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What is Photosynthesis: Light Reaction, Dark Reaction, and Significance

A botany graduate, Nithya Venkat enjoys writing about plants that help sustain life on planet Earth.

Overall Process of Photosynthesis

Overall Process of Photosynthesis

Photosynthesis is a process that takes place in plants, algae, and certain species of bacteria to make food needed to grow and reproduce. Plants take in water from the soil, light energy from the sun, carbon dioxide from the air and synthesize food with the help of the chlorophyll pigment present in the leaves.

The process of photosynthesis is a photochemical reaction in which solar energy is converted into chemical energy through a series of reactions.

During photosynthesis, plants produce glucose molecules from water and carbon dioxide and release oxygen into the surrounding air. Some of these glucose molecules provide energy to grow and reproduce, and the rest of the glucose molecules are stored in the plant cells in the form of carbohydrates for future use.

Plants, algae and certain species of bacteria that can produce their food are called autotrophs.

Where does photosynthesis take place?

A leaf is made up of three layers of cells, upper epidermis, lower epidermis, and a middle mesophyll layer. Photosynthesis takes place in the cells of the mesophyll layer.

These cells contain organelles called chloroplasts. Chloroplasts have green pigments called chlorophyll; these pigments are responsible for absorbing light energy from the sun.

Structure of Chloroplast

Structure of Chloroplast

Structure of a Chloroplast

The wall of the chloroplast is made of two membranes, an inner membrane, and an outer membrane.

The chloroplast also has a third internal membrane system called the thylakoid membrane. The thylakoid membrane forms a network of flattened discs called thylakoids that are arranged in stacks called grana. Thylakoids also have an inner and outer membrane, and the space between these two membranes is called the lumen.

The space that lies inside the chloroplast wall but outside the thylakoid membrane is called the stroma.

Stoma on the surface of the leaf

Stoma on the surface of the leaf

Photosynthesis is a photochemical reaction that requires water, carbon dioxide, and light energy.

Light energy is absorbed from the sun, water from the soil and carbon dioxide from the surrounding air to produce glucose, oxygen, and carbohydrates.

The exchange of oxygen and carbon dioxide takes place through small pores called stomata present in the lower surface of the leaf.

The Two Stages of Photosynthesis

Photosynthesis takes place in two stages - the light reaction and the dark reaction. The light reaction takes place only in the presence of sunlight and therefore it is also called the light-dependent reaction.

The dark reaction can take place in the presence or absence of sunlight. Therefore, it is also known as the light-independent reaction.

The first stage of photosynthesis (light reaction) takes place in the thylakoid membrane of the chloroplasts.

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Photosystems involved in Photosynthesis

There are two photosystems present in the thylakoid membrane – photosystem I (PS I) and photosystem II (PS II).

PS I and PS II are made up of many pigments and complex proteins with a pair of chlorophyll molecules in the center.

In the center of photosystem I one there is a pair of chlorophyll molecules called P700, and in the center of photosystem II, there is a pair of chlorophyll molecules called P680. These molecules are the main reactive centers in the photosystems.

Both photosystems PS I and PS II harvest light energy efficiently. The light reaction starts in PS II and then moves on to PS I.

The photosystems are named according to the order in which they were discovered.

Light Reaction/Light Dependent Reaction

Light Reaction/Light Dependent Reaction

The photosynthesis that is explained below is called C3 photosynthesis because the carbon fixation results in the production of a three-carbon sugar molecule.

C3 photosynthesis takes place in cool, moist weather conditions when plants can keep their stomata open to let carbon dioxide enter the plant cells.

Light Reaction/Light Dependent Reaction

The light reaction takes place in the thylakoid membranes of chloroplasts using light energy to produce ATP and NADPH molecules that are essential for the next stage of photosynthesis.

When light hits the surface of the leaf, the pigments in PS II capture the light energy and passes it on from one pigment to the next till it reaches P680, the main reaction center. (Sunlight can also directly activate PS II and initiate the light reaction stage of photosynthesis.)

The light energy boosts an electron in P680 to a high energy level, and once it gains enough energy, it leaves PS II and moves to the nearest electron acceptor molecule.

Once P680 gives up an electron, it becomes positively charged resulting in P680+, a strong oxidizing agent that attracts and captures the electron from the water. This electron passes through plastocyanin ( a protein involved in electron transfer) and replaces the electron that has escaped from PS II.

Once water loses its electrons, it splits into two hydrogen ions and one oxygen molecule that escapes into the surrounding air providing the oxygen that we breathe.

The energized electron that leaves PS II moves from a high energy state to a low energy state through an electron transport chain. It loses energy as it moves down the energy gradient. Plastoquinone (a complex protein molecule) acts as the electron carrier from PS II to the cytochrome b6-f complex.

The energy lost is used to pump hydrogen ions from the stroma into the lumen across the thylakoid membrane; this results in a high concentration of hydrogen ions inside the membrane. (The hydrogen ions that are released when water splits also adds to the concentration gradient). The pumping of hydrogen ions is facilitated by the cytochrome b6-f complex.

The hydrogen ions flow down the gradient through ATP synthase facilitating the addition of a phosphate group to ADP producing an ATP molecule.

When photons strike PS I, an electron in PS I is excited and enters a high energy level and moves to the nearest electron acceptor. The high energy electron moves down a second electron transport chain losing energy as it moves and this energy is used to reduce NADP to NADPH.

