CIE Categories Archives: 7. Human Nutrition

7.5) Absorption

7.5) Absorption


  • The small intestine is the region where digested food is absorbed.
  • Most absorption happens in the ileum. This is the longest part of the small intestine and is between 2-4 metres long.
  • The small intestine has a large internal surface area for absorption to happen quickly and efficiently.
  • Glucose and amino acids pass into the bloodstream.
  • Fatty acids and glycerol pass into the lacteals connected to the lymphatic system.
  • Water is absorbed in both the small intestine and the colon, but the most absorption of water happens in the small intestine.


The ileum is efficient in the absorption of digested food:

  • It is fairly long and presents a large absorbing surface to the digested food.
  • Its internal surface is greatly increased by circular folds bearing thousands of tiny projections called villi. These villi are about 0.55mm long and may be finger-like or flattened in shape.
  • The lining epithelium is very thin and the fluids can pass rapidly through it. The outer membrane of each epithelial cell has microvilli, which increase by 20 times the exposed surface of the cell.
  • There is a dense network of blood capillaries (tiny blood vessels) in each villus.
  • Network of blood capillaries – transports glucose and amino acids away from the small intestine in the blood
  • Internal structure called a lacteal – transports fatty acids and glycerol away from the small intestine in the lymph

  • The hepatic portal vein transports absorbed food from the small intestine to the liver.
  • Digested and undigested foods have different outcomes once they have passed through the alimentary canal (gut).



Assimilation is the movement of digested food molecules into the cells of the body where they are used.

  • Glucose is used in respiration to provide energy.
  • Amino acids are used to build new proteins.
  • Fats are built into cell membranes and other cell structure and is also an important source of energy for cell metabolism.



The small intestine absorbs most of the water in the contents of the gut. By the time the contents reach the end of the small intestine, most of the digested food has also been absorbed.

The remaining material consists of:

  • water
  • bacteria (living and dead)
  • cells from the lining of the gut
  • indigestible substances – such as cellulose from plant cell walls


The colon is the first part of the large intestine. It absorbs most of the remaining water. This leaves semi-solid waste material called faeces. The faeces are stored in the rectum, the last part of the large intestine. Egestion happens when these faeces pass out of the body through the anus.

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7.4) Chemical digestion

7.4) Chemical digestion

Involves enzymes.

  • Breaking down large molecules to small molecules.
  • The large molecules are usually not soluble in water while the smaller ones are.
  • The small molecules can be absorbed through the epithelium of the alimentary canal, through the walls of the blood vessels and into the blood.


  • Amylase breaks down starch to simpler sugars.
  • Protease breaks down protein to amino acids.
  • Lipase breaks down fats to fatty acids and glycerol.


    Where digestion happens:

    • Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine.
    • Lipases catalyse the breakdown of fats and oils into fatty acids and glycerol in the small intestine.
    • Amylase catalyses the breakdown of starch into maltose in the mouth and small intestine.
    • Maltase catalyses the breakdown of maltose into glucose in the small intestine.


    Digestion of protein:

    • Several proteases which break down proteins.
    • Pepsin is secreted in the stomach.
    • Pepsin acts on proteins and breaks them down into soluble compounds called peptides.
    • Trypsin is secreted by the pancreas in an inactive form, which is changed to an active enzyme in the duodenum.
    • Breaking down proteins to peptides.


    The stomach:

    • The stomach produces hydrochloric acid.
    • It kills many harmful microorganisms (bacteria) that might have been swallowed along with the food.
    • The enzymes in the stomach work best in acidic conditions – at a low pH.


    Functions of HCL in gastric juice:

    • Creates a very acid pH of 2.
    • This pH is important because it denatures enzymes in harmful organisms in food, such as bacteria
    • It provides the optimum pH for the protein-digesting enzyme pepsin to work.



    After it has been in the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions – but the food is acidic after being in the stomach. Bile is a substance produced by the liver and stored in the gall bladder.

    • The bile emulsify the fat – they break them up into small droplets with a large surface area, which are more efficiently digested by lipase.
    • Bile is slightly alkaline and has the function of neutralising the acidic mixture of food and gastric juices as it enters the duodenum.
    • This is important because enzymes secreted into the duodenum need alkaline conditions to work at their optimum rate.
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7.3) Mechanical digestion

7.3) Mechanical digestion


Types of teeth and functions:


Incisor – biting off pieces of food

Canine – holding and cutting food

Premolar – tearing and grinding food

Molar – chewing and grinding food


Tooth structure:


Crown: The part of the tooth that is visible above the gum line.

Gum: Is a tissue the overlays the jaws.

Root: The rest of the teeth, embedded in the jaw bone.

Enamel: Covers tooth crown. Enamel is very hard (harder than bone), and prevents the tooth from decaying.

Dentine: Located under the enamel, this looks quite similar to bone. Not as hard as enamel.

Pulp: Found at center of tooth, and contains blood vessels, nerves and soft tissues which delivers nutrients to your tooth.

Cement: The layer of bone-like tissue covering the root. It is not as hard as enamel.

