Photosynthesis in Human and Social Biology

Introduction to Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, storing it in the bonds of organic molecules (primarily glucose). This fundamental biological process sustains life on Earth by producing oxygen and organic compounds.

Overview of Photosynthesis

Light Energy Chloroplast Glucose + O₂

Figure 1: Simplified overview of photosynthesis

The Photosynthesis Equation

The overall chemical equation for photosynthesis is:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Leaf Structure and Adaptations

Leaves are specially adapted for photosynthesis:

1. Cuticle

Waxy layer that prevents water loss

2. Upper Epidermis

Transparent layer allowing light penetration

3. Palisade Mesophyll

Tightly packed cells with many chloroplasts

4. Spongy Mesophyll

Air spaces for gas exchange

5. Lower Epidermis

Contains stomata for gas exchange

6. Vascular Bundles

Xylem and phloem for transport

Cuticle Upper Epidermis Palisade Mesophyll Spongy Mesophyll Lower Epidermis Stomata Vascular Bundle

Figure 2: Cross-section of a leaf showing photosynthetic structures

Stages of Photosynthesis

1. Light-Dependent Reactions

Occur in the thylakoid membranes of chloroplasts:

Light H₂O O₂ ATP NADPH Thylakoid Membrane

Figure 3: Light-dependent reactions in the thylakoid membrane

2. Light-Independent Reactions (Calvin Cycle)

Occur in the stroma of chloroplasts:

CO₂ ATP NADPH Glucose RuBP PGA PGAL Calvin Cycle

Figure 4: The Calvin cycle (light-independent reactions)

Factors Affecting Photosynthesis

Factor Effect Explanation
Light Intensity Increases rate up to a point More light provides more energy for light reactions
Carbon Dioxide Concentration Increases rate up to a point CO2 is a reactant in Calvin cycle
Temperature Increases rate up to optimum Enzyme activity increases with temperature
Water Availability Affects stomatal opening Water is a reactant and maintains turgor
Chlorophyll Concentration Limits light absorption More chlorophyll can capture more light
Light Intensity Rate of Photosynthesis Limiting factors CO₂ Concentration Rate of Photosynthesis Temperature Rate of Photosynthesis Enzyme denaturation

Figure 5: Graphs showing factors affecting photosynthesis rate

Importance of Photosynthesis

Glossary of Terms

Chlorophyll
Green pigment in chloroplasts that absorbs light energy.
Thylakoid
Flattened sacs in chloroplasts where light-dependent reactions occur.
Stroma
Fluid-filled space in chloroplasts where Calvin cycle occurs.
Photolysis
Splitting of water molecules using light energy.
NADPH
Electron carrier molecule produced in light reactions.
RuBP (Ribulose Bisphosphate)
5-carbon compound that binds with CO2 in Calvin cycle.
PGA (Phosphoglycerate)
3-carbon compound formed when CO2 binds with RuBP.
PGAL (Glyceraldehyde-3-Phosphate)
3-carbon sugar produced in Calvin cycle used to make glucose.

Self-Assessment Questions

1. Write the balanced chemical equation for photosynthesis.
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
2. Name two adaptations of leaves for photosynthesis.
1) Broad, flat shape for large surface area. 2) Thin structure for short diffusion distance. 3) Many chloroplasts in palisade layer. 4) Stomata for gas exchange. 5) Transparent epidermis for light penetration.
3. What are the products of the light-dependent reactions?
ATP, NADPH, and oxygen (O2)
4. Explain why the Calvin cycle is called "light-independent".
The Calvin cycle doesn't require light directly - it uses the ATP and NADPH produced by the light-dependent reactions. However, it indirectly depends on light because it needs these products.
5. How does temperature affect the rate of photosynthesis?
Increasing temperature increases rate up to an optimum as enzyme activity increases. Beyond this, enzymes denature and rate decreases sharply.
6. What happens to the oxygen produced during photosynthesis?
Most is released into the atmosphere through stomata, where it becomes available for aerobic respiration in plants and animals.
7. Why is photosynthesis important for food chains?
Photosynthesis produces glucose that plants use for growth. This biomass becomes food for herbivores, which in turn are food for carnivores, forming the basis of all food chains.
8. What would happen to a plant kept in the dark?
It would eventually die because it couldn't photosynthesize to produce glucose for energy. It would use up its stored reserves and be unable to make more without light.
9. How does carbon dioxide concentration affect photosynthesis?
Increasing CO2 concentration increases the rate up to a point, as it's a reactant in the Calvin cycle. Beyond this, other factors become limiting.
10. Describe the role of chlorophyll in photosynthesis.
Chlorophyll absorbs light energy, exciting electrons that are passed along electron transport chains. This energy is used to split water and produce ATP and NADPH.

Summary of Key Points