10 - Different wavelengths & photosynthesis

What do chloroplasts contain & what do these absorb?

Photosynthetic pigments which absorb different wavelengths of light:

• this means that more light can be absorbed in total, increasing the rate of photosynthesis

What happens to wavelengths of light that is not absorbed?

It is reflected → filters will block out certain wavelengths (& therefore colours) of light from reaching the chloroplast

How can the rate of photosynthesis be measured?

Measuring the volume of gas produced → this gas is assumed to be oxygen (carbon dioxide from respiration is ignored)

What is equipment is needed for this practical?

• Pondweed & beaker

• Sodium hydrogencarbonate

• Aluminium foil

• Spatula & light filters

• Photosynthometer

• Paperclip & ruler

• Bench lamp

• Thermometer & stop clock

What is the method for this practical?

1. Place a piece of pondweed in a beaker of water in a thermostatically controlled water bath

2. Cover one side of the beaker with tin foil to block out the light & cover the other side of the beaker with one of the light filters

3. Add half a spatula of sodium hydrogencarbonate to the water to provide CO₂, mix & leave for 5 minutes

4. Place the bench lamp at a set distance from the beaker

5. Set up the photosynthometer & leave for 5 minutes to allow the pondweed to equilibriate

6. Record the volume of gas produced during this time using a gas syringe

7. Replace the filter with another colour of filter & repeat the experiment (using the same pondweed)

What is the risk, safety precaution & in emergency for the hazard: biohazard

Risk:

• contamination

Safety precaution:

• use disinfectant

• wash hands with soap after handling

In emergency:

• seek assistance

What is the risk, safety precaution & in emergency for the hazard: lamps

Risk:

• temporary damage to eyes

Safety precaution:

• don’t look directly at the lamp & wear protective eyewear (e.g. sunglasses)

In emergency:

• wait for after-image to disappear

• seek appropriate assistance if needed

What is the risk, safety precaution & in emergency for the hazard: sodium hydrogencarbonate

Risk:

• avoid inhalation & contact with eyes

Safety precaution:

• wear eye protection

• avoid contact with skin

In emergency:

• flood eye/cuts with cold water

• seek medical attention if necessary

What graph should be plotted for this practical?

Colour of filter (x-axis) against volume of gas produced (y-axis)

What is the conclusion from this practical?

• The volume of gas produced is proportional to the rate of photosynthesis

• The greatest volume of gas will be produced when there is no filter used, as all wavelengths of light can be absorbed

• All filters will decrease the volume of gas, but a green filter will decrease it the most (chloroplasts don’t absorb much green light; it’s mostly reflected)