Holaa!

2. CELLS

First, we must remember some ideas about cells:

All living beings are made of cells.
Cells are the basic unit living beings are made of, so they are the structural andfunctional unit of all living beings.
All cells come from preexisting cells.

We say that cells are microscopic because they are too small to be seen with the naked eye: they measure from 0.001 mm to 0.1mm in size.

We find three basic structures in cells:

The cell membrane is the thin layer which separates the inside of the cell from its sourronding environment. It regulates the substancesexcanged between the inside
The cytoplasm is a gelatinous, semi-transparent fluid which fills most cells. It contains structures called organelles which are responsible of cell functions.
Thegenetic material (DNA) is a fibre-like substance that controls the cell activities.
Some cells have their genetic material loose in the cytoplasm (like bacteria), this are called prokaryoticcells.
The most of the cells have their genetic material enclosed in a structure called nucleus, this are called eukaryotic cells.

Prokaryotic cell | Eukaryotic cell |

2.1. Animal andplant cells

Cells contain a great variety of different organelles in their cytoplasm. Some organelles like mitochondria can be found in all cells but there are other organelles that are exclusive toonly one type of cell:

Plant cells contain chloroplasts (that perform photosynthesis) and a rigid wall called the cell wall, made of cellulose.
Animal cells contain centrioles, specialorganelles which take part in cell division and in cillia and flagella formation. Cilia and flagella are responsible for the cell movement.

The next organelles can be found in both animal and plantcells:

Cell membrane.
Nucleus.
Rough ER (Endopasmic reticulum). Where proteins are formed.
Ribosomes. Where proteins are stored and processed.
Vacuoles. Sacs which contain storage...