Photosynthesis: Definition, Early Findings, and Pigments

Photosynthesis: Definition

Photosynthesis is the conversion of carbon dioxide and water into carbohydrates by plants with the help of light energy.

The entire reaction of photosynthesis can be expressed in the following manner:

   6CO2 + 6H2O (Sunlight and chlorophyll) ————— C6H12O6 + 6O2

Water is the hydrogen donor in green plants, and it undergoes oxidation to make oxygen.

There are two stages to photosynthesis: light reaction and dark reaction. In light reaction,  light is required for the reaction to take place. In this stage, light energy is captured and used to make the energy-storage molecules ATP and NADPH. The Dark reaction is a reaction that is unaffected by light. To capture and reduce carbon dioxide, a Dark reaction is used. The term “Dark reaction” doesn’t indicate that it happens in the dark.

Photosynthesis is important since

1. It is the principal food source.

2. It releases oxygen into the environment.

Early Findings

1. Jan Ingenhousz: Experiment with aquatic plants in light and dark – He discovered that sunlight is required for plant purification activities.

2. Julius Von Sachs: Plants produce glucose and store it as starch in their green portions.

3. T.W. Engelmann: Spilt light utilising prism into 7 colours (VIBGYOR) – Green Algae Cladophora in a suspension of aerobic bacteria – Bacteria were utilised to detect O2 evolution sites.

 4. Cornelius van Niel: He experimented with purple and green bacteria and established that photosynthesis is a light-dependent process in which CO2 is reduced to carbohydrates using hydrogen from H2O. He concluded that oxygen comes from H2O, not CO2.

Photosynthesis Site

 Leaves, stems, and floral portions that are green (sepal)

In chloroplast, the membrane system is responsible for capturing light energy as well as the creation of ATP and NADPH. The stroma contains enzymes that convert CO2 to carbohydrates (sugars )

Chloroplast – found in mesophyll cells of leaves 

Photosynthesis Pigments

Four different types of pigments can be found in leaves: Chlorophyll A, Chlorophyll B, Xanthophylls and Carotenoids

*Photosystems are pigments that are grouped into two separate photosystems in the thylakoid membrane (PS 1 & PS 11 )

*Each PS has one type of chlorophyll – a, as well as other pigments held together by proteins.

*Chlorophyll creates the reaction centre (where the actual reaction occurs), while other pigments form the light-harvesting complex (LHC) known as antennae.

*PS 1 has a p700 reaction centre (chlorophyll –absorbs light at 700 nm).

*PS 11 has a p680 reaction centre (chlorophyll –absorbs light at 680 nm).


In Plants and Algae, the chloroplast is the cell organelle where photosynthesis takes place. About 10 to 100 chloroplasts can be found in a normal plant cell. A membrane surrounds the chloroplast. An inner, outer, and intermediate membrane make up this membrane. The stroma is an aqueous fluid that exists within the membrane.

 In the stroma, there are stacks of thylakoids. A granum is a type of stack. A thylakoid is a disc that has been flattened. It is held together by a membrane. Within the membrane is the lumen, also known as the thylakoid space. Photosynthesis takes place in the thylakoid membrane. It contains membrane protein complexes that are both integral and peripheral. The membrane also contains pigments that absorb light energy. Photosystems are made up of protein complexes and pigments.

Chlorophyll is the pigment that absorbs the most light. Plants also absorb light energy with the help of carotenes and xanthophylls. Chlorophyll is also used by algae to absorb light.

These pigments are encased in unique antenna proteins found in plants and algae. The colours are arranged in these proteins in such a way that they can operate together perfectly. A light-harvesting complex is another name for this type of protein.

Chloroplasts are found in every cell in a plant’s green sections, although the leaves store the majority of the energy. For every square millimetre of leaf, the mesophyll can contain between 450,000 and 800,000 chloroplasts. The leaf’s surface is consistently coated with a water-resistant waxy cuticle that protects it from excessive water evaporation. It also reduces the absorption of UV or blue light, which helps to keep things cool. The leaf’s epidermal layer is translucent. It permits light to flow through to the palisade mesophyll cells, which are responsible for the majority of photosynthesis.

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