Digestion and Absorption

Question 1

Can large biological molecules cross cell membranes?

Answer 1

Large biological molecules in food are too big to cross cell membranes. This means they can’t be absorbed from the blood into the gut

Question 2

What happens to large molecules during digestion?

Answer 2

During digestion, large molecules are broken down into smaller molecules, which can move across cell membranes. This means they are easily absorbed from the gut into the blood, to be transported around the body for use by the body cells

Question 3

How can large biological molecules be broken down?

Answer 3

Through hydrolysis reactions - hydrolysis reactions break bonds by adding water

Question 4

During hydrolysis what are carbohydrates, fats and proteins broken down into?

Answer 4

Carbohydrates are broken down into disaccharides, then monosaccharides. Fats are broken down into fatty acids and monoglycerides. Proteins are broken down into amino acids

Question 5

Where are digestive enzymes produced?

Answer 5

Digestive enzymes are produced by specialised cells in the digestive systems of mammals

Question 6

What are carbohydrates broken down by?

Answer 6

Carbohydrates are broken down by amylase and membrane-bound disaccharides

Question 7

What is amylase?

Answer 7

Amylase is a digestive enzyme that catalyses the conversion of starch (a polysaccharide) into the smaller sugar maltose (a disaccharide). This involves the hydrolysis of the glycosidic bonds in starch

Question 8

Where is amylase produced?

Answer 8

Amylase is produced in the salivary glands (which release amylase into the mouth) and also by the pancreas (which releases amylase into the small intestine)

Question 9

What are membrane-bound disaccharides?

Answer 9

Membrane-bound disaccharides are enzymes that are attached to the cell membranes of epithelial cells lining the ileum

Question 10

What do membrane-bound disaccharides do?

Answer 10

They help to break down disaccharides into monosaccharides. This involves the hydrolysis of glycosidic bonds

Question 11

How can monosaccharides be transported across cell membranes?

Answer 11

Monosaccharides can be transported across cell membranes of the ileum epithelial cells via specific transporter proteins

Question 12

What are lipids broken down by?

Answer 12

Lipase with the help of bile salts

Question 13

What do lipase enzymes do?

Answer 13

Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids

Question 14

Where are lipases made and where do they work?

Answer 14

Lipase are made in the pancreas. They work in the small intestine

Question 15

What do bile salts do?

Answer 15

Bile salts are produced in the liver and emulsify lipids - this means they cause the lipids to form small droplets

Question 16

Why are bile salts important in the digestion of lipids?

Answer 16

Several small lipid droplets have a larger surface area than one large lipid droplet. So bile salts forming small droplets increases the surface area of lipids that’s available for lipases to work on

Question 17

What happens once lipids have been broken down into monoglycerides and fatty acids?

Answer 17

Once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles

Question 18

What are two types of proteins?

Answer 18

Endopeptidases and exopeptidases

Question 19

What are peptidases?

Answer 19

These are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between the amino acids

Question 20

What do endopeptidases do?

Answer 20

Endopeptidases act to hydrolyse peptide bonds within a protein

Question 21

Give three examples of endopeptidases

Answer 21

1. Trypsin and chymotrypsin - these are synthesised in the pancreas and secreted in the small intestine
2. Pepsin - this is released into the stomach by cells in the stomach lining. Pepsin only work in acidic conditions, provided by the hydrochloric acid in the stomach

Question 22

What do exopeptidases do?

Answer 22

Exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules. They remove single amino acids from proteins

Question 23

What are dipeptidases?

Answer 23

Dipeptidases are exopeptidases that work specifically on dipeptides. They act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them

Question 24

How are monosaccharides absorbed across the ileum epithelium?

Answer 24

Glucose is absorbed by active transport with sodium ions via a co-transporter protein. Galactose is absorbed in the same way as glucose. Fructose is absorbed via facilitated diffusion through a different transporter protein

Question 25

How are monoglycerides and fatty acids absorbed across the ileum epithelium?

Answer 25

Micelles help to move monoglycerides and fatty acids towards the epithelium. Micelles constantly break up and reform so they release monoglycerides and fatty acids, allowing them to be absorbed. Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane

Question 26

How are amino acids absorbed across the ileum epithelium?

Answer 26

Amino acids are absorbed via co-transport. Sodium ions are actively transported out of the ileum epithelial cells into the blood. This creates a sodium ion concentration gradient. Sodium ions can then diffuse from the lumen of the ileum into the epithelial cells through a sodium-dependant transporter proteins, carrying amino acids with them