Practical

Question 1

Describe the practical to investigate the rate of photosynthesis

Answer 1

1. Place a boiling tube 10 cm away from an LED light source- this LED light source doesn’t emit a lot of heat, this controls the temperature- too much heat would affect the temperature of the experiment
2. Pour sodium hydrogen carbonate solution into the boiling tube- this solution releases CO₂ which is required for photosynthesis
3. Place pondweed into the boiling tube with the cut end at the top
4. Then wait for 5 minutes to allow the pondweed to acclimatise to the conditions in the boiling tube- you will see bubbles of gas being released, these bubbles are oxygen which is a product of photosynthesis
5. Then using a stopwatch time 1 minute and count the number of bubbles of oxygen released and record this number into a table
6. Then repeat this two more times and calculate the mean number of bubbles released in one minutes
7. Then repeat the entire experiment, this time increasing the distance between the LED light source and the boiling tube by 10cm each time until you reach 40cm

Question 2

Why is an LED light source used in this experiment?

Answer 2

They don’t emit alot of heat
This controls the temperature as too much heat will affect the temperature of the experiment

Question 3

Why is sodium hydrogen carbonate solution used in this experiment?

Answer 3

This solution releases CO₂ which required for photosynthesis

Question 4

What are the two problems with this experiment?

Answer 4

1. The oxygen bubbles may be released too fast to count accurately
2. The oxygen bubbles may be different sizes

Question 5

How do you solve the problems with this experiment?

Answer 5

Measure the volume of oxygen released instead of counting the bubbles

Question 6

Describe how you can measure the volume of oxygen released in this experiment

Answer 6

1. Place a funnel on top of the boiling tube and catch the bubbles in a measuring tube filled with water
2. Measure the volume of oxygen released in the measuring cylinder

Question 7

What is the inverse square law?

Answer 7

Light intensity = 1/distance²

As when the distance INCREASES the light is being SPREAD over a larger area of space, the square of the distance!

Question 8

How does the inverse square law relate to photosynthesis?

Answer 8

When the distance from the light source is doubled the light intensisty is decreased by a factor of four
Light intensity affects the rate of photosynthesis, this means that the rate of photosynthesis is also decreased by a factor of four

Question 9

What is the independant variable in this practical?

Answer 9

The distance between the light source and the boiling tube

Question 10

What is the dependant variable in this practical?

Answer 10

The number of oxygen bubbles released

Question 11

What are the control variables in this practical?

Answer 11

The temperature
The concentration of carbon dioxide

Question 12

How do you calculate the rate of reaction/photosynthesis?

Answer 12

Number of oxygen bubbles released/time