Page 8 - ASOP ROT Study Guide

2 . 1 Chemical Level
Living material is made up of chemicals - atoms that are organized (joined together) in various
ways and are the simplest level within the structural hierarchy. The chemical level of
organization includes all the chemicals that are essential for life. The chemical level includes the
tiniest building blocks of matter, atoms, which combine to form molecules, like water. In turn,
molecules combine to form organelles, the internal organs of a cell.
2 . 2 Cellular Level
Cells are the basic units of life. They require food and oxygen. Cells can reproduce and can exist
in isolation and range in size from 7.5 micrometers to 300 micrometers (a micrometer is a
thousandth of a millimeter). In a typical animal cell the nucleus, which contains the genetic
material and controls the cell's activities, is surrounded by cytoplasm, which consists of a
semi-fluid substance in which the activities of the cell take place. The cell contents are surrounded
by a cell membrane which controls substances moving into or out of the cell. (refer to Figure 1)

FIGURE 1. SOME OF THE DIFFERENT TYPES OF CELLS FOUND IN THE HUMAN BODY

The shape and size of a cell and the structures it contains vary according to the function of the cell.
A human being starts life as a single cell (the fertilized egg) which is 300 micrometers in diameter
and carries the genetic material needed for the development of the new human being. This cell
divides again and again until a clump of cells is formed. At this stage all the cells are similar.
Then the cells start to group into different types as different parts of the genetic material in the cells
is 'switched on' - this is called differentiation. This is followed by the development of tissues,
organs and systems until the whole human organism is complete.