Cells
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.
Small Cells ---- Big Organisms
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 the 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. This means that a cell that is about to divide actually has 4 copies of its DNA for a very short time.
What is a Cell
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.
Cells contain all the necessities for life, water, nutrients, minerals proteins, enzymes, fats and
carbohydrates. In prokaryote cells these can be fairly loosely distributed, but in multicellular cells they
are often stored in special areas. In multicellular organisms the cells are specialised to perform particular
jobs such as storage, support, growth, transport of resources or defence of the organism. To fill these
various roles the cells end up becoming very different from each other. The three images above are all
examples of plant cells while the 3 below are all animal cells. There are many many more types of cells,
so many in fact that you can spend your whole life studying just some of these cells and how they work,
to do this you would become a 'Cell Biologist'.