Photosynthesis - Practical Skills

Question 1

What is photosynthesis?

Answer 1

The process by which plants synthesise glucose using light energy from the Sun

Question 2

Describe how you would investigate the effect of light intensity on the rate of photosynthesis in an aquatic plant

Answer 2

• Place algal beads in a boiling tube at a fixed distance from a light source
• Fill the boiling tube with bicarbonate indicator and record the initial colour
• After 30 minutes, record the final colour and find the pH using a colour standard
• Repeat several times at different distances/light intensities, and calculate the mean for each distance
• Plot a graph of light intensity (x) against rate of photosynthesis (bubbles per minute, y)

Question 3

Why is it best to use an LED light source rather than a lamp?

Answer 3

A lamp will emit more heat energy than an LED - therefore it may change the temperature of the experiment and affect the rate of photosynthesis

Question 4

What is the relationship between the distance from a light source and light intensity?

Answer 4

Inverse square law: light intensity is directly proportional to 1/distance^2

Question 5

How can the measurement of oxygen production be made more accurate?

Answer 5

If the pondweed is placed under a filter funnel with an inverted measuring culler over the spout the volume of oxygen produced can be determined

Question 6

Why is sodium hydrogen carbonate solution used during this experiment?

Answer 6

It releases CO2, which is used by the pondweed during photosynthesis

Question 7

Why can pH be used as an indication of the rate of photosynthesis?

Answer 7

Dissolved carbon dioxide, which makes the solution acidic, is taken up during photosynthesis. The higher the rate of photosynthesis, the greater the increase in pH in the given time.

Question 8

What is a source of error in this practical?

Answer 8

The colour changes may be subjective and determining the pH using a colour standard may be difficult