Download What is a Cell? All living things are made up of cells. Each of us has

What is a Cell?
All living things are made up of cells. Each of us has about 50 million million cells - an enormous
number which is difficult to imagine. Each cell is a sort of bag made from a sort of skin called a
membrane. The inside of a cell is watery and jelly-like. Cells are very small - you can't see them just
using your eyes. You need to use a microscope, which makes them look many times bigger that they
actually are.
If a cell is cut in half, it will not survive. So a cell can be considered as the smallest part of an organism
that can survive on its own. Some organisms have only one cell, while more complicated organisms are
made out of lots of cells. All cells have a membrane, which separates them from the outside world. The
membrane protects the cell, and allows the cell to be selective about what is allowed in. The membrane
is alive too, and can detect and respond to changes in the outside environment.
Are all cells the same?
No, they're not. Plant cells are different than animal cells. Plant cells in a root are different to those in
the stem or in the leaf. Animal cells, including the cells in our bodies are all sorts of different shapes
and sizes.
Cells are the units which all organisms are made from. Different cells do different things. Some
organisms consist of only one cell, like amoeba, or bacteria. Other, more complex organisms, are made
up of lots of different types of cells. For example, muscle cells are long and fat and work together in
bundles to let us move about whereas skin cells are flat and fit together like a jigsaw, covering our
bodies. In contrast, nerve cells are long and thin and can carry messages from one place to another.
Despite all the differences between types of cells, there are also similarities. For instance, all cells have
membranes which allow some substances to pass through, but not others. All cells contain DNA
(deoxyribonucleic acid) for storing information and RNA (ribonucleic acid) for building proteins.
Proteins are used in all cells for forming the structure of the cell itself, and regulating the cells reactions.
Where do new cells come from?
New cells come from old cells. When the time is right, an animal cell or a plant cell divides into two,
forming two new cells. These are called daughter cells. The two new cells are exactly the same as the
original cell. This process is called cell division.
You may be thinking, "But that's not right - they should have half of what was in the original cell!"
Although a cell may be to small to see, its not stupid - before it divides it makes an extra copy of
everything in its nucleus. This means that the two daughter cells get a complete nucleus. This is
important because the nucleus contains the 'recipe' which is used to tell the cell what to d o, including
telling it how to divide to make new cells. They do share the cytoplasm but they can make more and end
up the same size as their parent cell.
What is a single-celled animal?
A single-celled animal is an animal cell that lives on its own, without other cells. It can move around. It
can get its own food. It gets rid of waste. It divides to make more cells. It can tell something about
what is happening around it. the single cells of a multicellular animal (one made up of many cells such
as a tiger, an insect or a person) could not do this. They need to live surrounded by lots of other cells.
What is a single-celled plant?
A single celled plant, such as an algae, is a plant cell that lives on its own. It does not need to be part of
larger plant structure. Algae always live in water.
Levels of Organization
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CELLS: LEVEL ONE
Within a multicellular organism there is a division of labor. Division of labor means that the work of
keeping the organism alive is divided among the different parts of the body. Each part has a specific job
to do. And as each part does its special job, it works in harmony with all the other parts. The
arrangement of specialized parts within a living thing is sometimes referred to as levels of organization.
Cells, of course, are the first level of organization.
TISSUES: LEVEL TWO
In any multicellular organism, cells rarely work alone. Cells that are similar in structure and function
are usually joined together to form tissues. Tissues are the second level of organization. Bone cells in
your body from bone tissue, a strong sokid tissue that gives you shape and support. Blood cells in your
body are part of blood tissue, a liquid tissue responsible for transporting food and oxygen throughout the
body.
ORGANS: LEVEL THREE
Tissues are further organized in organs, the third level of organization in living things. Organs are
groups of different tissues that work together. Your heart, for example, is an organ made up of muscle
tissue, blood tissue, and nerve tissue. You are probably familiar with the names of many of the body
organs. The brain, stomach, kidneys, and skin are some examples.
ORGAN SYSTEMS: LEVEL FOUR
Like cells and tissues, organs seldom work alone. They "cooperate" with one another and form specific
organ systems. Organ systems are the fourth level of organization in living things. An organ system is a
group of organs working together to perform a specific function for the organism.
ORGANISMS: LEVEL FIVE
You are an organism. Dogs, trees, and butter cups are also organisms. Even a unicellular bacterium is
an organism. An organism is an entire living thing that carries our all the basic life fuctions. The
organism is the fifth level of organization.
Cells, tissues, organs, organ systems organisms--each level of organization interacts with every other
level. The smooth fuctioning of a complex organism is the result of all it various parts working together.
Cell Organelles
Mitochondria
The mitochondrion is the powerhouse of the cell. It is the site of respiration. It has a double membrane.
