An overview of cell

An overview of the cell

If we observe an onion peel or human cheek cells under a microscope, we will find that the cells in an onion peel show a distinct cell wall as its outermost boundary.  The cell membrane lies just inwards to the cell wall.  On the other hand, in a human cheek cell, the outermost delimiting structure is the cell membrane. In both, plant and animal cells, a centrally located dense membrane-bound structure called the nucleus is present. This nucleus is known to contain hereditary material i.e. DNA. 
Cells that contain a membrane-bound nucleus are known as Eukaryotic cells, whereas the cells in which the nucleus is not defined by a membrane are called Prokaryotic cells. In both prokaryotic as well as eukaryotic cells, the volume of the cell is composed of a semi-fluid matrix called the Cytoplasm. This cytoplasm is the primary site of cellular activities which are important for a living cell.

    Figure 1: Basic structure of a cell.

Along with the nucleus, a eukaryotic cell also contains cell organelles which are distinct membrane-bound structures. These organelles are responsible for carrying out different metabolic functions.  These cell organelles are absent in prokaryotic cells. Some important organelles of a cell are:
a)    Mitochondria: are responsible for generating the majority of the chemical energy required by the cell's metabolic activities.
b)    Endoplasmic Reticulum: is responsible for protein synthesis.
c)    Golgi complex: aids in the processing and packaging of proteins and lipid molecules, particularly proteins destined for cell export.
d)    Lysosomes: act as the digestive system of a cell.
e)    Vacuoles: help in the storage and disposal of various waste substances in a cell.
f)    Ribosomes: are the site for protein synthesis in the cell. They are not bound by a membrane.
g)    Centrosome: is also not bound by a membrane. It is present in animal cells and helps in cell division.

Cells can be differentiated based on size, shape, and activities. The smallest cell i.e. Mycoplasma is only 0.3µm long and is even smaller than bacteria. On the other hand, an Ostrich egg, the largest known single cell, is about 15cm to 18 cm long and wide. Cells can be of varying shapes such asdisc-shaped (RBCs), columnar (Goblet cells in the intestine), polygonal (Hydra), cuboid (kidneys), branched (Neuron), elongated (tracheid),and even irregular (Amoeba and WBCs).
 

AN OVERVIEW OF THE CELL
A typical cell possesses three major elements — outer envelope, genetic material and cytoplasm.

Outer Envelope: A cell is surrounded by an outer membrane called plasma membrane or plasmalemma. It isolates the cell interior. A distinct cell wall lies on its outer side in plant cells. Cell wall provides protection, rigidity and shape to cells.

Genetic Material: It represents hereditary material that not only controls the functioning of the cell but also contains information for forming the whole organism. Genetic material is DNA. In eukaryotes it is enclosed inside the nucleus as chromatin material. The latter appears as chromosomes during cell division. In prokaryotes, the genetic material lies freely inside the cytoplasm as coiled structure called nucleoid.

Cytoplasm: It is semifluid matrix that occupies the interior of cell between nuclear region and outer envelope. Cytoplasm is the area of major cellular or life activities which keep the cell in living state. Certain functions are associated with special cytoplasmic structures called organelles. Organelles are of three types (i) Membrane less, e.g., ribosomes, centrioles, (ii) Single Membranous, e.g., endoplasmic reticulum, Golgi complex, lysosomes, microbodies, sphaerosomes. (iii) Double Membranous, e.g., mitochondria, plastids (in plant cells).

Size and Shape

Cells differ greatly in size, shape and activities. For example, Mycoplasma, the smallest cell, are only 0.3μm in length while bacteria could be 3 to 5μm. The largest isolated single cell is the egg of an ostrich, Acetabularia, a unicellular green alga is about 10 cm in length.

Cell of alga Caulerpa may be upto one metre. Among multicellular organisms, human red blood cells are about 7.0 μm in diameter, nerve fi bres are the longest, upto 90 cm to few metres.

The upper limit or cell size or cell volume is determined by number of factors like :

(i) Metabolic Activity : Metabolically active cells are small in size while less active ones are large, e.g., sperm (active) and egg (passive).

(ii) Nucleocytoplasmic Ratio: Nucleus controls the metabolic activities of the cytoplasm. A higher nucleocytoplasmic ratio provides more efficient metabolic working. (iii) Surface

Volume Ratio : Active cells possess a higher surface : volume ratio. This occurs in small cells, elongated cells and cells with surface invaginations or ingrowths like microvilli of absorptive cells.

Cells also vary greatly in their shape.

They may be disc-like, polygonal, columnar, cuboid, thread like, or even irregular.

The shape of the cell may vary with the function they perform. e.g., RBCs are biconcave to pass through capillaries and carry O2; WBCs are irregular to perform phagocytosis, nerve cells are long to conduct impulses, sperms have tail for motility etc.

Types of cells

There are two basic types of cells i.e., prokaryotic cells and eukaryotic cells. They are differentiated based on organisation of bio membranes, variety of cytoplasmic organelle and complexity of nuclear material.