NUTRITION (plants)
Nutrition is the process by which living organisms obtain food substances and use them to provide energy and materials for their growth, activities and reproduction. There are two types of nutrition; autotrophic and heterotrophic.
Autotrophic nutrition is the type of nutrition in which organisms are able to manufacture food for themselves using raw materials in their environment. It is divided into two; photosynthetic and chemosynthetic. While heterotrophic nutrition is the type of nutrition in which organisms cannot manufacture their own food but have to depend on others.
Photosynthesis
Photosynthesis is the process by which green plants manufacture their food using carbon dioxide and water in the presence of sunlight. This is common in green plants and some unicellular organisms like the Chlamydomonas and Euglena. Green plants generally contain chloroplast in their cells which in turn contain chlorophyll, the green pigment responsible for splitting water using the energy gotten from the sun. Photosynthesis is important not only to plantsbut also to animals, which depends on plants directly or indirectly for food. It is represented by the equation:
sun
6H2O + 6CO2C6H12O6 + 6O2 . chloropyll
Watercarbondioxidestarchoxygen
In the light stage of photosynthesis which occurs during day time, the chlorophyll traps energy from the sun which it uses as a catalyst to split water molecule into its components; hydrogen and oxygen. The dark stage occurs at night. Here the hydrogen produced in the first stage reacts with carbon dioxide to produce the simpler sugars like glucose, and oxygen is given off as waste product.
The gases carbon dioxide and oxygen move in and out between the surrounding air and the leaves through stomatal openings on the leave surface. Water from the soil enters the root hairs by osmosis. From the root hairs, it is conducted upwards through the stem to the leaves.
Most of the sugars produced are changed into starch in the leaf cells. They are stored in this insoluble form until night time when the starch is converted back into sugars (soluble form) to be transported to other parts of the plant where they are used up or stored. This movement of sugar is known as translocation.
Leaf structure
The leaves are the main photosynthetic organs. As such, the structure of the leaf is specially adapted for carrying out this function. A leaf consists of a leaf stalk and a leaf blade or Lamina. The lamina (the large wide surface) allows a large amount of chloroplast to be exposed to sunlight, and has a network of veins running through it. The veins carry water and mineral salts from the soil to the leaves, and the organic substances made in the leaves to the rest of the plant. The leaf stalk usually holds the lamina in the best position for the sun’s ray to fall in it at right angle throughout the day.
External structure of the leaf Internal structure of a leaf
A transverse section of the leaf viewed under the microscope consist of an upper and lower epidermis, a mesophyll and the vascular bundles.
The epidermis occurs on each surface of the leaves and is covered by the cuticle, a waxy waterproof material that gives the leaf a shiny appearance. It also prevents water loss and protects the leaf against mechanical injury.
The mesophyll is the part where photosynthesis occurs. It consists of the palisade mesophyll and the spongy mesophyll.
Vascular bundles are connective tissues transporting fluids through the glands. They contain the xylem vessels and sieve-tubes. The form the veins of the leaves. They also contain the phloem tissues.
Chemosynthesis
In chemosynthetic nutrition, the organisms get their carbon from the atmospheric carbon dioxide or some other form of carbonates, and their energy comes from the breakdown of inorganic components. Examples of organisms that feed chemosynthetically are unicellular bacteria like Nitrobacter and Nitrosomonas. Non-green plants are also chemosynthetic.