Digestive system

Digestion

• The process of breaking down nutrients (fats protein and carbohydrates) into products small enough to be absorbed into the blood and cells

• Catabolic pathway

2 Types of digestion

• Mechanical

  • Physically breaking down food into smaller pieces

  • Chewing/tearing by teeth

  • Muscular contractions in the stomach

  • Mixing (segmentation) in the small intestine

• Chemical

  • Breaking down larger molecules into smaller ones by chemical means

  • Uses the enzymes in the digestive system

    • Saliva

    • Stomach

    • Small intestine

The Human Digestive System

• Function

  • Ingest food

  • Digest food (mechanically and chemically)

  • Moves food along the gastrointestinal tract (GI)

  • Absorbs food (molecule) into blood

  • Egest materials not absorbed

Digestion in the mouth

• Mechanical digestion (by teeth)

  • Incisors (biting and cutting)

  • Canine (tearing)

  • Premolars

  • Molars (crushing and grinding)

• Chemical digestion (by teeth)

  • Salivary amylase (enzyme)

    • Digests carbohydrates into smaller polysaccharides

  • Mucus

    • Protects the soft lining of the digestive system

    • Lubricates food for easier swelling forming a bolus

  • Buffer

    • Neutralise the acid in food to prevent tooth decay

  • Antibacterial chemicals

    • Kills bacteria that enter the mouth with the food

Swallowing

• Epiglottis

  • Closes the airway when swallowing to prevent choking

• Peristalsis

  • Involuntary muscle contraction that helps food to move along

Layers of Gastrointestinal Tract

• Inner layer = Mucosa

  • Shape allows for a larger SA meaning it can absorb more substance

• Submucosa

• Double layer of muscle

  • Longitudinal (pushes like a wave)

  • Circular (contract)

• Serosa

Digestion in the stomach

• Chemical digestion of protein

  • Pepsin

    • Protease

    • Needs a very acidic environment

    • Proteins are broken down into smaller chains of amino acids known as polypeptides

• Mechanical digestion of lipids, proteins and carbohydrates

  • Occurs by waves of movement along the stomach wall

  • Converts food into a thick soupy liquid called chyme

Ulcers

• Caused by bacterial infection of the stomach

• H.pylori bacteria

• Antibiotics available

Pancreas

• Produces and secretes pancreatic juice into the small intestine which contains protease, lipase and amylase

• produces buffers to neutralise the acid from the stomach

Liver

• Amino acids to glucose

• Metabolism of fat

• Synthesis of triglycerides

• Synthesis and regulation of cholesterol

• Deamination

• Production of bile

• Detoxification

• Heat production

Deamination

• When the amino acids exceed the requirement, it is removed, removing the amine group and converting the rest into fat

• The amine group is converted into ammonia which is then combined with CO2 making urea

Bile and Gallbladder

• Bile’s main purpose is to help with digestion

  • Composed of bile salts and pigment

• 600-1000ml of bile is produced each day and stored in the gallbladder

The small intestine

• Mechanical digestion

  • Mixing by segmentation (peristalsis)

  • Bile emulsify fat

• Chemical digestion

  • Pancreatic juices containing amylase, lipase and protease

  • Intestinal juices

• Absorption through villi and microvilli

  • Finger-like projection

  • Increases SA for absorption

Different substance absorption

• Monosaccharides: Active transport into the blood capillaries

• Amino acids: Active transport into the blood capillaries

• Fatty acids and glycerol: Simple diffusion into the villi cells

• Fat-soluble vitamins: With the fatty acids and glycerol

• Water soluble vitamins: Simple diffusion into blood capillaries

• Water: into villi cells by osmosis

Large intestine

• Water absorption

• Bacteria break down the remaining organic molecules, producing gases

Large intestine (colon)

• Re-absorbs water

  • 9 litres of water every day in digestive juices

  • 90% of that water is reabsorbed

    • If not enough water is absorbed it causes diarrhoea

    • If too much is absorbed it causes constipation

  • Water is absorbed through osmosis

Large intestine (bacteria)

• Escherichia coil (E.coil)

  • Digests cellulose: fruits and vegetable

  • Vitamin K and B

  • Generates gases: methane

Mouth

• Breaks down food

• Kills germs

• Digests starch

• Moistens food

Stomach

• Breaks down food

• Kills germ

• Digests protein

• Stores food

Pancreas

• Produces enzymes to digest proteins and carbs

Liver

• Produces bile (which is stored in the gallbladder)

• Breaks up fats

Small intestine

• Breaks down food

• Absorbs nutrients

Large intestine

• Absorbs water

Left after digestion

• Water

• Undigested food material

• Bacteria

• Bile pigments

• Cell remains from intestinal walls