KS3 Science · National Curriculum · Biology: Material Cycles & Energy
Mini-Lesson
Material cycles and energy
This mini-lesson walks you through the KS3 Biology topic Material cycles and energy: how plants make food by photosynthesis, how all living things release energy by respiration, and how energy flows through an ecosystem.
Work through each screen, answer the questions as you go and collect ⭐ stars. Press Start when you're ready.
Where does life's energy come from?
It all starts with the Sun
Almost every living thing on Earth depends on the Sun. But animals cannot use sunlight directly — only green plants and algae can capture it, through a process called photosynthesis.
Plants turn light energy into chemical energy, stored inside glucose (a sugar).
Animals get their energy by eating plants, or by eating other animals that ate plants.
Photosynthesis also tops up the oxygen in the air that we breathe.
Big idea: plants are producers — they make their own food. Everything else in a food chain is a consumer that ultimately relies on them.
Photosynthesis · word equation
The recipe plants use
In photosynthesis a plant takes in carbon dioxide and water, and — using energy from light — builds glucose and releases oxygen:
carbon dioxide + water → glucose + oxygen(light energy is needed, trapped by the green pigment chlorophyll)
Reactants (the ingredients): carbon dioxide + water.
Products (what is made): glucose + oxygen.
Watch out: read the arrow left to right. Ingredients (raw materials) go on the left; the things that are made go on the right. Getting the equation the wrong way round is one of the most common mistakes.
Quick check
Name the ingredients
?Which pair are the reactants (the raw materials taken in) for photosynthesis?
The leaf
Where photosynthesis happens
Photosynthesis mostly takes place in the leaves. A leaf is beautifully adapted to catch light and swap gases with the air:
A leaf is broad and flat (big surface for light), thin (gases move in and out easily), packed with chloroplasts, and dotted with tiny holes called stomata.Inside the cell
Chloroplasts and chlorophyll
Inside a leaf cell are tiny green structures called chloroplasts. They contain a green pigment, chlorophyll, which absorbs light — this is what actually traps the Sun's energy so photosynthesis can happen.
The green chloroplasts are where light is absorbed. No chlorophyll, no photosynthesis — which is why leaves are green.Quick check
What traps the light?
?Leaves are green because of a pigment that absorbs light for photosynthesis. What is this green pigment called, and where is it found?
What glucose is for
Making sugar isn't the end
Photosynthesis makes glucose, but a plant doesn't just leave it sitting there. It uses glucose in several ways:
Respiration — broken down to release energy for the plant's own cells (yes, plants respire too!).
Storage as starch — insoluble, so it can be stored in leaves, roots and seeds.
Making other substances — such as cellulose for cell walls, and (with nitrogen from the soil) proteins for growth.
Common mistake: people say "photosynthesis makes energy". It doesn't — it makes glucose, which stores chemical energy. The energy is only released later, by respiration.
Quick check
Storing the sugar
?Plants convert some of the glucose they make into a substance for storage because it is insoluble. What is this storage substance?
Rate of photosynthesis
Speeding it up (and slowing it down)
How fast a plant photosynthesises depends on three main things. Give the plant more of these and, up to a point, it makes food faster:
More light, more carbon dioxide and a warmer temperature all speed photosynthesis up — but if the temperature gets too high the plant's enzymes stop working.
Whichever factor is in shortest supply holds the rate back. On a dull, cold day it might be light or temperature; in a bright greenhouse it is often carbon dioxide.
Sort it
What's limiting the plant?
Tap the factor that is most likely holding back photosynthesis in each case.
Respiration · why we need it
Releasing the stored energy
Respiration is the process that releases energy from glucose so cells can use it. It happens in every living cell — plants, animals, fungi and microbes — all the time, day and night.
Cells need that energy for jobs such as:
moving and contracting muscles;
keeping warm (in mammals and birds);
building bigger molecules, so the organism can grow and repair itself.
Watch out: respiration is not the same as breathing. Breathing just moves air in and out of the lungs. Respiration is the chemical reaction inside cells that releases energy.
Aerobic respiration · word equation
Respiration with oxygen
Aerobic respiration uses oxygen to break glucose down completely. It releases the most energy and is the type your cells use most of the time:
glucose + oxygen → carbon dioxide + water(this reaction releases energy for the cell to use)
Aerobic respiration is almost the reverse of photosynthesis — but the two happen for opposite reasons.Quick check
Products of respiration
?In aerobic respiration, glucose reacts with oxygen. Which two products are made?
Anaerobic respiration · in humans
Respiring without enough oxygen
During hard exercise your muscles may need energy faster than your blood can deliver oxygen. They switch to anaerobic respiration — releasing energy without oxygen:
glucose → lactic acid(anaerobic respiration in human muscle — no oxygen needed)
It releases energy quickly, but much less per glucose than aerobic respiration.
It builds up lactic acid, which makes muscles ache and tire.
After exercise you keep breathing hard to bring in extra oxygen to break down the lactic acid — this "extra" is called the oxygen debt.
Quick check
Sprinting flat out
?A sprinter's leg muscles run short of oxygen. Which substance builds up as they respire anaerobically?
Anaerobic respiration · microorganisms
Fermentation in microbes
Microorganisms such as yeast can also respire anaerobically. In yeast this is called fermentation, and it makes different products from human muscle:
glucose → ethanol + carbon dioxide(anaerobic respiration in yeast — this is fermentation)
The ethanol (alcohol) is used to make beer and wine.
The carbon dioxide makes bubbles that cause bread dough to rise.
Compare: humans → lactic acid; yeast → ethanol + carbon dioxide. Both are anaerobic (no oxygen), but the products are different.
Quick check
Baking and brewing
?Yeast is used to make bread rise and to brew beer. Which two products does yeast make when it ferments glucose?
A common misconception
Do plants photosynthesise or respire?
Both! A very common mistake is to think plants only photosynthesise and animals only respire. In fact:
Plants respire all the time, day and night, to release energy — just like animals.
Plants only photosynthesise in the light, because that step needs light energy.
So in bright daylight a plant usually photosynthesises faster than it respires, and gives out oxygen overall. In the dark it only respires, taking in oxygen.
Remember: "plants make their own food" does not mean "plants don't respire". They do — otherwise they could not use the energy their glucose stores.
Quick check
Night-time in the garden
?What is a plant doing at midnight, in complete darkness?
Ecosystems · energy flow
Energy flows through a food chain
Photosynthesis and respiration link every living thing into one big flow of energy. Light energy enters through plants and passes along the food chain:
At each step, a lot of energy is lost — used up in movement and released as heat during respiration. That is why there is less energy available higher up the chain.
Quick check
Following the energy
?In the food chain grass → rabbit → fox, where does the energy originally come from, and why is there less of it available to the fox than to the rabbit?
Sort it
Photosynthesis or respiration?
Tap a statement, then tap the process it belongs to.