GCSE · Biology · AQA · Spec 8461

Rate of photosynthesis

Move a lamp closer to pondweed and it fizzes faster with oxygen, until suddenly moving it closer changes nothing. So what's holding the plant back now?

Biology · Photosynthesis

Turn up the light. Then turn it up again.

You're starting in dim light. Drag Light intensity up and watch the rate ring climb, until it suddenly stops climbing. Read the pill underneath to see what took over, then raise that and watch the ceiling lift. To explore temperature, set light and CO₂ high first. This is a model: it shows the pattern, not real measurements.

☀

Light 20%

CO₂

CO₂ 60%

T°

25°C

Chloroplast

carbon dioxide + water → glucose + oxygen

Glucose

O₂

Oxygen

H₂O

Water (vapour)

Light intensity20%

CO₂ concentration60%

Temperature25°C

Limiting factor: Light — raise this to push the rate higher.

Exam line: The limiting factor is whichever one is in shortest supply. Raising that factor raises the rate; raising any other factor does nothing.

Watch out: A flat line doesn't mean photosynthesis has stopped. The plant is still photosynthesising at a steady rate. It just can't go any faster until the new limiting factor is raised.

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

	Its job in photosynthesis	Raise it and the rate…	Why the rate stops rising (or falls)
Light intensity
Carbon dioxide concentration
Temperaturethe odd one out
Amount of chlorophyll

Each cell hides a short answer and the reason behind it. Predict before you tap.

Biology · Practical

Pondweed and a lamp: measuring the rate

Pondweed in water, gas collecting in a measuring cylinder

Pondweed in water, gas collecting in a measuring cylinder. Distance from lamp: 100 cm. How does the distance of the lamp from the pondweed, and so the light intensity, affect the rate of photosynthesis?

How does the distance of the lamp from the pondweed, and so the light intensity, affect the rate of photosynthesis?

PredictThe lamp moves from 100 cm to 20 cm away. What happens to the oxygen collected in three minutes?

Distance from lamp (cm)

Timer0.0 min

Pondweed releases oxygen as it photosynthesises, so it fizzes with tiny bubbles. Collect that gas for three minutes with the lamp at each distance, and the amount tells you the rate. These readings are illustrative: they show the pattern, not real results. Only the lamp distance changes. A thermometer in the water checks that the temperature isn't rising.

Exam line: Only the distance should change. The thermometer is there because a lamp can warm the water, and if the temperature rose, you couldn't tell whether light or warmth had changed the rate.

Watch out: Bubbles come in different sizes, so counting them is a rougher measure than reading the volume of gas collected.

Higher

Biology · Light intensity

Halve the distance. How much brighter?

The lamp is 40 cm from the pondweed. You move it to 20 cm, half the distance. How many times brighter is the light reaching the plant now?

Your estimate

4.5×

The lamp is 40 cm from the pondweed. You move it to 20 cm, half the distance. How many times brighter is the light reaching the plant now? Estimate between 1× and 8×.

1×8×

Exam line: Halve the distance: intensity × 4. Double the distance: intensity ÷ 4. A third of the distance: intensity × 9.

Watch out: Four times the light doesn't mean four times the rate. In the pondweed results, moving from 40 cm to 20 cm took the rate from 1.0 to 1.5 cm³/min. The rate rises with light intensity, but not in proportion to it: the curve bends over as it heads for the level where another factor takes over.

Higher

Biology · Reading rate graphs

What's holding the rate back on the flat part?

A graph shows the rate of photosynthesis against light intensity, with two lines. Both were measured at the same temperature. One line is for low carbon dioxide concentration and one is for high carbon dioxide concentration. Both lines rise and then go flat, but the high carbon dioxide line goes flat at a much higher rate.

Look at the flat part of the LOW carbon dioxide line. Which is closest to what you think is limiting the rate there?

Nothing: photosynthesis has stopped, because the line is flat.Light, because light intensity is the factor along the x-axis.Carbon dioxide, because the high carbon dioxide line reaches a higher rate.

How sure are you?

I'm sureI think soI'm guessing

Nothing: photosynthesis has stopped, because the line is flat.

This is a common misconception

Flat means the rate isn't changing, not that it's zero. Read the y-axis: the plant is still photosynthesising, at a steady rate. It just can't go any faster at this carbon dioxide concentration.

Light, because light intensity is the factor along the x-axis.

This is a common misconception

Light is limiting only on the rising part, where more light gives a faster rate. On the flat part, extra light changes nothing, so light can't be what's holding the rate back there.

Carbon dioxide, because the high carbon dioxide line reaches a higher rate.

This matches the evidence

Exactly. The second line is the evidence: at the same light intensity and temperature, more carbon dioxide gives a faster rate. So on the flat part of the low line, carbon dioxide is in shortest supply: it's the limiting factor.

The limiting factor is the one whose increase would raise the rate. It isn't automatically the factor on the x-axis.

Exam line: To name the limiting factor on a graph, ask: which factor, if I raised it, would make the rate go up?

Higher

Biology · Greenhouses

Should the grower pay for more light?

