Describe how the rate of photosynthesis of pondweed can be measured.

Count the oxygen bubbles released in a fixed time, or collect the gas given off and measure its volume in a fixed time.

Explain why a beaker of water is placed between the lamp and the pondweed.

The water absorbs heat from the lamp, so the temperature of the pondweed stays constant and does not affect the results.

Explain why sodium hydrogencarbonate solution is added to the water surrounding the pondweed.

It supplies a steady, plentiful supply of carbon dioxide, so carbon dioxide does not hold back the rate of photosynthesis.

Explain why the pondweed is left for a few minutes after the lamp is moved before any bubbles are counted.

It gives the plant time to settle at the new light intensity, so the reading measures the rate at that intensity.

Edexcel GCSE Biology 1BI0

Key Concepts in Biology

Topic overview Eukaryotic and prokaryotic cells Specialised cells and their adaptations Microscopes and magnification calculations Size, scale and units in cell biology Enzyme action and denaturation Factors affecting enzyme activity Enzymes in synthesis and digestion, and rate calculations Food tests and the energy in food Diffusion, osmosis and active transport Osmosis in potatoes and percentage change in mass

Cells and Control

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Genetics

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Plant Structures and Their Functions

Topic overview Photosynthesis as an endothermic reaction Limiting factors in photosynthesis Light intensity and the rate of photosynthesis Root hair cells, xylem and phloem Transpiration and translocation Leaf structure for photosynthesis and gas exchange Factors affecting the rate of transpiration Plant adaptations to extreme environments Plant hormones and their commercial uses

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Plant Structures and Their Functions

Light intensity and the rate of photosynthesis

3 things to know
Paper 2

Overview

Measuring the rate of photosynthesis in pondweed, the variables and controls in the lamp-distance investigation, improving on counting bubbles, the inverse square relationship and calculating relative light intensity as 1 ÷ distance², why light limits the rate in dim light, and hypotheses, predictions and whether data support them.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain why the pondweed investigation is done in a darkened room, how light intensity depends on lamp distance, and what happens to the bubbles as the lamp is moved closer.

Key term

In an investigation into how light affects photosynthesis in pondweed, what is the independent variable and how is it changed?

Key term

Name three variables that must be kept the same when investigating how light affects the rate of photosynthesis in pondweed.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe how the rate of photosynthesis of pondweed can be measured.
Model answer
Count the oxygen bubbles released in a fixed time, or collect the gas given off and measure its volume in a fixed time.
2
In an investigation into how light affects photosynthesis in pondweed, what is the independent variable and how is it changed?
Model answer
The independent variable is light intensity, changed by placing the lamp at different measured distances from the pondweed.
3
Name three variables that must be kept the same when investigating how light affects the rate of photosynthesis in pondweed.
Model answer
Temperature of the water, carbon dioxide concentration, and the same piece of pondweed for every reading.
4
Explain why a beaker of water is placed between the lamp and the pondweed.
Model answer
The water absorbs heat from the lamp, so the temperature of the pondweed stays constant and does not affect the results.
5
Explain why sodium hydrogencarbonate solution is added to the water surrounding the pondweed.
Model answer
It supplies a steady, plentiful supply of carbon dioxide, so carbon dioxide does not hold back the rate of photosynthesis.
6
Explain why the pondweed is left for a few minutes after the lamp is moved before any bubbles are counted.
Model answer
It gives the plant time to settle at the new light intensity, so the reading measures the rate at that intensity.

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Key Concepts in Biology

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ANitrates cause an algal bloom that blocks the light.

BWater plants die and decay.

CThe decomposers use up the dissolved oxygen.

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