Chapter # 07: Plant Nutrition

How Plants Obtain Nutrition:-

Plants fulfill their food requirements by preparing its own food using the
process of photosynthesis and through nitrates and water in soil.
What is Photosynthesis:-
Green plants are autotrophs. They use small molecules like carbon-dioxide
from air and water from soil (that are readiIy available around them) and build them up
into large organic molecules of carbohydrate glucose by using light energy from
sunlight. This chemical reaction is known as Photosynthesis. The glucose that is made is
then usually turns to starch and stored in leaves.

Chemical Equation for Photosynthesis:-

6CO2 + 12H2O C6 H12 O6 + 6H2O + 6O2

Word Equation:-

Carbon dioxide + water Glucose + Water + Oxygen

Intake of Water:
For majority of plants, roots absorb the water necessary for photosynthesis from soil.
A few millimeters behind the tip of every root lie root hair cells. The very numerous
root hair cells provide a very large surface area for the uptake of water and of ions
from the soil to leaves.

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Intake of Carbon Dioxide:
For majority of plants leaves
- absorb carbon-dioxide from air
- absorb sunlight energy
- manufacture carbohydrates
- release waste product oxygen
 The exchange of carbon dioxide and Oxygen in photosynthesis occurs through
Stomata (small openings) of leaves which are always open in day light.

Importance of Chlorophyll:
Chlorophyll is the substance that gives leaves and stems their green color. It is made of
Magnesium and is able to absorb energy from light and use it to split water molecules
into hydrogen and oxygen. The oxygen escapes from the leaf and the hydrogen
molecules are added to carbon dioxide molecules to form sugar.
Importance of Sunlight:
Sunlight provides the energy necessary to combine carbon-dioxide and water
molecules to form carbohydrates. That energy becomes locked away within the
carbohydrate as chemical energy.

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Storage of Glucose:
The first carbohydrate made during photosynthesis is glucose, but this is soluble,
affecting concentration and enzyme action within a cell. It is therefore usually converted
to insoluble carbohydrate starch. Starch is first stored in chloroplasts where it has
just been made by photosynthesizing cell. It is then converted to sucrose to be carried
to storage organs of the plant (roots or stem). Here, sucrose is reconverted to starch.
 A tuber of the potato plant is an example of a plant storage organ that stores
starch.
Carbon-dioxide Enters the leaf through Stomata:
In day light when photosynthesis occurs, the carbon-dioxide in the leaf is rapidly used
up. The carbon-dioxide concentration in the leaf becomes lower than that in the
atmospheric air, so a diffusion gradient is established. Therefore, carbon-dioxide
diffuses from the surrounding air through the stomata into the air spaces inside the leaf.
The surface of the mesophyll cells are always covered by a thin film of water so that
carbon-dioxide can dissolve in it. The dissolved carbon-dioxide then diffuses into the
cells as a solution.

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How a Leaf is Involved in process of Photosynthesis
1. Carbon dioxide diffuses down a concentration gradient from the atmosphere
through the stomata into the intercellular spaces of leaf.
2. Carbon dioxide dissolves in the film of water which surrounds the mesophyll
cells.
3. Carbon dioxide diffuses in solution into the mesophyll cells and passes to
chloroplasts where photosynthesis occurs.
4. Sugar made by photosynthesis is carried away (Translocated) from the leaf in
the phloem of the vascular bundles.
5. Oxygen diffuses from the mesophyll cells into the intercellular spaces and out
through the stomata, across a concentration gradient into the atmosphere.
Uses of glucose synthesized during photosynthesis:
1. Glucose is first used by the cells during respiration to provide energy for
cellular activities
2. Glucose is also used for the formation of cellulose cell walls.
3. The glucose in the leaf can react with nitrates brought to leaves to form amino
acids. Excess amino acids are carried to all parts of plants, especially the
growing regions of the plants, to build new protoplasm of cells.

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 4. Fats are also formed from glucose in the leaves and stored. These fats may be
used in cellular respiration or for forming new protoplasm, for example the cell
surface membrane.
Importance of Photosynthesis:
1. During photosynthesis, light energy is converted to chemical energy which is
stored within carbohydrate molecules. Fats, proteins and other organic
compounds can be formed from carbohydrates (as both fats and proteins also
have C,H,O in their structure). All these substances eventually become the food of
organisms. Organisms thus obtained chemical energy directly or indirectly from
plants, because plants are the producers in food chains.
2. Photosynthesis helps to purify the air by removing carbon-dioxide from the air.
At the same time, photosynthesis produces oxygen. This oxygen is used by living
organisms in respiration and thus is used in sustaining most living things.
3. Coal is formed from trees and it contains a store of energy obtained from
sunlight through photosynthesis. When coal is burnt this energy is set free. Coal
is a fuel for driving machines. Coal can also be converted into liquid fuels such as
gasoline and diesel.
4. Photosynthesis also provides food and energy to animals and human beings.

