The last page ended with a problem: a low surface area to volume ratio limits how much a cell can exchange. Some cells exist entirely to move substances across their membranes, and they have evolved striking ways of pushing that ratio back up.
📚 What you need to know
Cells that move a lot of material across the membrane need to maximise surface area to volume ratio.
The larger the surface area compared with the volume, the faster the rate of substance movement.
A red blood cell (erythrocyte) delivers oxygen from the lungs to respiring cells.
It is flattened and biconcave, which maximises surface area and minimises volume, so oxygen diffuses in and out quickly.
Red blood cells have no nucleus, leaving more room for haemoglobin.
Proximal convoluted tubule cells in the kidney reabsorb vital substances such as glucose and mineral ions from the glomerular filtrate.
They have microvilli on the apical membrane and invaginations (infoldings) on the basal membrane, both increasing surface area.
They also contain very many mitochondria, because reabsorption uses active transport and therefore ATP.
The principle behind both examples
For a cell whose job is exchange, the membrane is the bottleneck. Every glucose molecule reabsorbed and every oxygen molecule loaded has to cross it, so the more membrane there is per unit of cytoplasm, the faster the cell can work.
There are two ways to raise the ratio, and biology uses both:
Reduce the volume — flatten the cell, or throw out anything that takes up space and is not needed.
Increase the surface area — fold the membrane into projections or infoldings so far more of it is packed into the same space.
Red blood cells
A red blood cell, or erythrocyte, has one job: carry oxygen from the lungs to respiring tissues. Everything about it follows from that.
Compare it with a sphere of the same volume: the biconcave disc has more surface and no point inside it is far from the membrane.
Feature
What it allows
Flattened, biconcave shape
Maximises surface area and minimises volume, so the SA:V ratio is high and oxygen diffuses in and out faster
Thin in the middle
Shortens the diffusion distance from the membrane to the centre of the cell
No nucleus
Leaves more room for haemoglobin, so each cell carries more oxygen
No mitochondria
The cell cannot respire aerobically, so it does not consume any of the oxygen it is carrying
Small overall size
Can squeeze through the narrowest capillaries
Flexible membrane
Deforms to pass through vessels narrower than the cell itself
The “no mitochondria” point is the one that separates strong answers from average ones. It is not just about making room — a red blood cell that respired aerobically would be using up the very oxygen it is supposed to be delivering. That is a genuinely elegant piece of adaptation.
Proximal convoluted tubule cells
The proximal convoluted tubules are tiny tubes in the outer region of the kidney. Their job is to reabsorb vital substances — glucose, amino acids and mineral ions — out of the glomerular filtrate and back into the blood before they are lost in urine.
That means moving an enormous quantity of material across two membranes, and it explains everything about the cell’s structure.
Microvilli face the filtrate and take substances in; invaginations face the capillary and pass them out. Both are the same trick applied at opposite ends.
Feature
Where it is
What it allows
Microvilli
Apical membrane, facing the tubule lumen
A very large surface area for absorbing substances out of the filtrate
Invaginations
Basal membrane, facing the blood capillary
A very large surface area for passing those substances into the blood
Many mitochondria
Throughout the cytoplasm
Supply the ATP needed for active transport, since much reabsorption is against a gradient
Many carrier proteins
In both membranes
Carry out the specific transport of glucose, amino acids and ions
The join to earlier work. Sodium-dependent glucose cotransport, which you met under active transport, happens in exactly these cells. The microvilli are where the cotransporter proteins sit, and the mitochondria are there to power the sodium-potassium pumps that set up the gradient.
🧠
Apical and basal
Apical shares a root with “apex” — the top, facing the tube. Basal shares a root with “base” — the bottom, facing the blood. Microvilli stick out at the apex; invaginations fold in at the base.
Worked examples
WE 1
Explaining a shape
Explain how the shape of a red blood cell is adapted to its function. (3 marks)
Point 1: the shape
It is flattened and biconcave rather than spherical.
Point 2: the effect
This maximises surface area while minimising volume, giving a high surface area to volume ratio, and shortens the diffusion distance to the centre.
Point 3: the function
Oxygen can therefore diffuse in quickly at the lungs and out quickly at the respiring tissues.
biconcave means more surface, less volume, shorter distancethe question says “shape”, so the no-nucleus point is a bonus rather than the answer
WE 2
Reasoning from an unusual feature
Suggest why a mature red blood cell contains no mitochondria. (2 marks)
Reason 1: space
Removing them leaves more room for haemoglobin, so each cell can carry more oxygen.
Reason 2: the clever one
Without mitochondria the cell cannot respire aerobically, so it does not use up the oxygen it is transporting.
more room for haemoglobin, and none of the cargo is consumedthe second reason is what examiners are hoping to see — give both
WE 3
Linking three features together
Explain how proximal convoluted tubule cells are adapted for the reabsorption of glucose. (4 marks)
Point 1: the apical surfaceMicrovilli on the apical membrane greatly increase the surface area in contact with the filtrate.
Point 2: the basal surfaceInvaginations on the basal membrane increase the surface area facing the blood capillary.
Point 3: the proteins
A large surface area holds many carrier proteins, so more glucose can be transported at once.
Point 4: the energy
Many mitochondria supply the ATP needed, because glucose is reabsorbed against its concentration gradient.
two folded surfaces, many carriers, and the ATP to run them4 marks, four features — and every one must be followed by what it achieves
💡 Exam tips
Say biconcave, not “dented” or “dimpled”. It is the technical term the mark scheme uses.
Always name the ratio in full: surface area to volume ratio.
Distinguish microvilli (projections sticking out) from invaginations (folds going in), and say which membrane each is on.
Connect many mitochondria to ATP and then to active transport. Three steps, as always.
Give a number if you know one: a red blood cell is about 7.5 µm across.
For any “how is this cell adapted?” question, pair each feature with its function in the same sentence.
⚠ Common mistakes
Saying red blood cells have “no organelles”. They lack a nucleus and mitochondria, but they still have a membrane and cytoplasm full of haemoglobin.
Describing a red blood cell as round. Seen from above it is round; the point is that it is a disc, not a sphere.
Putting microvilli on the wrong membrane. They are apical, facing the filtrate.
Explaining mitochondria as “for energy”. Say ATP, and say what the ATP is used for.
Saying the folds increase volume. They increase surface area while barely changing volume — that is the whole point.
Confusing reabsorption with filtration. Filtration happens at the glomerulus; reabsorption happens in the tubule.
Up next: Examples of Specialised Cells — four more, chosen because each solves a completely different problem: gas exchange, contraction, and getting two cells to fuse into one.
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