3.3 Digestion and absorption

Question 1

What is amylase?

Answer 1

• The enzyme produced in the salivary glands and pancreas.
• Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose.

Question 2

What is maltase?

Answer 2

• The dissacharidase maltase is produced in the lining of the ileum (small intestine)
• It is not released into the lumen but is part of the cell-surface membranes of the epithelial cells, so is referred to as a membrane-bound disaccharide
• It hydrolyses the maltose from starch into α-glucose.

Question 3

What is sucrase?

Answer 3

• Sucrose is found in many natural foods, especially fruits
• Sucrase is a membrane-bound disaccharide
• It hydrolyses the single glycosidic bond in the sucrose molecule.
• This hydrolysis produces the two monosaccharides glucose and fructose.

Question 4

What is lactase?

Answer 4

• Lactose is found in milk, and milk products.
• Lactase is a membrane-bound disaccharide
• It hydrolyses the single glycosidic bond in the lactose molecule.
• This hydrolysis produces the two monosaccharides glucose and galactose.

Question 5

What is lipase?

Answer 5

• Lipids are hydrolysed by enzymes called lipases
• Lipases are produced in the pancreas and hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides (a glycerol molecule with a single fatty acid molecule attached)

Question 6

What are endopeptidases?

Answer 6

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

Question 7

What are exopeptidases?

Answer 7

• They hydrolyse the peptide bonds on the end amino acids of the peptide molecules formed by endopeptidases
• They progressively release dipeptides and single amino acids.

Question 8

What are dipeptidases?

Answer 8

• 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 9

What are peptidases?

Answer 9

• Proteins are large, complex molecules that are hydrolysed by peptidases
• There are endopeptidases, exopeptidases and dipeptidases.

Question 10

Why do endopeptidases work first?

Answer 10

• This exposes more ends of the amino acid chains for the exopeptidases to work on
• It can create 4 ends, but if exopeptidases worked first, there would only be 2 ends to work on, which would be slower

Question 11

What happens during digestion?

Answer 11

During digestion, large, insoluble molecules are hydrolysed to smaller molecules that can be absorbed across the cell membrane of the intestinal epithelial cells

Question 12

What 2 stages does digestion take place?

Answer 12

• Physical breakdown
• Chemical digestion

Question 13

What is physical breakdown?

Answer 13

• If the food is large, it is broken down into smaller pieces by structures such as the teeth
• This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion
• Food is churned by the muscles in the stomach wall and this also physically breaks it up

Question 14

What is chemical digestion?

Answer 14

• This hydrolyses large, insoluble molecules into smaller, soluble ones, through enzymes
• All digestive enzymes function by hydrolysis, the splitting up of molecules by adding water to chemical bonds that hold them together
• Enzymes are specific so more than one is needed to hydrolyse a large molecule
• Usually one enzyme hydrolyses a large molecule into sections and these sections are then hydrolysed into smaller molecules

Question 15

What are the three main important digestive enzymes?

Answer 15

• Carbohydrases - hydrolyse carbohydrates, ultimately to monosaccharides.
• Lipases - hydrolyse lipids into monoglycerides and fatty acids.
• Proteases - hydrolyse proteins, ultimately to amino acids.

Question 16

What is the first part of the process of carbohydrate digestion?

Answer 16

• Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing.
• The salivary amylase starts hydrolysing any starch in the food to maltose. It also contains mineral salts that help to maintain the pH around neutral, the optimal pH for amylase
• The food is swallowed and enters the acidic stomach, which denatures the amylase and prevents further hydrolysis of the starch.

Question 17

What is the second part of the process of carbohydrate digestion to do with the pancreas?

Answer 17

• The food is passed into the small intestine, where it mixes with the secretion from the pancreas called the pancreatic juice.
• This contains pancreatic amylase, which continues the hydrolysis of any remaining starch to maltose.
• Alkaline salts are produced by both the pancreas and the intestinal wall to maintain pH at neutral.
• Muscles in the intestine wall push the food along the ileum, its epithelium lining produces maltase.
• This hydrolyses the maltose from starch breakdown into α-glucose.

Question 18

What is the human digestive system?

Answer 18

• Made up of a long muscular tube and its associated glands
• The glands produce enzymes that hydrolyse large molecules into small ones ready for absorption
• The digestive system is therefore an exchange surface through which food substances are absorbed

Question 19

What is bile?

Answer 19

• It’s really alkaline
• Not an enzyme or an acid

Question 20

What is the role of bile?

Answer 20

• Emulsification
• Neutralisation of stomach acids

Question 21

What enzymes are involved in carbohydrate digestion and where are they found?

Answer 21

• amylase in the mouth
• maltase, sucrase, lactase in the membrane of small intestine

Question 22

Where are lipids digested?

Answer 22

small intestine

Question 23

What needs to happen before lipids can be digested?

Answer 23

they must be emulsified by bile salts produced by the liver. This breaks down large fat molecules into smaller, soluble molecules called micelles, increasing surface area

Question 24

How are lipids digested?

Answer 24

lipase hydrolyses the ester bond between the monoglycerides and fatty acids

Question 25

Which enzymes are involved in protein digestion?

Answer 25

endopeptidases
exopeptidases
dipeptidases

Question 26

How are certain molecules absorbed into the ileum despite a negative concentration gradient?

Answer 26

through co-transport

Question 27

Which molecules require co-transport?

Answer 27

amino acids and monosaccharides

Question 28

Explain how sodium ions are involved in co-transport.

Answer 28

Sodium ions are actively transported out of the cell into the lumen, creating a diffusion gradient. Nutrients are then taken up into the cells along with sodium ions

Question 29

Why do fatty acids and monoglycerides not require co-transport?

Answer 29

the molecules are non-poplar, meaning they can easily diffuse across the membrane of the epithelial cells