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Respiration:
At Night, in the absence of light, Plant uses Oxygen from air and releases carbon-dioxide in
the surrounding. This process is called respiration (opposite of photosynthesis).That is why it
is not recommended to sleep under trees at night as it causes breathing problems.

Experiments of Photosynthesis:
Investigation 6.1
Test for Starch in Leaf:
1. Remove a green leaf from a plant that has been exposed to sunlight for several
hours.
2. Put the leaf in boiling water for few minutes. This will convert the glucose in leaf
to starch and rupture the cell wall so that later iodine can enter the leaf and react
with starch.
3. Put the boiled leaf in a boiling tube containing some alcohol. This will remove all
the chlorophyll from leaf as alcohol will react with chlorophyll.
4. Rinse the leaf with tap water.
5. With the help of dropper, put few drops of iodine over the leaf.
6. Color of leaf changes to blue black that will confirm starch.

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Investigation 6.2
Whether Sunlight is necessary for Photosynthesis:
1. De-starch a potted plant by placing it in dark for 2 days.
2. Remove one leaf and test it for starch by boiling in water, alcohol and then
dropping iodine.
3. Sandwich the leaf between two pieces of black paper. Fasten the black paper
with paper clips.
4. Place the plant in strong sunlight for few hours.
5. Remove the leaf and test for starch.
6. Region where black paper covered the leaf remained brown. The rest of leaf
stained blue/black.
 Only the region where light was able to reach had made starch. Thus light is
necessary for photosynthesis.

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Investigation 6.3
Whether Chlorophyll is necessary for Photosynthesis:
1. De-starch a plant with variegated leaves, e.g Duranta by placing it in dark for 2
days. (A variegated plant has leaves which are green where chlorophyll is
present and white where there is no chlorophyll).
2. Expose this plant to strong sunlight for few hours.
3. Remove the leaf and test for starch with iodine.
4. The white areas of leaf stain Brown due to absence of chlorophyll while green
areas stain blue/black due to presence of chlorophyll.
 Since starch has been made only where there was chlorophyll, thus chlorophyll is
necessary for photosynthesis.

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Investigation 6.4
Whether Carbon-dioxide is necessary for Photosynthesis:
1. De-starch two potted plants by placing them in dark for 2 days.
2. Enclose the pots in polythene bags so that no carbon-dioxide comes from soil of
plant. Secure the plants to the plant stem.
3. Place one pot in the bell jar A that has no supply of carbon-dioxide as soda lime
and potassium hydroxide rapidly absorb carbon-dioxide.
4. Leave the whole apparatus in strong sunlight for few hours.
5. Set up a control in glass jar B using pebbles and water in place of soda lime and
potassium hydroxide respectively.
6. Leave the whole apparatus in strong sunlight for few hours.
7. Remove a leaf from each plant and test them for starch.
 Leaf from jar A stains brown as no starch was made due to absence of carbon-
dioxide.
 Leaf from jar B stain blue/black as starch was made due to availability of
carbon-dioxide in the jar.

Investigation 6.5
Whether Oxygen is given off during Photosynthesis:
1. Place a fresh water plant like Hydrilla as shown in figure.
2. Dissolve a little Sodium Hydrogen Carbonate in water in the beaker. This
provides carbon-dioxide to the plant.

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 3. Place the apparatus in strong sunlight for few hours.
4. Gas bubbles will form on the leaves in the tube. These bubbles will rise up the
test tube. When the tube is half filled with gas, remove the tube by placing a
thumb over its mouth.
 The glow of glowing splinter will greatly increase when brought near the gas.
This will confirm that bubbles were of Oxygen gas.

Investigation 6.6
Investigate the Effect of Varying Light Intensities on the Rate of
Photosynthesis:
1. Set up the apparatus as shown in figure with the cut end of the water plant facing
upwards.
2. Air bubbles are given off from cut end of the plant.
3. Allow sometime for the plant to adapt to the conditions provided before taking
readings.
4. When the bubbles are produced at a regular rate count the number of bubbles
over a period of 5 minutes.
5. Repeat this a few times to obtain the average rate.
6. Repeat step 4 with the light source closer to plant, e.g 40cm, 30cm, 20cm and
10cm.
7. Note that the nearer the light source is to the beaker, the higher the light
intensity that the plant is exposed to.

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 8. As the lamp is moved further away, then intensity of light reaching plant
decreases. As the light intensity decreases so does the number of bubbles
released by the cut stem over the same period of time.
 Thus the rate of photosynthesis decreases with the decreased light intensity and
vice versa.

