Cells consist of a protoplasm enclosed within a membrane,
which contains many biomolecules such as proteins and nucleic acids.
Organisms can be classified as unicellular (consisting of a single cell;
including most bacteria)
or multicellular (including plants andanimals).
While the number of cells in plants and animals varies from species to species,
humans contain about 100 trillion (10)
cells. Most plant and animal
cells are visible only under the microscope, with dimensions between 1 and
100 micrometres
The cell was discovered by Robert Hooke in 1665, who named the biological unit
for its resemblance to cells inhabited by Christian monks in a monastery. The cell theory,
first developed in 1839 by Matthias Jakob Schleiden and Theodor
Schwann, states that all organisms are composed of one or more
cells, that all cells come from preexisting cells, that vital functions of
an organism occur within cells, and that all cells contain the hereditary
information necessary
for regulating cell functions and for transmitting information to the next
generation of cells. Cells emerged on Earth at least 3.5 billion years ago.
Cells have
many parts, each with a different function. Some of these parts, called
organelles, are specialized structures that perform certain tasks within the
cell. Human cells contain the following major parts, listed in alphabetical
order:
Cytoplasm
Within
cells, the cytoplasm is made up of a jelly-like fluid (called the cytosol) and
other structures that surround the nucleus.
Cytoskeleton
The
cytoskeleton is a network of long fibers that make up the cell’s structural
framework. The cytoskeleton has several critical functions, including determining
cell shape, participating in cell division, and allowing cells to move. It also
provides a track-like system that directs the movement of organelles and other
substances within cells.
Endoplasmic reticulum (ER)
This
organelle helps process molecules created by the cell. The endoplasmic
reticulum also transports these molecules to their specific destinations either
inside or outside the cell.
Golgi apparatus
The Golgi
apparatus packages molecules processed by the endoplasmic reticulum to be
transported out of the cell.
Lysosomes and peroxisomes
These
organelles are the recycling center of the cell. They digest foreign bacteria
that invade the cell, rid the cell of toxic substances, and recycle worn-out
cell components.
Mitochondria
Mitochondria
are complex organelles that convert energy from food into a form that the cell
can use. They have their own genetic material, separate from the DNA in the
nucleus, and can make copies of themselves.
Nucleus
The
nucleus serves as the cell’s command center, sending directions to the cell to
grow, mature, divide, or die. It also houses DNA (deoxyribonucleic acid), the
cell’s hereditary material. The nucleus is surrounded by a membrane called the nuclear
envelope, which protects the DNA and separates the nucleus from the rest of the
cell.
Plasma membrane
The plasma
membrane is the outer lining of the cell. It separates the cell from its
environment and allows materials to enter and leave the cell.
Ribosomes
Ribosomes are organelles that
process the cell’s genetic instructions to create proteins. These organelles
can float freely in the cytoplasm or be connected to the endoplasmic reticulum.
