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Photosynthesis
Plants, unlike animals, produce their
own food. They are capable of
taking light energy from the Sun and
use it to change (convert) readily
available chemicals into energy rich
compounds that they can use as
food. Water from the soil and
carbon dioxide from the air are
combined to form glucose. The light
energy needed for this reaction to
occur is captured by a green pigment
in the leaves of the plant called
chlorophyll. The whole process is
called photosynthesis.
Light energy from the Sun is essential for photosynthesis. The glucose
made in this process is important as it can be broken down to provide energy
or it can be converted into more complex substances such as starch, fats or
protein.
Reactions which take in energy, such as this one, are endothermic reactions
and photosynthesis is the most important
endothermic reaction on Earth!!
A starch molecule is made up (consists) of
many, many glucose molecules bonded
together in a long chain. The long chain curls
up on itself to form a very compact molecule
that looks a bit like a spring.
Glucose molecules are very small, soluble and reactive. If there is more than
enough (excess) glucose in the leaf, it is turned into starch. Starch is
insoluble and very compact and so is an ideal way of storing glucose. The
starch forms macroscopic grains that can be seen in the cells. This store of
glucose can be broken down and used when energy is needed.
Testing a Leaf for Starch.
This test is used to determine if a leaf has been photosynthesising. If
starch is present in the leaf it will go blue-black in colour when iodine
solution is added.
The Procedure
Why boil the leaf?
Why place the boiled leaf in ethanol?
Why rinse the boiled leaf again in the hot water?
Factors that affect the Rate of Photosynthesis
What are the factors that affect how fast photosynthesis occurs in plants?
What happens if one of these factors changes?
What type of variable does that factor become – dependent or independent?
Which axis of a graph is the dependent variable recorded?
Light
Below are some results from an experiment carried out using a lamp at
varying distances from a water plant called Elodea.
Distance of lamp
from Elodea (cm)
Number of
Bubbles (mins)
100
80
60
40
20
0
13
12
31
41
55
76
Plot a graph of these results
Analyse the graph. What does it tell you about the results? What can you
conclude?
When the light is very close to the Elodea the rate of photosynthesis
increases a lot. What other factor, apart from light from the lamp, could
explain this big increase?
Look at the graph below.
What does line A tell you about the effect of light intensity on the rate of
photosynthesis?
Why does the graph level out?
What do lines B and C show?
Analysing these results should show that the rate of photosynthesis is
controlled by many factors.
The actual rate is determined by the factor in shortest supply (the one that
there is least of).
This factor is called the LIMITING FACTOR.
In summer, when there is plenty of bright light, carbon dioxide (CO2) is
usually a limiting factor.
In winter, light is often a limiting factor.