The cell is commonly known as the basic building block of life and is the smallest unit of life that is classified as a living thing. An organism is a cell or collection of cells that is capable of living processes, these are:
- Responding to stimuli
- Growth and development
- Taking in and gaining energy from food
Organisms can be unicellular (are made up of one cell only) or multicellular (made up of many cells). There are 2 types of cell which are called prokaryote and eukaryote. Prokaryotic cells are smaller and lack a cell nucleus whereas eukaryotic cells have one. I'll be focusing on eukaryotic cells from this point onwards because this is what human (animal) cells are called. Eukaryotic cells are typically between 1 and 100 micrometres in length.
The human body contains an estimated 10-100 trillion cells, the actual number is very hard to estimate. It is also thought that only around 10% of the cells found in and on the human body are human cells. The rest are non-human microbial cells.
Within cells there are sub-cellular components known as organelles. In the same way that the human body has specialised organs to carry out specific functions, the cell can be considered to be a miniature body with all of its personal organs. These are the most common organelles in eukaryotic cells:
- Cytoskeleton; this structure maintains the shape of the cell and also anchors the organelles in place. Much like the skeleton of our bodies.
- Cell nucleus; this is the cell's information centre and the most readily recognisable organelle in eukaryotic cells.
- Mitochondria; these organelles carry out respiration and are like power plants for the cell, they generate energy by a process called oxidative phosphorylation to produce ATP.
- Endoplasmic reticulum; this is an organelle that serves to transport molecules which need to be modified for some specific purpose or sent to other parts of the cell.
- Golgi apparatus; this processes and packages macromolecules (macro meaning large) such as proteins which are produced within the cell.
- Ribosomes; these are structures composed of two subunits made from RNA and protein molecules which synthesise proteins using information from RNA molecules made in the nucleus.
- Centrosomes; these produce microtubules (which form the cytoskeleton) and also manipulate the cytoskeleton itself. This strucutre is necessary for cell division.
- Vacuoles; these are essentially enclosed solutions contained within a membrane. They serve to store food and waste.
Cells are also enveloped by a semi-permeable membrane. This acts as a kind of gate which allows some but not all particles into the cell.