Photosynthesis is part of Biology. Below are notes on 9 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

C4 pathway (Hatch-Slack pathway): Anatomy, biochemistry, and ecological significance

  • C4 pathway (Hatch-Slack pathway): An alternative carbon fixation pathway for efficient photosynthesis, especially in hot, dry climates.
  • C4 Plant Examples: Sugarcane, Maize (Corn), Sorghum, Amaranthus, millet, and many tropical grasses.
  • Anatomy: Characterized by Kranz anatomy ("wreath-like" arrangement) of photosynthetic cells.
  • Kranz anatomy involves two distinct photosynthetic cell types: Mesophyll cells (outer) and Bundle Sheath cells (inner, surrounding vascular bundles).
  • Mesophyll Cells: Loosely arranged, contain chloroplasts, but lack RuBisCO.
  • Bundle Sheath Cells: Large, thick-walled, tightly packed, contain large, agranal chloroplasts (lack grana), and are rich in RuBisCO.
  • Biochemistry - Step 1 (Mesophyll Cells):
    • Atmospheric CO2 is first accepted by Phosphoenolpyruvate (PEP), a 3carbon compound.
    • Enzyme: PEP carboxylase (PEPcase) – high affinity for CO2, and no oxygenase activity (minimizes photorespiration in mesophyll).
    • First stable product: Oxaloacetic acid (OAA), a 4-carbon compound.
    • OAA is converted to Malate or Aspartate, then transported to the bundle sheath cells.
  • Biochemistry - Step 2 (Bundle Sheath Cells):
    • Malate/Aspartate undergoes decarboxylation, releasing CO2 and a 3carbon compound (Pyruvate).
    • The released CO2 then enters the Calvin cycle (C3 pathway) within the bundle sheath cells, fixed by RuBisCO.
    • Pyruvate returns to the mesophyll cells and is regenerated into PEP using 2 ATP molecules (catalyzed by Pyruvate dikinase).
  • Energy Cost: The C4 pathway requires an additional 2 ATP molecules per CO2 fixed compared to the C3 pathway, primarily for PEP regeneration.
  • Ecological Significance:
    • Adapted to environments with high temperatures, high light intensity, and low atmospheric CO2 concentrations.
    • Minimizes photorespiration by creating a high CO2 concentration around RuBisCO in the bundle sheath cells.
    • Achieves higher photosynthetic rates and better water use efficiency compared to C3 plants under these challenging conditions.

Ecological and agricultural importance of photosynthesis

  • Photosynthesis is the fundamental process by which green plants (autotrophs) synthesize their own food, primarily glucose (a carbohydrate).
  • During photosynthesis, solar/light energy is converted into chemical energy (potential energy), which is stored in the bonds of organic molecules like carbohydrates.
  • Autotrophs utilize solar energy to convert atmospheric carbon dioxide (CO₂) and water into carbohydrates and oxygen.
  • Oxygen (O₂) is a key gaseous byproduct of photosynthesis, released into the atmosphere, predominantly during daylight hours.
  • The oxygen released through photosynthesis is essential for the aerobic respiration of most living organisms, thus sustaining life.
  • A significant decrease in plant life would lead to a substantial reduction in atmospheric oxygen levels.
  • Carbohydrates, such as glucose (which is often converted and stored as starch), are the primary organic substances produced in plant leaves during photosynthesis.
  • Photosynthesis fulfills the carbon and energy requirements of autotrophic organisms, forming the base of almost all food chains and ecosystems.

Factors affecting the rate of photosynthesis

  • Light Intensity: Rate increases with intensity up to a saturation point; Red & Blue light are most effective.
  • CO₂ Concentration: A major limiting factor; rate increases with CO₂ concentration up to a saturation point (CO₂ is a key raw material).
  • Temperature: Affects enzyme activity (especially in dark reactions); an optimal temperature range exists.
  • Water: Essential raw material; water stress leads to stomatal closure, reducing CO₂ uptake.
  • Chlorophyll: Essential pigment for light absorption; its quantity and health impact the rate.
  • Oxygen (O₂): A by-product of photosynthesis, not a requirement; high O₂ can inhibit photosynthesis (photorespiration).
  • Product of Photosynthesis: Glucose, primarily stored as starch.
  • Starch Test: Iodine solution turns blue-black in the presence of starch; chlorophyll must be removed (e.g., by boiling in alcohol) for a clear observation.
  • Compensation Point: The light intensity at which the rate of photosynthesis equals the rate of respiration, resulting in no net gaseous exchange (CO₂ uptake = CO₂ release, O₂ release = O₂ uptake).

Light-dependent reactions: Photosystems (PS I & PS II), electron transport chain

  • Light-dependent reactions occur in the thylakoid membranes of chloroplasts.
  • Chlorophyll absorbs light energy (photons).
  • Light energy is converted into chemical energy (ATP and NADPH).
  • Two Photosystems (PS) are involved: Photosystem II (PS II or P680) and Photosystem I (PS I or P700).
  • PS II absorbs light, excites electrons, and initiates the splitting of water (photolysis).
  • Photolysis of water: H₂O splits into protons (H⁺), electrons (e⁻), and oxygen (O₂).
  • Electrons from photolysis replace those lost by PS II; O₂ is released as a byproduct.
  • Electrons flow from PS II through an electron transport chain (ETC) to PS I, generating ATP via photophosphorylation.
  • PS I receives electrons from the ETC and, upon absorbing light, passes them to reduce NADP⁺ to NADPH.
  • The principal function is to generate ATP and NADPH (collectively called assimilatory power).
  • ATP and NADPH are then utilized in the light-independent (dark) reactions to reduce carbon dioxide to carbohydrates.
  • Key events: Absorption of light energy by chlorophyll, conversion of light energy to chemical energy, splitting of water molecules.
  • Processes that do NOT occur: Chlorophyll degradation, oxidation of carbon to carbon dioxide, or reduction of carbohydrates into carbon dioxide.

