Gas exchange, membranes and transport — Edexcel International A Level Biology
Gas exchange surfaces and Fick's law, the mammalian lung, membrane structure, osmosis and the ways substances cross membranes.
Revision notes
8 short notes, in the order of the specification. Each one in short:
A gas exchange surface lets gases diffuse quickly because it has a large surface area to volume ratio, is very thin and keeps a steep concentration gradient. Fick's law states that the rate of diffusion is proportional to surface area multiplied by concentration difference, divided by the thickness of the exchange surface.
Mammalian lungs are adapted for rapid gaseous exchange by millions of alveoli that give a huge surface area, walls only one cell thick, and a dense capillary network. Ventilation and blood flow keep oxygen and carbon dioxide concentration gradients steep, so diffusion is fast.
A cell membrane is a phospholipid bilayer, about 7 nm thick, with hydrophilic heads facing outwards and hydrophobic tails facing inwards. Proteins, cholesterol, glycoproteins and glycolipids are embedded in it. The membrane is partially permeable and controls which substances enter and leave the cell.
The fluid mosaic model is a scientific model of the membrane: a flexible phospholipid bilayer in which protein molecules are scattered like tiles in a mosaic and can move sideways. Like any model it is an interpretation of data, and it replaced earlier ideas when new evidence was found.
Beetroot cells contain a red pigment (betalain) in their vacuoles. Damage to the cell membranes lets the pigment leak into the surrounding water. By changing temperature or alcohol concentration and measuring how much pigment is released, you can compare the effects on membrane permeability.
Osmosis and water potential
PracticalOsmosis is the net movement of free water molecules through a partially permeable membrane, down a water potential gradient, from a region of higher water potential to one of lower (more negative) water potential. Water potential is measured in kilopascals, and pure water has a water potential of zero.
Passive transport moves substances across a membrane down a concentration gradient without using ATP. Diffusion occurs directly through the phospholipid bilayer; facilitated diffusion uses channel proteins or carrier proteins for ions and polar molecules, which cannot cross the hydrophobic core of the bilayer.
Active transport moves substances across a membrane against their concentration gradient, using energy from ATP and carrier proteins. Endocytosis and exocytosis move large particles and bulk materials into and out of cells in vesicles formed from, or fusing with, the cell-surface membrane. Both also need ATP.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
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23 cards: flip them, mark what you knew, and practise the rest.
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The whole of membranes, proteins, dna and gene expression on one page, so you can see where this subtopic fits.
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Quick check questions
State the three properties of a gas exchange surface.
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A large surface area to volume ratio, a thin surface (short diffusion distance) and a large concentration difference.
What happens to the rate of diffusion if the thickness of the exchange surface doubles?
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It halves, because the rate is inversely proportional to the thickness (Fick's law).
Why can oxygen cross the phospholipid bilayer, but sodium ions cannot?
Show answer
Oxygen is small and non-polar, so it dissolves in the hydrophobic centre. Ions are charged and are repelled by it.
Define osmosis.
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The net movement of free water molecules through a partially permeable membrane, down a water potential gradient.
What is the role of ATP in active transport?
Show answer
It is hydrolysed to ADP and Pᵢ, releasing the energy that changes the shape of the carrier protein.
Frequently asked questions
What are the adaptations of the alveoli for gas exchange?
Alveoli give a large surface area to volume ratio through their huge number. Their walls and the capillary walls are one cell thick, so diffusion distance is short. A dense capillary network and ventilation maintain steep concentration gradients, and elastic fibres help air to be breathed out.
What is Fick's law of diffusion?
Fick's law says the rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the exchange surface. A large, thin surface with a steep gradient therefore gives rapid diffusion, which is why gas exchange surfaces have all three features.
What is the fluid mosaic model of the cell membrane?
The fluid mosaic model describes a membrane as a phospholipid bilayer in which proteins are scattered like a mosaic. It is fluid because the phospholipids and many proteins can move sideways. It is a model built from experimental evidence, such as freeze-fracture images and cell-fusion experiments.
How does osmosis differ from diffusion?
Osmosis is a special case of diffusion for water. It is the net movement of free water molecules through a partially permeable membrane down a water potential gradient. Diffusion is the movement of any particles down a concentration gradient and does not need a membrane.
What is the difference between facilitated diffusion and active transport?
Facilitated diffusion is passive: substances move down a concentration gradient through channel or carrier proteins, with no ATP. Active transport moves substances against a gradient using carrier proteins and energy from the hydrolysis of ATP. Both are specific to the molecules they transport.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026