GCSE · Biology · Edexcel · Spec 1BI0

Core practical: light intensity and rate of photosynthesis

You can’t watch photosynthesis speed up, but you can collect the oxygen it gives off — and measure it.

Biology · Pondweed practical

How fast is the pondweed photosynthesising?

Pondweed under an inverted funnel, with the oxygen collected as a volume reading (schematic)
06121824OXYGEN COLLECTED0 cm³oxygen is the waste product we collect

Pondweed under an inverted funnel, with the oxygen collected as a volume reading (schematic). Distance from the lamp: 100 cm. How does the distance of the lamp (and so the light intensity) change the rate of photosynthesis in pondweed?

How does the distance of the lamp (and so the light intensity) change the rate of photosynthesis in pondweed?

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

Distance from the lamp (cm)

Timer0.0 min

Move the lamp, run the practical and watch the oxygen collect. The readings are invented to show the pattern — they are illustrative, not real results — and the drawing is a schematic stand-in that gives a live volume reading. Hazards: electricity near water (risk of electric shock — check cables are undamaged and keep electrical items away from beakers of water), broken glassware (risk of cuts) and spilled liquids (risk of slipping).

Watch out: These numbers are invented to show the pattern. Your own results will not match them exactly — which is one reason each distance is repeated.

Biology · Variables

Change it, measure it, or keep it the same?

Sort each variable into the job it does in the pondweed investigation.

Still to sort

Independent variable (0)

The one thing you change on purpose.

Where the line is: You choose its values; you do not measure it as the result.

Dependent variable (0)

The thing you measure to see the effect.

Where the line is: It responds to the change you made; you do not set it.

Control variable (0)

Kept the same every time.

Where the line is: It could affect the result, so it is not allowed to change.

6 of 6 still to sort.

Each thing in the pondweed practical has one job. Pick an item, then the job you think it has.

Watch out: Moving the lamp changes the light intensity — the lamp is not what you measure.

Biology · Rate calculation

Finish the rate calculation

In one run, the gas in the measuring cylinder reads 1.0 cm³ at the start and 7.0 cm³ after three minutes (invented numbers). Calculate the rate of photosynthesis.

  1. Volume of gas produced = final volume − initial volumeThe cylinder already held some gas at the start, so find how much was added.
  2. missing step
Which line is step 2?

Two ways to measure the oxygen

Counting bubblesvsCollecting gas in an inverted measuring cylinder

Same dependent variable, different accuracy.

Focus

What you record

Counting bubbles

The number of bubbles released

Collecting gas in an inverted measuring cylinder

The volume of oxygen gas collected

The insight

The rate is worked out from a volume of gas, so the cylinder records the thing you actually need.

Bubble size

Counting bubbles

Bubbles are of different sizes and volumes, so each one counts the same whatever its size

Collecting gas in an inverted measuring cylinder

The size of the bubbles does not matter

How the gas is measured

Counting bubbles

Counted one by one

Collecting gas in an inverted measuring cylinder

The oxygen displaces the water in the cylinder, and the volume is read off

Accuracy

Counting bubbles

May not give accurate results

Collecting gas in an inverted measuring cylinder

Measures the actual volume of gas more accurately

Predict, then check

Think about what a lamp does besides shine.

The lamp is very close to the pondweed. Besides the light, what else might change?

Biology · What is the light doing?

What is the light doing?

In the pondweed practical, the nearer the lamp, the more oxygen is collected.

Which idea is closest to what you think the light is doing?
How sure are you?

Exam line: Light transfers the energy for the reactions; the rate rises with light intensity until another factor limits it.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Turn the pondweed’s waste oxygen into a measurement

What you need to know

  • Pondweed gives off oxygen when it photosynthesises. Oxygen is a waste product, so more oxygen means faster photosynthesis.
  • Closer to the lamp means higher light intensity, because farther away the light is spread over a larger area.
  • Have a goSam is sure of it: "Move the pondweed away from the lamp and the light gets MORE intense — it’s had longer to build up." Do you agree?

    No. Further from the lamp, the light intensity is lower.

    Light spreads over a larger area as the distance grows, so less of it reaches each part of the pondweed.

  • Independent variable: light intensity, changed by moving the lamp. Dependent variable: the volume of oxygen produced.
  • Keep the time the gas is collected for, the temperature, the pondweed’s mass or size and the volume of water the same.
  • Have a goA classmate collects gas for three minutes at 100 cm but for five minutes at 20 cm. Why does that make it a poor comparison?

    The collection time is a control variable and should be the same each time.

    If time changes as well as light intensity, you cannot tell which one caused the difference in gas volume.

