Photosynthesis and respiration as an energy pair
The two equations look like mirror images, and students often assume they cancel out. They do not — and understanding why is the heart of bioenergetics.
Lesson overview
What you'll learn in this lesson
Understand the reactants in, and products of, photosynthesis, and a word summary for photosynthesis
Key learning points
- • The word equations
- • Why they do not cancel
- • Where the glucose goes
- • Limiting factors
This lesson at a glance
- 30 minutes
- 25 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 25
Watch first
Photosynthesis and respiration as an energy pair
Photosynthesis and respiration running side by side in a living plant.
In a nutshell
Understand the reactants in, and products of, photosynthesis, and a word summary for photosynthesis
What you already know
Part 2 of 25
Before we start
What you already know
You should already know that plants make food using light and that all living things respire.
Key words
Part 3 of 25
Key words
Words you'll need today
- Photosynthesis
- Light energy converted to chemical energy in glucose.
- Respiration
- Releasing energy from glucose in every living cell.
- Limiting factor
- The factor currently holding back the rate.
- Chlorophyll
- The green pigment that absorbs light for photosynthesis.
- Endothermic
- A process that takes in energy from its surroundings.
Reset break
Part 4 of 25
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 5 of 25
Learning cycle 1 of 2
Two equations, two purposes
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 6 of 25
Learn
The word equations
Carbon dioxide + water → glucose + oxygen, driven by light and chlorophyll, is photosynthesis: endothermic, in chloroplasts, in the light. Glucose + oxygen → carbon dioxide + water is aerobic respiration: exothermic, in mitochondria, every cell, all the time.
Explore the idea
Part 7 of 25
Learn
Why they do not cancel
A plant photosynthesises faster than it respires during the day, so it accumulates biomass. At night only respiration runs. Net gas exchange therefore reverses across a 24-hour cycle, which is a favourite exam trap.
Reset break
Part 8 of 25
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 9 of 25
Quick check
Part 10 of 25
Learning cycle
Part 11 of 25
Learning cycle 2 of 2
Rates and limiting factors
A short piece of teaching, then a check to make sure it has landed.
Reset break
Part 12 of 25
Pause
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Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 13 of 25
Learn
Where the glucose goes
Glucose is used in respiration, converted to starch for storage, built into cellulose for cell walls, turned into lipids in seeds, or combined with nitrates to make amino acids. 'Plants make food' is true but far too vague at this level.
Explore the idea
Part 14 of 25
Learn
Limiting factors
Light intensity, carbon dioxide concentration and temperature can each limit the rate. Increase light and the rate rises, then plateaus when something else becomes limiting. Above about 40 °C the rate collapses because enzymes denature — that is not a plateau, it is a fall.
Quick check
Part 15 of 25
Reset break
Part 16 of 25
Pause
That's sitting 4 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 17 of 25
Common mix-ups
Part 18 of 25
Common mix-ups
Things people often get wrong
- Plants respire all the time, not only at night.
- A plateau on a light-intensity graph means another factor is limiting, not that light stopped mattering.
- Photosynthesis takes in energy; respiration releases it.
Challenge round
Part 19 of 25
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Reset break
Part 20 of 25
Pause
That's sitting 5 of 6 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 21 of 25
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Carbon dioxide + water → glucose + oxygen, driven by light and chlorophyll, is ____: endothermic, in chloroplasts, in the light.
Challenge round
Part 22 of 25
Game · Recall cards
Photosynthesis takes place in...
Card 1 of 4
Exit quiz
Part 23 of 25
Exit quiz
Show what you've learned
6 questions, marked together at the end. Nothing is timed.
1. Reactants of photosynthesis are...
2. Respiration occurs in...
3. Above 40 °C the rate falls because...
4. Amino acids need glucose plus...
5. At night a plant...
6. Photosynthesis is described as...
Mastery quiz
Part 24 of 25
Marked quiz
End of lesson quiz: Photosynthesis and respiration as an energy pair
4 questions, marked with the reasoning shown. No timer.
1. Photosynthesis takes place in...
2. Which is exothermic?
3. A rate graph plateaus because...
4. Glucose is stored in plants as...
Lesson round-up
Part 25 of 25
Lesson round-up
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