Light-independent reactions: Calvin cycle (C3 pathway) mechanism

  • The Calvin cycle (C3 pathway) occurs in the stroma of chloroplasts.
  • It is a light-independent reaction, utilizing ATP and NADPH generated during the light reactions.
  • The primary function is to convert atmospheric Carbon Dioxide (CO2) into carbohydrates (e.g., glucose).
  • The cycle proceeds in three main stages: Carboxylation, Reduction, and Regeneration.
  • Carboxylation:
    • CO2 is accepted by a 5-carbon compound, Ribulose-1,5-bisphosphate (RuBP).
    • This reaction is catalyzed by the enzyme RuBP carboxylase-oxygenase (RuBisCO).
    • RuBisCO is the most abundant enzyme on Earth.
    • The first stable product formed is two molecules of 3-phosphoglyceric acid (3-PGA), a 3-carbon compound, which gives the pathway its "C3" designation.
  • Reduction:
    • 3-PGA is converted into glyceraldehyde-3-phosphate (G3P or triose phosphate) using ATP for phosphorylation and NADPH for reduction.
    • This stage consumes 2 ATP and 2 NADPH for every molecule of CO2 fixed.
  • Regeneration:
    • The remaining G3P molecules are used to regenerate RuBP to continue the cycle.
    • This regeneration step consumes 1 ATP for every molecule of CO2 fixed.
  • To synthesize one molecule of glucose (a 6carbon sugar), the Calvin cycle must operate 6 times (fixing 6 molecules of CO2).
  • The total energy consumed for synthesizing one molecule of glucose is 18 ATP (6 CO2 3 ATP/CO2) and 12 NADPH (6 CO2 2 NADPH/CO2).

Photolysis of water and oxygen evolution

  • Oxygen (O₂) is released as a by-product during photosynthesis.
  • The O₂ evolved during photosynthesis originates exclusively from the splitting (photolysis) of water (H₂O).
  • This process, known as photolysis of water, occurs on the inner side of the thylakoid membrane.
  • Photolysis of water requires light energy.
  • Essential mineral ions for the photolysis of water include Mn²⁺ (manganese) and Cl⁻ (chloride) ions.

Photorespiration (C2 pathway) and its implications

  • Photorespiration (C2 pathway) is a wasteful process primarily in C3 plants.
  • It occurs when the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) acts as an oxygenase instead of a carboxylase.
  • RuBisCO's oxygenase activity increases under conditions of high O2:CO2 ratio, high temperature, and high light intensity.
  • Instead of CO2, RuBisCO binds O2 to RuBP (Ribulose-1,5-bisphosphate).
  • This oxygenation produces one molecule of 3-PGA (3-Phosphoglycerate) and one molecule of 2-carbon Phosphoglycolate.
  • The pathway involves three cell organelles: chloroplasts, peroxisomes, and mitochondria.
  • Phosphoglycolate is dephosphorylated to glycolate in chloroplasts.
  • Glycolate enters peroxisomes, is converted to glyoxylate, then to glycine.
  • Glycine enters mitochondria, where two molecules of glycine are converted into one molecule of serine, releasing CO2 and ammonia.
  • Serine returns to peroxisomes, is converted to glycerate, which then enters chloroplasts to be phosphorylated back to 3-PGA, consuming ATP.
  • Photorespiration consumes ATP (2 ATP) and NADPH (2 NADPH for converting glycerate to PGA) but does not produce any sugar or ATP.
  • It leads to a net loss of previously fixed CO2 (through the CO2 release in mitochondria).
  • This process reduces the photosynthetic efficiency and yield of C3 plants by 25-50%, especially in hot and dry environments.
  • C4 plants have evolved mechanisms (e.g., C4 cycle, Kranz anatomy) to suppress photorespiration by maintaining a high CO2 concentration around RuBisCO.

Photosynthetic

machinery:

Chloroplast

structure and function

  • Chloroplasts are the primary sites of photosynthesis in eukaryotic plant cells.
  • Chloroplasts are the organelles where solar energy is converted into ATP (chemical energy).
  • Chlorophyll, the green pigment essential for photosynthesis, is located within chloroplasts in eukaryotic plants.
  • In photosynthetic prokaryotic bacteria, chlorophyll is associated with membranous vesicles, as they lack chloroplasts.

Photosynthetic pigments: Types, absorption and action spectra

  • Photosynthetic pigments are molecules that absorb light energy for photosynthesis.
  • Types of Pigments: Chlorophylls, Carotenoids, Phycobilins.
  • Chlorophylls: Primary pigments, responsible for green color in plants.
    • Chlorophyll-a: Primary photosynthetic pigment; appears blue-green. Absorbs strongly in blue-violet and red wavelengths (peak absorption).
    • Chlorophyll-b: Accessory pigment; appears yellow-green. Absorbs blue and orange light, transfers energy to Chlorophyll-a.
    • Chlorophylls absorb red and blue light wavelengths while reflecting green light, making plants appear green.
    • Contain Magnesium (Mg) as the central metal ion.
    • Synthesis of chlorophyll requires sunlight. Lack of light causes etiolation (paleness/yellowing), as seen when grass is covered.
  • Carotenoids: Accessory pigments.
    • Carotenes: Orange pigment.
    • Xanthophylls: Yellow pigment.
    • Absorb light in blue-violet regions; transfer energy to Chlorophyll-a. Protect chlorophyll from photo-oxidation.
  • Phycobilins: Accessory pigments (found in cyanobacteria and red algae).
    • Phycocyanin: Blue-green pigment.
    • Phycoerythrin: Red pigment.
  • Absorption Spectrum: A graph showing the amount of light absorbed by pigments at different wavelengths.
    • Chlorophylls show highest absorption in blue-violet and red light.
    • Green light is the least absorbed by green plants, leading to reflection.
  • Action Spectrum: A graph showing the rate of photosynthesis at different light wavelengths.
    • Closely matches the absorption spectrum of chlorophyll-a, with highest photosynthetic rates in red and blue light.
    • Lowest photosynthetic rate occurs in green light.
  • Experiments: Variegated leaves (e.g., Lady Plymouth plant) are suitable to test that chlorophyll is necessary for photosynthesis, as only the green (chlorophyll-containing) parts photosynthesize.

