Human Digestion.

Heterotrophs

Organisms that cannot make their own food.

3 main types of heterotrophs:

1. Herbivores eat plant material only e.g squirrels.

2. Carnivores eat other animals only e.g pike.

3. Omnivores eat both plant and animal material e.g mice.

Autotrophs

Organisms that can make their own food.

Digestion

The breakdown of large food molecules into smaller, soluble ones.

Alimentary canal

A series of connected organs along a long tube, extending from the mouth to the anus.

Main parts of the Digestive System:

• mouth.

• oesophagus.

• stomach.

• small intestine.

• large intestine.

• rectum.

• anus.

Accessory Organs:

• salivary glands.

• tongue.

• liver.

• gall bladder.

• pancreas.

Human nutrition occurs in stages:

1. Ingestion.

2. Digestion.

3. Absorption.

4. Egestion.

Forms of digestion

Mechanical digestion:

The physical breakdown of food into smaller pieces.

Forms of digestion

Chemical digestion:

The breakdown of food into smaller molecules by enzymes, stomach acid or bile.

1. Mouth:

Physical digestion by teeth

• Chewing reduces the food to smaller pieces which increases the surface area for enzymes to work on. Using teeth and tongue the food is formed into a bolus ready for swallowing.

Types of teeth:

• Incisor = sharp, chisel-shaped for cutting/biting.

• Canine = long, pointed fangs for gripping and tearing.

• Molars & Premolars = rounded tops for grinding & crushing.

Human dental formula

2 (I 2 C 1 PM 2 M 3) (side view: top)

2 1 2 3 bottom

• (Permanent total = 32. Milk/deciduous/child set = 20, has no molars)

Chemical digestion by …

Salivary amylase, produced by salivary glands. Saliva consists of water, salts, mucin, amylase and lysozyme. Mucin and water help to soften and dissolve food so that we can taste and swallow it.

starch ———- salivary amylase ———- maltose

^{( pH 7.5 alkaline )}

2. Oesophagus:

Peristalsis

The rhythmic contraction and the relaxation of the muscles of the gut wall. It pushed the food down to the stomach. Fibre rich foods aid peristalsis e.g wholemeal pasta, wholemeal bread.

3. Stomach:

• Food then enters the stomach when the cardiac sphincter muscle relaxes. The stomach is a muscular, expandable, j shaped bag.

• It warms food to 37 degrees for optimum enzyme activity.

• Gastric glands in the wall produce gastrin hormone which causes the production of gastric juice.

• Gastric juice consists of: HCI, pepsinogen and mucus.

HCI

• Converts the inactive enzyme pepsinogen to pepsin.

• Sterilises food i.e kills microorganisms that enter.

• Loosens fibrous and cellular foods.

• Denatures salivary amylase.

Pepsin

• Pepsinogen is the inactive form. Acid conditions of the stomach convert pepsinogen to pepsin.

• Pepsin converts proteins to peptides. → chemical digestion , pepsin is an enzyme.

Mucus

• A thick, sticky, alkaline substance lining the wall of the stomach, protecting it from self digestion by acid and enzymes.

4. Small Intestine

• A narrow, 6 meter-long tube in the abdomen.

• Functions: digestion & absorption.

• Divided into 2 main parts:

  • The duodenum.

  • The ileum.

• The inner lining is not smooth. It is thrown into folds.

• The surface of these folds are covered by thousands of villi.

• Villi increase the surface area for the release of enzymes and the absorption of digested food.

4. (A) Duodenum

• The walls secrete intestestinal juice, bile and pancreatic juice.

• The juices, particularly the pancreatic juice contains enzymes to continue the digestive process.

• Peristalsis shunts the food along the tube.

Digestion in the small intestine

• The wall of the small intestine contains a mixture of enzymes, including:

  • maltase (breaks down maltose).

  • lactase (breaks down lactose).

  • sucrase (breaks down sucrose).

• Maltose, lactose and sucrose are disaccharides.

• The end products of the digestion of these disaccharides are simple sugars like glucose, which can be absorbed into the blood.

5. Pancreas

• The pancreas is an organ that lies under the stomach.

• It secretes pancreatic juice into the duodenum via the pancreatic duct.

• Pancreatic juice contains water, sodium bicarbonate and enzymes.

• The sodium bicarbonate from the pancreas neutralises acid chyme, giving the duodenum a pH of 7-8.

Enzymes made in the pancreas

ENZYME

Pancreatic amylase

Pancreatic lipase

Trypsin

ACTION IN THE DUODENUM

Breaks down starch to maltose

Breaks down lipids to FA’s and glycerol

Breaks down proteins to peptides

6. Liver

• The liver is the largest gland in the body, other than the skin.

• It lies to the right-hand side of the stomach, underneath the diaphragm.

Liver

Functions:

• Produces bile.

• Makes sodium hydrogen carbonate to neutralise acid.

• Storage of fat-soluble vitamins A, D, E, K, glycogen and mineral - iron is released from the breakdown of old red blood cells by the liver and spleen. Iron is stored before being recycled to make new red blood cells in the bone marrow.

• Detoxification of poisonous substances e.g drugs & alcohol.

