Diffusion only ever takes you downhill. If a cell needs to move something uphill, or move a great deal of something at once, it has to pay — and the currency is ATP.
📚 What you need to know
Active transport is the movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.
It goes against (up) a concentration gradient and requires carrier proteins, often called pumps.
The energy comes from ATP, which is hydrolysed so that a phosphate can be transferred to the pump, changing its shape.
The sodium-potassium pump moves three Na+ out and two K+ in per ATP, creating an electrochemical gradient across the membrane of a nerve cell.
Cotransport is the coupled movement of two substances by one carrier protein; indirect active transport uses the gradient of one to move the other.
The sodium-dependent glucose cotransporter in the ileum and kidney absorbs glucose alongside sodium ions.
Bulk transport moves large quantities at once inside vesicles: endocytosis in, exocytosis out. Both need energy and both need a fluid membrane.
Phagocytosis is the bulk intake of solids; pinocytosis is the bulk intake of liquids.
Active transport
Learn this definition
the movement of molecules and ions across a cell membrane, from a region of lower concentration to a region of higher concentration, using energy from respiration
Compare that with facilitated diffusion and the two differences jump out. Both use carrier proteins. Only active transport goes up the gradient, and only active transport uses ATP. Everything else is the same.
The mechanism is worth understanding rather than memorising:
The molecule or ion binds to a specific site on the carrier protein.
ATP is hydrolysed and a phosphate group is transferred to the protein. This is called phosphorylation.
Gaining that phosphate makes the protein change shape, which carries the binding site — and its cargo — to the other side of the membrane.
The cargo is released, the phosphate is released, and the protein returns to its original shape ready to go again.
Notice that ATP does not push anything through a hole. It changes the shape of a protein. That is worth saying explicitly in an answer, because it shows you understand the mechanism rather than just the label.
The sodium-potassium pump
This is the named example you need, and it is the reason a nerve cell can fire at all.
Three positive charges out for two in. That imbalance is the electrochemical gradient a nerve cell later uses to fire.
🧩 The pumping cycle, step by step
Three sodium ions bind to the pump from the inside of the axon.
ATP attaches and transfers a phosphate to the pump. This phosphorylation makes it change shape, so it now opens to the outside.
The three sodium ions are released out of the axon.
Two potassium ions from outside enter and bind to their own sites.
The phosphate is released, which changes the shape of the pump again.
The two potassium ions are released inside the axon, and the pump is back where it started.
Because three positive ions leave for every two that enter, the inside of the cell ends up negatively charged compared with the outside. That difference in both charge and concentration is called an electrochemical gradient. When a nerve cell is stimulated, sodium channels open and sodium ions rush back in down that gradient, reversing the charge across the membrane and generating a nerve impulse.
An exchange transporter. The sodium-potassium pump moves two different substances in opposite directions at the same time, which is what “exchange transporter” means. Keep that phrase for it; a pump that only moves one thing is not an exchange transporter.
Cotransport and indirect active transport
Cotransport is the coupled movement of two substances across a membrane by the same carrier protein. Coupled means they happen at the same time and cannot happen separately.
The clever part is indirect active transport. Instead of spending ATP to move a substance uphill, the cell lets a different substance run downhill through the same protein, and uses that as the driving force. ATP is still involved, but only earlier on, to set up the gradient in the first place.
Sodium-dependent glucose cotransport
The best-known example sits in the epithelial cells lining the mammalian ileum. It is how glucose is absorbed from your food into your blood, and the sequence is easier if you start at what looks like the wrong end.
Sodium-potassium pumps on the far side of the epithelial cell actively transport sodium ions out of the cell and into the blood. This lowers the sodium concentration inside the cell.
Sodium ions from the gut now move down their concentration gradient into the cell, through the cotransporter protein.
As they do, glucose is dragged in with them through the same protein — even though glucose is moving against its own concentration gradient.
Glucose then moves down its gradient out of the cell and into the blood, through a separate glucose channel protein.
Only step one uses ATP directly. The transport of glucose itself does not, which is exactly why it is called indirect active transport. The same process reabsorbs glucose in the kidney, which is why healthy urine contains no glucose at all.
Examiners know the order feels backwards, and they say so. Starting with the sodium-potassium pump is the only way the rest makes sense: without that pump lowering the sodium inside the cell, there is no gradient, so no sodium flows in, so no glucose comes with it.
