1. Introduction to Photosynthesis in Higher Plants
Photosynthesis is a fundamental biological process in higher plants, making it an essential topic for NEET (National Eligibility cum Entrance Test) aspirants. A strong understanding of photosynthesis will not only help you score well in NEET but also build a solid foundation in plant physiology.
In this article, we will cover the key concepts of photosynthesis and provide a comprehensive set of Multiple Choice Questions (MCQs) designed specifically for NEET preparation.
Photosynthesis is the process by which green plants convert solar energy into chemical energy, synthesizing glucose and releasing oxygen. This process occurs in chloroplasts and is crucial for sustaining life on Earth. Higher plants perform photosynthesis efficiently, ensuring adequate energy production for their growth and development.
Key aspects of photosynthesis:
- It occurs in the chloroplasts of mesophyll cells.
- It involves light-dependent (light) reactions and light-independent (dark) reactions.
- Oxygen is released as a byproduct.
- The process provides energy (ATP and NADPH) required for plant metabolism.
2. Chloroplast: The Photosynthetic Organelle
Chloroplasts are the cellular organelles where photosynthesis takes place. They are double-membrane structures containing stroma, thylakoids, and grana. The thylakoid membranes house the pigments required for capturing light energy.
Key components of chloroplasts:
- Grana: Stacks of thylakoids where light-dependent reactions occur.
- Stroma: Fluid-filled space where the Calvin cycle (light-independent reactions) occurs.
- Thylakoids: Contain photosynthetic pigments like chlorophyll.
3. Photosynthetic Pigments and Their Functions
Photosynthetic pigments play a vital role in capturing light energy. They include:
- Chlorophyll a – The primary pigment involved in photosynthesis.
- Chlorophyll b – An accessory pigment that broadens the absorption spectrum.
- Carotenoids (Carotenes and Xanthophylls) – Help in light absorption and protection from photodamage.
These pigments work together to absorb light of different wavelengths, ensuring maximum efficiency in photosynthesis.
4. Light-Dependent Reactions (Photochemical Phase)
Light-dependent reactions occur in the thylakoid membranes and convert light energy into ATP and NADPH. The main processes include:
- Photolysis of Water: Splitting of water molecules into oxygen, protons, and electrons.
- Electron Transport Chain (ETC): Electrons are transferred through a series of proteins, generating ATP and NADPH.
- Oxygen Evolution: Oxygen is released as a byproduct.
These reactions provide the necessary energy for the Calvin cycle.
5. Calvin Cycle: The Dark Reaction
The Calvin cycle, also known as the light-independent reaction, occurs in the stroma of the chloroplast. This cycle utilizes ATP and NADPH from the light reactions to fix carbon dioxide and synthesize glucose.
The three phases of the Calvin cycle:
- Carbon Fixation: CO₂ is fixed into organic molecules by the enzyme RuBisCO.
- Reduction: ATP and NADPH convert 3-PGA into G3P (a precursor to glucose).
- Regeneration: RuBP (Ribulose-1,5-bisphosphate) is regenerated for continued CO₂ fixation.
6. Factors Affecting Photosynthesis
Several factors influence the rate of photosynthesis, including:
- Light Intensity: Higher light intensity increases the rate of photosynthesis up to a saturation point.
- Carbon Dioxide Concentration: Increased CO₂ enhances photosynthesis until the saturation level is reached.
- Temperature: Optimum temperature (25-35°C) is essential; extreme temperatures inhibit enzyme activity.
- Water Availability: Water shortage limits photosynthesis as it is required for photolysis.
7. Photorespiration: A Wasteful Process
Photorespiration is a process where RuBisCO binds with oxygen instead of CO₂, leading to energy loss. It occurs under high oxygen concentrations and reduces the efficiency of photosynthesis.
8. C4 and CAM Pathways: Adaptations in Photosynthesis
Some plants have evolved alternative pathways to minimize photorespiration:
- C4 Pathway (e.g., Maize, Sugarcane): Fixes CO₂ into a four-carbon compound before entering the Calvin cycle.
- CAM Pathway (e.g., Cacti, Pineapple): Absorbs CO₂ at night to conserve water.
9. Significance of Photosynthesis in Higher Plants
Photosynthesis is essential because it:
- Produces oxygen required for life.
- Forms the basis of the food chain.
- Provides energy in the form of glucose.
- Helps in the carbon cycle and reduces atmospheric CO₂.
10. MCQs on Photosynthesis for NEET
- Which pigment is primarily responsible for photosynthesis?
a) Chlorophyll a
b) Chlorophyll b
c) Carotenoids
d) Xanthophyll - In which organelle does photosynthesis occur?
a) Mitochondria
b) Chloroplast
c) Nucleus
d) Ribosome - The oxygen released during photosynthesis comes from:
a) Carbon dioxide
b) Water
c) Glucose
d) Chlorophyll - Which enzyme is responsible for carbon fixation in the Calvin cycle?
a) ATP synthase
b) RuBisCO
c) NADP reductase
d) Pyruvate kinase - Which pathway is found in desert plants to reduce water loss?
a) C3
b) C4
c) CAM
d) None of the above
Which is the first stable product of the Calvin cycle?
a) Phosphoglyceric acid (PGA)
b) Glucose
c) Oxaloacetic acid (OAA)
d) Phosphoenolpyruvate (PEP)
Which photosystem is involved in cyclic photophosphorylation?
a) PS I only
b) PS II only
c) Both PS I and PS II
d) None of the above
In which part of the chloroplast do light reactions occur?
a) Stroma
b) Thylakoid membrane
c) Outer membrane
d) Inner membrane
The process of photophosphorylation leads to the formation of:
a) ATP
b) NADPH
c) Both ATP and NADPH
d) FADH₂
Which of the following statements about C4 plants is incorrect?
a) They minimize photorespiration.
b) They perform Calvin cycle in mesophyll cells.
c) They have Kranz anatomy.
d) They initially fix CO₂ into a four-carbon compound.
