IB Biology HLGas Exchange SystemsPaper 1 & 2~11 min read
Gas Exchange in Organisms
Every cell in your body needs oxygen and needs to get rid of carbon dioxide. A single-celled amoeba can let both gases wander in and out on their own. You cannot. The reason has nothing to do with being complicated – it is a piece of simple maths about size.
📚 What you need to know
Cell respiration releases energy as ATP. Aerobic respiration uses oxygen and makes carbon dioxide.
Gas exchange is the movement of those two gases between an organism and its environment, and it happens by diffusion.
The rate of diffusion depends on surface area, the concentration gradient and the diffusion distance.
Small organisms have a large surface area to volume ratio and a short diffusion distance, so diffusion alone is enough.
As an organism gets bigger, the SA:V ratio falls and the diffusion distance grows, so it needs a specialised gas exchange surface.
A good exchange surface is permeable, thin, moist and large in area.
The concentration gradient is kept steep by a dense capillary network, continuous blood flow and ventilation.
Respiration and gas exchange are not the same thing
Students lose easy marks here every year, so get it straight before anything else.
Term
What it actually is
Where it happens
Cell respiration
A chemical process that oxidises glucose and releases energy as ATP
Inside every living cell
Gas exchange
The physical diffusion of oxygen and carbon dioxide across a surface
At a respiratory surface, such as an alveolus or a leaf air space
Respiration is the reason the gases need to move. Gas exchange is the moving. One is chemistry inside a cell, the other is physics across a membrane.
If a question says “explain gas exchange” and your answer talks about glycolysis and ATP, you have answered a different question. Write about diffusion, gradients and surfaces instead.
What controls how fast diffusion happens
Three things decide the rate. Learn them as one sentence you can write in any answer.
Rate of diffusion depends on
rate ∝ (surface area × concentration difference) ÷ diffusion distance
Surface area – a bigger surface means more places for molecules to cross at the same time, so the rate goes up.
Concentration gradient – the steeper the difference between the two sides, the faster the net movement.
Diffusion distance – the further a molecule has to travel, the slower it arrives. This one is on the bottom of the fraction, so a shorter distance means a faster rate.
Every single adaptation in this whole topic – alveoli, gills, spongy mesophyll, capillary beds – is just one of those three factors being pushed in the helpful direction.
Why size is the problem
An amoeba is a good size for diffusion. Oxygen only has to cross about 0.01 mm of cytoplasm to reach the middle of the cell, and the whole outer membrane is available as a surface. Diffusion supplies it easily.
Now make the organism bigger. Volume grows faster than surface area does, because volume is a cube of the length and area is only a square of it. So the surface has to serve more and more inside for every square centimetre it has.
Triple the length and the ratio drops from 6:1 to 2:1. The cube in the middle has three times less surface for every unit of inside to supply.
So a large, active animal runs into two problems at once:
Its outer surface is too small for the amount of living tissue inside it.
Its middle is too far from the outside for diffusion to reach in any useful time.
There is a third problem people forget. The outside of a large animal is built for protection – skin, scales, fur, a shell. Those are thick, dry and waterproof, which is exactly the opposite of what a gas exchange surface needs to be. The body cannot use its outside as a lung even if the area were enough.
Active makes it worse. A mouse and a lizard of the same mass do not need the same amount of oxygen. Warm, active animals respire much faster, so they need more oxygen per second and need to remove carbon dioxide faster too. High activity is what forces the evolution of a specialised exchange organ, not size alone.
What a specialised gas exchange surface looks like
Once diffusion through the body surface stops working, the organism has to grow a surface that is good at it. Every one of these features maps straight back to the three factors above.
The two gases cross the same wall in opposite directions at the same time, each moving down its own gradient.
