The Energy Factory

The Energy Factory 🏭

Every plant is a tiny factory that turns **light** into **food**. The process is **photosynthesis**, and it's the doorway through which almost all the energy in living things enters the food chain. Master the equation, what controls its speed, and what the plant does with the sugar it makes.

The reaction ☀️

In the leaves, using energy from **light** (absorbed by **chlorophyll**), plants combine two simple raw materials into glucose: **carbon dioxide + water → glucose + oxygen** It is an **endothermic** reaction — energy is transferred **into** it from the environment (the light).

Complete the equation

carbon dioxide + _____ → _____ + oxygen

Energy in or out?

Photosynthesis is described as **endothermic**. What does that mean?

• Energy is transferred into the reaction from the environment
• Energy is released to the surroundings
• No energy is involved at all
• It only works in hot conditions

The balanced symbol equation ⚗️

**Higher tier:** the same reaction as balanced symbols — **6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂** The big numbers (coefficients) make the atoms balance: 6 carbons, 12 hydrogens and 18 oxygens on each side.

Which is balanced?

Which is the correctly **balanced** symbol equation for photosynthesis?

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• CO₂ + H₂O → C₆H₁₂O₆ + O₂
• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 3O₂
• 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂

What controls the rate? 📊

Speed up or slow down photosynthesis by changing three **limiting factors**: • **Light intensity** — more light, faster (up to a point). • **Carbon dioxide concentration** — more CO₂, faster (up to a point). • **Temperature** — warmer speeds it up, but too hot **denatures** the enzymes. A **limiting factor** is whichever one is in shortest supply — it caps the rate.

Read the graph

On a graph of rate against **light intensity**, the rate rises then **levels off** (plateaus). At the plateau, why does adding more light not speed things up?

• Light is no longer the limiting factor — something else (CO₂ or temperature) now limits the rate
• The plant has run out of chlorophyll
• Photosynthesis has stopped completely
• The light is now too bright and damages the plant

Light and distance 💡

**Higher tier:** in the RP6 pondweed experiment, light intensity depends on how far the lamp is: light intensity is proportional to **1 / distance²** (the inverse-square law). So doubling the distance doesn't halve the light — it quarters it (2² = 4). You must **show this calculation**, not just say "closer is brighter".

Do the maths

An interactive activity.

What the glucose is for 🍬

The glucose a plant makes isn't just for energy. It is used to: • **Respire** — release energy. • Make **amino acids** (combined with nitrate) → proteins. • Store as insoluble **starch**. • Make **fats and oils** for storage (e.g. in seeds). • Make **cellulose** to strengthen cell walls.

Match the use

Don't mix them up

Which correctly compares **photosynthesis** with **respiration**?

• Photosynthesis is endothermic (takes energy in); respiration is exothermic (releases it)
• Both reactions release energy to the surroundings
• Photosynthesis releases energy; respiration stores it
• Photosynthesis happens only at night

In the exam 🎓

Factory mastered. Grade-9 habits for photosynthesis: • Learn the **word equation** (and HT the balanced **symbol** equation); it is **endothermic**. • On a limiting-factor graph, name **which factor limits** at each part, and explain the **plateau** by naming the **new** limiting factor. • HT: light intensity ∝ **1/distance²** — show the calculation. And never confuse photosynthesis (endothermic) with respiration (exothermic).