ACME SMART PUBLICATION
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).