19. Plant Nutrition

Practice MCQs: Photosynthesis

C4 pathway (Hatch-Slack pathway): Anatomy, biochemistry, and ecological significance

  1. The Xylem in plants is responsible for –

    Asked in: Co operative Supervisor & Environment Supervisor (Group C) Exam 2023 · MPSC CS Prelims 2019

    1. Transport of water
    2. Transport of food
    3. Transport of amino acid
    4. Transport of oxygen
    Show answer

    Answer: A. Transport of water

    Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. Xylem's main function is to transport water, but it also transports some nutrients and provides physical support. C. Sunflower D. Potato Ans: A - C₄ photosynthesis is an adaptation that improves carbon fixation efficiency in certain environmental conditions. - Other common C₄ plants include maize, sorghum, millet, and many tropical grasses.

  2. Which of the following is C4 plant?

    Asked in: UPPSC Upper Mains 2014

    1. Paddy
    2. Soya bean
    3. Maize
    4. Wheat
    Show answer

    Answer: C. Maize

    C4 plants have a specialized mechanism for carbon fixation (C4 photosynthesis) that is more efficient in hot, dry, and high-light environments compared to C3 plants like paddy, soybean, and wheat.

Ecological and agricultural importance of photosynthesis

  1. Plants prepare glucose in the process of :

    Asked in: UPSC CDS 1 2022

    1. respiration
    2. photosynthesis
    3. degradation
    4. mineral absorption
    Show answer

    Answer: B. photosynthesis

    Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigment. During photosynthesis, plants convert light energy into chemical energy in the form of glucose (a sugar). This process uses carbon dioxide and water as raw materials and releases oxygen as a byproduct.

  2. In photosynthesis

    Asked in: CGPSC Prelims Exam 2017

    1. Solar energy converts into heat energy
    2. Solar energy converts into kinetic energy
    3. Solar energy converts into potential energy
    4. Light energy converts into mechanical energy
    Show answer

    Answer: C. Solar energy converts into potential energy

    The fundamental energy conversion during photosynthesis is the transformation of light energy from the sun into chemical energy. This chemical energy is stored in the bonds of organic molecules, primarily glucose, synthesized from carbon dioxide and water. Chemical energy stored in bonds is a form of potential energy. - Option A is incorrect; while some solar energy becomes heat, this is not the primary conversion of photosynthesis. - Option B is incorrect; solar energy is not converted into kinetic energy in photosynthesis. - Option C is correct; solar energy (light energy) is captured and used to create chemical bonds in glucose molecules, storing potential energy. - Option D is incorrect; light energy is not converted into mechanical energy in photosynthesis.

  3. Direction to the following two questions (Q. Nos. 29 & 30). Read the two statements Assertion (A) & Reason (R). Choose the correct answer from the four choices (a), (b), (c) or (d) as given below : Statement-I (A) : The Process of energy conversion from Solar heat energy to Chemical energy is known as photosynthesis. Statement-II (R) : Autotrophs use Solar energy to convert carbon dioxide and water to produce carbohydrates and finally other biochemical molecules to support life.

    Asked in: KPSC KAS Prelims 2014

    1. Both (A) and (R) are correct and (R) explains (A).
    2. Both (A) and (R) are correct, but (R) does not explain (A).
    3. (A) is correct, but (R) is false.
    4. (A) is false, but (R) is true.
    Show answer

    Answer: D. (A) is false, but (R) is true.

    Photosynthesis is the process used by autotrophs (like plants, algae, and some bacteria) to synthesize foods (carbohydrates) from carbon dioxide and water using solar energy (specifically, light energy). Solar heat energy is less directly involved in the primary energy conversion step compared to light energy (photons).

  4. The sunlight energy used by green plants in photosynthesis is transformed into

    Asked in: MPPSC CS Prelims 2018

    1. chemical energy
    2. physical energy
    3. Both (A) and (B)
    4. None of the above
    Show answer

    Answer: A. chemical energy

    This process is a fundamental example of energy transformation where light energy is converted into chemical energy.

  5. The gaseous product of a process in plants is a requirement for another vital process that releases energy. Given below are four combinations of the process and product. Identify the correct answer.

    Asked in: UPSC NDA 1 2021

    1. Respiration and Nitric oxide
    2. Transpiration and Water vapour
    3. Photosynthesis and Oxygen
    4. Germination and Carbon dioxide
    Show answer

    Answer: C. Photosynthesis and Oxygen

    - Photosynthesis is the process by which plants convert light energy into chemical energy (glucose) using carbon dioxide and water, releasing oxygen as a byproduct. - Respiration is the process by which organisms break down organic molecules (like glucose) to release energy. Aerobic respiration requires oxygen and produces carbon dioxide and water. - The question describes a gaseous product (oxygen from photosynthesis) being a requirement (for respiration) of a process that releases energy.

  6. Carbon and energy requirements of autotrophic organisms are fulfilled by:

    Asked in: UPSC NDA 2 2015

    1. Photosynthesis
    2. Gluconeogenesis
    3. Glycogenesis
    4. External sources
    Show answer

    Answer: A. Photosynthesis

    Autotrophs obtain their carbon and energy through processes like photosynthesis or chemosynthesis.

