How Plants Get Their Food 1

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How Plants Get Their Food (1)
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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 57g. van
Helmont concluded that the tree had made 74kg of
new growth from water alone
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• van Helmonts experiment was effective in showing
  that the plants 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 174kg 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
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• 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

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Animals get their food
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Plants make their food by photosynthesis
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Photosynthesis
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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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C6H12O6

6O2

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
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• 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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Chlorophyll
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• 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
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cell wall
chloroplast
nucleus
cytoplasm
vacuole
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All the reactions to combine CO2 and H2O take
place in the chloroplast
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sunlight
palisade cell of leaf
water
in the chloroplast, carbon dioxide and water
combine to make sugar
carbon dioxide
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Cell structure of a leaf The palisade cells are
in the uppermost layers of the leaf
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Carbohydrates
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• 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?
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• 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)
How plants get their food (2)
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Other Food
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• 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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fruits
other sugars
energy
protein
GLUCOSE
e.g. seed germination
cytoplasm
starch
cellulose
cell walls
storage e.g. starch in potato
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Nitrates
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• Nitrates are present in the soil, dissolved in
  water
• The plants take up nitrates in the soil water
• The nitrates are conducted through the roots to
  the stem and then to the leaves
• In the leaves, the nitrates and glucose are
  combined to make proteins
• This process is called assimilation

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Mineral salts
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• Nitrates are not the only salts that plants need
  to take in from the soil
• They need phosphates, sulphates, iron, potassium
  and magnesium salts
• 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
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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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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 in the
  soil and are taken up in solution by the roots.

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QUESTIONS
In the questions which follow, choose the best
answer from the four alternatives
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Question 1
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• 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
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• A by-product of photosynthesis is
• (a) Water vapour
• (b) Oxygen
• (c) Carbon dioxide
• (d) Nitrogen

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Question 3
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• The plant needs to take in nitrates in order to
  make
• (a) Protein
• (b) Cellulose
• (c) Starch
• (d) Sugars

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Question 4
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• Chlorophyll is present only in
• (a) The cytoplasm
• (b) The vacuole
• (c) The cell wall
• (d) The chloroplasts

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Question 5
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• 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
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• Which mineral salt is needed for making protein?
• (a) Magnesium salt
• (b) Sulphates
• (c) Phosphates
• (d) Nitrates

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Answer (1)
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• Incorrect

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Answer (2)
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• Correct