Cell structure

Introduction

All living things are made of cells. Even though cells are the main structural and functional units of living organisms they aret not all the same. They vary enormously in their size, shape and internal structure. For example, bacteria cells can be seen only through the microscope. A hen's egg is actually one cell - the largest cell there is.

In this chapter we are going to discuss the internal structure of a biological cell. We will talk about different types of cells and their main differences. See Image 1

Cell - up close and personal

All biological cells are similar to each other but they are never identical - even within the same tissue or organism. Plants and animals have different cellular structures and so do different tissues and organs within the same organism. See video 'Varying cells'

In this chapter we are going to discuss just the general structure of a biological cell. We will talk about the differences between plant and animal cells in the next chapter.

There are different components within the cellular cytoplasm called organelles. They all do different jobs and vary depending on the cell's function.

All cells have:

• DNA - the cell's genetic information carrier. In some cells DNA is in a separate cell structure called a nucleus and in others, it just "floats" in the cell's cytoplasm in the form of a nucleoid.
• Membrane - the outer thin protective layer
• Cytoplasm - jelly-like matter inside the membrane.

The cell nucleus is a very important organelle. It controls all the processes within the cell. Some cells have a nucleus and some do not. All cells are divided into two large groups: cells with a nucleus, called eukaryotic cells, or simply, eukaryotes; and cells without a nucleus, called prokaryotic cells, or prokaryotes.

Some cells have multiple nuclei. They are actually healthy normal cells called coenocytes. Cells like this are found in some fungi, algae and some unicellular organisms from the protozoa group.

The table below will help you to better understand the differences between prokaryotes and eukaryotes.

The table above gives only general and very simplified picture of the structure of cells. All cells' organelles are very complex and vary enormously between eukaryotes and prokaryotes, plants and animals, and different body tissues and organs.

Prokaryotes - first living forms

The first living cells on Earth were prokaryotic cells. Prokaryotes are much simpler in structure than eukaryotes. Prokaryotes were well adapted to the harsh environment of the young planet Earth. They lived in water and were anaerobic types of organisms, meaning that oxygen was poisonous to them. Because at that time there wasn't any oxygen, they thrived. Some of these early organisms started to use sunlight as a source of energy in the process called photosynthesis. Oxygen is one of the "waste" products of photosynthesis. The photosynthesizingprokaryotes released oxygen which eventually turned the Earth into a place suitable for other living organisms. Blue-green cyano-bacteria were some of the first prokaryotic cells. This type of cell can still be found today.

Prokaryotes reproduce by cell division which is also called asexual reproduction. See Image 2

Eukaryotes

Eukaryotic cells are also microscopic in size but they are much bigger than prokaryotic ones. Eukaryotes are much more complex in structure and have more cellular organelles. Scientists believe that some organelles are the evolutionary result of mutual coexistence between prokaryotic and eukaryotic cells. In other words, millions of years ago some organelles used to be prokaryotic cells living on their own. Organelles like mitochondria and chloroplasts still have their own DNA, and divide the same way they did when they were free living organisms. Eukaryotic cells protect them from oxygen and provide a stable environment with all the necessary nutrients.

Eukaryotes can reproduce by asexual or sexual reproduction. Sexual reproduction means the new organism is produced by the mating of two cells. See Image 3