Plant Physiology: Processes and Mechanisms

Posted on Mar 10, 2025 in Biology

Root Pressure

Root pressure is maintained by water accumulation in the root tissues. This generates pressure on xylem sap, pushing it upwards. Guttation is evidence of this phenomenon, typically in tropical jungles.

Gas Exchange

Stomata

Stomata are specialized structures of the epidermis formed by two kidney-shaped cells, called guard cells, which define a space between them, the ostiole. After entering the ostiole, CO2 diffuses into the intercellular spaces of the tissue, enters the cells, and reaches the chloroplasts. The oxygen produced in photosynthesis exits through the ostiole. Plants must control the opening and closing of stomata to achieve a balance between water loss and gas exchange. When the plant needs CO2, a proton pump is put into operation, ejecting protons from the cell. This creates a deficit of positive charges, compensated by the entry of potassium ions. The increased intracellular saline concentration causes water to enter via osmosis, opening the stoma. When water loss by perspiration cannot be offset by absorption, the plant closes its stomata to ensure survival. This mechanism is regulated by abscisic acid. Other hormonal factors include temperature and light. High temperatures stimulate cellular respiration, increasing CO2 concentration between the mesophyll cells. This response prevents excessive water loss. Light activates photosynthesis, increasing CO2 consumption and triggering stomatal opening.

Lenticels

Lenticels are small bumps in the cork tissue of stems and branches of woody plants that also facilitate gas exchange.

Photosynthesis

Photosynthesis is an anabolic process that converts light energy into chemical energy. This energy is used to synthesize organic molecules from inorganic compounds such as CO2 and water, releasing oxygen. There are two sub-phases:

  • Light-dependent reactions: Occur in the thylakoids of chloroplasts. Light rays excite electrons, raising their energy level. Some electrons are transferred to a chain of carrier molecules, releasing energy used by ATP synthase to make ATP. Remaining electrons are transferred to NADP+, forming NADPH. Chlorophyll retrieves electrons from water, releasing oxygen.
  • Light-independent reactions (Calvin Cycle): Occur in the stroma of chloroplasts. A set of chemical reactions lead to the synthesis of glucose from CO2 and water, using energy and reducing power from ATP and NADPH. The first reaction is CO2 fixation, catalyzed by the enzyme Rubisco.

Cellular Respiration

Cellular respiration is a catabolic process that requires oxygen. Its function is the degradation of organic matter to extract the energy contained in its bonds. It takes place in the mitochondria.

Excretion and Secretion

Excretion is the elimination of substances that are unusable by the organism. Secretion removes substances with a physiological significance. Examples of secretion and excretion include resin, scents, nectar, and latex.

Phytohormones

Phytohormones are chemical substances produced by poorly differentiated tissues, such as meristematic or embryonic tissues. They travel to target organs to exert their function.

Tropisms

Tropisms are slow, permanent movements that depend on the direction of the stimulus. They can be positive (towards the stimulus) or negative (away from the stimulus).

  • Plagiotropic: Inclined movement dependent on the distribution of the plant hormone.
  • Phototropism: Plant growth generated by light.
  • Gravitropism: Plant growth induced by the direction of gravity.
  • Thigmotropism: Plant growth stimulated by contact with a solid.
  • Chemotropism: Plant growth caused by the presence of chemical substances.

Nasties

Nasties are fast, reversible movements in response to an external factor. Unlike tropisms, the direction of the stimulus does not influence them.

  • Thermonasties: Caused by temperature.
  • Photonasties: Caused by light.