The nucleus, the membrane and the cytoplasm

The plasma membrane, a selective boundary

Every cell is bounded by a plasma membrane, a thin envelope that separates it from the outside environment. It is not a simple watertight barrier: it lets certain substances through (water, oxygen, nutrients) and blocks others, somewhat like a selective filter. It is what allows the cell to control what comes in and what goes out.

The cytoplasm, where everything happens

Inside the membrane is the cytoplasm, a watery gel in which all the organelles are bathed. It is where most of the cell's chemical reactions take place: the manufacture of molecules, the breakdown of nutrients, the transport of substances from one organelle to another.

The nucleus, control center

The nucleus is the most visible organelle under a light microscope, usually rounded and clearly delimited by its own envelope (the nuclear envelope). It contains DNA, the molecule that carries all the information needed for the cell's functioning and reproduction. The nucleus is often called the "control center": it is what indirectly directs the manufacture of all the molecules the cell needs.

Not all cells have a well-defined nucleus: in bacteria, the DNA floats directly in the cytoplasm, without a nuclear envelope. These cells are called prokaryotic cells, as opposed to eukaryotic cells (animal, plant, fungal), which have a true nucleus.

A logical organization

• Membrane: boundary and control of exchanges.
• Cytoplasm: working environment, where the organelles circulate.
• Nucleus: control center, contains the genetic information.

Keep in mind that these three elements are present, in one form or another, in almost all living cells: this is the basic structure common to all cellular life.