• Produces heat to warm the blood and body.

• Formation of cholesterol.

Cholesterol

A lipid-like substance, needed in cell membranes and to make many hormones.

Bile

• Made in liver and stored in the gall bladder.

• Consists of water, bile salts, bile pigments, cholesterol and phospholipids.

Functions:

• Emulsifies fats.

• Contains sodium bicarbonate which helps to neutralise chyme from stomach.

• Excretes pigments biliverdin and bilirubin, which are made from dead red blood cells.

• Excess cholesterol is excreted in bile.

More about the liver

• The liver has a huge blood supply:

  • The hepatic portal vein carries digested nutrients from the small intestine to the liver.

  • The hepatic artery brings oxygenated blood to the liver.

  • The hepatic vein carries deoxygenated blood and other nutrients from the liver.

• Nutrients travel from the liver to cells in the body where they are needed.

• In this way, the food we eat reaches the various parts of our body.

4. (B) Ileum

Most of the absorption of digested food into circulatory system occurs in the ileum.

• Glucose, amino acids, water-soluble vitamins and minerals are absorbed by diffusion and active transport into the blood capillaries. These capillaries unite to form the hepatic portal vein, which carries food to the liver.

• Fatty acids, glycerol, cholesterol and fat-soluble vitamins pass into the lacteals and are carried through the lymph system.

How the absorption occurs:

• The movement of digested nutrients from the intestine to the blood is carried out by:

  • Diffusion.

  • Active transport.

• Diffusion does not require energy.

• In active transport, energy (ATP) is used to move nutrients from the intestine into the blood using specific transport proteins on the membrane of the villus surface.

• This ensures all the digested nutrients are fully absorbed into the blood.

Adaptations for increased absorption

Adaptations of villi for more absorption

• Large numbers (increase the surface area).

• Large numbers of microvilli (increase the surface area).

• Wall are only one cell thick.

• Have a rich blood supply.

Adaptations of small intestine for more absorption

• It is very long.

• Has numerous villi and microvilli (which increase surface area).

• The walls of the villi are very thin.

• There is a rich blood supply to carry away water-soluble products.

7. Large Intestine

• 1.5 meters long and wider than the small intestine.

Functions: water reabsorption & elimination of faeces.

• Caecum - ends in appendix. May play a role in immunity.

• Colon - reabsorbs water (& mineral salts) - prevents dehydration.

• Rectum - stores semi-solid waste.

• Anus - waste passes out of the body as faeces i.e egestion.

• Waste consists of undigested food, mucus, cells from intestinal lining, bacteria and bile pigments.

Symbiotic bacteria

Bacteria that live on or in another organism involving benefit.

• In the colon, they produce vitamins B and K, which are then absorbed by the body.

• They also digest some cellulose.

• Both organisms are benefiting from the relationship.

• This type of symbiosis is known as mutualism.

Benefits of symbiotic bacteria

• Makes vitamins B and K.

• Protects us against pathogenic bacteria that may colonise our large intestine.

• May protect us against cancer, as some of the chemicals they produce inhibit the growth of tumour cells.

Storage of Digested Nutrients

Carbohydrates

Proteins

Lipids

Excess glucose is stored in the liver and muscles as glycogen. If glycogen stores are full, excess sugar can be converted to fatty acids

If there’s excess amino acids from digested protein, the liver can convert them to glucose for energy or store as glycogen. Some excess amino acids can be converted into urine.

Excess fatty acids are stored within adipose tissue, specialised cells under the skin. This tissue acts as a long-term energy reserve, providing heat insulation and protection as well.

Digestion

• Variety of foods: A varied diet which includes fruits, vegetables and wholegrains contributes to a healthy digestive system.

• Fibre: The collective term for indigestible carbohydrates, e.g cellulose.

  • It acts as a prebiotic, promoting the growth of beneficial gut bacteria.

  • It also adds bulk to stool, aiding smooth passage and preventing constipation.

• Fats: Healthy unsaturated fats, found in the sources like olive oil and avocados, can aid digestion and nutrient absorption - helps lower cholesterol.

  • Excessive saturated and trans fats (fried food) from process food can slow down digestion.

• Sugar: Excessive sugar intake can disrupt the balance of good and bad bacteria in the gut.

• Hydration: Water is the medium for enzyme reaction, helping to break down food.

  • It also keeps stool soft for easier passage and prevents constipation.

• Age and gender: Older people produce less stomach acid and digestive enzymes and need more water, fibre and vitamins.

  • Males generally require more carbohydrates and proteins than females.

  • Females require more mineral nutrients, especially iron.

  • Pregnant women require additional nutrients, including folic acid (vitamin B9) which is needed for embryonic spinal development.

Role of fibre in the diet

Benefits:

• It stimulates peristalsis - helps to keep the contents of the gut moving. Fibre absorbs water and provides bulk which gives the muscle of the intestine something to push against. Helps prevent constipation. Fibre absorbs water and this keeps the faeces soft.

• It soaks up cholesterol and toxic chemicals which helps to reduce the risk of colon cancer, heart disease and diabetes.

• May help prevent over-eating. Eating fibre-rich foods e.g wholegrain bread and cereals often make you feel full.