Gas exchange surfaces and ventilation in mammals, gas exchange in plants and insects, the transport of oxygen and carbon dioxide in blood, and the structure of the circulatory and transport systems.

What this dot point is asking

CCEA wants you to describe gas exchange surfaces and ventilation in mammals, gas exchange in plants and insects, how oxygen and carbon dioxide are transported in blood, and the structure of the circulatory and plant transport systems.

Gas exchange surfaces and ventilation

These features all increase the rate predicted by Fick's law: $\text{rate of diffusion} \propto \dfrac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$. Ventilation in mammals uses the diaphragm and intercostal muscles to change thoracic volume and pressure, drawing air in (inspiration) and forcing it out (expiration), which keeps the concentration gradient at the alveoli steep. Insects are small, so the tracheal system delivers oxygen directly to cells without needing blood to carry it.

Transport of gases in blood

The sigmoid curve arises from cooperative binding: binding the first oxygen changes the shape of haemoglobin, making the next oxygens easier to bind, which is why the curve steepens in the middle. Most carbon dioxide is carried as hydrogencarbonate ions ($\text{HCO}_3^-$) formed in red blood cells when carbon dioxide reacts with water (catalysed by carbonic anhydrase), with smaller amounts dissolved in plasma or bound to haemoglobin as carbaminohaemoglobin. Fetal haemoglobin has a higher oxygen affinity (its curve is shifted left), so it takes oxygen from the mother across the placenta.

Circulatory and plant transport systems

The mammalian circulation is a closed double system: arteries carry blood at high pressure away from the heart and have thick muscular elastic walls, veins return blood at low pressure and have valves to prevent backflow, and thin one-cell-thick capillaries allow exchange with tissues. In plants, xylem carries water and minerals upward in the transpiration stream by cohesion-tension (water columns held together by hydrogen bonding and pulled up as water evaporates from leaves), and phloem translocates sugars from source to sink by mass flow.

Examples in context

Example 1. Altitude training and haemoglobin. Athletes who train at high altitude experience a lower partial pressure of oxygen in the air, so their blood is less saturated. The kidneys release more erythropoietin, stimulating production of extra red blood cells and haemoglobin. On returning to sea level, the athlete carries more oxygen per unit of blood, improving endurance. This illustrates how oxygen transport by haemoglobin can be tuned to the environment, and why the oxygen dissociation curve is central to performance physiology.

Example 2. Emphysema and reduced surface area. In emphysema, often caused by smoking, the walls between alveoli break down, merging many small alveoli into fewer large air spaces. The total surface area for gas exchange falls sharply, so (by Fick's law) the rate of oxygen uptake drops and the person becomes breathless on exertion. This shows directly why a large surface area matters: losing it cripples gas exchange even when ventilation is normal.

Try this

Q1. State three features of an efficient gas exchange surface. [3 marks]

• Cue. Large surface area, thin (short diffusion distance), moist, and a maintained steep concentration gradient.

Q2. Explain how the Bohr effect helps deliver oxygen to respiring tissues. [2 marks]

• Cue. A higher carbon dioxide level shifts the dissociation curve right, so haemoglobin releases more oxygen where it is needed.

Q3. Explain how water moves up the xylem from the roots to the leaves. [3 marks]

• Cue. Water evaporates from leaves (transpiration), creating tension; cohesion between water molecules by hydrogen bonding holds the column together so it is pulled up as one continuous stream.