Biology - AQA - Unit 2 - B4. Active Transport

Active transport uses energy to move molecules and ions across plasma membranes, usually against a concentration gradient. Carrier proteins and co-transporters are involved in active transport.

The process is pretty similar to facilitated diffusion - a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side.

• Active transport usually moves solutes from a low to a high concentration

• in facilitated diffusion, they always move from a high to a low concentration.

• Active transport requires energy-facilitated diffusion does not. ATP undergoes a hydrolysis reaction, splitting into ADP and P, (inorganic phosphate). This releases energy so that the solutes can be transported.

Co-transporters are a type of carrier protein. They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.

Figure 2 shows the co-transport of sodium ions and glucose. Sodium ions move across the membrane down their concentration gradient. This moves glucose across the membrane too, against its concentration gradient.

Glucose is absorbed into the bloodstream in the small intestine. In the mammalian ileum the concentration of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen (middle) of the ileum by co-transport.

Step 1
Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood, by the sodium-potassium pump.

This creates a concentration gradient-there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell.

Step 2
This causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their concentration gradient. They do this via the sodium-glucose co-transporter proteins.

The co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.

Step 3
Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion.

• The speed of individual carrier proteins the faster they work, the faster the rate of active transport.

• The number of carrier proteins present the more proteins there are, the faster the rate of active transport.

• The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can’t take place.

• Net movement of particles from an area of higher concentration to an area of lower concentration.

• Passive process - doesn’t require energy.

• Net movement of particles from an area of higher concentration to an area of lower concentration.

• Uses carrier proteins and channel proteins to aid the diffusion of large molecules and charged particles through the membrane.

• Passive process-doesn’t require energy.

• Movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential.

• Passive process - doesn’t require energy.

• Movement of molecules, usually from an area of lower concentration to an area of higher concentration.

• Uses carrier proteins and co-transporters to transport molecules.

• Active process - requires energy.