  7. Green plant commonly :

    Asked in: OPSC CS Prelims 2015

    1. Releases CO₂ in day time and O₂ in night
    2. Releases O₂ in day time and CO₂ in night time
    3. Releases both O₂ and CO₂ in day as well as night
    4. Releases both O₂ and CO₂ in day and only O₂ in night
    Show answer

    Answer: B. Releases O₂ in day time and CO₂ in night time

    - Photosynthesis: CO₂ + Water + Light Energy → Glucose + O₂ (occurs only in light) - Respiration: Glucose + O₂ → CO₂ + Water + Energy (occurs continuously) - Daytime: Photosynthesis > Respiration (Net: O₂ release, CO₂ uptake) - Nighttime: Respiration > Photosynthesis (Photosynthesis is absent) (Net: O₂ uptake, CO₂ release)

  8. Which one of the following is the process involved in photosynthesis?

    Asked in: OPSC CS Prelims 2023 · UPSC IAS 2014

    1. Potential energy is released to form free en‐ ergy
    2. Free energy is converted into potential energy and stored
    3. Food is oxidized to release carbon dioxide and water
    4. Oxygen is taken, and carbon dioxide and water vapour are given out
    Show answer

    Answer: B. Free energy is converted into potential energy and stored

    Photosynthesis is the process by which light energy (a form of free energy) is captured by chlorophyll and other pigments and converted into chemical energy stored in the bonds of glucose molecules (a form of potential energy). This chemical energy is then used by the plant for its metabolic processes and can be transferred to other organisms that consume the plant. Options A, C, and D describe aspects of respiration or other biological processes, not the primary energy conversion in photosynthesis.

  9. Plants capable of performing photosynthesis belong to which one among the following types of organisms?

    Asked in: UPSC CAPF 2013

    1. Heterotrophs
    2. Saprotrophs
    3. Autotrophs
    4. Chemoheterotrophs
    Show answer

    Answer: C. Autotrophs

    Organisms are classified based on how they obtain energy and carbon. - Autotrophs are organisms that can produce their own food using light, water, carbon dioxide, or other chemicals. Plants capable of photosynthesis use light energy (photoautotrophs) to synthesize organic compounds from inorganic sources like CO2 and water. - Heterotrophs obtain carbon by consuming other organisms (plants or animals). - Saprotrophs are heterotrophs that feed on dead organic matter. - Chemoheterotrophs obtain both energy and carbon from consuming organic compounds. Plants that perform photosynthesis are the primary producers in most ecosystems and are classified as autotrophs.

  10. Consider the following statements describing the functions of plants : 1. Photosynthesis is a process to convert light energy from the Sun into chemical energy i.e., Carbohydrates are synthesized from carbon dioxide and water using sun light 2. Fermentation or Zymology is a metabolic process that converts carbohydrate to alcohols and carbon dioxide Which of the statements given above is/are correct ?

    Asked in: UPSC CAPF 2014

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Show answer

    Answer: C. Both 1 and 2

    Statement 1 is correct. Photosynthesis is the primary metabolic process in most plants (and other photoautotrophs) where light energy is converted into chemical energy in the form of glucose and other carbohydrates using carbon dioxide and water. Statement 2 is correct. Fermentation is a metabolic process that can occur in plants, particularly under anaerobic or low-oxygen conditions (e.g., in flooded roots or germinating seeds). Alcoholic fermentation, as described, converts carbohydrates into ethanol and carbon dioxide. While not as central a function as photosynthesis, it is a metabolic pathway found in plants. Zymology is the study of fermentation. The description of the process is accurate.

  11. If all the plants and trees vanish from the earth, the gas which will decreases is:

    Asked in: RPSC RAS Prelims 2012

    1. Caron dioxide
    2. Nitrogen
    3. Water vapour
    4. Oxygen
    Show answer

    Answer: D. Oxygen

    Photosynthesis by plants is the main source of atmospheric oxygen.

Factors affecting the rate of photosynthesis

  1. A leaf was plucked from a plant on a sunny day and kept for 2 minutes in boiling water. It was subsequently immersed in boiling alcohol and treated with iodine solution. What will be the final colour of the leaf after the test?

    Asked in: UPSC CDS 1 2023

    1. Colourless
    2. Green
    3. Blue
    4. White
    Show answer

    Answer: C. Blue

    This experiment is a standard test for the presence of starch in a leaf, which is a product of photosynthesis. Keeping the leaf on a sunny day ensures starch production. Boiling water kills the cells. Boiling alcohol removes the chlorophyll, making the leaf pale so the colour change with iodine is visible. Iodine solution turns blue-black in the presence of starch.

  2. Which one of the following is NOT a requirement for photosynthesis ?

    Asked in: UPSC NDA 2 2023

    1. Chlorophyll
    2. CO₂
    3. Water
    4. O₂
    Show answer

    Answer: D. O₂

    - Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. - The main reactants required for photosynthesis are carbon dioxide (CO₂), water (H₂O), and light energy. Chlorophyll is the pigment that captures light energy. - The general equation for photosynthesis is: 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ (glucose) + 6O₂.

  3. The point at which the rate of photosynthesis is equal to the rate of respiration is called

    Asked in: ASO 2025

    1. Respiratory quotient
    2. Equivalence point
    3. Photosynthetic quotient
    4. Compensation point
    Show answer

    Answer: D. Compensation point

    This question relates to plant physiology and the balance between photosynthesis and respiration, particularly under specific light conditions.

  4. Which is not required for photosynthesis ?

    Asked in: Combined (Group C) Exam 2023

    1. Sunlight
    2. Oxygen
    3. Chlorophyll
    4. Carbon dioxide
    Show answer

    Answer: B. Oxygen

    Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigment. The overall equation is: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2.

