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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5.3 Appreciating the Movement of Chromosomes During Mitosis & Meiosis

Failure of chromosomes to move in an orderly manner during cell division
1. Most of the time,  the mechanism that separate chromosomes in meiosis work in an orderly manner, but sometimes homologous chromosomes fail to separate. This is call non-disjunction.

2. If non-disjunction occurs, half the daughter cells produced have an extra chromosome (n+1), whilst the other half have a chromosome missing (n-1).

3. The result of non-disjunction may be genetic disorders such as Down’s syndrome, Klinefelter’s syndrome and Turner’s syndrome.

Non-disjunction in Meiosis I

Non-disjunction in Meiosis II

4. In plants, non-disjunction sometimes affects all pairs of homologous chromosomes, resulting in the formation of diploid (2n) gamete. Fertilisation involving a diploid gamete with a haploid (n) gamete produces a triploid (3n) zygote, whilst fertilisation involving two diploid gametes produces a tetraploid (4n) zygote.

5. Triploid and tetraploid plants ofter have advantageous features, such as increased size, hardness and resistance to diseases.


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5.2 Meiosis

A cell with two sets of chromosomes (one set from the male parent and the other from the female parent) is referred to as diploid (symbol 2n). Somatic cells or non-reproducing body cells are diploid.

A cell with a single set of chromosomes is referred to as haploid (symbol n). Gametes or sex cells are haploid.

Human somatic cell has 46 (23 pairs) chromosomes while the ovum or sperm has 23 chromosomes. In humans n= 23.

There are 23 pairs of homologous chromosomes in each humans somatic cell. Members of a homologous pair are identical in length and in the position of the centromere and can be identified by their characteristic shape.

Gametes are produces by  a process called meiosis.

Meiosis is a division of the nucleus to produce four daughter cells each containing half the chromosome number of the parent nucleus. Meiosis is associated with sexual reproduction.

Meiosis is preceded by an interphase during which the cell replicates its DNA and organelles.

Meiosis (reduction division)

1. During meiosis,  the cell undergoes DNA replication once, followed by two nuclear divisions.

First meiotic division (meiosis I):

  • The behaviour of chromosomes differs from mitosis. In meiosis I, homologous chromosomes pair up and exchange DNA whereas chromatids remain connected to each other. Meiosis I is divided into four phases: prophase I, metaphase I, anaphase I and telophase I.

Second meiotic division (meiosis II):

  • The behaviour of chromosomes are typical of mitosis. Meiosis II is divided into four phases: prophase II, metaphase II, anaphase II and telophase II.

2. Prophase I

  • Chromosomes shorten and thicken and each is seen to comprise two chromatids joined at the centromere.
  • The homologous chromosomes pair up. Each pair of homologous chromosome is called a bivalent.
  • The maternal and paternal chromatids intertwine to form crosses or chiasmata (singular: chiasma).
  • The formation of chiasmata results in exchange of DNA between maternal and paternal chromosomes, a process called crossing over.
  • Nuclear membrane breaks down and the nucleoli disappear.
  • Centrioles migrate to the poles and the spindle forms.

3. Metaphase I

  • The bivalents become arranged around the equator of the spindle, attached by their centromeres.
  • The arrangement is completely random relative to the orientation of other bivalents, leading to genetic variation in the gametes.

4. Anaphase I

  • The spindle fibres contract and pull the homologous chromosomes, centromeres first, towards the poles of the spindle.
  • One of each pair is pulled to one pole, its sister chromosome to the opposite pole.

5. Telophase I

  • The chromosomes reach their opposite poles. The chromosomes for two haploid sets, one set at each end of the spindle.
  • The nuclear membrane forms around each set of chromosomes, the spindle fibres disappear and the chromatids uncoil.
  • Cytokinesis usually occurs and two haploid cells are formed.
  • The nucleus may enter interphase but no further DNA replication occurs.

