Cells can be found in many different types, all suited to best fulfil its function – this is known as cell differentiation.

When we are conceived, the first cells to form are all the same, simple clones. These are known as stem cells, but these will soon differentiate into the various cell types that make up our body. Stem cells are highly prized in science since they can form any other cell type and can therefore be used to repair tissues that the body would otherwise not be able to repair properly such as the spinal cord and in the future it is expected that stem cells can release someone from a paralysis or return sight to the blind. The current obstacle to this is ethical: the only current, reliable source we have of stem cells is from embryos and these cannot be harvested without the destruction of the embryo. However, this is outside the scope of this article.

The internal structure of a cell is highly suited to its function – this allows it to carry out its task more efficiently but may also sacrifice its independence, hence the reason we need many cell types to keep the body functioning. Single celled organisms such as amoeba have to be able to carry out every task it needs to survive, very much a jack-of-all-trades, quite different from a differentiated human cell which will perform a few functions very well, but cannot survive without other types of cell performing functions it had lost by differentiation. An organelle is a specialised part of a cell which performs a specific function for the cell. We will now have a look at some of these organelles.

The Nucleus

The nucleus is the chemical centre of a cell – it contains all the genetic material of the cell and controls the cell’s activities. Looking through a microscope, the nucleus appears as a dark blob inside the cell, but this does not hint at the sheer complexity of this organelle. It is commonly known as ‘the brain of the cell’, directing the cell through the production of proteins. Almost all cells have a nucleus: one major exception is the red blood cell, which does not have a nucleus so it can have more available area for carrying oxygen.

The Mitochondrion (Plural: Mitochondria)

Mitochondria are small capsule shaped organelles that provide the cell with energy. Sugars are broken down and converted through respiration into energy, without which the cell would die. Cells with more mitochondria are therefore cells that require more energy – muscle and sperm cells will have a lot of mitochondria to suit their function.

Endoplasmic Reticulum

This oddly named organelle is responsible for the structuring and internal compartmentalising of the cell. Cell products are stored within the sheet-like membranes of the endoplasmic reticulum for later use. The endoplasmic reticulum is also an important site for the production of proteins and carbohydrates. Again, cells which produce a lot of products will have larger, better developed endoplasmic reticulum to best suit their function.

Ribosomes

Ribosomes are small protein-producing molecules. They are usually found on the endoplasmic reticulum and are extremely important for protein synthesis. They occur in large numbers throughout the cell.

The Golgi Apparatus

An extension of the endoplasmic reticulum, the Golgi apparatus appears to be composed of many membrane sheets or flattened sacs. The role of the Golgi apparatus is to modify the products of the cells before release. Proteins and carbohydrates can be combined to make glycoproteins, enzymes produced, lysosomes and lipids (fats) modified. When the Golgi apparatus releases one of its completed products, it is sealed in a membrane, known as a vesicle. When reaching the main cell membrane, the barrier between the cell and its environment, the vesicle will fuse with it and disgorge its products out of the cell.

Lysosomes

Lysosomes appear as small bubbles in the cytoplasm (a jelly like liquid inside the cell), and its role it to store the various products and wastes of the cell. It is important that the contents of the lysosome are not just released into the cell because the product may be harmful to the cell. For example the cells that release acid into the stomach would destroy themselves if the acid was not sealed in a lysosome while inside the cell. When a cell dies or if it needs to destroy itself (perhaps due to a viral infection), specialised lysosomes break down. These lysosomes will contain powerful enzymes which effectively dissolve the cell.

Not all cells will have the above organelles, and they will occur with differing frequency within cells – mostly related to the cell function