BY2 WJEC Essay Questions '09 onwards

Describe the way in which a normal heart beat is initiated and controlled [10].

• Heart muscle is myogenic.
• It can contract without any nerve stimulation.
• The stimulus to contract originates in the sinoatrial node (SAN).
• Which controls the rate of beating.
• It is situated in the wall of the right atrium.
• Electrical impulse from the SAN causes the two atria to contract.
• Thin layer of connective tissue prevents the stimulus spreading to the ventricles.
• At the bottom of the wall seperating the two atria is the atrioventricular node AVN.
• This delays the impulse (about 0.1 sec) before passing it onto the ventricles.
• The impulse is sent to the tip of the ventricles.
• Along the Bundle of His.
• And is conveyed upwards along the branching Purkinje fibres.
• Causing a wave of ventricular contraction starting from the lowermost part of the ventricle.
• The SAN may be stimulated by variuos factors to change it's pacing. 
• E.g. hormones (adrenalin), exercise, body temperature.

Explain how the reproductive strategies of animals have changed as a result of the evolution of life on land. Comment on the advantages of the mammalian strategies. [10]

• Gametes are shed in water.
• fertilisation is external.
• heavy waste of gametes which fail to fuse.
• embryo is entirely dependant on yolk supply for it's development.
• many hazards - predation etc. = large waste of embryos.
• finding suitable conditions for development is a completely random process.
• terrestrial mammals have internal fertilisation.
• placing of gametes/ female gametes are not shed.
• greater certainty of fertilisation. number of eggs has been much reduced.
• internally developing embryo not dependant soley on yolk/ importance of placenta.
• high level of protection from external hazards during development.
• in general more time/ energy/ resources devoted to fewer offspring.
• contact between parent and offspring/ parental care.
• e.g. provision of dens/ burrowing/ herding/ protection from predation etc.

Describe how the leaf structure of a flowering plant is adapted for gaseous exchange and photosynthesis [10].

• Leaves are flat and have a large surface area to abrosb maximum light/ CO2/ gas exchange.
• Leaves can grow towards the light to expose maximum area.
• Leaves are thin to allow light to penetrate their lower layers.
• Cuticle and epidermis are transparent to allow light to penetrate mesophyll.
• Palisade cells packed with chloroplasts to increase photosynthetic capacity/light absorption.
• Chloroplasts can move/ circulate inside cells to gain best positions for absorbing light. 
• Spongy mesophyll cells are moist to allow for easier gas exchange.
• Xylem to supply water and/ or phloem to remove sugar.
• Leaves thin to reduce distance for diffusion.
• Air spaces in spongy mesophyll allow circulation of gasses/ cut down distance for diffusion into cells/ gradient for uptake.
• Stomatal pores permit entry and exit of gases/ sub stomatal/ intercellular spaces in spongy mesophyll also supply carbon dioxide and remove oxygen.
• Waxy cuticle on upper leaf surface reduces water loss by evaporation.
• Stomatal pores in lower epidermis reduce water loss by evaporation.
• Guard cells can close stomatal pores to reduce water loss.

Plants have two distinct transport systems. Describe how the structure of each is adapted to it's function [10].

• Xylem transports water and mineral salts from the roots to the leaves.
• Phloem transports the soluble products/ sucrose/ sugars/organic molecules/ amino acids/ source to sink of photosynthesis from leaves to other parts of the plant.
• Xylem is made of vessels and tracheids.
• Xylem vessels form continuous tubes.
• Their secondary wall is lignified to waterproof and  stop them collapsing under suction/ tension.
• Mature xylem vessels are dead and cytosplasm has gone so there is nothing to impede flow.
• Tracheids have tapered ends that fit together and pits to allow transfer of water between cells.
• Phloem is made of sieve tubes and companion cells.
• Sieve tube cells have perforated sieve/ end plates that allow solutes to pass through.
• Suger solution flows in both directions.
• Companion cells have organelles,responsible for keeping sieve tube cells alive (Provide ATP).
• Any reference to fibres or paranchyma in either.
• Cohesion- tension/ capillarity/ adhesion in xylem.
• Correct use of terms in correct context: reference to mass flow/ cytoplasmic streaming/ protein filaments in phloem.

Describe the structure of the circulatory system in a mammal [10].

