3.3 DIGESTION

Question 1

What is the simple definition of digestion?

Answer 1

When large biological molecules are hydrolysed to smaller molecules that can be absorbed across the cell membrane.

Question 2

What enzymes break down carbohydrates?

Answer 2

Carbohydrases

Question 3

What are carbohydrates broken down into?

Answer 3

Monosaccharides (MoS)

Question 4

Outline how carbohydrates are digested, from mouth to stomach.

Answer 4

1. MOUTH
   Amylase hydrolyses the glycosidic bond in starch, breaking it down into maltose (DiS)
2. STOMACH
   Food is swallowed and the stomach acid (HCl) denatures amylase to stop the hydrolysis.
3. ILEUM
   Food passes into the ileum where:
   - Maltase hydrolyses maltose into 2 x glucose.
   - Sucrase hydrolyses sucrose into glucose & fructose.
   - Lactase hydrolyses lactose into glucose & galactose.

Question 5

How are the monosaccharides absorbed by the cells lining the ileum?

Answer 5

Glucose is transported by co-transport with sodium.

Question 6

What enzyme digests lipids?

Answer 6

Lipases

Question 7

Where are lipases made and found?

Answer 7

• MADE in the pancreas

- FOUND in the ileum

Question 8

What are lipids broken down into?

Answer 8

2 Fatty acids & a monoglyceride (glycerol w/ 1 fatty acid)

Question 9

What is emulsification?

Answer 9

Bile salts split up lipids into tiny droplets called ‘micelles’.

Question 10

What is the need for emulsification?

Answer 10

After emulsification, the micelles provide a bigger surface area so the lipases can hydrolyse lipids faster.

Question 11

What happens when lipases interact with bile salts?

Answer 11

Lipids are broken down into monoglycerides and fatty acids, and each stays attached to bile salts as micelles.

Question 12

How are monoglycerides and fatty acids absorbed in the ileum?

Answer 12

1. Micelles move through the villi lumen and are broken down into the monoglyceride and fatty acids (leave micelles) when in contact with the epithelial cells lining the villi.
2. Monoglycerides and fatty acids are non-polar, so pass straight across the membrane into the epithelial cells (EC).
3. In EC, monoglycerides & fatty acids move to the endoplasmic reticulum and are recombined into triglycerides.
4. Triglycerides join with lipoproteins & cholesterol to form chylomicrons at the Golgi apparatus.
5. Chylomicrons move out of EC by exocytosis and enter the lacteal in the villi - joining the lymphatic system, and then the bloodstream.

Question 13

What enzymes digest proteins? (3)

Answer 13

• Endopeptidases
• Exopeptidases
• Dipeptidases

Question 14

What do endopeptidases do?

Answer 14

Hydrolyse peptide bonds in amino acids at the centre of a polypeptide to make smaller groups of amino acids.

Question 15

What do exopeptidases do?

Answer 15

Hydrolyse peptide bonds from the smaller groups created by endopeptidases (terminal amino acids) into dipeptides.

Question 16

What do dipeptidases do?

Answer 16

Hydrolyse bonds between dipeptides to release single amino acids.

Question 17

How are monosaccharides and amino acids absorbed into the bloodstream?

Answer 17

By co-transport with sodium into the epithelial cells and then facilitated diffusion into the bloodstream.

Question 18

How is the ileum adapted for absorption?

Answer 18

Has multiple villi to increase the surface area of the ileum.

Question 19

How are villi adapted for absorption?

Answer 19

• Very thin walls so has a short diffusion distance
• Has muscle to move things along and maintain a concentration gradient.
• Copious blood supply to carry away nutrients and maintain a gradient.
• Also have microvilli for increased SA.