How Plants Get Their Food (1)
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How do plants get their food ?
90.72kg soil
90.20kg
soil
In the 17th Century, A Belgian physician, van Helmont, set up an
experiment in which he planted a willow sapling in a weighed
amount of soil.
The soil was watered but nothing else was added. After 5 years, the
tree had gained 74kg in weight but the soil had lost only 52g.
van Helmont concluded that the tree had made 74kg of new growth
from water alone
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van Helmont’s experiment was effective in
showing that the plant’s food did not come
from the soil.
But he had overlooked the fact that air was
available to the plant as well as water.
Could it be that the plant made 74kg of
material from just air and water?
This might seem unlikely but we now know
that plants do indeed make their food from
carbon dioxide from the air and water from
the soil.
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Feeding
Animals get their food by eating plants, or other
animals
Carnivores eat animals
Herbivores eat plants
Plants make their own food
They combine carbon dioxide from the air with water
and dissolved salts from the soil
Plants do NOT get their food from the soil
The first stage by which plants make food is
called PHOTOSYNTHESIS
Animals get their food …
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or (c) other animals
by eating
plants or ...
... plant
products,
Plants make their food by photosynthesis
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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
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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
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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
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carbon
dioxide
water
sunlight
(energy)
water
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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
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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
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epidermis
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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)
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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
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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
How
Getget
Their
Food
(2)(2)
HowPlants
plants
their
food
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Other Food
Glucose and starch are carbohydrates
Carbohydrates can be oxidised during
respiration to produce energy
Plants need more than carbohydrates
They need proteins for making new
cytoplasm and cells for growth
To make proteins plants combine glucose with
compounds of nitrogen, (nitrates)
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energy
e.g. seed germination
other sugars
GLUCOSE
protein
cytoplasm
starch
cellulose
cell walls
storage e.g. starch in potato
fruits
Ions
When a salt such as potassium nitrate dissolves
in water it separates into two ions, a potassium
ion and a nitrate ion
KNO3
K+ + NO3-
The potassium ion (K+) carries a positive charge.
The nitrate ion (NO3-) carries a negative charge
These ions move freely and independently in the
soil water and it is in this form that they are
taken up by plants
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Nitrates
Nitrate ions are present in the soil, dissolved in
water
The plants take up nitrate ions in the soil water
The nitrate ions are conducted through the roots
to the stem and then to the leaves
In the leaves, the nitrate ions and glucose are
combined to make proteins
This process is called assimilation
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Mineral ions
Nitrates are not the only ions that plants need
to take in from the soil
They need phosphate, sulphate, iron,
potassium and magnesium ions
This is the reason why farmers and gardeners
add fertiliser to the soil
These fertilisers usually contain nitrates,
phosphates and potassium (NPK)
Effects of fertilisers
These are experimental strips of wheat. Varying amounts and
types of fertiliser have been added to the soil to see which give the
best plant growth
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Average yearly wheat yields from
experimental plots
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3000
Chemical
fertilizer
Farmyard
manure
Kg per hectare
2500
No
magnesium
No
phosphate
2000
1500
1000
500
0
No
manure
No
nitrate
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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
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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
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Question 2
A by-product of photosynthesis is
(a) Water vapour
(b) Oxygen
(c) Carbon dioxide
(d) Nitrogen
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Question 3
The plant needs to take in nitrate ions in order
to make
(a) Protein
(b) Cellulose
(c) Starch
(d) Sugars
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Question 4
Chlorophyll is present only in
(a) The cytoplasm
(b) The vacuole
(c) The cell wall
(d) The chloroplasts
Question 5
The food made by photosynthesis is
transported round the plant in the form of
(a) Glucose
(b) Sucrose
(c) Starch
(e) Cytoplasm
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Question 6
Which mineral ions are needed for making
protein?
(a) Magnesium ions
(b) Sulphate ions
(c) Phosphate ions
(d) Nitrate ions
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Incorrect
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Correct