Gas exchange surfaces and cell membranes

Efficient gas exchange depends on three structural properties:

• A large surface area to volume ratio
• A thin exchange surface (short diffusion distance)
• A steep concentration difference (concentration gradient) maintained across the surface

These three properties are summarised quantitatively in Fick's Law, which states that the rate of diffusion is proportional to (surface area × concentration difference) divided by the thickness of the exchange surface:

rate of diffusion ∝ (surface area × concentration difference) ÷ membrane thickness

The mammalian lung shows all three adaptations clearly:

• Millions of alveoli provide a vast total surface area of around 70 m².
• The squamous epithelial walls of the alveoli and the capillary endothelium are each only one cell thick, giving very short diffusion distances.
• Continuous ventilation and blood flow maintain steep concentration gradients between the alveolar air and the blood.

Together these adaptations allow rapid gas exchange to meet the body's metabolic demand.

Cell membranes themselves are partially permeable barriers built from a phospholipid bilayer with proteins, cholesterol, and carbohydrates embedded throughout.

The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes this arrangement and is supported by evidence such as freeze-fracture electron microscopy and fluorescent labelling of membrane proteins.

The model also demonstrates how science evolves: older models such as the Davson–Danielli model, which depicted proteins as continuous layers coating both sides of the bilayer, were replaced when new techniques revealed data inconsistent with that structure.

Membrane permeability can be tested experimentally using beetroot, where increased temperature or alcohol (ethanol) concentration disrupts the bilayer and causes the red betalain pigment to leak out, which can be measured quantitatively using a colorimeter.

Exam questions typically expect:

• Explicit links between structural features and Fick's Law
• Accurate descriptions of the fluid mosaic model components
• Clear identification of the variables in the practical investigation