PHOTOSYNTHESIS

Photosynthesis

 

Photosynthesis:   Photosynthesis can be defined as the synthesis of     carbohydrates (glucose) from inorganic materials like   CO₂   and H₂O with the help of solar energy trapped by pigments like chlorophyll is called photosynthesis.

 

Chloroplast: The site, where photosynthesis occurs, is called a chloroplast.

Photons: The small discrete energy packets of light are called photons.

 

Thylakoids: The individual sac-like structure of grana is called thylakoids.

 

Chromatography: The technique of separation of plant pigments using mobile and stationary phase as per their molecular weight is called chromatography.

 

Stomata: The small opening present on the surface of the leaf is called stomata.

 

Quantasome: The unit of photosynthesis is called quantasome. It is found in the thylakoid membrane of the chloroplast.

 

Photolysis of water: The splitting of H₂O in presence of sunlight into the hydrogen and oxygen is called photolysis of water.

 

Quantum energy: The minimum amount of energy required to boost the electron to the higher energy level is called quantum energy.

 

Photo-oxidation: It is the degradation of a protein surface in presence of O₂ or O_3. This effect is facilitated by radiant energy such as UV or artificial light. It is the most significant factor in the degradation of chlorophyll-a molecule.

 

Chlorophyll: It is a green photosynthetic pigment found in plants, algae and cyanobacteria. Chlorophyll mostly absorbs blue light and to a lesser extent red light of electromagnetic spectrum.

 

Chlorophyll-b: It has an aldehyde group (-CHO). It is also a major photosynthetic

 pigment. It is yellow-green in colour. Its molecular formula is C₅₅H₇₀O₆N₄Mg.

 

Chlorophyll -a:   It has a methyl group (-CH₃). It is a major photosynthetic pigment. It is bright or blue-green in colour, it’s molecular formula is C₅₅H₇₂O₅N₄Mg .

 

Carotenoids: These are the coloured accessory pigments which not only help in absorption of light but also impart colour to the various parts of the plants. Carotene C₄₀H₅₆  and Xanthophyll C₄₀H₅₆O₂  are types of carotenoids.

 

Absorption spectrum:   A curve which shows the amount of light absorbed at each wavelength is termed as Absorption spectrum.

 

Action Spectrum: The curve that shows the rate of photosynthesis at different wavelength is called Action spectrum.

 

Photophosphorylation: The process of formation of ATP from ADP and iP (inorganic phosphate) in presence of sunlight is called photophosphorylation.

 

Light reaction:    It is the first phase of photosynthesis in which solar energy is trapped by chlorophyll and converted into chemical energy in the form of ATP and the reducing power  NADPH_.

 

Reaction centre or Core complex:   The site in the chloroplast that receives the light energy trapped by chlorophyll and other accessory pigments and initiates the electron transfer process is called reaction centre.

 

Light harvesting or antenna molecule: The accessory pigments and other chlorophyll molecules    harvest solar energy and pass it to the reaction centre i.e. chlorophyll-a are called Light-harvesting complex or antenna molecule.

 

Photosystem or Pigment system: The core complex (chlorophyll-a) along with light-harvesting complex (all accessory pigments) together called photosystem or pigment system.

 

Assimilatory power: Chemical energy generated as a result of a light reaction in the form of ATP and NADPH (reducing power) to obtain food in the form of carbohydrates is called assimilatory power.

 

Chemiosmosis: the process of formation of ATP due to the movement of ions (H⁺ ions) down their electrochemical gradients across the semipermeable plasma membrane is called chemiosmosis.

 

Cyclic Photophosphorylation: The formation of ATP during the cyclic transfer of electron is called cyclic photophosphorylation. It involves PS-I only.

 

Non-Cyclic Photophosphorylation: The formation of ATP during the non-cyclic transfer of electron is called non-cyclic photophosphorylation. It involves both PS-I and PS-II. This process leads to photolysis of water along with the formation of ATP, NADPH.

 

Dark reaction:     It is the second phase of photosynthesis in which the fixation of CO₂ occur to form the food i.e. carbohydrate (Glucose). To Synthesize 1 molecule of Glucose, a total of 6 Calvin cycle, 18 ATP and 12 NADPH₂ are used.

 

Photorespiration: It is the wasteful process in which the loss of carbon dioxide occurs during day time is called Photorespiration. It generally occurs under the conditions like high temperature, bright light, high oxygen and low CO₂ concentration. It involves three organelles chloroplast, peroxisomes and mitochondria. It is also called PCO (photosynthetic carbon oxidation) cycle.

 

C₄ Pathway / Hatch-Slack Pathway: It is an alternate pathway for CO₂ fixation found

in some tropical and sub-tropical grasses and some dicotyledonous plants. C₄-pathway

can avoid photorespiration. In this pathway, the first product of CO₂ fixation is a

4C-compound, oxaloacetic acid. Hence such plants are called C₄ plants.

 

Kranz anatomy:  The specialized structure in C₄-plants where the mesophyll cells are clustered around the bundle sheath cells in a ring-like fashion. The number of chloroplast in bundle sheath cells are more than that in the mesophyll cells.

 

CAM – Crassulacean Acid Metabolism: It is an alternative pathway of CO₂ fixation found in plants living in dry and hot climates e.g. cacti. In CAM plants, stomata are scotoactive. (Active during the night). CAM plants fix CO₂ via C₄ pathway during the night and reduce CO₂ during day time via C₃ pathway.

 

Acidification: Formation of malic acid during the dark reaction is called acidification.

 

Deacidification:  In a CAM plant, malate is formed and stored in vacuole during the night. During day time, malate undergoes decarboxylation and releases CO₂ for C₃ cycle. This is called a deacidification.