What is Photosynthesis in Higher Plants Class 11: Definition & Process

Definition and Importance of Photosynthesis in Higher Plants

Photosynthesis is a biochemical process that occurs in green plants, algae, and some bacteria, enabling them to synthesize food from inorganic substances. In higher plants, photosynthesis primarily takes place in the leaves, specifically within the chloroplasts.

Definition: Photosynthesis is the process by which green plants use sunlight to convert carbon dioxide ($CO_2$) and water ($H_2O$) into glucose ($C_6H_{12}O_6$) and oxygen ($O_2$).

Importance:

• Provides energy for plant growth and development.
• Produces oxygen necessary for respiration in living organisms.
• Forms the base of the food chain in ecosystems.
• Helps in maintaining atmospheric $CO_2$ balance.

This process is crucial for sustaining life on Earth and is a key topic in Class 11 NCERT Biology syllabus.

Structure of Chloroplasts and Their Role in Photosynthesis

Chloroplasts are the organelles where photosynthesis occurs. They are found mainly in the mesophyll cells of leaves.

Key features of chloroplasts:

• Double membrane envelope.
• Internal fluid called stroma.
• Thylakoid membranes stacked into grana.
• Contain chlorophyll pigments.

Role in photosynthesis:

• Thylakoids house chlorophyll that absorbs light energy.
• Light reactions occur in thylakoid membranes.
• Dark reactions (Calvin cycle) take place in the stroma.

The arrangement of chloroplasts maximizes light absorption, making photosynthesis efficient in higher plants.

The Two Main Stages of Photosynthesis: Light and Dark Reactions

Photosynthesis consists of two major stages:

1. Light Reactions (Photochemical Phase):

• Occur in the thylakoid membranes.
• Require light energy.
• Light energy excites chlorophyll, producing ATP and NADPH.
• Water molecules split (photolysis) releasing oxygen.

2. Dark Reactions (Biosynthetic Phase or Calvin Cycle):

• Occur in the stroma.
• Do not require light directly.
• Use ATP and NADPH to convert $CO_2$ into glucose.

Overall reaction:

$$6CO_2 + 12H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$$

Understanding these stages helps Class 11 students grasp how plants convert solar energy into chemical energy.

Factors Affecting Photosynthesis in Higher Plants

Several environmental and internal factors influence the rate of photosynthesis:

• Light Intensity: Photosynthesis rate increases with light up to a point.
• Carbon Dioxide Concentration: Higher $CO_2$ levels boost photosynthesis.
• Temperature: Optimal temperature range (25-35 °C) is necessary.
• Water Availability: Water stress reduces photosynthesis.
• Chlorophyll Content: More chlorophyll means higher photosynthetic capacity.

Students should understand these factors for practical and theoretical questions.

Comparison Between Photosynthesis and Respiration in Plants

Photosynthesis and respiration are complementary processes in plants. Here is a comparison:

This comparison clarifies the energy flow in plants and is important for Class 11 exams.

Worked Example: Calculating Glucose Produced in Photosynthesis

Problem: How many molecules of $CO_2$ are required to produce one molecule of glucose during photosynthesis?

Solution: The photosynthesis equation is:

$$6CO_2 + 12H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$$

This shows that 6 molecules of $CO_2$ are required to produce 1 molecule of glucose ($C_6H_{12}O_6$).

Formula:

$$\text{Glucose molecules} = \frac{1}{6} \times \text{CO}_2 \text{ molecules used}$$

This simple calculation helps students understand stoichiometry in photosynthesis.