The human body is made up of millions of billions of cells organized into tissues.
There are about 200 different types of cells. All animal cells, as different as they are, share common characteristics.
A cell measures less than a tenth of a millimeter (...) Under the optical microscope, only a membrane and a nucleus are visible, which bathe in cytoplasm.
The plasma membrane separates the extracellular space from the intracellular space. It consists of a 8 nanometers thick lipid bilayer, in which there are various proteins that act as channels for the transfer of ions and nutrients. There are also receptors capable of fixing hormones.
Cytoplasm is the living environment of the cell. It consists of a semi-viscous substance: Cytosol.
The nucleus bathes in the cytoplasm. It is delimited by a double discontinuous membrane: called the inner and outer nuclear membranes. Nuclear pores allow the exchange of molecules between the cytoplasm and the nucleus.
The nucleus has a dense central zone, the nucleolus, around which the nucleoplasm is located. The nucleolus contains long chromatin fragments containing the DNA. These fragments condense during cell division to form chromosomes.
But the nucleus is not the only structure in the cytoplasm. An observation through an electron microscopic reveals other organelles such as the Rough Endoplasmic Reticulum. It is a membrane network formed from the outer nuclear membrane. It has on its surface small particles called ribosomes. The Endoplasmic Reticulum ensures the biosynthesis of proteins. In a way, it is the factory where proteins are made.
The Golgi apparatus is a membrane network that packages and sorts proteins. The vesicles that form it contain these proteins. Some are released out of the cell by exocytosis. The Golgi apparatus is the center of distribution for the proteins.
It also produces lysosomes that contain various digestive enzymes. Lysosomes help to recycle molecules that have become useless, and to degrade foreign materials like bacteria. They play a major role in the recycling of molecules and the disposal of cellular debris.
All of these mechanisms consume energy. Mitochondria are organelles that produce energy in the form of ATP. This production takes place during cellular respiration thanks to the degradation of glucose in the presence of oxygen. Mitochondria are the cells power station.
Some cells have accessory structures such as flagellum, cilia, or other organelles such as the chloroplasts of chlorophyllian plant cells; (...) but all have in common the different structures presented here, necessary for their functioning, their defense, or their reproduction.