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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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 …

by eating plants or ...

... plant products,

or (c) other animals

Plants make their food by photosynthesis

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6

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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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 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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cell wall

nucleus

chloroplast

cytoplasm vacuole

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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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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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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)

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

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

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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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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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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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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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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A by-product of photosynthesis is

(a) Water vapour

(b) Oxygen

(c) Carbon dioxide

(d) Nitrogen

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The plant needs to take in nitrate ions in order to make

(a) Protein

(b) Cellulose

(c) Starch

(d) Sugars

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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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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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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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