Archive for category Chapter 6: Nutrition

Assimilation of digested food

Some of the products of digestion are brought directly to the liver for processing, in preparation for metabolic processes or assimilation. Assimilation takes place in the cells where the nutrients are used to form complex compounds or structural components.

  • The liver acts as a checkpoint which controls the amount of nutrients released into the blood circulatory system.
  • Most of the glucose is converted into glycogen and stored in the liver. When the blood sugar level falls and the body needs energy, the stored glycogen is converted back to glucose. Glucose is distributed throughout the body by the circulatory system. When the glucose molecules reach the cells, they are oxidised to release energy during cellular respiration. When the glycogen store in the liver is full, excess glucose is converted into lipids by the liver.


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6.5 Absorption and Assimilation of Digested Food

In the ileum, two processes occur which are digestion and absorption.

The process of digestion is completed in the ileum to produce simple sugars ( glucose, fructose and galactose), amino acids, glycerol and fatty acids.

The process of absorption also occurs in the ileum to absorb the products of digestion into the blood capillaries and to be used by the cells in the body.

The products of digestion are absorbed into the body by small finger-like projections called villi (singular: villus) in the walls of the small intestine. Each villus contains a network of blood capillaries and a lymphatic vessel called lacteal in the centre of the villus.

Absorption of digested food

  • Nutrient absorption involves both diffusion and active transport. Initially, glucose, amino acids, water-soluble vitamins and minerals diffuse into the epithelial cells and are absorbed into the capillaries. Subsequently, the transport of the remaining nutrients across the epithelial lining involves active transport during which energy is used.
  • In contrast, glycerol and fatty acids enter the epithelial cells where they recombine to form tiny droplets of lipids, which then move into the lacteals. Fat-soluble vitamins are also absorbed into the lacteals to be transported together with lipids.
  • The lacteals converge into larger vessels of the lymphatic system. The fluid carrying lipids and fat-soluble vitamins enters the lymphatic system which forms a network throughout the body. 
  • The contents are then drained into the right lymphatic duct and thoracic duct before being emptied into the bloodstream through the subclavian veins. Capillaries that drain water-soluble nutrients away from the villi converge into hepatic portal vein, which leads to the liver. From here, the nutrients are transported to all cells in the body.
  • Together with the small intestine, the colon reabsorbs almost 90% of the water and minerals. In the colon, water and minerals are reabsorbed into the cells lining the colon, and subsequently into the bloodstream, so that we do not constantly lose them.

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6.4 Food Digestion

The process that breaks down complex food substances to simpler, soluble molecule small enough for body to absorb is called digestion.

Carbohydrates, proteins and lipids can be broken down into their component monomers or units through hydrolysis. Digestion breaks down starch into glucose, proteins into amino acids and lipids into glycerol and fatty acids.

Digestion of carbohydrates, proteins and lipids

  • Digestion involves both physical and chemical processes. Physical digestion involves the breaking up of large pieces of food into smaller pieces by mechanical means. By breaking up food into smaller pieces, mechanical digestion increases the surface area of the food available for chemical digestion. During this process, digestive enzymes break down complex food molecules into smaller molecules which enter the bloodstream to be transported to the whole body.
Digestion in the mouth
  • Maltose is not one of the small molecules that can be absorbed by the intestinal lining. An additional digestive process occurs further along the alimentary canal to convert maltose to glucose. The thoroughly chewed food is rolled by the tongue into a mass called a bolus in preparation for swallowing.
Digestion in the stomach
  • The stomach is a thick-walled, sausage-shaped organ situated below the diapharagm. The epithelial lining of the stomach contains gastric glands. These glands secrete gastric juice which consists of mucus, hydrochloric acid and the enzymes pepsin and rennin. The amount of hydrochloric acid secreted is so great that the stomach normally has a pH of around 2.0. Such a high acidity is sufficient to destroy most bacteria that are present in food.
  • Pepsin starts the hydrolysis of large protein molecules into smaller chains of polypeptides.
  • Rennin coagulates milk by converting the soluble milk protein, caseinogen, into the insoluble casein.
Digestive system in ruminants and rodents
  • Ruminants and rodents obtain most of their energy from the breakdown of cellulose of plant cell walls. The enzyme cellulase is required to break down cellulose but is not produced by these animals.
  • Ruminants like cows and goats have stomachs which are divided into four chambers, namely rumen, reticulum, omasum and abomasum.
  • The rumen and reticulum are specialised compartments which harbour large communities of bacteria and protozoa. These microorganisms are able to produce cellulase that digest cellulose.
  1. Partially chewed food is passed to the rumen, the largest compartment of the stomach. Here, cellulose is broken down by the cellulase produced by bacteria. Part of the breakdown products are absorbed by the bacteria, the rest by the host.
  2. As the food enters the reticulum, the cellulose undergoes further hydrolysis. The content of the reticulum, called the cud, is then regurgitated bit by bit into the mouth to be thoroughly chewed. This process helps soften and break down cellulose, making it more accesible to further microbial action.
  3. The cud is reswallowed and moved to the omasum. Here, large particles of food are broken down into smaller pieces by peristalsis. Water is removed from the cud.
  4. The food particles finally move into the abomasum, the true stomach of the cow. Here, gastric juice containing digestive enzymes completes the digestion of proteins and other food substances. The food then passes through then small intestine to be digested and absorbed in the normal way.
In the rodents like rats, the caecum and appendix are enlarged to store the cellulose-digesting bacteria. The breakdown products pass through the alimentary canal of rabbits twice. The faeces in the first batch are usually produced at night. The faeces are then eaten again to enable the animals to absorb the products of bacterial breakdown as they pass through the alimentary canal for the second time. The second batch of faeces becomes drier and harder.

