Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
There are two main types of cell in the world – the simpler cells such as bacteria are prokaryotic cells, and cells like ours are eukaryotic cells. The difference is that a eukaryotic cell has a nucleus, a membrane-bound section which holds the cell's main DNA.
The membranes (the cell membrane which surrounds the whole thing, and the nuclear membrane which divides the nucleus from the rest of the cell) are made of lipids - fatty molecules.
Now, an important thing to remember whenever you see a biological image is that it is always an oversimplification. The image above is obviously really, really simple (I'll fill in some more detail in a second!) - but however much detail anyone fills in, the description will never be perfect. That's the beauty of science, or the beauty of the world: there's always more to know, about everything.
So here's a bit more detail, one important thing that isn't mentioned in the diagram above. The cell membrane has a massive amount of proteins embedded in it - molecules which stick out of the membrane to serve one of many purposes. Some proteins serve as channels to transport things from one side of the membrane to the other, some serve as sensors which detect chemical signals in the environment and trigger processes inside the cell as a result... and there are many other things that can be done.
The cell itself has many, many contents apart from just the nucleus. What I've called the "cytoplasm" is densely packed with specialised areas for certain jobs, with structural proteins, and with chemical messengers passing from one part of the cell to another.
A virus is not a cell. An individual virus, floating around (in the blood, for example), is called a "virion" or "virus particle". Let's have a look at a HIV virus particle:
You can see that the cell and the virus have some things in common. The virus particle has a membrane too, and it's made of the same sort of molecules (lipids). It also has protein sticking through its membrane - only one, though: the "env" protein. I've drawn it in the picture (the pink bits), because env is very important in the HIV life cycle. The viral core is where the virus keeps its genetic code, so in that sense it's a bit like a cell's nucleus - but the viral core is not surrounded with a lipid membrane. The viral core is simply two strands of RNA, wrapped in proteins, all bundled up into a conical shape.
Other viruses: Not all viruses are as complicated as this. Some are simply strands of DNA or RNA with a protein coat (some don't even have the protein coat). Some are very odd shapes indeed, not sphericalish like HIV.
The last thing I want to mention about cells and virus particles is the blobbiness of membranes. The lipid molecules which make up a membrane aren't bound to each other in a fixed position - they can "float about" in the membrane. If you bring two membranes close enough together, they can even "blob" together and become one:
This picture shows a HIV particle in the process of fusing with a cell. This kind of fusion doesn't normally happen when two cells, for example, come together - special processes need to be initiated in order for fusion to happen. In the above case, it's HIV's env protein which helps it fuse with human cells by latching on to a protein called CD4 embedded in the membrane of certain human cells.