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, so diffusion across the body surface alone remains sufficient for them
• CThe SA:V ratio is independent of body size, because surface area and volume increase in direct proportion to each other
• DA small SA:V ratio increases the rate of diffusion per unit volume, so large organisms exchange gases easily

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 dissolves in the haemolymph and is circulated to all the respiring tissues by the dorsal tubular heart
• BAir enters through spiracles and travels along tracheae and tracheoles directly to respiring tissues✓
• CGas exchange occurs across the moist gill lamellae, which are ventilated by a counter-current flow of water
• DAlveoli lined with a thin moist epithelium provide the main surface for the diffusion of oxygen into the blood

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, so oxygen levels equalise quickly
• BIt removes the need for a large surface area on the gill filaments and lamellae
• CIt enables oxygen to be actively pumped into the blood using ATP from respiration
• 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 rises above ventricular pressure during atrial systole
• BThe ventricles relax and refill with blood during ventricular diastole
• CPressure in the aorta rises above the pressure in the relaxed 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, so it stays largely unsaturated even in arterial blood
• COxygen binds independently to each haem group, with no interaction between the four subunits
• DHaemoglobin cannot become fully saturated even at the high partial pressures found in the lungs

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, so oxygen is released at a constant rate as pO₂ falls
• DThe curve is unaffected, because carbon dioxide is carried only by the 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 molecules across the root endodermis by ATP-powered carrier proteins located in the Casparian strip
• BRoot pressure generated in the root cortex alone, pushing water up the whole height of a tall tree
• CEvaporation of water from the leaf mesophyll (transpiration) creating tension transmitted down continuous water columns✓
• DTranslocation of sucrose along the phloem sieve tubes, which drags the water column up with it from the roots to the leaves

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 pores of the sieve plates, down the concentration gradient from the leaf
• BBy active loading involving co-transport, which lowers the water potential of the sieve tube so water enters by osmosis✓
• CBy evaporation from the leaf surface, which draws sucrose into the sieve tubes along with the water of the transpiration stream
• DBy passive diffusion of sucrose down a hydrostatic pressure gradient running back from the sink tissues to the source leaves

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 muscular walls for strength, a small total surface area, and a poor capillary blood supply
• BA waterproof waxy cuticle, many stomata that can open and close, and large air spaces in the spongy mesophyll 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 stacked gill lamellae, maintaining a diffusion gradient along their whole length

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 are forced open and blood is ejected from the ventricles into the aorta and the pulmonary artery
• CThe ventricles contract and the atrioventricular valves are pushed shut, producing the first of the heart sounds
• DBoth the atria and the ventricles relax and the heart fills passively with blood

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.

Why do small organisms not need a specialised gas exchange system?

• ATheir surface area to volume ratio is small
• BTheir metabolic rate per gram is extremely low
• CTheir surface area to volume ratio is large✓
• DTheir cells respire anaerobically most of the time

Answer: A small body has a large surface area relative to its volume, so diffusion across the surface alone supplies every cell quickly enough. As size increases the ratio falls and diffusion distances grow, which is why larger organisms need exchange surfaces and a transport system.

What makes the alveoli efficient for gas exchange?

• ALarge surface area, thick walls and a good blood supply
• BSmall surface area, thin walls and a limited blood supply
• CSmall surface area, thick walls and a limited blood supply
• DLarge surface area, thin walls and a good blood supply✓

Answer: Millions of alveoli give an enormous surface area, their walls are one flattened cell thick, and the dense capillary network keeps the concentration gradient steep by constantly removing oxygen. All three features raise the rate given by Fick's law.

What does Fick's law state about the rate of diffusion?

• AIt rises with area and distance, and falls with gradient
• BIt falls with area and gradient, and rises with distance
• CIt rises with area and gradient, and falls with distance✓
• DIt rises with distance and gradient, and falls with area

Answer: Rate ∝ (surface area × concentration difference) / diffusion distance. Every adaptation of an exchange surface increases one of the top two terms or reduces the bottom one.

How does counter-current flow in fish gills increase oxygen uptake?

• ABlood and water flow in opposite directions along the lamellae✓
• BBlood and water flow in the same direction along the lamellae
• CBlood flows much faster than the water crossing the lamellae
• DWater is held stationary against the lamellae while blood flows

Answer: Because the two flows oppose, blood always meets water with a slightly higher oxygen concentration along the whole length of the lamella, so a diffusion gradient is maintained throughout. Parallel flow would let the two equilibrate about halfway and uptake would fall sharply.

How is water mainly moved up the xylem?

• ATranspiration pull, aided by active pumping in each vessel
• BRoot pressure alone, generated by mineral ions in the root
• CTranspiration pull, aided by cohesion between water molecules✓
• DTranslocation from the phloem into the adjacent xylem

Answer: Evaporation from the leaves lowers water potential and pulls the column upward; hydrogen bonding holds the column together (cohesion) and to the vessel walls (adhesion). Root pressure contributes only a little, and translocation is a separate phloem process.

What does the mass flow hypothesis explain?

• AMovement of sucrose from source to sink in the phloem✓
• BMovement of sucrose from sink to source in the phloem
• CMovement of water from leaves to roots in the xylem
• DMovement of mineral ions from soil into the root hairs

Answer: Sucrose is actively loaded at the source, lowering water potential so water follows from the xylem and raises hydrostatic pressure. That pressure drives sap to the sink, where sucrose is unloaded. Xylem transport is a separate, passive mechanism.

Why do insects have spiracles that can close?

• ATo reduce heat loss when gas exchange demand is low
• BTo increase the rate of oxygen entry into the tracheae
• CTo reduce water loss when gas exchange demand is low✓
• DTo allow haemoglobin to load oxygen more efficiently

Answer: Spiracles open into the tracheal system, so any opening also loses water vapour. Closing them between bouts of activity conserves water, which matters greatly for a small terrestrial animal. Insect blood does not carry oxygen.

Why is a double circulatory system an advantage to mammals?

• ABlood pressure is restored before reaching the body tissues✓
• BBlood pressure is reduced before reaching the body tissues
• CBlood passes through the heart only once per full circuit
• DBlood carries both oxygen and carbon dioxide in one vessel

Answer: Blood loses pressure crossing the lungs, so returning it to the heart lets it be pumped again at high pressure to the body. That keeps delivery rapid enough for a high metabolic rate — a single circulation cannot do both jobs at once.

What causes the Bohr shift?

• AHigher carbon dioxide raises haemoglobin's oxygen affinity
• BHigher carbon dioxide lowers haemoglobin's oxygen affinity✓
• CLower temperature lowers haemoglobin's oxygen affinity
• DLower carbon dioxide lowers haemoglobin's oxygen affinity

Answer: Respiring tissue produces carbon dioxide, which lowers pH and changes haemoglobin's shape so it releases oxygen more readily — the dissociation curve shifts right. That delivers most oxygen precisely where demand is greatest.