The white folded structure is the inner membrane. Most of aerobic respiration occurs along this
membrane. The inner membranes is ruffled. It has a very large surface area. These ruffles are called
cristae. Mitochondria have their own DNA and manufacture some their own proteins. It is thought that
the mitochondrion evolved from symbiotic bacteria that took up residence inside the first eukaryotic
cells.
Rough ER
The endoplasmic reticulum is a series of double membranes that loop back and forth between the cell
membrane and the nucleus. These membranes fill the cytoplasm but you cannot see them because they
are very transparent. The rough E.R. has ribosomes attached to it. This gives it its texture. Th ese
ribosomes manufacture proteins for the cell. The ribosomes are the organelles which manufacture
proteins. They are made of two separate parts. These structures are both made of ribosomal RNA.
Smooth ER
The endoplasmic reticulum is a series of double membranes that loop back and forth between the cell
membrane and the nucleus. These membranes fill the cytoplasm but you cannot see them because they
are very transparent. There are two distinct types of E.R.: The rough E.R. has ribosomes and is the site of
protein synthesis; the smooth E.R. has no ribosomes. The smooth E.R. lacks ribosomes. It acts as a
pathway throughout the cytoplasm. It runs from the cell membrane to the nuclear membrane and
throughout the rest of the cell. The smooth E.R functions much like a pathway between the nuclear
membrane and the cell membrane. It also produces lipids for the cell.
Cell Membrane
The Cell membrane performs a number of critical functions for the cell. It regulates all that enters and
leaves the cell; in multicellular organisms it allows self recognition. In order to understand the function
of the cell membrane you must understand its structure. With the membrane color coded you can see the
the various proteins (in purple) that drift around in the double layer of lipids.
Golgi Body
The golgi body is responsible for packaging proteins for the cell. Once the proteins are produced by the
rough E.R. they pass into the sack like cisternae that are the main part of the golgi body. These proteins
are then squeezed off into the little blebs which drift off into the cytoplasm.
Nucleus
The nucleus is called the headquarters of the cell. It is a large dark spot in eukaryotic cells. It controls all
cell activity. Zoom in close an you will see that the nuclear membrane has many pores. The nuclear
membrane is continuous with the E.R. With the outer membrane removed it is much easier to see the
contents of the nucleus. The thick ropy strands are the chromatin. The large solid spot is the nucleolus.
The nuclelous is a knot of chromatin. It manufactures ribosomes. The nucleolus is a spot of condensed
chromatin. It is responsible for the manufacture of ribosomes. The chromatin is DNA in its active form.
It is a combination of DNA and histone proteins. It stores the information needed for the manufacture of
proteins.
Cytoplasm
Cytoplasm is the jelly-like substance found between the plasma membrane and the nucleus in a cell. The
cytoplasm is made up of amino acids, sugars, carbohydrates, and fatty acids. However, it is made up
mostly of water. The chemicals found in the cytoplasm help the cell to carry out life processes. The
organelles contained in the cytoplasm are very important for the maintenance of the cell. The cytoplasm
is constantly moving or streaming.
Vacuole
Vacuoles are like storage bubbles in the cytoplasm. Plant cells generally have larger vacuoles than
animal cells. Vacuoles can store food that needs to be digested, water, and waste. The indigestible
waste can wait in the vacuole until the vacuole meets up with the cell membrane and squirt the wastes
outside. Vacuoles in plant cells get their strength from the water pressure in the cell sap vacuole.
Lysosome
Lysosomes are called suicide sacks. They are produced by the golgi body. They consist of a single
membrane surrounding powerful digestive enzymes. With the outer membrane removed it is much easier
to see the contents of the lysosome. Those lumpy brown structures are digestive enzymes. They dissolve
bacteria and other foreign bodies. Under some conditions the lysosomes in a cell will break open and a
cell will self destruct in a process called autolysis (giving rise to the name "suicide sacks"). Take a close
look at the enzymes here. They help protect you by destroying the bacteria that your white blood cells
engulf. They also cause autolysis which is the way you body gets rid of old or damaged cells. Lysosomes
act as a clean up crew for the cell.
Centrioles
Centrioles are found only in animal cells. They function in cell division. Zoom in and notice the 9 groups
of 3 arrangement of the protein fibers.
Chloroplast
The chloroplast is the site of photosynthesis. It consists of a double membrane. With the outer
membrane removed it is much easier to see the contents of the chloroplast. The stacks of disk like
structures are called the grana. The membranes connecting them are the the thylakoid membranes. The
membranes that you see here are the site of photosynthesis. It is here that the energy harnessing process
of photosynthesis occurs.
Cell Wall
Plants lack certain structures that animals have that offer support. Plant cells do not have skeletons to help
support their weight or give them structure. Instead they require a cell wall to help with support. The cell wall
is made up of cellulose. The cellulose and the pressure created from a full cell sap vacuole help to create
enough strength to hold up larger plants. Woody plants need something additional to help make them stronger;
this extra layer is made up of pectin.