Lesson 40 - Co-transport

Question 1

How does the Sodium-Potassium pump work?

Answer 1

1. The sodium potassium pump binds 3 sodium ions and a molecule of ATP.
2. The ATP is hydrolyzed, meaning only an inorganic phosphate molecule is left in the binding site. This provides energy to change the shape of the channel. The sodium ions are driven through the channel.
3. The sodium ions are released to the outside of the membrane, and the new shape of the channel allows 2 potassium ions to bind.
4. Release of the phosphate allows the channel to revert back to its original form, releasing the potassium ions on the inside of the membrane.

Question 2

How are sodium and glucose/amino acids transported from the small intestine lumen cells and into the bloodstream?

Answer 2

1. Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood by the sodium-potassium pump. 2. This creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ilium than inside the cell.
2. This causes sodium ions to diffuse out of the lumen of the ileum into the epithelial cell, down their concentration gradient. They do this via the sodium-glucose co-transporter proteins.
3. The co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.
4. Glucose diffuses out of the cell into the blood, down its concentration gradient through a protein channel, by facilitated diffusion.

Question 3

Cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions. Explain how. (3 marks)

Answer 3

• Sodium ions are actively transported from ileum cell to blood
• Maintains diffusion gradient for sodium to enter cells from gut (and with it, glucose)
• glucose enters by facilitated diffusion with sodium ions

Question 4

Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some of these cells have membranes with a carrier protein called NHE3. NHE3 actively transports 1 sodium ion into the cell in exchange for 1 proton (hydrogen) out of the cell. How does NHE3 do this? (3 marks)

Answer 4

• Co-transport
• Uses (hydrolysis of) ATP
• Sodium ion and proton bind to the protein
• Protein changes shape (to move sodium ion and proton across the membrane)

Question 5

Which type of transport is bulk transport?

Answer 5

Active transport, it requires energy

Question 6

What does ‘bulk transport’ transport (as opposed to what ‘diffusion’, ‘osmosis’ and ‘active transport’ transport?

Answer 6

• Diffusion, osmosis and active transport are responsible for the transport of individual molecules/ions across cell membranes
• Bulk transport can transport larger quantities of materials into or out of cells

Question 7

What are examples of what ‘bulk transport’ transports?

Answer 7

• Large molecules such as proteins or polysaccharides
• Parts of cells
• Whole cells eg. bacteria

Question 8

What are the name of bulk transport into and out of cells?

Answer 8

• Bulk transport into cells = endocytosis
• Bulk transport out of cells = exocytosis

Question 9

How does exocytosis work?

Answer 9

vesicle (e.g. a secretory vesicle from the Golgi apparatus) fuses with cell surface membrane and contents (e.g. large molecules such as proteins) released from the cell (secreted) by exocytosis.

Question 10

How does endocytosis work?

Answer 10

Vesicle forms as cell surface membrane surrounds/folds around large molecules outside the cell. Vesicle buds off from cell membrane allowing the molecules to enter the cell inside the vesicle by endocytosis