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Written by Tim Sheppard MBBS BSc. Created 6/1/10; last updated 14/8/12

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What is an organelle?

The organelles are those parts of the cell which make things happen. They are the substance of the cell, the machines of the cell, the things in which all of the exciting processes happen. They are parts of the cell with a specific function.

Organelles are usually surrounded by membranes to keep them in one specific area, and they exist to make sure that a particular function can be carried out. They include things like the nucleus - the brain of the cell which sends out all the orders - or mitochondria, the 'power houses' of the cell, which produce energy in the form of ATP.

A cell is therefore filled with organelles, and the organelles are surrounded by cytosol. The cytosol and the organelles together (except for the nucleus) make up the cytoplasm.

One of the most important features of organelles is that they are wrapped up in membranes to keep them separate from each other. Because each organelle has a specific purpose, it is kept from the others. This prevents it from mixing up two processes, and ensures the cell can function properly.

This article is going to describe a few of the organelles you can find in human cells, but its important to remember that there are loads of different organelles that you could find if you were to pick a cell at random. There are lots of processes which a bacteria might be carrying out which a human cell doesn't need to. Or a plant, for instance, needs to carry out photosynthesis, and has specialised organelles for that which a human simply doesn't have.

What is the cytoplasm?

The cytoplasm is basically everything in the cell except for the nucleus - a sea of little factories carrying out the jobs they were created to do. The cytoplasm is therefore made up of organelles and something called the cytosol. The cytosol is a liquid that surrounds all the organelles, made up predominantly of water, but also of lots of ions and big molecules.

The concentrations of ions in the cytosol is incredibly important, because it affects what happens at the membrane of the cell - including the resting membrane potential. The concentration of potassium inside a cell is usually about 120mM, whereas outside the cell it is more like 5mM (i.e. the the concentration of potassium inside the cell is about 24 times the concentration of potassium outside).

As well as potassium, sodium is an important ion in the cytosol. Its concentration is much higher outside the cell (about 145mM) than inside (about 10mM). This means that when a sodium channel opens, sodium will flow into the cell - which is how an action potential happens.

Although most of the cytosol is water, with lots of ions dissolved in it, it also has bigger molecules that bulk it out and can have an important effect in organising what goes on in the cytosol.

What is the endoplasmic reticulum?

The endoplasmic reticulum is a series of tubes that runs through the cytoplasm of a cell, usually starting around the nucleus and working its way out. It acts like a factory for the cell, producing lots of chemicals and getting them transported to the relevant part of the cell. In fact there are two main types of endoplasmic reticulum making different things, named on the basis of what they look like through a microscope.

The rough endoplasmic reticulum (or RER) is covered in ribosomes, which are involved in making proteins. Because it is dotted with these structures, it looks like it has a rough surface through a microscope. The smooth endoplasmic reticulum (or SER) lacks these ribosomes, so it looks smooth through a microscope. The endoplasmic reticulum can swap between the two types depending on how much one type is needed.

Another organelle with a similar structure (and name!) is the sarcoplasmic reticulum which is found in muscle cells and used for storing calcium.

The main function of the rough endoplasmic reticulum is making proteins. Because it is basically connected to the outer part of the nucleus, mRNA from the nucleus can come and be translated. The ribosomes on the surface only form when translation is ready to commence, and don't stay as a permanent part of the membrane. The proteins are happily produced, and then delivered to where they need to go (e.g. the Golgi apparatus) by the delivery vans: vesicles. The RER is also able to make proteins that are inserted straight into the membrane - e.g. for ion channels that need to live in the membrane.

The main function of the smooth endoplasmic reticulum is making fats and steroids, as well as dealing with drugs and helping with carbohydrate metabolism. Again, it is made up of tubes and vesicles, which can carry the products to where they need to be.

What is the Golgi apparatus?

The Golgi apparatus is a bit like the sorting office of the cell. It's where all sorts of packages arrive, and where they're sorted - changed, if they need to be - and then shipped off to where they need to go.

Most of the packages that the Golgi apparatus deals with come from the endoplasmic reticulum - the factory of the cell. Packages molecules - especially proteins, but also lipids - are delivered to the Golgi apparatus, which fuse with the Golgi membrane and deposit their load into it. The Golgi then makes certain changes, such as adding carbohydrates to the end of proteins, and then sending them out.

This isn't all that it does, however. It's also involved in the manufacture of certain things - like lysosomes, or some of the substances that make up the extracellular matrix (the substance that lives outside cells). In fact, we don't understand everything that the Golgi apparatus does - but we do know that it is key in the processes detailed above.

What is a vesicle?

The vesicle is like the delivery van, running goods from one end of the cell to the other. They are made up of a phospholipid bilayer membrane around the outside (or sometimes more than one) and some kind of contents on the inside. This bubble of goods floats through the cell to wherever it needs to go, and the membrane can join with another membrane in order to deposit its contents - e.g. inside the endoplasmic reticulum, or to throw its contents out of the cell in exocytosis.

Because vesicles have their contents separated from the rest of the cytoplasm by a bilayer, they are a great place for chemical reactions to take place. In fact, certain specialised kinds of vesicles exist for this very purpose.

Lysosomes are little bubbles of enzymes that are made by the Golgi apparatus to break down big molecules like proteins and carbohydrates.

Peroxisomes are another type of vesicle which is used to get rid of waste hydrogen peroxide which is generated in certain reactions that go on in the cell.

Finally, the autophagosome is a special vesicle which breaks down bits of the cytoplasm when it's finished with.

So, vesicles may be incredibly simple, but they are also extremely important for basic function of the cell.

What is a ribosome?

Ribosomes are tiny little machines that either float through the cytoplasm or sit on the surface of the rough endoplasmic reticulum (ER), producing proteins - in fact, it is these little machines that give the rough ER its rough appearance.

Each ribosome is made up of two subunits - one that is called the 40s subunit, and one is called the 60s subunit. Each of these is made up of particular proteins and ribosomal RNA in a special way that allows it to do its job.

The ribosome exists to perform a job called translation, which means reading the code from RNA and turning it into a protein.

What other organelles are there?

There are countless organelles which aren't listed above - many of which do not exist at all in human cells. Human cells don't have the need to perform photosynthesis in order to obtain energy from light, so they don't have chloroplasts or chlorosomes. They have a nucleus so they don't need a nucleoid to contain their DNA.

The nucleus, as mentioned, is needed to store genetic material, and because DNA codes for all the proteins, it is a bit like the brain of the cell. It is described in more detail elsewhere. Human cells also contain mitochondria in order to carry out oxidative phosphorylation - to 'make' energy; again, that is described in more detail elsewhere.

Human cells can also contain a number of other organelles which are not listed above. These include myofibrils, which are used in muscles for contraction, and the centrosome. The centrosome is made up of two centrioles (which are barrel-like structures) surrounded by dense protein; it plays an important role in cell division, and although it is considered an organelle, it is not surrounded by the normal phospholipid membrane.

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