  • Set-up: 10 cm of pondweed, cut edge up, under an inverted funnel raised on plasticine, with a water-filled measuring cylinder upside down over it.
  • Put the beaker 100 cm from the light, note the starting gas volume, switch on, and collect for three minutes.
  • Repeat with the lamp at 80, 60, 40 and 20 cm, and repeat each distance so you can check the results are repeatable.
  • Volume of gas = final volume − initial volume. Rate of photosynthesis = volume of gas ÷ time in minutes.
  • Have a goA cylinder starts at 0.5 cm³ and reads 6.5 cm³ after three minutes. What is the rate of photosynthesis?

    2 cm³ per minute

    Take away the start first (6.5 − 0.5 = 6 cm³), then divide by the time in minutes (6 ÷ 3). Dividing 6.5 by 3 would count gas that was already there.

  • Work out the mean at each distance, then plot rate against distance with a smooth line of best fit.
  • Light is not a reactant or a product: it transfers the energy for the reactions. The rate rises with light intensity until another factor limits it.

The big picture

In the pondweed practical you change light intensity by moving the lamp, collect the oxygen the pondweed produces, and turn the gas volume into a rate. The rate increases as the pondweed moves nearer the lamp, until another limiting factor takes over.

Key points

1Oxygen is a waste product of photosynthesis, so the volume of oxygen collected measures the rate.
2Light intensity is changed by the distance of the lamp: nearer means higher intensity.
3Control variables: collection time, temperature, pondweed mass or size, and volume of water.
4Volume of gas = final − initial. Rate = volume of gas ÷ time in minutes, in cm³ per minute.
5The rate increases as the pondweed is moved nearer the lamp, until another limiting factor takes over.

Worked example

Problem

At one distance the experiment is run three times. The volumes of oxygen produced in three minutes are 14 cm³, 16 cm³ and 15 cm³ (invented numbers). Find the rate to plot for this distance.

⚠ Watch out

Dividing by the time in seconds instead of minutes, or forgetting to take away the starting gas volume before dividing.

🧠

Memory hook

Nearer lamp, more light; more light, more oxygen; more oxygen per minute, faster photosynthesis — until something else limits it.

✓

Check yourself

Without scrolling up: say what you change, what you measure, three things you keep the same, and how the readings become a rate.

Flashcards

(15)
What is light’s role in photosynthesis?
Light is not a reactant or a product. It transfers the energy needed for the chemical reactions of photosynthesis to take place.
Which substances does photosynthesis use, and which does it produce?
It uses water and carbon dioxide, and produces glucose and oxygen.
What is light intensity?
The amount of light that reaches a given surface in a given period of time.
What is a limiting factor?
A condition that, when in short supply, slows down or limits the rate of photosynthesis. Light, carbon dioxide concentration, temperature and water availability can all be limiting factors.
Why does the volume of oxygen tell you the rate in pondweed?
Oxygen is a waste product of photosynthesis, so the more the pondweed photosynthesises, the more oxygen gas is produced.
In the pondweed practical, what are the independent and dependent variables?
Independent: light intensity, changed by moving the light source closer. Dependent: the volume of oxygen gas produced.
Name four control variables in the pondweed practical.
The time the gas is collected over, the temperature, the mass or size of the pondweed and the volume of water.
Which distances are used, and how long is gas collected at each?
The lamp is placed 100, 80, 60, 40 and 20 cm from the pondweed, with gas collected for three minutes at each, and each distance repeated.
Why is the funnel raised off the bottom of the beaker on plasticine?
So the water mixes and carbon dioxide is available to the pondweed.
Why does moving the lamp closer raise the light intensity?
Farther away, the light is spread over a larger area. Closer, it is spread over less, so more reaches the pondweed.
A lamp very close to the pondweed may cause a problem. What, and how can it be reduced?
The lamp may warm the pondweed, and temperature affects the rate. A large beaker of water helps shield it, or an LED bulb can be used.
How do you work out the rate from the cylinder readings?
Volume of gas = final volume − initial volume. Rate = volume of gas ÷ time in minutes, in cm³ per minute.
How are the results analysed and what do they show?
Calculate the mean at each distance, then plot rate against distance or light intensity with a smooth line of best fit. The rate increases as the pondweed is moved nearer the lamp.
Name the three hazards in the pondweed practical.
Electricity (shock: check cables are undamaged and keep electrical items away from water), broken glassware (cuts) and spilled liquids (slipping).
Why is a measuring cylinder better than counting bubbles?
Bubbles are of different sizes and volumes, so counting may be inaccurate. Oxygen displacing the water in the cylinder measures the actual volume of gas more accurately.

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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