Photosynthesis in Higher Plants – Class 11 Biology Notes
Introduction to Photosynthesis
Photosynthesis is the process by which green plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose (a type of sugar). This process is essential for life on Earth, as it produces the oxygen we breathe and forms the foundation of the food chain. In higher plants, photosynthesis occurs primarily in the leaves, inside specialized structures called chloroplasts.
1. Photosynthetic Pigments
Photosynthesis requires pigments that can absorb light energy. The most important pigments are:
- Chlorophyll: The green pigment found in chloroplasts that absorbs light energy for photosynthesis. There are two types:
- Chlorophyll a: The main pigment involved in photosynthesis, absorbs primarily blue and red light.
- Chlorophyll b: Assists chlorophyll a by absorbing light in the blue and red-orange wavelengths.
- Carotenoids: Yellow or orange pigments that assist in light absorption and protect the plant from harmful light energy. Examples include carotene and xanthophyll.
2. The Process of Photosynthesis
Photosynthesis consists of two main stages:
- Light Reaction (Light-Dependent Reaction)
- Dark Reaction (Light-Independent Reaction or Calvin Cycle)
2.1. Light Reaction (Light-Dependent Reaction)
The light reaction takes place in the thylakoid membranes of the chloroplasts and requires light energy to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are used in the dark reaction.
Steps of the Light Reaction:
- Absorption of Light Energy:
- Chlorophyll absorbs light energy, exciting electrons to a higher energy state.
- Splitting of Water (Photolysis):
- Water molecules are split into oxygen, protons (H+), and electrons. This process releases oxygen as a by-product.
- Formation of ATP and NADPH:
- The excited electrons pass through the electron transport chain (ETC), which leads to the synthesis of ATP.
- Electrons are transferred to NADP+, converting it into NADPH.
The end products of the light reaction are ATP, NADPH, and oxygen.
2.2. Dark Reaction (Calvin Cycle or Light-Independent Reaction)
The dark reaction takes place in the stroma of the chloroplast and does not require light to occur. It uses the products of the light reaction (ATP and NADPH) to convert carbon dioxide (CO2) into glucose.
Steps of the Calvin Cycle:
- Carbon Fixation:
- Ribulose bisphosphate (RuBP), a 5-carbon compound, combines with CO2 in the presence of the enzyme RuBisCO to form a 6-carbon compound.
- Reduction Phase:
- The 6-carbon compound immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA).
- ATP and NADPH are used to reduce 3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar.
- Regeneration of RuBP:
- Some of the G3P molecules are used to regenerate RuBP, using ATP, so the cycle can continue.
- Formation of Glucose:
- The remaining G3P molecules are used to form glucose and other carbohydrates.
3. Factors Affecting Photosynthesis
Several environmental and internal factors affect the rate of photosynthesis in plants. These include:
3.1. Light Intensity:
- Light is essential for photosynthesis. As light intensity increases, the rate of photosynthesis also increases, but only up to a point. After reaching a certain intensity, the rate plateaus because other factors become limiting.
3.2. Carbon Dioxide Concentration:
- Increasing the concentration of CO2 enhances the rate of photosynthesis, as it is one of the raw materials used in the Calvin Cycle. However, the effect is significant only up to a certain concentration.
3.3. Temperature:
- Photosynthesis is temperature-dependent. As temperature increases, the rate of photosynthesis increases until it reaches an optimum temperature (usually around 25-30°C in most plants). Beyond this point, the rate decreases due to enzyme denaturation.
3.4. Water Availability:
- Water is essential for photolysis in the light reaction. Lack of water can lead to stomatal closure, reducing CO2 intake and limiting photosynthesis.
3.5. Chlorophyll and Other Pigments:
- The presence of adequate chlorophyll and other pigments is crucial for capturing light energy efficiently. If the chlorophyll content is reduced (e.g., due to nutrient deficiency), the rate of photosynthesis will decrease.
4. Photosynthesis Equation
The overall chemical equation for photosynthesis is:6CO2+6H2O+light energy→C6H12O6+6O2
This equation shows that six molecules of carbon dioxide and six molecules of water, using light energy, are converted into one molecule of glucose and six molecules of oxygen.
MCQs – Photosynthesis in Higher Plants
1. The primary function of chlorophyll in photosynthesis is to:
a) Split water molecules
b) Absorb light energy
c) Convert glucose into energy
d) Transport electrons
Answer: b) Absorb light energy
2. The light reaction of photosynthesis occurs in the:
a) Stroma
b) Thylakoid membrane
c) Cytoplasm
d) Mitochondria
Answer: b) Thylakoid membrane
3. Which of the following is produced during the light reaction of photosynthesis?
a) Oxygen
b) Carbon dioxide
c) Glucose
d) Water
Answer: a) Oxygen
4. Which enzyme catalyzes the fixation of carbon dioxide in the Calvin Cycle?
a) ATP synthase
b) RuBisCO
c) NADP reductase
d) Phosphofructokinase
Answer: b) RuBisCO
5. The end products of the light reaction are:
a) Oxygen, glucose, and NADPH
b) Oxygen, ATP, and NADPH
c) ATP, CO2, and glucose
d) Oxygen, glucose, and CO2
Answer: b) Oxygen, ATP, and NADPH
6. The Calvin Cycle occurs in the:
a) Cytoplasm
b) Stroma
c) Thylakoid membrane
d) Mitochondria
Answer: b) Stroma
7. Which of the following factors does NOT directly affect the rate of photosynthesis?
a) Light intensity
b) CO2 concentration
c) Temperature
d) Soil pH
Answer: d) Soil pH
8. The light-independent reactions of photosynthesis are also known as:
a) The Calvin Cycle
b) The Electron Transport Chain
c) The Krebs Cycle
d) The Citric Acid Cycle
Answer: a) The Calvin Cycle
9. The electron transport chain in photosynthesis is responsible for:
a) Splitting water molecules
b) Forming glucose
c) Generating ATP and NADPH
d) Fixing carbon dioxide
Answer: c) Generating ATP and NADPH
10. In the photosynthesis process, which pigment absorbs light energy?
a) Carotene
b) Chlorophyll
c) Xanthophyll
d) Both a and b
Answer: d) Both a and b