Cell as a Physiological Unit
Living organisms are made up of structural and functional units called cells. Every cell is made up of several biomolecules and organelles. The organelles are membrane-bound structures involved in specific functions. The organelles are absent in prokaryotic cells; yet a prokaryotic cell also can carry out all the physiological functions required for its metabolism.
The plant cell can be referred to as a physiological unit as it has the capabilities of carrying out physiological functions as a single entity.
The cell wall is a non-living outer limiting layer of the plant cell which provides shape and rigidity to the cell.
The cell wall is followed by the plasma membrane or cell membrane which is selectively permeable involved in the absorption of water by osmosis, and other substances like ions and low molecular weight biomolecules.
The protoplasm which is the physical basis of life includes cytoplasm, nucleus and cellular organelles which are involved in the various physiological processes taking place within the cell. The protoplasm is a water retentive colloid holding 90% of water which is a universal solvent.
The plant cells are characterised by a prominent vacuole holding the cell sap and functioning as an osmo-regulatory organelle of the cell.
Thus the cell is the principal seat of metabolic events.
Among the cell organelles, the Chloroplast is the most important structure unique to plant cells and involved in the most important physiological process called Photosynthesis. This process is the source of food for life on earth.
The organelle of the plant cell involved in cellular respiration is the mitochondrion. This organelle is concerned with the oxidation of food substances to release CO2, water and energy in the form of Adenosine Triphosphate (ATP).
The synthesis of proteins and their transport is carried out by the ribosomes and the endoplasmic reticulum.
Cell secretion takes place by the organelle called the Golgi body which is referred to as the dictyosome in plants.
The plant is thus able to co-ordinate the physiological activities of the various organelles and behaves as a perfect physiological unit.
Thus the cell is the seat of important physiological functions.