3.3.3 Digestion and Absorption

Question 1

What happens during digestion?

Answer 1

Large biological molecules are hydrolysed to smaller

molecules that can be absorbed across cell membranes

Question 2

What are the enzymes involved in the digestive system?

Answer 2

Carbohydrases: hydrolyses carbohydrates, ultimately into monosaccharides
Lipases: hydrolyse lipids into glycerol and fatty acids
Proteases: hydrolyse proteins, ultimately into amino acids

Question 3

What are the steps of starch digestion in humans?

Answer 3

Mouth: saliva contains mineral salts which maintain a neutral optimum pH and salivary amylase which starts to hydrolyse starch
Stomach: acidic which denatures amylase so hydrolysis stops
Ileum: pancreatic juice contains pancreatic amylase which converts starch to maltose and alkaline salts which maintain the optimum neutral pH, muscles push the food along
Maltase is a part of the cell surface membranes of the epithelial cells which line the ileum (membrane-bound disaccharide)
Maltase hydrolyses maltose into alpha glucose

Question 4

What is the process of lipid digestion?

Answer 4

Hydrolysed by enzymes called lipases
Lipases hydrolyse the ester bond in triglycerides to form fatty acids and monoglycerides
Lipids are firstly split up into tiny droplets called micelles by bile salts which are produced by the liver
This is emulsification which increases surface area of the lipids so the action of lipases is sped up

Question 5

How are proteins digested?

Answer 5

They are hydrolysed by enzymes called proteases

Question 6

What are the types of peptidases? (no explanation)

Answer 6

Endopeptidases, exopeptides, dipeptides

Question 7

What is an endopeptidase?

Answer 7

They hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules

Question 8

What is an exopeptidase?

Answer 8

Hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids

Question 9

What is a dipeptidase?

Answer 9

They hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum

Question 10

How do villi increase the efficiency of absorption?

Answer 10

Increase the surface area for diffusion
Very thin walled which reduces diffusion distance
Contain muscle so can mix the contents of he ileum to maintain the concentration gradient
Well supplied with blood vessels so the blood carries away molecules to maintain a concentration gradient
Epithelial cells lining the villi possess microvilli which are finger-like projections that increase the surface area for absorption

Question 11

How are triglycerides absorbed?

Answer 11

Monoglycerides and fatty acids are in association with bile salts
The micelles, through the movement of material, comes into contact with the epithelial cells lining the ileum
Then the micelles break down, releasing monoglycerides and fatty acids
These are non polar molecules so easily diffuse across the cell surface membrane
Once inside, they are transported to the ER where they are recombined to form triglycerides
Starting in the ER and continuing in the Golgi, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons
Chylomicrons move of of the epithelial cells by exocytosis
They then enter lymphatic capillaries called lacteals which are found in the centre of each villi
From here they pass, via lymphatic vessels, into the blood system
The triglycerides are hydrolysed in the endothelial cells of blood capillaries where they diffuse into cells

Question 12

How are micelles formed?

Answer 12

One end of the bile salt is soluble in fat and hydrophobic, other is reversed
This means that they can arrange themselves with their ends in fatty acid droplets and their lipophobic ends sticking out
This prevents fat droplets from sticking to each other to form large droplets, leaving tiny droplets (micelles)

Question 13

What is the structure of the ileum?

Answer 13

The wall of the ileum is folded and possesses finger-like projections called villi
They have thin walls lined with epithelial cells on the other side of a network of blood capillaries

Question 14

How are amino acids and monosaccharides absorbed?

Answer 14

Amino acids, fructose, glucose and galactose are absorbed via diffusion and co-transport

Question 15

Which organelles are numerous/well developed in an epithelial cells of the ileum?

Answer 15

ER to re-synthesise triglycerides
Golgi apparatus to form chylomicrons
Mitochondria to provide ATP for co-transport

Question 16

What are the main parts of the human digestive system?

Answer 16

Oesophagus
Stomach
Ileum
Large intestine
Rectum
Salivary glands
Pancreas

Question 17

What is the oesophagus?

Answer 17

It carries food from the mouth to the stomach

Question 18

What is the stomach?

Answer 18

A muscular sac
Has an inner layer which produces enzymes
Stores and digests food, especially proteins
It has glands which produces enzymes to digest protein

Question 19

What is the ileum?

Answer 19

A long muscular tube
Also known as the small intestine
Food is further digested in the ileum by enzymes that are secreted by glands in its walls
Inner walls of the ileum are folded into villi Microvilli protude from each epithelium of tbe villi cells
This produces a large surface area which is better for absorption

Question 20

What is the large intestine?

Answer 20

The long tube surrounding the ileum
Absorbs water
Most of the water that is absorbed is water from the secretions of the many digestive glands

Question 21

What is the rectum?

Answer 21

The final section of the intestines

Faeces is stored here before periodically being removed via a process called egestion

Question 22

What are the salivary glands?

Answer 22

Situated near the mouth
They pass their secretions via a duct into the mouth
Secretions contain amylase which hydrolyses starch into maltose

Question 23

What is the pancreas?

Answer 23

A large gland situated below the stomach
Produces pancreatic juice
Secretion contains:
   Proteases to hydrolyse proteins
   Lipase to hydrolyse lipids
   Amylase to hydrolyse starch

Question 24

What are the two stages of digestion?

Answer 24

Physical breakdown

Chemical digestion

Question 25

What is the process of physical breakdown of food?

Answer 25

Food is broken down into smaller pieces
E.g. by the teeth or by muscles in the stomach churning food
This provides a larger surface area for chemical digestion

Question 26

What is chemical digestion?

Answer 26

When large, insoluble molecules are hydrolysed into smaller and soluble ones via enzymes

Question 27

What are the common disaccharides that are hydrolysed in the digestive system?

Answer 27

Maltose into alpha glucose
Sucrose into glucose and fructose
Lactose into glucose and galactose

Question 28

How are disaccharides broken down in the digestive system?

Answer 28

The single glycosidic bond is is hydrolysed

Question 29

Where are villi found?

Answer 29

Situated at the interface between the lumen (cavity) of the intestines and the blood and other tissues inside the body