Photosynthesis is the process by which chlorophyll-containing organisms—green plants, algae, and some bacteria—capture energy in the form of light and convert it to chemical energy.

Virtually all the energy available for life in the earth's biosphere—the zone in which life can exist—is made available through photosynthesis.

 

OVERVIEW OF PHOTOSYNTHESIS

Photosynthesis can be represented by the equation:

light
 

CO2 + H2O
 

CARBOHYDRATE + O2
 
 


    

 

 It shows that photoautotrophs can synthesise carbohydrate using carbon dioxide, water and light energy.

 

Photosynthesis consists of two stages:

            -a series of light-dependent reactions that are temperature independent

            -a series of light independent reactions that are temperature-dependent.

 

The rate of the first series, called the light reaction, can be increased by increasing light intensity (within certain limits) but not by increasing temperature. In the second series, called the dark reaction, the rate can be increased by increasing temperature (within certain limits) but not by increasing light intensity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


CHLOROPLASTS: THE SITE OF PHOTOSYNTHESIS

 

Photosynthesis takes place within cells, in organelles called chloroplasts that contain the chlorophylls and other chemicals, especially enzymes, necessary for the various reactions. Chloroplasts are generally 4 to 6 microns in length, 1 to 2 microns in width, and somewhat disc- or ellipse-shaped.  Each chloroplast is surrounded by an envelope composed of two lipoprotein membranes. Inside is an extensive array of green membranes called thylakoids, in a granular fluid known as the stroma.  The thylakoid membranes occur in pairs and enclose a lumen space, forming disks that tend to be stacked into ordered structures called grana.  The grana are connected sporadically by unstacked thylakoids.  This structural complexity plays a role in controlling the interactions between different components.

 

 

 

 

 

 

 

 

 

 

 

LIGHT DEPENDENT REACTIONS

 

The first step in photosynthesis is the absorption of light by pigments.

Chlorophyll is the most important of these because it is essential for the process. It captures light energy in the violet and red portions of the spectrum and transforms it into chemical energy through a series of reactions.

The chemicals involved are organised into units of the chloroplasts called thylakoids, and the pigments are embedded in the thylakoids in subunits called photosystems. Light is absorbed by the pigments, raising their electrons to higher energy levels. The energy is then transferred to a special form of chlorophyll a called a reaction centre.

 

Two photosystems, numbered I and II, are now recognised.

1 Light energy is first trapped by photosystem II, and the energised electrons are boosted to an electron receptor.

            2 They are replaced in photosystem II by electrons from water molecules, and oxygen is released.

3 The energised electrons are passed along an electron transport chain back to photosystem I. Light absorbed by photosystem I is then passed to its reaction centre, and energised electrons are boosted to its electron acceptor.

4 They are passed by means of another transport chain to energise the coenzyme nicotinamide adenine dinucleotide phosphate, or NADP, resulting in its reduction to NADPH.

5 The electrons lost by photosystem I are replaced by those passing along the electron transport chain from photosystem II.

6 ATP is produced using energy from the movement of protons (hydrogen ions) through a chemiosmotic channel

            7 The light reaction ends with the energy yield stored in the ATP and NADPH.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Summary:

 Formation of NADPH (sent to light independent reactions)

 Formation of ATP (sent to light independent reactions)

 Water is broken down

 Oxygen is produced as a by-product

 
           

 

 

 

 

 

 

 

LIGHT INDEPENDENT REACTIONS: CARBON FIXATION

The dark reaction takes place in the stroma (matrix) of the chloroplast, where the energy stored in the ATP and NADPH is used to reduce carbon dioxide to organic carbon. This is accomplished through a series of reactions known as the Calvin cycle, driven by the energy in the ATP and NADPH.

At each turn of the cycle one molecule of carbon dioxide enters and is initially combined with a five-carbon sugar called RuBP (ribulose 1,5-biphosphate) to form two molecules of a three-carbon compound called GP(glycerate -3 phosphate). Three turns of the cycle—each of which consumes one molecule of carbon dioxide, two of NADPH, and three of ATP—produce a three-carbon molecule, G3P(glyceraldehyde 3-phosphate), two molecules of which combine to form a six-carbon sugar, glucose. The RuBP is regenerated with each turn of the cycle.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


ATP and NADPH are used by the light independent reactions to drive a series of reduction reactions to produce carbohydrates.

ADP and NADP go back to the light dependent reactions.

 

Summary:

Formation of carbohydrates

CO2 required

NADP formed (sent to light dependent reactions)

ADP formed (sent to light dependent reactions)

 
 
 

 

 

 

 

 

 

 


Thus, the net effect of photosynthesis is the temporary capture of light energy in the chemical bonds of ATP and NADPH through the light reaction, and the permanent capture of the energy in glucose through the light independent reactions.