All living things are made of cells, the smallest units of life, and there are only two main types of cells:
prokaryotes, which are the primitive, simple cells of bacteria and their bacteria-like cousins archaea
Article Summary: Animal cells are a type of eukaryotic cell with a nucleus, membrane-bound organelles and no cell wall. Here is a summary of their structure and function.
Animal Cell Structures, Functions & Diagrams
Membranous Organelles of Animal Cells
All membranous organelles are enclosed by the same type of material as the plasma membrane, and can therefore ship materials to each other, via the endomembrane system, in which a piece of a membrane-bound organelle breaks off from one organelle, travels, and then fuses with a different membrane-bound organelle. Material can also enter (endocytosis) or exit (exocytosis) the cell via this method.
Nucleus: Often the largest and most visible organelle in an animal cell, the nucleus is bound by a double-layer nuclear membrane and contains the genetic material (the genome) of the cell. It is also filled with fluid, called nucleoplasm and may contain one or more nucleoli (regions where ribonucleic acid RNA is synthesized).
Endoplasmic reticulum: A network of hollow tubes, called the endoplasmic reticulum (ER), extends off the nuclear membrane. There are two types of ER. One is rough endoplasmic reticulum, which is studded with ribosomes and involved in making and shipping proteins. Ribosomes are the protein making machinery of the cell, and cells contain many thousands of these tiny nonmembrane-bound organelles. Some ribosomes are attached to the rough ER, others float freely in the cell. The other type of ER, called smooth endoplasmic reticulum, does not have ribosomes, and is involved in the synthesis and transport of lipids.
Stained cheek cell with oral bacteria @1000xTM.
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eukaryotes, which include the more advanced cells of animals, plants, fungi, protozoans, algae, and slime & water molds
Eukaryotic cells share many similar characteristics, including some of the following structures, which can be found in the cells of animals:
Cell Envelope & External Structures of Animal Cells
Glycocalyx: Some animal and protozoan cells have this sticky outer layer anchored to the plasma membrane. Gycocalyces help animal cells stick to each other and protect cells from dehydration. This layer is not present in cells that have a cell wall, such as plants, algae and fungi.
Cilia and flagella: Constructed of microtubules covered with plasma membrane (analogous to an arm or leg bone covered in skin), these external appendages, present in some animal cells, aid in cell movement and in moving materials around the outside surface of the cell.
Plasma membrane: All cells have plasma membranes. In eukaryotic cells, this barrier between the inside and the outside of the cell is made mainly of phospholipids, proteins and sterols.
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