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Cell
A cell is the smallest, most basic unit of
life, that is capable of existing by itself.
Cells carry out the chemical processes
that are necessary for life to exist. They
use energy and reproduce themselves.
The bodies of living organisms are made
up of cells.

Some organisms are made up of only one
cell. Human beings, on the other hand, are
much more complex and are made up of
billions of cells. Something made of two
cells or subdivisions is known as bicellular.
The various components of a cell.
 
ARE ALL CELLS THE SAME?

No. There are millions of different types of cells, with different structures, sizes, and
special jobs to do in the body. In general, however, cells in the human body have the
same basic structure. Cells perform many of the complex tasks that are necessary for
life. For example, red blood cells carry oxygen in the body, white blood cells destroy
invading organisms, and nerve cells send electrochemical (electric and chemical)
impulses throughout the body. When groups of cells of the same type come together
with non-living material, they are called tissues. Muscle, for example is a type of tissue.
The fact that tissues can become so specialized shows how specialized many of the
cells are that make up the tissues.
When several different types of tissue come
together, they are called an organ. The heart is
an example of an organ. When a number of
organs come together, they are called a system.
An example is the digestive system, which is
made up of organs such as the stomach and the
intestines.

The intestine is a tube shaped structure that
stretches from an opening in the stomach to the
anus (rear end) and occupies most of the lower
parts of the belly.
WHAT IS A CELL MADE OF?

As you can see in the picture below, a cell is made up of many parts.

Here is a brief description of some of the parts that you see labeled above:

PLASMA CELL MEMBRANE: This is the outer surface of the cell. It holds the cell together and allows
some substances to pass through it, but keeps others out. It is also known as the cell membrane, the
plasma membrane, and the plasmalemma.

NUCLEUS: This is the center of a cell. Think of it as the cell's headquarters, controlling the activities
within it. It has two layers on the outside and surrounds a jelly-like substance known as nucleoplasm
(also known as karolymph). Inside of the nucleoplasm are one or more small, rounded bodies known as
nucleoli (see listing below). The nucleoplasm also contains structures called chromosomes, which
contain naturally occurring substances known as proteins and a substance known as DNA (an
abbreviation for deoxyribonucleic acid). Chromosomes are made from thread-like bodies known as
chromatin. So chromatin also holds DNA.

DNA is a chain of many connected genes. Genes contain coded instructions for how proteins should be
constructed and how certain bodily characteristics should develop. For example, genes control the
natural color of people's eyes and hair, and whether they will be male or female. It is an important job for
all cells to make copies of DNA.

ENDOPLASMIC RETICULUM (ER): A complex system of folded, flat sacs that provide a large area for
fluid to be stored and for reactions to occur. ER with ribosomes (see next listing) is known as rough ER.
ER without ribosomes is known as smooth ER.

RIBOSOMES: Small, round particles that build up proteins. Cells make many copies of proteins with the
help of ribosomes. Most of the ribosomes are attached to the endoplasmic reticulum.

LYSOSOMES: Round sacs that contain special proteins known as enzymes. The lysosomes take in
foreign substances such as bacteria and the enzymes inside the lysosomes destroy them.

MITOCHONDRIA: Rod-shaped bodies that break down simple substances to provide energy. Cells that
require a lot of energy, such as muscle cells, have many mitochondria.

GOLGI: A special area of smooth ER (see above) that collect and distribute substances made in the
cell. Golgi also change and package proteins. Golgi look like a bunch of plates stacked together. Golgi is
also known as Golgi complex, Golgi apparatus, and dictyosome.

In addition to the parts mentioned above, there are other parts of a cell that also important. Here is a
brief description of some of the parts that you do not see labeled in the above picture:

CYTOPLASM: A gooey substance that fills up a cell. It contains everything in the cell other than the
nucleus (see above) and the outer surface of the cell. Cytoplasm is made up of fluid and structures
known as organelles (see below).

