Exchange & Transport Systems

As an organism increases in size, its surface-area-to-volume (SA:V) ratio changes. Which statement is correct?

• ALarger organisms have a smaller SA:V ratio, so they are more likely to need specialised exchange surfaces✓
• BLarger organisms have a larger SA:V ratio, making diffusion across the body surface sufficient
• CSA:V ratio is independent of body size
• DA small SA:V ratio increases the rate of diffusion per unit volume

Answer: As size increases, volume rises faster than surface area, so the SA:V ratio decreases. Diffusion across the outer surface alone then cannot meet metabolic demands, so larger organisms evolve specialised, highly folded exchange and transport systems.

Insects exchange respiratory gases without a blood pigment for oxygen transport. Which feature of the insect gas-exchange system is correctly described?

• AOxygen is carried dissolved in the haemolymph to all tissues
• BAir enters through spiracles and travels along tracheae and tracheoles directly to respiring tissues✓
• CGas exchange occurs across moist gill lamellae ventilated by water
• DAlveoli provide the main surface for diffusion of oxygen

Answer: Insects take in air through spiracles in the body wall. The air passes along the tracheae and finer tracheoles, which deliver oxygen directly to the respiring tissues, where gases diffuse (and tracheal fluid can move to aid this). Insects do not use a respiratory pigment to transport O₂.

Fish gills use a counter-current flow system. Why does this make gas exchange more efficient than a parallel (concurrent) flow?

• AIt allows water to flow in the same direction as blood, equalising oxygen levels quickly
• BIt removes the need for a large surface area on the gill lamellae
• CIt enables oxygen to be actively pumped into the blood using ATP
• DIt maintains a diffusion gradient for oxygen across the entire length of the gill lamellae✓

Answer: In counter-current flow, water and blood move in opposite directions, so blood always meets water with a higher oxygen concentration along the whole lamella. This sustains a diffusion gradient across the entire length, allowing more oxygen to be absorbed than parallel flow (which equilibrates partway).

During the cardiac cycle, the semilunar valves (aortic and pulmonary) open when:

• AAtrial pressure exceeds ventricular pressure
• BThe ventricles relax during diastole
• CPressure in the aorta exceeds pressure in the ventricles
• DVentricular pressure rises above the pressure in the aorta and pulmonary artery✓

Answer: During ventricular systole, contraction raises ventricular pressure above that in the aorta/pulmonary artery, forcing the semilunar valves open so blood is ejected. When ventricular pressure falls below arterial pressure, the valves close, preventing backflow.

The oxygen dissociation curve for adult human haemoglobin is S-shaped (sigmoid). What does the steep middle section of the curve indicate?

• AA small fall in partial pressure of oxygen causes a large release of oxygen, aiding unloading at respiring tissues✓
• BHaemoglobin has a very low affinity for oxygen at all partial pressures
• COxygen binds independently to each haem group with no interaction
• DHaemoglobin cannot become fully saturated even at high oxygen partial pressures

Answer: The sigmoid shape arises from cooperative binding: binding of the first O₂ makes further binding easier. On the steep part, a small drop in pO₂ (as at respiring tissues) causes a large fall in saturation, so a lot of oxygen is released where it is needed.

The Bohr effect describes how an increase in carbon dioxide concentration affects the oxygen dissociation curve. What is the effect, and why is it advantageous?

• AThe curve shifts left, increasing oxygen affinity so more oxygen is loaded at the tissues
• BThe curve shifts right, reducing oxygen affinity so more oxygen is unloaded at actively respiring tissues✓
• CThe curve becomes a straight line, allowing constant oxygen release
• DThe curve is unaffected because CO₂ binds only to plasma proteins

Answer: Higher CO₂ (and the resulting lower pH) lowers haemoglobin's affinity for oxygen, shifting the dissociation curve to the right. At respiring tissues (high CO₂), this causes haemoglobin to release more oxygen exactly where demand is greatest.

Water moves up the xylem from roots to leaves in the transpiration stream. According to the cohesion-tension theory, what generates the tension that pulls water upward?

• AActive transport of water by the root endodermis using ATP
• BRoot pressure alone pushing water up the entire height of a tall tree
• CEvaporation of water from the leaf mesophyll (transpiration) creating tension transmitted down continuous water columns✓
• DTranslocation of sucrose dragging water up the phloem

Answer: Evaporation of water from mesophyll cell surfaces (transpiration) lowers water potential in the leaf and creates tension. Because water molecules are cohesive (hydrogen bonding), they form continuous columns, so tension is transmitted down the xylem, pulling water up from the roots.

In the mass flow hypothesis of phloem translocation, how is sucrose loaded into the phloem at a source such as a leaf?

• ABy simple diffusion of sucrose directly through the sieve plate
• BBy active loading involving co-transport, which lowers the water potential of the sieve tube so water enters by osmosis✓
• CBy evaporation drawing sucrose into the sieve tubes
• DBy passive diffusion of sucrose down a pressure gradient from sink to source

Answer: At the source, sucrose is actively loaded into companion cells and sieve tubes (using H⁺ co-transport). This lowers the water potential in the sieve tube, so water enters from the xylem by osmosis, raising hydrostatic pressure and driving mass flow toward sinks.

Which set of adaptations correctly describes the human alveoli as an efficient gas-exchange surface?

• AThick walls for strength, small total surface area, and a poor blood supply
• BA waterproof waxy cuticle and many stomata for ventilation
• CThin (one-cell-thick) walls, a large total surface area, and a rich capillary supply maintaining a steep diffusion gradient✓
• DA counter-current flow of air and blood across gill lamellae

Answer: Alveoli provide a very large total surface area, walls only one flattened cell thick (short diffusion distance), and a dense capillary network. Continuous blood flow and ventilation maintain steep concentration gradients for O₂ and CO₂, maximising diffusion.

During atrial systole in the cardiac cycle, which event occurs?

• AThe atria contract, forcing remaining blood through the open atrioventricular valves into the ventricles✓
• BThe semilunar valves open and blood enters the aorta
• CThe ventricles contract and the atrioventricular valves close
• DBoth atria and ventricles relax and the heart fills passively only

Answer: In atrial systole the atria contract while the atrioventricular (bicuspid and tricuspid) valves are open, pushing the final portion of blood into the relaxed ventricles. Ventricular contraction and AV-valve closure follow in ventricular systole.