Principles of Biochemistry

1.1.3 Structure and function of Cell

The cell is the basic unit of life. All organisms are made up of cells (or in some cases, a single cell) and depend on cells to function normally. There are two major classifications of cells: the prokaryotes and eukaryotes. The prokaryotes (Greek “pro” -primitive; “karyon” – nucleus) lack a nucleus. e.g. bacteria. Eukaryotic cells (Greek “eu” -good; “karyon” - nucleus) have a membrane enclosed nucleus encapsulating their DNA. e. g. Cells of animals, plants and fungi.

a. Prokaryotes: Prokaryotes are almost always single-celled, except when they exist in colonies. Reproduce by means of binary fission. 

b. Eukaryotes: Eukaryotic cells range in size from 1 to 100 micrometers in diameter and have volume a thousand to a million times that of a typical prokaryotic cell. Cells are covered by a cell membrane and come in many different shapes. The contents of a cell are called the protoplasm.

i. Protoplasm: It is differentiated in to plasma membrane (=plasmalemma or cell membrane), cytoplasm, nucleus and vacuoles. Cytoplasm is distinguishable in to cytoplasmic matrix and endoplasmic reticulm.  Cytoplasmic matrix is also called hyaloplasm. It is a polyphasic colloidal system which exists in two states, sol and gel. The gel form usually occurs near the plasma membrane. This region is sometimes called ectoplast in contrast to sol region known as endoplast. Ectoplast is firmer. It is quite conspicuous on the free sides of the cells. In protozoans, ectoplast is prominent on all sides. Cytoplasmic matrix is generally in perpectual motion. The phenomenon is called cyclosis, cytoplasmic or protoplasmic streaming. In the cytoplasmic matrix are embedded a large number of cell organells or organised protoplasmic subunits having specific functions.

ii. Organelle:  An organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own membrane. Organelles are identified by microscopy, and can also be purified by cell fractionation.

Eukaryotic cells contain several types of organelles, while prokaryotic cells contain a few organelles (ribosomes) and none that are bound by a membrane. There are also differences between the kinds of organelles found within different eukaryotic cell types. Plant cells for example, contain structures such as a cell wall and chloroplasts that are not found in animal cells.

The parts of a plant cell and an animal cell, important organelles and their functions are described herewith.