What is the primary function of RuBisCO in photosynthesis?
a) ATP synthesis
b) Water splitting
c) Carbon fixation
d) NADPH production
Which pigment is known as an accessory pigment in photosynthesis?
a) Chlorophyll a
b) Chlorophyll b
c) NADP
d) ATP synthase
Which of the following processes releases oxygen?
a) Glycolysis
b) Krebs cycle
c) Photolysis of water
d) Electron transport chain
Which environmental factor is least likely to directly affect the light-dependent reactions of photosynthesis?
a) Light intensity
b) Temperature
c) CO₂ concentration
d) Water availability
In CAM plants, carbon dioxide is fixed at:
a) Night
b) Day
c) Both day and night
d) None of the above
The Z-scheme refers to:
a) Calvin cycle
b) Electron transport chain in photosynthesis
c) ATP synthesis in mitochondria
d) Glycolysis
Which compound acts as the primary CO₂ acceptor in C4 plants?
a) RuBP
b) PEP
c) PGA
d) G3P
Which enzyme is responsible for the photolysis of water?
a) Cytochrome oxidase
b) ATP synthase
c) Oxygen-evolving complex
d) NADP reductase
The final electron acceptor in the non-cyclic photophosphorylation is:
a) Water
b) Oxygen
c) NADP⁺
d) FAD
Which of the following plants follows the C3 pathway?
a) Maize
b) Sugarcane
c) Wheat
d) Pineapple
During which stage of photosynthesis is ATP produced?
a) Light-dependent reaction
b) Calvin cycle
c) Photorespiration
d) Krebs cycle
What is the major function of carotenoids in photosynthesis?
a) Capturing CO₂
b) Absorbing green light
c) Protecting chlorophyll from photooxidation
d) Generating ATP
What is the role of the enzyme PEP carboxylase in C4 plants?
a) Fixation of CO₂ in mesophyll cells
b) Regeneration of RuBP
c) Splitting of water molecules
d) Reduction of NADP⁺
What happens to the rate of photosynthesis at very high light intensities?
a) Increases indefinitely
b) Decreases due to photoinhibition
c) Remains constant
d) Stops completely
Which of the following molecules is the first electron donor in the light reaction?
a) NADP
b) ATP
c) H₂O
d) FADH₂
The enzyme responsible for ATP synthesis during photosynthesis is:
a) RuBisCO
b) NADP reductase
c) ATP synthase
d) Cytochrome C
What is the correct order of electron flow in the light-dependent reactions?
a) PS I → PS II → NADPH
b) PS II → PS I → NADPH
c) NADPH → PS I → PS II
d) ATP → NADPH → PS II
Which of the following statements about the light-independent reactions is true?
a) They occur in the thylakoid membrane.
b) They generate ATP directly from sunlight.
c) They use ATP and NADPH to fix carbon dioxide.
d) They require oxygen as a reactant.
Which part of the visible spectrum is most effective for photosynthesis?
a) Green and yellow
b) Red and blue
c) Orange and violet
d) Infrared and ultraviolet
Which of the following is a direct product of the Calvin cycle?
a) Glucose
b) ATP
c) G3P
d) Pyruvate
Answer Key for Additional MCQs
(c) G3P
(a) Phosphoglyceric acid (PGA)
(a) PS I only
(b) Thylakoid membrane
(c) Both ATP and NADPH
(b) They perform Calvin cycle in mesophyll cells
(c) Carbon fixation
(b) Chlorophyll b
(c) Photolysis of water
(c) CO₂ concentration
(a) Night
(b) Electron transport chain in photosynthesis
(b) PEP
(c) Oxygen-evolving complex
(c) NADP⁺
(c) Wheat
(a) Light-dependent reaction
(c) Protecting chlorophyll from photooxidation
(a) Fixation of CO₂ in mesophyll cells
(b) Decreases due to photoinhibition
(c) H₂O
(c) ATP synthase
(b) PS II → PS I → NADPH
(c) They use ATP and NADPH to fix carbon dioxide
(b) Red and blue
11. Answer Key and Explanations
- (a) Chlorophyll a – It is the primary pigment that directly participates in photosynthesis.
- (b) Chloroplast – The site of photosynthesis in plant cells.
- (b) Water – Oxygen is released due to the photolysis of water.
- (b) RuBisCO – This enzyme catalyzes the fixation of CO₂ in the Calvin cycle.
- (c) CAM – CAM plants fix CO₂ at night to prevent water loss.
12. Conclusion
Understanding photosynthesis is crucial for NEET aspirants. This article has covered the essential concepts and NEET-oriented MCQs to aid in your preparation. Keep practicing MCQs to reinforce your understanding.
13. FAQs
Glucose (C6H12O6).
Why is photosynthesis important for higher plants?
It provides energy and organic matter for growth and development.
Which pigment absorbs maximum light?
Chlorophyll absorbs maximum light in the red and blue spectrum.
What is the role of NADPH in photosynthesis?
It reduces the power of the synthesis of glucose.
Why do C4 plants have an advantage in hot climates?
They minimize photorespiration and maximize carbon fixation.
What is the end product of the Calvin cycle?
Glucose (C6H12O6).
