The Human Body

Cell is the basic unit of life.These are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste,and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consists of a single cell. Plants, animals and fungi are multicellular, that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and effieciency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpuate life.
Cells vary considerably in size. The smallest cell is a type of known as mycoplasma and among the largest are the nerve cells that runs down the giraffe's neck. Human cells also display a variety of sizes from small RBC to liver cells that may be ten times larger. About 10,000 average-sized human cells can fit on the head of a pin. The shape of a cell also varies a lot. In plant, the cells typically resembles boxes or cubes while in animals the cells may be flat as in skin, or elongated as in muscles and different other shapes in blood cells and in matrix of connective tissue.
Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure and also participate in cell functions. For eg: a tightly organized arrangement of lipids, proteins, and protein-sugar compounds forms the plasma membrane, or outer boundry, of certain cells. The organelles, membrane bound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. The deoxyribonucleic acid contains the hereditary information for cells, and another nucleic acid, ribonucleic acid works with DNA to build the thousands of protiens the cell needs for its functoning.
As the body contains organs in the same way a cell contains organelles. Some common cell organelles of a cell are mitochondria, chloroplast(only in plant cell), centrosome( only in animal cell), endoplasmic reticulum, golgibodies etc. Though nucleus is not a cell organelle, it is the most important part of a cell. It is the brain of the cell that contains genetic information stored in a sequence inside a DNA. A DNA expresses itself in the form of protein. A particular sequence of nucleic acid in the DNA codes for a particular protein. If this sequence in DNA is altered than we call it point mutation. A single misplacement of a nucleic acid may cause a huge change in the cell. Sometimes mutation may be lethal also.