Plant Nutrition
1. Photosynthesis
The
fundamental process by which plants manufacture carbohydrates from raw
materials using energy from light.
Carbon dioxide + Water ------(light)------> Glucose + Oxygen
6CO2 +
6H2O ------(light)------> C6H12O6
+ 6O2
Photosynthesis occurs in the leaves of a plant.
How
photosynthesis happens
•
Carbon
dioxide and water enter the cell
•
The
cell traps light energy using chloroplasts.
•
The
energy is used to split water (H2O) into hydrogen and oxygen
•
The
oxygen is excreted outside the leaf to the atmosphere as a waste product
•
The
hydrogen reacts with carbon dioxide forming glucose.
Carbon dioxide supply
Land plants get
their carbon dioxide from the air. There is only about 0.04% of it in the air
but since the plants continuously carry out photosynthesis, there is always a
concentration gradient. The carbon dioxide diffuses into the plants through
tiny holes in the leaf called stomata. Then these molecules move through the
air spaces between the spongy cells, through the cell walls, cell membranes and
cytoplasm of the spongy and palisade cells, and finally into their
chloroplasts.
Water supply
The water is absorbed
by the roots of the plants. Then it is transported upwards through a hollow tube
called the xylem vessel, until it reaches the leaf where photosynthesis takes
place. It enters the leaf through holes in the xylem. Excess water leaves the
cell through the stomata; this is called “transpiration.”
Light supply
The leaves are always
exposed to sunlight at daytime. The green pigment, chlorophyll, found in the
chloroplasts of plant cells absorbs light energy from the sun. The light penetrates
the transparent layers on the leaf until it reaches the mesophyll layer, where
photosynthesis takes place. Palisade cells are nearer to the surface of the
leaf than the spongy cells, so they receive more of the light and make more
photosynthesis. Guard cells are also able to photosynthesize.
Uses of produced glucose
- For respiration
- Converted into starch and stored
- Turned into sucrose and transported to
other parts of the plants
LIMITING FACTORS
Something present in the environment in such short supply that it restricts
life processes.
There are several factors affecting the rate of photosynthesis. The
most important ones are:
- Light intensity
-
Concentration of carbon dioxide
-
Temperature
Light intensity
In the first part of the curve, as light intensity increases, the rate
of photosynthesis increases. It is then a limiting factor as the rate of the
reaction is being kept low or limited by the lack of light.
However, it eventually reaches a point where the plant is
photosynthesizing as fast as it can. Light stops being a limiting factor as
even if more light is shone on the plant, the rate of the reaction is still the
same. This is probably because another limiting factor is holding it back.
Carbon dioxide
CO2 is a key limiting factor. In perfect conditions, a lack of CO2
holds back the rate or reaction. Like light intensity, as a plant is given more carbon dioxide it will
photosynthesize faster. This is so until it reaches its maximum. From here
onwards, something else must be the limiting factor.
Temperature
The chemical reactions of photosynthesis can only take place very slowly
at low temperature, so a plant can photosynthesize faster on a warm day than on
a cold one. However, plants can’t photosynthesize either if it gets too hot.
Greenhouses
A green house is a placed covered by
transparent polythene. In green houses, the limiting factors of photosynthesis
are eliminated, and the plants are provided the most suitable conditions for a
healthy, rapid growth.
The soil in green houses is fertilized and
very rich in mineral ions, assuring healthy, large yields. More carbon dioxide
is supplied to the crops for faster photosynthesis. The polythene walls and
ceiling allow heat waves and light rays only to enter and prevent harmful
waves, thus providing a high light intensity and optimum temperature, sometimes
a heating system is used too. A watering system is also present. The
disadvantages of green houses are that it is too small to give a large yield
and that it is expensive.
2. Leaf structure
Upper Epidermis:
it is a layer of cells that covers the leaf and protects it. It is covered by a
layer of wax called cuticle.
Mesophyll Layer:
·
Palisade Mesophyll: a layer of palisade cells which carry out most of photosynthesis
·
Spongy Mesophyll: a layer of spongy cells beneath the palisade layer, they carry out
photosynthesis and store nutrients.
Vascular
Bundle: it is a group of phloem and xylem vessels
that transport water and minerals to and from the leaves.
Lower
Epidermis: similar to the upper epidermis, only that
it contains a special type of cells called guard cells. Guard cells are
a specialized type of cells that control the passage of carbon dioxide into the
cell and the passage of oxygen out of the cell by opening and closing the
stomata (a hole in the leaf through which gases pass) so guard cells are
responsible for the gas exchange.
How guard cells work
At daytime, the guard cells open the stomata to allow gaseous exchange,
this occurs according to the following steps:
•
Sunlight increases the potassium concentration in
the vacuoles of the guard cells, the water potential decreases making a
gradient between the guard cells and the surrounding epidermal cells.
•
Water moves by osmosis into the guard cells from the
epidermal cells.
•
The water raises the pressure inside the guard
cells.
•
The cell wall adjacent to the stomata is thicker and
less stretchable then the cell wall on the other side.
•
The pressure expands the whole cell except for the
inner cell wall, creating a curve and a pore between the two guard cells.
•
The stoma opens.
At night however, the mechanism is opposite:
•
Potassium level decreases in the vacuole of the
guard cells,
•
Water potential increases in the cell and water
diffuses out of it,
•
The guard cells straighten up because of low
pressure closing the stoma.
3. Mineral requirements
The plant is also in need for mineral ions
to control chemical activities, grow, and produce materials. The most important
minerals are:
- Mg+2 (magnesium
ions): they are important
for the production of chlorophyll (chlorophyll synthesis). A lack of it results
in lack of photosynthesis and wilting of the leaves.
- Nitrates: these are the sources of
nitrogen; they are required to make amino acids and proteins by combining with
glucose. A lack of it results in deformation of the plant structure making it
small and weak.
Fertilizers:
Sometimes the soil is lacking of the mineral
ions needed. This problem can be solved by adding fertilizers to the soil. Fertilizers
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. But there are disadvantages of
fertilizers, such as:
- Excess minerals and
chemicals can enter a nearby river polluting it and creating a layer of green
algae on the surface of it, causing lack of light in the river, thus preventing
the aqua plants photosynthesizing.
- When living organisms in
the river or lake die, decomposers such as bacteria multiply and decay, respire
using oxygen. Eutrophication takes place eventually.
PSB
It has been shown that
when we get touched by a tickling motion by another, our somatosensory cortex
and the anterior cingulated cortex get affected and these two create what we
know as the tickling sensation. We have millions of nerve endings under our
skin and when these get stimulated by touch for example, the message gets sent
to the brain and then processes it to know whether or not it is important. This
is why it is not possible to tickle ourselves as the brain knowns the action is
coming and this message therefore becomes an unnecessary piece of information
and doesn't react. As for when a different person does the same thing to us we
react differently as it is an exterior stimulation.
