Plant Physiology

Physiology is the branch of biology that studies the functions and processes within a living organism. Thus, plant physiology deals with studying the major functions of different structures in a plant and their interactions. Even though plants appear passive and are mostly sessile, they constantly perform complex physiological activities like photosynthesis, respiration, mineral nutrition, transportation of gas (like O₂ and CO₂) and nutrients and growth.

What is Plant Physiology?

Plant physiology includes the study of all the dynamic processes required by a plant to exist. The majority of these activities occur at the cell or tissue level. Thus, there is a close association between the structure (anatomy) and function (physiology) of the plants.

Important aspects of Plant Physiology

Here are some key aspects:

Photosynthesis

Photosynthesis is an anabolic process that helps the autotrophs to prepare the primary organic food (glucose) that supports the food chain for all living organisms. The event takes place within the chloroplasts of green parts of a plant through light-dependent and light-independent phases. In photosynthesis, oxygen is released to the atmosphere (sustaining the inhalation of the living organisms) and produces glucose (stored as starch within the plant body).

Respiration in Plants

Respiration begins with glycolysis. In aerobic respiration, glucose undergoes complete oxidation of the various organic substances that release and utilise the energy stored in the molecules. It contains three major steps that occur in the cytosol (glycolysis) and mitochondria (Krebs’ Cycle, Electron Transport Chain and Oxidative Phosphorylation).

Mineral Nutrition

Mineral nutrition gives insights into identifying the essential elements of plants. It recognises the essentiality, mechanism of absorption and deficiency symptoms of various mineral elements. Macro and micronutrients absorbed by the plants maintain essential processes like osmotic concentration of cell sap, cell-membrane permeability, enzyme activity, etc.

Transport of Nutrients and Gas

Vascular tissues (xylem and phloem) help in the transport of nutrients and water from roots to all the parts of a plant through translocation. Transport of water and nutrients in the plants might be unidirectional or multidirectional. There might be passive transport (diffusion or facilitated diffusion) or active transport (mediated by pumps- the specific membrane proteins). At the cellular level, osmosis allows the transport of molecules in and out of the cells.

Transpiration

The tiny pores called stomata, present in the leaves, remove excess water through the crucial process called transpiration. In some cases, a small amount of water loss may also occur through the cuticle, though its primary role is to reduce transpiration.

Plant Growth and Development

A precise sequence of events leads to the maturation of a plant from a single-celled zygote. The zygote develops and differentiates into tissues and organs, which result in the organisation of a mature plant. Development is the total set of these events, the underlying genetic instructions.

Growth is the irreversible, permanent increase in size of a cell or organ, accompanied by metabolic processes. Plants have meristems and thus retain their capacity to grow throughout their lives. Plant growth regulators also control the growth.

Summary

Plants synthesise their food via photosynthesis, carry out respiration, gain mineral nutrition, transport nutrients and carry out essential processes like transpiration. They grow and develop, complemented by various hormones and specific systems.

Frequently Asked Questions (FAQs)

Q1. Write the chemical equation for aerobic respiration in plants.

A. In a plant cell, during aerobic respiration (in the presence of oxygen), a glucose molecule undergoes three major steps to produce carbon dioxide, water and energy (in the form of ATP) as the end products. The chemical reaction is

C₆H₁₂O₆ (glucose) + 6O₂ → 6CO₂ + 6H₂O + 36 ATP (energy)

Q2. Why is the transport of water through xylem a passive process, but the sugar transport through phloem is active?

A. Transport of water through xylem is driven by the physical forces like transpirational pull, cohesion and adhesion of the water droplets, without the expense of energy. Transport of water through xylem is passive, whereas sucrose transport through phloem requires active loading and unloading of sugars.