Light-dependent reactions: Photosystems (PS I & PS II), electron transport chain

  1. Which one of the following statements about the process of photosynthesis is correct ?

    Asked in: UPSC CDS 2 2022 · UPSC NDA 2 2021

    1. Light energy is directly converted to kinetic energy which oxidizes carbon dioxide.
    2. Light energy is directly converted to chemical energy which causes reduction of water.
    3. Chlorophyll absorbs the chemical energy of light which causes oxidation of carbon dioxide.
    4. Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.
    Show answer

    Answer: D. Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.

    - Chlorophyll absorbs light energy from sunlight. - This light energy is used to split water molecules (photolysis of water) into oxygen, protons, and electrons. This is the oxidation of water. - The energy captured and the electrons/protons generated are then used to reduce carbon dioxide into glucose (a sugar). This is the reduction of carbon dioxide. Thus, chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.

  2. Which of the following processes does not take place during photosynthesis ?

    Asked in: HPPSC HPAS Prelims 2016

    1. Absorption of light energy by chlorophyll
    2. Conversion of light energy
    3. Splitting of water molecules into hydrogen and oxygen
    4. Chlorophyll degradation
    Show answer

    Answer: D. Chlorophyll degradation

    Photosynthesis is the process by which light energy is converted into chemical energy by plants and other organisms. The core processes involve light absorption, energy conversion, and the splitting of water, ultimately leading to the synthesis of sugars.

  3. फोटोͧसंथेͧसस एक प्रͩक्रया है जो ͩक हǐरत पादपों को उनके भोजन का ͧसंथेसाइज करने में मदद करती है। घटनाओं का सोपानी पात तथा के ͧमकल अͧभͩक्रयाएँ एक ͪवͧशष्ट अनुक्रम में होती हैं। उस घटना का चयन कीǔजए जो फोटोͧसंथेͧसस में नहीं होती है।

    Asked in: HAS Prelims 2021

    1. क्लिोरोlफलि द्वारा प्रकाश ऊजार्त का अविशोषण
    2. काबर्तन ल्डिाइऑक्साइल्डि के åरल्डिक्शन से काबर्बोहाइड्रेट
    3. काबर्तन के ऑक्सील्डिेशन से काबर्तन ल्डिाइऑक्साइल्डि
    4. जलि lविभाजन
    Show answer

    Answer: C. काबर्तन के ऑक्सील्डिेशन से काबर्तन ल्डिाइऑक्साइल्डि

    - Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigment. - Key events in photosynthesis include: - Absorption of light energy by chlorophyll (Option A). This is the initial step of the light-dependent reactions. - Splitting of water molecules (hydrolysis or photolysis) (Option D). This occurs in the light- dependent reactions, releasing electrons, protons, and oxygen. - Reduction of carbon dioxide to carbohydrates (Option B). This occurs in the light- independent reactions (Calvin cycle), where CO2 is fixed and reduced using the energy and reducing power generated during the light reactions, ultimately forming glucose (a carbohydrate). - Oxidation of carbon to carbon dioxide (Option C) is the process that occurs during respiration, where organic molecules (like glucose) are broken down in the presence of oxygen to release energy, producing carbon dioxide as a byproduct. This is the reverse of what happens to carbon in photosynthesis.

  4. Photosynthesis is a process that helps green plants synthesize their food. A cascade of events and chemical reactions take place in a specified sequential order. Choose an event that does not take place in photosynthesis.

    Asked in: HAS Prelims 2021

    1. Absorption of light energy by chlorophyll
    2. Reduction of carbon dioxide to carbohydrate
    3. Oxidation of carbon to carbon dioxide
    4. Splitting of water
    Show answer

    Answer: C. Oxidation of carbon to carbon dioxide

    Photosynthesis is a process where light energy is converted into chemical energy in the form of glucose (a carbohydrate). This involves the reduction of carbon dioxide and the oxidation of water.

  5. What is the principal function of the light- dependent reactions of photosynthesis ?

    Asked in: MPSC CS Prelims 2023

    1. To transfer the NADH₂ and ATP
    2. To utilize the NADPH and ATP
    3. To generate the NADPH and ATP
    4. All of the above
    Show answer

    Answer: C. To generate the NADPH and ATP

    The light-dependent reactions convert light energy into chemical energy stored in ATP and NADPH, which are then used to power the synthesis of glucose in the light-independent reactions.

  6. Which one of the following is not a function of chlorophyll?

    Asked in: UPSC CAPF 2021

    1. Splitting of water
    2. Absorption of photon
    3. Oxidation of carbohydrate
    4. Conversion of light energy into chemical en‐ ergy
    Show answer

    Answer: C. Oxidation of carbohydrate

    Chlorophyll's main functions in photosynthesis include the absorption of light energy (photons), initiation of the light-dependent reactions which include the splitting of water molecules (photolysis) to produce oxygen, protons, and electrons, and facilitating the conversion of light energy into chemical energy in the form of ATP and NADPH. The oxidation (breakdown) of carbohydrates, such as glucose, to release energy (ATP) is the process of cellular respiration, which occurs in mitochondria (or cytoplasm/cell membrane in some organisms) and is distinct from the function of chlorophyll and photosynthesis.

  7. In the photosynthesis, assimilatory power is generated during the following process :

    Asked in: Agri. Officer 2020

    1. light reaction
    2. dark reaction
    3. photolysis of water
    4. ETS
    Show answer

    Answer: A. light reaction

    Assimilatory power (ATP and NADPH) is produced during the light reactions of photosynthesis.

  8. Which one of the following process occurs during photosynthesis? - (1) Reduction of carbohydrates into carbon dioxide. - (2) Conversion of light energy to chemical energy. - (3) Emission of light energy by chlorophyll. - (4) Conversion of hydrogen and oxygen into water.

