Because water molecules cohere (stick together) (because of hydrogen bonds); 4. So water travels in a single column; 5. A water bubble would break the column; In daylight, most of the water evaporates from the leaves but some is used by the plant. Describe the ways in which this water could be used by the plant. (6) - ANSWER-1. (water is used in) the light-dependent reactions of photosynthesis; 2. electrons from water enable ATP production / H+ are used to reduce NADP / produces O2; 3. (water can be used in) hydrolysis reactions within the plant; 4. to create turgor; 5. as a solvent for transport; 6. as a medium for chemical reactions; 7. component of cells / cytoplasm; Describe two features you would expect in the leaves of a tree adapted to a dry environment. Explain how each feature helps the tree's survival. (6) - ANSWER-Sunken stomata; water evaporation into pit creates local humidity; increased humidity reduces gradient for water evaporation; close arrangement of stomata; diffusion shells of individual stomata overlap; interferes with water diffusion and slows evaporation; restriction of stomata to lower side of leaf; rate of air movement below leaf less/ heating effect of sun less; gradient for water evaporation reduced/ water molecules have less kinetic energy; thick cuticle/wax/suberin (on upper surface); (wax/suberin )waterproof; water unable to diffuse onto surface to evaporate, presence of trichomes/ hairs; surface traps water close to leaf surface; increased humidity reduces gradient for water evaporation; reduced leaves/spines/small surface area to volume; less surface area for evaporation; more distance across leaf for water to diffuse; rolled leaves; stomata enclosed in localised humidity; increased humidity reduces gradient for water evaporation; Xylem transports water through a plant. Describe and explain how the cells of xylem are adapted for this function. (5) - ANSWER-Thick cell w