how is light needed for photosynthesis + how does oxygen production show the rate of photosynthesis

how can you show that light is needed for photosynthesis

place a plant in a dark environment (e.g. a cupboard) for around 48 hours

cut a leaf from the plant and test it for starch with iodine solution

the leaf wont turn blue-black

this shows that light is needed for photosynthesis because no starch has been made

why do we need to place the plant in the dark environment for 48 hours

to ensure that it has used up all its starch stores

label the apparatus of how to test how light intensity shows photosynthesis

light source

ruler to vary distance from plant

water + sodium hydrocarbonate

canadian pondweed

small O₂ bubbles

clamp

water in capillary tube

syringe

ruler

O₂ bubble

why is canadian pondweed used for this experiment

rate at which the pondweed produces oxygen corresponds to the rate at which it is photosynthesizing

the faster the rate of oxygen production, the faster the rate of photosynthesis

how does light intensity affect the rate of photosynthesis

gas syringe should be empty to begin with

add sodium hydrocarbonate to water to release CO₂ into the solution

a source of white light is placed at a specific distance from pondweed

pondweed is left to photosynthesize for a set amount of time

as it photosynthesises the oxygen released will be collected in the capillary tube

at the end of experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the bubble is measured

this is proportional to the volume of O₂ produced

experiment is repeated with the light source places at different distances from the pondweed

what variables must be controlled

temperature of water

time the pondweed is left to photosynthesize