Photosynthesis 2 Question)

8/3/2019 Photosynthesis 2 Question)

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How Plants Get Their Food (1)


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How do plants get their food ?

73.943kg

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 57g.

van Helmont concluded that the tree had made 74kg of new growth

from water alone

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74kg 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.


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van Helmonts experiment was effective inshowing that the plants food did not come

from the soil.

But he had overlooked the fact that airwasavailable to the plant as well as water.

Could it be that the plant made 174kg ofmaterial from just airand water?

This might seem unlikely but we now know

that plants do indeed make their food fromcarbon dioxide from the air and water fromthe soil.

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FeedingAnimals 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 waterand dissolved salts from the soil

Plants do NOTget their food from the soil

The first stage by which plants make food iscalled PHOTOSYNTHESIS

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Animals get their food

by eating

plants or ...

... plantproducts,

or (c) other animals

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

CO2

CO2

CO2

CO2

CO2

CO2

H2O

H2O

H2O

H2O

H2O

H2O

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

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 and

water combine to

make sugar

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palisade cellof leaf


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Cell structure of a leafThe palisade cells are in the

uppermost 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 andcellulose (in cell walls)

Carbohydrate molecules contain the

elements carbon, hydrogen and oxygen

Living organisms can easily change onecarbohydrate 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 chemicalprocesses in the cell ( by respiration)

(b)turned into sucrose and transported toother parts of the plant

or

(c) turned into starch and stored in the cell asstarch grains

In darkness the starch is changed back intoglucose and transported out of the cell

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How plants get their food (2)How Plants Get Their Food (2)


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Other Food

Glucose and starch are carbohydrates

Carbohydrates can be oxidised during

respiration to produce energyPlants need more than carbohydrates

They need proteins for making new

cytoplasm and cells for growthTo make proteinsplants 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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Nitrates

Nitrates are present in the soil, dissolved inwater

The plants take up nitrates in the soil waterThe nitrates are conducted through the roots

to the stem and then to the leaves

In the leaves, the nitrates and glucose arecombined to make proteins

This process is called assimilation

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Mineral salts

Nitrates are not the only salts that plants needto 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 soilThese 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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A l h t i ld f 22


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Averageyearly wheat yields fromexperimental plots

0

500

1000

1500

2000

2500

3000

Kg

perhectare

No

manure

Farmyard

manure

Chemical

fertilizer

No

phosphate

No

nitrate

No

magnesium

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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 combinedwith 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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TO SUM UP


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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 nitrates 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 salt is needed for making

protein?

(a) Magnesium salt

(b) Sulphates

(c) Phosphates

(d) Nitrates

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Answer (1)

Incorrect

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Answer (2)

Correct

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Photosynthesis 2 Question)

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