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Requirements
of photosynthesis
By
Joanna George-Johnson, Contributor
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| Students
prepare ahead of their next session recently. - Photo by Anthony Minott |
Last
week we took a brief look at the photosynthesis equation: Though
not shown in the above equation, we also know that sunlight and chlorophyll are
requirements of photosynthesis. From what we know about chemical equations, the
substances on the left are those required for the process to occur, and those
on the right are the substances produced as a result of the process. This week,
we look at how these substances enter the leaf and how they influence the rate
of photosynthesis.  | | Photosynthesis
equation |
Carbon
dioxide Carbon
dioxide concentration in the air is extremely low, up to 0.03 per cent. Leaves,
therefore, must be very efficient at absorbing it. However, the cells which need
CO2 are in the middle of the leaf, that is, the mesophyll;
carbon dioxide must move from the atmosphere to the mesophyll. The
gas is being used up in the leaf, therefore, it will be at a lower concentration
in the mesophyll than in the atmosphere. It moves down its concentration gradient
from higher concentration (outside the leaf) to lower concentration (inside the
leaf), by the process of diffusion. This
occurs through the stomata. Inside the air spaces, the CO2
continues to diffuse between cells, through the cell walls, cell membranes, chloroplast
membranes into the chloroplast where it is utilised. Water
Water
in the soil is at a higher concentration than in the root hair cells. Water, therefore,
enters these cells. It is vital to note that at the same time water is also being
used up in the mesophyll, as well as being lost through the stomata by transpiration.
A concentration
gradient is thus set up between the atmosphere and the soil, the water travelling
through the xylem vessel up the stem and into the vascular bundles in the leaf.
There,
it travels by osmosis between the mesophyll cells, and into the chloroplasts of
photosynthetic cells. Transpiration, therefore, serves to supply the mesophyll
cells with water. Sunlight
The
upper epidermal cells are transparent, therefore, sunlight easily enters the leaf.
The thinness of the leaf also means that the sunlight radiates easily to the cells
where it is required. How
does the presence of these substances alter the rate of photosynthesis? By
simply looking at the chemical equation, it is quite easy to appreciate that having
more of the substances on the left (reactants) will mean that more of the substances
on the right (products) will be produced. However,
at very high concentrations of reactant this does not hold true. When
no light is present, photosynthesis does not occur at all. However, as light intensity
increases, so does the rate of photosynthesis. Light intensity, therefore, is
acting as a limiting factor, as the fact that it is not at optimum means that
it limits the rate at which photosynthesis can occur. As
light intensity increases above optimum, the rate of photosynthesis levels off
as other factors such as temperature and CO2 concentration
become limiting to photosynthesis. (See
graph at below) - Carbon
dioxide concentration
The
same principle applies to CO2 concentration; increased CO2
increases the rate of photosynthesis up to a point. After this point, increased
CO2 has no effect of photosynthesis. From
a chemical standpoint, particles need to collide with enough energy for them to
react. Increasing temperature increases the rate and energy with which the reactants
of photosynthesis collide, thus increasing the rate of photosynthesis. Above
a certain temperature the rate may stop increasing, or even decrease! This is
because stomata close at very high temperatures, decreasing CO2
uptake. Can you think of another reason this happens? (Hint: enzymes) Wind
removes the layer of water vapour saturated air from around the stomata. This
makes the diffusion gradient between the air space and the atmosphere steeper,
thus increasing transpiration rate. The mesophyll, therefore, gets more water,
and the rate of photosynthesis increases. Questions
1.
Sketch a graph showing the effects of temperature on the rate of photosynthesis.
2. Explain,
giving two reasons, why extremely high temperatures decrease photosynthesis rate.
3. Where
on the leaves would one generally find stomata? 4.
What structures should mesophyll cells possess to carry out photosynthesis efficiently?
Joanna
George-Johnson teaches at Ardenne High School masterbio@gmail.com.
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