Chapter 28: Human Nutrition

Nutrition

The process by which an organism obtains and uses its food.

Types of Nutrition

Autotrophic

• When an organism makes its own food.

• Most autotrophs are green plants.

• They make food using sunlight, CO2 and water (photosynthesis).

Heterotrophic

• When an organism cannot make its own food.

• Instead, they feed on food made by other organisms.

Heterotrophs

Herbivore

• Can only eat plant material
  e.g. Sheep, Cattle, Deer

Carnivore

• Can only eat animal material
  e.g. Seals, Dogs, Cats

Omnivore

• Eat both plant and animal material
  e.g. Humans, Bears, Badgers

Digestion

• Physical and chemical breakdown of large food molecules into smaller, soluble molecules.

Function

• Break down large food particles until they are small enough to pass into body cells.

• Allow food to be digested in a single location (mouth, acid in stomach)

• Individual cells do not have to contain a full range of digestive enzymes.

Stages in Human Nutrition

Consists of the alimentary canal (gut).

Four stages include:

• Ingestion

• Digestion

• Absorption

• Egestion

Ingestion

Taking of food into the alimentary canal.
This occurs at the mouth.

Digestion

Is the mechanical and chemical breakdown of food into smaller, soluble pieces. Mechanical breakdown involves the teeth, peristalsis and muscular churning.

Absorption

The taking of the digested material from the alimentary canal into the bloodstream. Absorption takes place through the membranes of the stomach and villi in the small intestines.

Assimilation then occurs. This is when the products of digestion are used in cells for chemical reactions.

Glucose → Respiration

Fats → Adipose Tissue

Amino Acids → Proteins, Enzymes, Hormones

Egestion

Removal of the unabsorbed, undigested material.

The Mouth

Mechanical digestion begins in the mouth by the chewing action of teeth.

Incisors: Sharp for slicing and biting

Canines: Pointed to grip, stab, and tear food

Premolars: Crush and chew food

Molars: Crush and chew food

Mechanical Digestion
Physical breakdown of food

• Makes food particles easier to swallow

• Small pieces of food provide large surface areafor ennzymes to work.

Chemical Digestion
Chemical breakdown of food

• Carried out by amylase

• Secreted by 3 pairs of salivary ducts (under tongue, back of jaws, in cheeks)

The Oesophagus

• Food is pushed to the back of the mouth into a ball called a bolus. The epiglottis covers the trachea which ensures the bolus passes down the oesophagus.

• It is a muscular tube, 25cm long.

• Food moves down the oesophagus to the stomach in a process called peristalsis.

• Peristalsis is the alternatre contraction and relaxation of the muscles in the walls of the oesophagus. Helps to break down the food further.

• Churns food in the stomach and mixes it with gastric juices.

• In small intestine, pushes food back and forth which allow longer contact time between food and enzymes and wall of intestine for absorption.

The Stomach

• Food leaves the oesophagus and enters the stomach through a ring of muscle called the cardiac sphincter muscle.

• It is a muscular bag which can hold up to 1 litre of food for 3-4 hours, all the time being churned and mixing with enzymes.

• Stomach lining is heavily folded, forming gastric glands, which produce hydrochloric acid, enzymes and mucous.

• This mixing results in the food being turned into a creamy liquid called chyme. It leaves the stomach and enters the small intestine via the pyloric sphincter muscle.

• Stomach cells are tightly packed together and can be replaced rapidly.

Hydrochloric Acid

Makes the stomach environment acidic (pH 1-2).

HCL activates the enzyme protease, kills bacteria that may have been ingested with the food, loosens fibrous and cellular foods and denatures salivary amylase.

Pepsinogen

Is an inactive enzyme which is activated when in contact with HCL. Pepsinogen is converted to the active enzyme pepsin. Pepsin converts proteins to peptides.

Doesn’t come into contact with stomach lining.

Mucous

Protection from self digestion as it is alkaline and lines the stomach.

The Small Intestine

Narrow, but is 6m-7m long.

Further digests food and absorbs products of digestion into the blood stream and lymph system.

Duodenum

25 centimetres long, is where most digestion occurs.

Food entering duodenum from stomach receives juices from pancreas, liver, and duodenum wall.

Glands in Small Intestine

Pancreas

• Located under stomach

• Secretes insulin and pancreatic juices through the pancreatic duct. Pancreatic juices contain water, sodium bicarbonate (neutralise chyme) and pancreatic enzymes.

• Pancreatic amylase converts starch to maltose.

• Pancreatic lipase converts lipids to fatty acids and glycerol.

• Trypsin (pancreatic protease) converts proteins to peptides.

Liver

• Located on the right handside of the stomach.

• Has huge blood supply including hepatic artery, hepatic vein, hepatic portal vein.

• Produces bile.

• Detoxifies blood.

• Breakdown of red blood cells.

• Storage (glyocgen, vitamin A and D, minerals (Fe, Cu, Zn)).

