Photosynthesis
* Photosynthesis is a physiochemical process in which light is used to prepare food.
* Cornelius Van Niel found that photosynthesis is a light-dependent reaction in which a hydrogen atom reduces carbon dioxide to form carbohydrates.
* He also inferred that water is the hydrogen donor which is oxidised O2.
* Chloroplast is found in the mesophyll cells of leaves.
* The four pigments involved in photosynthesis are chlorophyll a, chlorophyll b, carotenoids and xanthophylls.
Light Reaction
* Light reaction are the first stage of photosynthesis. It is the photochemical phase of photosynthesis.
* Light reaction takes place in the thylakoid membranes inside the chloroplast. It includes photosynthetic pigments that are arranged into light-harvesting complexes within photosystem I and photosystem II.
* Light reaction consists of the following four stages-light absorption, water splitting, release of oxygen and formation of high energy intermediates ATP and NADPH.
* Water splitting contributes electrons that replace the electrons in PS II and protons that help in ATP synthesis.
* The process of synthesising high-energy compounds like ATP by cells is called phosphorylation.
* The formation of ATP requires a membrane, a proton pump, a proton gradient and an enzyme, ATPase.
The Calvin cycle
* Light reaction results in ATP, NADPH and oxygen.
* The ATP and NADPH are utilised in the synthesis of carbohydrate in the presence of carbon dioide and water.
* The three stages of Calvin Cycle are carboxylation, reduction and the chloroplast.
* The Calvin Cycle or the dark reaction takes place in the stroma of the chloroplast.
* The formation of a molecule of glucose through Calvin Cycle requires 18 ATP molecules and 12 NADPH molecules.
The C4 Pathway
* In C4 plants, the first product of carbon fixation is a 4 carbon compound. Hence it is called the C4 Pathway.
* The transverse section of C4 leaves shows a specialised arrangement of cells around the vascular bundles.
* The anatomy of a leaf in which the vascular bundles are encircled by two layers of cells is called Kranz anatomy.
* In the C4 Pathway, carbon dioxide is fixed in the mesophyll cells.
* A molecule of carbon dioxide, to be fixe through the C4 Cycle, requires 30 ATP molecules.
Photorespiration
* During photorespiration, RUBP combines with oxygen to from one molecule of phosphoglycerate and one molecule of phosphoglycolate in the presence of the enzyme RuBP oxygenase.
* Photorespiration lowers the efficiency of photosynthesis in plants.
* During photorespiration, there is a loss of carbon in plants and the process also utilises energy in the form of ATP.
* Photorespiration has a pronounced effect in C3 plants during hot dry conditions.
* Photorespiration is a wasteful process in C3 plants.
* C4 plants bypass the photorespiratory pathway that results in higher crop yields.
Factors Affecting Photosynthesis
* Photosynthesis is influenced by several internal and external factors.
* Internal factors include:
• Number, size, age and orientation of leaves.
• Mesophyll cells and chloroplasts
• Internal carbon dioxide concentration
• Amount of chlorophyll
* External factors include:
• Availability of sunlight
• Light intensity
• Incident light
• Temperature
• Atmospheric carbon dioxide concentration
• Water
* Blackman’s Law of Limiting Factors state that “If chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed.”
* At low light intensity, incident light and the rate of photosynthesis are linearly related.
* When C4 plants are subjected to higher temperatures, they register a greater increase in the rate of photosynthesis than C3 plants.
* Water stress caused by lack of sufficient water forces the stomata to close, which automatically means less carbon dioxide for plants.