cell

Summarizing: The major levels of organization in the body, from the simplest to the most complex are: atoms, molecules, organelles, cells, tissues, organs, organ systems, and the human organism.

Cells are the most basic unit of life at the smallest level of organization. Cells can be prokaryotic (without nucleus) or eukaroyotic (with nucleus). The four categories of tissues are connective, muscles, epithelial, and nervous tissues. Organs are made of different types of tissues and perform complex functions.

Cells are considered the basic units of life in part because they come in discrete and easily recognizable packages. That’s because all cells are surrounded by a structure called the cell membrane — which, much like the walls of a house, serves as a clear boundary between the cell’s internal and external environments.

A water molecule consists of three atoms; an oxygen atom and two hydrogen atoms, which are bond together like little magnets. The atoms consist of matter that has a nucleus in the centre. The difference between atoms is expressed by atomic numbers.

There are three fundamental units we’ll deal with in this course: length, time, and mass. We’ll see shortly how other quantities of physical interest can be made out of these three units.

The smallest thing that we can see with a ‘light’ microscope is about 500 nanometers. A nanometer is one-billionth (that’s 1,000,000,000th) of a meter. So the smallest thing that you can see with a light microscope is about 200 times smaller than the width of a hair. Bacteria are about 1000 nanometers in size.

The electron is, as far as we know, one of the fundamental, indivisible building blocks of the universe. It was the first Standard Model particle ever discovered.