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Plants are referred to as autotrophs, i.e. being able to manufacture their food (Organic compound from inorganic materials such as water, carbon dioxide)

Autotrophs generally have two modes of nutrition which are chemosynthesis and photosynthesis (holophytic).



This is a type of nutrition in which organisms depend directly or indirectly on autotrophs for food. All animals, fungi, protozoa and some bacteria belong to this group and they are referred to as heterotrophs



(a)        Holozoic Nutrition

This is the mode of feeding in which food is ingested in the form of complex organic substances which is digested and assimilated into the body. Holozoic organisms are grouped into carnivores, herbivores, omnivores and scavengers.


(b)        Parasitic Nutrition

This is the mode of feeding in which certain organisms (parasites) feed on other organisms (host). Parasites can be endoparasites (tapeworm, liver fluke) or ectoparasites (lice, ticks) depending on whether they live inside or outside the host. Plant parasites include dodder, mistletoe. Parasites have special structures such as haustoria, sucker, hooks, and muscular lips e. t. c.


(c)        Saprophytic Nutrition

This is feeding process in which certain organisms (saprophytes) feed on non living organic matter i. e. plant and animal remains. Saprophytes secrete digestive enzymes into the dead organic matter to digest the food (extracellular digestion) and the soluble food substances are then absorbed into their body. Examples include fungi i.e. mucor, mushroom, yeast and some bacteria


(d)        Symbiotic Nutrition

This is a mode of feeding in which two organisms of different species called symbionts live together and derive mutual benefits e.g. nutrients or food, shelter, moisture e. t. c from each other. Examples are nitrogen-fixing bacteria called Rhizobia, which live in root nodules of legumes, algae and fungi in a lichen.


(e)        Carnivorous / Insectivorous Nutrition

This is a feeding process in which some plants that have special devices trap and digest insects. These plants grow in poor soil and proteins obtained from the insects are used to supplement their nitrogenous compound requirement. These plants secrete enzymes which help in digesting their victim. Examples include bladderwort (Ultricularia), Pitcher plant, sundew and venus fly trap



Plants are referred to as autotrophs, i.e. being able to manufacture their food (Organic compound from inorganic materials such as water, carbon dioxide)

Autotrophs generally have two modes of nutrition which are chemosynthesis and photosynthesis



This is the process by which green plants synthesize organic compounds from inorganic raw materials in the presence of chlorophyll and light.


6C02 + 6H20                                                     C6H12O6               +       602

(Inorganic materials)          Chlorophyll             (organic compound)      (by product)

The main product of photosynthesis is sugar (carbohydrate) while the oxygen given up is a by-product which is released into the atmosphere.



These are as follows:

  1. Photosynthesis occurs in the chloroplast of cells in the presence of light.
  2. The raw materials required are low energy containing inorganic compound i.e. CO2 and H2O
  3. The final product is a high energy containing compound (C6H12O6 )n Photosynthesis, therefore is an anabolic process (metabolic building up process)
  4. The energy required is obtained from solar energy through the chlorophyll (the green pigment present in the chloroplast of green plants.
  5. The carbon dioxide and oxygen diffuse in and out (respectively) between the surrounding air and leaf passing through the stomata opening on the leaf surface. These gases are stored in insoluble form until the day is dark. Then, the sugar is converted and transported to other parts of the plants where it is needed. The movement of the sugar is called translocation.
  6. Leaf is the main photosynthetic organ of the plant although all green parts of a plant are photosynthetic. The main site of photosynthesis in a leaf is the pallisade mesophyll.




Photosynthesis involves two stages

  1. Light phase: it takes place during the day. It involves four stages
  • Activation of chlorophyll: the chlorophyll trap light energy from the sun and get energized
  • Photolysis of water: splitting of water molecules by light energy H2O                             H+     +       OH—                                                                                                                              OH                            H2O
  • Hydrogen transfer by NADP (NADP     +     H+        NADPH2)
  • Formation of ATP from ADP: this takes place to store energy for the dark phase reaction
  1. Dark phase: In the dark phase of photosynthesis (occurring in the stroma of chloroplast in the presence of NADPH and ATP), neither light energy nor chlorophyll is needed. This stage takes place in the dark or at night.


During this phase, hydrogen ion and carbon dioxide molecule combine chemically under the control of enzymes to form the simple sugar.

4H+ + CO2                 CH2O   +   H2O

Simple sugar forms the chemical structural basis for other carbohydrates, lipids and proteins



The enzyme-rich product of photosynthesis is used by plants and animals in the following ways:

  1. The source of energy for their metabolic activities
  2. It is needed in food cycle being the only process that can trap sun energy.
  3. All animals and other heterotrophic plants are directly or indirectly dependent on green plants for food.
  4. Photosynthesis assists in the purification of the environment by removing carbon (iv) oxide from it while oxygen is released into it.
  5. Basis for manufacturing other complex organic compounds such as lipids, proteins



The occurrence of photosynthesis in plants can be shown by experiments. Experiment is carried out to show the importance of carbondioxide, sunlight energy, chlorophyll. There is the need to prove that oxygen is given off.


