Cell Biology

Cell:
The cell is the basic structural, functional and biological unit of all known living organisms. Cells are the smallest unit of life that is classified as a living thing, and are often called the "building blocks of life".
Discoverer:
The cell was discovered by Robert Hooke in 1665. The cell theory, first developed in 1839 by Matthias Jakob Schleiden andTheodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[4] Cells emerged on Earth at least 3.5 billion years ago.
Cell Theory:
In biology, cell theory is a scientific theory that describes the properties of cells, the basic unit of structure in every living thing. The initial development of the theory, during the mid-17th century, was made possible by advances in microscopy; the study of cells is called cell biology. Cell theory is one of the foundations of biology.
The three parts to the cell theory are as described below:
All living organisms are composed of one or more cells.
The cell is the basic unit of structure, function, and organization in all organisms.
All cells come from preexisting, living cells.
Cell Shape:
There are over 200 different types of cells within the human body. These cells all vary in size, shape and diameter. Most cells range between ten and fifteen micrometers in diameter, however, some cells, such as the human egg cell, are much larger than this, with a diameter of roughly 100 micrometers. The human eggs cell is just barely visible to the naked eye. Some of the longest cells include nerve cells, which can be as long as a meter, but are so thin; they are invisible to the naked eye. Cells, although they range in size and shape, cannot become too large, or they may become unable to support their own functions, or could burst.
Cells are found in many different shapes and sizes. Some of the most common cell shapes include; squamous, cuboidal, columnar, polygonal, spheroid, discoid, fusiform and fibrous.
Squamous
Squamous cells are thin and flat, with a slight bulge where the nucleus lies. These cells are commonly compared to the appearance of a fried egg. These cells are most abundant in the skin and the lining of the esophagus.
Polygonal
Polygonal cells, much like their name implies, are polygonal in shape, with five or more sides. Sometimes these sides are elongated in such a manner that they form a stellate, or star-like shape.
Cuboidal
Cuboidal cells are square-like in shape and are typically as tall as they are wide. This type of cell is commonly found in the liver.
Columnar
Columnar cells are similar to cuboidal cells, however, they are taller than they are wide. This type of cell is commonly found in the lining of the intestines.
Spheroid
Spheroid cells, sometimes referred to as ovoid cells, range from circular to ovular. Examples of spheroid and ovoid cells include fat cells and human egg cells.
Discoid
Discoid cells are shaped much like a disc, or a Frisbee. An example of a discoid cell includes red blood cells.
Fusiform
Fusiform cells are often thought of as spindle-shaped; thick in the center and tapered at the ends. These cells make up the smooth muscles.

Cell Category
There are two main groups of cells, prokaryotic and eukaryotic cells. They differ not only in their appearance but also in their structure, reproduction, and metabolism. However, all of the cells belong to one of the five life kingdoms. The greatest difference lies between cells of different kingdoms. The following diagram shows the five kingdoms: monera, protista, plantae, fungi, and animalia
Prokaryotic cells
The prokaryotes are a group of organisms whose cells lack a membrane-bound nucleus (karyon). The organisms whose cells do have a nucleus are called eukaryotes. Most prokaryotes are unicellular organisms, although a few such as myxobacteria have multicellularstages in their life cycles or create large colonies like cyanobacteria
Example: E. coli bacterium, Salmonella bacterium, Bacillus bacterium
Eukaryotic cells
A eukaryote is an organism whose cells contain a nucleus and other organelles enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried.
Example: Animals, plants, fungi, and protists are eukaryotes.
Difference between Prokaryotic cells and Eukaryotic cells | Eukaryotic Cell | Prokaryotic Cell | Nucleus: | Present | Nuclear body / No Nucleus | Number of chromosomes: | More than one | One--but not true chromosome: Plasmids | Cell Type: | Usually multicellular | Usually unicellular (some cyanobacteria may be multicellular) | True Membrane bound Nucleus: | Present | Absent | Example: | Animals and Plants | Bacteria and Archaea | Genetic Recombination: | Meiosis and fusion of gametes | Partial, undirectional transfers DNA | Lysosomes and peroxisomes: | Present | Absent | Microtubules: | Present | Absent or rare | Endoplasmic reticulum: | Present | Absent | Mitochondria: | Present | Absent | Cytoskeleton: | Present | May be absent | DNA wrapping on proteins. | Eukaryotes wrap their DNA around proteins called histones. | Multiple proteins act together to fold and condense prokaryotic DNA. Folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein. | Ribosome: | Larger (80S) | Smaller (70S) | Vesicles: | Present | Present | Golgi apparatus: | Present | Absent | Chloroplasts: | Present (in plants) | Absent; chlorophyll scattered in the cytoplasm | Flagella: | Microscopic in size; membranebound; usually arranged as nine doublets surrounding two singlets | Submicroscopic in size, composed of only one fiber | Permeability of Nuclear Membrane: | Selective | not present | Plasma membrane with steroid: | Yes | Usually no | Cell wall: | Only in plant cells and fungi (chemically simpler) | Usually chemically complexed | Vacuoles: | Present | Present | Cell size: | 10-100um | 1-10um |