The Energy Factory

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.

Work out the equation from the evidence

A pondweed is lit brightly and gas bubbles collect at the top of the tube. The gas relights a glowing splint, so the product leaving the plant is _____. Take the dissolved carbon dioxide out of the water and the bubbling stops, which shows that _____ is one of the two raw materials. The plant gains mass although nobody ever feeds it, because the other product, _____, is being built inside the leaf. The second raw material is the one the roots draw up: _____.

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.

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

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.