2. 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
2
90.72kg 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.
90.20kg
soil
3. van Helmontâs experiment was effective in
showing that the plantâs 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.
3
4. Feeding
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
4
5. Animals get their food âŠ
by eating
plants or ...
... plant
products,
or (c) other animals
Plants make their food by photosynthesis
5
6. Photosynthesis
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 to
make the sugar, glucose (C6H12O6)
6CO2 + 6H2O = C6H12O6 + 6O2
Oxygen (O2) is a by-product of this reaction
8. 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
9
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
10
12. 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
12
palisade cell
of leaf
13. Cell structure of a leaf
The palisade cells are in the
uppermost layers of the leaf
epidermis
palisade cell ( photosynthesis)
vessel (carries water)
stoma (admits air)
13
14. Carbohydrates
âą Glucose is one example of a carbohydrate
âą Other examples are starch, sucrose and
cellulose (in cell walls)
âą Carbohydrate molecules contain the
elements carbon, hydrogen and oxygen
âą Living organisms can easily change one
carbohydrate into another
14
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
15
16. How plants get their food (2)How Plants Get Their Food (2)
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 proteins plants combine glucose
with compounds of nitrogen, (nitrates)
17
18. GLUCOSE
storage e.g. starch in potato
starch
fruitsother sugars
e.g. seed germination
energy
cytoplasm
protein
cell walls
cellulose
18
19. 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
20. 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
19
21. 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)
20
22. 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
21
23. Average yearly wheat yields from
experimental plots
0
500
1000
1500
2000
2500
3000
Kgperhectare
No
manure
Farmyard
manure
Chemical
fertilizer
No
phosphate
No
nitrate
No
magnesium
22
24. 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.
23
TO SUM UP
26. 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
25
27. Question 2
A by-product of photosynthesis is
(a) Water vapour
(b) Oxygen
(c) Carbon dioxide
(d) Nitrogen
26
28. Question 3
The plant needs to take in nitrate ions in order
to make
(a) Protein
(b) Cellulose
(c) Starch
(d) Sugars
27
29. Question 4
Chlorophyll is present only in
(a) The cytoplasm
(b) The vacuole
(c) The cell wall
(d) The chloroplasts
28
30. Question 5
The food made by photosynthesis is
transported round the plant in the form of
(a) Glucose
(b) Sucrose
(c) Starch
(e) Cytoplasm
29
31. Question 6
Which mineral ions are needed for making
protein?
(a) Magnesium ions
(b) Sulphate ions
(c) Phosphate ions
(d) Nitrate ions
30
Notice how little the leaves overlap each other. This enables them to trap the maximum amount of sunlight. the importance of this will be explained later in the presentation
Most fungi and bacteria get their food by breaking down organic matter, such as plant and animal remains (detritus). They then absorb the soluble breakdown products.
These organisms are called detritivores.
The name âPhotosynthesisâ is derived from âphotoâ (light) and âsynthesisâ (building up).
Plants synthesize their food with the aid of sunlight. (Slides 9 and 10).
When a plant is photosynthesising, it is taking in carbon dioxide and giving out oxygen.
Plants which live in ponds, streams etc. are immersed in the water they need for photosynthesis
This reaction is summarised by the equation
6CO2 + 6H2O = C6H12O6 + 6O2
Without sunlight, photosynthesis could not take place.
Without photosynthesis, plants could not survive.
Without plants, most animals would die out because, ultimately, animals depend on plants for their food.
e.g. sunlight >>>> plants >>>> herbivores >>>> carnivores
The carbon dioxide comes from the air.
The water comes from the soil.
The energy comes from sunlight.
It is not only the leaves that contain chlorophyll; any green part of a plant, leaves, leaf stalk, stem, sepals will contain chloroplasts.
These are called palisade cells and they are present in the upper layers of a leaf where most sunlight is absorbed.
The chloroplasts are present in the cytoplasm lining the cell
The water travels from the roots, through the stem and into the leaf in a system of vessels.
The carbon dioxide diffuses into the leaf through tiny pores called stomata.
The palisade layer traps most of the sunlight.
The vessel carries water from the stem to the leaf.
Carbon dioxide enters through the stoma and diffuses through the air spaces between the cells.
The chemistry of carbohydrates is dealt with in the section âChemicals of Living Cellsâ.
Before it is transported, glucose is converted to sucrose. Two glucose molecules combine to make a molecule of sucrose.
2C6H12O6 = C12H22O11 + H20
glucose sucrose
It is the sucrose which is transported throughout the plant
Pyracantha. Some of the food made in the leaves is sent to the berries
Carbohydrates may be
(a) oxidised to provide energy for chemical reactions.
(b) turned into starch and stored in storage organs such as potatoes and parsnips.
(c) turned into cellulose which builds the cell walls.
(d) Combined with nitrogen (from nitrates) to make amino acids , which are combined to make proteins
Proteins are needed for making the cell structures, e.g. cytoplasm, nucleus, chloroplasts.
The plant can grow only by making new cells and cell structures
Strictly speaking, it is not nitrates salts that are taken up but nitrate ions. When a salt such as potassium nitrate, is dissolved in water it splits into positively charged potassium ions and negatively charged nitrate ions.
KNO3 becomes K+ and NO3- The plant may take up either or both of these ions.
Nitrates are needed for making proteins.
Phosphates are needed for DNA and for chemical reactions involving energy release.
Sulphates are needed for some proteins.
Iron is needed for certain enzyme reactions.
Magnesium is needed for making chlorophyll.
Some of these plots have received different types of fertiliser.
Some have had only manure added to the soil.
Some had neither manure nor fertiliser for many years
The chemical fertilizer contains all the mineral ions needed by the plants. The other fertilizers are lacking in one of the essential ions. Which of these mineral ions appears to be the most important?