A cell is the basic and fundamental unit of life.
It is composed of constituents called biomolecules.
Biomolecules are essential for the maintenance of all the metabolic processes that occur in a cell.
These include carbohydrates, lipids, proteins, enzymes, nucleic acids and many other small molecules like primary and secondary metabolites.
This chapter comprises of study of each biomolecule in an systematic manner along with suitable examples.
b) Components of a Cell
A cell comprises of protoplasm in which all organelles are suspended.
All the constituents comprise of both organic and inorganic components.
Protoplasm can be defined as the fluid which upholds the living content of a cell i.e. all the structures of cell along with the fluid enclosed within the limits of cell membrane.
It comprises of two regions, i.e., Nucleoplasm and Extranuclear Protoplasm.
It was first observed by Corti, 1772.
3. PHYSICAL PROPERTIES OF PROTOPLASM
The properties of protoplast can be best explained by Colloidal Theory given by Fisher 1894, Hardy 1899 and Wilson 1925 after the electron microscopic observation of the protoplasm.
This theory states that protoplasm is a polyphasic colloidal system i.e. it is partly a true solution and partly a colloidal solution.
Colloidal solution comprises of two stages:
The liquid, gaseous, or solid phase in a two-phase system in which the particles of the dispersed phase are distributed are called dispersion medium or continuous phase or external phase.
This phase comprises of finely divided particles as colloidal particles, droplets, or bubbles of one substance distributed through another substance. This phase is also known as discontinuous phase or internal phase.
Based on dispersion medium and dispersed phase colloids can be enlisted into following four types:
|1||Sol||Dispersion medium is liquid and dispersed phase is solid.||Protoplasm in majority of living cells.|
|2||Gel||Dispersion medium is solid and dispersed phase in liquid||Protoplasm in skin cells|
|3||Emulsion||Both medium and phase are liquid||Blood plasma composed of both sol and emulsion.|
|4||Aerosol||Dispersion medium is gas and dispersed phase in solid particles||Smoke|
The physical properties of protoplasm are enlisted below:
- Odourless, translucent, polyphasic colloidal solution, viscous, granular elastic, pH = 6.8, Refractive index = 1.4 and viscosity= 2-20 centipoises.
- Either Sol or gel. Sol stage provides cyclosis (environmental conditions like temperature causes changes in properties of protoplasm and as a result in protoplasm endocellular movement is observed known as cyclosis), Brownian movements (colloid particles carry electric charge and thus remain in continuous random motion called as Brownian movement) and high reactivity to protoplasm while as gel stage provides elasticity, contractibility, rigidity and viscosity.
- Possess surface tension and solutes like proteins and lipids having less surface tension form a delimiting membrane at surface called as Plasma membrane. Plasma membrane possess rapid power of regeneration.
4. BIOLOGICAL PROPERTIES OF PROTOPLASM
Possess irritability (i.e. sensitivity of protoplasm to external stimuli) and conductivity (i.e. transmission of stimuli). Such characteristic is more prominent in cells like nerve cells, muscle cells.
Metabolic reactions occur in protoplasm which includes catabolic and anabolic reactions. Anabolic reactions are constructive reactions e.g. synthesis of various biomolecules and catabolic reactions are destructive reactions like oxidation of food.
Power of assimilation i.e. the capability to use external material to resynthesise into a new form, respiration and excretion are also properties of protoplasm.
Protoplasm being considered as living matter of the cell also is capable of producing some non-living substances e.g. Yolk bodies, lipid droplets, pigments etc.
5. CHEMICAL NATURE OF PROTOPLASM
Protoplasm chemically reacts as a weak alkali. It is soluble in dilute alkali and acids but solidifies when treated with strong acids or alcohols. It is unstable and during chemical analysis readily decomposes into H2O, NH3 and CO2, etc.
Chemical analysis shows that protoplasm essentially consists of 34 elements out of which about 12 elements are present universally.
Oxygen, carbon, hydrogen and nitrogen are 4 basic elements that comprise 99% of protoplasm. Therefore; these are called major constituents of the protoplasm.
Sulphur, phosphorus, potassium, iron, magnesium, calcium, sodium and chloride are 8 other elements that are present in quantities of less than 1%.
These elements are usually not present in a free state but are found as various compounds such as phosphorus is the main component of adenosine triphosphate (ATP), deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Many other trace elements are required for various physical activities like osmosis and diffusion as well as biochemical activities such as impulse conduction, etc. Water in protoplasm is about 85% to 90% approximately. The water acts as the dispersion medium in which other elements are suspended.
The water occurs in two forms i.e. free water, and bound water. 95% of the total cellular water is free water in which various inorganic substances and organic compounds are found dissolved. Bound water which is loosely linked with protein molecules by hydrogen bonds or other forces comprises remaining five percent of the total cellular water.
Generally, the dry protoplasm shows the following constitution:
Protein 45% , Carbohydrates 25% , Lipids 25% , Other substances 5%.
These various compounds may be either of organic nature comprising: C, H, N, O, or of inorganic nature consisting of salts, gases and some elements in free State, like S, Fe, P, Cl, etc.
It is about 98% of the mass of living organism is composed of just six elements including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.
It is colourless, odourless, tasteless and transparent liquid.
It comprises of small polar molecules that readily pass through the cell membranes
Water is an excellent solvent because both non-electrolyte and electrolyte polar substances can form hydrogen bonds with it and join its lattice structure.
Medium of various metabolic reactions.
High specific heat.
Minimises temperature variations.
65-70% of total body weight.
In human body 55% of water occurs as intracellular water while 45% of water is present extracellularly in the form of blood, lymph etc.
It helps in maintenance of turgidity of cell as well as in growth and lubrication.
Occurs in protoplast in ionised form.
Responsible for electric conductivity, protoplasm irritability and conductivity.
Participate in salt-bridge linkage.
Play role as cofactors in enzymatic activities.
Examples are enlisted below in Table:
Regulation of osmotic pressure and chemical exchange of protoplasm with respect to its environment.
Ions like Na+, K+, Mg+2 participate in roles like: Nerve Induction, muscle contraction, blood clotting, bone formation, stomatal opening and closing etc.
Salts and bases have their role in preventing pH variations such as Carbonic acid Bicarbonate buffer system.