6. Prophase II

  • The nucleoli disappear and the nuclear membrane breaks down.
  • The centrioles divide and move to opposite poles.
  • Spindle fibres develop.
  • Chromosomes condense and move to the equator of the spindle.

7. Metaphase II

  • The chromosomes arrange themselves on the equator of the spindle.

8. Anaphase II

  • The centromeres divide and are pulled by the spindle fibres to opposite poles, carrying the chromatids with them.

9. Telophase II

  • The chromatids uncoil and become indistinct.
  • The spindle fibres disappear.
  • The nuclear membrane and the nucleoli reform.
  • Cytokinesis occurs and four haploid cells are formed from one parent cell.



Enjoy the Mitosis Song.

Let’s enjoy the song after a long-time learning about Mitosis from the previous topic. :)

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5.1 Mitosis

Cell division starts with the division of the nucleus.

There are two forms of nuclear division: mitosis and meiosis.

Mitosis is a division of the nucleus to produce two new daughter cells containing chromosomes identical to the parent cells.

Significance of mitosis

1. Growth: Mitosis allows a zygote to produce more cells in order to grow into a multicellular organism.

2. Repair and replacement: Mitotic cell division allows the body to repair itself, or even regenerate following injury. For example, a house lizard will regenerate a tail that is lost to a predator. Mitosis also allows a multicellular organism to maintain its tissues, many of which require frequent replacement, for example skin cells and blood cells.

3. Asexual reproduction: Mitosis provides the basis of asexual reproduction, in which offsprings are formed from a single parent. the offsprings are called clones.

The cell cycle

1. Cell division is just a small part of the cycle of cell growth and asexual reproduction known as the cell cycle.

2. The cell cycle is defined as the period from the formation of a cell by division to the point when that cell divides itself. The length of a cell cycle is very variable, depending on the type of cells.

Interphase (G1, S G2 phase)

1. Interphase is not a ‘resting phase’. During interphase, the cell is metabolically active and is involved in protein and DNA synthesis.

2. Interphase may account for 90% of the total cell cycle.

3. Interphase is divided into 3 shorter phases G1, S and G2 respectively.

4. During G1 (gap or growth phase 1) phase, the cell is sensitive to internal and external signals that help it decide whether to divide or not. Once decided to divide, the cell becomes metabolically active. The cytoplasm increases in volume due to the synthesis of a new proteins and organelles.

5. During S (synthesis) phase, DNA replicates and two sister chromatids form from each chromosome. In animal cells, the centrioles duplicate.

6. During G2 (gap or growth phases 2) phase, organelles and proteins necessary for cell division are synthesised.

Mitosis (M phase)

1. Mitosis is a continuous process, but it may be subdivided into four main phases, based on the appearance and behaviour of the chromosomes:

2. Prophase

  • The chromosomes condense, that is, they shorten and thicken and finally become visible under the light microscope.
  • Each chromosome consists of sister chromatids attached at point called the centromere, The two sister chromatids correspond to identical molecules of DNA formed during the S stage.
  • The nucleoli disappear, the nuclear membrane breaks down, and the centrioles migrate to opposite poles of the cell. Centrioles are absent in plant cells.

Click on the video below to watch more detail explanation:

3. Metaphase

  • The spindle fibres are fully formed.
  • All chromosomes are arranged with their centromeres along the equator of the spindle.

Here is the video about metaphase. Enjoy it :)

4. Anaphase

  • Anaphase begins with the separation of the centromeres.
  • The sister chromatids are drawn to opposite poles of the cell. Once the sister chromatids are separated they are referred to as daughter chromosomes.
  • The poles move further apart, lengthening the cell.

Can’t get it? Watch this video! :)

5. Telophase

  • Telophase begins when the two sets of daughter chromosomes have reached the two poles of the cell.
  • The spindle fibres disintegrate, the nuclear membrane forms around each set of daughter chromosomes, and the nucleoli reappear.
  • The chromosomes uncoil and become less visible under the light microscope.

