[Part 4]- Required practicals🔬

Question 1

Describe [the required practical of] how to investigate the effect of light intensity, on the rate of photosynthesis. 🔆

Answer 1

1) start by taking a boiling tube, and placing it 10cm away from an LED light source

2) fill the boiling tube with sodium hydrogen carbonate solution- this releases carbon dioxide, which is needed for photosynthesis.

3) put a piece of pondweed into the boiling tube, with the cut end at the top.

4) leave this for five minutes to acclimatise to the conditions in the boiling tube.

5) bubbles of gas [oxygen] should be produced, from the cut end of the pondweed

6) start a stopwatch, counting the numbers of bubbles produced one minute.

7) then repeat this one more time, and then calculate: the mean number of bubbles produced in one minute.

8) repeat the experiment again from: 20cm, 30cm and then 40cm

Question 2

What are the problems with the investigating the effect of light intensity, on the rate of photosynthesis? 🔆

Answer 2

• the number of bubbles can be too fast to count accurately
• and the bubbles are not always the same size. A large bubble, would count the same as a small bubble
• to solve both these problems, measure the volume of oxygen produced.

Question 3

Why is an LED used instead of a normal lightbulb ? 🔆

Answer 3

• an LED is used, because it doesn’t release much heat- too much heat would change the temperature of the experiment.

Question 4

What happens if a normal light bulb is used, rather than a LED ? 🔆

Answer 4

• ; if a normal light bulb is used, place a beaker of water between the light and the boiling tube- this would absorb the heat produced by the bulb.

Question 5

Describe steps one to two of [the required practical about] how to investigate the effect of light intensity, on the rate of photosynthesis. 🔆

Answer 5

1) start by taking a boiling tube, and placing it 10cm away from an LED light source

2) fill the boiling tube with sodium hydrogen carbonate solution- this releases carbon dioxide, which is needed for photosynthesis.

Question 6

Describe steps three to four of [the required practical about] how to investigate the effect of light intensity, on the rate of photosynthesis. 🔆

Answer 6

3) put a piece of pondweed into the boiling tube, with the cut end at the top.

4) leave this for five minutes to acclimatise to the conditions in the boiling tube.

Question 7

Describe steps five to six of [the required practical about] how to investigate the effect of light intensity, on the rate of photosynthesis. 🔆

Answer 7

5) bubbles of gas [oxygen] should be produced, from the cut end of the pondweed

6) start a stopwatch, counting the numbers of bubbles produced one minute.

Question 8

Describe steps seven to eight of [the required practical about] how to investigate the effect of light intensity, on the rate of photosynthesis. 🔆

Answer 8

7) then repeat this one more time, and then calculate: the mean number of bubbles produced in one minute.

8) repeat the experiment again from: 20cm, 30cm and then 40cm

Question 9

Referring to the improved method, describe [the required practical of] how to investigate the effect of light intensity, on the rate of photosynthesis 🔅

Answer 9

1) place the pond-weed under a funnel, and catch the bubbles in a measuring cylinder, filled with water.

2) use the measuring cylinder, to measure the volume of the oxygen produced.

Question 10

What is inverse square law ? 🔅

Answer 10

• inverse square law, is when the light intensity decreases in proportion, to the square of the distance.
• ; with inverse square law, if the distance is halved, the light intensity will be four times greater.
• though if you double the distance, the light intensity will be four times smaller.

Question 11

If the distance is doubled, what will happen to the light intensity? Explain your answer. 🔅

Answer 11

• though if you double the distance, the light intensity will be four times smaller.
• This is because we need light for photosynthesis, therefore this also causes the number of oxygen bubbles to fall by four times.

Question 12

What is the formula for inverse square law ? 🔅

Answer 12

• light intensity ⍺ 1➗distance²
• [light intensity is 1, over distance squared]

Question 13

What are the hazards to the pondweed practical ? 🔅

Answer 13

• There is a potential allergy risk from the pondweed.
• The Lamp [source of light] may get hot.
• and keep water away from electrical power outlets and wiring- it poses a risk of electrocution.