Parietal cells Cells that secrete hydrochloric<br>acid in the human stomach cells Paneth cells Intestinal cells that help<br>fight bacteria Xylem vessels in Tilia (lime) wood - Xylem vessels are dead cells that provide support and transport water and mineral ions. Annular (ring) and spiral thickenings of lignin can be seen stomata - The lower epidermis of a leaf showing stomata with their guard cells. These structures control the exchange of gases in the leaf and limit water loss Vascular bundle in sunflower stem - The red walls are of xylem vessels, at least two of them have spiral thickening as seen bythe loose cut ends of the lignin stained red. These are dead cells that carry water and dissolved minerals Nearby cells are phloem that carry sugars and other products of photosynthesis. ion bulbion bulbion bulbion bulb

Animal Cells
Human and other animal cells, the building blocks of life


A simple cell diagram,
what you can see with a simple microscope

A more complex cell diagram,
what you can see with a more sophisticated microscope with particular lighting and staining techniques

Basics

We are all made of cells, loads and loads of them as they are well titchy. The average human (full grown) is made of about 37 trillion (reference) which is a stupidly large number.  There are enough cells in about 1/4930 th of your body (about 2/3rd of one of your little fingers) to give one each to every one of the 7.5 billion people currently on earth. If they were able, they could then each use the information in that one cell to clone an entire new you. Or alternatively, you have enough cells so that if each one was cloned, you could populate 4,930 earths with 7.5 billion people on each one.

Needless to say, cells are microscopic and cannot be seen with the naked eye, it wasn't until 1665 when Robert Hooke invented the first simple microscope that cells were first seen at all. These days cell biology is considered to be one of the most important areas for scientific research.

Cells are the building blocks of life, they are made of the same kinds of materials and have a common structure wherever they are in your body. They are also recognizably very similar regardless of the species they come from. Your cells are very similar in appearance to those of a tiger, a termite and a terrapin.

All cells have a cell membrane around them, this is very important, far more than you might think, it controls what enters and leaves the cell. Inside the cell is the nucleus, this contains the DNA of the cell and controls what the cell does. The rest of the cell is filled with the cytoplasm, this is mainly made of water but is quite a thick liquid, it is where the chemical reactions of the cell take place.

Human cells are in the region of 20 micrometers (microns) across. A micrometer is a thousandth of a millimeter or a millionth of a meter. 50 cells of this size end to end would stretch a millimeter and it would take about 2,500 to cover this full stop. No wonder it takes so many to build a human!

New cells are made by a process called mitosis, where one cell splits into two new ones, you need a cell to make a cell.

Specialized cells

You see this drawing to the left, what is it supposed to be? That's right, it's a man, you spotted it. However you've never actually seen anyone that looks like that though have you? No you haven't, stop being silly.

The simple diagram above is of a generalized cell, it's a bit slicker than my hastily drawn stick man but it is to a real cell what a stick man is to a real man. It is recognizable and the main bits are there but you won't actually see any cells that look like that. Real cells are usually differentiated, that is they have developed into something more specialized that has a special job of its own.

When the cells that make you up were first made by mitosis and you looked like the picture of the 3 day old 8 cell embryo on the slider at the top of this page they were more like the diagram of a generalized cell. They were what are called stem cells that could become any of the kinds of cells in your body. As the cells get older, they become more specialized, their shape changes and they have a job that is theirs for life.




A nerve cell (purple), the part with the yellow cells carries the nerve impulse and can be many centimeters long, the yellow parts are other cells that act as electrical insulators1



Sperm cell, the nucleus is in the head, the rest of the cell provides power and a tail for swimming



Human blood cells, a red blood cell, left has no nucleus so more oxygen carrying pigment can fit inside it. The middle cell is a platelet also without a nucleus that is involved in blood clotting. The cell on the right is a white blood cell, part of the immune system, it fights disease.



Muscle cells, inter-meshed fibers help the cells to contract, they are unlike other cells in that there are many nuclei scattered through the very large cells.

More about animal cells

In order to do everything that cells do, including building you from a starting point of two cells the size of microscopical dots, running your body, hopping, jumping, skipping, thinking, weeing, sweating, talking etc., there has to be more in cells than the very simple diagram above. There are lots of things in cells called organelles, these are the  "little organs" of the cell which carry out the process that the cell has to do to make it work on its own, with the other cells around it and in the organism of which it is a part. Here are some of the more important ones.

 

  • Cell membrane - a fluid barrier that contains the cell and controls what leaves and enters.
  • Nucleus - contains the DNA, the genetic material of the cell, controls the cell and carries the blueprint of the whole organism.
  • Cytoplasm - a watery jelly-like liquid mainly water, where many reactions of the cell take place.
  • Mitochondrion - where respiration takes place, this being the process that releases energy from food. The power plants of the cell. Mitochondria - plural.
  • Ribosome - these are responsible for making proteins using information from the DNA in the nucleus and raw materials in the cytoplasm.
  • Endoplasmic reticulum (ER) -  the "test tubes and beakers" of the cell, where chemical reactions take place without contacting other parts of the cell or cytoplasm. It may be rough endoplasmic reticulum, RER or smooth, SER. The rough is made that way by ribosomes attached to the outside. The ER is made of the same material as the cell membrane.
  • Golgi apparatus - a specialized part of the endoplasmic reticulum that modifies proteins after they have been made packaging them in bags of membrane for transport within the cell or secretion out of the cell.
  • Lysosome - membrane bound sacs containing digestive enzymes that re-cycle old organelles and other waste material, they also destroy ingested bacteria and viruses.
  • Centrosome - these organize microtubules, the scaffolding of the cell, these are also important in cell division.

The diagram below shows how different membrane systems work together in a cell, in this case to make a protein that is secreted by the cell. The information to make proteins is held in the nucleus, selected information to make a particular protein leaves through the nuclear pores, the ribosomes of the rough endoplasmic reticulum make the raw protein. This is then modified in the smooth endoplasmic reticulum before final processing in the golgi apparatus (named after Mr. Golgi). The finished article is then packaged into secretory vesicles before fusing with the cell (plasma) membrane, the protein then leaves the cell.