Download Photosynthesis Learning Outcomes Photosynthesis

National 5 Biology – Living Cells – Photosynthesis
Photosynthesis
Photosynthesis, ATP, ADP, chloroplast, chlorophyll, photolysis, carbon
fixation, starch, cellulose
Learning Outcomes
You will be able to:

Describe the two stage process of photosynthesis.

Give details of the summary word equation for photosynthesis.

Describe what happens during the light reaction.

Describe what happens during the carbon fixation.

Describe the possible uses of sugar made during photosynthesis.
Photosynthesis
Photosynthesis is the chemical reaction which takes place in plant cells in
order to produce sugar using light energy from the sun.
The word equation for this reaction is:
In this topic we're going to zoom in on the steps involved in photosynthesis in
more detail and consider the effect of different conditions on the rate of
photosynthesis. Before we begin however, we're going to be mentioning the
molecule ATP in both this topic and the next, so it makes sense to explain what
this is a little first.
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National 5 Biology – Living Cells – Photosynthesis
ATP
You don't need to know what ATP stands for, but it might help you understand
how it works. ATP stands for Adenosine Triphosphate. So, it's a molecule with
three (tri) phosphate groups attached. The best way to imagine ATP is like the
charged battery of the cell. If a cell wants to do anything which requires energy
(such as active transport) then it needs some energy in the form of ATP. The
cell releases the energy by breaking ATP up into ADP (the D standing for Di
instead of Tri), which has only two phosphate groups attached, and a separate
inorganic phosphate molecule (Pi). To recharge the ADP to ATP, energy is
required to add a phosphate molecule. These processes are summarised in the
following diagram:
Learning Activity 1
1. State the name of the raw materials needed for photosynthesis.
2. State the name of the products of photosynthesis.
3. State what other essential requirements are needed for photosynthesis
to take place.
4. Copy the equation for photosynthesis, Circle the raw materials in red,
essential requirements in yellow and products in blue produce a key to
show this.
5. What is ATP and how is it made?
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National 5 Biology – Living Cells – Photosynthesis
Chemistry of Photosynthesis
Photosynthesis is a series of enzyme controlled chemical reactions occurring in
two stages:
1. the light reaction (photolysis)
2. carbon fixation
Photolysis: The light reactions of photosynthesis take place in the chloroplasts
of the plant cell. Here, the green pigment chlorophyll absorbs light energy from
the sun. This energy is used to do two things. Some energy is used to produce
ATP as discussed above. This ATP acts as an energy store and can be used in
the second stage of photosynthesis. At the same time, light energy is used to
split water into hydrogen and oxygen. The hydrogen also goes to the second
stage of photosynthesis whereas the majority of the oxygen diffuses out of
the cell, and out of the leaf, as a waste product.
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National 5 Biology – Living Cells – Photosynthesis
Carbon Fixation: The second stage of photosynthesis also takes place in the
chloroplast, but does not require chlorophyll as it does not require light energy
directly. During Carbon Fixation the ATP and hydrogen from the Light
Reactions is used to convert Carbon Dioxide into the sugar glucose.
Uses of Sugar by Plants
The sugar which is produced by photosynthesis can be used in lots of different
ways by the cell. Some of it will be used for respiration. However, some of it will
be joined together into long chains to form molecules of starch. This acts as an
energy store for the plant, which is why storage organs such as the food store in
seeds and tubers (potatoes) are packed full of starch. The plant can also join the
sugar molecules together to form a different form of long-chained molecule called
cellulose. This is the molecule which makes up the majority of the structure of
plant cell walls which helps give the plant structure and support.
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National 5 Biology – Living Cells – Photosynthesis
Learning Activity 2
1. Describe the role of chlorophyll in the light reaction and the energy
change which takes place.
2. Explain what happens to the hydrogen which is released.
3. Insert and complete the ‘Light Reaction diagram’ into your notes.
4. State where carbon fixation takes place.
5. Name the molecule which combines with the CO2.
6. Name the end product which is formed.
7. Insert and complete the ‘Carbon Fixation diagram’ into your notes.
8. Explain the fate of sugar produced during photosynthesis.
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National 5 Biology – Living Cells – Photosynthesis
Limiting Factors of Photosynthesis
Carbon dioxide, light intensity, temperature, limiting factor
Learning Outcomes
You will be able to:

Describe the limiting factors of photosynthesis.

