Living Cells
Photosynthesis
Sunlight as a source of energy
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Green plant use sunlight energy to synthesis their
own food, in a process called photosynthesis
The process involves the capture of light energy by
the plant = Energy fixation
The light is trapped by the pigment chlorophyll,
which is present in the chloroplasts of the plant cells
Once trapped the light energy is converted to
chemical energy, in the form of ATP
Summary of photosynthesis
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The raw materials used in photosynthesis are carbon dioxide
(absorbed from the atmosphere, through stomata on the leaf
surface) and water (from the soil, which travels up through the
plant in vessels called xylem)
The plant converts these raw materials with the help of
sunlight energy (trapped by chlorophyll), into a plant food
source (carbohydrate) glucose (stored as starch) and a by
product oxygen (released through the stomata)
A carbohydrate is a compound containing the chemical
elements carbon, hydrogen and oxygen (See sheets pg 33)
Carbon dioxide + Water + light energy
Carbohydrate + Oxygen
Sunlight as a source of energy
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Activity 1.13 – Comparing photosynthesis in light
and dark conditions pg 34
Remember that the sunlight energy is absorbed in the
first place by chlorophyll in the chloroplasts
Activity 1.14 – Comparing photosynthesis in the
absence / presence of CO2 pg 35 (top)
Photosynthesis occurs as a set of reactions;
 Photolysis
 Carbon fixation
Photolysis
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‘Photo’ = light ,
‘lysis’ = split
(See sheets pg 35)
Photolysis involves the splitting of water molecules
into the elements oxygen and hydrogen, using light
energy
Photolysis provides the hydrogen and chemical
energy in ATP, needed for stage 2 of photosynthesis
The oxygen is a by-product
Carbon fixation
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This involves the hydrogen (produced by
photolysis) combining with carbon dioxide
(absorbed from the atmosphere) to form the
carbohydrate glucose (C6H12O6)
The carbon becomes ‘fixed’ into the
carbohydrate with the hydrogen
(See sheets top pg36)
Conversion of glucose to other
carbohydrates
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During photosynthesis the plant will continue to
make glucose, however, not all of this glucose is used
by the plant at any one time
The spare glucose is synthesised into the larger more
complex starch and cellulose carbohydrates
The starch is insoluble but can be reversibly
converted to glucose when required
The cellulose is combined together to form ribbonlike fibres which is a large component of plant cell
walls
These (reactions) conversions involve enzyme
controlled reactions (See sheets bottom pg 36)
Factors affecting the rate of
photosynthesis
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Carbon + Water + Light
dioxide
intensity
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The 3 things required for photosynthesis are carbon
dioxide, water and light energy
The correct temperature is also required (enzymecontrolled)
Water is usually in plentiful supply and rarely in
shortage
However, concentrations of carbon dioxide, light
intensity and temperature can become limited, and are
said to be limiting factors. (See sheets top pg 37)
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Carbohydrate + Oxygen
The Elodea Bubbler experiment
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The elodea bubbler experiment can be used to
measure the effect of altering light intensity and
carbon dioxide concentration, on the rate of
photosynthesis
The apparatus can be set up as shown on pg 37 of
your sheets, and the number of oxygen bubbles given
off over a fixed period of time (usually per minute), is
measured, and this will show the rate at which
photosynthesis is occurring (See sheets pg 37/38/39)
Familiarize yourself with the reasons for using each
piece of apparatus.
Early crops in horticulture
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It is clear from our knowledge of photosynthesis that plants require a good supply
of water, light energy and carbon dioxide to grow and develop properly, as well as a
suitable temperature
If any of the limiting factors are in short supply for a crop of plants, then the rate of
photosynthesis will be limited, and therefore the maximum quantity and quality of
crop will not be obtained
Farmers / Horticulturists can take steps to ensure the best possible conditions are
maintained for the optimum quality and quantity of crop production (See sheets
bottom pg39)
Suitable
temperature
– Greenhouse / thermostatically controlled heating system
Light intensity
– Electric lighting
Carbon dioxide
concentration – Added supply pumped into greenhouse