Basics

At a microscopic level, every living thing is made up of cells. A bacterium only has one cell, but you have about 100 trillion.

Your 100 trillion cells may be one of about 200 different kinds, in different shapes and sizes. Each kind has its own role to play. In order to do this, the cells group together into different types of tissue, which in turn form organs and structures, like the heart. These organs work within systems, like the cardiovascular system, which allow the body to function. Our muscles are made of muscle cells, and our bones of bone cells. There are even very specialized types of cells for our fingernails and each of the delicate parts of our eyes.

Chromosomes, which are made up of DNA molecules, are found within the nucleus of the cell. A gene is one section of a DNA molecule that contains the code for the production of a protein.

Proteins, produced by our genes, are essential to life. Our bodies are constructed of them and depend on them. Protein absorbed from the food that we eat provides our bodies with the necessary building material and energy that is essential to its proper functioning.

Mutations in genetic instructions can alter the production of proteins, which can lead to sickness or even death.

Inside the cell

We've known about the existence of cells and have studied them for hundreds of years. As scientists developed ever more powerful and sophisticated magnifying devices, we have been able to look further into the features and behaviours of cells. Still, we do not completely understand all of their possible functions. It seems the more we study them, the more we discover about their organization and complexity.

The cell is the basic unit of life. It is the smallest entity that can exist as an independent living system. The proper functioning of your body depends on what happens within your many different types of cells.

bviously, there are major differences between types of cells. Muscle cells, which can contract, have to be quite different from bone cells. Human nerve cells have long, thin fibres that may extend from the spinal cord to the toes, while blood cells have no projecting fibres at all. Plant cells have a unique ability to use light as a source of energy.

Biologists make a distinction between cells based on how they are organized. Eukaryotic cells are more complex and highly structured, with membrane-bound 'organelles'. They include cells of protozoa, higher plants and animals. Prokaryotic cells are less structured, and do not have membrane-bound organelles. They include simple cells like bacteria.

Types of Cells

We can classify all living organisms according to five kingdoms:

Animals
Plants
Fungi
Protoctista
Bacteria

Organisms in the first four kingdoms are eukaryotes, while bacteria are considered to be prokaryotes.

Bacteria are the most numerous organisms on Earth. Billions of them may be found in a handful of mud, or in your mouth! A single bacterium is a self-contained, living cell. Bacteria have no nuclear membrane, and so their genetic material is contained in a single circular molecule of DNA in the 'nucleoid' area.

Protoctista - Protoctista is the catch-all category for organisms that do not quite fit into the other kingdoms. They are one-celled or multi-celled eukaryotes, with a nuclear membrane. Some of them, like amoeba, look like typical animals cells, but they are single-celled organisms. Others, like Euglena, have features of both animal and plant cells.

Fungi must absorb their food molecules directly from other living or non-living organic sources. They usually have branching multicellular threads called hyphae that develop from spores.

Animal cells have a nucleus, mitochondria, endoplasmic reticulum, ribosomes, golgi bodies and lysosomes. Unlike plant cells, they do not have chloroplasts, a cell wall nor a vacuole. Animal cells are surrounded only by a membrane, which allows the cell to be flexible and yielding.

Plant cells have a nucleus, mitochondria, endoplasmic reticulum, ribosomes, golgi bodies and lysosomes, like animal cells. However, a plant cell is surrounded by a cell wall made of cellulose, which makes the cell more rigid.

In addition, plant cells contain vacuoles that hold cell sap among other things, and chloroplasts, which contain the green pigment chlorophyll. It is inside the chloroplasts that the process of photosynthesis takes place and the plant produces its own food.

Source: http://www.nature.ca/genome/03/c/03c_e.cfm