The cell is the fundamental structural and functional unit of all known living organisms, representing the smallest entity capable of carrying out all the essential processes of life. The study of cells, or cell biology, began with the invention of the microscope, which allowed scientists like Robert Hooke to first observe and name cells in the 17th century. Subsequent discoveries by numerous scientists culminated in the formulation of the Cell Theory, principally credited to Matthias Schleiden and Theodor Schwann. This foundational theory states that all living organisms are composed of cells and products of cells, and that all new cells arise from pre-existing cells, a concept later added by Rudolf Virchow.

All cells, despite their incredible diversity in shape and size, can be fundamentally categorized into two major types: prokaryotic and eukaryotic.

Prokaryotic cells are structurally simpler and evolutionarily older. They are characterized by the absence of a true, membrane-bound nucleus; instead, their genetic material, a single circular DNA molecule, is located in a region of the cytoplasm called the nucleoid. Prokaryotic cells also lack other membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. They possess ribosomes for protein synthesis, but these are of the 70S type. The cell is enclosed by a cell membrane, which is often surrounded by a rigid cell wall (except in mycoplasma) and, in some cases, an outer glycocalyx layer. Bacteria, cyanobacteria, and archaea are all examples of prokaryotic organisms.

Eukaryotic cells, on the other hand, are more complex and are found in protists, fungi, plants, and animals. Their defining feature is the presence of a true nucleus, which houses the cell’s genetic material (DNA) organized into linear chromosomes. The cytoplasm of a eukaryotic cell is highly compartmentalized, containing numerous membrane-bound organelles that perform specific functions, allowing for a greater degree of specialization and efficiency.

A detailed examination of a eukaryotic cell reveals a complex and highly organized internal structure. The cell is enveloped by a selectively permeable cell membrane (or plasma membrane), which regulates the passage of substances into and out of the cell. In plant and fungal cells, this membrane is further enclosed by a rigid cell wall, which provides structural support and protection.

The internal environment of the cell is the cytoplasm, a jelly-like substance that contains all the organelles. The nucleus acts as the cell’s control center, directing cellular activities and containing the blueprint for all cellular proteins. It is enclosed by a double-membraned nuclear envelope.

Many organelles function as part of a coordinated endomembrane system. The endoplasmic reticulum (ER) is a network of membranes that exists in two forms: the rough ER, studded with ribosomes and involved in protein synthesis and modification, and the smooth ER, which is involved in lipid synthesis and detoxification. The Golgi apparatus (or Golgi complex) receives proteins and lipids from the ER, modifies, sorts, and packages them into vesicles for transport to other destinations. Lysosomes are small vesicles containing digestive enzymes that break down waste materials and cellular debris. Vacuoles are large, membrane-bound sacs, particularly prominent in plant cells, that are involved in storage, waste disposal, and maintaining turgor pressure.

Energy conversion is handled by two key organelles. Mitochondria, often called the “powerhouses” of the cell, are the sites of cellular respiration, where glucose is broken down to produce ATP, the main energy currency of the cell. In plant cells and some protists, plastids are present. The most important of these are chloroplasts, which contain chlorophyll and are the site of photosynthesis, the process of converting light energy into chemical energy.

Ribosomes are non-membranous structures responsible for protein synthesis and are found in both prokaryotic and eukaryotic cells (though they are larger, 80S type, in eukaryotes). They can be found free in the cytoplasm or attached to the rough ER.

The structural framework and internal organization of the cell are maintained by the cytoskeleton, a complex network of protein filaments including microtubules, microfilaments, and intermediate filaments. This network provides mechanical support, enables cell movement, and facilitates the transport of organelles within the cell. In many cells, motility is achieved through specialized structures like cilia and flagella. Finally, in animal cells, the centrosome, containing a pair of centrioles, plays a crucial role in organizing microtubules and is vital for cell division.