Learn · Biology · Class 9

Bioenergetics

How plants capture sunlight, and how every living thing releases the energy again.

8 parts
Scroll to begin ↓
📘 DefinitionTwo opposite processesPart 1 of 8
Inside a leafwaxy cuticle + upper epidermispalisade layer — most chloroplastsspongy layer — air spaceslower epidermisstomata — CO₂ in, O₂ and water vapour outsunlight
A leaf in cross-section: light enters the top, gases move through the stomata below.
🧮 EquationsRespiration, aerobic and anaerobicPart 3 of 8
✏️ Worked exampleLimiting factorsPart 4 of 8
🌍 In the real worldWhy this shapes agriculturePart 5 of 8
⚠️ Common mistakesWhere marks get lostPart 6 of 8
✅ SummaryWhat to rememberPart 7 of 8
✏️ Worked exampleWatch one worked throughPart 8 of 8
1📘 DefinitionPart 1 / 8

Two opposite processes

Bioenergetics is the study of how energy flows through living things. Two processes do the work, and they are near-opposites.

Photosynthesis: plants use light energy to build glucose from carbon dioxide and water.

Respiration: all living things break glucose down again to release usable energy.

Every bit of energy in your body traces back to sunlight captured by a plant. That is true whether you eat the plant or eat an animal that did.

2🧩 The partsPart 2 / 8

Photosynthesis, and where it happens

carbon dioxide + water →(light, chlorophyll)→ glucose + oxygen
6CO2 + 6H2O → C6H12O6 + 6O2

It happens in chloroplasts, which contain the green pigment chlorophyll that absorbs light.

A leaf is built for the job: broad and flat for surface area, thin so gases diffuse quickly, a palisade layer packed with chloroplasts near the top, air spaces below, and stomata underneath to let CO2 in and O2 out.

3🧮 EquationsPart 3 / 8

Respiration, aerobic and anaerobic

Aerobic respiration: with oxygen, in the mitochondria. Releases a lot of energy.

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Anaerobic respiration: without oxygen. Much less energy, because the glucose is only partly broken down.

in muscle: glucose → lactic acid + a little energy
in yeast: glucose → ethanol + CO2 + a little energy

Lactic acid build-up causes the burn in your muscles during a sprint, and creates an oxygen debt you repay by breathing hard afterwards.

4✏️ Worked examplePart 4 / 8

Limiting factors

Question. A graph shows photosynthesis rate against light intensity. It rises, then flattens. Explain the shape.

The rising part. Light is the limiting factor, more light, more photosynthesis.

The flat part. Something else has become limiting: carbon dioxide concentration or temperature. Adding more light now changes nothing.

How to answer well: name which factor is limiting in each region, and say that the rate is set by whichever factor is in shortest supply.

Test it: if you raise CO2 and the line rises again, CO2 was the limiting factor. That is exactly how commercial greenhouses are optimised, and why they burn CO2 generators.

5🌍 In the real worldPart 5 / 8

Why this shapes agriculture

Greenhouses control all three limiting factors at once, extra light, added CO2, and heating, to raise yield well above field conditions.

Crop spacing. Plants too close together compete for light, so each gets less and yields fall. Correct spacing is applied bioenergetics.

Food chains. Energy is lost at each transfer, so far more people can be fed from a hectare of wheat than from a hectare grazed by cattle: the reason grain dominates in densely populated regions.

Fermentation. Anaerobic respiration in yeast produces the CO2 that makes bread rise and the ethanol in fermented drinks and biofuel.

6⚠️ Common mistakesPart 6 / 8

Where marks get lost

  • "Plants respire only at night." They respire all the time. In daylight photosynthesis simply exceeds it.
  • "Plants take in CO₂ and give out O₂" as the whole story. They also take in O₂ for respiration.
  • Unbalanced equations. Both need the 6s.
  • Saying anaerobic respiration in muscle makes ethanol. That is yeast; muscle makes lactic acid.
  • "Chlorophyll makes food." It absorbs light; the reactions make the food.
7✅ SummaryPart 7 / 8

What to remember

  • 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light, chlorophyll).
  • C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, in mitochondria.
  • Anaerobic: lactic acid in muscle, ethanol + CO₂ in yeast.
  • Limiting factors: light, CO₂, temperature.
  • Plants respire constantly.
8✏️ Worked examplePart 8 / 8

Watch one worked through

Here is one done slowly, with the reason for every move. Then one for you.

Watch one first

Write the equation for photosynthesis and explain why a plant also respires at night.

  1. Write the balanced equation.
    6CO₂ + 6H₂O →(light)→ C₆H₁₂O₆ + 6O₂
    Include the light and chlorophyll above the arrow. They are conditions, not reactants, so they do not sit in the equation itself.
  2. Note that photosynthesis makes glucose but does not release its energy.
    photosynthesis = stores energy
    Glucose is a store. A plant cannot use stored energy for anything until it breaks it down again.
  3. Respiration is how the stored energy is released, and it never stops.
    C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
    Notice it is the photosynthesis equation reversed. Every living cell respires, day and night, plant or animal.
  4. At night only respiration runs, because there is no light.
    day: photosynthesis + respiration night: respiration only
    The common error is thinking plants respire only at night. They respire all the time; at night it is just the only one happening.

Answer: Photosynthesis stores energy as glucose; respiration releases it and runs constantly, so at night only respiration is happening

Now you try

Which gas does a plant give out during photosynthesis? One word.

Show the answer

oxygen

Practise Bioenergetics

Everything on QuizByt for Class 9 Biology that goes with this lesson: read it first, then test yourself while it is fresh.

🧠 5 quizzes on Bioenergetics (2 easy, 2 medium, 1 hard)

Work up from easy to hard: the hard quiz asks the Class 9 exam-style questions on Bioenergetics.

🖨️ Printable worksheet

Bioenergetics on paper for Class 9, with room to write the working out. Preview the first page free.

🎮 Games for Class 9 Biology

Memory and reasoning games at the right level for Class 9. Passing a quiz on Bioenergetics earns the credits that open them.

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