Plant Nutrition
Introduction
All living organisms need to take many different substances into their
bodies.
Taking in useful substances is called feeding, or nutrition.
Animals cannot make their own food. They depend on others for their
food.
Green plants make their own food by the process of photosynthesis.
Photosynthesis as the fundamental process by which plants
manufacture carbohydrates from raw materials using energy from
light.
Photosynthesis
https://www.youtube.com/watch?v=3pD68uxRLkM
Green plants make the carbohydrate glucose from carbon dioxide and
water. At the same time, oxygen is produced. Green plants use the
energy of sunlight for this.
The reaction is therefore called photosynthesis ( photo means light,
and synthesis means manufacture).
The reaction requires light energy, which is absorbed by a green
substance called chlorophyll.
Photosynthesis takes place in leaf cells. These contain chloroplasts,
which are tiny structures containing chlorophyll.
Chlorophyll is the pigment which makes plant look green.
When sunlight falls on a chlorophyll molecule, some of the energy in
the light is absorbed.
The chlorophyll molecule then releases the energy.
The released energy makes carbon dioxide combine with water, with
the help of enzymes inside the chloroplast.
The glucose that is made contains energy that was originally in the sunlight.
So, in this process, light energy is transferred to chemical energy.
The full equation for photosynthesis is
The balanced equation for photosynthesis is
Carbon Dioxide
Carbon dioxide is obtained from air.
Only about 0.04% of the air is carbon dioxide.
Therefore, the leaf must be very efficient at absorbing it.
Plants get carbon dioxide from the air through their leaves.
The carbon dioxide diffuses through small holes in the underside of the leaf
called stomata. (singular: stoma plural: stomata)
The lower part of the leaf has loose fitting cells, to allow carbon dioxide to
reach the other cells in the leaf. This also allows the oxygen produced in
photosynthesis to leave the leaf easily.
Sunlight
A leaf usually has a large surface area, so that it can absorb a lot of light.
Its top surface is protected from water loss, disease and weather damage by
a waxy layer.
The upper part of the leaf is where the light falls, and it contains a type of cell
called a palisade cell.
This is adapted to absorb a lot of light. It has lot of chloroplasts.
In dark , a plant cannot photosynthesise at all.
In dim light, it can photosynthesise slowly.
The rate of photosynthesis increases as the light intensity increases until at a
certain light intensity the rate does not increase any further.
Increasing the light intensity increases the energy available to the plant for
photosynthesis.
The effect of increasing light intensity on the rate of photosynthesis
Intake of carbon dioxide
Carbon dioxide diffuses in for photosynthesis through stomata of a leaf.
Stomata are small pores (holes) in the epidermis that allow gases to diffuses
into and out of the leaf.
Stomata are usually in the lower epidermis, but some plants like water lilies
have them in the upper epidermis.
Oxygen made in photosynthesis diffuses out.
Stomata are opened and closed by guard cells.
Water passes into the guard cells by osmosis. This makes them bend so the
stomata opens.
At night the stomata close. Water passes out of the guard cells by osmosis
and they straighten and move closer together so closing the stomata pores.
The stomata also close in hot, dry weather to help prevent the plant wilting.
Open stoma and closed stoma
Intake of water
Xylem is the tissue of vascular plants that transports water and nutrients from
the soil to the stems and leaves.
The xylem vessels are long thin tubes with no cell contents when mature.
They have thickened cell walls, impregnated with a material called lignin,
which can form distinct patterns in the vessel walls.
Cross section of Dicot leaf
https://www.youtube.com/watch?v=co0JdqUlycg
Cuticle - waterproof layer that also cuts down the water lost by evaporation.
Secreted by cells of upper epidermis.
Upper epidermis - single layer of cells with no chloroplasts. Thin and
transparent (allows light to pass through). Acts as a barrier.
Palisade mesophyll - the palisade cells contain lots of chloroplasts. Most
photosynthesis occurs here. Receive CO2 by diffusion from air spaces in the
spongy mesophyll.
Spongy mesophyll - more rounded cells with lots of air spaces between them.
Xylem vessels - bring water and ions to the leaf
Phloem tubes - carry sugar & other organic nutrients made by plant from the
leaves to the rest of the plant.
Lower epidermis - act as a protective layer. Stomata are present to regulate
the loss of water vapour (transpiration). Site of gaseous exchange into and
out of the leaf.
Stomata - each stomata is surrounded by a pair of guard cells. Guard cells-
control whether the stoma is open or closed. Water vapour passes out during
transpiration. CO2 diffuses in and O2 diffuses out during photosynthesis.
Plant nutrients
Nutrients are needed for healthy growth of plants. They are used for a variety
of purposes in plants.
If these nutrients are lacking in the soil then plants do not grow well and show
certain symptoms known as deficiency symptoms.
Importance of nitrate and magnesium ions
Nitrates
Plants absorb nitrate ions from the soil, through their root hairs
Nitrate ions combine with glucose to make amino acids
Amino acids bond together to make proteins
Deficiency cause poor growth, especially of leaves. The stem becomes weak,
lower leaves become yellow and die, while upper leaves turn pale green.
Nitrate deficiency: growth severely restricted, few stems, yellowing of older foliage
Magnesium
It is a building block of the chlorophyll, which makes leaves appear green
Plants absorb magnesium ions from the soil solution
Used for the manufacture of chlorophyll
Each chlorophyll contains one magnesium atom
Deficiency makes leaves turn yellow from the bottom of the stem upwards
and eventually stops photosynthesis
Magnesium deficiency in potato plant
Sometimes the soil is lacking of the mineral ions needed, this problem can be
solved by adding fertilisers to the soil.
Fertilisers are chemical compounds rich in the mineral ions needed by the
plants. They help the plants grow faster, increase in size and become
greener, they simply make them healthier and increase the crop yield.
A major problem with the use of fertilisers occurs when they are washed off
the land by rainwater into rivers and lakes.
Nutrients from fertilisers can cause eutrophication in water
Testing a leaf for starch
Iodine solution is used to test for starch. A blue black colour shows that starch is
present.
http://brilliantbiologystudent.weebly.com/testing-a-leaf-for-the-presence-of-starch.h
tml
Questions
1. Why was the leaf put into boiling water?
2. Why did the alcohol become green?
3. Why was the leaf put into alcohol after being put into boiling water?
To see if light is needed for photosynthesis
http://brilliantbiologystudent.weebly.com/is-light-is-necessary-for-photosynthesis.ht
ml
Questions
1. Why was the plant destarched before the beginning of the experiment?
2. Why was part of the leaf left uncovered?
3. What do your results tell you about light and photosynthesis?
To see if chlorophyll is needed for photosynthesis
https://www.youtube.com/watch?v=x17XhQJ3PxI
