6.10 Absorption of products of digestion and also 4.5

Question 1

How is the ileum adapted for digestion

Answer 1

. Wall of ileum is folded into microvilli, which increases surface area of ileum to accelerate rate of absorption

. They have thin walls that are one cell thick, that are lined with epithelial cells (that contain microvilli).
Microvilli can contain carrier proteins for diffusion, facilitated diffusion, and active transport can take place

. There is a rich network of blood capillaries for absorbing the products into the bloodstream

Question 2

Villi are in the ileum between the lumen of intestines, and the blood / body tissues

How are they adapted for absorbing products of digestion

Answer 2

. Increase surface area for diffusion
. Very thin walled so reduce diffusion distance between capillaries and small intestine

. Contain muscle that allow them to move to mix contents of the ileum
This helps maintain diffusion gradients so products of digestion are absorbed into the blood and they are replaced by new material rich in nutrients, from the stomach constantly

. Good blood capillary supply so blood can carry away the absorbed molecules and hence maintain a diffusion gradient
. The epithelial cells lining the villi possess microvilli which further increase surface area for absorption by containing carrier proteins

Question 3

How are amino acids and monosaccharides absorbed by the ileum into epithelial cells

Answer 3

Amino acids are produced by hydrolysis of proteins by proteases
Monosaccharides are produced by hydrolysis of starch to maltose to glucose etc

They are both absorbed into the bloodstream by diffusion and co-transport

Question 4

How are triglycerides transported from the ileum to epithelial cells

How do micelles help this, what do they contain

Answer 4

. The monoglycerides and fatty acids are produced in digestion with lipase and bile which emulsifies the lipids so there is a larger surface area for lipase to hydrolyse

. Structures formed are micelles which are tiny
. Micelles contain bile salts and the fatty acids and monoglycerides
. Micelles make them more water soluble
. The micelles bring the fatty acids and monoglycerides to the lining of ileum, epithelial cells

. The micelles break down, releasing the monoglycerides and fatty acids
These are both non polar, and lipid soluble so can diffuse across the cell membrane into the epithelial cells

Question 5

Once in the epithelial cells, how are monoglycerides and fatty acids transported to blood stream

Answer 5

. In the epithelial cells, the fatty acids and monoglycerides are transported to ER in the cell where they are recombined to form triglycerides
. Then go to golgi apparatus, the triglycerides are modified and processed, and are associated with lipoproteins to form chylomicrons / vesicles

Chylomicrons / vesicles are adapted for lipid transport
. These move out of epithelial cells by exocytosis to enter the lymphatic capillaries called lacteals that are in each villi

from here they pass via lymphatic vessels into the bloodstream

The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells

Question 6

What is diffusion

Answer 6

The net movement of molecules or ions from a region of high to low concentration

Question 7

How does diffusion absorb amino acids and monosaccharides from the ileum into the blood

How is a concentration gradient maintained

Answer 7

. There is a higher concentration of amino acids and monosaccharides in the ileum than in the bloodstream

. So they move by facilitated diffusion with carrier proteins into the blood

. A concentration gradient is maintained as the heart is constantly circulating blood to remove all the amino acids and monosaccharides away from the ileum

Question 8

Why is active transport also needed for the absorption of amino acids and glucose from the ileum to the blood

Answer 8

. Diffusion can only result in the concentrations of either side of intestinal epithelium being equal, so not all the available amino acids / monosaccharides will be absorbed, and some may be wasted

. As a result active transport can occur to absorb all the glucose and amino acids into the blood

Question 9

How does co-transport help the amino acids and monosaccharides in the ileum be absorbed into the bloodstream

Answer 9

. Either glucose or amino acids are absorbed into cells along with sodium Na+ ions that have been actively transported out of a sodium potassium pump:

• Sodium ions leave epithelial cells by active transport into the blood
  A protein carrier molecule found in epithelial cell membrane does this
• This means there is a higher concentration of sodium ions in the lumen of intestine than inside the epithelial cells
• So sodium ions enter epithelial cells by facilitated diffusion down the concentration gradient through a co-transport protein in the cell membrane
  And in doing so they can carry a glucose or amino acid

Question 10

How does the co transport actually work with sodium ions and amino acids / glucose

Why is it indirect active transport

Answer 10

Whilst sodium ions diffuse down their concentration gradient from the ileum to the epithelial cells , the glucose is moving against its concentration gradient

The sodium ion concentration gradient is what powers the movement of glucose and amino acids into the cells.
So it is not the ATP directly doing this

This makes it indirect active transport