O Level Revision : Combined Science - Plant Nutrition

The sun is the ultimate source of energy for all living things. Green plants can use energy from the sun to make food in a process called photosynthesis. All other organisms directly or indirectly depend on the food made by plants. Photosynthesis produces carbohydrates and oxygen. Plants require other nutrients such as proteins, fats and mineral salts. Plants are affected by pests and diseases. Pests and diseases can be controlled using chemical methods and cultural methods.


Photosynthesis is the manufacture of food by green plants, using carbon dioxide, water and sunlight energy.

Word equation for photosynthesis


Carbon dioxide + water  →  carbohydrate + oxygen


Factors affecting photosynthesis

Four factors affect the process of photosynthesis. These are:

  • Carbon dioxide
  • Light
  • Chlorophyll
  • Water

Points to note about experiments on the factors affecting photosynthesis

  • Presence of starch in leaves is evidence that photosynthesis has occurred.
  • Leaves must be de-starched before setting up experiments. De-starching is allowing the plant to use up all the starch it has produced while it has been placed in conditions where it can produce more starch.
  • De-starching is achieved by placing plant in darkness for at least 24 hours.
  • In these experiments, a control has all conditions needed for photosynthesis. The purpose of the control is to enable comparison with the actual experiments.

 Testing for starch in a leaf


  • boiling ethanol hot water
  • leaf
  • (folded up)

The leaf is boiled in alcohol. Ethanol is an alcohol.   Methylated s pirit  is   alcohol   that has been mixed with benzoate.




1. Boil leaf in water

- to kill leaf/stop chemical reactions

- leaf becomes soft

2. Boil leaf in alcohol/ethanol

/methylated spirit

- to decolourise the leaf/remove chlorophyll (chlorophyll masks colour changes of iodine later)

- ethanol changes from colourless to green

- leaf becomes brittle

3. Dip leaf in hot water

- to soften the leaf

- leaf becomes soft

4. Add a few drops of iodine solution

- to test for starch

- leaf turns blue black if starch is present

- colour of iodine does not change/remains brown if starch is not present

Alcohol is heated using a water bath and not heated directly because it is inflammable.

Carbon dioxide is necessary for photosynthesis

Carbon dioxide is necessary for photosynthesis. Comparing observations between control experiment and actual experiment shows that carbon dioxide is needed for photosynthesis to take place.

 Light is necessary for photosynthesis


  • The part of leaf covered with opaque material (paper/foil) is the actual experiment.
  • The part of leaf exposed to light is the control experiment.


The uncovered part turns blue-black when tested for starch while the covered part remains brown.


Light is necessary for photosynthesis.

Chlorophyll is necessary for photosynthesis

  1. a) A variegated leaf is used  b)         After testing for starch


When tested for starch green parts turn blue-black while non-green parts remain brown.


Chlorophyll is necessary for photosynthesis.

End products of photosynthesis are:

  1. Oxygen (gas)                       2.   Carbohydrates

Identification of end products of photosynthesis

  • Carbohydrates are identified using the starch test in a leaf outlined in previous sections.
  • Oxygen is identified using the following set up:

 Oxygen is released during photosynthesis


  • Light
  • Bubbles of gas collected
  • Inverted test tube
  • Beaker
  • Inverted filter funnel
  • Aquatic plant
  • Water containing bicarbonate of soda to provide more carbon dioxide
  • Support to keep funnel off bottom


  • Bubbles rise from the water weed.
  • Gas collects in the test-tube.
  • The gas relights/rekindles a glowing splint.


Oxygen is released during photosynthesis.

Fate of the end products of photosynthesis

(i)   Oxygen diffuses out of the leaf into the environment with some being used during respiration. (ii)  Carbohydrates may be:

  1. a) translocated → glucose transported to other parts of the plant.
  2. b) stored → some glucose converted to insoluble starch and stored in leaves or special

storage structures e.g. fruits, seeds, roots or tubers.

  1. c) made into structures → glucose used to make cellulose which forms cell walls.

Transverse section of a leaf

Parts of a leaf and their functions






Outer cover

Reduces water loss

Lower epidermis

Lower under cover

Has stomata for gaseous exchange

Palisade layer

Tightly packed

Most photosynthesis occurs here

Spongy layer

Has many air spaces

Allows movement of gases into and out of leaf

Mesophyll layer

Spongy layer + palisade layer

Main food making tissue of plants


Small openings mainly on lower epidermis

Allow gaseous exchange (oxygen out and carbon dioxide in)

Vascular bundle

Xylem and phloem vessels

Xylem vessels transport water and salts

Phloem vessels transport food


Contain chlorophyll

Site of photosynthesis

Adaptations of leaves for photosynthesis

(i)   Surface area: large surface area for absorption of light and carbon dioxide to enable  more photosynthesis.

