Digestion

Question 1

What is digestion?

Answer 1

The process by which the large molecules in the food we eat are hydrolysed into smaller more soluble molecules that can be absorbed by the body.

Question 2

What two ways does digestion occur in the body?

Answer 2

1) Physical breakdown, where large food particles are mechanically broken into smaller ones by the action of the teeth and churning food in the stomach

2) Chemical breakdown where larger, insoluble molecules are broken down into smaller, soluble ones by enzymes.

Question 3

What does hydrolysis mean?

Answer 3

Broken down by the addition of water.

Question 4

How is starch digested?

Answer 4

• Saliva contains the enzyme amylase that hydrolyses starch into maltose.
• Pancreatic amylase allows hydrolysis of any undigested starch molecules into maltose.
• Maltase is an enzyme found in the cell membranes of cells lining the small intestine.
• Maltase catalyses the hydrolysis of maltose into molecules of glucose.

Question 5

How are proteins digested?

Answer 5

• Endopeptidases: Hydrolyse peptide bonds between amino acids in the central region forming smaller peptide molecules.
• Exopeptidases: Hydrolyse the peptide bonds between the terminal amino acids to release dipeptides or single amino acids.
• Dipeptidases: They are membrane bound. They hydrolyse peptide bonds between two amino acids of a dipeptide producing amino acids.

Question 6

Why is it beneficial to digest polypeptides in the middle to produce smaller peptides?

Answer 6

To increase the surface area and provide more ends for exopeptidases.

Question 7

How are lipids digested?

Answer 7

• Bile salts emulsify large lipid droplets into smaller droplets to increase the surface area.
• Lipase hydrolyse the ester bonds found in triglycerides to form fatty acids and glycerol and sometimes monoglycerides.

Question 8

How does the presence of microvilli increase absorption?

Answer 8

Increase surface area for absorption as more channels and carries can be present in the membrane.

Question 9

How do large numbers of mitochondria within a cell affect the rate of active transport?

Answer 9

Provides lots of energy in the form of ATP for active transport. Large numbers will increase the rate.

Question 10

How does a co-transport protein recognise sodium ions and glucose molecules but nothing else?

Answer 10

Na+ and glucose have a complementary shape to a binding site on the receptor protein, which has a specific tertiary structure.

Question 11

How is glucose and amino acids absorbed?

Answer 11

• Active transport of sodium ions out of the epithelial cell into the blood.
• Forms a concentration gradient so sodium ions can diffuse into the epithelial cells from gut lumen by facilitated diffusion.
• As sodium ions diffuse, the glucose/amino acids is taken with the sodium ions. This is co-transport.
• Glucose/amino acid builds up inside the epithelial cells and moves into the blood by facilitated diffusion using a carrier protein.

Question 12

How are lipids absorbed?

Answer 12

• Bile salts stay associated with fatty acids and glycerol to form tiny droplets called micelles.
• Micelles are soluble in water and transport their contents to the epithelial cells.
• Micelles come into contact with the epithelial cells and glycerol and fatty acids are released.
• These are non-polar molecules and diffuse across the cell surface membrane by simple diffusion.
• Inside the cells, triglyceride molecules reform, forming chylomicrons within the golgi and are transported out of the cell by exocytosis.
• These are absorbed into vessels called lacteals and then into the blood.