Photosynthesis In Higher Plants: Processes And Diagram

Process Of Photosynthesis In Higher Plants

Photosynthesis is a biochemical process through which green plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose. It is the greatest natural process that sustains life on Earth because it is the main source of energy for all living organisms. Photosynthesis occurs largely in the leaves of a plant in the chloroplasts of plant cells.

• Energy Conversion: Photosynthesis converts radiant light energy into chemical energy.

• Chemical Equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2

• Chloroplasts: Plant cells contain these specialised organelles for photosynthesis.

• The Bottom of the Food Chain: Primary producers that convert solar energy into a form capable of being consumed by heterotrophs.

• The Global Carbon Cycle: Sustains the level of carbon dioxide in the atmosphere, which balances climate change.

Overview Of The Process In Higher Plants

• Two Major Stages: Light-dependent reactions and Calvin Cycle.

• Light-Captured Energy: Absorbed by chlorophyll. These processes end up stimulating electrons.

• Photolysis of Water: Yields dioxygen, ejecting electrons and protons.

• Energy Carriers Produced: ATP, NADPH — Energy carriers produced during the light dependent reactions that are used in the Calvin cycle

• Carbon Fixation: CO2 + organic molecules.

Leaf Structure

Leaves are the main organs of photosynthesis in higher plants. They are modified to acquire light effectively and to exchange gases involved in the process.

Cuticle

• Waxy Layer: Prevents loss of water by evaporation.

• Protection: Prevents attack by pathogens and injury by an organism.

Epidermis

• Single Layer of Cells: Outer protective layer.

• Transparent: To allow penetration of light to the chloroplasts.

Mesophyll Cells

There are two types

Palisade Mesophyll:

• Columnar Cells: Closely arranged with several chloroplasts.

• Primary Site of Photosynthesis: To allow for maximum absorption of light rays

Spongy Mesophyll:

• Closely Arranged Cells. It is very prudent for gas exchange.

• Air Spaces: Pockets of air, which make diffusion of CO2 and O2 possible.

Stomata and Guard Cells

• Stomata: These are pores on the surface of the leaf through which gas exchange takes place.

• Guard Cells: They control the opening and closing of a stoma by regulation according to water loss and gas

• Light Absorption: It absorbs sunlight from the chlorophyll present in the mesophyll cells.

• Gas Exchange: In turn, it absorbs CO2 and releases O2 through the stomata.

• Water Uptake: It carries water from the roots to the leaves through xylem.

Organelles Of The Photosynthesis Machinery

Photosynthesis takes place within specialised organelles found in plant cells.

Chloroplast

• The structure of the chloroplast is different because it has a crucial role in absorption and conversion of light energy.

• Organisation: A chloroplast is a double membrane-bound organelle.

Outer and Inner Membrane:

• Double Membrane: The membrane surrounding the chloroplast.

• Intermembrane Space: The space within the outer and inner membranes.

Thylakoid Membranes

• Thylakoids: Disc-shaped sacs that contain chlorophyll and other pigments.

• Grana: Stacks of thylakoids where light-dependent reactions occur.

Stroma

• Fluid Matrix: Surrounds the thylakoids.

• Site of the Calvin Cycle: The location within the chloroplast where CO2 is converted into glucose.

Function Of Chloroplasts

• Absorb Light Energy: The light is absorbed by the chlorophyll and other pigments.

• Convert Light Energy into Chemical Energy: ATP and NADPH are produced in light-dependent reactions.

• Glucose Synthesis: Calculus Cycle incorporates ATP and NAD.

Photosynthetic Pigments

Photosynthetic pigments are molecules that absorb specific wavelengths of light and participate in the light energy capture process of photosynthesis.

Types of Pigments

Chlorophyll a

• Primary Pigment: Participates directly in the light reactions.

• Absorbs Blue-Violet and Red Light: Reflects green and is the reason plants appear to be green.

Chlorophyll b

• Accessory Pigment: Broadens the spectrum of colors that can be absorbed.

• Absorbs Blue and Red-Orange Light: Assists chlorophyll a by capturing light energy.

Carotenoids

• Accessory Pigments: Among the carotenoids are.

• Absorb Blue and Green Light: Protection of chlorophyll from the damage of excess light.

Function of Pigments in Absorption of Light Energy

• Absorption: Pigments trap light energy and carry it to the reaction centre.

• Energy Transfer: Energy electrons are transferred to the electron transport chain.

• Photoprotection: Carotenoids absorb energy to protect chlorophyll from photo-oxidation.

Mechanism Of Photosynthesis

Photosynthesis among higher plants consists of two principal steps, light-dependent reactions and the Calvin Cycle.

Light-dependent Reactions

• Photolysis: It releases oxygen, electrons, and protons.

• Oxygen evolution: The by-product formed in this process is oxygen.

• Formation of ATP and NADPH: ATP Synthesis in the thylakoid membrane through chemiosmosis.

• NADPH Production: Through reduction of NADP+ using electrons from water.

Light Independent Reactions (Calvin Cycle)

• Carbon Fixation: CO2 is attached to a five-carbon molecule called ribulose bisphosphate or RuBP with the help of the enzyme Rubisco.

• Formation of 3-PGA: It produces 3-phosphoglycerate.

• Reduction Phase: G3P is reduced to glyceraldehyde-3-phosphate using ATP and NADPH.

• Regeneration of RuBP: It is most essential to regenerate RuBP for the repetition of the cycle.

Factors Affecting Photosynthesis

The rate of photosynthesis in higher plants is affected by the following environmental factors.

• Higher the light intensity, the higher the photosynthesis up to a saturation point.

• Light saturation point is the state after which no further increase in photosynthesis occurs.

• Too much of it damages the photosynthetic apparatus.

Carbon Dioxide Concentration

• Increase Rate: With a higher level of CO2, the rate increases.

• CO2 Saturation Point: The point at which the rate no longer increases.

• Acts As a Limiting Factor: The unavailability of carbon dioxide often limits the photosynthesis rate in a natural environment.

Temperature

• Optimal Range: Each plant has its optimal range for temperature to carry out photosynthesis.

• Enzyme Activity: Activities of the enzymes related to photosynthesis are affected by this factor.

• High Temperature: A high temperature can cause photorespiration and reduce efficiency.

Water Availability

• Essential for Photolysis: Water is a substrate in light-dependent reactions.

• Turgor Pressure: Supports cell turgor and stomata opening.

• Water Stress: Stomata may close to prevent water loss and reduce carbon assimilation.

Importance Of Photosynthesis

Photosynthesis is vital for life on Earth; it influences a number of ecological and biological functions.

Oxygen Production

• Oxygen Evolution: It liberates oxygen to the atmosphere.

• Aerobic Life Support: It ensures respiration in most organisms.

Basis for Food Chains

• Primary Producers: Solar energy is converted into chemical energy in glucose.

• Energy Transfer: The energy transfers from the producers to the consumers in the ecosystem.

Effects on the Global Carbon Cycle

• Carbon Sequestration: The plant absorbs C02 from the atmosphere via its photosynthesis and, in the process, depleting carbon dioxide levels in the atmosphere.

• Climate Regulation: Helps in moderating climate change by controlling carbon levels.

Comparison Between C3, C4, and CAM Plants

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