Testing a leaf for starch

The leaf to be tested is detached from the plant and put into boiling water for about 10-15mins. This kills the protoplasm of the leaf and makes iodine to easily penetrate the starch granules.

The boiled leaf is put into alcohol. This is to extract the chlorophyll from the leaf. The leaf becomes white and brittle. It is then dipped into hot water to soften the tissues.

The bleached leaf is spread on a white tile a few drop of iodine is added to it. The colour of the leaf is then tested against a light source.

If the leaf appears blue-black in colour, it contains starch, if the colour is yellowish-brown, it contains no starch


To show that light is necessary for photosynthesis.

Two potted plants are kept in darkness or 48-72 hours to make it starch-free or carry out the above experiment in destarching leaves. One of the potted plants remains in darkness while the other is exposed to sunlight for 4 hours. Leaves from the potted plants exposed to light turns blue-black when tested with iodine while the leaves kept in darkness remains brown. This shows that without light, photosynthesis cannot take place


To show that oxygen is given off as a by-product of photosynthesis

In this experiment, gas is collected in the test tube placed over the green plant and tested with a glowing splint. If it re-kindles the glowing splint, it shows that oxygen is given off because it is the only colourless gas that re-kindles a glowing splint.


Experiment to show that chlorophyll is necessary for photosynthesis

The same experiment in destarching a leaf is carried out here


Experiment to show that carbon (IV) oxide is necessary for photosynthesis

In this experiment, the leaves are destarched. Two potted plants are placed in two bell jars labeled A and B. Each of the potted plants is place on a Vaseline glass plate to prevent air containing carbon (iv) oxide from leaking into the bell jar. A dish containing lime water is placed in one of the jars to absorb carbon (iv) oxide. Both jars are left exposed to sunlight for about 4 hours and tested for starch.

See also  Characteristics of living things



The two major conditions necessary for photosynthesis are

  1. External factors which include light, water, carbon (iv) oxide and mineral salt.
  2. Internal factors include chlorophyll and enzymes.

Temperature is partly external (from sunlight) and partly internal (from the chemical reactions taking place in plant



This is the process by which non-green plants (e.g. Nitrosomonas, Nitrobacter, iron bacteria etc) synthesize organic compound from inorganic materials (carbon dioxide and water). The energy used in this process is got from inorganic substances such as ammonia and by hydrogen sulphite.

H2S     +     O2                                        S        +        H2O      +     chemical energy


H2O    +      CO2                                         CH2O




Mineral elements are required for normal growth and development of green plants. In addition to Carbon, oxygen, and hydrogen, other mineral requirements are discussed below. Unlike glucose and other carbohydrates made from carbondioxide and water by green plants, mineral salts are obtained from the soil.



These are mineral nutrients that are required by plants in large quantities. They include the following;

Macronutrients Importance Deficiency
Nitrogen(N) 1.    Protein synthesis

2.    Nucliec acid synthesis

3.    Proper root development

4.    Component of chlorophyll and enzymes

1.  Stunted growth and chlorosis

2.  Poor flower and fruit formation

Phosphorus(P) 1.    Formation of co-enzymes and proteins

2.    Formation of DNA, RNA and ATP

3.    Stem, root, fruit and seeds formation

1. Poor root development

2. Stunted growth

Sulphur(S) Formation of certain proteins in the protoplasm 1. Stunted growth

2. Chlorosis

Potassium(K) 1. Protein synthesis

2. Cell membrane formation

3. Activates respiration and photosynthesis

4. Crop maturation

1. Premature death

2. Leaf margin turns yellow and brown

Magnesium(Mg) 1. Formation of chlorophyll

2. Helps in cell division

3. Activates many co-enzymes

1. Poor growth

2. Chlorosis

Calcium(Ca) 1. Formation of cell wall

2. Helps in storage of protein and carbohydrates in roots and tubers

3. Gives rigidity to plants

1. Death of terminal buds

2.  Roots develops poorly

3. Stunted growth

Iron(Fe) 1. Formation of chlorophyll

2. Formation of protein

1. Poor growth

2. Yellowing of leaves



These are mineral nutrients required by plants in small quantities. They include the following;

Micro nutrients Importance Deficiency
Zinc(Zn) 1. Necessary for the synthesis of the starting material of auxin

2. Activates some enzymes

3.  Plays a role in the synthesis of protein

1. Poor growth
Copper(Cu) Component of respiratory enzymes


Poor growth
Manganese(Mn) Activates some enzymes involved in cellular respiration Death of shoot
Boron(B) Translocation of sugar and calcium Poor growth of pollen tubes
Chlorine(Cl) Necessary for oxygen release during photosynthesis Affects growth
Molybdenum(Mb) 1. Necessary for nitrogen fixation by nitrogen-fixing bacteria

2. Activates the enzyme that reduces nitrate to nitrite

Poor growth


See also




Scheme of work



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