Organization of Life

Cell Theory

The smallest unit of living things is the cell. All living things come from cells, and all parts of organisms are made of cells. Cells come from other cells.

Cell Structure

All cells have an outer membrane to regulate what can pass in and out of it. Cells also have small organs inside them that have different tasks, called organelles. Cells also have a nucleus, which contains chromosomes. The chromosomes contain information about he cell’s genetic makeup.


Metabolism is the process by which cells turn nutrients into energy. Carbohydrates are turned to energy, which is then used to either carry out the cell’s functions or to produce proteins.


Enzymes are chemical in a cell that act as catalysts for the metabolism process. A catalyst is a chemical that speeds up or assists a chemical reaction.

Specialized Cells 

In the body, specialized cells carry out the tasks of the different body systems. A bone cell’s primary task may be to provide rigid support. A nerve cell (pictured at left) may transmit information through the body using electric pulses. A blood cell may carry nutrients to other cells through the blood.


Cells and single cell organisms reproduce through a process called mitosis. In most animals, sexual reproduction occurs, which means that cells from a male and female parent combine to form an offspring.


Mitosis is the process by which individual cells reproduce. First the chromosomes in the nucleus of the cell copy themselves. Then the nucleus splits into two nuclei. Then the rest of the cell divides and becomes two cells. Cells in the body are constantly reproducing to replace ones that have died.


Meiosis is a process similar to mitosis used to create cells used in sexual reproduction. The chromosomes in meiosis, however do not copy themselves. This creates cells with half the number of chromosomes. These cells combine with cells from the other parent to form a whole cell, called a zygote, which then reproduces to create an offspring.









Cell Diagram:  Openstax; Nerve Cell:  Wellcome Images