Digestion And Absorption

Question 1

What is the function of digestion?

Answer 1

To break down large insoluble molecules into small soluble molecules

Question 2

What is physical digestion?

Answer 2

The chewing action of the teeth and the churning of the stomach physically break down food into smaller pieces, providing a large surface for chemical digestion.

Question 3

What is chemical digestion?

Answer 3

The hydrolysis of polymers into monomers by enzymes

Question 4

Describe the process of digestion in the human body.

Answer 4

• teeth and tongue in the mouth break up the food into small pieces with larger surface area, and form it into a ball. The salivary glands secrete saliva which contains water to dissolves soluble substances, mucus for lubrication and amylase to digest starch. The food is swallowed down the oesophagus.
• the stomach wall secrete gastric juice which contains hydrochloric acid to kill bacteria, mucus to lubricate and some endopeptidase enzymes.
• the duodenum (first 30cm of small intestine) is where almost all digestion takes place. Pancreatic juice is secreted from the pancreas to the duodenum by the pancreatic duct and the juice contains amylase and endo/exopeptidases. Bile is secreted by the liver, stored in the gull bladder, it contains bile salts which aid lipid digestion, and it is alkali so neutralises the stomach acid.
• the ileum (rest of the small intestine) is the site of final digestion and absorption. To maximise the rate of absorption the ileum has: a large surface area, network of blood capillaries to minimise diffusion distance.
• the large intestine is the site of water absorption.

Question 5

Describe starch digestion in the mouth, duodenum and ileum.

Answer 5

mouth: salivary glands release amylase which hydrolyses starch into maltose
duodenum: pancreas secretes amylase into duodenum which hydrolyses starch into maltose
ileum: membrane-bound maltose hydrolyses maltose into Alpha glucose

Question 6

describe sucrose digestion in the ileum

Answer 6

membrane bound sucrose is hydrolysed into glucose + fructose

Question 7

describe lactose digestion in the ileum

Answer 7

membrane bound lactase is hydrolysed into glucose + galactose

Question 8

describe lipid digestion in the duodenum

Answer 8

lipids are emulsified by bile salts into smaller lipid droplets, this increases the surface area.
the pancreas secretes lipase into duodenum which hydrolyses lipids into fatty acids + triglycerides

Question 9

describe protein digestion in the stomach, duodenum and ileum

Answer 9

• stomach endopeptidase hydrolyses peptide bond between amino acids in the middle of the polypeptide chain, into a series of smaller peptide molecules
• duodenum pancreas secretes endopeptidases and exopeptidases into the duodenum.
  exopeptidase hydrolyses the peptide bond on the ends, due to action of endopeptidases
  -ileum membrane-bound dipeptidase hydrolyses dipeptides into amino acids

Question 10

summary of all digestion

Answer 10

Mouth, teeth mechanically break down food into smaller chunks, increase SA for enzymes to do hydrolysis actions. Starch hydrolysed into maltose by salivary amylase

Stomach, stomach churns food, acid secreted, endopeptidases hydrolyse proteins into shorter polypeptides

duodenum, lipids emulsified into smaller lipid droplets, lipids => fatty acids and monoglycerides by lipase, polypeptides => shorter polypeptides by endopeptidases, polypeptides => amino acids by exopeptidases, starch => maltose by amylase

ileum, dipeptides => amino acids by membrane bound dipeptidases, maltose => glucose by membrane-bound maltase, lactose => glucose + galactose by membrane-bound lactase, sucrose => glucose + fructose by membrane-bound sucrase

Question 11

what are the products of digestion?

Answer 11

• amino acids
• monosaccharides
• fatty acids
• monoglycerides

Question 12

how is the ileum adapted to be a good exchange surface?

Answer 12

• large surface area for more carrier + channel proteins for facilitated diffusion…
• short diffusion distance
• ileum folded into villi and each epithelial cell has microvilli
• thin walls, thin layer of epithelial cells
• maintenance of the concentration gradient
• contraction and relaxation of smooth muscle lining the ileum, moves the digestion products along
• circulation blood flow moves blood along, replacing blood with high conc of glucose ect. with fresh blood

Question 13

describe absorption of glucose and amino acids via co-transport

Answer 13

1. sodium ions actively transported out of epithelial cells by the Na+K+ pump into the blood
2. lowers the concentration of Na+ ions in the cells, creating a conc. grad. with the lumen of ileum for Na+ ions to enter the cell
3. Na+ ions diffuse from the lumen of ileum into the epithelial cell down the concentration gradient via Na+ glucose co-transporters or Na+ amino acid co-transporters (both down Na+ conc.gradient)
4. glucose diffuses out of cell into blood
   or
   amino acids diffuse out of cell into blood

Question 14

describe the absorption of fatty acids through epithelial cells

Answer 14

1. fatty acids and monoglycerides arrive at epithelial cells lining the ileum, arranged into micelles
2. individual fatty acids and monoglycerides diffuse across the membrane into the epithelial cell
3. fatty acids and monoglycerides reformed into triglycerides in the smooth endoplasmic reticulum
4. triglycerides are modified in the golgi body by adding proteins and making chylomicrons
5. chylomicrons exit the base of the epithelial cell via exocytosis and enter the lymphatic capillaries
6. they are returned to the blood stream much later