Explain the impact of limiting factors on photosynthesis and growth.
Limiting Factors
The presence or absence of various factors can limit the rate of
photosynthesis. As the products of photosynthesis are used in cell growth, any
limit in the rate of photosynthesis will affect the rate of growth also.
Possible Limiting Factors in Photosynthesis include:
-
Carbon dioxide
Light intensity
Temperature
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National 5 Biology – Living Cells – Photosynthesis
Carbon Dioxide: As you know, carbon dioxide is a crucial molecule in
photosynthesis. It is the raw material in the carbon fixation process. If the
Light Reactions are proceeding at their maximum rate but there is a low
concentration of carbon dioxide in and around the leaf, then the rate of
photosynthesis will be limited, as will the rate of growth of the cell and the
plant.
Light Intensity: Light intensity is obviously crucial for the rate of
photosynthesis as it provides the energy source for the Light Reactions, which
in turn transfer the energy to Carbon Fixation through Hydrogen and ATP. If all
other raw materials are present in abundance but there is a low level of light
intensity, then the rate of photosynthesis will be limited as will growth.
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National 5 Biology – Living Cells – Photosynthesis
Temperature: Photosynthesis is a series of enzyme-catalysed chemical
reactions, and just like any series of enzyme-catalysed reactions it can be
affected by temperature. Low temperatures can result in a reduced rate of
photosynthesis and growth, however high temperature can also greatly limit the
rate of photosynthesis and growth. Why? Because photosynthesis is catalysed
by enzymes and so high temperatures will result in denaturation.
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National 5 Biology – Living Cells – Photosynthesis
So, let's bring it all together now...can you interpret the following graph now?
What is the limiting factor at points A, B & C on the graph and why?
A: This is the easy one remember. The line is going up, which means the rate of
photosynthesis increased when the light intensity (on the horizontal axis) was
increased. Therefore, the limiting factor at point A is light intensity.
B: The graph has levelled off here, so a higher light intensity doesn't result in a
higher rate. Something other than light intensity must be limiting. On this graph
it also shows that the experiment was repeated at two carbon dioxide
concentrations. At point B the rate increased when the carbon dioxide
concentration increased, therefore carbon dioxide concentration was limiting.
C: It is more difficult to decide what is limiting at point C. It is definitely not
light intensity as the graph has levelled off meaning a higher light intensity
didn't result in a higher rate. It could be carbon dioxide concentration again, or
it could be temperature.
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National 5 Biology – Living Cells – Photosynthesis
Learning Activity 1
1. What is meant by a ‘limiting factor’?
2. Name the possible limiting factors of photosynthesis.
3. Sketch the graph shown above into your notes.
4. Label the part of the graph where light is acting as a limiting factor, label
the part of the graph where light is no longer a limiting factor.
5. State the relationship between light intensity and rate of photosynthesis.
6. Use the data in the table below to plot a line graph of results.
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National 5 Biology – Living Cells – Photosynthesis
7. Label the part of the graph where carbon dioxide is the limiting factor,
and the part of the graph where carbon dioxide is not the limiting factor.
8. State the relationship between carbon dioxide concentration and the rate
of photosynthesis.
9. Use the data in the table below to plot a line graph of results.
10. Describe the effect of increasing temperature on the rate of
photosynthesis for the first part of the graph.
11. Why does this happen?
12. Photosynthesis cannot occur at all, at very high temperatures. Explain
this?
13. Add a label to your graph to show where temperature is not acting as a
limiting factor.
Learning Activity 2
Collect the experiment card ‘The Effect of Varying Light Intensity’.
Carefully follow the instructions to complete the experiment.
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National 5 Biology – Living Cells – Photosynthesis
Extension Task
Use any resources available to you to find out about producing early crops in
horticulture or agriculture.
Explain how early crops can be produced in horticulture, with reference to:
-
supplementary lighting
-
carbon dioxide enrichment
-
heating.
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