Active transport

Where it happens

Active transport happens across cell membranes wherever a cell must take in or push out a substance against its gradient, for example roots absorbing mineral ions and the gut absorbing glucose.

The three examples the syllabus expects you to know are root hair cells absorbing mineral ions such as nitrate from very dilute soil water, the epithelial cells of the ileum absorbing glucose and amino acids once the concentration in the lumen has fallen below that in the blood, and the cells of the kidney tubule reabsorbing glucose and mineral ions from the filtrate back into the blood. In each case the cell membrane is the site of the process, and each cell contains abundant mitochondria close to the membrane to supply the energy needed.

Inputs and outputs

• Input: the substance to be moved, such as glucose, amino acids or mineral ions, on the low-concentration side of the membrane.
• Input: ATP produced by aerobic respiration in the mitochondria, which is hydrolysed to release energy.
• Input: a specific carrier protein embedded in the phospholipid bilayer of the plasma membrane.
• Output: the same substance, unchanged, released on the high-concentration side of the membrane.
• Output: ADP and phosphate, which return to the mitochondria to be rebuilt into ATP.
• Output: the carrier protein returned to its original shape, ready to bind the next molecule.

The steps

1. A substance is present at a lower concentration on one side of the membrane and a higher concentration on the other, so diffusion alone would move it the wrong way for the cell's needs.
2. The substance binds to a specific site on a carrier protein that spans the membrane, on the low-concentration side.
3. A molecule of ATP from respiration binds to the carrier protein and is broken down to ADP and phosphate, releasing energy.
4. The energy changes the shape of the carrier protein, so that the binding site now faces the other side of the membrane.
5. The substance is released on the high-concentration side, having been moved against its concentration gradient.
6. The carrier protein returns to its original shape and the cycle repeats as long as ATP and substance molecules are available.
7. The mitochondria continually regenerate ATP from ADP and phosphate so that the process can continue.

Why it matters and how it is controlled

Active transport lets cells absorb useful substances even when they are more concentrated inside, such as mineral ions in root hair cells and glucose in the small intestine. This is why such cells have abundant mitochondria.

The rate of active transport depends on the rate of respiration, because respiration supplies the ATP. Anything that slows respiration slows active transport: a lower temperature, a shortage of oxygen, or a shortage of glucose.

A respiratory poison such as cyanide stops ATP production and therefore stops active transport, while diffusion and osmosis carry on unaffected. This difference is the standard experimental evidence that active transport needs energy.

The number of carrier proteins in the membrane also sets a limit. Each carrier is specific to one substance, so a cell adjusts what it absorbs by changing the carriers it makes.

Once every carrier is occupied, adding more substance cannot raise the rate further, which is why a graph of rate against concentration levels off for active transport but keeps rising for diffusion.

How it is examined

You may be asked to define active transport, to contrast it with diffusion and osmosis, or to explain why a cell doing a lot of it has abundant mitochondria.

Data questions often present the uptake of an ion by root tissue with and without oxygen, or with and without a respiratory inhibitor, and ask you to explain the difference. The expected explanation names the energy source, ATP from aerobic respiration, and links the missing oxygen or the inhibitor to a fall in ATP and therefore a fall in uptake.

A table comparing diffusion, osmosis and active transport under headings such as direction, energy, carrier proteins and examples is a common structured item.

Common misconceptions

Worked exam-style question

Question. A student measured the uptake of nitrate ions by pieces of root tissue in two flasks. Flask A was bubbled with air; flask B was bubbled with nitrogen gas.

After one hour the tissue in flask A had absorbed far more nitrate than the tissue in flask B. (a) Name the process by which nitrate ions are absorbed from dilute soil water.

(b) Explain the difference in uptake between the two flasks. (c) State two structural features of a root hair cell that support this process.

Model answer. (a) Active transport. (b) Nitrate is absorbed against its concentration gradient, which requires energy in the form of ATP from aerobic respiration.

In flask A, oxygen is available, so respiration and ATP production continue and carrier proteins keep moving nitrate into the cells. In flask B, nitrogen displaces oxygen, so aerobic respiration stops, little ATP is made and uptake falls to the small amount that can occur by diffusion.

(c) A root hair cell has a long extension that increases the surface area of membrane for carrier proteins, and it contains abundant mitochondria to supply ATP.

Source:SRC-DSKP-EN

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