Bulk transport
Diffusion, osmosis and active transport all move individual molecules or ions. Sometimes a cell needs to move something far too large for any protein — a whole bacterium, for example — or a great quantity of material at once. For that it uses vesicles.
In both cases a piece of membrane is being rebuilt, which is only possible because the bilayer is fluid.
Process
Direction
What happens
Example
Phagocytosis
Into the cell
Bulk intake of solid material, forming a phagocytic vacuole
A white blood cell engulfing a bacterium
Pinocytosis
Into the cell
Bulk intake of liquid in small vesicles
Cells taking up droplets of extracellular fluid
Exocytosis
Out of the cell
Secretory vesicles fuse with the plasma membrane and release their contents
Pancreatic cells secreting digestive enzymes
Both directions are active processes: forming a vesicle, moving it and fusing it all cost ATP. Both also depend on membrane fluidity, because the bilayer has to bend, break and reseal. That is the direct link back to the previous page.
🧠
Endo and exo
Endo- means inside, as in endoskeleton. Exo- means outside, as in exoskeleton. Endocytosis brings material in; exocytosis sends it out. And phago- means eat (solids), pino- means drink (liquids).
Worked examples
WE 1
Comparing two protein-based processes
Both facilitated diffusion and active transport use carrier proteins. Explain two ways in which they differ. (4 marks)
Difference 1: direction
Facilitated diffusion moves substances down a concentration gradient, whereas active transport moves them against one.
Difference 2: energy
Facilitated diffusion is passive and needs no ATP; active transport requires ATP, which is hydrolysed to change the shape of the carrier protein.
direction of movement, and whether ATP is used4 marks for two differences means two sentences each — state the difference, then explain it
WE 2
Interpreting an inhibitor experiment
Cells are treated with a respiratory inhibitor that stops ATP production. Uptake of substance X falls to almost zero, while uptake of substance Y is unaffected. Explain these results. (3 marks)
Point 1: substance X
X must be taken up by active transport, which needs ATP from respiration to change the shape of the carrier protein.
Point 2: why it stops
With no ATP being made, the pumps cannot work, so uptake of X stops.
Point 3: substance Y
Y must be taken up by a passive process such as diffusion or facilitated diffusion, which uses the concentration gradient rather than ATP.
X is actively transported; Y crosses passivelyrespiratory inhibitors are the classic way an exam distinguishes active from passive
WE 3
Explaining indirect active transport
Glucose is absorbed into an epithelial cell of the ileum against its concentration gradient, yet no ATP is used by the cotransporter protein itself. Explain how this is possible. (4 marks)
Point 1: set up the gradient
Sodium-potassium pumps actively transport sodium ions out of the cell into the blood, using ATP. This lowers the sodium concentration inside the cell.
Point 2: the downhill movement
Sodium ions from the gut then move down their concentration gradient into the cell through the cotransporter.
Point 3: the coupling
Glucose is carried in by the same protein at the same time, even though this is against its own gradient.
Point 4: name it
The energy therefore comes indirectly from the ATP spent earlier by the sodium-potassium pump.
the sodium gradient, created using ATP, drags the glucose inalways start this answer with the sodium-potassium pump, even though it is not step one in the gut
💡 Exam tips
Say against or up the concentration gradient. Do not just say “actively transported” and hope.
Name ATP and say what it does: transfers a phosphate that changes the shape of the carrier protein.
Memorise the ratio: 3 sodium out, 2 potassium in, 1 ATP. It is worth an easy mark.
Use the term electrochemical gradient for the sodium-potassium pump, not just “concentration gradient”.
For cotransport, always mention that ATP is used to establish the gradient, not to move the glucose.
Link bulk transport to membrane fluidity. Examiners like to see topics joined up.
⚠ Common mistakes
Saying active transport uses channel proteins. It uses carriers only. A channel is just a hole and cannot push anything uphill.
Getting the pump ratio backwards. Three sodium out, two potassium in.
Writing that ATP “gives the ion energy”. The energy changes the shape of the protein.
Calling cotransport passive. Glucose moves uphill, so the process as a whole is active — just indirectly.
Assuming exocytosis is passive because material is leaving. Vesicle formation and fusion both cost ATP.
Mixing up phagocytosis and pinocytosis. Solids are eaten, liquids are drunk.
Up next: Cell Adhesion — the last piece. How membranes let cells hold on to each other, which is the whole reason you are a body rather than a puddle.
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