Photosynthesis: Definition, Reaction, Diagram and Process (2024)

Process of Photosynthesis: Understanding photosynthesis is simple yet complex when we dive deeper into the process. Here, the basic to moderate information on photosynthesis is provided along with the definition, reaction, diagram and process.

Photosynthesis in Higher Plants: Various processes are occurring inside the living organism. Each of these processes has its essentiality and is thus important for the survival of the organisms. One such process is photosynthesis. Photosynthesis is a term that almost every science enthusiast has known and studied. However, it is not easy to memorise all the steps and processes of photosynthesis by heart. This article elaborates on the important aspects related to photosynthesis, including its definition, raw materials, reaction, location of occurrence, and factors that affect its rate.

Photosynthesis Simple Definition

Photosynthesis is the food-synthesising process in green plants. Some species of algae and certain bacteria also perform this process to make their food.

Photosynthesis is also called the photo-biochemical process as it requires light, a living system and certain chemicals to synthesise energy. This process is fundamental to life on Earth as it provides the primary source of energy for nearly all organisms.

Raw Materials for Photosynthesis

The process of photosynthesis requires three main raw materials to synthesise energy in the form of ATP (Adenosine Triphosphate). Those three raw materials are; Carbon dioxide, Water, and Light.

In the presence of light with optimum wavelength the electrons in the photosystems get excited which further leads to the breakdown of water and carbon dioxide molecules to synthesise ATP and oxygen.

Photosynthesis Reaction

The overall chemical reaction for photosynthesis can be simplified as:

Light Reaction of Photosynthesis

This is the first phase of photosynthesis that occurs in the presence of light and is thus called a light-dependent reaction. This reaction occurs in the thylakoid membranes of chloroplasts.

Steps:

Absorption of Light: Chlorophyll and other pigments absorb light energy.
Water Splitting: Light energy is used to split water molecules into oxygen, protons, and electrons.
Production of ATP and NADPH: The electrons travel through the electron transport chain, creating a proton gradient that helps produce ATP. Electrons are then used to reduce NADP+ to NADPH.
End Result: Oxygen (released into the atmosphere), ATP, and NADPH (used in the Calvin cycle).

Dark Reaction of Photosynthesis (Calvin Cycle)

As the name suggests, this reaction can occur in the absence of light and is thus also named a light-independent reaction of photosynthesis. This reaction occurs in the stroma of chloroplasts. American biochemist Melvin Ellis Calvin discovered the dark reaction and thus, it is also called the Calvin Cycle or C3 cycle.

Steps:

Carbon Fixation: Carbon dioxide is attached to a five-carbon sugar, ribulose bisphosphate (RuBP), by the enzyme Rubisco, forming a six-carbon compound that immediately splits into two three-carbon molecules, 3-phosphoglycerate (3-PGA).
Reduction Phase: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.
Synthesis of Glucose: The remaining G3P molecules are used to form glucose and other carbohydrates.
Outputs: Glucose (which can be converted into other carbohydrates like starch and cellulose).

Difference between light and dark reactions of Photosynthesis.

Location of Photosynthesis

The process of photosynthesis is majorly performed in the mesophyll cells of a leaf. The reason is the high chloroplast concentration in mesophyll cells. Inside the leaf cells chloroplasts are found that are the main organelle to perform photosynthesis.

Chloroplasts are the main sites of photosynthesis in plant cells. These organelles contain pigments like chlorophyll that capture light energy and convert it into chemical energy.
Within the chloroplasts, there are disc-like structures called thylakoids. Thylakoids are stacked in some regions to form structures called grana (singular: granum). The thylakoid membranes are the sites of the light-dependent reactions of photosynthesis. These membranes contain chlorophyll and other pigments, as well as the proteins and enzymes necessary for the light reactions.
The stroma is the fluid-filled space that surrounds the thylakoids inside the chloroplast. It is the site of the light-independent reactions (Calvin cycle) of photosynthesis. The stroma contains enzymes that facilitate the conversion of carbon dioxide and other compounds into glucose.

