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How Plants Get Their Food )
Photosynthesis
Green plants take in carbon dioxide (CO2)
from the air
They take up water (H2O) from the soil
The plants combine the CO2 with the H2O to
make the sugar, glucose (C6H12O6)
6CO2 + 6H2O = C6H12O6 + 6O2
Oxygen (O2) is a by-product of this reaction
CO2
H 2O
CO2
H2O
H2O
CO2
C6H12O6
CO2
H2O
+
6O2
CO2
CO2
H2O
H 2O
6 molecules of carbon dioxide combine with 6 molecules of water
to make one molecule of glucose and 6 molecules of oxygen
Energy
It takes energy to make CO2 combine with
H2O
This energy comes from sunlight
The energy is absorbed and used by a
substance called chlorophyll
carbon
dioxide
water
sunlight
(energy)
water
Chlorophyll
Chlorophyll is a green coloured chemical
It is present in the leaves of green plants
The chlorophyll in the cells is packaged into
tiny structures called chloroplasts
The next slide shows a diagram of leaf cells
with their chloroplasts
Leaf cells with chloroplasts
chloroplast
cell wall
nucleus
cytoplasm
vacuole
All the reactions to combine CO2 and H2O take
place in the chloroplast
sunlight
palisade cell
of leaf
water
in the chloroplast,
carbon dioxide and
water combine to
make sugar
carbon dioxide
epidermis
palisade cell ( photosynthesis)
Cell structure of a leaf
The palisade cells are in the
uppermost layers of the leaf
vessel (carries water)
stoma (admits air)
Carbohydrates
• Glucose is one example of a carbohydrate
• Other examples are starch, sucrose and
cellulose (in cell walls)
• Carbohydrate molecules contain the
elements carbon, hydrogen and oxygen
• Living organisms can easily change one
carbohydrate into another
What happens to the glucose?
The glucose made by the chloroplast is either
(a) used to provide energy for the chemical
processes in the cell (by respiration)
(b) turned into sucrose and transported to
other parts of the plant
or
(c) turned into starch and stored in the cell as
starch grains
In darkness the starch is changed back into
glucose and transported out of the cell
energy
e.g. seed germination
other sugars
GLUCOSE
protein
cytoplasm
starch
cellulose
cell walls
storage e.g. starch in potato
fruits
TO SUM UP
Plants combine carbon dioxide from the air, and water
from the soil to make glucose.
The energy needed for this process comes from sunlight
The sunlight is absorbed by chlorophyll contained in the
chloroplasts of the leaf.
The glucose can be used for energy or to make other
substances.
To make other substances, the glucose must be combined
with other chemical elements such as nitrogen and
potassium.
These chemical elements are present as ions in the soil
and are taken up in solution by the roots.
QUESTIONS
In the questions which follow, choose the best
answer from the four alternatives
Question 1
For a plant to make glucose it needs
(a) CO2 and H2O
(b) CO2, H2O and sunlight
(c) CO2, H2O, sunlight and chlorophyll
(d) CO2, H2O, sunlight, chlorophyll
and nitrates
Question 2
A by-product of photosynthesis is
(a) Water vapour
(b) Oxygen
(c) Carbon dioxide
(d) Nitrogen
Question 3
Chlorophyll is present only in
(a) The cytoplasm
(b) The vacuole
(c) The cell wall
(d) The chloroplasts
Question 4
The food made by photosynthesis is
transported round the plant in the form of
(a) Glucose
(b) Sucrose
(c) Starch
(e) Cytoplasm