The Energy Factory
How plants turn light into food: the photosynthesis equation, the factors that limit its rate, and everything glucose is used for.
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The Energy Factory
Every plant is a tiny factory that turns **light** into **food**. The process is **photosynthesis**, and it is the doorway through which almost all the energy in living things enters the food chain. 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.
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.
Balance the equation
Complete the balanced symbol equation for photosynthesis: _____CO₂ + _____H₂O → C₆H₁₂O₆ + _____O₂
What controls the rate?
Speed photosynthesis up or slow it down by changing three factors. The higher-tier detail about the lamp belongs here too, because it is how light intensity is actually varied in RP6:
Plot the rate
A pondweed experiment measured the rate of photosynthesis at different light intensities. The x-axis is light intensity (arbitrary units); the y-axis is the rate (bubbles per minute). Plot the six points: 0 → 0, 1 → 2, 2 → 4, 3 → 5, 4 → 5, 5 → 5.
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, such as carbon dioxide 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
Do the maths
Light intensity ∝ 1 / distance². If the lamp is moved to twice its distance from the pondweed, the light intensity drops to one-Nth of the original. What is N?
What the glucose is for
The glucose a plant makes is not just for energy. It is used to: • **Respire**: release energy. • Make **amino acids**, combined with nitrate, which build proteins. • Store as insoluble **starch**. • Make **fats and oils** for storage, for example in seeds. • Make **cellulose** to strengthen cell walls. Name the use AND what it is for. "Storage" on its own does not say whether you mean starch, oils or cellulose.
Match the use
- Used in respiration
- Made into amino acids
- Stored as starch
- Made into cellulose
- Made into fats and oils
- Releases energy for the cell
- Builds proteins
- An insoluble energy store
- Strengthens cell walls
- An energy store, e.g. in seeds
Don't mix them up
Which correctly compares **photosynthesis** with **respiration**?
- Photosynthesis is endothermic and takes energy in; respiration is exothermic and releases it
- Both reactions release energy to the surroundings
- Photosynthesis releases energy; respiration stores it
- Photosynthesis happens only at night
Do not mix them up
One step above asks how photosynthesis and respiration differ, and until now nothing in this module put them side by side. This is the discriminator, and the rest of the grade-9 habits sit under it: learn the **word equation** and, for higher tier, the balanced **symbol** equation; on a limiting-factor graph name **which** factor limits at each part and explain the plateau by naming the **new** limiting factor; and show the **1/distance²** calculation rather than saying that closer is brighter.