National 5 Biology Course Notes
Unit 1 : Cell Biology
Part 7 :
Photosynthesis
Word equation for photosynthesis
In photosynthesis, a plant uses light energy absorbed by chlorophyll to convert carbon dioxide and
water into glucose. Oxygen is also produced as a by-product and is released from the plant.
Photosynthesis can be summarised by this word equation:
Carbon Dioxide + Water
Light
Glucose + Oxygen
Absorbed
by chlorophyll
However photosynthesis is not a single chemical reaction, it is a series of reactions, each controlled by its
own enzyme.
Two stages of photosynthesis
The photosynthesis reactions can be divided into two main stages:
1. Light reactions – This stage uses light energy trapped by chlorophyll
2. Carbon fixation – In this stage, which does not require light, carbon dioxide is converted to glucose
using substances produced in the light reactions
Light reactions
These reactions use light energy from the sun which is trapped by chlorophyll in the chloroplasts.
There are two light reactions:
Photolysis
Light energy absorbed by chlorophyll is used to split water into oxygen and hydrogen atoms
Water
Hydrogen + Oxygen
Production of ATP
Light energy absorbed by chlorophyll is used to change ADP + phosphate to ATP
ADP + Pi
ATP
[Pi = phosphate]
This reaction changes light energy into chemical energy (stored in ATP)
Oxygen produced by photolysis is released from the plant as a by-product.
The other products of the light reactions – hydrogen atoms and ATP are used in the second stage of
photosynthesis, carbon fixation
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
Carbon dioxide
Glucose
ATP ADP + Pi
The hydrogen needed to change carbon dioxide to glucose
comes from splitting water (photolysis).
The energy needed to make glucose from CO2 comes from
changing ATP (from the light reactions) 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
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
3. Carbon dioxide concentration
Limiting factor graphs
The graph shows the effect of
light intensity on the rate of photosynthesis
Rate of photosynthesis
2. Temperature
Light intensity
X
Up to point X, increasing light intensity will increase the rate of photosynthesis, so light intensity is the limiting factor
After X, increasing light intensity does not increase the rate of photosynthesis, so light intensity is no longer the limiting
factor (the limiting factor is now either temperature or carbon dioxide concentration)
When the graph is like this
the variable on the X axis is the limiting factor
When the graph is like this
the variable on the X axis is not the limiting factor