Active

Question 1

What is active transport?

Answer 1

the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

Question 2

How is ATP used in active transport

Answer 2

In active transport ATP is used to:
• directly move molecules
• individually move molecules using a concentration gradient which has already been set up by (direct) active transport. This is known as co-transport and is further explained in Topic 4.5.

Question 3

How does active transport differ from passive transport

Answer 3

Metabolic energy in the form of ATP is needed.
• Substances are moved against a concentration gradient, that is from a lower to a higher concentration.
• Carrier protein molecules which act as ‘pumps’ are involved.
• The process is very selective, with specific substances being transported.

Question 4

Direct active transport

Answer 4

The carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it.
• The molecule or ion binds to receptor sites on the carrier protein.
• On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and a phosphate molecule. As a result, the protein molecule changes shape and opens to the opposite side of the membrane.
• The molecule or ion is then released to the other side of the membrane.
• The phosphate molecule is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated. The phosphate molecule then recombines with the ADP to form ATP during respiration.

Question 5

Increasing the rate of movement across membranes

Answer 5

The epithelial cells lining the ileum possess microvilli
These are finger-like projections of the cell-surface membrane about 0.6 pm in length.. The microvilli provide more surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place. Another mechanism to increase transport across membranes is to increase the number of protein channels and carrier proteins in any given area of membrane

Question 6

The role of diffusion in absorption

Answer 6

As carbohydrates and proteins are being digested continuously, there is normally a greater concentration of glucose and amino acids within the ileum than in the blood. There is therefore a concentration gradient down which glucose moves by facilitated diffusion from inside the ileum into the blood. Given that the blood is constantly being circulated by the heart, the glucose absorbed into it is continuously being removed by the cells as they use it up during respiration. This helps to maintain the concentration gradient between the inside of the ileum and the blood

Question 7

Role of active transport in absorption

Answer 7

At best, diffusion only results in the concentrations either side of the intestinal epithelium becoming equal. This means that not all the available glucose and amino acids can be absorbed in this way and some may pass out of the body. The reason why this does not happen is because glucose and amino acids are also being absorbed by active transport. This means that all the glucose and amino acids should be absorbed into the blood.

Question 8

Cotransport

Answer 8

This term is used because either glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium-potassium pump

Question 9

Co transport of a glucose molecule

Answer 9

Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell-surface membrane of the epithelial cells.
2
This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.

Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them.
4
The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.