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How Plants Get Their Food (1). 2. 90.20kg soil. 90.72kg soil. How do plants get their food ?. 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.

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2

90.20kg soil

90.72kg soil

How do plants get their food ?

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


3

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 airwas available to the plant as well as water.

Could it be that the plant made 74kg of material from just airand 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.


Feeding

4

Feeding

Animals get their food by eating plants, orother 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 NOTget their food from the soil

The first stage by which plants make food is called PHOTOSYNTHESIS


Animals get their food …

or (c) other animals

... plant

products,

by eating

plants or ...

5

Plants make their food by photosynthesis


Photosynthesis

6

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


7

CO2

H2O

CO2

H2O

H2O

CO2

CO2

H2O

CO2

H2O

CO2

H2O

C6H12O6

+

6O2

6 molecules of carbon dioxide combine with 6 molecules of water

to make one molecule of glucose and 6 molecules of oxygen


Energy

9

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

sunlight

(energy)

water

water

9


Chlorophyll

10

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

11

Leaf cells with chloroplasts

cell wall

chloroplast

nucleus

cytoplasm

vacuole


All the reactions to combine co 2 and h 2 o take place in the chloroplast

12

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)

vessel (carries water)

stoma (admits air)

13

Cell structure of a leaf

The palisade cells are in the

uppermost layers of the leaf


Carbohydrates

14

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

15

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 Plants get their Food (2)

How Plants Get Their Food (2)

How plants get their food (2)


Other food

17

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 proteinsplants combine glucose with compounds of nitrogen, (nitrates)


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fruits

other sugars

energy

protein

e.g. seed germination

GLUCOSE

cytoplasm

starch

cellulose

cell walls

storage e.g. starch in potato


Ions

KNO3 K+ + NO3-

When a salt such as potassium nitrate dissolves in water it separates into two ions, a potassium ion and a nitrate ion

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

19

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


Mineral ions

20

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

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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


Average yearly wheat yields from experimental plots

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Averageyearly 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 as ions 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


Question 1

25

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

26

Question 2

A by-product of photosynthesis is

(a) Water vapour

(b) Oxygen

(c) Carbon dioxide

(d) Nitrogen


Question 3

27

Question 3

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

(a) Protein

(b) Cellulose

(c) Starch

(d) Sugars


Question 4

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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

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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


Question 6

30

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


32

Correct


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