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6.3 Malnutririon

1. When a person does not take balanced diets, he or she is said to suffer from malnutrition.

2. A malnourished person either eats an insufficient or excessive amount of certain nutrients.

3. Excessive or insufficient intake of certain nutrients can lead to diseases. For example, a deficiency in proteins causes kwashiorkor.

4. Deficiency in vitamin B, can cause beri-beri and deficiency in vitamin C can cause scurvy.

5. Deficiency in calcium can cause rickets in children and osteoporosis in adults. Deficiency in iron (ferum) can cause anaemia.

6. To prevent deficiency diseases, our diet should contain all the vitamins and minerals that our body needs. However, these nutrients must not be taken in excess. Excessive intake of vitamins, especially vitamins A, D, E and K is toxic to our liver.

7. Osteoporosis is a condition in which bones loses calcium and become porous and brittle. We should take enough calcium to prevent osteoporosis.

8. Excessive intake of nutrients can be harmful to our health.

9. Excessive intake of calcium can cause calculus (stones) formation in the kidneys and gall bladder.

10. Excessive intake of carbohydrates can cause diabetes mellitus.


6.2 Balanced Diet

The foods that constitute a balanced diet should contain the major nutrients which include carbohydrates, proteins and lipids, as well as vitamins, minerals, water and roughage or dietary fibre.

Daily Energy Requirement

  • The amount of heat generated from the combustion of one gram of food is known as the energy value of the food. The unit of energy value is joule per gram (J/g). 4.2 joules of energy are needed to raise the temperature of 1g of water by 1’C.
  • The three main energy-providing organic molecules are lipids, carbohydrates and proteins.

Nutrient Content in Food


  • Vitamins provide no energy but are essential for the maintenance of good health and efficient metabolism.
  • Water-soluble vitamins include vitamins B and C.
  • The fat-soluble vitamins are A, D, E and K.


  • Major minerals, called macrominerals, are required in relatively large quantities such as calcium, phosphorus, sodium, potassium and chlorine.
  • Microminerals such as ferum, iodine and zinc are required in trace amounts of less than 20 mg per day.

Roughage or dietary fibre

  • Dietary fibre refers to the indigestible part of plant food which consists mainly cellulose.
  • Roughage aids peristalsis which allows gut muscles to grip food and move it along quickly through the digestive tract. This prevents the build-up of toxic substances in the rectum, which can lead to bowel cancer.
  • Deficiency of roughage  in a person’s diet can lead to constipation and other disorders of the large intestine. Constipation can be prevented by taking enough fibre and water.


  • Water makes up about 70% of the total body weight. It also serves as a transport medium for nutrients and waste substances

Food Guide Pyramid


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6.1 Types of Nutrition

Nutrition is the entire process by which organisms obtain energy and nutrients from food, for growth, maintenance and repair of damaged tissues.

Living organisms can be divided into two main groups based on their nutritional habits: autotrophs and heterotrophs.

1. Autotrophs are organisms which practise autotrophic nutrition. Autotrophs are able to manufacture their own food,  either by photosynthesis or by chemosynthesis.

  • Photosynthesis is the process through which green plants, called photoautotrophs, produce organic molecules from carbon dioxide and water using light as a source of energy.
  • Chemosynthesis is the process by which chemoautotrophs, which include certain types of bacteria, synthesise organic compounds without the help of light. These organisms obtain energy by oxidising inorganic substances such as hydrogen sulphide and ammonia.

2. Heterotrophs are organisms that cannot synthesise their own nutrients. Heterotrophs may practise holozoic nutrition, saprophytism or parasitism.

  • In holozoic nutrition, the organisms feed by ingestion solid organic matter which is subsequently digested and absorbed into their bodies.
  • Saprophytes  feed on dead and decaying organic matter. They include bacteria and fungi which digest the food externally before the nutrients are absorbed.
  • Parasite obtains nutrients by living on or in the body of another living organism, the host. The parasite absorbs readily digested food from its host.

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