What You Need to Know
✓ Photosynthesis definition and word equation
✓ Role of chlorophyll and chloroplasts
✓ How plants use and store glucose
✓ Why minerals matter for plant growth
✓ Experimental investigation methods
✓ Balanced chemical equation
✓ Limiting factors (temperature, light, CO₂)
✓ Leaf structure and adaptations
Photosynthesis: The Basics
What Is Photosynthesis?
Photosynthesis is the process by which green plants manufacture carbohydrates from raw materials using energy from light. Think of it as a plant's superpower: they can turn sunlight into food!
The simple truth: Plants take carbon dioxide from the air and water from the soil, and with the help of sunlight, transform these into glucose (sugar) and oxygen. The oxygen is released as a waste product—which is lucky for us because we breathe it!
Word Equation
⚠️ Common Exam Mistake: Students often say "light energy" is a raw material. It's not! Light is the energy source that drives the reaction, but only carbon dioxide and water count as raw materials. Examiners look for this distinction.
Where Does Photosynthesis Happen?
Photosynthesis occurs in the chloroplasts of plant cells—tiny structures inside leaf cells where all the magic happens. The process requires three things to work:
- Carbon dioxide (from the air)
- Water (from the roots)
- Light energy (from the sun)
Practice Question 1
State the two raw materials required for photosynthesis to occur.
Chlorophyll: The Light Absorber
What Is Chlorophyll?
Chlorophyll is a green pigment found inside chloroplasts within plant cells. Its job is simple but absolutely critical: to absorb light energy from the sun and convert it into chemical energy stored in glucose molecules.
Why are plants green? Because chlorophyll reflects green light and absorbs other colours (mainly red and blue). The green light bounces off, so we see plants as green!
The Role of Chlorophyll
Chlorophyll doesn't just absorb light and hope for the best. It specifically:
- Absorbs light energy (especially red and blue wavelengths)
- Transfers this energy into chemical form
- Powers the synthesis of glucose from CO₂ and water
🔑 Key Concept: Photosynthesis will not occur in the absence of chlorophyll. No chlorophyll = no light absorption = no photosynthesis. This is why variegated (partly white) leaves only photosynthesise in their green areas.
Practice Question 2
Explain why a plant's leaf appears green.
What Happens to the Glucose?
The Many Fates of Glucose
Once a plant makes glucose through photosynthesis, it doesn't just sit there. Glucose is incredibly versatile and can be used in several ways:
Storage
Converted into starch — Plants store excess glucose as starch in their leaves and roots. Starch is insoluble, so it doesn't affect osmosis in cells, and it's easy to store for later use. When the plant needs energy (e.g., at night), it breaks starch back down into glucose.
Structure
Converted into cellulose — Cellulose is the main component of plant cell walls. Plants link thousands of glucose molecules together to build strong, rigid structures.
Energy
Used in respiration