    Asked in: Asst. Engineer Comb. Comp. Exam 2018

    1. (1) only
    2. (2) only
    3. (3) only
    4. (4) only
    Show answer

    Answer: B. (2) only

    Photosynthesis is the process used by plants, algae, and cyanobacteria to convert light energy into chemical energy, through a process that uses sunlight, water, and carbon dioxide. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water.

  9. In which of the following processes, solar energy being converted into chemical energy?

    Asked in: BJS 2023

    1. Burning
    2. LASER
    3. Photosynthesis
    4. Power supply
    Show answer

    Answer: C. Photosynthesis

    Photosynthesis is a fundamental biological process that sustains life on Earth by converting light energy into chemical energy stored in organic compounds.

  10. Where does light reaction take place in plants ?

    Asked in: Co operative Supervisor & Environment Supervisor (Group C) Exam 2023

    1. In stroma
    2. In thylakoids
    3. In cytoplasm
    4. In golgi bodies
    Show answer

    Answer: B. In thylakoids

    Photosynthesis occurs in chloroplasts, which have an inner and outer membrane. Inside, there is a fluid-filled space called the stroma and a system of interconnected flattened sacs called thylakoids, often arranged in stacks called grana. Light reactions occur on the thylakoid membranes.

Light-independent reactions: Calvin cycle (C3 pathway) mechanism

  1. In the process of Photosynthesis, how many molecules of A.T.P. are involved in the synthesis of each molecule of Glucose?

    Asked in: CGPSC Prelims Exam 2020

    1. 16
    2. 18
    3. 20
    4. 24
    Show answer

    Answer: B. 18

    The synthesis of glucose occurs during the light- independent reactions (Calvin cycle) of photosynthesis. To fix 6 molecules of CO2 and produce one molecule of glucose, the cycle consumes 18 molecules of ATP and 12 molecules of NADPH. These ATP and NADPH are produced during the light- dependent reactions.

  2. In plants, which enzyme is found in largest quantity ?

    Asked in: MPSC CS Prelims 2014

    1. β galactosidase
    2. RuBP carboxylase
    3. Enterokinase
    4. Heterokinase
    Show answer

    Answer: B. RuBP carboxylase

    RuBP carboxylase-oxygenase, commonly known as Rubisco, is the enzyme responsible for the first major step of carbon fixation in photosynthesis, where carbon dioxide is converted into energy-rich molecules. Due to its critical role in supporting virtually all life on Earth (by producing the organic matter that fuels ecosystems) and its relatively slow catalytic rate (requiring plants to synthesize large quantities), Rubisco is considered the most abundant enzyme in plants and on Earth overall.

  3. Which one of the following statements is correct?

    Asked in: UPSC CAPF 2018

    1. In photosynthesis by green plants, carbon dioxide is used for making carbohydrate.
    2. During photosynthesis, plants use oxygen for making carbohydrate.
    3. The carbohydrate generated by plants comes from soil.
    4. The carbohydrate generated by plants comes from urea.
    Show answer

    Answer: A. In photosynthesis by green plants, carbon dioxide is used for making carbohydrate.

    In photosynthesis, plants use carbon dioxide and water, with the help of light energy and chlorophyll, to synthesize carbohydrates and release oxygen. Carbon dioxide is the source of carbon atoms in the carbohydrate.

  4. The first stable product of the dark reaction of photosynthesis in C₃ plants is

    Asked in: Agri. Officer 2020

    1. 3-PFALR
    2. 3-PGA
    3. 1, 3-bis PGA
    4. DHAP
    Show answer

    Answer: B. 3-PGA

    The Calvin cycle involves several key steps: 1. Carbon Fixation CO₂ + RuBP → Unstable 6-C intermediate → 2 molecules of 3-PGA. 2. Reduction 3-PGA is converted to 1,3-bisphosphoglycerate, and then to glyceraldehyde-3-phosphate (G3P), which is an energy-rich sugar phosphate. 3. Regeneration of RuBP Some G3P molecules are used to synthesize glucose, while others are used to regenerate RuBP.

  5. What is the role of the enzyme RuBisCO in photosynthesis ?

    Asked in: ASO 2025

    1. It fixes carbon dioxide to produce carbo‐ hydrate
    2. It regulates stomatal opening and closing for gaseous exchange
    3. It catalyzes the splitting of water molecules to generate oxygen
    4. It is involved in the electron transport chain to generate ATP
    Show answer

    Answer: A. It fixes carbon dioxide to produce carbo‐ hydrate

    RuBisCO is central to the carbon fixation stage of photosynthesis, which is the first step in converting atmospheric carbon into organic molecules. It is a key enzyme in the Calvin cycle.

  6. Which gas is absorbed by plants during the process of photosynthesis?

    Asked in: JUNIOR laboratory assistant 2025

    1. Carbon dioxide
    2. Oxygen
    3. Nitrogen
    4. Hydrogen
    Show answer

    Answer: A. Carbon dioxide

    Plants absorb carbon dioxide during the process of photosynthesis.

Photolysis of water and oxygen evolution

  1. The by-product of photosynthesis is

    Asked in: BPSC Prelims Exam 65th

    1. CO₂
    2. O₂
    3. energy
    4. sugar
    Show answer

    Answer: B. O₂

    Oxygen (O₂) is released as a gaseous by-product during the light-dependent reactions of photosynthesis when water molecules are split (photolysis).

  2. In plants, which one of the following gases is released during Photosynthesis ?

    Asked in: UPSC NDA 1 2015

    1. Carbon dioxide
    2. Oxygen
    3. Hydrogen
    4. Methane
    Show answer

    Answer: B. Oxygen

    Knowing the inputs and outputs of the photosynthesis process.

