Plant Nutrition

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Welcome to class!

In today’s class, we will be talking about plant nutrition. Enjoy the class!

Nutrition is the process of providing or obtaining the food necessary for health and growth.

A nutrient is a substance that provides nourishment essential for the maintenance of life and for growth.

There are two major modes or types of nutrition. They are

Autotrophic nutrition
Heterotrophic nutrition.

Autotrophic nutrition

Autotrophic nutrition is a process in which the organism (green plants) produces their food from the simple inorganic materials such as water and carbon dioxide.

Plants are referred to as autotrophs, i.e. they are able to manufacture their food. There are two types of autotrophic nutrition. They are Photosynthesis (holophytic) and Chemosynthesis.

Photosynthesis:

This is the process by which green plants synthesize organic compounds from inorganic raw materials in the presence of chlorophyll and light.

light

6C0₂ + 6H20 —–> C₆H₁₂O₆+ 60₂

The main product of photosynthesis is sugar (carbohydrate) while the oxygen given up is a by-product which is released into the atmosphere.

Mechanism of photosynthesis

Photosynthesis involves two stages

Light phase: it takes place during the day. It involves four stage

Activation of chlorophyll: the chlorophyll traps light energy from the sun and get energized
Photolysis of water: light energy splits water molecules H₂O —–>H⁺+ OH⁻ + OH^— —-> H₂O + O₂
Hydrogen transfer by NADP (NADP + H⁺NADPH₂)
Formation of ATP from ADP: this takes place to store energy for the dark phase reaction

Dark phase: In the dark phase of photosynthesis (occurring in the stoma of chloroplast in the presence of NADPH and ATP), neither light energy nor chlorophyll is needed. This stage takes place in the dark or at night.

During this phase, hydrogen ion and carbon dioxide molecules combine chemically under the control of enzymes to form simple sugar.

4H⁺ + CO₂ ——> CH₂O + H₂O

Importance of photosynthesis

The products of photosynthesis are the source of energy for their metabolic activities.
It is the only process in the food cycle that can trap sun energy.
All animals and other heterotrophic plants are directly or indirectly dependent on green plants for food.
Photosynthesis helps in the purification of the environment by removing carbon (iv) oxide from it while adding oxygen into it.
It provides the basis for the manufacture of other complex organic compounds such as lipids, proteins.

Conditions (factors) necessary for photosynthesis to take place.

The conditions necessary for photosynthesis can be grouped into two:

External factors: which are carbon(iv)oxide,water,light and temperature.
Internal factors: which are enzymes and chlorophyll.

Experiments in photosynthesis

The occurrence of photosynthesis in plants can be shown by experiments. There are experiments to show the importance of carbon dioxide, sunlight energy, chlorophyll. There is a need to prove that oxygen is given off.

Test for starch in the leaf:

Starch is tested in the leaf to show that photosynthesis took place.

Procedure for test for starch

Detach leaf to be tested from the plant after 4 to 6 hours of exposure to the sun to enable it to form starch;
Put the leaf in boiling water for about 30 seconds to kill the cells;
Put the boiled leaf in 70% warm alcohol or ethylated spirit in the water bath to decolourize the leaf;
Rinse decolourized leaf in running cold water to soften the leaf tissues and remove alcohol
Spread out the leaf on white tiles and add a few drops of iodine solution;
Allow to stand for a few minutes;
It turns blue-black showing or indicating the presence of starch.

To show that light is necessary for photosynthesis

Take the potted plant and keep it in a dark place for 2-3 days so that the leaves get detached.
Cover a part of one of its leaves with the strip of black paper. Make sure that you cover both the sides of the leaf.
Now place this plant in sunlight for 3—4 hours.
Pluck the selected covered leaf and remove the black paper covering it
Test the leaf for starch following the procedure above.
The part of the leaf-covered with strip of black paper will test negative for starch showing that light is necessary for photosynthesis.

To show that oxygen is given off as a by-product of photosynthesis

In this experiment, gas is collected in the test tube placed over the green plant and tested with a glowing splint. If it re-kindles a glowing splint, it shows that oxygen is given off because it is the only colourless gas that re-kindles a glowing splint.

Two potted plants are de-starched by placing them in a dark cupboard for 24hours;
A leaf each from both plants are then tested for starch to ensure they have no starch.
The potted plants are placed on board each.
A beaker containing caustic soda is placed beside plant A; to absorb carbon dioxide;
While a beaker containing water is placed beside plant B as control;
They are both covered with bell jars labelled A and B respectively;
The rim or edge is smeared with vaseline or petroleum jelly to make it airtight;
The setup is exposed to sunlight for 4-6 hours;
A leaf each from both plants are plucked and tested for starch.
It will be observed that leaf from plant A is negative while leaf from plant B is positive.

Mineral elements

Plant cells require mineral salts or elements obtained from the soil for healthy growth and development. These minerals are called plant nutrients.

Classes of plant nutrients

Macronutrient or major elements: These are nutrients required in large quantities for plant growth and development Eg nitrogen, hydrogen, sulphur, etc.
Micronutrients or trace elements: These are nutrients that are required by the plants in small quantities for their growth and development e.g. zinc, copper and cobalt.

In our next class, we will be talking about the Nutrient Cycle in Nature. We hope you enjoyed the class.

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.