National 5 Biology
Unit 1
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7 Photosynthesis
Covered
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do this?
1. Photosynthesis (a)
Photosynthesis is a series of enzyme-controlled reactions which allow green
plants to make their own food.
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2. The summary equation for photosynthesis.
Light energy
Carbon dioxide + water
sugar + oxygen
Chlorophyll
3. Chloroplasts are disc shaped organelles enclosed in a double membrane.
(Covered in Cell Biology Unit 1. Cell Structure)
4. Photosynthesis occurs in two stages:
1. Light reactions (photolysis)
2. Carbon fixation
5. In stage one: Light reactions (photolysis) light energy from the sun is trapped
by chlorophyll in the chloroplasts and is converted into chemical energy in the
form of ATP
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6. In the light reaction stage (photolysis), water is split to produce hydrogen and
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concentration.
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12. I have an understanding of limiting factor graphs.
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oxygen. Excess oxygen diffuses from the cell and is released as a by-product.
7. The Hydrogen and ATP molecules are transported and used in the 2nd stage
of photosynthesis. In summary photolysis is the process by which energy from
sunlight is used to split water molecules into hydrogen and oxygen and provide
ATP.
8. In stage two: Carbon fixation the hydrogen produced by photolysis is
combined with carbon dioxide from the air to form sugar (glucose). ATP (from
first stage) provides the energy needed to carry out a series of enzyme controlled
reactions to produce this sugar (glucose).
9. Photosynthesis (b)
The chemical energy in the glucose produced can be used for
1) respiration
2) Stored as starch
3) converted to cellulose
10. Photosynthesis (c)
A limiting factor in photosynthesis is anything which, when in short supply,
reduces or limits the rate of photosynthesis taking place.
11. Limiting factors include; Temperature, Light intensity and Carbon Dioxide
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Unit 1
Unit 1
Pupil Summary Notes
Photosynthesis is the process which allows green plants to make their own food.
It can be summarised with the word equation
Light
Carbon Dioxide + Water
Glucose + Oxygen
Chlorophyll
Raw materials
Products
This takes place in the chloroplast of the plant cell.
Testing for Starch
The glucose the plant makes during photosynthesis can be stored in the leaves as starch. Testing a
leaf for starch can show if the plant has been photosynthesising.
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Chemistry of Photosynthesis
Light Reactions (Photolysis)
This is the first stage of photosynthesis and involves the splitting of water into oxygen and
hydrogen using energy captured from sunlight by chlorophyll. This reaction also produces
ATP.
Light energy from
the sun captured by
chlorophyll
Light energy
Water
ADP + Pi
ATP
Oxygen
Released as a
by-product
Used in the second stage
– Carbon Fixation.
Hydrogen
Due to requirement of light energy to break apart water molecules this reaction is classified as
light dependent, meaning it will only occur in the presence of light.
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Carbon Fixation
This is the second stage of photosynthesis. It involves a series of enzyme controlled
reactions joining together carbon dioxide and hydrogen to form glucose. This reaction requires
ATP and hydrogen, passed on from photolysis, and cannot occur without the products of
photolysis.
Carbon Dioxide
Enzyme controlled
Glucose
Hydrogen
ATP
ADP + Pi
The glucose produced during photosynthesis can be used in various ways by the plant cell
1. Used immediately for energy to power cell process, such as mitosis or protein
synthesis.
2. It can be converted to starch for long term storage.
3. It can be converted to cellulose and used to create cell walls
1.
Limiting Factors
A limiting factor is a factor that by its presence or absence controls the rate at which a reaction
happens.
For photosynthesis there are various factors that can affect the rate of photosynthesis

Light Intensity

Temperature

Carbon dioxide concentration
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National 5 Biology
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Increasing light intensity or carbon dioxide concentration should increase the rate of
photosynthesis, until the plant is receiving as much as it can utilise at which point they cease
being a limiting factor
C - 0.50% CO2
Y
B - 0.10% CO2
X
A - 0.01% CO2
At the point labelled Y the limiting factor is the light intensity as the rate of photosynthesis
increases as the light intensity increases.
At the points labelled X the rate of photosynthesis shows no further increase even if the light
intensity is increased, therefore another factor such as carbon dioxide concentration must be
limiting the rate of photosynthesis.
Temperature usually works in the same way. Remember, however, that above a certain
temperature the enzymes controlling photosynthesis will become denatured and the rate of
photosynthesis will actually start decreasing. This is shown on the following graph.
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Homework questions
1a)
The following word equation shows the first stage of photosynthesis
light energy
water
oxygen + hydrogen + ATP
Name this stage of photosynthesis
Describe what happens to each of the products
Oxygen
Hydrogen
ATP
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2a)
Unit 1
The graph below shows how the rate of photosynthesis is affected by light
intensity at different concentrations of carbon dioxide
High
High cabon dioxide
concentration
Rate of
photosynthesis
(units)
Low cabon dioxide
concentration
low
high
Light intensity (units)
What factor was limiting the rate of photosynthesis at points 1 and
2?
Name one other factor which may limit the rate of photosynthesis
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Unit 1
3a)
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Unit 1
3a) iii
3a) iv
3b)
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