  3. The oxygen evolved during photosynthesis comes from splitting of

    Asked in: UPSC NDA 2 2018

    1. water
    2. carbon dioxide
    3. oxygen
    4. light
    Show answer

    Answer: A. water

    - The overall equation for photosynthesis is 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂. - Experiments using isotopic tracing (specifically, using water labeled with the heavy oxygen isotope ¹⁸O) demonstrated that the ¹⁸O appeared in the evolved oxygen gas, while using carbon dioxide labeled with ¹⁸O resulted in the ¹⁸O appearing in the glucose product, not the oxygen gas.

  4. During photosynthesis, O₂ of the atmosphere is fixed with the help of H₂O and sunlight to synthesize carbohydrate and O₂ is evolved by the splitting of

    Asked in: UPSC CISF AC EXE 2020

    1. CO₂
    2. H₂O
    3. NO₃⁻
    4. PO₄³⁻
    Show answer

    Answer: B. H₂O

    - Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigment. - The overall equation for photosynthesis is $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H} _{12}\text{O}_6 + 6\text{O}_2$. - The process occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). - The light-dependent reactions take place in the thylakoid membranes of chloroplasts. In this stage, light energy is absorbed by chlorophyll, leading to the splitting of water molecules ($H_2O$). This process is called photolysis. - Photolysis of water ($2H_2O \rightarrow 4e^- + 4H^+ + O_2$) releases electrons (used in the electron transport chain to produce ATP and NADPH), protons ($H^+$), and oxygen gas ($O_2$) as a byproduct. This oxygen is released into the atmosphere. - The light-independent reactions (Calvin cycle) take place in the stroma of chloroplasts and use the ATP and NADPH produced during the light reactions to fix carbon dioxide ($CO_2$) from the atmosphere and synthesize carbohydrates. - The question contains a slight inaccuracy by stating "$O_2$ of the atmosphere is fixed with the help of $H_2O$ and sunlight...". It should be "$CO_2$ of the atmosphere is fixed...". However, the second part of the sentence correctly asks what $O_2$ is evolved from. - Based on the mechanism of photosynthesis, the oxygen gas evolved comes directly from the splitting of water molecules during the light-dependent stage.

  5. The photolysis of water during photosynthesis occurs in the presence of following mineral(s) :

    Asked in: Agri. Officer 2020

    1. Mn²⁺ ion
    2. Mg²⁺ ion
    3. Mn²⁺ and Cl⁻ ion
    4. K⁺ and Cl⁻ ions
    Show answer

    Answer: C. Mn²⁺ and Cl⁻ ion

    Manganese (Mn²⁺) and chloride (Cl⁻) ions are essential for the photolysis of water during photosynthesis.

  6. The source of oxygen released during photosynthesis is

    Asked in: Additional Private Secretary Exam 2017

    1. Carbon dioxide
    2. Water
    3. Sugar
    4. Pyruvic Acid
    Show answer

    Answer: B. Water

    Water is the source of oxygen released during photosynthesis.

  7. The oxygen is a product of photosynthesis that comes from

    Asked in: Assistant Conservator of Forest (ACF) 2012

    1. Carbon dioxide
    2. Carbon absorbed from soil
    3. Oxides of mineral elements
    4. water
    Show answer

    Answer: D. water

    The oxygen released during photosynthesis is derived from the photolysis of water (H₂O).

Photorespiration (C2 pathway) and its implications

  1. If lipid is the respiratory substrate, the respiratory quotient will be –

    Asked in: Food Safety Officer 2019

    1. one
    2. less than one
    3. more than one
    4. zero
    Show answer

    Answer: B. less than one

    The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration. For carbohydrates, RQ is 1. For fats, the oxidation involves more oxygen and produces less carbon dioxide compared to carbohydrates, resulting in an RQ of about 0.7.

Photosynthetic machinery: Chloroplast structure and function

  1. Solar energy is converted into ATP in

    Asked in: BPSC Prelims Exam 65th

    1. mitochondria
    2. chloroplast
    3. ribosome
    4. peroxisome
    Show answer

    Answer: B. chloroplast

    Chloroplasts are the site of photosynthesis where light energy is converted into chemical energy (ATP and NADPH).

  2. Chlorophyll in photosynthetic prokaryotic bacteria is associated with

    Asked in: UPSC CDS 2 2019

    1. plastids
    2. membranous vesicles
    3. nucleoids
    4. chromosomes
    Show answer

    Answer: B. membranous vesicles

    Chlorophyll in photosynthetic prokaryotes is not contained within plastids but is located in specialized internal membrane structures within the cytoplasm.

  3. Photosynthesis that converts light energy into chemical energy by the plants, involves the use of green pigments of leaves called chlorophyll. The only structure in a leaf that hosts chlorophyll is :

    Asked in: UPSC CDS 2 2024

    1. Nucleus
    2. Ribosome
    3. Chloroplast
    4. Chromosome
    Show answer

    Answer: C. Chloroplast

    - Chloroplasts are the site of photosynthesis in plant cells. - Chlorophyll, the main light-absorbing pigment, is located on the thylakoid membranes within chloroplasts.

  4. Photosynthesis occurs in :

    Asked in: HPPSC HPAS Prelims 2017

    1. Nucleus
    2. Mitochondria
    3. Chloroplast
    4. None of the above
    Show answer

    Answer: C. Chloroplast

    Chloroplasts are specialized organelles in plant cells that contain chlorophyll, the pigment necessary to capture light energy for photosynthesis.

  5. In which organelle does photosynthesis occur?

    Asked in: lower devision clerk 2025

    1. Mitochondria
    2. Chloroplast
    3. Ribosome
    4. Cytoplasm
    Show answer

    Answer: B. Chloroplast

    Chloroplasts are specialized organelles found in plant cells and eukaryotic algae that conduct photosynthesis. They contain chlorophyll, the pigment that captures light energy.

