Cells are amazing, variable, beautiful, functionally superb, a concept of genius, they work alone or in groups with equal ease. Cells are the basic unit of life, all living things,---- are made up of one or more cells. Organisms that exist as single cells are called unicellular and organisms that are made up of groups of cells working together are called multicellular. There are two kingdoms of unicellular organisms (Archaea and Bacteria ), and three kingdoms of multicellular organisms (Animals, Fungi and Plants), and one kingdom which contains a mixture of both unicellular and multicellular organisms (the Protista). Nearly everything you see without a microscope is an animal a fungus or a plant and therefore a multicellular being (the exceptions to this rule are eggs, eggs, before they are fertilised and start to divide are the largest single cells around). Because all living things are made up of cells, and because we desire to understand ourselves and the other living things around us it makes sense to learn something about cells. This page is an introduction to form and function of cells.
All living things are divided into two major groups depending on how their cells are set up, these two groups are the Prokaryotes, and the Eukaryotes
Eukaryotes have areas inside the cell separated off from the rest of the cell by membranes, like the cell membrane (see below). These areas include the nucleus, numerous mitochondria and other organelles such as the golgi body, and or chloroplasts within each of their cells. These areas are made distinct from the main mass of the cells cytoplasm by their own membrane in order to allow them to be more specialised. You can think of them as separate rooms within your house. The nucleus contains all the cell's DNA, the Mitochondria are where energy is generated, chloroplasts are where plants trap the suns energy in photosynthesis. There are exceptions to every rule of course, and in this case the most obvious two are the red blood cells of animals and the sieve tube elements of plants, which, though living, have no nucleus and no DNA, normally these cells to do not live very long.
Prokaryotes do not have a nucleus, mitochondria or any other membrane bound organelles. In other words neither their DNA nor any other of their metabolic functions are collected together in a discrete membrane enclosed area. Instead everything is openly accessible within the cell, though some bacteria have internal membranes as sites of metabolic activity these membranes do not enclose a separate area of the cytoplasm. See Cells: The Basis of Life
All the Prokaryotes (Bacteria and Archaea) are unicellular, only Eukaryotes:- the Protista, some Fungi and some Plants are multicellular. In most single celled organisms the cells are all the same most of the time in any given species. In multicellular organisms individual groups of cells have become specialised to perform particular roles in the life of the organism. The life of the organism is dependant on the correct working of all the different groups, each of which is dependant on all the others for its continued existence. In simple multicellular organisms such as sponges all the cells are very similar, in more complicated multicellular organisms the degree of specialisation of cells is much greater resulting in cells that are very different from one another. In humans, there are about 40 trillion cells all told. They occur in 1014 different types cells making up over 200 different kinds of tissues.
Most cells are tiny, too small to be seen with the naked eye, yet we can easily see the trees and elephants etc. which are made up of trillions of cells working together. Where do all the cells come from?? One cell becomes two cells by slowly dividing in half, these halves then grow to full size before they themselves divide. This is the golden rule whether you are a single celled bacteria or a 100 ton blue whale. Even you and I started life as a single cell which divided to become 2, then both of these divided to make the total 4 and so on, and so on until you are grown up and have 40 trillion cells. Even then some cells keep dividing, making new cells to replace ones that get worn out or damaged. In normal everyday cells, and single celled organisms, the DNA within the cell is copied before the cell divides so that both the new cells, normally called daughter cells, can have their own copy. This is very important because the new cells would not know what to do without their DNA. Each new cell also gets half the other contents of the old cell so that it has enough of all the important bits to be getting on with. Scientists know a whole lot more about how this happens and I might write something about it one day but this is enough for now. The exception to this rule are reproductive cells, eggs and sperm. Because these cells are destined to join with another reproductive cell to make a new being in the future they only have half as much DNA as ordinary cells, thus when they unite with another reproductive cell the new organism has the full amount of DNA. To understand this you have to realise that normal cells have two copies of their DNA (one from each parent). This means that a cell that is about to divide actually has 4 copies of its DNA for a very short time.
OK, so now we know a bit about how important cells are but what really is a cell. The simplest answer is that a cell is a container, like a box or a bottle or a jar. It has an inside and an outside, and something like a wall in between to let us know where the outside begins and the inside ends. This 'something like a wall in between' is called a cell membrane. All cells have a cell membrane. It is the cell membrane that keeps the insides in and the outside out. Though like your house it has doors and windows in it to let things in and out. A cell membrane is very flexible and the cell can change shape quite easily. However some cells have given up this flexibility for greater strength and protection in the form of a 'cell wall'. A cell wall is not flexible so cells that have one have a constant shape. Most, but not all bacteria and archaea have cell walls, many protista, all plants and all fungi also have a cell wall around every cell. Animal cells however never have a cell wall. The cell wall is built outside of the cell membrane so it can protect the cell. So things that need to get into or out of the cell have to go through two sets of doors, one in the cell wall and one in the cell membrane.
|Cell Membranes||Inside a Cell||Cell Walls|
|What is a Living Thing ??||This Living World||The Six Kingdoms|