Cytology (generally known as cell biology) is the topic of this part of the Atlas, which focuses primarily on the organization of the cell. But what is a cell? The following may be a good definition: cells are the anatomical and functional units of living organisms. Cells may be alone or in groups to form multicellular organisms. A cell is the simplest molecular organization that is considered as alive. Three cell lineages are known to be present on Earth: archaea and bacteria, which are unicellular prokaryotes, and eukaryotes, which can be unicellular or form multicellular organisms. Prokaryotes (prior to the presence of a nucleus) usually do not have internal compartments surrounded by membranes, while eukaryotes (with a true nucleus) always contain internal membranous organelles. The nucleus is a characteristic compartment of eukaryotes.
Cells, whether prokaryote or eukaryote, are highly organized sets of molecules. In fact, cells have many internal compartments with specific functions. Let's say that a cellular compartment is a space, delimited or not by a membrane, where a necessary or important function for the cell is performed. One of the compartments found in every cell is the cell membrane, also known as plasmalemma or plasma membrane, which encloses all the other cell compartments. The plasma membrane is a semipermeable barrier that separates the inner cell space from the outer cell space.
Eukaryotic cells have internal compartments delimited by membranes. The nucleus is one of them. It is bounded by a double membrane and contains the genetic material known as DNA. DNA stores the information the cell needs to carry out survival and reproduction tasks. The space between the nucleus and the plasma membrane is filled with the cytosol, an aqueous gel that contains a wide variety of molecules involved in structural and metabolic functions, homeostasis, signaling, and many other requirements of cellular life. For example, the ribosomes are involved in protein synthesis, the cytoskeleton is necessary for internal cell organization and mobility, many enzymes and co-factors are in charge of metabolism. Between the cell membrane and the nucleus, surrounded by the cytosol, there are many organelles, membrane-bounded compartments, which accomplish functions such as digestion, respiration, photosynthesis, metabolism, intracellular transport, secretion, energy production, storage, etc. Mitochondria, chloroplasts, peroxisomes, lysosomes, endoplasmic reticulum, and vacuoles are some of these organelles. The cytoplasm is the cytosol plus all organelles, excluding the nucleus. See Figures 1 and 2.
Compared to eukaryote cells, prokaryote cells (bacteria and archaea) are generally described as lacking organelles. If organelles are defined as membrane-bound compartments, it is true that most prokaryote cells lack organelles. However, there are internal membrane-bound compartments in some prokaryote cells. The membranes of these compartments are usually continuous with the plasma membrane, that is, they are actually inward folds (invaginations) of the plasma membrane or are formed by detaching these invaginations from the plasma membrane. Four types of internal prokaryote membrane-bound compartments have been described: thylakoids, chlorosomes, magnetosomes and carboxysomes.
In the following pages we will take a tour through the different parts of the eukaryotic cell and its environment. Some aspects of cell function will not be addressed in depth here, such as gene expression or cell metabolism. It would take a large amount of space which would undermine the idea we want to give about the cell. Furthermore, there are many Internet sites dedicated to those subjects. The different "places" of the cell that we are going to "visit" are shown in the right side panel.