• Production of heat.

• Protein Metabolism
  - Excess amino acids cannot be stored in the body. They are broken down by the liver into urea (deamination) and excreted.
  - Liver also produces plasmaproteins, such as fibrinogen needed for blood clotting.

• Carbohydrate Metabolism
  - Excess glucose converted to glycogen and stored in liver cells.

• Lipid Metabolism
  - Excess carbohydrates are converted to fat and transported from liver to other cells in the body or stored under the skin.

• Bile
  - Produced in liver and stored in gall bladder and enters the duodenum through the bile duct. Bile is a green-yellow colour that is partly formed from dead red blood cells and also contains bile salts and bile pigments.
  - Bile salts emulsify fats, they break up fats into fat droplets which makes it easier for lipase to work.
  - Does not contain any enzymes. It is alkaline (sodium hydrogen carbonate) and neutralises chyme.

Enzymes

Amylase

Saliva consists of water, salts, mucous, amylase and lysozyme. It is only active for a short while as it is inhibited by acid in the stomach.

Function: Breaks down starch

Production Site: Salivary glands, travels through small ducts to mouth.

pH: 7-7.5

Products: Starch → Maltose

(Protease) Pepsin

Function: Breaks down proteins to peptides

Production Site: Stomach lining

pH: 1-2

Products: Polypeptides

Trypsin

Function: Breaks down proteins to peptides

Production Site: Pancreas

pH: 7-7.5

Products: Polypeptides

Lipase

Function: Breaks down lipids to fatty acids and glycerol.

Production Site: Pancreas

pH: 7-8 (pH in duodenum)

Products: Fatty acid and glycerol

Digestive Enzymes

Enzyme: Carries out the action.

Substrate: Substance the enzyme acts on.

Products: Substance created.

Lipases

Enzymes that digest lipids (fats and oil)

e.g. Lipase → Lipids = Fatty Acids + Glycerol

Proteases

Enzymes that digest proteins (Protein → Amino Acids)

e.g. Pepsin → Protein = Amino Acids

Trypsin → Protein = Amino Acids

Carbohydrases

Enzymes that break down carbohydrates.

e.g. Amylase → Starch = Maltose

Maltase → Maltose = Glucose

Ilium

• Wall of the ilium secretes enzymes which complete the progress of digestion.

• By the time food enters the ilium it is almost completely digested.

• The main function of the ilium is absorption of nutrients.

Villus

• The inner lining of the small intestine is not smooth. Instead, it is highly folded.

• Each fold has thousands of villi (finger-like projections).

• In addition to this, each villus has about 600 microvilli.

• These villi and microvilli increase the surface area for the release of enzymes and the absorption of digested material.

Capillaries in Villi

• The walls of villi are only one cell thick. Inside villi there is a rich blood supply in the form of blood capillaries.

• Water and soluble nutrients such as amino acids, glucose, vitamins, and minerals are absorbed through the villi and into the blood stream. They travel in the hepatic portal vein to the liver and transported from here onwards.

Lacteal in Villi

• Digested lipids (fatty acids and glycerol) do not pass directly into the blood stream. Instead, they pass into the lymph vessels in villi. These lymph vessels are called lacteals.

• Inside the wall on the villi they reform into fats and are coated in protein. They then pass into the lacteals. The fats are transported in the lymph and rejoin the blood stream at the subclavian veins in the neck. The protein coat dissolves and is finally transported to the liver.

Adaptations of The Small Intestine for Absorption

• 500m long.

• Thousands of villi and microvilli.

• Intestinal wall only one cell thick.

• Rich blood supply to carry water soluble products.

• Each villus has a lymph supply (lacteal) to carry away fats.

The Large Intestine

• Larger in diameter compared to small intestine

• 1.5 metres long

• Consists of the caecum, appendix, colon and rectum

• Caecum and appendix have no known functions in humans (vestigal)

The Colon

• Muscular tube with smooth inner mucus-secreting lining, material moves along by peristalsis. The mucus secreted by the lining lubricates the material and helps it move along

• Main function is absorption of water back into blood

• The remaining semi-solid waste is called faeces and is stored in the rectum before being excreted through the anus

Symbiotic Bacteria

• Bacteria in the colon feed on the waste and in return produce vitamins B and K which is absorbed by the body. This relationship between bacteria and humans, where both benefit is called symbiosis

• Some bacteria also help digest material such as cellulose

• These beneficial bacteria also prevent the growth of bad bacteria

Balanced Diet

A balanced diet is one that contains all the essential nutrients in the correct amounts

There are seven components:

• Carbohydrates

• Proteins

• Lipids

• Vitamins

• Minerals

• Fibre

• Water

Fibre

• Produces bulk which gives muscles of the gut wall something to push against

• Absorbs water (prevents constipation)

• Prevents bowle (colon) cancer

• Makes you feel full which prevents overeating