Photosynthesis: An overview

Photosynthesis

Photosynthesis is the process by which green plants and certain other organisms like algae and cyanobacteria transform light energy into chemical energy. The basic process occurring here is capturing of light energy of sun and converting it into chemical energy; such as sugar.

Types of photosynthesis:

There are two types of photosynthetic process:

1.   Oxygenic photosynthesis:

2.   Anoxygenic photosynthesis:

The general principles of anoxygenic and oxygenic photosynthesis are very similar, but oxygenic photosynthesis is the most common and is seen in plants, algae and cyanobacteria.

1.Oxygenic photosynthesis:  light energy transfers electrons from water (H₂O) to carbon dioxide (CO₂), to produce carbohydrate. In that,the CO₂ is "reduced," or receives electrons, and the water becomes "oxidized," or loses electrons. Ultimately, oxygen is produced along with carbohydrates.

2.Anoxygenic photosynthesis: it occurs under anaerobic condition

 

Anoxygenic photosynthesis uses electron donors other than water      and does not produce oxygen hence is known as anoxygenic photosynthesis.

The process typically occurs in bacteria such as  purple bacteria and green Sulfur bacteria.

Where photosynthesis occur?

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast.

 For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope composed of an outer membrane and an inner membrane.

Within the double membrane are stacked, disc-shaped structures called thylakoids.

  Thylakoids contains chlorophyll, a pigment that absorbs certain portions of the visible spectrum and captures energy from sunlight.

Chlorophyll gives plants their green color and is responsible for the initial interaction between light and plant material, as well as numerous proteins that make up the electron transport chain.

 thylakoid membrane encloses an internal space called the thylakoid lumen. A stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is the stroma or “bed.”

Stages of photosynthesis: (The photosynthesis process)

I photosynthetic process in plants can be divided into four stages, each occurring in a defined area of the chloroplast:

1.    absorption of light

2.    electron transport leading to the reduction of NADP⁺ to NADPH

3.    Generation of ATP

4.    conversion of CO₂ into carbohydrates (carbon fixation).

All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1-3 are catalyzed by proteins in the thylakoid membrane. 

The enzymes that incorporate CO₂ into chemical intermediates and then convert it to starch are soluble constituents of the chloroplast stroma .The enzymes that form sucrose from three-carbon intermediates are in the cytosol.

Light Reaction and Dark Reaction:

 

 photosynthesis occurs in two phases – light reaction and dark reaction.

Light Reaction

The light reaction is a light-dependent process which includes a series of events such as

·       Light absorption

·       Hydrolysis

·       The release of oxygen

·       Formation of ATP and NADPH.

The light reaction of photosynthesis initiates only when it is supplied with light energy.

The photosystem is the arrangement of pigments including chlorophyll within thylakoids.

There are two photosystems in plants:

• Photosystem I (PS-I)
• Photosystem II (PS-II)

 

Photosystem I   - absorbs light at a wavelength of 700 nm, whereas Photosystem II - absorbs light at a wavelength of 680 nm.

The light reaction occurs in the thylakoids of the chloroplast. When the light hits, chlorophyll a get excited to higher energy state followed by a series of reactions. This energy is converted into energy molecules ATP and NADPH by using PS I and PS II.  Also, hydrolysis occurs and releases oxygen.

Dark Reaction

Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from the carbon dioxide and water molecules.

The dark reaction occurs in the stroma of the chloroplast where they utilize the products of the light reaction.

Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin cycle.

In the Calvin cycle, the ATP and NADPH formed during light reaction drives the reaction and convert 6 molecules of carbon dioxide into one sugar molecule i.e. glucose.

 

Photosynthetic reaction:

The overall photosynthetic reaction can be summarized as below:

6CO₂ + 12H₂O + Light Energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

 

Here, six molecules of carbon dioxide (CO₂) combine with 12 molecules of water (H₂O) using light energy. The end result is the formation of a single carbohydrate molecule (C₆H₁₂O₆, or glucose) along with six molecules each of breathable oxygen and water.

The reactions of plant photosynthesis are divided into those that require the presence of sunlight and those that do not. Both types of reactions take place in chloroplasts:

 

        light-dependent reactions in the thylakoid

        light-independent reactions in 

Importance of photosynthesis:

·       During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen, which are then used to produce a high energy molecule ATP in the process of respiration

        Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

·       It contributes to carbon cycle as it involves building of carbon     based materials from carbon dioxide

·        it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth.

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CONCLUSION:

Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar energy and convert it into chemical energy such as sugar (e.g.; glucose, fructose etc.) which is then used by other living creatures and hence drives chain of energy.

In conclusion photosynthesis occurs in the presence of sunlight. The plant takes up carbon dioxide and water, thus increasing the pH.

 In the absence of sunlight there was no photosynthesis occurring in green plants however the plant continued to respire, producing carbon dioxide.

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