6.1 Digestion and Absorption

Question 1

What does the mouth do?

Answer 1

Voluntary control of eating and swallowing. Mechanical digestion of food by chewing and mixing with saliva, which contains lubricants and enzymes that start starch digestion.

Question 2

What does the oesophagus do?

Answer 2

Movement of food by peristalsis from the mouth to the stomach.

Question 3

What does the stomach do?

Answer 3

Churning and mixing with secreted water and acid which kills foreign bacteria and other pathogens in food, plus initial stages of protein digestion.

Question 4

What does the small intestine do?

Answer 4

Final stages of digestion of lipids, carbohydrates, proteins and nucleic acids, neutralising stomach acid, plus absorption of nutrients.

Question 5

What does the pancreas do?

Answer 5

Secretion of lipase, amylase and protease.

Question 6

What does the liver do?

Answer 6

Secretion of surfactants in bile to break up lipid droplets.

Question 7

What does the gall bladder do?

Answer 7

Storage and regulated release of bile

Question 8

What does the large intestine do?

Answer 8

Re-absorbtion of water, further digestion especially of carbohydrates by symbiotic bacteria, plus formation and storage of feces.

Question 9

What is the structure of the small intestine?

Answer 9

On the outside you have the serosa, which is an outer coat.
Then you have a layer of muscle, which has longitudinal muscle with circular muscle inside.
Then you have the sub mucosa which is a tissue layer containing blood and lymph vessels.
Then you have the mucosa which is the lining and has the epithelium on it that absorbs nutrients.

Question 10

What is peristalsis?

Answer 10

The contraction of circular and longitudinal muscle layers of the small intestine mixes the food with enzymes and moves it along the gut.
The wall of the small intestine has smooth muscle and not striated muscle so it is small cells rather than long fibres. Waves of muscle contraction move down the small intestine, then circular muscle contracts behind it to stop the food going backwards. This happens slowly so there is lots of time for absorption.

Question 11

Where is the muscle in the small intestine controlled?

Answer 11

enteric nervous system

Question 12

How do you vomit?

Answer 12

It is not peristalsis but rather the abdominal muscles.

Question 13

How does the pancreas work?

Answer 13

The pancreas secretes enzymes into the lumen of the small intestine.
- Some of it makes insulin and glucagon but the rest
- Makes enzymes to be secreted.
Small groups of gland cells cluster around the ends of tubes called ducts into which enzymes are secreted. Enzymes are synthesised on the rough endoplasmic reticulum and then processed by the Golgi apparatus, then secreted by exocytosis.

Question 14

What is starch broken down into?

Answer 14

Maltose by amylase.

Question 15

What is lactose?

Answer 15

Glucose and galactose

Question 16

What is sucrose?

Answer 16

Glucose and fructose

Question 17

What are villi?

Answer 17

Villi are finger like projections on the surface of the epithelium, on the mucosa over which absorption is carried out.
They absorb monosaccharides, any of the 20 amino acids, fatty acids, mineral ions,

Question 18

Methods of absorption in the small intestine?

Answer 18

Nutrients need to pass through the small intestine wall into the blood. The nutrients must first be absorbed by the epithelium cells on the mucosa, and then they need to pass out of these cells through the plasma membrane.

TRIGLYCERIDES:
- During digestion these are broken down into fatty acids and monoglycerides. These are then absorbed by simple diffusion, as they can pass through the membrane.

• Fatty acids can also be absorbed by facilitated diffusion as there are fatty acid transporters.
• Once inside they are combined again so cannot diffuse out. They join with cholesterol to form droplets and these become coated in phospholipids and protein, these are released by exocytosis through the plasma membrane on the inner side of the villus, they are either carried away by the lymph system or by the blood in the capillaries.

GLUCOSE
- Glucose is polar and therefore cannot pass through the membrane by simple diffusion because it is hydrophilic.

• Sodium potassium pumps use ATP and pump sodium ions from the cytoplasm into the interstitial spaces inside the villus and potassium ions in. This creates a low concentration of sodium ions inside.
• Because of this low concentration gradient a sodium glucose pump the other side starts to bring sodium in and with it a molecule of glucose.
• Glucose channels then, allow glucose to leave and enter the interstitial space by the villi.

Question 19

How are Triglycerides absorbed?

Answer 19

TRIGLYCERIDES:
- During digestion these are broken down into fatty acids and monoglycerides. These are then absorbed by simple diffusion, as they can pass through the membrane.

• Fatty acids can also be absorbed by facilitated diffusion as there are fatty acid transporters.
• Once inside they are combined again so cannot diffuse out. They join with cholesterol to form droplets and these become coated in phospholipids and protein, these are released by exocytosis through the plasma membrane on the inner side of the villus, they are either carried away by the lymph system or by the blood in the capillaries.

Question 20

How is glucose absorbed?

Answer 20

GLUCOSE
- Glucose is polar and therefore cannot pass through the membrane by simple diffusion because it is hydrophilic.

• Sodium potassium pumps use ATP and pump sodium ions from the cytoplasm into the interstitial spaces inside the villus and potassium ions in. This creates a low concentration of sodium ions inside.
• Because of this low concentration gradient a sodium glucose pump the other side starts to bring sodium in and with it a molecule of glucose.
• Glucose channels then, allow glucose to leave and enter the interstitial space by the villi.

Question 21

How is starch digested?

Answer 21

Amylose has bonds between the 1 and 4 carbons, amylase breaks these bonds as long as there is a chain of at least 4 glucose monomers.
Amylose is therefore digested into a mixture of two and three glucose fragments called maltose and maltriose.

Amylopectin is a branched form of amylose, amylase cannot break the 1,6 bonds. These sections that cannot digest are called dextrins. Dextrinase therefore has to break these down. Maltase and maltotriose digest the two forms of broken down amylose and then dextrinase takes care of the rest.

It is absorbed through sodium glucose pumps, and then glucose channels take it into the interstitial space. It then absorbs into the capillaries. These capillaries are in the sub mucosa and the blood is taken to the liver where excess glucose can be absorbed by the liver cells and is converted to glycogen for storage.

Question 22

How can we model the small intestine?

Answer 22

Using dialysis tubing.