Co-transport

Question 1

What are microvilli and where can they be found?

Answer 1

• They are finger-like projections of the cell-surface membrane (about 0.6um in length)
• They are found on epithelial cells lining the ileum
  (Collectively termed ‘brush border’ as under a light microscope they look like bristles on a brush)

Question 2

How can microvilli increase transport across membranes?

Answer 2

• They provide more surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion + active transport can take place
• Increase the no. of protein channels + carrier proteins in any given area of membrane (i.e. increase their density)

Question 3

Describe the role of diffusion in absorption of glucose in the ileum

Answer 3

• As carbohydrates + proteins are being digested continuously, there is normally a greater conc. of glucose + amino acids within the ileum than in the blood
• So, there is a conc. grad. down which glucose moves by facilitated diffusion from inside the ileum into the blood
• As 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 conc. grad. between inside ileum + blood
• This means rate of movement by facilitated diffusion across epithelial cell-surface membranes is increased

Question 4

Describe the role of active transport in absorption of glucose in the ileum

Answer 4

• Diffusion only results in the concentrations on either side to become equal - means not all available glucose + amino acids can be absorbed this way + some may pass out of body
• The reason why this doesn’t happen is because glucose + amino acids are also being absorbed by active transport - means all the glucose + amino acids should be absorbed into the blood
• The actual mechanism by which they are absorbed from the small intestine is an example of co-transport - this term is used as either glucose/amino acids are drawn into cells along with Na+ ions that have been actively transported out by sodium-potassium pump

Question 5

What are villi?

Answer 5

Villi are 1mm projections of the wall of the ileum (different to microvilli as microvilli are projections o the surface of epithelial cells)

Question 6

Describe the overall process of absorption of glucose in the ileum

Answer 6

1. Na+ ions are actively transported out of epithelial cells, by Na-K (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 much higher conc. of Na+ ions in lumen of intestine than inside the epithelial cells
3. Sodium ions diffuse into epithelial cells down this conc. grad. through a different type of protein carrier (co-transport protein) in cell-surface membrane. As Na+ ions diffuse in through this 2nd carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them
4. The glucose/amino acids pass into blood plasma by facilitated diffusion using another type of carrier

Question 7

Explain what makes the movement of glucose molecules an indirect form of active transport

Answer 7

Both Na+ ions & glucose/amino acid molecules move into the cell, but while the Na+ ions move down their conc. grad. , the glucose molecules move against their conc. grad. It is the Na+ ion conc. grad. , rather than ATP directly, that powers the movement of glucose + amino acids into cells. This makes it an indirect rather than a direct form of active transport