Investigation 6.7
Investigate the Effect of Different Temperatures on the Rate of
Photosynthesis:
1. Set up the apparatus as shown in figure with the cut end of the water plant facing
upwards.
2. Place a lamp of 60W bulb 10cm away from the plant. Keep this distance constant
throughout the investigation.
3. Add ice cold water to the water bath to keep the temperature at 5oC.
4. Allow sometime for the plant to adapt to conditions provided before taking the
readings.
5. Count the number of bubbles over a period of 5min.
6. Repeat this a few times to obtain an average rate.
7. Repeat step 5 at the different temperatures like 15oC, 25oC, 35oC, 45oC, 55oC,
65oC and 75oC.
8. The higher the temperature (up to around 45oC), the more bubbles are released
per unit time.
 Thus, higher the temperature (up to about 45oC), the faster rate of
photosynthesis.

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Investigation 6.8
Investigate the Effect of Different Carbon-dioxide Concentrations on
the Rate of Photosynthesis:
1. Set up the apparatus as shown in figure with the cut end of the water plant facing
upwards.
2. Place a lamp of 60W bulb 10cm away from the plant. Keep this distance constant
throughout the investigation.
3. Conduct the investigation at room temperature.
4. Use different concentrations of Sodium Hydrogen Carbonate solutions like
0.01M, 0.02M, 0.03M, 0.04M, 0.05m, 0.06M upto 0.10M. (These are proportional
to carbon-dioxide concentration in the solutions)
5. When the bubbles are coming out at a regular rate, measure the rate of bubbling
for each concentration of sodium hydrogen carbonate solution.
6. More carbon-dioxide becomes available to the plant as more sodium hydrogen
carbonate is added.
7. As more carbon-dioxide becomes available, so the number of bubbles released
by the cut stem in three minutes increases.
8. As the amount of carbon-dioxide available to the plant increases, so does the
amount of carbon-dioxide released from it. The increased amounts of available
carbon-dioxide increase the rate of photosynthesis.
Importance of Control experiment:
A control experiment is used to make sure that the results of our experiments are
valid. This is done by making a comparison in the experiment. For example, in
experiment of carbon-dioxide necessary for photosynthesis, we used potassium
hydroxide in one jar to absorb carbon-dioxide, but for comparison we used water in
other jar which do not absorb carbon-dioxide so this is control experiment (The
comparison of the actual experiment).
FACTORS AFFECTING RATE OF PHOTOSYNTHESIS:-

i) Light:-

Light affects the rate of photosynthesis in two ways:

 Wavelength of Light

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  Intensity of Light

White light coming from sun has the combination of various colours i.e. Violet,
indigo, blue, green, yellow, orange & red [VIBGYOR]. These colors range in
wavelength from 400-700nm wavelength.
All these colors of light are not equally absorbed by plant. Since Blue &
Red color have maximum wavelengths, so they are easily absorbed by plants to
make glucose. Since green color has the shortest wavelength so they are least
absorbed by plant & ultimately plant leaves appear Green.
 At High light intensity photolysis of water produces a lot of oxygen for
photosynthesis.
Graph shows as the light intensity increases, rate of photosynthesis also increase
as long as the other factors are in adequate supply.

ii) Carbon dioxide:-

As CO2 is the raw material for photosynthesis so its increase in concentration

(0.13%) increases the rate of photosynthesis & decreases in concentration (0.03%)
decrease the rate of photosynthesis.
Above a certain concatenation, rate of plant’s photosynthesis does not increase.

 Temperature: -

Increasing the temperature (25-45 C) allows the reaction between carbon

dioxide & hydrogen to take place more rapidly, thus increasing the rate of
photosynthesis producing maximum food for the plant.
 LIMITING FACTORS IN PHOTOSYNTHESIS:-

Any factor that will limit the rate of photosynthesis, even though all other factors may
be optimum is called limiting factor. These limiting factors are
 Light Intensity
 CO2 concentration
 Temperature
These three factors affect the rate of photosynthesis. The rate of photosynthesis is
governed by these three factors even available in shortest supply.

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Limiting Factors: -
In other words the factors that directly affect the rate at which photosynthesis can take
place are limiting factors.

In 1905 when investigating factors affecting the rate of photosynthesis,

Blackman formulated the law of limiting factors. This states that any change in the level
of limiting factors will affect the rate of photosynthesis.
Case I:

Initially as light intensity increases, the rate of photosynthesis increases from 0 to A.

Here, light intensity is the limiting factor. Beyond point A, light is no longer the
limiting factor since the rate of photosynthesis remains constant even though the light
intensity increases. Some other factor, possibly the temperature or the carbon-
dioxide concentration becomes the limiting factor that causes the leveling of graph
along AB.

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Case II:

Increasing the temperature from 20oC to 30oC while keeping the carbon-dioxide
concentration constant, does not bring about a large increase in the rate. This shows
that the temperature in not an important limiting factor in graph1.