• Double circulation.
• Right atrium connected to ventricle through tricuspid valve.
• Left atrium connected to ventricle through bicusid/mitrial valve.
• Vena cava brings deoxygenated blood from the body to the right atrium.
• Pulmonary artery carries blood from right ventricle to lungs.
• Pulmonary vein brings blood from lungs to left atrium.
• Aorta carries blood from left ventricle to general body circulation.
• Backflow is prevented by semi-lunar valves.
• Heart is a pump driven by muscle which is thickest in left ventricle.
• The coronary artery supplies blood to the heart muscle.
• Aorta then branches into arteries and smaller arterioles supplying all body tissues.
• Arteries- small lumen, thick layer of elastic tissue, endothelial lining.
• Veins- wide lumen, little elastic tissue/ thin muscle layer, endothelial lining.
• Veins have (pocket) valves along their length.
• Capillaries- endothelium only/ one cell thick, connection between arteries and veins.

Describe the structure of the lead of a flowering plant, explaining how this structure is adapted to the functions of the leaf. [10]

• Leaf is flat and thin/ large surface area to volume ratio.
• Large surface area for light trapping.
• Thin to give short diffusion pathway into leaf.
• Transparent layer of epidermis.
• Covered by cuticle.
• Prevents water loss.
• Palisade mesophyll- vertically arranged packed cells or contain abundant chloroplasts.
• Arrangements gives maximum light absorption.
• Spongy mesophyll with large intercellular spaces.
• Allows room for movement of respiratory gases and water vapour.
• Diffusion of gases in and out of leaf through stomata.
• Stomatal pore surrounded by two guard cells.
• Change in water potential/ pressure bends guard cells to open stomata.
• This change is related to photosynthetic activity so gas exchange only occurs when it is required.
• Closure at other times prevents water loss.

Describe the theory of evolution, as put forward by Charles Darwin, suggesting evidence which supports his views [10].

• Darwin recognised that species did change/ put forward a theory as to how they changed.
• Mutation qualified.
• Overproduction.
• Numbers remain constant/ high mortality rate/ struggle for survival.
• Variation e.g. beak size or shape/ rats / moths.
• Competiton.
• Individuals with a beneficial variation survive/ survival of the fittest.
• Pass on their beneficial characteristic.
• Repeats generation after generation.
• Details of beak adaptation, seed, insects, fruit etc.
• Natural selection.
• Adaptive radition.
• Morphologically similar to mainland form/ common ancestor.
• Similarities of proteins/enzymes./Similarities of DNA/genes.
• Fossil evidence/ Pentadactyl.

Compare the dentition of a carnivore and a grazing herbivore. Describe how the gut region of a ruminant is adapted to its diet [10]. NOTE: cannot in this case use the reverse argument unless stated.

• Small flat top incisors/ horny pad.
• Canines absent/ small indistinguishable front incisors.
• Teeth continuously growing.
• Carnivores, canines large (backward pointing, killing and holding prey).
• Herbivore cheek teeth larger surface area for grinding/ interlock in WM arrangement.
• Carnassial.
• Sharp cutting edges on teeth. 
• Diastema, manipulation of food.
• First three chambers in 'stomach' modified oesophagus. Large volume, longer gut.
• Symbiotic. Ref. anaerobic conditions.
• Ref. reverse peristalsis. chewing cud.
• Cellulose digestion by bacteria.
• Jaw articulation.
• Ref. urea in saliva to provide nitrogen 

Discuss the adaptations shown by different terrestrial animals for gas exchange. [10]

• Large surface area.
• Moist surface area.
• Short diffusion pathway.
• Circulatory system with blood pigments e.g. haemoglobin.
• Internal lungs minimise loss of water/ heat.
• Ventilation mechanism.
• Ensures fresh oxygen is brought to gas exchange surface area.
• (frogs) Inactive amphibian uses it's moist skin for gas exchange.
• Active amphibian uses lungs.
• Tadpole stage uses gills.
• (reptiles and birds) More efficient lungs than amphibians.
• Air sacs act as bellows.
• (insects) Have a branched chitin lined system/ presence of tracheae.
• With openings called spiracles.
• Gases exchange directly with tissues - no blood pigment present.

Using a suitably drawn and labelled graph, explain what is meant by the oxygen dissociation curve for normal adault mammalian haemoglobin and, using additional lines of the graph, explain how this differs according to the environment in which the organism lives [10].

