National 5 Biology
Unit 1
1.3 Photosynthesis
Pupil Course Notes
Topic 7
Photosynthesis
Pupil Notes
Word equation for photosynthesis
Light
Carbon Dioxide + Water
Glucose + Oxygen
Absorbed by
chlorophyll
Photosynthesis reactions
There are 2 main stages
Light reactions
The first stage needs light and has 2 reactions
Photolysis
Light energy absorbed by chlorophyll is used to split water into oxygen and
hydrogen atoms
water
hydrogen + oxygen
Hydrogen is used in the second stage (carbon fixation), oxygen is released as
a by-product
Production of ATP
Light energy absorbed by chlorophyll is used to change ADP + phosphate to
ATP
ADP + Pi
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ATP
[Pi = phosphate]
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National 5 Biology
1.3 Photosynthesis
Pupil Course Notes
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 stage requires ATP and hydrogen, passed on from the light
reactions
Glucose
Carbon Dioxide
ATP
ADP + Pi
The hydrogen needed to change
carbon dioxide to glucose comes
from splitting water.
The energy needed to make glucose
from CO2 comes from changing
ATP back to ADP and Pi
How the plant uses glucose
The glucose produced during photosynthesis can be used in various ways by
the plant cell
1. It can be broken down during respiration to release energy in
respiration
2. It can be converted to starch and stored.
3. It can be converted to cellulose and used to create cell walls
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Page 2 of 3
National 5 Biology
1.3 Photosynthesis
Pupil Course Notes
Limiting Factors
A limiting factor is something that slows the rate of a process when it is in
short supply.
Limiting factors for photosynthesis are
1. Light Intensity
2. Temperature
3. Carbon dioxide concentration
Limiting factor graphs
The graph shows the effect of light intensity on the rate of photosynthesis
Rate of
photosynthesis
Light intensity
X
Up to point X, light intensity is the limiting factor, since increasing light
intensity will cause an increase in the rate of photosynthesis.
After X, increasing light intensity does not increase the rate of
photosynthesis, so light intensity is no longer the limiting factor
When the graph is like this
When the graph is like this
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the variable on the X axis is the limiting factor
the variable on the X axis is not the limiting factor
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