The digestive system

What is

• ingestion

• egestion

• digestion

• secretion

• large insoluble food mols are take in

• large insoluble, undigestable food mols are removed directly no digestion

• large insol mols are digested into smaller more sol mols

• waste products of digestion are removed

Explain the first stage of digestion

The mouth/ buccal cavity

the food large insoluble molecules are ingested and two types of digestion occur

1. mechanical - tongue, lips, teeth and cheek muscles breakdown larger food molecules into smaller pieces. covers the food in saliva

2. chemical - saliva is secreted which contains amylose which breaks down starch (polysaccharide) into disaccharides of maltose. Saliva also emulsifies the food into a bolus for easy movement down the oesophagus

Explain the second stage of digestion

Oesophagus

Muscular walls of circular (contact behind to move food down) and longitudinal (relaxed to let food down) muscle that undergo peristalsis (a series of contractions and relaciones by circular and long. muscle to assist the movement of food). this moves the bolus into the stomach

Explain the third stage of digestion

Stomach

1. mechanical digestion - the stomach walls contain muscles that contact and relax to emulsify food in gastric juices and HCL and to break down food directly

2. chemical

• Oxyntic cells - secrete HCL which kills bacteria, provides low pH 2, activates pepsinogen (inactive form) by low pH into pepsin to break down proteins into amino acids

• goblet cells - secrete mucus that acts as a barrier to prevent self-digestion and low pH damage

• myzogenic cells - secrete pancreatic juice that contains lipase, amylose and tripsinogen that is activated by stomach acid into tripsin which breakdown

What is bile and what is its role

Bile is a substance that is made in the pancreas, secreted from the liver and stored in the gall bladder.

Role = emulsify fats as hydrophobic and raises pH

Explain the next stage of digestion

Duodenum - small intestine

• located at the top of the SI that connects ileum to stomach

• pancreatic juices containing lipase and tripsinogen are secreted

• peristalsis

Explain the next stage of nutrition

ILEUM - SMALL INTESTINE

Structure and role

1. Mucosa - inner most layer/lining - secretes mucus from goblet cells to protect the walls and lubricate the food. epithelium cells with microvilli (small hair like projections with high SA:V for absorption of products of digestion. They also have lots of mitochondria for ATP for AT of mols.) - digestive enzymes are embedded too - multase, lactase, sucrase, endo and exo peptidases to digest further.

2. crypt of lieberkuhn

3. brunners gland - alkaline secretions - neutralise acid

4. sub-mucosa - contains lymph/ lacteal and blood vessels (that go to the heptic portal vein to the liver/ subclavian vein) for absorption of nutrients and nerves to control peristalsis

5. muscle layers - peristalsis - circular and long. muscle.

6. serosa - outer most layer - connective tissue to protect intestine and prevent friction with other organs

Explain the final stages of digestion

1. THE LARGE INTESTINE - COLON

• absorbs excess water and mineral salts and ions that weren’t absorbed earlier

• moves food in bolus via peristalsis

2. ANUS - site of egestion

Explain how proteins, carbs, lipids, ions, minerals and water are digested, absorbed and transported in more detail

1. PROTEINS

Proteins are broken down from large polypeptide teritiary structures into small polypeptide chains and then finally into amino acids

they are 3 types of protease

1. endopeptidase - breaks down the chains at the middles into smaller chains

Types

• pepsinogen in stomach (inactive to prevent autolysis) is activated by HCL due to low pH to form pepsin to digest into smaller chains

• trypsinogen - inactivate and secreted by pancreas into duodenum where activated into trypsin by enterokinase - prefers alkaline 8.0

2. exopeptidase - breaks down the chains at the ends - carboxylic groups and amino groups. secreted from pancreas into intestines

3. dipeptidase - splits dipeptides into AA embedded in the epithelium

They are absorbed into blood stream by FD and carried in capillaries

2. CARBOHYDRATES

They are broken down from large polysaccharides into disaccharides in the mouth and SI lumen and into monosaccharides via epithelium membrane. maltase, lactase and sucrase are embedded so that when absorbed the di are broken down further. The products eg. glucose are then soluble so must be actively transported by co-transport with sodium ions through the membrane of epi cell. They then diffuse via FD into blood

3. LIPIDS eg. triglyceride

They are broken down by lipase from pancreas juices in stomach and duodenum into fatty acids and glycerol. They then diffuse directly across membrane as polar into lacteals of lymphatic system to subclavian vein and reform triglycerides.

4. IONS - AT or FD

5. MINERALS - all transport

6. WATER - osmosis or channel proteins