Plant Physiology: Water Transport and Photosynthesis Mechanisms

Transport of Water and Salts

Transpiration

The force that causes water to ascend from the soil to the leaves is called transpiration. As water evaporates from the leaf, it creates a suction force that draws more water into the roots, allowing it to climb through the interior of the stem to replace the evaporated moisture. This process is known as the transpiration stream, driven by the cohesive forces between water molecules.

Water loss through transpiration is strictly regulated by the specialized cells that surround the stomata.

The Importance of Photosynthesis

Energy flows through the biosphere, with approximately 1% of solar radiation captured by plants during photosynthesis. Plants store light energy in chemical form within their cells, specifically in the covalent bonds of organic molecules such as carbohydrates.

All living organisms transform these organic molecules to access the energy stored in their chemical bonds for the synthesis of their own compounds, with a portion of this energy being lost as heat.

Phases of Photosynthesis

Light-Dependent Phase

The synthesis of organic molecules requires Carbon (C), Hydrogen (H), and Oxygen (O), while proteins and nucleic acids also require Nitrogen (N) and Phosphorus (P). This phase occurs in the thylakoid membrane and involves the following processes:

• a) Photodissociation of water: Light energy causes the breakage of water molecules into hydrogen and oxygen.
• b) Formation of ATP and NADPH: These are produced through a succession of chemical reactions within the thylakoid membrane.

Dark Phase (Calvin Cycle)

Taking place in the stroma of the chloroplast, this phase uses the H2, NADPH, and ATP generated previously to reduce CO2 and form organic compounds. This process is known as the Calvin cycle.

Cellular Respiration

A portion of the carbohydrates produced by photosynthesis is used in cellular respiration to provide the plant with necessary energy. The remainder is converted into starch and stored in various parts of the plant. During this process, carbohydrates are oxidized and transformed back into CO2 and H2O, which are released externally, while the released energy powers various biological functions. During the day, plants simultaneously perform both photosynthesis and cellular respiration.