Chapter 7: Nutrition in Plants

Photosynthesis:

Is the metabolic reaction occurring in plants in which light energy converts raw materials; carbon dioxide & water into glucose & oxygen. Glucose can be stored in cells & used as an energy source via respiration.

Equation for photosynthesis:

Conditions essential for photosynthesis:

• Temperature:

  • At optimum temperatures plant photosynthesize the most.

• Light intensity:

  • As it increases, the rate of photosynthesis increases.

  • But at a point light intensity becomes a limiting factor and it does not affect further.

• Carbon dioxide concentration:

  • As CO2 increases, rate of photosynthesis increases.

What happens to glucose in plants:

• It provides energy for cellular activities.

• Excess glucose is converted in sucrose which is transported to storage organs eg. seeds, stem tubers and root tubers and then it is converted to starch as a storage form. During the day, in large amount glucose is made and temporarily stored in leaves of plant. At night, photosynthesis stops so starch is converted as glucose and used again.

• Glucose in leaves react with nitrates and other mineral salts.

• It also reacts with nitrates and other mineral salts brought to the leaf to form amino acids. They help in forming new protoplasm in cells and help in growth.

Importance of Photosynthesis:

• All carbohydrates, fats, proteins and other organic compounds produced eventually become food for animals, since animals directly or indirectly depend depend upon plants.

• During photosynthesis sunlight energy is converted into chemical energy, which is stored; when animals feed on plants they obtain this energy from them.

• Coal a fossil fuel is formed when dead trees and plants are decomposed.

• It helps to purify the air; by taking in in CO2 and giving off O2.

The Leaf: Nature’s food making factory

• The whole leaf mainly consists of a lamina or a petiole.

Petiole: holds the lamina away from the stem so that lamina can obtain sufficient sunlight and air.

Lamina: helps obtains maximum amount of photosynthesis for photosynthesis.

• Venation:

  • Veins carry water and mineral salts to the cells in the lamina and carry manufactured food from it to other parts of the plant.

  

• Internal structure of the Lamina:

  • Has upper epidermis it is made up of a single layer of closely packed cells, covered on the outside by a cuticle.

  • Cuticle protects the leaf, prevents excessive evaporation of water, and ‘focuses’ on the light onto the mesophyll.

  • Mesophyll is the main site for photosynthesis. two regions of the mesophyll can be recognized, namely as palisade tissue and spongy tissue.

  • Palisade tissue consists of one or two layers of closely packed, long and cylindrical cells. These cells contain numerous chloroplasts which enable them to absorb maximum sunlight for photosynthesis.

  • Chloroplasts are oval shaped structures which contain chlorophyll.

  • Spongy tissue are loosely arranged so that numerous large intercellular air spaces occur among them.

  • Lower epidermis consists of a single layer of closely packed cells covered by an outer layer of cuticle. It contains many minute openings called stomata.

  • In stomata gaseous exchange occur; CO2 and O2 diffuse in and out respectively.

  • Guard cell differ from epidermal cell in the following ways:

    • Guard cells are bean shaped in surface view, while the epidermal cells are in irregular shapes.

    • Contain chloroplasts, so they can manufacture food photosynthesis. (NOTE: epidermal cells do not contain chloroplasts).

• The entry of carbon dioxide into the leaf:

  • CO2 concentration in the the leaf becomes lower than that in the atmospheric air i.e a diffusion gradient exists. So CO2 diffuses from the external environment through the stomata into the systems of air spaces in the leaf.

• The entry of water and mineral salts into the leaf:

  • Veins of plant contain xylem and phloem.

  • The xylem brings water and dissolved mineral salts to the leaf from the soil via roots.

  • Once the cells receives all of its raw materials it starts manufacturing food by photosynthesis; the sugars (glucose) made are transported via the phloem to all plant cells.

How is the leaf adapted to photosynthesis?

Mineral nutrients in plants:

• Nitrate ions are essential for plants growth and synthesis of protoplasm

• Magnesium ions are essential constituent of chlorophyll. They give green colour to the plant