On a bright summer day, a grower's greenhouse tomatoes are photosynthesising at a rate limited by carbon dioxide concentration. The grower is thinking of paying to install extra lamps. Explain whether this is a good use of money. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Light is not the limiting factor, because carbon dioxide concentration is. (1 mark)
So extra lamps would not increase the rate of photosynthesis, or the tomato crop. (1 mark)
The grower's costs would rise with no extra crop, so it would reduce profit or waste money. (1 mark)
Raising the carbon dioxide concentration would raise the rate, but it is only worth it if the extra crop is worth more than the cost. (1 mark)

Model answer

It is not a good use of money. The rate is limited by carbon dioxide concentration, not light, so extra lamps would not make the tomatoes photosynthesise any faster or produce a bigger crop. The grower would pay for the lamps and the electricity but get nothing back, so profit would fall. It would make more sense to raise the carbon dioxide concentration, because that is the limiting factor, but only if the value of the extra tomatoes is greater than the cost of adding the carbon dioxide.

Money spent on a factor that isn't limiting buys nothing. Find the limiting factor first, then check that raising it pays for itself.

Exam line: In any greenhouse question, find the limiting factor first. Only spending on that factor can raise the rate, and even then it only pays if the extra crop is worth more than it costs.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why a plant speeds up when you give it more light — and why, at some point, more light stops helping.

What you need to know

Four factors affect the rate of photosynthesis: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.
The limiting factor is the one in shortest supply. Raising it raises the rate; raising any other factor does not.
Light, carbon dioxide and chlorophyll make the rate rise and then level off. Temperature is different: above the optimum, enzymes are denatured and the rate falls.
You can measure the rate from the oxygen a water plant releases: rate = volume of oxygen ÷ time, for example in cm³/min.
Higher: light intensity is inversely proportional to the square of the distance from the light source.
Higher: the factors interact, and a grower only gains by raising the limiting factor, and only if the extra crop is worth more than it costs.

The big picture

The rate of photosynthesis depends on light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. Whichever factor is in shortest supply is the limiting factor: raising it raises the rate, while raising the others changes nothing. You can measure the rate by collecting the oxygen that pondweed gives off, and calculate it as volume ÷ time. At Higher level, you also use the inverse square law for light intensity, pick out the limiting factor on graphs with two or three lines, and apply limiting factors to greenhouse costs.

Key points

1On a rate graph, the factor on the x-axis is limiting only where the line is still rising.

2A flat section means a steady rate, not zero. A different factor has become limiting.

3If a second line with more carbon dioxide, or a warmer temperature, rises higher, that factor was limiting the lower line's flat section.

4Moving a lamp closer raises light intensity, so the rate rises until another factor takes over.

5In the pondweed investigation, only the lamp distance changes; a thermometer checks that the water temperature doesn't rise.

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Worked example

Problem

In another pondweed investigation, the oxygen was collected for 5 minutes at each distance: 10 cm → 6.0 cm³, 20 cm → 4.5 cm³, 30 cm → 2.5 cm³, 40 cm → 1.5 cm³, 50 cm → 1.0 cm³. Work out the rate of photosynthesis at each distance, then choose a sensible scale for plotting the rate on the y-axis.

⚠ Watch out

Assuming every factor behaves like light. Light and carbon dioxide make the rate rise and then level off, but temperature doesn't just level off. Past the optimum, the enzymes are denatured and the rate falls.

🧠

Memory hook

A barrel built from planks of different heights only fills to the shortest plank. Make the others taller and the level won't budge. Light, carbon dioxide and temperature are the planks; the shortest is the limiting factor. (Overheat the temperature plank and it shrinks again.)

✓

Check yourself

Explain why moving a lamp closer to pondweed makes the rate of photosynthesis rise at first, but eventually makes no difference.

Flashcards

(12)

Which four factors affect the rate of photosynthesis?

Light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.

What is a limiting factor?

The factor in shortest supply. It stops the rate going any higher, however much of the other factors there is.

Why does the amount of chlorophyll affect the rate?

Chlorophyll absorbs light. Less chlorophyll means less light energy is absorbed, so the rate is lower.

Why does the rate fall at high temperatures?

The enzymes that control photosynthesis are denatured, so they stop working properly.

Why does carbon dioxide concentration affect the rate?

Carbon dioxide is a raw material: the plant turns it, with water, into glucose.

How can you measure the rate of photosynthesis in pondweed?

Count the oxygen bubbles, or measure the volume of oxygen collected, over a fixed time such as three minutes.

How do you calculate a rate of photosynthesis from gas collected?

Rate = volume of oxygen ÷ time taken, for example in cm³/min.

Why monitor the water temperature in the pondweed experiment?

So you can be sure the temperature isn't rising, and that only light intensity is changing the rate.

On a volume of oxygen against time graph, what does a steeper line show?

A faster rate of photosynthesis: more oxygen made every minute.

On a rate against light intensity graph, where is light the limiting factor?

On the rising part, where more light still gives a faster rate.

Higher: what happens to light intensity when you halve the distance from the lamp?

It goes up four times, because intensity is inversely proportional to distance squared.

Higher: why would a grower not pay to heat a greenhouse when light is limiting?

Heating wouldn't raise the rate, so the cost would buy no extra crop and cut profit.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Turn up the light. Then turn it up again.

Pondweed and a lamp: measuring the rate

Halve the distance. How much brighter?

What's holding the rate back on the flat part?

Should the grower pay for more light?

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