Photosynthesis and Respiration
~8 min read
- Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Chlorophyll absorbs light energy.
- Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Mitochondria are the site.
- Reverse processes: Photosynthesis builds glucose using light; respiration releases energy by breaking glucose.
Photosynthesis and respiration are the two great chemistries of life — one stores light energy in glucose, the other releases it as ATP. CDS/OTA asks about chlorophyll, glucose production and the site of respiration.
Photosynthesis — Building Glucose
- Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.
- Site: chloroplasts — specifically the thylakoid membranes (light reaction) and stroma (Calvin/dark reaction).
- Chlorophyll (green pigment) absorbs light energy. It is the only place in a leaf that hosts the chemistry of photosynthesis.
- Light reaction splits water (photolysis), releases O₂, produces ATP and NADPH.
- Dark reaction (Calvin cycle): uses ATP/NADPH to fix CO₂ into glucose.
- Mg²⁺ is the central atom in chlorophyll — traps sunlight energy.
Cellular Respiration — Releasing Energy
- Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~38 ATP.
- Site: mitochondria (Krebs cycle in matrix; electron transport chain on inner membrane). Glycolysis happens in cytoplasm.
- Three stages: glycolysis → Krebs cycle → electron transport chain.
- Aerobic: uses oxygen, produces ~38 ATP. Anaerobic: no O₂, produces only 2 ATP plus lactic acid (muscle) or ethanol (yeast).
Comparison Table
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Where | Chloroplasts (green plants only) | Mitochondria (all living cells) |
| Raw materials | CO₂ + H₂O + light | Glucose + O₂ |
| Products | Glucose + O₂ | CO₂ + H₂O + ATP |
| Energy | Light → chemical (stored) | Chemical → ATP (released) |
| When | Only in light | Continuous, day and night |
| Direction | Anabolic — builds | Catabolic — breaks down |
Special Adaptations — Xerophytes
- Plants adapted to dry conditions have smaller leaves, waxy cuticle, stomata in sunken pits to reduce water loss.
- They have fewer stomata (not more), often sunken to slow transpiration.
- Cacti use a modified photosynthesis pathway (CAM) — open stomata only at night.
CDS/OTA PYQ Examples
Q: Plants prepare glucose in the process of:
(a) respiration (b) photosynthesis (c) degradation (d) mineral absorption
Answer: (b) Photosynthesis. [CDS-I 2022]
Q: Which statement about photosynthesis is correct?
(a) Light → kinetic energy oxidising CO₂ (b) Light → chemical energy reducing water (c) Chlorophyll absorbs chemical energy of light (d) Chlorophyll absorbs light energy which causes splitting of water and reduction of CO₂
Answer: (d) Chlorophyll absorbs light → splits water → reduces CO₂ to glucose. [CDS-II 2022]
Q: The only structure in a leaf that hosts chlorophyll is:
(a) Nucleus (b) Ribosome (c) Chloroplast (d) Chromosome
Answer: (c) Chloroplast. [CDS-II 2024]
Q: Which is NOT a feature of xerophytes?
(a) Smaller leaves (b) Large number of stomata (c) Waxy cuticle (d) Stomata in pits
Answer: (b) Xerophytes have FEWER stomata, not more. [CDS-II 2024]
Drill Photosynthesis and Respiration for CDS/OTA
CDS/OTA-pattern items on Photosynthesis and Respiration with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is photosynthesis essential for life on Earth?
It produces O₂ for respiration and locks light energy into glucose — the starting point of all food chains. Without it, atmospheric oxygen would disappear within decades.
Do plants only respire at night?
No — plants respire 24/7 like all living cells. During day, photosynthesis exceeds respiration so the plant releases net O₂. At night, only respiration runs, so the plant releases CO₂.
Why is the dark reaction misnamed?
It doesn't require darkness — it just doesn't need light directly. It uses ATP/NADPH from the light reaction. Better called the Calvin cycle or light-independent reaction.