Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane and containing a membrane-bound nucleus and organelles. Unlike the cells of the two other eukaryotic kingdoms, plants and fungi, animal cells don't have a cell wall. This feature was lost in the distant past by the single-celled organisms that gave rise to the kingdom Animalia.
The lack of a rigid cell wall allowed animals to develop a greater diversity of cell types, tissues, and organs. Specialized cells that formed nerves and muscles -- tissues impossible for plants to evolve -- gave these organisms mobility. The ability to move about by the use of specialized muscle tissues is the hallmark of the animal world. (Protozoans locomote, but by nonmuscular means, i.e. cilia, flagella, pseudopodia.)
The animal kingdom is unique amongst eukaryotic organisms because animal tissues are bound together by a triple helix of protein, called collagen. Plant and fungal cells are bound together in tissues or aggregations by other molecules, such as pectin. The fact that no other organisms utilize collagen in this manner is one of the indications that all animals arose from a common unicellular ancestor.
Animals are a large and incredibly diverse group of organisms. Making up about three-quarters of the species on Earth, they run the gamut from sponges and jellyfish to ants, whales, elephants, and -- of course -- human beings. Being mobile has given animals the flexibility to adopt many different modes of feeding, defense, and reproduction.
The earliest fossil evidence of animals dates from the Vendian Period (650 to 544 million years ago), with coelenterate-type creatures that left traces of their soft bodies in shallow-water sediments. The first mass extinction ended that period, but during the Cambrian Period which followed, an explosion of new forms began the evolutionary radiation that produced most of the major groups, or phyla, known today. Vertebrates (animals with backbones) are not known to have occurred until the Ordovician Period (505 to 438 million years ago).
- Centrioles are self-replicating organelles made up of nine bundles
of microtubules and are found only in animal cells. They appear to
help in organizing cell division, but aren't essential to the
- For single-celled eukaryotes, cilia and flagella are essential for
the locomotion of individual organisms. In multicellular organisms,
cilia function to move fluid or materials past an immobile cell as
well as moving a cell or group of cells.
- The endoplasmic reticulum is a network of sacs that manufactures,
processes, and transports chemical compounds for use inside and
outside of the cell. It is connected to the double-layered nuclear
envelope, providing a connection between the nucleus and the
- The Golgi apparatus is the distribution and shipping department
for the cell's chemical products. It modifies proteins and fats
built in the endoplasmic reticulum and prepares them for export to
the outside of the cell.
- The main function of these microbodies is digestion. Lysosomes
break down cellular waste products and debris from outside the cell
into simple compounds, which are transferred to the cytoplasm as new
- Microfilaments are solid rods made of globular proteins called
actin. These filaments are primarily structural in function and are
an important component of the cytoskeleton.
- These straight, hollow cylinders, composed of tubulin protein, are
found throughout the cytoplasm of all eukaryotic cells and perform a
number of functions.
- Mitochondria are oblong shaped organelles that are found in the
cytoplasm of every eukaryotic cell. In the animal cell, they are the
main power generators, converting oxygen and nutrients into energy.
- The nucleus is a highly specialized organelle that serves as the
information and administrative center of the cell.
- Microbodies are a diverse group of organelles that are found in
the cytoplasm, roughly spherical and bound by a single membrane.
There are several types of microbodies but peroxisomes are the most
- All living cells have a plasma membrane that encloses their
contents. In prokaryotes, the membrane is the inner layer of
protection surrounded by a rigid cell wall. Eukaryotic animal cells
have only the membrane to contain and protect their contents. These
membranes also regulate the passage of molecules in and out of the
- All living cells contain ribosomes, tiny organelles composed of
approximately 60 percent RNA and 40 percent protein. In eukaryotes,
ribosomes are made of four strands of RNA. In prokaryotes, they
consist of three strands of RNA.