Case III:

When the temperature remains constant and the carbon-dioxide concentration of the
environment is raised to 0.13%, the rate of photosynthesis greatly increases. This
indicates that carbon-dioxide concentration is the limiting factor in AB in graph 1.

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Case-IV

Increasing the temperature from 20oC to 30oC while keeping carbon-dioxide

concentration constant at 0.13%, causes a large increase in the rate of photosynthesis.
Therefore, the temperature of the surrounding is the limiting factor in EF in graph
3.

External structure of Leaf:

Leaf Lamina :
It has large surface area to trap maximum sunlight for photosynthesis. Lamina is also
very thin for easy exchange of gases (Carbondioxide and Oxygen).

Midrib:
From midrib a network of veins travel in the leaf. It mainly supplies water and mineral
salts to all cells of the leaf and carry manufactured food from these cells to other plant
parts.

Petiole:
It holds the lamina away from stem so that the leaf lamina can obtain maximum sunlight
for photosynthesis.

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Internal structure of leaf:

1. Upper Epidermis:
The leaf lamina has an upper epidermis made up of a single layer of closely packed
cells. The upper epidermis is covered on outer side by a waxy cuticle.
Functions:
 The cuticle protects the enclosed leaf tissue and prevents excessive
evaporation of water.
 It is transparent and single-celled in thickness to allow the sunlight to
penetrate inside leaf.

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  Epidermal cells (both upper and lower) do not contain chloroplast.
2. Mesophyll:
Mesophyll lies between the lower and upper epidermis.
Function:
 These cells contain numerous chloroplasts which enable them to absorb
maximum sunlight for photosynthesis.
a) Palisade Mesophyll:
The palisade mesophyll consists of one or two layers of closely packed, long and
cylindrical cells joined end to end without any spaces.
Function:
 These cells contain numerous chloroplasts which enable them to absorb
maximum sunlight for photosynthesis.
b) Spongy Mesophyll:
These cells are irregular in shape. There are numerous large intracellular air spaces
among them to allow for rapid diffusion of gases through the leaf.
Functions:
 These cells contain fewer chloroplasts than palisade mesophyll tissue.
However, they also carry out photosynthesis.
 Transport tissues like xylem and phloem (also called vascular bundles
together) are also present here that transports water and sugars in entire plant
respectively.
 All mesophyll cells both (palisade and spongy) are covered with a thin layer of
moisture so that carbon-dioxide can dissolve in it and both then utilized as raw
material for photosynthesis.
3. Lower Epidermis:
Beneath the mesophyll is the lower epidermis. Like upper epidermis, lower epidermis
also consists of a single layer of closely packed cells covered by an outer layer of cuticle.
Function:
 It reduces water loss through epidermal cells.
4. Stomata:
The lower epidermis contains large number of tiny and minute openings called stomata.

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The concentration of stomata on the lower side of the leaf is much higher than upper
epidermis.
Function:
 Stomata are specifically involved in exchange of gases, e.g carbon-dioxide and
oxygen.
 Moreover, loss of water in transpiration also occurs through stomata.
5. Guard Cells:
 Guard cells are bean shaped structures that contains chloroplasts, so they can
manufacture their own food (sugars) by photosynthesis.
 Guard cells can control the rate of diffusion of gases into and out of the leaf by
controlling the size of stomata.
 The cell wall near the stomata is thicker than elsewhere in the cell.
How Guard cells Control Size of Stomata:
Stomata generally open in the presence of light and closes at night.
During Day Light:
 The concentration of Potassium ions (K+) increases in the guard cells as guard
cells contain chloroplasts. The light energy is converted into chemical energy
which pumps the potassium ions into the guard cells from neighboring
epidermal cells. This lowers the water potential in guard cells.
 Water from neighboring epidermal cells enters the guard cells by end-osmosis
so they swell and become turgid.
 The guard cells have a thicker cellulose cell wall on one side of the cell (the
side around stomatal pore). Hence, the swollen guard cells become more curved
and pull the stomata open.
During Night:
 The Potassium ions (K+) accumulated in the guard cells during the day diffuse
out of the guard cells.
 This increases the water potential in the guard cells and water leaves them by
Ex-osmosis.
 Thus guard cells become flaccid and the stomata closes.

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Mineral Nutrition:-
1. Nitrates:-
These are up taken by the plant from soil through root hairs by Active
Transport. Nitrates are needed by the plant to synthesize amino acids which
ultimately forms proteins in plant.
Deficiency:-
Deficiency of nitrates cause stunted growth of plant.
2. Magnesium Ions:-
These ions are needed for the synthesis of chlorophyll is leaf.
Deficiency:-
Its deficiency causes yellowing of leaves because chlorophyll is not formed.

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