Biology B4 - required photosynthesis practical

explain the 1st step of the photosynthesis practical

1. boiling tube and place it 10cm away from an LED light source

why do we use an LED light source

because they don't release too much heat, too much would change the temperature of the experiment

if an normal light bulb is used what do you have to do

place a beaker of water between the bulb and tube to absorb the heat produced by the bulb

2nd step?

fill the tube with sodium hydrogen carbonate solution

why do we use sodium carbonate solution

releases the carbon dioxide needed for photosynthesis

step 3

put a piece of pondweed into the boiling tube with the cut end at the top

step 4

leave for 5 minutes to acclimatise to the conditions in the boiling tube

what should start to happen

bubbles of gas being produced from the cut end of the pondweed. this gas is oxygen being released by photosynthesis

step 5

start a stop watch and count the number of bubbles produced in one minute

step 6

repeat 2 more times and calculate the mean number of bubbles produced in one minute

step 7

repeat experiment all over again from 20cm, then 30cm, 40cm,

what are the two main problems with this practical

1. bubbles can be too fast to count accurately

2.bubbles are not always the same size - a large bubble would count as a small bubble

how do we solve this problem

measure the volume of oxygen produced instead of counting bubbles

explain the steps of this experiment

1. place the pondweed under a funnel & catch the bubbles in a measuring cylinder

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

if we plot the mean number of bubbles per minute or volume of oxygen per minute against the distance from the lamp to the pondweed, what does the graph look like - explain the graph too.

if we double the distance then number of bubbles per minute falls by a factor of 4.

what do scientists call this

the inverse square law

why does this happen

if we double the distance, the light intensity falls by 4 times, because we need light for photosynthesis this causes te number of bubbles produced per min to fall 4 times as well