How Plants Get Their Food (1)
How do plants get their food ?
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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90.72kg soil
In the 17th Century, A Belgian physician, van Helmont, set up anexperiment in which he planted a willow sapling in a weighed amount of soil.
90.20kg soil
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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FeedingAnimals 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
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Animals get their food …
by eating plants or ...
... plant products,
or (c) other animals
Plants make their food by photosynthesis
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Photosynthesis6
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 tomake the sugar, glucose (C6H12O6)
6CO2 + 6H2O = C6H12O6 + 6O2
Oxygen (O2) is a by-product of this reaction
C6H12O6
CO2
CO2
CO2
CO2
CO2
CO2
H2O
H2O
H2O
H2O
H2O
H2O
6O2
+
6 molecules of carbon dioxide combine with 6 molecules of waterto 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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sunlight(energy)
waterwater
carbon dioxide
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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
cell wall
nucleus
chloroplast
cytoplasm vacuole
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All the reactions to combine CO2 and H2O take place in the chloroplast
sunlight
water
carbon dioxide
in the chloroplast,carbon dioxide andwater combine tomake sugar
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palisade cell of leaf
Cell structure of a leafThe palisade cells are in theuppermost layers of the leaf
epidermis
palisade cell ( photosynthesis)
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
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How plants get their food (2)How Plants Get Their Food (2)
Other Food
Glucose and starch are carbohydratesCarbohydrates can be oxidised during
respiration to produce energyPlants need more than carbohydratesThey need proteins for making new
cytoplasm and cells for growthTo make proteins plants combine glucose
with compounds of nitrogen, (nitrates)
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GLUCOSE
storage e.g. starch in potato
starch
fruitsother sugars
e.g. seed germination
energy
cytoplasm
protein
cell walls
cellulose
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When a salt such as potassium nitrate dissolves in water it separates into two ions, a potassium ion and a nitrate ion
Ions
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
Nitrates
Nitrate ions are present in the soil, dissolved in water
The plants take up nitrate ions in the soil waterThe nitrate ions are conducted through the
roots to the stem and then to the leavesIn the leaves, the nitrate ions and glucose are
combined to make proteinsThis 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)
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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
0
500
1000
1500
2000
2500
3000
Kg
per
hec
tare
No manure
Farmyard manure
Chemical fertilizer
Nophosphate
No nitrate
Nomagnesium
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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.
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TO SUM UP
QUESTIONS
In the questions which follow, choose the best answer from the four alternatives
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Question 1For 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
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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
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