3.3.3 Digestion

Digestion - large, insoluble molecules are hydrolysed into smaller and more soluble molecules to be transported across membranes and assimilated

Physical Digestion - Large molecules into smaller molecules ( action of the teeth and churning food in the stomach to increase SA)

Chemical Digestion - Larger insoluble molecules are broken down by enzymes

Digestion System

• Salivary Glands (mouth) - secrete enzyme amylase through a duct in the mouth to hydrolyse starch into maltose.

• Oesophagus - Food → Stomach - Muscles allow for muscle contractions to push food along (peristalsis)

• Stomach - muscular sac with inner layer which produces enzymes to digest and store food (especially proteins) - includes glands which produce enzymes to hydrolyse proteins. The stomach also contains highly acidic hydrochloric acid which has a mucus lining to prevent damage to the stomach

• Duodenum - Short section of the small intestine from the stomach - chemicals and enzymes from the pancreas and gall bladder are made there.

• Pancreas - Large gland below the stomach which secretes proteases, lipase and amylase

• Gall Bladder - small pouch which stores bile produced by the liver

• Ileum - long section of the small intestine where food is digested in the ileum of enzymes produced by its walls and glands with the ileum folds into villi ( increased SA) and fold further into microvilli for better absorption.

• Large Intestine - absorbs water

• Rectum - faeces stored there before being removed through excretion

Digestion of Carbohydrates

In the mouth:

• Salivary amylase hydrolyses starch in the mouth through the salivary glands in the mouth during chewing

• The salivary amylase hydrolyses the glycosidic bonds in the starch molecule to produce maltose(disaccharide) - other components maintain pH to prevent enzyme denaturing.

In the small intestine :

• Once the starch reaches the small intestine it mixes with pancreatic juice which contains pancreatic amylase which continues hydrolysis of starch into maltose

• Muscles in small intestine push food down to the ileum which contains the enzyme maltase(membrane-bound disaccharide) which hydrolyses the glycosidic bond in maltose to alpha glucose to be absorbed into the blood.

Digestion of Proteins

Proteins are large, complex molecules hydrolysed by peptidases/proteases (enzymes) which occurs in the stomach and continues in the small intestine

• Endopeptidases - hydrolyses the peptide bonds between amino acids in the middle of protein molecules to form smaller peptide molecules - produced by the stomach and the pancreas

• Exopeptidases - hydrolyses the peptide bonds between terminal amino acids of the peptide molecule formed by endopeptidases to form dipeptides or amino acids - produced the pancreas and the small intestine wall

• Dipeptidases - hydrolyses the peptide bond between two amino acids of a dipeptide to form single amino acids - these enzymes are membrane-bound and found on the cell membrane of the small intestine epithelial cells on the ileum.

Digestion of Lipids

Lipids are hydrolyses by lipases produced in the pancreas and work in the small intestine by hydrolysing ester bonds in triglycerides and monoglycerides

• Lipids are split into tiny droplets by bile salts produced by the liver by emulsification and increases SA so lipases can begin to hydrolyse lipids into monoglycerides and fatty acids

• Bile (alkaline) - neutralises the stomach acid in the small intestine and provides optimum pH for lipase

• When the lipid is hydrolysed by lipase the monoglycerides and fatty acids form small micelles to help products of digestion be absorbed across the epithelial lining of the small intestine.

Absorption of Digestion Products

• Many Villi = Larger SA for diffusion

• Villi have thin walls = Short diffusion pathway

• Rich Blood Supply from capillaries = Maintain Diffusion gradient

• Epithelial cells have many villi = Further increases SA for diffusion

• Epithelial cells having many mitochondria = Large amounts of ATP for active transport from respiration for transport into the blood

Monosaccharides

• Glucose absorbed by active transport with Na ions through the co-transporter protein

• Galactose is absorbed also through the co-transporter protein

• Fructose is absorbed by facilitated diffusion through a different transporter protein

• Amino Acids are absorbed similarly to glucose using the co-transport of Na ion concentration rather than ATP which helps with the movement of glucose/amino acids into the cell

Monoglycerides and Fatty Acids

• Monoglycerides and Fatty Acids are associated with bile salts which emulsify the lipid droplets in micelles

• Micelles have contact with the epithelial cells in the ileum when they break down releasing monoglycerides and fatty acids

• Non-polar molecules easily diffuse across the cell membrane into the epithelial cells

• Once inside they are transported to the SER where they are formed into triglycerides

• In the SER they are then transported into the golgi which associates the triglycerides with cholesterol and lipoproteins to form chylomicrons

• Chylomicrons move out of the epithelial cells by exocytosis into the lymphatic capillaries then into the blood stream

• Triglycerides in chylomicrons are hydrolysed by the enzyme in cells in the blood capillaries where they diffuse into cells