Photosynthesis In Higher Plants Class 11 Notes Biology Chapter 13 - CBSE

Chapter : 13

What Are Photosynthesis In Higher Plants ?

Photosynthesis

Chemiosmotic Hypothesis Of ATP Formation

Photosynthesis is an enzyme regulated anabolic process of producing organic compounds in presence of chlorophyll from carbon dioxide and water with sunlight as a source of energy

$$6\text{CO}_{2} + 12\text{H}_{2}\text{O}\xrightarrow[\text{Chlorophyll + Enzymes}]{\text{Sunlight}}\\\text{C}_{6}\text{H}_{12}\text{O}_{6} + 6\text{H}_{2}\text{O} +6\text{O}_{2}\uarr$$

Proposed by Mitchell in 1961. ATP synthesis is linked to development of proton gradient across the membrane of thylakoid and mitochondria.

Calvin Cycle/c3 Cycle/reductive Pentose Sugar Phosphate Pathway

Malvin Calvin, Benson and their colleagues used radioactive 14C and Chlorealla and Scenedesmus algae to discover that first CO₂ fixation product is 3-carbon organic compound (3- hosphoglyceric acid) or PGA. Later on a new compound was discovered which contain 4-carbon called Oxaloacetic Acid (AAO). On the basis of number of carbon atoms in first stable product they are named C₃ and C₄ pathway. Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.

Photorespiration

It is the light dependent process of oxygenation of RuBP and release of carbon dioxide by photosynthetic organs of plants.
Photorespiration decreases the rate of photosynthesis when oxygen concentration is increased from 2-3% to 21%.
Presence of light and higher concentration of Oxygen results in the binding of RuBisCO enzyme with O₂ to form phosphoglyceric acid and phosphoglycolate.

$$\text{RuBisCO + O}_{2}\xrightarrow{}\\\text{PGA + phosphoglycolate}$$

This pathway involves Chloroplast, Peroxisome and Mitochondria. Photorespiration does not occur in C₄ plants.

Difference Between C₃ Plants And C₄ Plants

C₃ Plants

The leaves do not have Kranz anatomy.
Photorespiration occurs.
RuBisCO is the first acceptor of CO₂.
PGA is the first stable product.
Plants are adapted to all climates.
Mesophyll cells perform complete photosynthesis.

C₄ Plants

The leaves show Kranz anatomy in leaves.
Photorespiration does not occur.
PEP is the first acceptor of CO₂.
OAA is the first stable product.
Plants are adapted to tropical climate.
Mesophyll cells perform only initial fixation.

Factors affecting Photosynthesis

Light
Carbon dioxide
Temperature
Water

Blackman’s Law Of Limiting Factors

If a 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.

Chloroplast

Membranous System

Grana lamellae
Stroma lamelle

Fluid stroma

Pigments Involved In Photosynthesis

Chlorophylls

Green colored pigment Light is required for their synthesis Soluble in organic solvents

Carotenoid

Yellow to yellow orange colour pigments occur alone inside chromoplast and occur alongwith
chlorophylls inside chloroplast.

Phycobilins

They are hot water soluble pigment.
They lack Mg and phytol tail.

Cyclic Photophosphorylation

Difference Between Cyclic And Non-cyclic Photophosphorylation

It is performed by photosystem I independently.
An external source of electron is not required.
It synthesizes only ATP.
It occurs only in stromal or intergranal thylakoids.

It is performed by collaboration of both PS I and PS II.
The process requires an external electron donor.
It synthesizes ATP and NADH both.
It occurs in the granal thylakoids only.