Photosynthesis and Respiration
1. Photosynthesis — Making Food
PHOTOSYNTHESIS is the process by which GREEN PLANTS make their own food (glucose) using CARBON DIOXIDE and WATER in the presence of SUNLIGHT and CHLOROPHYLL.
Word Equation
Carbon dioxide + Water → Glucose + Oxygen (in presence of sunlight and chlorophyll)
Chemical Equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Site of Photosynthesis
- Organelle: CHLOROPLAST (contains the green pigment CHLOROPHYLL).
- Location: Mainly in LEAVES (also in green stems).
2. Requirements for Photosynthesis
| Requirement | Role | Source |
|---|---|---|
| Carbon dioxide | Raw material for glucose | Air (enters through STOMATA) |
| Water | Raw material, splits to provide H⁺ and electrons | Soil (absorbed by roots) |
| Sunlight | Provides ENERGY for the reaction | Sun |
| Chlorophyll | AbsorBS light energy and converts to chemical energy | Chloroplasts in leaf cells |
Experiment to Show Chlorophyll is Necessary
A variegated leaf (green + white) is tested for starch with iodine.
- Green parts turn BLUE-BLACK (starch present → photosynthesis occurred).
- White parts remain BROWN (no starch → no photosynthesis).
3. Factors Affecting Photosynthesis
| Factor | Effect |
|---|---|
| Light intensity | Increase → Increase in photosynthesis (up to a point) |
| CO₂ concentration | Increase → Increase (up to a point) |
| Temperature | Optimum 25-30°C. Too high or too low decreases rate. |
| Water availability | Water shortage causes STOMATA to close, reducing CO₂ intake |
4. Respiration
RESPIRATION is the process of BREAKING DOWN glucose to release ENERGY.
Types of Respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed | YES | NO |
| Products | CO₂ + H₂O + Energy | Lactic acid / Ethanol + CO₂ + Energy |
| Energy released | HIGH (38 ATP per glucose) | LOW (2 ATP per glucose) |
| Location | Mitochondria | Cytoplasm |
| Occurs in | Most organisms | Yeast, some bacteria, muscle cells (temporarily) |
Word Equations
Aerobic: Glucose + Oxygen → Carbon dioxide + Water + Energy C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Anaerobic (in yeast): Glucose → Ethanol + Carbon dioxide + Energy C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy
Anaerobic (in muscle): Glucose → Lactic acid + Energy C₆H₁₂O₆ → 2C₃H₆O₃ + Energy
5. Comparison: Photosynthesis vs Respiration
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Occurs in | Plants only (green parts) | All living organisms |
| Organelle | Chloroplast | Mitochondria (aerobic), Cytoplasm (anaerobic) |
| Energy | ABSORBS (requires sunlight) | RELEASES (breaks down glucose) |
| Reactants | CO₂ + H₂O | Glucose + O₂ (aerobic) |
| Products | Glucose + O₂ | CO₂ + H₂O + Energy |
| Gas exchange | Takes in CO₂, releases O₂ | Takes in O₂, releases CO₂ |
| When | Daytime (sunlight needed) | All the time (day and night) |
Important
- Plants carry out BOTH photosynthesis AND respiration.
- During the day: Photosynthesis rate > Respiration rate (net O₂ release).
- At night: ONLY respiration occurs (net CO₂ release).
6. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Photosynthesis equation and requirements | 3 marks | Very High |
| Factors affecting photosynthesis | 2 marks | High |
| Aerobic vs anaerobic respiration | 3 marks | Very High |
| Comparison of photosynthesis and respiration | 2-3 marks | High |
| Experiments (variegated leaf, starch test) | 3 marks | Medium |
Common Mistakes
- Writing the equation for photosynthesis BACKWARDS (CO₂ + H₂O → glucose + O₂, NOT the reverse).
- Thinking plants do NOT respire (they DO — all living things respire).
- Saying respiration only occurs at night (it occurs ALL the time).
- Confusing chlorophyll (pigment) and chloroplast (organelle).
Self-Test (5 Questions)
Q1. The site of photosynthesis is: (1 mark)
- A) Mitochondria
- B) Chloroplast
- C) Nucleus
- D) Cytoplasm
Q2. 'Write the word equation for aerobic respiration.' (2 marks)
Q3. 'Name two factors that affect the rate of photosynthesis.' (2 marks)
Q4. 'What is the difference between aerobic and anaerobic respiration in terms of oxygen?' (1 mark)
Q5. 'Why do plants release CO₂ at night?' (2 marks)
Answers
A1. B) Chloroplast. A2. Glucose + Oxygen → Carbon dioxide + Water + Energy. A3. Light intensity, CO₂ concentration, temperature, water availability (any two). A4. Aerobic respiration REQUIRES oxygen. Anaerobic respiration occurs WITHOUT oxygen. A5. At night, plants do NOT photosynthesise (no sunlight). They only respire. Respiration releases CO₂. So plants release CO₂ at night.
