• Photosynthesis involves a series of chemical reactions, all of which take place inside chloroplasts
  • A general equation for photosynthesis is:

Carbon (IV)Oxide+Water light energy—Glucose+Oxygen chlorophyll

6CO2+6H2O light C6H12O+6O2 chlorophyll

  • The reaction occurs in two main phases or stages
  • The initial state requires light and it is called the light dependent stage or simply light stage
  • It takes place on the lamellae surfaces
  • Its products are used in the dark stage
  • The dark stage does not require light although it occurs in the light and is called light independent stage


  • Two reactions take place that produce raw materials for the dark stage:
  • Light energy splits the water molecules into hydrogen and oxygen
  • This process is called photolysis
  • The hydrogen is taken up by a hydrogen acceptor called Nicotinamide adenine dinucleotide phosphate (NADP) while oxygen is released as a by-product 2H2O(l) light energy4H+O2photolysis
  • Light energy strikes the chlorophyll molecules and sets in motion a series of reactions resulting in the production of a high energy molecule called adenosine triphophate (ATP)

Dark Stage

  • This stage involves the fixation of carbon i.e

the reduction of carbon (IV) oxide by addition of hydrogen to form carbohydrate

  • It uses the products formed during the light stage
  • ATP

Carbon (IV) oxide + Hydrogen — Carbohydrates

  • The synthesis of carbohydrates does not take place in a simple straight line reaction as shown in the equation above
  • It involves a series of steps that constitute what is known as the Calvin cycle
  • Carbon (IV) oxide is taken up by a compound described as a carbon (IV) oxide acceptor
  • This is a 5-carbon compound known as ribulose biphosphate and a six carbon compound is formed which is unstable and splits into two three-carbon compounds
  • Hydrogen from the light reaction is added to the three carbon compound using energy (ATP) from the light reaction
  • The result is a three carbon (triose) sugar, (phosphoglycerate or PGA)
  • This is the first product of photosynthesis
  • Glucose, other sugars as well as starch are made from condensation of the triose sugar molecules
  • The first product is a 3-carbon sugar which condenses to form glucose (6-C sugar)
  • From glucose, sucrose and eventually starch is made
  • Sucrose is the form in which carbohydrate is transported from the leaves to other parts of the plant
  • Starch is the storage product
  • Other substances like oils and proteins are made from sugars
  • This involves incorporation of other elements e.g. nitrogen, phosphorus and sulphur

Factors Influencing Photosynthesis

  • Certain factors must be provided for before photosynthesis can take place
  • The rate or amount of photosynthesis is also influenced by the quantity or quality of these same factors

Carbon(IV) Oxide Concentration

  • Carbon (IV) oxide is one of the raw materials for photosynthesis
  • No starch is formed when leaves are enclosed in an atmosphere without carbon (IV) oxide
  • The concentration of carbon (IV) oxide in the atmosphere remains fairly constant at about 0.03% by volume
  • However, it is possible to vary the carbon (IV) oxide concentration under experimental conditions
  • Increasing the carbon (IV) oxide concentration up to 0.1 % increases the rate of photosynthesis
  • Further increase reduces the rate

Light Intensity

  • Light supplies the energy for photosynthesis
  • Plants kept in the dark do not form starch
  • Generally, increase in light intensity up to a certain optimum, increases the rate of photosynthesis
  • The optimum depends on the habitat of the plant
  • Plants that grow in shady places have a lower optimum than those that grow in sunny places


  • Water is necessary as a raw material for photosynthesis
  • The amount of water available greatly affects the rate of photosynthesis
  • The more water available, the more the photosynthetic rate, hence amount of food made
  • Effect of water on photosynthesis can only be inferred from the yield of crops
  • It is the main determinant of yield (limiting factor in the tropics)


  • The reactions involved in photosynthesis are catalysed by a series of enzymes
  • A suitable temperature is therefore necessary
  • The optimum temperature for photosynthesis in most plants is around 30″C
  • This depends on the natural habitat of the plant
  • Some plants in temperate regions have 20°C as their optimum while others in the tropics have 45°C as their optimum temperature
  • The rate of photosynthesis decreases with a decrease in temperature below the optimum
  • In most plants, photosynthesis stops when temperatures approach O°C although some arctic plant species can photosynthesise at -2°C or even -3°C
  • Likewise, increase in temperature above the optimum decreases the rate and finally the reactions stop at temperatures above 40°c due to enzyme denaturation
  • However, certain algae that live in hot springs e.g. Oscilatoria can photosynthesise at 75°C


  • Chlorophyll traps or harnesses the energy from light
  • Leaves without chlorophyll do not form starch

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