Hehe, don’t worry. This is the video:

6. Cytokinesis

  • Cytokinesis is the process of cytoplasmic division to form two daughter cells.
  • Cytokinesis usually begins before nuclear division is completed.
  • In cytokinesis the organelles become evenly distributed between the two daughter cells.
  • In animal cell, a cleavage furrow forms at the equator of the cell and deepens until the daughter cells separate.
  • In plant cell, the Golgi apparatus buds off carbohydrate-filled vesicles that line up along the cell’s equator.
  • The vesicles fuse, producing the cell plate. The cell plate extends outwards to the existing cell wall and separates the two daughter cells.

The video quality may not be good, but the process inside does happen in animals and humans.

7. Most animal cells are capable of mitosis.

8. Only specialised groups of plant cells called meristems are capable of mitosis.

9. There are 3 type of meristems.

- Apical meristem

  • These are found at the tips of shoots and roots. Apical meristems are responsible for the increase in length of plants.

- Lateral meristems

  • These are found in stem and roots. Lateral meristems contribute to an increase in girth.

- Intercalary meristems

  • These are found at nodes in monocotyledonous plants. Intercalary meristems contribute to a an increase in length of monocots.

Controlled and Uncontrolled Mitosis

Controlled Mitosis

1. Regeneration is the ability to restore lost or damaged tissues, as well organs or limbs.

2. Regeneration involves controlled mitosis.

3. Some lizards drop a jumping and twisting tail to entice a pursuing predator, and then regenerate itself a new tail ready for the next encounter.

4. Another type of regeneration is the healing of wounds. Whenever we have a cut on our skin, the healing takes place over a period of time because new cells are made to replace the destroyed and damaged cells.

5. Many plants are capable of total regeneration, that is,  the formation of a whole plant from a leaf, stem or root. For example, if a Begonia leaf together with its petiole is detached and laid on damp sand, roots develop at the end of the petiole and vegetative buds are generated on the lamina. Entire new plants develop from these buds.

Uncontrolled Mitosis

1. Cancer is a disorder of the body’s growth in which cells multiply due to uncontrolled mitosis.

2. Tumour cells undergo mitosis without cytokinesis. This process produces single cells with many nuclei.

3. The result is a population of abnormal cells called tumours.

4. Tumours of two types: benign tumour and malignant tumour.

5. The cells in a benign tumour normally grow slowly and remain constrained in one area, but the cells of a malignant tumour grow uncontrollably and destroy other tissues.

6. Cancer kills more than six million people worldwide each year.

Cloning plants by tissue culture

1. Cloning is the production of one or more individual plants or animals that are genetically identical to another plant or animal.

2. Commercial plant growers can clone plants by a technique called tissue culture.

3. Tissue culture is a technique or process of keeping tissues alive and growing in a culture medium. Scientists are able to produce whole plants using cells, tissues or organs from different parts of a plant. Tissue culture is also called cell culture or micropropagation.

4. Advantages of tissue culture

  • Can produce plants that are difficult to reproduce by traditional methods.
  • Many clones can be produced quickly in large numbers.
  • The plantlets are free from diseases.

5. An outline of plant tissue culture

  •  All apparatus and materials used in this technique must be sterilised.
  • The surface of a leaf is sterilised with ethanol or dilute sodium hypochlorite solution.
  • The leaf is then cut into small pieces. The small pieces of plant tissue are called explants.
  • The explants are then placed inside a test tube containing nutrient agar and growth hormones.
  • After six to eight weeks, the explants develop new shoots.
  • The shoots are then cut free from the explants, and placed in a flask containing a new medium that helps roots to develop.
  • The rooted plantlets are then transferred to soil and kept in a controlled environment until fully acclimatised.
  • From one original plant, hundreds of genetically identical plant could be produced.

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