Unit 2 Fundamentals of Cop Physiology Part 2 Photosynthesis , Light and Dark Reaction, C3 ,C4 cycle

 Unit - 2.   part 〽️ 2

Fundamentals of crop physiology 

Photosynthesis :- 

 Photosynthesis is a physio – chemical process in which green plants convert high energy to chemical energy (organic Compound sugar). 

The chemical energy stored in the form of sugar, which are created are from water and carbon dioxide. 

 -  Photosynthesis is also used by the algae to convert the solar energy into chemical energy. 

- Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen.

- Leaves contains the microscopic cellular organalls known as chloroplasts. 

- Each chloroplasts contains a green color pigment is called chlorophyll. 

-  light energy is absorbed by chlorophyll molecules whereas carbon dioxide & oxygen enter through the tiny pores of stomata located in the epidermis of the leaves. 

- Another by-product of photosynthesis is sugars such as glucose and fructose. 

Photosynthesis is Occurs in :- 

  Chloroplasts are the sites of photosynthesis in the plants and blue-green algae. 

All green plants of a plants, including the green stems, green leaves and sepals – floral parts comprises of chloroplasts – green color plastids. 

These cell organelles are present only in plant cell and are located within the mesophyll cells of leaves. 

Factors affecting Photosynthesis :- 

Light Intensity 

The concentration of CO2 

Temperature 

Water 

Pollution 

Sites of Photosynthesis :- 

Photosynthesis tales place in special organelles known as chloroplasts. 

This organelle has its own DNA, genes and hence can synthesize its own proteins. 

Chloroplasts are consists of Stroma, fluid and stack of thylakoids known as grana. 

There are three important pigments present in the chloroplast that absorb light energy, chlorophyll a , chlorophyll b , and carotenoids. 

Pigments involved in Photosynthesis :- 

There are four different types of pigments present in leaves. 

Chlorophyll a 

Chlorophyll b 

Xanthophylls 

Carotenoids

Chlorophyll :-

 It is green colored pigment that traps blue and red light. 

Chlorophyll is a lipid soluble pigment. 

It has following important features :- 

The central metal ion in chlorophyll is Mg. 

Chlorophyll is cyclopentanone ring (ring V) fused to pyrrole ring III. 

Ring IV of chlorophyll is esterified to a long chain alcohol.

 

Chlorophyll is subdivided into “chlorophyll a” , “chlorophyll b” and “chlorophyll c”. 

Chlorophyll a is a widely present in all the photosynthetic cells. 

Pigments like chlorophyll b , xanthophylls and carotenoids are called accessory pigments. 

A bacterial variant of chlorophyll known as bacteriochlorophyll cab absorb infrared rays. 

Difference between Chlorphylll a & chlorophyll b :- 

Chlorophyll a

Chlorophyll b 

It is blue – green. 

It is Olive green. 

It is the essential photosynthetic pigment 

It is accessory photosynthetic pigment.  

Pyrrole ring II contain Methyl group 

Pyrrole ring contain II aldehydes groups 

It absorb red wavelength than violet blue wavelength of light. 

It absorb violet blue wavelength than RED wavelength of light.

It is C55H72O5N4Mg 

It is C55H70O6N4Mg 

Carotenoids :- 

these are yellow, orange or red-coloured pigments that absorb bluish – green light, xanthophyll and carotenes are examples of carotenoids. 

Functions of Carotenoids :- 

It act as accessory light harvesting pigment. 

It capture the light energy and feed it to the photochemical reaction systems. 

It acts as quencher as well as scavenger. 

Stages of Photosynthesis :- 

Light dependent reaction or light reactions 

Light independent reaction or Dark Reactions

Light reaction of Photosynthesis :- 

The light reaction, Grana trap the light energy and synthesis ATP and NADH. This set of reaction called light reactions. 

Photosynthesis begins with the light reactions which is carried out only during the day in the presence of sunlight. 

