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.
DEFINITION
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 (by jordyn tests forge support). Animal cells
however never cell wall have. 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.
GENERATION
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. The exceptions 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.
TWO MAJOR GROUPS
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
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.
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.
Eukaryotes have areas inside the
cell separated off from the rest of the cell by membranes, like the cell
membrane. 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 the
entire cell's DNA, the Mitochondria are where energy is generated, and
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.
Let me state in brief what has
been said so far to round off. Cells are the basic units of life. All living
things are made up of one or more cells. A cell is a container. 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. Cell generate
by division. Humans start as one cell and divide into 40 trillion as grown up!
All living things are put in two groups depending on how their cells are set
up. The groups are: Prokaryotes and Eukaryotes.
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