The protoplasm is the living component of the plant cell and involves four parts (i) the cytoplasm, (ii) the vacuoles, (iii) a number of organelles and (iv) the nucleus.
In young cells, the vacuoles are many, small and scattered, whereas in a mature cell, there is a single large vacuole occupying the centre of the cell and the cytoplasm forms a thin peripheral layer around the vacuole.
Many theories have been put forth to expalin the physical nature of the protoplasm.
Butschili in 1882, said that the protoplasm is a semi-transparent, viscous and slimy substance, essentially a liquid possessing a foaming or alveolar structure.
Wilson Fischer (1894) considered the protoplasm as a polyphase colloidal system.
This theory is a widely accepted one as the protoplasm is seen to exhibit the properties of colloids.
The protoplasm forms a colloidal system composed of a water phase in which mineral matter is dissolved, also having a protein phase, a fat phase and many minor phases. So it is said to be a polyphase colloidal system.
The protoplasm exists mostly as a sol (which is semi-liquid) but sometimes it becomes rigid and is viewed as a gel (which is sem-solid).
The particles of the protoplasm show an erratic zig-zag movement. This random motion, caused by the uneven bombardment of particle is called Brownian movement.
The scattering of a beam of light by the particles of a colloid is termed tyndall effect. This is a property of the protoplasm also.
The particles of the protoplasm cannot be filtered through ordinary filter paper but can be filtered through ultrafilters such as millipore filters.
The particles of the colloid carry an uniform electric charge.
When the particles of a colloid lose their charges they tend to aggregate and increase in size. As a result they fall out and get precipitated. In other words protoplasm loses its living property.
These properties of the protoplasm indicate that it is a living substance and has rightly been termed as the physical basis of life (vivum fluidum).
The major constituent of the protoplasm is water which makes up 90% of it. The dry matter has several organic and inorganic substances. Proteins and other nitrogen-containing compounds constitute the bulk of organic matter. Liquids like fats and oils are also present in small amounts. Compounds consisting of chlorides, phosphates, sulphates and carbonates of magnesium, potassium, sodium, calcium and iron are also present.
Since the protoplasm contains all the chemical constituents required for life, it has been called the 'Physical and Chemical Basis of Life' .