Chemical composition of plasma membrane

Lipids are the major components of the cell membrane as they form the continuous structural frame of the cell membrane. Lipids such as phospholipids, glycolipids, and steroids are found in membranes.

The lipid molecule possess both polar hydrophilic (water loving) and non-polar hydrophobic (water repelling) ends. The hydrophilic region is in the form of a head, while the hydrophobic part contains two tails of fatty acid. Hydrophobic tail is present towards the centre of the membrane. This structures results is the formation of lipid bilayer known as unit membrane/biological membrane/cell membrane.

A plasmid is usually a circular (sometime linear), double strandad DNA, that can replicate itself autonomously. These are found in the cytoplasm of the bacterial cell, plasmids normally remain separated from the chromosome, but sometimes may integrate into it temporarily and replicate with it incidentally.

Role of Plasmids in Bacteria

Plasmids are the extrachromosomal circular, independently replicating unit besides nucleoid in the bacterial cell. Plasmids are used to transfer the information from one cell to another, i.e., transfer of important genes (e.g., they may confer resistance of particular, antibiotics to their bacterial cells), enabl to metabolise a nutrient, which normally a bacteria is unable to. It also helps in conjugation of bacteria. These days, plasmids are used in a variety of recombination experiments, as cloning vectors. A such example of plasmid vector is given below in figure.

Histones are the basic group of globular proteins that have a high content of basic amino acis, i.e., arginine and lysine. Histones forms the part of the chromosomal material in eukaryotic cells.

There are five types of histone proteins $\mathrm{H}_1, \mathrm{H}_2 \mathrm{~A}, \mathrm{H}_2 \mathrm{~B}, \mathrm{H}_3$ and $\mathrm{H}_4$. Four of them $\left(\mathrm{H}_2 \mathrm{~A}, \mathrm{H}_2 \mathrm{~B}, \mathrm{H}_3\right.$ and $\left.\mathrm{H}_4\right)$ occur in pairs to produce histone octamer called Nubody or core of nucleosome.

Functions Histones bear positively charged ends, that attaract negetively charged strands of DNA. Histones provides a medium around which DNA winds, and they play a vital role in gene regulation, as they act as gene repressor. They form the chromatin material and provide stability as are not coagulated by heat.

All organisms are made up of cells, which further organises themselves into tissues, organs and organ systems. Thus, forming the building blocks of organisms cells also the property of totipotency, capable of developing into a new organism. Besides, forming the structural unit they perform different specialised functions in the same way as each organ or system carries out in an organism. Thus, exhibiting division of labour i.e., cell organelles are specific in their functions.

	Structure	Function	Diagram
1.	Cell membrane all cell possess a phospholipid based cell membrane.	The cell membrane is selectively permeable, i.e., only selected material can pass through it.
2.	Cytoplasm It is a watery solution containing controlled concentration of organic and inorganic compounds.	It functions as a site for metabolism and provide energy and material for growth and reproduction.
3.	Nucleus It essentially consists of DNA, the nuclear matrix or the nucleoplasm containing nucleolus and chromatin.	It serves to store and transmit information to direct the synthetic activities of the entire cell. In also transfers the genetic information required for growth and reproduction.
4.	Golgi apparatus (Golgi complex) They constitute of many flat disc shaped sacs or cistermae of $0.5 \propto \mathrm{~m}-1.0 \propto \mathrm{~m}$ diameter. These stacks are arranged parallel to each other.	These are mainly involved in packaging the materials to be delivered either to intracellular targets or secreted outside the cells.
5.	The Endoplasmic Reticulum (ER) ER is often an extensive 3- dimensional network of intra cellular membranes formed by three elements- cisternae, tubules and vesicles	The ER bearing ribosomes on it surface is called rough endoplasmic reticulum (CRER). These are involved in protein synthesis and secretion. The ER without ribosomes are called smooth endoplasmic reticulum involved in synthesis of lipids like steroidal hormones.
6.	Mitochondria Mitochondria is a double membrane bound structure with outer and inner membrane dividing its lumen into two compartments, i.e., outer membrane forming of organelle and inner membrane forming number of infoldings called the cristae.	Mitochondria are the sites for aerobic respiration. It is known as power house of the cell since producing cellular energy in the form of ATP.
7.	Lysosomes These are membrane bound vesicular structures, formed by the process of packaging in the Golgi apparatus.	These are rich in many types of hydrolytic enzymes (hydrolases lipases, proteases, carbohydrates).
8.	Vacuoles The vacuole is the membrane bound space found in the cytoplasm, it contain water, sap, excretory product and other material not useful for the cell. The vacuole is bound by single membrane called tonoplast.	In plants the tonoplast facililates the transport of a number of ions and other materials against concentration gradients into the vacuole.

All these factors, i.e., structural and functional attributes it to be called a living cell.

(a) Rudolf Virchow (1855) first explained that the cells divide and new cells are formed from the pre-existing cells (omnis cellula e cellula). Virchow was the first to demonstrate that the cell theory applies to diseased tissue as well as to healthy tissue. He also engaged his research in the fields of archeology and anthropology.

(b) Schleiden and Schwann observed the cells and cell membranes for the first time. They proposed the cell theory which explains that the animal and plant cells are composed of cells and product of cells.

Theodore Schwann (1839) studied different types of animal cells and reported that cells had a thin outer layer which is called as plasma membrane. Schleiden also concluded on plant studies, i.e., the presence of cell wall is a unique character of plant cells. Thus, they proposed that plants and animals are composed of cells and their products.