Digestion and absorption

Question 1

Major parts of digestive system

Answer 1

Oesophagus - carries food from the mouth to the stomach.
Stomach - muscular sac with inner layer that produces enzymes, needed to store and digest food, especially proteins.

Duodenum - digestion continues using enzymes from pancreas which enter via the pancreatic duct, secretions from the liver via the gall bladder assist the activity of enzymes.
Ileum - long muscular tube, food is further digested by the enzymes produced by its walls. Inner walls are folded into villi that have tiny projections called microvilli, for large surface for absorption of products of digestion into bloodstream.

Large intestine - absorbs water.
Rectum - faeces are stored before being removed via the anus in a process called egestion.

Salivary glands - secrete enzyme amylase, which hydrolyses starch into maltose.
Pancreas - large gland below the stomach, secretes pancreatic juice that contains proteases to hydrolyse proteins, lipase to hydrolyse lipids, and amylase to hydrolyse starch.

Question 2

Digestion consists of:
1. physical breakdown
2. chemical digestion

Answer 2

Physical breakdown
- food is broken down into smaller pieces, making it possible to ingest and increasing the surface area for chemical digestion.
- teeth and tongue in the mouth, churning in the stomach

Chemical digestion
- enzymes hydrolyse large, insoluble molecules into smaller, soluble ones.
- carbohydrases, proteases, lipases

Question 3

Carbohydrate digestion

Answer 3

1. Starch - salivary amylase and pancreatic amylase - maltose (starts in the mouth, stops in the stomach, continues in the duodenum)
2. Maltose - maltase - a-glucose (at the lining of the ileum)

*disaccharides: maltose / sucrose / lactose
*membrane-bound disaccharidases: maltase / sucrase / lactase

*acid in the stomach denatures salivary amylase and prevents further hydrolysis of starch
*pancreatic amylase in pancreatic juice continues digestion of starch

Question 4

Lipid digestion

Answer 4

1. Lipids - bile salts - micelles (small intestine)
2. Micelles - lipase - lipids (small intestine)
3. Triglycerides - lipase - fatty acids and monoglycerides (small intestine)

*lipases hydrolyse the ester bond in tryglycerides to form fatty acids and monoglycerides
*emulsification of lipids by bile salts forms micelles and increases the surface area

Question 5

Protein digestion

Answer 5

1. Protein - endopeptidase - polypeptides (lumen of the gut)
2. Polypeptides - exopeptidase - dipeptides (lumen of the gut)
3. Dipeptides - dipeptidase - amino acids (inside cell-surface membranes of epithelial cells)

*endopeptidase hydrolyses peptide bonds in the center of a polypeptide chain increasing surface area for exopeptidase
*exopeptidase hydrolyses peptide bonds at the end of a polypeptide chain releasing singular amino acids or dipeptides
*membrane-bound dipeptidases hydrolyse the peptide bonds between 2 amino acids in dipeptides, found inside the cell-surface membrane of epithelial cells

Question 6

Adaptation of the ileum for absorption

Answer 6

vili:
- increase surface area for diffusion
- very thin walls reducing diffusion distances
- muscle movement maintains the diffusion gradient
- well supplied with blood vessels to maintain the diffusion gradient
- microvilli are finger-like projections on the cell-surface membrane that further increase the surface area for absorption

Question 7

Sodium-potassium pump / Absorption of glucose and amino acids

Answer 7

1. Sodium ions are constantly transported out of epithelial cells into the blood which maintains a higher concentration of sodium ions in the ileum compared to the inside of epithelial cells
2. Sodium ions move into the epithelial cells through protein carriers (co-transport proteins) by diffusion, carry amino acid or glucose molecules into the cell too
3. Glucose / amino acids pass into the blood plasma by facilitated diffusion through carrier proteins

*when the concentration of glucose and amino acids in the lumen of the ileum and epithelial cells is equaled by diffusion, active transport occurs
*sodium ions move into the cell down their concentration gradient, while glucose and amino acids move against their concentration gradient
*this is indirect form of active transport as it’s the concentration gradient of sodium ions rather than ATP directly, that moves glucose and amino acids into the cells

Question 8

Absorption of lipids

Answer 8

1. Micelles break down releasing fatty acids and monoglycerides coming in contact with epithelial cells
2. Fatty acids and monoglycerides enter the epithelial cells by diffusion
3. Reformed to triglycerides in the ER
4. Associated with cholesterol and lipoproteins in the Golgi apparatus to form chylomicrons
5. Chylomicrons leave the cell by exocytosis and enter the lacteal, pass via lymphatic vessels into the blood system