VACUOLES: Sacs in the cytoplasm that are filled with fluid. Vacuoles either remove substances or
contain fluid brought into the cell.

CENTRIOLES: Two bodies that are necessary for cells to divide. In animal cells, they are located right
outside of the nucleus. Centrioles are also found in some types of plant cells.

NUCLEOLI: Small, rounded bodies in the nucleus (see above) that produce the parts of the ribosomes
(see above). The parts of the ribosomes are taken outside of the nucleus and put together in the
cytoplasm (see above). The nucleoli contain a substance known as RNA (ribonucleic acid) that is
important in building up proteins. RNA is also present in the cytoplasm (see above). It is an important job
for all cells to make copies of RNA.

PLASTIDS: Tiny bodies found in plant cells that contain substances important for the plant to live.

PEROXISOMES: Structures that help make poisonous substances no longer harmful.

ORGANELLES: A name for all of the small bodies in the cell that perform necessary roles regarding the
chemical reactions inside a cell. Lysosomes, ribosomes, endoplasmic reticulum, golgi, centrioles,
nucleoli, mitochondria, peroxisomes, and plastids are all types of organelles. See above for a
description of each type of organelle.

CYTOCENTRUM: An area of cytoplasm (see above) that contains one or two centrioles (see above),
but contains no other organelles (see above).

PROTOPLASM: A rarely used term for all the materials found inside a cell. See this detailed entry on
protoplasm.

DO ALL CELLS HAVE A NUCLEUS?

No. There are organisms made of one cell (known as prokaryotes) that do not contain a true nucleus
surrounded by two layers. Red blood cells, which carry oxygen in the body, do not contain a nucleus. A
cell with a true nucleus is called a eukaryote.

CAN A CELL HAVE MORE THAN ONE NUCLEUS?

Yes. Some cells have more than one nucleus. These types of cells are called binuclear, binucleate, or
multinucleate. When a cell has more than one nucleus, this is usually because there is a great deal of
cytoplasm in the cell. More cytoplasm means that there are more structures inside the cell that needs to
be controlled, thus the need for more than one nucleus. Please see the section above for a description
of cytoplasm and the structures within it.

Some types of cells that have more than one nuclei include cells that destroy bone, some cells in the
liver, and skeletal muscle cells. The liver is the largest organ in the body and is responsible for filtering
(removing) harmful chemical substances, producing important chemicals for the body, and other
important functions. Skeletal muscles are muscles that are connected at either or both arms or legs with
the skeleton of the body.

CAN ALL CELLS BE REPLACED AFTER THEY DIE?

No. When certain cells in the body die, they can never be replaced naturally. However, there are some
cells that continue to function for some time, even after death. Some cells die due to natural processes
(e.g., aging), disease (e.g., cancer), or elective medical procedures. An example of the latter is
advanced teeth whitening (such as what is offered by Easy Dental VA), which kills cells that
cause unsightly stains,
CAN CELLS BE SEEN?

Yes, but usually it requires using an instrument known as a microscope to see a cell. A microscope is an
instrument that makes things appear bigger when you look through it. Seeing a red blood cell (a cell that
carries oxygen in the body) would require using a microscope because it is only .0003 inches. However,
you would not need a microscope to see some types of nerve cells, which can be 3 feet or more in
length. The quality, degree, or condition that cells are present is known as cellularity.

DOES THE WORD "CELL" HAVE ANY OTHER MEANINGS IN THE FIELD OF MEDICINE?

Yes. Cell has the following additional meanings in the field of medicine:

1. A small closed, or partly closed open area.

2. A container of glass or other solid material in which chemical reactions that produce electricity take
place. Cells, in this sense of the word, are also used to hold substances for evaluating the measurement
of light intensity.

WHAT IS THE ORIGIN OF THE WORD, CELL?

Cell comes from the Latin word "cella" meaning "a storeroom."
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