Photosynthesis is the process by which green plants (and some other organisms) use LIGHT ENERGY to make their own food.
It converts two simple raw materials — carbon dioxide and water — into glucose and oxygen.
Photosynthesis is an ENDOTHERMIC reaction — it takes in energy (from light) and stores it as chemical energy in glucose molecules.
This is the opposite of respiration, which releases energy from glucose.
Photosynthesis is the foundation of almost all food chains on Earth — it is the process that converts the Sun's energy into a form that living organisms can use.
Where Photosynthesis Happens
Photosynthesis takes place in the CHLOROPLASTS — organelles found in the cells of green plants and algae.
Chloroplasts contain a green pigment called CHLOROPHYLL.
Chlorophyll absorbs LIGHT ENERGY — mainly red and blue wavelengths of light.
Green light is REFLECTED by chlorophyll — this is why plants appear green to our eyes.
Not all plant cells have chloroplasts:
Leaf palisade cells — packed with chloroplasts (maximum photosynthesis).
Root cells — no chloroplasts (underground, no light available).
Storage cells — no chloroplasts (no photosynthesis needed).
The Equation for Photosynthesis
Word equation:
carbon dioxide + water → glucose + oxygen
Symbol equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This equation requires LIGHT ENERGY — usually written above the arrow.
REACTANTS (what goes in):
Carbon dioxide (CO₂) — absorbed from the air through stomata in the leaves.
Water (H₂O) — absorbed from the soil by root hair cells, transported up to leaves through xylem.
PRODUCTS (what comes out):
Glucose (C₆H₁₂O₆) — a sugar used by the plant for energy and building materials.
Oxygen (O₂) — released as a by-product through stomata.
Photosynthesis vs Respiration
Students often confuse these two processes. Here is the key difference:
PHOTOSYNTHESIS:
Occurs ONLY in light.
Occurs ONLY in cells that have chloroplasts.
Takes in CO₂ and water → makes glucose and O₂.
Stores energy in glucose (endothermic).
RESPIRATION:
Occurs ALL THE TIME — day and night.
Occurs in ALL living cells (including animals, bacteria).
Breaks down glucose + O₂ → CO₂ and water.
Releases energy from glucose (exothermic).
In BRIGHT LIGHT: rate of photosynthesis > rate of respiration → net release of oxygen.
In LOW LIGHT: rate of photosynthesis < rate of respiration → net release of carbon dioxide.
At the COMPENSATION POINT: rate of photosynthesis = rate of respiration → no net gas exchange.
⚠️ Common Mistake
Photosynthesis only happens in LIGHT and only in cells with CHLOROPLASTS. Respiration happens ALL THE TIME in ALL living cells. Never say 'plants don't respire' — they do, constantly. In bright light they photosynthesise faster than they respire, which is why we see net O₂ production.
📐 Key Equations
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy)
📌 Key Note
Photosynthesis: CO₂ + H₂O → glucose + O₂. Needs light energy. Occurs in chloroplasts. Endothermic — stores energy. Chlorophyll absorbs red and blue light, reflects green.
🎯 Matching Activity — Photosynthesis — Match the Term
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Reactant
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Product
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Where it happens
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Energy type
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Glucose — used for energy, building and storage
Chloroplasts — contain chlorophyll that absorbs light
Endothermic — light energy absorbed and stored in glucose
Carbon dioxide — enters through stomata from the air
Water — absorbed by roots, travels up through xylem
Oxygen — released as a by-product through stomata
🧪 Required Practical
🔬 RP5 — Investigate the effect of light intensity on the rate of photosynthesis using aquatic plants (e.g. Elodea or Cabomba). Count oxygen bubbles per minute at different distances from a lamp.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 4
1. What is the word equation for photosynthesis?
2. Why do plants appear green?
3. In a dark room, does a plant carry out photosynthesis?
4. Where exactly in a plant cell does photosynthesis take place?
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