Photosynthesis: Definition, Meaning, Formula, Process, Diagram, Process

What is Photosynthesis?

Photosynthesis is the biochemical process by which green plants, some algae, and some bacteria convert light energy to chemical energy in organic compounds, mainly glucose. The process involves capturing light energy by such organisms using chlorophyll, changing carbon dioxide and water into glucose and oxygen. All this transformation keeps the plants alive and generates power for other living organisms through the food chain.

Photosynthesis is essential for life on Earth. It is the most significant source of biomolecules for almost all living organisms, sitting at the basis of the food chain. The oxygen that is evolved as a byproduct of photosynthesis is required for the respiration of most living organisms, including human beings. Photosynthesis also plays a very important role in controlling atmospheric carbon dioxide levels and thus, in regulating the Earth's climate.

Also Read:

• Photosynthesis in Higher Plants
• Photosynthetic Pigments
• Difference between Photosystem 1 and 2
• Respiration in Plants

Photosynthesis Reaction

Photosynthesis is the process where green plants, algae, and some bacteria capture the energy of sunlight and turn it into chemical energy.

Photosynthesis may be represented by the general equation:

6 CO₂ + 6 H₂O + light energy → C₆H

Historically, scientists like Jan Ingenhousz and Joseph Priestley contributed a lot towards the elucidation of photosynthesis, its basic principles and its requirement for light.

Given below is the photosynthesis diagram:

Structure of Chloroplasts

Chloroplasts are present in plant cells, responsible for photosynthesis consequently have a highly specialised structure. The principal parts or structures include:

• Thylakoids: Membrane-bound compartment with light-dependent reactions.

• Grana: Stack of thylakoids.

• Stroma: Fluid found surrounding the thylakoids where light-independent photosynthesis, Calvin Cycle, occurs.

• Chlorophyll: Green pigment capturing light energy.

Structure of Chlorophyll

Chlorophyll is one of the major pigments in photosynthesis that captures light energy and gives green colour to plants. Understanding its working mechanism in the process of photosynthesis thus requires knowing its structure.

• Molecular Structure: The molecule of chlorophyll contains a porphyrin ring—a large ring structure comprising a magnesium ion within its nucleus.
• Types of Chlorophyll: There exist two main types of chlorophyll, chlorophyll a and chlorophyll b. Both are commonly found in vegetation.
• Function: Chlorophyll is a green pigment that generally absorbs light in the blue and red regions of the visible spectrum, reflecting very little green light.

Photosynthetic Pigments

Stages of Photosynthesis

Photosynthesis occurs in two fundamental stages: light-dependent reactions and the Calvin Cycle. It used to be called light-independent reactions. The two stages collaboratively engage in converting light energy into the chemical energy housed within glucose.

Light-Dependent Reactions

• Location: Light-dependent reactions occur within the thylakoid membranes of the chloroplasts.

• Absorption of Light: Chlorophyll and other pigments absorb light energy, and the excited electrons undergo excitation.

H₂O → 4 H⁺ + 4 e⁻ + O₂

ATP Formation And NADPH Formation

• Electrons excited as described are passed down an electron transport chain, where ATP and NADPH are produced.

• Formation of Glucose: The G3P molecules formed combine to form glucose and other carbohydrates.

The light-dependent reactions are the first of the two phases in photosynthesis, and they occur in the initial stage.

• It is in these reactions that the chlorophyll contained in the light is being absorbed.

• The absorbed energy of the light is transferred into chemical energy in the form of NADPH and ATP.

• The reactions depend on light and occur along the thylakoid membranes of a chloroplast.

• Location: thylakoid membrane

• Sunlight and Chlorophyll: Upon the arrival of light

• Photolysis of Water Sunlight splits water molecules to produce oxygen, protons, and electrons.

• Energy from excited electrons creates ATP and NADPH through the electron transport chain.

The Calvin Cycle (Light-Independent Reactions)

The Stroma of the chloroplast is the site of the actions in the Calvin cycle.

• CO₂ fixation: CO₂ is incorporated into a 5-carbon sugar (RuBP) by the ribulose bisphosphate car.

• RuBP Regeneration: Some of the G3P produced is used in the formation of glucose and other carbohydrates, while some combine with ATP to regenerate more RuBP.

• The Calvin cycle, otherwise light-independent reactions, occurs in the stroma of the chloroplasts.

• This process utilises the ATP and NADPH acquired from the light-dependent reactions to synthesise glucose from carbon dioxide.

• Location: Stroma

• Role of ATP and NADPH: Provides the energy and reducing power for carbon fixation and sugar synthesis.

Steps of the Calvin Cycle

• Carbon Fixation: CO₂ is fixed into a 5-carbon sugar (RuBP) by the enzyme Ru­bisco.

• Reduction Phase: ATP and NADPH reduce 3-PGA to G3P.

• RuBP Regeneration: ATP regenerates RuBP from G3P.

Factors Affecting Photosynthesis

Several environmental factors affect photosynthesis and therefore its efficiency and rate. Understanding them is crucial for optimising the growth and productivity of plants.

• Light Intensity: It is one of the limiting factors of photosynthesis up to a given point. The higher the light intensity, the higher the rate.

• Carbon Dioxide Concentration: The level of CO₂ may limit the rate of photosynthesis; higher values may result in higher rates.

• Temperature: Enzymatic activities linked with photosynthesis are temperature-dependent.

• Water: Availability of water is a necessity for photolysis and the general water needs of a typical plant.

C3, C4, And CAM Pathways

• The different plants show variation in the photosynthetic pathways, and it seems that each one of them is adapted to some specific environment.

• So, C3, C4, and CAM plants have their strategies to take up the process of photosynthesis in that particular environment.

Differences Between C3, C4, and CAM Plants

• C3: This is the most common pathway; RuBP is used for carbon fixation.

• C4: Minimises photorespiration; adapted to high light intensity and temperature.

• CAM: Reduces transpiration; adapted to arid conditions.

• Adaptations: Each type has evolved in some particular ecological habitats.

Examples

• C3: Wheat, rice

• C4: Maize, sugarcane

• CAM: Cacti, succulents

Importance of Photosynthesis

Photosynthesis is essential for life on Earth. There will be many ecological implications, but photosynthesis also derives some practical benefits for the living world. As such, photosynthesis is among the most important biological processes.

• Role in the Carbon Cycle: CO₂ is converted into organic compounds, leading to a decrease in atmospheric CO₂.

• Production of Oxygen: Formulation of the oxygen.

• The base of the Food Chain: It serves as the basic source of energy for all trophic levels.

• Agriculture and Renewable Energy Applications: Improved crop yields and artificial photosynthesis are being researched as a means of sustainable energy production.

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