Prokaryotic and eukaryotic cells: differences, scheme and summary of biology. Characteristics, function and structure.
The cell
The part of biology that studies the cell is called cytology. It began in 1667 with the first microscopic observations, carried out by Robert Hooke.
In the 19th century, the cell theory was developed , which was based on three hypotheses:
- the cell is the smallest living unit capable of independent life;
- each cell is born from another cell, reproduces and dies;
- All organisms are made up of cells.
The size of the cell is extremely small: thousandths of a millimeter (um). Based on the structural complexity there are two types of cells: the prokaryotic cell and the eukaryotic cell .
The prokaryotic cell
The prokaryotic cell is typical of microscopic single-celled organisms, such as bacteria. The prokaryotic cell is made up of:
- from a rigid wall that separates it from the external environment;
- from a cell membrane that provides for the exchange of substances with the outside;
- from a cytoplasm, a gelatinous substance that occupies the inside of the cell, which contains very small granules: ribosomes.
All chemical reactions occur in the cytoplasm or at the membrane level. The prokaryotic cell does not have a nucleus and the DNA is in the form of a single, large, circular molecule that occupies an irregularly shaped area in the cytoplasm.
The eukaryotic cell
The eukaryotic cell is the fundamental unit of the vast majority of living things, such as animals, plants, and bacteria. It is ten times larger than a prokaryotic cell.
It is surrounded by a cell membrane or plasma membrane, which separates the cytoplasm from the external environment. In the cytoplasm are the organelles or cellular organelles, relatively isolated compartments where specific chemical reactions take place.
The cellular organelles are:
- ribosomes, where protein synthesis occurs;
- the endoplasmic reticulum, where lipids are synthesized and some proteins are processed;
- the Golgi complex, a center for the modification and sorting of certain substances;
- mitochondria, the cell’s powerhouse, where respiration takes place;
- lysosomes, where waste products are broken down.
The plastic membrane
All eukaryotic cells are surrounded by a membrane called the plasma or cell membrane , which separates the internal space of the cell from the external environment.
This structure has a thickness between 0.05 um and 0.01 um, regulates the exchange of substances with the external environment and allows chemical reactions to occur relatively independently. The membrane is therefore essential for the homeostasis of the cell.
The plasma membrane is made up of phospholipids and proteins . It is a film made up of a double layer of phospholipids to which some proteins are added.
Proteins can pass completely through the bilayer (intrinsic proteins), or protrude from the inside or outside of the membrane (extrinsic proteins).
According to this model, called the fluid mosaic model, the membrane is neither rigid nor non-deformable, but has a certain fluidity.
Cells need a continuous exchange of substances with the environment around them, from which they take the molecules necessary for metabolic activities and into which they release waste products from chemical reactions.
The plasma membrane allows the passage of only some substances and is defined as selectively permeable.
Small molecules cross the membrane thanks to diffusion, a passive phenomenon that consists in the movement of molecules from an area where they are more concentrated to an area where their concentration is lower.
When there is a difference in concentration between the external environment and the cytoplasm, an equilibrium between the two solutions can be achieved by the movement of water. The difference in concentration on the two surfaces of the membrane determines a pressure, called osmotic pressure.
As a result of this force, some water molecules move from the more dilute solution, where they are in greater quantity, to the more concentrated solution, where they are in less quantity. This process is called osmosis.
