Biology › Membranes and transport across cells › Active transport, co-transport and moving things in bulk

Active transport, co-transport and moving things in bulk

A gradient can be climbed, but only by paying for it. Carrier proteins that hydrolyse ATP, a pump that shifts three sodium ions out for every two potassium in, the trick the ileum uses to absorb the last of your glucose, and the membrane folding itself around cargo too large for any protein.

Before this Diffusion and facilitated diffusion · ATP as the cell's immediate energy source

What you should be able to do

• Explain how a carrier protein moves a substance against its concentration gradient, and where the energy comes from.
• Describe the sodium-potassium pump, quoting what moves in each direction for each ATP hydrolysed.
• Work through co-transport of glucose in the ileum as an ordered sequence, and say which step uses ATP.
• Distinguish endocytosis from exocytosis and give a named example of each.
• Compare all the transport mechanisms by direction, proteins used and energy required.

Paying to go the wrong way

Diffusion runs downhill and stops when the gradient is gone. A cell that only had diffusion available could never concentrate anything: root hair cells could not take up nitrate ions from soil where nitrate is scarcer than in the cell, and your ileum could not absorb the last of a meal's glucose once blood glucose had risen above gut glucose. Active transport is what makes those possible, and it is defined by direction and by cost.

Active transport

The movement of a substance across a membrane against its concentration gradient, using a carrier protein and energy from the hydrolysis of ATP.

Carrier protein

An intrinsic protein with a binding site specific to one substance, which changes shape to move it across the membrane.

The mechanism is worth writing out as a sequence, because questions ask for it in that form. The substance binds to a specific site on the carrier protein, on the side it is starting from. ATP binds to the protein and is hydrolysed to ADP and inorganic phosphate. The phosphate group attaches to the protein, and that attachment changes the protein's tertiary structure, so the binding site now faces the other side of the membrane and holds the substance more weakly. The substance is released. The phosphate detaches, the protein returns to its original shape, and it is ready again.

Three features fall out of that description and all three are examined. The carrier is specific, so a cell controls what it accumulates by controlling which carriers it makes. The rate plateaus when every carrier is occupied, exactly as facilitated diffusion does. And the rate depends on respiration — cyanide, which blocks aerobic respiration, stops active transport within minutes, while facilitated diffusion carries on. That last observation is the classic experimental way of telling the two apart, and it is a better answer than 'active transport uses energy' whenever a question hands you data.

Cells that do a lot of it look the part. An epithelial cell in a kidney tubule or the ileum is packed with mitochondria, and its exchange surface is folded into microvilli. Neither adaptation is decorative: one supplies the ATP, the other supplies the surface area.

The sodium-potassium pump

The best-studied example sits in the surface membrane of virtually every animal cell and runs continuously for the life of the cell.

For each ATP hydrolysed, the pump moves three sodium ions out of the cell and two potassium ions in. Both movements are against the ion's gradient: sodium is already more concentrated outside, potassium already more concentrated inside, and the pump is what made that true in the first place.

Because the exchange is uneven, the pump is electrogenic — it moves net positive charge out and leaves the inside of the cell more negative than the outside. That contributes to the resting potential of a neurone, which is where you will meet this pump again. It is also expensive: in a resting cell roughly a quarter of all the ATP produced goes on this one protein, and in nerve tissue the share is far higher, because every impulse has to be paid for afterwards.

The gradients it maintains are useful in their own right, which is the idea the next section depends on. A steep sodium gradient across a membrane is a store of potential energy. Anything that lets sodium fall back down that gradient can be made to do work on the way.

Co-transport in the ileum

Glucose absorption in the small intestine is the standard example, and it is standard because it looks impossible at first. Glucose has to move from the gut lumen into the epithelial cell even when there is already more glucose in the cell than in the lumen. No diffusion will do that. Yet the protein doing it hydrolyses no ATP at all.

Written as a sequence, with the mark scheme's order:

Co-transport

The transport of two substances across a membrane by the same carrier protein at the same time, in which one moves down its concentration gradient and provides the means for the other to move against its own.

Because the process depends on the sodium gradient rather than on ATP at the co-transporter, it is sometimes called indirect or secondary active transport. Amino acids are absorbed the same way, and the kidney reabsorbs glucose from the filtrate in the proximal convoluted tubule using exactly this arrangement — which is why glucose appears in the urine of an untreated diabetic only once the co-transporters are saturated.

When the cargo is too big for any protein

A protein can carry an ion or a sugar. It cannot carry a bacterium, a droplet of fluid or a vesicle full of digestive enzymes. For those the membrane itself does the work, and it can, because it is fluid.

In endocytosis the membrane invaginates around the material, pinches off and forms a vesicle inside the cell. Where the cargo is solid the process is called phagocytosis: a neutrophil engulfing a bacterium forms a phagosome, which then fuses with a lysosome so that hydrolytic enzymes can digest the contents. Where the cargo is liquid it is pinocytosis. Where a receptor on the membrane binds a specific molecule first — the way cells take up cholesterol packaged as LDL — it is receptor-mediated endocytosis.

In exocytosis the sequence runs backwards. A vesicle, usually from the Golgi apparatus, moves to the cell surface membrane, the two membranes fuse, and the contents are released outside. Insulin leaving a β cell of the pancreas, digestive enzymes leaving an acinar cell, neurotransmitter leaving the presynaptic membrane — all exocytosis, all requiring ATP for the movement of the vesicle along the cytoskeleton and for the fusion itself.

Endocytosis

The bulk transport of material into a cell by invagination of the cell surface membrane to form a vesicle; requires ATP.

Exocytosis

The bulk transport of material out of a cell by fusion of a vesicle with the cell surface membrane; requires ATP.

Both processes depend on the membrane being fluid enough to bend, pinch and fuse, which is the fluid mosaic model doing something you can see the consequences of rather than something you have to take on trust.

All of it in one table

Comparison questions in this unit reward a table you have already built. Learn the columns rather than the rows: for any named process, you should be able to say which way it goes relative to the gradient, whether a protein is involved, and whether ATP is.

The awkward row is co-transport, and it is awkward on purpose. Glucose ends up somewhere it could not have reached by diffusion, so the outcome is active; the protein that moved it hydrolysed nothing, so the step is passive. Both statements are true, and an answer that holds them together — the ATP was spent earlier, at a different protein, on a different ion — is the one that scores.

In the exam

• Name the energy source precisely. 'Energy' is vague; 'ATP hydrolysed to ADP and inorganic phosphate' is what the mark scheme has written down.
• For the sodium-potassium pump, quote the numbers: three sodium out, two potassium in, per ATP. A described pump without its stoichiometry is half an answer.
• In co-transport questions, start with the pump. The sequence that begins 'sodium is actively transported out of the cell into the blood' picks up the marks that the sequence beginning with glucose never reaches.
• Say what the ATP does to the protein: the phosphate group binds and the tertiary structure changes, so the binding site faces the other way. That is the mechanism, not just the fuel.
• If a question gives you data on a respiratory inhibitor, use it. A substance whose uptake stops when respiration is blocked is being actively transported; one whose uptake continues is not.
• Endocytosis and exocytosis both need ATP, even though no carrier protein is involved. The energy is for moving and fusing the vesicle.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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