At the end of the light reaction stage, ATP and NADPH molecules are produced. These molecules are used in the next stage of photosynthesis, the dark reaction.

Calvin Cycle

Calvin Cycle

Dark Reaction/Light-Independent Reaction

Dark reaction or light-independent reaction uses ATP and NADPH from the light reaction along with carbon dioxide from the atmosphere to produce sugars. In this cycle ATP and NADPH is used to fix carbon dioxide to produce glucose. This carbon fixation is catalyzed by an enzyme called Rubisco which is abundant in plants and the most abundant protein on earth.

The entire reaction goes through a pathway called the Calvin Cycle that takes place in the stroma of the chloroplasts.

The dark reaction occurs in three stages -

  1. carbon fixation
  2. reduction
  3. regeneration

1. Carbon Fixation

A carbon dioxide molecule reacts with a five-carbon acceptor molecule called Ribulose-1,5 bisphosphate (RuBP) that results in six carbon compound that is unstable and immediately splits into two molecules of a three-carbon compound called 3-Phosphoglyceric acid also known as Glycerate-3-Phosphate.

This reaction is catalyzed with the help of an enzyme known as RuBisCo (ribulose -1,5 bisphosphate carboxylase/oxygenase).

In this stage, two 3-Phosphoglyceric acid molecules are produced for every one molecule of carbon dioxide that enters the cycle.

2. Reduction

In the second stage of the Calvin Cycle ATP and NADPH provide the energy needed to drive the conversion of 3-Phosphoglyceric acid molecules into molecules of Glyceraldehyde-3-Phosphate (G3P).

Each molecule of 3-Phosphoglyceric acid receives a phosphate group from ATP becoming a doubly phosphorylated molecule called 1,3-bisphosphoglycerate and ADP is released.

The 1,3-bisphosphoglycerate molecules are reduced by receiving two electrons from NADPH and loses one of its phosphate groups becoming a three-carbon sugar G3P (glyceraldehyde-3-phosphate) releasing NADP + and phosphate as byproducts.

Both ADP and NADP + that are released are used by the light reaction.

3. Regeneration

In this stage, some of the G3P molecules exit the Calvin Cycle and are used to make glucose, other sugar molecules, and carbohydrates needed by the plant and the rest are recycled to generate RuBP acceptor molecules.

Three turns of the Calvin Cycle are required to produce one G3P molecule that can leave the cycle and move towards the production of glucose.

In three turns of the Calvin Cycle –

3 CO2 combines with 3RuBP acceptors producing 6 molecules of Glyceraldehyde-3-Phosphate (G3P).

1 G3P molecule leaves the cycle and goes towards the production of glucose, other sugar molecules, and carbohydrates.

5 G3P molecules are recycled to regenerate 3 Ribulose-1,5 bisphosphate (RuBP) acceptor molecules

9 ATP molecules are converted into 9 ADP molecules (6 during fixation and 3 during regeneration).

6 NADPH is reduced to 6 NADP+ during the reduction.

One G3P molecule has three fixed carbon atoms. Therefore it takes two G3P molecules to build a six-carbon glucose molecule.

To produce one glucose molecule, it would take 6 CO2, 18 ATP, and 12 NADPH; it will take 6 turns of the Calvin Cycle to produce these molecules.

C4 Photosynthesis

The photosynthesis that results in the production of a four-carbon sugar molecule is called C4 photosynthesis. This type of photosynthesis occurs in the summer when the conditions are so hot that the plants close their stomata to prevent excess water from escaping. When the stomata are closed, then there is no way for the carbon dioxide to enter the plant cells.

In such conditions, plants adopt the C4 photosynthesis method in which an enzyme PEP carboxylase facilitates the production of a four-carbon sugar molecule.

Significance of Photosynthesis

Photosynthesis provides the energy for plants to grow and reproduce. It forms the base of the food pyramid. All living beings are directly or indirectly dependent on plants for food.

Photosynthesis is important because of the following -

  • it is the primary source of organic food
  • produces oxygen that is important for all living beings to breathe
  • removes carbon dioxide from the air and maintains a balance between carbon dioxide and oxygen

Application of Photosynthesis in Modern Technology

Artificial Photosynthesis is currently being researched to mimic plants to produce fuel from solar energy on a large scale, to cater to the increasing energy demands of the growing population. This research aims to provide cost-effective solutions to the production of energy from artificial photosynthesis.

Scientists are aiming to perfect the process of artificial photosynthesis keeping in mind the cost involved while transferring this technology from the laboratory to modern-day applications.

The perfection of photosynthesis will provide renewable, environment-friendly energy to fuel future production factories and reduce carbon dioxide emissions that is the primary cause of the greenhouse effect.

Improved artificial photosynthesis can provide a steady supply of food and oxygen for the future colonies and produce a self-sustaining atmosphere to live in space. It will help to increase the production of food crops in the agricultural sector and end scarcity of food in the near future.


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Photosynthesis Education

© 2019 Nithya Venkat


Nithya Venkat (author) from Dubai on April 19, 2019:

Peggy I do hope more research is conducted on photosynthesis to help benefit the world.

Peggy Woods from Houston, Texas on April 19, 2019:

Your article is so very interesting! I had no idea that there were a light reaction and a dark reaction aspect to photosynthesis, among other things you detailed. The most exciting thing to learn was that utilizing artificial photosynthesis may help the growing food shortage worldwide as well as reduce the greenhouse effect. Living in space may also be aided by having this new technology.

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