Active Transport & Co-transport

What is active transport?

The movement of substances from an area of lower concentration to an area of higher concentration against a concentration gradient.

“Active transport is an active process.” Explain what this means.

It requires metabolic energy which is provided by the molecule ATP.

What affects active transport?

Anything that affects ATP production (aerobic respiration) will affect active transport.

For example, cyanide (a respiratory inhibitor).

Give examples of active transport processes.

1. Uptake of mineral salts by plant roots

2. Protein synthesis

3. Muscle contraction

4. Nerve impulse transmission

Explain how the active transport of substances across cell membranes occurs.

• The molecule / ion being transported attaches to a receptor site on the carrier protein. This takes place on the side of the membrane where the substance is at a lower concentration.

• A molecule of ATP binds to the carrier protein from the inside of the cell.

• The ATP molecule undergoes hydrolysis; this produces phosphate and a molecule of ADP.

• Phosphate attaches to the carrier protein which causes it to change shape or rotate, transporting the molecule / ion to the other side of the membrane. Molecule / ion is released.

• Phosphate leaves the carrier protein, causing it to revert back to its original shape. ADP and phosphate later reform ATP during respiration.

Explain the graph depicting the active transport of molecules across the membrane.

There is an initial increased rate of uptake as the concentration gradient increases. This is due to the carrier proteins (pumps) actively pumping ions and molecules across the cell membrane.

Rate of active transport levels off at higher concentration differences. This is due to the pumps being full – this limits the rate of active transport.

Active transport is affected by respiratory inhibitors (which stop ATP production) as ATP is required for this process; rate of transport sharply drops after addition of a respiratory inhibitor such as cyanide.

Describe the type of carrier proteins needed for active transport.

The carrier proteins used in active transport are specific- each carrier protein will only transport one type of molecule / ion.

Cells which carry out lots of active transport will contain what?

Lots of mitochondria- produces ATP which is needed for active transport to occur.

What is co-transport?

Co-transport is a type of active transport that transports molecules and ions into cells together on the same carrier protein.

Give an example of a co-transport process.

Sodium and glucose co-transport in the ileum (small intenstine) and kidney nephron.

Explain the co-transport of sodium and glucose in the ileum (small intestine).

1. Sodium ions are actively transported out of epithelial cells lining the ileum into the blood, creating a low concentration of sodium ions within the cells.

2. Now, there is a higher concentration of sodium ions in the lumen of the gut, compared to the epithelial cells, causing sodium ions to diffuse into the epithelial cells via a co-transport protein. Glucose molecules will also attach to the carrier protein.

3. Finally, glucose molecules pass via facilitated diffusion into the blood and sodium ions by active transport.