Obviously, 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.

" ... a typical cell is like a miniature body containing tiny organs, called organelles. One organelle is the command center; others provide the cell with energy, while still others manufacture proteins and additional molecules that the cell needs to survive and to communicate with the world around it. The entire cell is enclosed in a fine 'skin', its membrane. This membrane not only keeps the cell intact, it also provides channels that open and close to allow selected molecules into and out of the cell." [Source: National Institute of General Medical Sciences]

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.

	Eubacteria.

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.

	Major anatomical features of Euglena.

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, Erysiphe.

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.

	Animal cell.

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.

	Plant cell.

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.