Structures that facilitate photosynthesis: Stomata are small openings on the surface of leaves that allow gases to enter and exit the leaf. Guard cells surround each stoma and control its opening and closing. Through the stomata, carbon dioxide enters the leaf, and oxygen and water vapour exit. This gas exchange is crucial for photosynthesis.

Process of Photosynthesis

The process of photosynthesis follows certain steps to achieve its end products. Those steps are mentioned below:

>Light-Dependent Phase of Photosynthesis:

Absorption of Raw Materials

Entry of gases like carbon dioxide into the leaf through stomata.
Water gets absorbed through roots and reaches leaves through xylem tissue.
Absorption of light energy that excites electrons, raising them to a higher energy level.

Water Splitting (Photolysis)

The absorbed light energy is used to split water molecules into oxygen, protons (hydrogen ions), and electrons. The reaction is:
2H₂O→4H⁺+4e⁻+O₂

Electron Transport Chain (ETC)

The excited electrons from chlorophyll are passed through a series of proteins embedded in the thylakoid membrane, known as the electron transport chain.
As electrons move through the ETC, their energy is used to pump protons into the thylakoid lumen, creating a proton gradient.

Formation of ATP (Photophosphorylation)

The proton gradient created by the ETC drives the synthesis of ATP from ADP and inorganic phosphate (Pi) through a process called chemiosmosis, facilitated by the enzyme ATP synthase.

Reduction of NADP+ to NADPH

At the end of the ETC, the electrons are transferred to NADP⁺ along with a proton (H⁺) to form NADPH.
NADPH is an electron carrier that will be used in the Calvin cycle.

>Light-Independent Phase of Photosynthesis

Carbon Fixation

Carbon dioxide is attached to a five-carbon sugar called ribulose bisphosphate (RuBP) by the enzyme Rubisco.
This forms a six-carbon compound that immediately splits into two three-carbon molecules of 3-phosphoglycerate (3-PGA).

Reduction Phase

ATP and NADPH produced in the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
ATP provides energy, while NADPH provides electrons for the reduction.

Regeneration of RuBP

Some molecules of G3P are used to regenerate RuBP, the five-carbon molecule needed to accept new carbon dioxide molecules.
This step requires ATP and allows the cycle to continue.

Synthesis of Glucose

The remaining G3P molecules can be used to form glucose and other carbohydrates, which serve as energy sources for the plant and other organisms that consume it.

Factors Affecting Photosynthesis

Photosynthesis is influenced by various environmental and internal factors. For example:

Light Intensity: The rate of photosynthesis generally increases with increasing light intensity up to a certain point.
Carbon Dioxide Concentration: An increase in carbon dioxide concentration typically enhances the rate of photosynthesis.
Temperature: Photosynthesis requires an optimum temperature to occur. Its rates decrease at extreme hot or cold temperatures.
Chlorophyll Content: Higher chlorophyll content increases the efficiency of light absorption.
Light Quality: Red and blue light are most effective for photosynthesis.
Nutrient Availability: Essential nutrients like nitrogen, magnesium, potassium, and phosphorus are crucial for the synthesis of chlorophyll, ATP, and other molecules involved in photosynthesis.

This is a complete guide on photosynthesis. For more such information on various other topics check the official page of Jagran Josh.

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Aspect	Light Reactions	Dark Reactions (Calvin Cycle)
Location	Thylakoid membranes of the chloroplasts	Stroma of the chloroplasts
Energy Source	Light energy (photons)	ATP and NADPH produced in the light reactions
Main Purpose	Convert light energy into chemical energy (ATP and NADPH)	Use ATP and NADPH to fix carbon dioxide into glucose
Key Inputs	Light, water, ADP, NADP+	Carbon dioxide, ATP, NADPH
Key Outputs	Oxygen, ATP, NADPH	Glucose, ADP, NADP+
Oxygen Production	Yes (from the splitting of water molecules)	No
Carbon Dioxide Usage	No	Yes
Time of Occurrence	Only during daylight	Can occur both day and night (as long as ATP and NADPH are available)
Primary Processes Involved	Photolysis, electron transport chain, photophosphorylation	Carbon fixation, reduction phase, regeneration of RuBP
Major Enzymes	Photosystems I and II, ATP synthase	Rubisco