CELL DIVISION
The cell membrane is also known as the plasma membrane or plasmalemma. It covers the entire cell and serves to hold it together. It also actively regulates what enters and what leaves the cell. It is only about 10 nanometers thick.
In animal cells (as well as in plant, protist and fungus cells) the nucleus is separated from the rest of the cell by the nuclear envelope. Such cells are called eukaryotic cells (from the Greek eu (true) karyon (kernel). This is to show the difference between prokaryotic cells, which do not have a true membrane-enclosed nucleus and are more primitively organised. Prokaryotic cells are only found among the bacteria and their close relatives.
The nuclear envelope is made up of two layers of membrane. These are very similar to the cell membrane but have many pores.
Within the nucleus is a prominent structure called the nucleolus (sometimes there are two or more nucleoli) and a network of thin threads called chromatin. The chromatin contains the hereditary material of the cell. The fluid that fills the rest of the space in the nucleus is called the nuclear sap.
The term "cytoplasm" is still used to designate all of the cell contents outside the nucleus but inside the cell membrane, although it is now known that cytoplasm is not the homogeneous substance it was once thought to be. One of the prominent organelles in the cytoplasm is the mitochondrion. 90% of the energy that eukaryotic cells get from oxidizing food molecules is developed there.
The Golgi complex is a stack of membranous sacs in which various molecules are manufactured.
Centrioles are cylindrical bundles of microtubules that give rise to longer spindle microtubules that separate the two duplicate sets of chromatin at the time of cell division.
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