Let’s define the cell

We see a lot of news and posts by cell biologists discussing the importance of investigate cell functions. Yet, to help all the other scientists and non-scientists out-there to really fully grasp why studying cells is so important, it is necessary to take a step back, and explain what a cell really is.

We call a Cell the smallest unit of a life. Every living organism on this planet is composed of cells. Some, like bacteria and yeast, exist as one single cell entity. Whereas, others are made of thousands and thousands of cells assembled like bricks into bigger structures that we call organs and tissues. This is the case of plants, animals, and humans. Speaking of human beings, it is curious to see that the ultimate number of cells/person is still under debate. Up to today, last published attempted count estimated that each person is a collage of roughly 37.2 trillions cells (Bianconi E et al, An estimation of the number of cells in the human body. Ann Hum Biol. 2013).

But what is a Cell? Scientists define a cell a structure composed of:

1. plasma membrane, a wall made of different types of lipid and proteins, acting like a wall, as separating the inner compartment to the surrounding environment;

2. cytosol, a fluid-like region enclosed within the plasma membrane and containing various organelles with different function, and filaments(cytoskeleton);

3. nucleic acids, macromolecules (as DNA or RNA) carrying the fundamental information for the cell function, fate and survival.

Cells are also defined by their capacity of:

• Growing and duplicate

• Self sustain themselves by producing their own energy

Depending on their structural organisation, we can distinguish:

Eukaryotic cells have membrane-bound organelles in the cytosol, and their DNA is organised in linear strings compacted into the nucleus (e.g. plant and animals);

Prokaryotic cells do not have membrane-bound organelles and they carry a circular DNA free in the cytosol (e.g. bacteria and yeast).

CURIOSITY: On the base of the previously listed characteristics, viruses do not meet the required criteria to be named cells. In fact, despite of viruses having a protein shell enclosing nucleic acids, they are incapable of self-sustain and reproduce. They can do so, only through infecting a host cell that carries the necessary machinery to grow and expand.

Nowadays, the definition of cell appears clear and precise. Yet, scientists developed this concept through centuries. In my next post, I will present a summary of how the cell was first discovered and how cell biology became a discipline.