Almost every living thing on Earth depends on one quiet process happening inside green leaves. Plants do something animals cannot: they make their own food out of nothing more than air, water, and sunlight. That process is photosynthesis, and once you see how it fits together it explains why plants are green, why we can breathe, and where the food chain really begins.
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View feesWhat Is Photosynthesis?
Definition: Photosynthesis
The Word and Balanced Equations
You will be expected to know photosynthesis as both a word equation and a chemical (symbol) equation. They say exactly the same thing.
Word Equation
(in the presence of light and chlorophyll)
Balanced Symbol Equation
six carbon dioxide + six water → one glucose + six oxygen
The Four Things a Plant Needs
| Requirement | Where it comes from | Why it is needed |
|---|---|---|
| Carbon dioxide | Air, through tiny pores (stomata) in the leaf | Supplies carbon to build glucose |
| Water | Soil, taken up by the roots | Supplies hydrogen to build glucose |
| Light | The Sun (or a lamp) | Provides the energy to drive the reaction |
| Chlorophyll | Chloroplasts inside leaf cells | Absorbs light energy (and makes leaves green) |
Why the Leaf Is Built for the Job
Leaves are not flat and thin by accident — almost every feature is an adaptation that makes photosynthesis more efficient.
Leaf Adaptations
- • Broad and flat — a large surface area to catch more light.
- • Thin — gases only have a short distance to travel in and out.
- • Stomata — pores (mostly on the underside) let CO₂ in and O₂ out.
- • Veins — bring water from the roots and carry glucose away.
- • Packed with chloroplasts — especially in the upper layer, near the light.
Limiting Factors: What Speeds It Up or Slows It Down
The rate of photosynthesis depends on three main conditions. A limiting factor is whatever is in shortest supply — it is the bottleneck that holds the whole process back, even if everything else is plentiful.
The Three Limiting Factors
- 1. Light intensity — more light, faster reaction (until something else runs short).
- 2. Carbon dioxide concentration — more CO₂, faster reaction.
- 3. Temperature — warmth speeds it up, but too hot damages the enzymes and it slows or stops.
A plant is in bright light and warm conditions but the rate stops rising. Why?
List what is plentiful
Find what is in short supply
State the conclusion
Why Photosynthesis Matters
The Bigger Picture
- • It makes the glucose that feeds the plant and every animal in the food chain.
- • It releases the oxygen that most living things need to breathe.
- • It removes carbon dioxide from the air, helping balance the atmosphere.
- Photosynthesis turns CO₂ + water into glucose + oxygen using light energy.
- Word equation: carbon dioxide + water → glucose + oxygen.
- Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Chlorophyll absorbs light; leaves look green because they reflect green light.
- Light and chlorophyll go above the arrow — they are not reactants.
- Limiting factors: light intensity, CO₂ concentration, and temperature.
- It supplies the food chain with energy and the air with oxygen.
Practice Problems
- 1
Write the word equation for photosynthesis.
Hint: Two reactants on the left, two products on the right.
- 2
Name the green pigment that absorbs light for photosynthesis, and say which cell structure contains it.
Hint: Pigment starts with 'chloro'; the structure does too.
- 3
Where does a plant get the carbon dioxide and the water it uses?
Hint: One comes through the leaf, the other through the roots.
- 4
A farmer keeps a greenhouse warm and well-lit but the plants stop growing faster. Suggest the limiting factor and one way to fix it.
Hint: Which of the three factors has not been increased?
- 5
Explain why leaves are usually broad, flat and thin.
Hint: Think about catching light and moving gases.
- 6
How are photosynthesis and respiration related?
Hint: Compare what each one uses and produces.
Next: Respiration and the Carbon Cycle
Photosynthesis is one half of the story — respiration is the other, and together they drive the carbon cycle that keeps the whole planet ticking. If biology equations and diagrams feel like a lot to hold in your head at once, start with a consultation and we will build them up one piece at a time.
