Digestion

Question 1

What are carbohydrates?

Answer 1

Important source of energy in diet, mostly in the form of plant based starch, some from glycogen if eating lots of meat
Glycogen is a long complex chain of monosaccharides called polysaccharides

Question 2

What is starch and glycogen?

Answer 2

Long chains of glucose joined by alpha1-4 glycosidic bonds are ingested and this is why amylase is called alpha-amylase as it breaks alpha bonds

Question 3

What is the composition of ingested carbohydrates?

Answer 3

We ingest disaccharides such as sucrose (glucose + fructose), lactose (glucose + galactose) and maltose (glucose + glucose) and a limited amount of monosaccharides (glucose)

Question 4

What are proteins?

Answer 4

Proteins are not a major source of energy, proteins are required for amino acids. There are 20 amino acids, 12 can synthesised, the others are essential and cannot be synthesised. Proteins come from diet/what we eat 50% and from proteins secreted into intestine, endogenous proteins, 50% (enzymes and immunoglobulins)

Question 5

What are lipids?

Answer 5

Lipids are not essential however are an important source of energy, includes fat soluble vitamins A, D, E and K, and are good at controlling rate of gastric emptying
When we ingest lipids the structure is a glycerol back bone with 3 fatty acids attached (<6 carbons are short fatty acids, more than 12 long)

Question 6

Why do we need chemical digestion?

Answer 6

We can only absorb nutrients as small molecules, monosaccharides, amino acids and fatty acids, chemical digestion reduces the size of nutrients to allow them to be absorbed. Occurs at the surface of food particles (mechanical digestion breaks up food and increases SA available for chemical digestion) and utilises digestive enzyme

Question 7

What are digestive enzymes?

Answer 7

Digestive enzymes are extracellular organic catalyses: enzyme + substrate form complex and then split into enzyme and products
Very specific - need different enzymes for different substrates
Have optimal pH - salivary = alkaline, gastric = acidic, small intestinal = alkaline
Large amounts of cellulose in diet which have long chains of B 1-4 glycosidic bonds which cannot be digested

Question 8

What are the two stage process?

Answer 8

Stage one: luminal digestion - mostly pancreatic enzymes, some pepsin and salivary amylase, secreted into lumen
Stage two: contact digestion - brush boarder enzymes in small intestine, completes digestion before absorption, involves enzymes produced by enterocytes and attached to brush boarder of enterocytes

Question 9

Carbohydrate - luminal digestion

Answer 9

Start with salivary and pancreatic amylase - long chain with alpha bonds, amylase breaks down alpha bonds into disaccharides, polysaccharides converted to oligosaccharides and disaccharides

Question 10

Carbohydrate - contact digestion

Answer 10

Brush boarder enzymes take disaccharides and break them into monosaccharides. Involves enzymes such as sucrase (breaks down sucrose), lactase (breaks down lactose) and maltase (breaks down maltose) which are bound to the brush boarder

Question 11

Proteins - luminal digestion

Answer 11

Start with long peptides, pepsin begins digestion and breaks proteins into long chains, enters small intestine where large chains of peptide bonds are broken down by series of enzymes (trypsin and chymotrypsin). Amino acids are snipped off COOH terminus by carboxypeptidase
Converts proteins into polypeptides

Question 12

Proteins - contact digestion

Answer 12

Involves peptidases which converts polypeptides and dipeptides into amino acids. Many types of peptidases are attached to brush boarder

Question 13

Chemical digestion of lipids

Answer 13

Lipid digestion occurs in the lumen of the small intestine, there is no contact digestion. Pancreatic lipase is the main digestive enzyme. Lingual lipase and gastric lipase have a minor role
Problem: digestive enzymes dissolved in aqueous luminal fluid however lipids are insoluble in water

Question 14

Chemical digestion of fats stages

Answer 14

Emulsification (motility)
Stabilisation (bile salts)
Digestion (hydrolysis, enzymes)
Formation of micelles (bile salts)

Question 15

What is emulsification?

Answer 15

Motility breaks up lipid droplets into small droplets which increases surface area for digestion. Occurs in stomach, retropulsion, and small intestine, segmentation, bile salts stabilise droplets

Question 16

What is stabilisation?

Answer 16

Stabilisation occurs in small intestine, bile salts are released into small intestine with arrival of food. Bile has a hydrophobic and negatively charged hydrophilic side which stabilises the emulsion in the small intestine and reduces the size of emulsion droplets (further increases SA)

Question 17

What is hydrolysis?

Answer 17

Hydrolysis occurs in small intestine at surface of emulsion droplets and involves lipase and cofactor colipase (both secreted by pancreas and collapse anchors lipase to surface of droplets). Lipase converts triglycerides to monoglycerides and free fatty acids

Question 18

What is the formation of micelles?

Answer 18

Products of fat digestion are insoluble in water especially monoglycerides and long chain fatty acids. Products are kept in solution through formation of micelles which are small droplets and consist of 20-30 molecules (bile salts, fatty acids and monoglycerides)