What is a Cell?

A cell is the most basic form of life, we humans are made up of billions of cells each working together to ultimately keep us alive (a process known as Homeostasis, but that is a topic for another day!). But cells in themselves are quite complex and vary dramatically, just like we humans do.

There is more than just one variety of cell, a bacterial cell varies greatly from a human cell and a human cell is different again to a plant cell. Whilst all cells in the human body contain all the same Deoxyribonucleic Acid (DNA, the genetic coding material, and again a topic for another day!), each cell type, so lets say a muscle cell and a hepatocyte (liver cell), will express it differently. But, there are two main cell types that you should know about and they are:

Prokaryotic:

These cells are generally associated with things such as bacteria, prions and archaea. They are much, much smaller than a Eukaryotic cell and are very distinctive. They have no defined nucleus meaning they have their DNA just floating around inside the cellular space, known as

cytosol.  They generally have a tail(s), known as a flagella(plural is flagellum) , to move around. They do contain ribosomes, which help synthesize proteins, but have no other definable organelles. They are contained by a first layer, known as a plasma membrane (made of  special lipids and proteins)  and then by another protective layer called a cell wall (made of peptidoglycans in bacteria). They reproduce through a process known as binary fission and have the ability to communicate to each other though little external structured called pilli(s).

(Diagram sourced from:http://www.biologycorner.com/APbiology/cellular/notes_cells1.html, 22/04/2012)

Eukaryotic:

Eukaryotic cells are much more complex than prokaryotic cells. They have a defined nucleus, within it a nucleolus. They also contain many membrane bound organelles, such as an endoplasmic reticulum (both smooth and rough), lysosomes (used in a process known as apoptosis as well as phagocytosis), golgi apparatus (a cellular packaging and sorting organelle) and a few other ones which I may cover in a little bit of detail down the track (they are not huge parts of cellular function).

 (Diagram sourced from: http://mrskingsbioweb.com/Biology.html, 22/04/2012)

The diagram is of a plant and animal cell, of which there are a few big differences.

Firstly, and most notably, the plant cell has a large vacuole taking up somewhere between 1/3 and 1/2 of the internal cellular space. This is how plants store water and nutrients for survival, animals do not need this function and hence do not have such a prominent vacuole.

Secondly, animal cells have mitochondria to produce energy in the form of Adenosine TriPhosphate (ATP). They use a process known as The Citric Acid Cycle (TCA cycle) to do this (this will be covered later on, it is a bit complex). This is compared to plant cells, which use sunlight and chlorophyll to produce ATP in the Chloroplast.

Lastly, an animal cell has a cell membrane enclosing the cell from the external environment. This is contrasted to a plant cell which has a cell wall. Why you may ask? Well, a plant has to be able to store water and nutrients for its survival. They store them in the vacuole, and when it is full it causes the cell to expand. A cell membrane (also known as a plasma membrane) has no real flexibility and if the cell gets too big it will explode (yes, it literally explodes, especially with red blood cells). However, in a plant the cell wall provides some rigidity and makes it virtually impossible for the cell to lyse (burst).

This is just a very, very basic run down of a very complex topic. I will go into more detail on all the organelles over time as understanding their function, and more importantly how they work together, is very interesting as well as ever evolving.