Cell Theory

Cell Theory is a way to describe the biology of living things. Cell theory says that the cell is the basic unit of life. Cells by themselves are alive, but they can also be part of a larger living thing. The smallest living organisms (like bacteria but not viruses) and the biggest ones (like humans and whales) are all made of cells. Very small organisms like bacteria and amoebas are only made of one cell each, so they are called unicellular organisms ("uni" means "one"). Larger organisms are made of many cells, and they are called multicellular organisms.

Definition: The Cell Theory is one of the basic principles of Biology. Credit for the formulation of this theory is given to German scientists Theodor Schwann, Matthias Schleiden, and Rudolph Virchow.

The Cell Theory states: * All living organisms are composed of cells. They may be unicellular or multicellular. * The cell is the basic unit of life. * Cells arise from pre-existing cells.

The modern version of the Cell Theory includes the ideas that: * Energy flow occurs within cells. * Heredity information (DNA) is passed on from cell to cell. * All cells have the same basic chemical composition.

Animal Tissues

1. Epithelial: In Greek, ἐπί (epi) means "on" or "upon", and θηλή (thēlē) means "nipple". Epithelial layers are avascular, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane. Epithelia can also be organised into clusters of cells that function as exocrine and endocrine glands.
Three principal morphologies associated with Epithelial cells: a. Squamous epithelium has cells that are wider than they are tall (flat and scale-like). b. Cuboidal epithelium has cells whose height and width are approximately the same (cube shaped). c. Columnar epithelium has cells