Cell : The Unit of Life

The cell is the fundamental structural and functional unit of all living organisms.It is first discovered by Robert Hooke in 1665 in cork.

Cell theory was proposed by Schleiden and Schwann in 1839.

The cells have been classified into two categories: Prokaryotic and Eukaryotic cells.

Prokaryotic cells are the primitive cells that lack a nuclear membrane and other membrane bound organelles.

Eukaryotic cells are the cells in which genetic material is bounded by a nuclear membrane.

Singer and Nicholson proposed Fluid Mosaic Model for the structure of cell membrane. The plasma membrane is selectively permeable and facilitates transport of several molecules.

The cell wall is found in plant cells and made of cellulose, hemicellulose, pectins, and proteins. This is the outermost boundary of plant cells, which gives shape to the cell and protects the cell from mechanical damage.
The endoplasmic Reticulum (ER) is a network of interconnected membrane-bound, flattened tubules. It is of two types- Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).ER helps in the transport of substances, synthesis of proteins, lipoproteins and glycogen.

The Golgi apparatus is a membrane-bound network of parallel, flattened, interconnected sacs and small vesicles.It is an important site for the formation of glycoproteins and glycolipids.

The lysosomes are membrane-bound spherical structures found in the cytoplasm of animal cells and protozoans. They are also known as suicide bags. These contain hydrolytic enzymes which can digest carbohydrates, proteins, lipids and nucleic acids.

The vacuoles are membrane-bound structures found in the cytoplasm. The membrane of the vacuoles is called tonoplast. It contains water, sap, excretory product and other materials not useful for the cell.

Mitochondria are double membrane-bound cell organelles, which help in production of energy for the cell in the form of ATP molecules. Thus, these are called “Power house of the cell”.

The plastids are double membrane-bound cell organelles found in plants. Based on the type of pigments, plastids can be classified into chloroplasts,chromoplasts and leucoplasts.

In plant cells,chloroplasts are responsible for trapping light energy essential for photosynthesis. In the plastid,the grana are the site of light reactions and the stroma of dark reactions.

The ribosomes are tiny, membraneless organelle composed of RNA and proteins. Each ribosome is composed of two subunits. Ribosomes are involved in protein synthesis. These occur freely in the cytoplasm or are associated with ER.

Cilia are small structures that help in locomotion of the cell or the movement of fluid surrounding the cell. Centrosomes and centrioles form the basal body of cilia and flagella.

Nucleus is a double-membraned spherical structure containing genetic material in the form of chromatin fibres. Chromatin fibres are the nucleoprotein fibres made of DNA, histones, non-histone proteins and RNA.  The chromosome is made of two arms called chromatids attached to centromere. Based on the position of the centromere, the chromosomes can be classified into four typesmetacentric, sub-metacentric, acrocentric and telocentric.

 

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