Site for Photosynthesis in Plants

In this site for photosynthesis in plants post we have briefly explained about definition, significance, and site of photosynthesis in green plants.

The plants get energy from sun by converting solar or radiant energy into chemical energy by the process of Photosynthesis in green plants, which acts as a driving force for both biotic and abiotic world.

Photosynthesis in green plants produces 1700 million tonnes of dry matter per year by fixing 75 × 1012 Kg of carbon every year. Photosynthetic organisms use only 0.2% of incident solar light on earth. Carbohydrates produced by photosynthesis in green plants are the basic raw material for respiration and also to produce many organic compounds.

It maintains atmospheric oxygen and carbon dioxide level. Photosynthesis in green plants consumes atmospheric carbon dioxide which is continuously added by the respiration of organisms. Photosynthesis in green plants is the major endergonic reaction. In this chapter, we will study about the energy yielding process of photosynthesis and various types of energy utilization processes to produce carbohydrates.

Site for Photosynthesis in In Green Plants


Photosynthesis in green plants is referred as photochemical oxidation and reduction reactions carried out with help of light, converting solar energy into Chemical energy. It is the most important anabolic process.

Plants and photosynthetic bacteria use simple raw materials like carbon dioxide water and with the help of light energy synthesize carbohydrates and evolve oxygen. The overall chemical equation for photosynthesis is:

Ruben and Kamen (1941) demonstrated six molecules of water as insufficient for the evolution of 6 molecules of O2 and modified the equation as:

Light Chlorophyll Photosynthesis in green plants is a collection of oxidation and reduction reactions (Redox reaction). Oxidation- Water is oxidised into oxygen (loss of electrons). Reduction–CO2 is reduced into Carbohydrates (gain of electrons).

In some bacteria, oxygen is not evolved and is called as non-oxygenic and anaerobic photosynthesis in green plants. Examples: Green sulphur, Purple sulphur and green filamentous bacteria.


Photosynthetic organisms provide food for all living organisms on earth either directly or indirectly. It is the only natural process that liberates oxygen in the atmosphere and balances the oxygen level.

The balance of oxygen and carbon dioxide is maintained in the atmosphere by the oxygen released by plants during photosynthesis in green plants and carbon dioxide released by human, animals etc. in the atmosphere.

Fuels such as coal, petroleum and other fossil fuels are from preserved photosynthetic plants. Photosynthetic organisms are the primary producers on which all consumers depend for energy. Plants provide fodder, fibre, fire wood, timber, useful medicinal products and these sources come by the act of photosynthesis in green plants.

Site of Photosynthesis


(a) 3D view of chloroplast (b) Sectional view of chloroplast, Image Source:


Chloroplasts are the main site of photosynthesis in green plants and both energy yielding process (Light reaction) and fixation of carbon dioxide (Dark reaction) that takes place in chloroplast.

It is a double wall membrane bounded organelle, discoid or lens shaped, 4–10 μm in diameter and 1–33 μm in thickness.

The membrane is a unit membrane and space between them is 100 to 200 Å. A colloidal and proteinaceous matrix called stroma is present inside.


A sac like membranous system called thylakoid or lamellae is present in stroma and they are arranged one above the other forming a stack of coin like structure called granum (plural grana).

Each chloroplast contains 40 to 80 grana and each granum consists of 5 to 30 thylakoids. Thylakoids found in granum are called grana lamellae and in stroma are called stroma lamellae.

Thylakoid disc size is 0.25 to 0.8 micron in diameter. A thinner lamella called Fret membrane connects grana. Pigment system I is located on outer thylakoid membrane facing stroma and Pigment system II is located on inner membrane facing lumen of thylakoid.

Grana lamellae have both PS I and PS II whereas stroma lamellae have only PS  I. Chloroplast contains 30–35% Proteins, 20–30% phospholipids, 5–10% chlorophyll, 4–5% Carotenoids, 70S ribosomes, circular DNA and starch grains.

Inner surface of lamellar membrane consists of small spherical structure called as Quantasomes. Presence of 70S ribosome and DNA gives them status of semi-autonomy and proves endosymbiotic hypothesis which says chloroplast evolved from bacteria.

Thylakoid contains pigment systems which produces ATP and NADPH using solar energy. Stroma contains enzyme which reduces carbondioxide into carbohydrates. In Cyanobacteria thylakoid lies freely in cytoplasm without envelope.

Further Readings