Food must be digested because the molecules are:

-insoluble and too big to cross membranes and be absorbed into the blood.

-polymers. Must be converted into monomers so they can be rebuilt and used by body cells.

Structure of the gut wall

Consists of four tissue layers surrounding a cavity. The proportions of the different layers vary depending on function of that aspect.

The four layers are:

• Serosa - tough connective tissue protecting the gut wall. Also reduces friction with other abdominal organs

• Muscular layer (inner circular muscles and longitudinal muscles), Coordinated waves of contractions (peristalsis).

• Submucosa - connective tissue containing blood and lymph vessels, which remove absorbed products of digestion. Also contains nerves that co-ordinate peristalsis.

• Mucosa - lines the gut wall. Epithelium secretes mucus, lubricating and protecting mucosa. In some regions it secretes digested foods, and in others it absorbs digested food.

Functions of the gut

1. Ingestion: food taken in through mouth (buccal cavity).

2. Digestion: breakdown of insoluble molecules into soluble ones. Mechanical (using teeth and muscle contractions) and chemical (use of digestive enzymes).

3. Absorption: passage of molecules and ions through gut wall into the blood.

4. Egestion: removal of waste not made by the body, such as undigested food (e.g. cellulose).

Mechanical digestion occurs in the buccal cavity (the mouth).

Food is mixed with saliva by the tongue and is chewed with teeth (mastication).

This increases the surface area of the food, allowing for more efficient use of enzymes.

Saliva (from salivary glands) contains:

• amylase - starts the breakdown of starch into maltose

• ions - maintains slightly alkali pH, optimum for amylase

• mucus - lubricates the food’s passage along oesophagus

Food is carried to the stomach, from the buccal cavity through the oesophagus.

The bolus of food gets there via peristalsis. This occurs when the longitudinal muscles contract and push food forwards, then they relax.

This creates a wave of contractions that move the bolus along the tube towards the stomach.

Bile is made in the liver, stored in the gall bladder and passes through the duodenum through bile duct.

-It contains no enzymes, but contains bile salts (amphipathic - contain hydrophobic and hydrophilic parts)

-Bile emulsifies lipids in the food. This involves breaking up large globules of lipids into smaller globules, increasing their surface area. This increases efficiency of lipase.

Bile is alkaline - neutralises the acid in food from the stomach. Also provides suitable pH for enzyme activity in the small intestine (acts as a buffer)

Food enters here from the oesophagus and remains here by the contraction of sphincters.

The walls contact rhythmically and mix up the food with gastric juices.

Gastric juices are secreted through gastric pits.

The juices include:

• Peptidases (enzymes that break down protein). Pepsinogen is secreted and activated by H+ ions to pepsin -an endopeptidase (this hydrolyses protein into polypeptides).

• HCL - this lowers the pH (to about 2), and is optimum for enzymes. Also kills most of the bacteria in the food.

• Mucus - secreted by goblet cells and forms a protective lining to the wall from the enzymes through the pancreatic duct.

The pancreas

The islet cells from this organ secrete pancreatic juices. They enter the duodenum through the pancreatic duct.

Many enzymes are secreted from here into the duodenum including

• Endopeptidases - hydrolyse protein to peptides.

• Trypsinogen - becomes converted into trysin (protease) by enterokinase.

• Amylase - digests any remaining starch into maltose.

• Lipase - hydrolyses lipids into fatty acids and glycerol

Sodium hydroxide is also found in these pancreatic juices.

This raises pH to slightly alkaline - neutralising acid from stomach and providing appropriate pH for enzyme activity.

Small intestine

It has 2 regions: duodenum and Ileum

Duodenum

It is the first 25cm and receives the secretions from liver and pancreas.

Contains villi that also secrete enzyme - endopeptidases and exopeptidases. These continue the break down of polypeptides and dipeptides and eventually into amino acids.

Also absorbs disaccharides and these are broken down intracelluarly using carbohydrases within the epithelial cells of the villi.

• maltase hydrolyses maltose into two glucose molecules into two glucose molecules

• sucrase hydrolyses sucrose into glucose and fructose

• lactase hydrolyses lactose into glucose and galactose

Ileum

It is mainly used for absorption and is well adapted. It is about 6m long in humans. It contains villi and microvilli - providing large surface area for absorption.

Absorption requires ATP for active transport (many mitochondria found in the epithelial cells)

• Amino acids are absorbed into the epithelial cells (active transport) and sometimes pass into capillaries (individually via facilitated diffusion)

• Glucose passes into the epithelial cells with sodium ions (co-transport). They move into capillaries - sodium via active transport and glucose via facilitated diffusion (some using active transport to prevent it leaving in faeces).

• Fatty acid and glycerol diffuse into epithelial cells and into the lacteals, lymph capillaries in villi.

• Minerals are into blood via diffusion, facilitated diffusion and active transport.

• Vitamins B and C are absorbed into the blood (water soluble). Vitamins A,D, and E are lipid soluble and dissolve into lacteals.

• Water is absorbed into epithelial cells via osmosis.

Large intestine

it is about 1.5m long and it is made up of the caecum, colon, appendix, and rectum.

Undigested food, mucus, bacteria and dead cells pass into the colon. Colon wall has fewer, but larger, villi than small intestine and is used for water absorption.

As the material passes along the colon, water is absorbed and the remaining material is semi- solid.

The semi- solid faecal matter is then stored in the rectum and is eventually egested as faeces, via the anus. This process is called defecation.

Adaptations of the gut wall

Oesophagus wall

• has smooth mucosa layer - allowing for bolus of food to pass along oesophagus easier.

• two large muscular layers - this allows for peristalsis

Stomach wall

• has 3 layers - allows for more contractions

• has folds in mucosa layer - this is where gastric pits are located

Small intestine wall

• contains villi and microvilli - this provides a very large surface area

Large intestine wall

• contains villi and microvilli - this provides a large surface area, it also absorbs water from food into the blood