Nerves: Each tooth and periodontal ligament has a nerve supply and the teeth are sensitive to a wide variety of stimuli.


Tooth decay:

Tooth decay happens when the hard outer enamel of the tooth is damaged. This can happen when bacteria in the mouth convert sugars into acids that react with the enamel. Bacteria can then enter the softer dentine inside.

Tooth decay can be prevented by:

  • avoiding foods with a high sugar content
  • using toothpaste and drinking water containing fluoride
  • regular, effective brushing to prevent the buildup of plaque (a sticky layer on the teeth)
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7.2) Alimentary canal

7.2) Alimentary canal


Ingestion: is the taking of substances such as food and drink into the body through the mouth.

Mechanical digestion: is the breakdown of food into smaller molecules without chemical change to the food molecules.

Chemical digestion: is the breakdown of large insoluble molecules into smaller soluble molecules.

Absorption: is the movement of digested food molecules and ions through the wall of the intestine into the blood.

Assimilation: is the movement of digested food molecules into cells of the body where they are used, becoming part of the cell.

Egestion: is the passing out of food that has not been digested or absorbed, as faeces, through the anus.


Diarrhoea: is the loss of watery faeces.

Treatment of diarrhoea is known as oral hydration therapy. This involves drinking plenty of fluids – sipping small amounts of water at a time to rehydrate the body.


Cholera: is a disease caused by the bacterium Vibrio cholera which causes acute diarrhoea.

When the Vibrio cholera bacteria are ingested, they multiply in the small intestine and invade its epithelial cells. As the bacteria become embedded, they release toxins which irritate the intestinal lining and lead to the secretion of large amounts of water and salts, including chloride ions. The salt decrease the osmotic potential of the gut contents, drawing more water from surrounding tissues and blood by osmosis. This makes the undigested food much more watery, leading to acute diarrhoea, and the loss of body fluids and salt leads to dehydration and kidney failure.



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7.1) Diet

7.1) Diet


Balance diet: is a diet in which all the components needed to maintain health are present in appropriate proportions.


Energy requirements:

The amount of energy we need varies. This is due to the following factors:

  • Age
  • Activity levels
  • Gender



The amount of energy we need tends to increase as we approach adulthood.

The energy needs of adults go down as they age.


Activity level:

People who are active tend to need more energy than sedentary people.



Females tend to have lower energy requirements than males.

Females, on average, a lower body mass than males, which has a lower demand on energy intake.


The effects of malnutrition in relation to starvation, coronary heart disease, constipation, and obesity: Malnutrition: is a condition where certain nutrients of a balanced diet are missing, in excess, or taken in the wrong proportions.



  • Occurs when a person has a severe deficiency of energy, nutrient and vitamin intake.
  • Prolonged starvation may cause organ damage, and if not treated properly, death.


Coronary heart disease:

  • Occurs when the diet contains too much fat.
  • Deposits of a fatty substance buildup in the arteries, which result in blood clots.
  • Blood supply to the heart can be reduced resulting in angina (chest pain when exercising).
  • And eventually a coronary heart attack.



  • Constipation occurs when one finds it difficult to poop 🙂
  • Common causes: lack of fibre intake, lack of water intake.



  • A person is considered obese if his/her body weight is 20%above the standard body weight.
  • An over-abundance of calorie intake, increased dependence on fast food and sugary foods have accelerated the number of obese people.
  • Suffer from high blood pressure, coronary heart disease and diabetes.



  • Is caused by the lack of vitamin C in the diet.
  • Symptoms include bleeding under the skin, swollen and bleeding gums and poor healing of wounds.


    • They are not digested or broken down for energy.
    • Mostly, they are not build into body structures.
    • They are essential in small quantities for health.
    • They are needed for chemical reactions in the cells, working in association with enzymes.

    Mineral salts:

    Mineral ions are only needed in small amounts to maintain a healthy body. A lack of the correct mineral ions in the diet also leads to deficiency symptoms.


Dietary fibre (roughage):

Dietary fibre consists of material in food that cannot be digested, in particular cellulose from plant cell walls.

Sources of fibre include:

  • fruit
  • vegetables
  • Cereals


Dietary fibre is important because it provides bulk, which helps the walls of the intestine move food and faeces along the gut. Lack of dietary fibre can lead to constipation.



About two-thirds of the human body is water. It is found in the cytoplasm of our cells and in body fluids like blood.

Sources of water include:

  • food
  • drinks
  • metabolic processes – such as aerobic respiration


Water acts as a solvent and as a transport medium.


Causes and effects of protein-energy malnutrition:


  • Kwashiorkor is caused by a lack of protein in the diet.
  • Infection, plant toxins, digestive failure can also cause kwashiorkor.
  • Symptoms include dry skin, pot-belly, changes to hair colour, weakness and irritability.
  • Marasmus is caused by a very poor diet with inadequate carbohydrates intake as well as a lack of protein.
  • Symptom include reduced fat and muscle tissue, skin is thin and hangs in folds.


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