• (DIAGRAM) With correct axes PPO2, saturation of haemoglobin with oxygen (%).
• Correct numbers.
• Correct shaped curve for adult haemoglobin, labelled.
• Correct position of curve for fetal haemoglobin, labelled.
• Correct position of curve for llama/ lugworm, labelled. (NOTE: lines not to go over 100%)
• (TEXT) Sigmoid shape (S shaped) more efficient.
• Easier O2 loading in lungs so fully saturated at relatively low partial pressure.
• Significance of thie for living at altitude.
• More O2 delivered to tissues.
• Reduced affinity for O2 at lower partial pressures.
• Bohr effect reduces haemoglobin affinity for O2/ more O2 is delivered to respiring tissues.
• Correct biological explanation for this - acidity reduces affinity Hb for O2.
• Ref. Myoglobin or position on graph.
• Correct explanation for llama/ lugworm curve position i.e. ref affinity higher.

Describe the uptake of water in plants from the soil into the xylem [7]. Explain the role of ions in this process [3].

• Water absorbed by the root hair cells.
• Water can be absorbed/ moves through cell walls.
• Moves across cortex via apoplast route.
• Moves across plasma membrane by osmosis.
• Water moves from the cytoplasm of one cell into the next via plasmodesmata.
• Called the symplast route.
• Water can also move through cytoplasm and vacuoles via vacular route.
• Casparian *****/ band in walls of endodermal cells.
• Made of suberin/ waterproof.
• Stops apoplast route/ water forced into symplast route.
• Ions absorbed into root hair cells by active transport.
• Ions lowering water potential in root hair cells.
• At the endodermis ions absorbed into cytoplasm by active transport/ uptake.
• Ions travel into xylem. Lowers water potential in xylem.
• lower water potential in root hair cells of xylem increasing osmotic gradient between soil and cell contents.

Explain why large. multi cellular organisms have evolved special surfaces for gaseous exchange [3]. Describe and explain how terrestrial mammals are adapted for gaseous exchange in air [7].

• Metabolic need proportional to volume/ larger organisms need more oxygen.
• Larger organisms external surface insufficient for gas exchange.
• Diffusion of respiratory gases proportional to surface area.
• Surface area: volume ratio is too small.
• diffusion distances too large
• Not enough O2 can diffuse to meet metabolic needs.
• Large number of alveoli to increase surface area.
• Internal lungs.To reduce water loss.
• Gaseous exchange takes place in the alveoli.
• Thin walls- reduce diffusion distance.
• Layer of moisture - for gasses to dissolve in.
• Blood supply maintains concentration gradient between alveolar air and blood.
• haemoglobin - transport of oxygen.
• Ventilation mechanism description. To replace stale air with fresh air/ continuous exchange of gases.

Distinguish between the two types of reproduction used by organisms and state the advantages and disadvantages of each [7].Explain how the reproductive strategies of flowering plants have lead to their success [3].

• Reference to sexual and asexual.
• Asexual produces offspring that are genetically identical/ clones.
• By mitosis.
• Allows rapid colonisation.
• If conditions change e.g. temp changeor disease.
• All individuals may die/ none may have resistance.
• Sexual reproduction produces offspring that are genetically different.
• Gametes produced by mitosis.
• Genetic variability allows a species to adapt to environmental change.
• Slower/ needs a partner (usually).
• Relationship with animals/ insects for pollination. Relationship with animals/ insects for seed dispersal.
• Pollen can survive dessication/ without water.
• Seed with stored food enables the embryo plant to grow until leaves form/ are exposed to sunlight.
• Seed has a resistant coat to withstand advers conditions.

Explain what is meant by transpiration. Describe the factors affecting transpiration[7]. Describe how you could use a potometer to investigate one of these factors [3].

• Transpiration is the loss/ evaporation of water vapour from inside the leaves and stem of a plant.
• Through stomata.
• Down a water potential gradient.
• High temperature increases rate of transpiration.
• Correct explanation of efffect of temp / increased kinetic energy.
• Plus correct explanation - decreased diffusion gradient.
• High light intensity increases rate.
• Causes stomatal opening.
• Set up under water with a continuous column of water, must be air tight,
• any description of how to change one factor may be apparent on diagram.
• Volume of water.
• To give a close approximation of transpiration rate.