Feature
Why it helps
Factor it improves
Permeable
Gases can actually pass through the surface
Makes diffusion possible at all
Thin tissue layer
Often one flat cell thick, so the trip is tiny
Short diffusion distance
Moist
Gases dissolve in the film of water before crossing
Diffusion across a membrane needs solution
Large surface area
Many molecules cross at once
Surface area
Keeping the gradient steep
A surface is only useful while there is a difference in concentration across it. If oxygen built up in the blood next to the alveolus, diffusion would slow down and then stop. Three things stop that happening.
A dense network of capillaries. This wraps the exchange surface in blood vessels, giving a huge area of contact.
Continuous blood flow. Blood carries oxygen away as fast as it arrives and brings fresh carbon dioxide in, so the difference across the wall never disappears. Blood is a good transport medium for both gases.
Ventilation. Air in lungs, or water over gills, is constantly replaced, so the air side of the gradient stays high in oxygen and low in carbon dioxide.
🧠
Remember the surface with PT-ML
Permeable, Thin, Moist, Large. Then add the two movers that keep the gradient going: blood flow and ventilation.
Worked examples
WE 1
Surface area to volume ratio
A cube-shaped model organism has sides of 4 cm. Calculate its surface area to volume ratio and state what this shows about diffusion. (3 marks)
Step 1: surface area
A cube has 6 faces, so SA = 6 × 4 × 4 = 96 cm²Step 2: volumeV = 4 × 4 × 4 = 64 cm³Step 3: divide and simplify
96 ÷ 64 = 1.5, so the ratio is 1.5 : 1
SA : V = 1.5 : 1this is much lower than the 6 : 1 of a 1 cm cube, so diffusion across the surface could not supply the inside fast enough
WE 2
Why an amoeba needs no lungs
Explain why a unicellular organism such as an amoeba does not need a specialised gas exchange surface. (3 marks)
Point 1: the ratio
It is very small, so it has a large surface area compared with its volume.
Point 2: the distance
No part of the cytoplasm is more than about 0.01 mm from the membrane, so the diffusion distance is very short.
Point 3: the conclusion
The rate of diffusion across the cell membrane is therefore fast enough to supply all the oxygen the cell needs and remove all its carbon dioxide.
Large SA:V + short distance = diffusion alone is enoughalways finish with the “therefore” sentence, that is usually the third mark
WE 3
Maintaining the concentration gradient
Outline how a concentration gradient for oxygen is maintained across a gas exchange surface. (3 marks)
Point 1: blood supply
A dense network of capillaries sits right against the surface, giving a large area of contact.
Point 2: blood flow
Blood flows continuously, so oxygen is carried away as soon as it crosses and the blood side stays low in oxygen.
Point 3: ventilation
Fresh air is drawn in, so the air side stays high in oxygen; the difference across the surface never runs out.
Blood removes it on one side, ventilation replaces it on the otherif the question says “explain”, add that a steeper gradient gives a faster rate of diffusion
💡 Exam tips
Learn the three factors as a set: surface area, concentration gradient, diffusion distance. Most “explain” marks come from naming one of them.
When you describe an adaptation, always add the because: “many alveoli, which gives a large surface area, so more oxygen diffuses at once”.
Use the phrase surface area to volume ratio, not just “surface area”, when comparing sizes.
Say gases move down a concentration gradient, not “from high to low” on its own.
Ratio answers must be simplified and written in the form x : 1.
Remember the fourth reason large animals need lungs: their outer surface is protective, thick and dry.
⚠ Common mistakes
Mixing up respiration and gas exchange. Respiration is chemistry in the cell; gas exchange is diffusion across a surface.
Saying big organisms have a small surface area. They have a huge surface area – it is the ratio to their volume that is small.
Forgetting the distance factor. A short diffusion distance gives a faster rate, because distance is on the bottom of the relationship.
Writing “oxygen is sucked in”. Nothing sucks. Gases diffuse passively down a gradient.
Saying diffusion needs energy. Diffusion is passive and uses no ATP.
Leaving out ventilation and blood flow in gradient questions and only writing about the surface itself.
Up next: Adaptations of Mammalian Lungs – how the human lung turns those four surface rules into an organ with a surface area bigger than a squash court.
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