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.
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.
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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.
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.
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.
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."