How does Photosynthesis Work | Step by Step Process

Photosynthesis, by definition, is the synthesis of carbohydrates in green plants by utilization of carbon dioxide and water in the presence of sunlight. This phenomenon provides food for all living organisms directly or indirectly.

264 gms of CO2 and 216 gms of water are utilized to produce 180 gms of glucose, 108 gms of water, and 192 gms of oxygen.

The green parts of plants, like the leaves, young stems, and buds, are involved in the process.

Interestingly, this process also occurs in microorganisms like blue-green algae, bacteria, and protozoa.

Hence, plants are the prime source of food in food chains.

But how does photosynthesis work?

The entire process of photosynthesis occurs in two major steps, like

1. Light Phase

2. Dark Phase.

The light phase is the reaction that occurs due to light. In contrast, the dark phase occurs without the dependence on light.

Light phase: This phase is dependent on light. Here, the light energy is used for

a) Photolysis of water

b) Phosphorylation of ADP.

Dark Phase: This occurs without the need for light. It consists of two reactions as

a) Carbon fixation

b) Assimilation, i.e., the formation of carbohydrates.

Photolysis of water:

This is defined as the breakdown of water and liberation of oxygen by the illuminated chloroplast in the presence of a hydrogen acceptor.

Hence, here, water molecule H2O is split to release Oxygen O2. In green plants, the Hydrogen acceptor in NADP+

Photophosphorylation:

It is the synthesis of ATP (Adenosine triphosphate) in the chloroplasts by utilization of light energy.

Thus by the two steps in the light phase, both ATP (chemical energy) and NADPH2 are produced. These are necessary to reduce carbon dioxide and thereby complete the dark phase.

Dark Phase: As mentioned above, this phase occurs in the daylight. However, the process is not directly dependent on light.

This phase has two critical steps, like the fixation of carbon and the assimilation, i.e., the formation of carbohydrates.

The fixation of carbon: This is the most complex process of all photosynthesis. It occurs by two methods, like

C3 plants and C4 plants are based on the amount of carbon in sugars of the respective plants.

In C3 plants, the process is called the Calvin cycle. This is again of three steps.

Calvin cycle

a) Carboxylation phase

b) Reduction phase

c) Regeneration phase.

Carboxylation phase:

Here 5 carbon compound ribulose biphosphate (RuBP) accepts CO2 in the presence of the enzyme RuBP carboxylase. So when 6 molecules of RuBP accept 6 CO2 molecules, it leads to the formation of 6 molecules of unstable hexose. These hexoses are immediately broken down into 12 molecules of 3-Phospho-glyceric acid (PGA).

Thus the first compound formed by taking CO2 is phosphoglyceric acid, which is a 3 carbon compound. Hence, the name C3 cycle and the plants in which it occurs are called C3 plants.

Reduction phase:

Here, the ATP and NADPH formed in the light phase are used up. First, the 12 phosphoglyceric acid molecules are activated by using 12 ATP molecules to form Diphosphoglyceric acid (DPGA). This is catalyzed by the enzyme phosphoglyceric phosphotransferase.

The 12 DPGA formed in the above step is reduced to 12 phosphoglyceraldehydes (PGAL) by NADPH2.

This reduction reaction is catalyzed by the enzyme Triosephosphate dehydrogenase.

This completes the carbon fixation, and the first sugar is formed in photosynthesis.

Regeneration phase or pentose phosphate pathway:

The 1/6th of total PGAL formed, i.e., 2 molecules, are used to form hexose. Meanwhile, the remaining 5/6th, i.e., 10 molecules, are used to form RuBP. Here, 6 ATP molecules are utilized.

Six triose phosphate molecules are condensed by aldolase to form 3 molecules of fructose 1,6-diphosphate.

These three fructose diphosphate molecules are dephosphorylated to 3 molecules of fructose 6-phosphate by the action of the enzyme fructose phosphatase.

Of the three molecules of fructose phosphate formed, one molecule is isomerized to glucose phosphate in the presence of the enzyme fructophospho isomerase. The glucose is finally converted into starch.

The remaining two molecules of fructose-6-phosphate  react with two molecules of PGAL to form 2 molecules of erythrose 4-phosphate in the presence of the enzyme transketolase.

Two molecules of erythrose 4-phosphate react with two molecules of DHAP to form 2 molecules of sedoheptulose diphosphate in the presence of transaldolase.

Two molecules of sedoheptulose diphosphate are dephosphorylated to form sedoheptulose monophosphate by the action of enzyme phosphatase.

Two molecules of sedoheptulose monophosphate and two molecules of PGAL react to form 2 molecules of xylulose 5-phosphate in the presence of the enzyme transketolase.

In the presence of the enzyme phospho-ketopentoisomerase, four molecules of xylulose 5-phosphate of reactions 3 and 6 isomerize into four molecules of ribulose 5-phosphate

The general formula for photosynthesis is as

Carbon dioxide reacts with water in the presence of light energy. The light energy needed for this reaction is absorbed in the chloroplasts of the plant cells.

The light has energy in the form of ATP  and is trapped in the form of chemical bonds inside the carbohydrate molecules.

Besides, oxygen is also released. Hence, it is beneficial for life on earth.

When these carbohydrates are consumed by animals, they get absorbed into the digestive system.

In the body cells, they are broken down into carbon dioxide and water, releasing energy in the form of ATP.

So, based on the above discussion, photosynthesis works only in the presence of

Light: Light can be from the sunlight or the electric bulb. This means plants can perform photosynthesis even at night if there is light from an electric source.

Chlorophyll: This is the pigment present in plant cell organelle, namely chloroplasts.

Chloroplasts are high in number in the cells of the leaves of plants. Hence, if you notice, leaves tend to bend towards the light.

In the evenings, they droop as there is no light. This behavior of leaves attempts to trap sufficient light energy required for photosynthesis.

This is a green pigment that can trap light energy. This trapped light energy is used to run a photosynthesis reaction.

Carbon dioxide: Water is always available to plant from the ground. Water is also necessary for the plant for the transport of sap within the body.

It is also needed to remove heat from the plant by transpiration.

However, carbon dioxide is needed only for photosynthesis. It is available from the atmosphere.

Even during respiration, plants release some amount of carbon dioxide.

So carbon dioxide is also mostly available.

Thus, both water and carbon dioxide form the carbohydrate, completing the photosynthesis process.

Frequently asked questions and answers.