Photosynthetic pigments: Types, absorption and action spectra

  1. Chlorophyll absorbs ______ wavelengths of the sunlight.

    Asked in: BPSC Prelims Exam 68th

    1. green and blue
    2. green and red
    3. red and blue
    4. More than one of the above
    Show answer

    Answer: C. red and blue

    The absorption spectrum of chlorophyll shows peaks in the blue-violet and red regions of the visible light spectrum, indicating that these wavelengths are most effective for photosynthesis.

  2. Which one of the following colours of the visible spectrum of light is least absorbed by the green plants?

    Asked in: UPSC Geoscientist 2022

    1. Violet
    2. Red
    3. Green
    4. Yellow
    Show answer

    Answer: C. Green

    - Photosynthesis primarily utilizes light from the blue and red ends of the visible spectrum, as chlorophyll pigments absorb these wavelengths most effectively. - Green light is poorly absorbed by chlorophyll, hence it is reflected or transmitted, making the plants appear green.

  3. Which one of the following would be a suitable plant to test that chlorophyll is necessary for photosynthesis?

    Asked in: UPSC Geoscientist 2023

    1. Lady Plymouth
    2. Cactus
    3. Cuscuta
    4. Oleander
    Show answer

    Answer: A. Lady Plymouth

    - Experiment requires comparison between parts of a plant with and without chlorophyll. - Variegated leaves are suitable for this purpose. - Green parts contain chlorophyll, non-green parts do not. - Testing for starch after photosynthesis reveals which parts photosynthesized.

  4. Leaves of most plants appear green because the chlorophyll present in it :

    Asked in: UPSC NDA 1 2024

    1. absorbs red and blue light while reflecting green light.
    2. absorbs green light only.
    3. absorbs green light while reflecting red and blue light.
    4. reflects red light and absorbs blue and green light.
    Show answer

    Answer: A. absorbs red and blue light while reflecting green light.

    Chlorophyll is the primary pigment in plants responsible for photosynthesis. It absorbs light most strongly in the blue and red regions of the visible spectrum. The green light from the spectrum is not absorbed efficiently; instead, it is reflected by the leaves. This reflected green light is what our eyes perceive, making the leaves appear green.

  5. Match the following algal pigments with their colour : Pigment Colour (a) Chlorophyll 1. Orange (b) Xanthophyll 2. Green (c) Carotenoid 3. Blue green (d) Phycocyanin 4. Yellow

    Asked in: TNPSC CS Prelims 2017

    1. (a)-2, (b)-4, (c)-1, (d)-3
    2. (a)-2, (b)-4, (c)-3, (d)-1
    3. (a)-1, (b)-2, (c)-3, (d)-4
    4. (a)-4, (b)-1, (c)-3, (d)-2
    Show answer

    Answer: A. (a)-2, (b)-4, (c)-1, (d)-3

    Algal pigments are light-absorbing molecules found in algae that capture energy from light for photosynthesis. Different pigments absorb light at different wavelengths and have characteristic colors.

  6. Statement I: When a piece of brick lying on grass for a number of days is removed, the covered part of the grass appears pale. Statement II: Synthesis of chlorophyll, which gives green pigment, occurs in presence of sunlight.

    Asked in: UPSC CAPF 2013

    1. Both the statements are individually true and Statement II is the correct explanation of Statement I
    2. Both the statements are individually true but Statement II is not the correct explanation of Statement I
    3. Statement I is true but Statement II is false
    4. Statement I is false but Statement II is true
    Show answer

    Answer: A. Both the statements are individually true and Statement II is the correct explanation of Statement I

    - Statement I describes an observed phenomenon: grass covered by an opaque object (like a brick) becomes pale. This is a common observation. - Statement II provides a scientific fact: Chlorophyll, the pigment responsible for the green color in plants, requires sunlight for its synthesis. This process is part of photosynthesis. - The paleness of the grass (loss of green color) occurs because the brick blocks sunlight, preventing the grass from synthesizing chlorophyll. Without chlorophyll, the green pigment is absent or significantly reduced. Therefore, Statement II directly explains why Statement I occurs.

  7. Which metal is present in chlorophyll?

    Asked in: WBPSC CS Prelims 2016

    1. Iron
    2. Zinc
    3. Aluminium
    4. Magnesium
    Show answer

    Answer: D. Magnesium

    Chlorophyll is a pigment essential for photosynthesis in plants and algae.

  8. Chlorophyll contains

    Asked in: UPPCS Prelims 2012

    1. Iron
    2. Copper
    3. Magnesium
    4. Manganese
    Show answer

    Answer: C. Magnesium

    Chlorophyll, the primary pigment responsible for absorbing light energy in photosynthesis, has a porphyrin ring structure with a central metal atom. This central metal atom is magnesium.

  9. Which pigment is primarily responsible for capturing light-energy during photosynthesis?

    Asked in: JUNIOR laboratory assistant 2025

    1. Xanthophyll
    2. Carotene
    3. Chlorophyll-a
    4. Chlorophyll-b
    Show answer

    Answer: C. Chlorophyll-a

    Chlorophyll-a is the primary pigment responsible for capturing light-energy during photosynthesis.

  10. Chlorophyll provides - A) green colour of a plant leaf - B) colour pigments of flower petals - C) yellow colouration of egg yolk - D) black colour of human hair

    Asked in: Horticulture Development Officer(HDO) 2018

    1. green colour of a plant leaf
    2. colour pigments of flower petals
    3. yellow colouration of egg yolk
    4. black colour of human hair
    Show answer

    Answer: A. green colour of a plant leaf