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