Development is defined as an ordered change or progress, often towards a higher, more ordered or more complex state. Development may take place with growth and growth may take place with development.
Population growth is the increase in the total mass of cells making the population, or the increase in the total number of cells of the population. Individual growth is the increase in mass of the individual cell.
WHY GROWTH OCCURS?
Growth is expressed as the division of a cell to form two cells and the enlargement of the newly divided cells. When we say that cells double in all their constituents and then divide in half, we are obviously describing the average case. When we observe individual cells, however, we find deviations from the average. In some instances, the growth rate is constant rather than accelerating.
Mass of cells doubles and halves only in an average way and not in an exact way. Cells divide when they are ready and they are ready only when they have completed certain preparations for division.
CRITICAL MASS THEORY:-
In 1908 the German biologist Richard von Hertwig proposed the theory of the critical mass. In Hertwig's view a growing cell eventually reaches a size at which the ratio between the masses represented by cytoplasm on the one hand and the cell nucleus on the other becomes limiting. When that limiting ratio is reached, he proposed, some instability sets which triggers cell division. The nucleus-cytoplasm relation theory, as hertwig called it, does not hold up very well when mass alone is considered to be the factor that triggers cell division. Many exceptions to this hypothesis have accumulated that, although the division of cells usually parallels, an increase in cytoplasmic mass, this increase alone is not generally accepted as the fundamental mechanism
controlling cell cycle.
AGE OF CELLS:-
Cells taken from old cultures are often referred to as old cells, The age of a single cell cannot be greater than the time between two divisions. A cell, when first formed by cell division, although often called a young cell, contains materials both genetic and protoplasmic, which pre-existed in the parent cell. Age of the cell depend on its life spam. The life spam begins when the daughter cell is completely separated from the parental cell, this is called inception. It ends when this daughter cell itself divide later to give new daughter cells, this is known as termination. The period between inception and termination is called the generation time. This is actually the visible generation time that is observed. We can define young population of cells as the one, which is still actively growing and contains fewer cells that it is possible to obtain from that culture under tha same conditions, but old population is defined as that one which has reached the maximum number of cells under similar conditions. There are three types of following generation times :-
1. The visible generation time which is the period between inception and termination. It depends on division of cell wall.
2. Nuclear generation time depends on division of the nucleus. It starts from the moment the nucleus divides to the complete ability to divide again in the new daughter cells.
Generation time is given the symbol t.
SYNCHRONOUS AND SYNCHRONIZED GROWTH:-
Synchronous growth cultures are produced without metabolic shock. But synchronized culture are two ones in which we produce step-wise growth, the synchrony is induced by physiological shock. The principal means of doing synchronous growth are as follows :
1. Separation by physical means of cells in the population that are all at the same stage of the cellular life cycle. Ex. by filteration, centrifugation etc.
2. Attachment of growing cells to a membrane, from which one product of cell division is shed. i.e., age selection.
BALANCED AND UNBALANCED GROWTH:-
Exponential growth, whether in batch or continuous cultures, is the balanced growth. That is all cellular constituents are manufactured at constant rates relative to each other. If nutrient levels or other environmental conditions change, unbalanced growth results because the rates of synthesis of cell components vary relative to one another until a new balanced stage is reached.
Unbalanced growth occurs when a bacterial population is shifted down from a rich medium to a poor one. The organism may previously have been able to obtain many cell components directly from the medium. When shifted to a nutritionally inadequate medium, they need time to make the enzyme required for the biosynthesis of unavailable nutrients. Consequently cell division and DNA replication continue after the shift-down, but net protein and RNA synthesis slow.
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