(ii)  Cross section of leaf: thin to enable light through, with short distance for carbon dioxide to diffuse to enable more photosynthesis.

(iii) Air spaces: to enable easy passage of carbon dioxide.

(iv) Presence of stomata: allow for exchange of carbon dioxide, water vapour and oxygen between leaf and atmosphere.

(v)  Veins: to carry water to and food away from leaves.

Mineral nutrition

  • Besides water from soil and carbon dioxide from atmosphere, plants also need mineral elements from the soil.
  • Nitrogen, phosphorous and potassium (major elements) are needed in larger quantities.
  • Iron, calcium, magnesium, boron, sulphur, molybdenum and zinc (trace elements) are needed in smaller quantities.

To show the effects of lack of nitrogen, phosphorous and potassium on plant growth

Type of culture solution

Observation (deficiency symptom)

1.   Complete culture solution

Normal growth

2.   Minus nitrogen

Stunted with chlorosis

3.   Minus potassium

Yellow/brown leaf margins, death

4.   Minus phosphorous

Stunted roots with purple leaf colouration



Nitrogen, phosphorous and potassium are necessary for normal plant growth.

  • Nitrogen is used for protein synthesis (growth).
  • Phosphorous is used for root growth and producing energy carrier molecules.
  • Potassium is used for good flower and fruit formation and maintaining osmotic balance.

Plant pests and diseases

Definition:   A pest is any organism that is harmful to human activities.

Major types of pests (i)   Tissue eating pests (ii)  Sap sucking pests (iii) Boring pests

Tissue eating pests

  • Eat parts of plants e.g. leaves, stems, roots.
  • Have mouthparts (jaws) that cut and chew tissues of plants e.g. locusts, cutworms, crickets , etc.
  • Signs of damage on plants include torn leaves, holes and gaps on plant organs.

Sap sucking pests

  • Suck plant juices.
  • Have needle-shaped mouthparts used to pierce to phloem vessels and suck juices e.g. aphids, red spider mite, leaf hoppers and stink bugs.
  • Signs of damage include spots on leaves (red spider mite), distorted leaves (aphids).


Boring pests

  • Bore or tunnel into plant tissues e.g. stems, fruits and seeds.
  • Have snouted (sharp pointed) mouth parts that bore or tunnel into plant tissues e.g. weevils, maize stalk borer, ball worms, grain borers etc.
  • Signs of damage include marked holes on fruits, stems and seeds.

Major plant diseases

  1. Bacterial wilt
  • Caused by bacteria (microscopic micro-organisms) which accumulate and block xylem vessels preventing water from reaching plant parts hence wilting occurs e.g tomatoes, potatoes, tobacco, sunflower etc.
  • Is prevalent in waterlogged soils.
  • Symptoms include blackened vascular bundles.

Control involves improving drainage of soils and using good quality seeds.

  1. Fungal rust
  • Caused by fungi (microscopic organisms) living in stems and leaves of plants e.g. maize, beans, sunflower etc.
  • Symptoms include leaves coated with light powder, rusty red spots on leaves.

Control of plant pests and diseases

  1. Cultural control

Uses farm operations that make it difficult for pests to establish themselves such as

  • early planting
  • weed removal
  • crop rotation
  • burning/burying crop residues

American bollworm on cotton can be controlled by any of the following methods:

  • spraying wood ash on plants
  • spraying liquid soap solution
  • using hens and bantams as predators
  • rotating cotton with maize


Advantages of cultural control methods

  • cheap  •   safe to use

Disadvantages of cultural control methods

  • slow  •   require constant repetition

Chemical control

  • Chemicals are purchased and applied to kill pests.
  • Chemicals used to kill pests are called pesticides.
  • Chemicals used to kill fungi are called fungicides.
  • Chemicals used to kill weeds are called herbicides.

Advantages of chemical control methods

  • quick  •   specific


Disadvantages of chemical control methods

  • expensive
  • affects other animals in the food chain
  • pests develop resistance with time

Safe use of chemicals

Involves taking safety precautions such as:

  • reading and strictly following instructions on their use
  • wearing protective clothing
  • practicing good hygiene
  • proper storage of chemicals
  • safe disposal of empty containers