Structure of Cells

In 1965, Robert Hooke observed a section of cork under microscope; he saw numerous chambers similar to those of beehive. He called the individual chambers as Cells. But he, in fact, observed only the walls of actual cells as he was viewing dead cells.

Subsequently, several scientists began observing various biological materials under the microscope and discovered different cell types. On the basis of all the available information, two German biologists, Mathias Schleiden and Theodor Schwann, in 1839, put forth the famous unifying theory of biology: The Cell Theory.

The cell theory establishes the following two major points:

(1) The cell is the structural unit of all living organisms, and

(2) All the living cells have structural and functional similarities

Many organisms are unicellular, while others are multicellular. As the biochemical and physical techniques advanced and microscopes improved, greater details were observed regarding the function and architecture of the cells. The different functions of cells are related to their various subcellular components.

In general, the cells have the following four main functions:

(1) All the cells are enclosed by a cell membrane which separates and protects the internal cell contents from the external environment.

(2) The cell contains and transmits genetic materials to its progeny cells. The genetic material contains the genetic information to govern the various cell activities and for the synthesis of various cellular components.

(3) The metabolic activities catalysed by enzymes take place within the cells.

(4) Cells also have the capacity of movement.

With the invention of electron microscope, the different cell components could be observed in much greater detail so that their size, shape and function could be determined (Table 2.1).

The cells are divided into two major categories on the basis of their subcellular architecture:

(1) Prokaryotic cells and

(2) Eukaryotic cells.

(1) Prokaryotic Cells:

A prokaryotic cell has only one membrane, the cell membrane or plasma membrane that forms a single compartment in which all cell components are enclosed. With some notable exceptions, e.g., mycoplasma etc., the prokaryotic cells have a cell wall on the outside of the plasma membrane.

Their cytoplasm contains DNA, RNA, ribosomes and other substances but it does not contain endoplasmic-reticulum (E.R), mitochondria, plastids and a true nucleus that is enclosed by a nuclear membrane (Fig. 2.1)

(2) Eukaryotic Cells:

Eukaryotic cells, on the other hand, possess many internal membrane surfaces called ER in addition to the plasma membrane which constitutes less than 5% of the total membranes present in eukaryotic cells. Plant cells also have a cell wall.

Cell cytoplasm has several distinct structures, viz., nucleus, endoplasmic reticulum, Golgi complex, mitochondria, plastids (only in plants), lysosomes, ribosomes etc. (Fig. 2.2). The differences between the organisation of prokaryotic and eukaryotic cells are listed in Table 2.2.

Viruses are very small particles (prokaryotes) which do not have typical cell like organisation; their genetic material (DNA or RNA) is enclosed in a protein coat. Viruses do not have cytoplasm and, as a result, are unable to perform any physiological functions on their own. They are able to multiply only within their host cell. Viruses are considered as living organisms just at the line distinguishing the living from non-living.

Several components are observable in electron micrographs of cells; these are listed below:

1. Cell was and related structures

2. Plasma membrane

3. Cytoplasm:

The cytoplasm has several distinct structures:

(i) Endoplasmic reticulum,

(ii) Golgi complex,

(iii) Lysosomes,

(iv) Peroxisomes,

(v) Glyoxysomes

(vi) Spherosomes,

(vii) Vacuole,

(viii) Centrosome,

(ix) Ribosomes,

(x) Chloroplasts (Plant cells),

(xi) Mitochondria.

Apart from these, there is a cytoskeleton which is composed of microtubules, actin filaments and intermediate filaments.

4. Nucleus.