Main Organ for Photosynthesis

2.4 Main Organ for Photosynthesis

Photosynthesis is needed by plants because:

  • The photosynthetic product is glucose,the primary source for cellular transpiration used to synthesise energy
  • The glucose produced can be used to from cellulose
  • The glucose produced can be converted into sucrose and transported to other parts of plant for metabolism or storage in the form of starch
  • Glucose can be converted into amino acids,proteins or fats for metabolism or storage
Chemical reaction of overall photosynthesis process

\(12\text{H}_2\text{O}+6\text{CO}_2\xrightarrow[\text{chlorophyll}]{\text{light energy}}\text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2+6\text{H}_2\text{O}\)\(12\text{H}_2\text{O}+6\text{CO}_2\xrightarrow[\text{chlorophyll}]{\text{light energy}}\text{C}_6\text{H}_{12}\text{O}_6+6\text{O}_2+6\text{H}_2\text{O}\)


Main organ in photosynthesis by having some adaptations:

Leaf shape
  • Broad-increases surface area to absorb more sunlight
  • Thin-allow penetration of sunlight to reach lower layer of cells in leaf
  • Transparent-allow sunlight to penetrate into leaf
  • Cuticle-to reduce water loss from leaf
Palisade mesophyll
  • Cells which have abundant of chloroplasts-can carry out photosynthesis
  • Cylindrical shape of cells-more cells can be arranged compactly
  • Cells arranged compactly-absorb sunlight maximally for photosynthesis
Spongy mesophyll
  • Cells which have chloroplasts-can carry out photosynthesis
  • Irregular shaped of cells and loosely arranged-to form air spaces for transpiration and gaseous exchange
  • Transport water and mineral salts from roots to mesophyll for photosynthesis
  • Transport organic products from leaves to other parts of plants
  • Pore that formed from two guard cells
  • For gaseous exchange and transpiration
Structure of chloroplast:-
  • Photosynthesis takes place in chloroplasts
  • Photosynthesis consists of light-dependent reactions and light independent reactions

The light-dependent reactions take place in thylakoid:

  • In light-dependent reactions,chlorophyll absorbs light energy to becomes active and then releases electrons
  • At the same time,photolysis of water occurs in which a water molecule is splitted into hydrogen ion,oxygen and molecules
  • The oxygen gas produced is released into atmosphere
  • The electron released from water molecule will replace the electrons released from the chlorophyll
  • The electrons flow from chlorophyll eventually received by hydrogen ions and NADP to form hydrogen atoms and NADPH
  • During the flow of electrons from chlorophyll to hydrogen ions,some energy released from the electrons is used to synthesis ATP
  • The hydrogen atoms,NADPH and ATP produced from the light-dependent reactions will be used by the light independent reaction in stroma

The light independent reactions occur in stroma and do not required sunlight:

  • Carbon dioxide gas diffuses into chloroplast 
  • The hydrogen atom , NADPH and ATP produced from the light-dependent reactions are used to reduce the carbon dioxide to produce glucose and water. 
  • The reduction process requires enzymes found in the stoma. 
  • The glucose produced form starch by condensation.

The simillarities between the light-dependent and light independent reactions:

  • Occurs in daytime
  • Occurs in chloroplasts
  • Involves chemical reactions with enzymes

Perbezaan antara tindak balas bersandarkan cahaya dan tindak balas tidak bersandarkan cahaya:

Light-dependent reaction  Light-independent reaction
Require light energy  Do not require light energy 
Occurs in granum and thylakoid  Occurs in stroma
Photolysis of water takes place Photolysis of water does not take place 
Produce ATP and NADPH Do not produce ATP and NADPH
Produce oxygen and water molecule Produce glucose
Factors affecting the rate of photosynthesis

Carbon dioxide concentration:

  1. The increase in carbon dioxide concentration increases the photosynthesis rate as long as there are no other limiting factors such as surrounding temperature and light intensity.
  2.  At P, photosynthesis rate is constant.
  3. As the concentration of carbon dioxide increases after P, the rate of photosynthesis remains unchanged.
  4. This is due to light intensity becoming the limiting factor.

Light intensity:

  1. Light is needed in the light-dependent reaction.
  2. If the concentration of carbon dioxide and temperature are constant, the rate of photosynthesis increases until it reaches its maximum point at noon.
  3. Graph I shows that the rate of photosynthesis increases with the increase of light intensity until it reaches a light saturation point at P.
  4. After point P, the increase in light intensity (from P to Q) is no longer increases the rate of photosynthesis because it is limited by other factors such as temperature and carbon dioxide concentration.
  5. Graph II shows when the concentration of carbon dioxide in the environment is increased to 0.13%, the rate of photosynthesis also increases.


  1. The reactions in photosynthesis are catalysed by enzymes.
  2. Therefore, changes of surrounding temperature will affect enzyme activity and also the rate of photosynthesis.
  3. The optimum temperature is different for different plant species but in general, the optimum temperature is between 25°C to 30°C.
  4. A very high temperature denatures the enzymes and the process of photosynthesis is stopped.
The effect of different light intensities and light colours on the rate of photosynthesis
  1. The rate of photosynthesis in plants is different throughout the day.
  2. Other than the light intensity factor, the rate of photosynthesis is also affected by the colour of light.
  3. Light spectrum consists of seven colours in a certain sequence (violet, indigo, blue, green, yellow, orange and red).
  4. Each colour has a different wavelength.
  5. The rate of photosynthesis is the highest in red and blue light.
  6. All of the red light is absorbed by chlorophyll.
  7. The blue light is absorbed by carotenoid pigments before being transferred to the chlorophyll.
  8. These two lights have enough amount of energy to excite electrons in the light-dependent reaction.