In plants, light dependent reactions takes place in the thylakoids membranes of chloroplasts. 

The Grana, membrane bound sac like structures present inside the thylakoids functions by gathering light and is called photosystems. 

The chemical equation in the light reaction of photosynthesis can be reduced to :- 

2H2O + 2NADP+ + 3ADP + 3Pi ( O2 + 2NADPH + 3 ATP 

Photophosphorylation:- 

  The process of forming ATP from ADP and inorganic phosphate by utilizing light energy is called photophosphorylation. 

In the light reaction start with the electron flow. 

The flow of electrons is of two types :- Cyclic and Non Cyclic photophosphorylation. 

Cyclic Photophosphorylation 

Non Cyclic Photophosphorylation 

Only photosystem I is involved 

Both photosystem I and II are involved 

P700 is the active reaction center. 

P0680 is the active reaction center

Electrons travel in cyclic manners 

Electrons travel in non – cyclic manner 

ATP molecules are produces 

Both NADPH and ATP molecules are produced. 

Water is not required 

Photolysis of water is present 

NADPH is not synthesized 

NADPH is synthesized 

Oxygen is not involve the by – product 

Oxygen is envolved as a by- product 

This process is predominant only in bacteria

This process is predominant in all green plants. 

Dark Reaction of Photosynthesis or Light independent reaction :- 

   Dark reaction also known as carbon – fixing reaction. 

It is a light independent process in which sugar molecules are formed the water and carbon dioxide molecules. 

The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reactions. 

Plants capture carbon dioxide from the atmosphere through stomata and processed to the calvin photosynthesis cycle. 

In the Calvin Cycle , the ATP And NADPH during light reactions drive the reaction and convert 6 molecules of CO2 into one sugar molecule or glucose. 

The chemical equation for the dark reaction can be reduced to : 

3CO2 + 6NADPH + 5H2O + 9ATP ( G3P + 2H + 2NADP+ +9ADP + 8Pi 

** G3P ( glyceraldehyde 3-phosphate 

 Pi ( Inorganic Phosphate 

Calvin Cycle :- 

 Calvin cycle is set off reaction performed by plants to reduce CO2 and other compounds into glucose. 

C3 pathway :- 

the majority of the plants produce 3-carbon acid is called 3-phosphoglyceric acid (PGA) as a first product during carbon dioxide fixation. Such pathway is known as C3 pathway which is also called Calvin cycle. 

Calvin cycle include the following steps :- 

Carbon Fixation 

Reduction 

Regeneration 

In the first step Calvin cycle , light independent reactions are initiated and carbon dioxide is fixed. 

In the second stage of C3 cycle , ATP and NADPH reduce 3PGA to G3P. ATP and NADPH are then converted into ATP and NADP+ 

In the  last stage, Ribulose 1,5 – bisphosphate (RuBP ) is regenerated. 

C4 cycle Pathway / Hatch and Slack Pathway :- 

  Every photosynthesis plant follows calvin Cycle, but some plants, there is primary stages to the Calvin cycle known as C4 pathway. 

Plant in tropical desert regions commonly follows the C4 pathways. 

Here, 4 carbon compound called oxaloacetic acid (OAA) is the first product by carbon fixation. Such plants are special and have certain adaptation as well.  

The C4 pathway initiates with a molecule called phosphoenolpyruvate (PEP) which is a 3- carbon molecules. 

This is the primary CO2 acceptor and the carboxylation takes place with the help of an enzyme called PEP carboxylase. They yield a 4-carbon molecule called (OAA) oxaloacetic acid. 

Eventually, it converted into another 4 – carbon compound called malic acid. Later, they are transferred from mesophyll cells to bundle sheath. Here , OAA is broken down into yield carbon dioxide and a 3-carbon molecule.

Corn, sugarcane and some shrubs are example of C4 pathway. 

Calvin pathway is a common pathway in both in C3 and C4 plants, but it takes only place in the mesophyll of the C3 plants but not in the C4 plants.