The basic structural and functional unit of life is the cell, a highly complex and organized, membrane-enclosed compartment filled with aqueous solution and functional structures. A cell can take up substances from its surroundings to make its own compositions or for energy supporting its activities. Cells can respond to stimuli and interact with their environment or communicate with each other. A cell can reproduce itself by dividing into two, when the genetic materials are transmitted. Cells may vary in size, shape, composition and behavior as they are designated to different roles or exposed to different environment.

Organisms could be either unicellular or multicellular. Unicellular organisms are composed of single cells, while multicellular organisms are composed of many cells that play different but cooperated roles. There are about 200 types of cells and about 10¹⁴ cells in an adult human body. Different types of cells are well organized together and work coordinately. A specialized group of cells join together to form a tissue. The tissues are the building blocks of the organs.

Cell biology is a subfield of biology that studies the structures and functions of the cell. Cell biology aims to discover how cells function (e.g., grow, proliferate, differentiate, communicate with the environment and each other, and transmit heredity information) in atomic, molecular, macromolecular, organelle and cellular scale.

Viruses were/are not considered as life by many biologists because they are not cellular structures and cannot replicate on their own. However, as the study of giant viruses suggests, viruses may be evolved from an ancient cellular ancestry. Thus, viruses are considered to be to non-cellular lives. A viroid consists solely of single-stranded circular RNA that does not encode any protein. All viroids found up to now infect plants and multiply themselves. Viroids are not life but may shed some light on the beginning of life evolution. A prion is the infectious protein isoform that can convert its normal form to the infectious isoform by conformational change.

In 1665, Robert Hooke observed cells in a piece of cork under his primitive microscope. In fact, he only saw the empty cavities of the cell wall, which seemed to be the empty houses left by their residents.

In 1674, Antonie van Leeuwenhoek first observed a living cell, a protist, and he called it animalcule.

From 1838 to 1839, German botanist Matthias Jakob Schleiden and zoologist Theodor Schwann first put forward the cell theory, which states that all living beings are composed of one or more cells and cells are the basic units of living beings.

In 1858, pathologist Rudolf Virchow added the third tenet to cell theory: All cells arise only from pre-existing cells. In addition, he also stated that all pathological phenomena of the body are based on cellular damage. The cell theory tells us that cells are not only the functional units of the human body but also the structural units of disease pathology. The essence of disease treatment is dealing with cells.

In 1931, Ernst Ruska, a German physicist, designed the first electron microscope (EM) with a magnifacation of about 17.4× and in 1933 he completed a new EM with a magnifacation of 12000×¹⁾. Now, A EM may have a magnification of up to 1000000× and a resolving power of 3Å (1Å = 0.1 nm) ²⁾. With electron microscope, cell biology reached subcellular and molecular level.

In 1953, James Watson and Francis Crick discovered the double helix structure of DNA and revealed the relationship between the structure and function of DNA in molecular level. Since 1970s, more attention has been paid to life activities in molecular level, and cell biology has been developed further into molecular cell biology.

Cells are divided into prokaryotic cells and eukaryotic cells based on whether they have a distinct membrane-enveloped structure called nucleus.

Typical prokaryotic cells have a diameter of about 1µm and make up organisms individually. Prokaryotic cells lack a membrane-bound nucleus and its naked DNA is not packaged with proteins. Prokaryotic cells do not have membrane-enclosed organelles. The prokaryotes are divided into two distinct domains, bacteria and archaea. Archaea was once misnamed as “archaebacteria” and thus “eubacteria” was once used for bacteria.

Bacteria are typically about 0.5-5.0 µm in diameter. Most bacterial species are spherical, rod-shaped or spiral-shaped. A spherical bacterium, such as Streptococcus pneumoniae, is called a coccus (plural: cocci). A rod-shaped bacterium, such as Shigella dysenteriae and Escherichia coli, is called a bacillus (plural: bacilli). A spiral-shaped bacterium, such as Spirillum minus, is called a spirillum (plural: spirilla). Bacteria may have some common cellular structures: cell wall that is mainly composed of peptidoglycan, capsule that lies outside the cell wall and is mainly composed of polysaccharide and polypeptide, cell membrane that is mainly composed with lipid bilayer and proteins, Nucleoid that is made of cyclic duplex DNA without repeated sequences and introns, self-replicable circular plasmids independent of the Nucleoid, Cytoplasm with 70S ribosomes that consist of larger 50S subunit and smaller 30S subunit, Flagella and pili as the locomotive organ.

Mycoplasma is an atypical genus of bacteria that does not have cell wall. It is the smallest cell that can live independently. They are parasitic and about 0.1 to 0.3µm in diameter. It can synthesize 700 different proteins. Several mycoplasma species are pathogenic for human and may cause inflammations.

Archaea are generally similar to bacteria in size and shape, however, they may inhabit the most extreme environment on the earth. However, the archaeal transcription and translation processes are similar to that of eukaryotic ones, while the archaeal metabolic and structural proteins are similar to bacterial ones.

Eukaryotic cells are complex in structure and highly diverse in morphology. They are 10 to 100μm in diameter. The defining feature of eukaryotic cells is that they have a membrane-enveloped “true” nucleus. In the cytoplasm, there are membranous organelles and non-membranous organelles besides ribosome. All animal, plant, fungi, and protist cells are eukaryotic.

Membranous structures are the organelles made of or surrounded by biological membrane, including cell membrane and cellular structures such as nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosome, and peroxisome.

Non-membranous structures refer to cellular structures without membrane surrounding, such as ribosome, centrosome, chromatin, chromosome, nucleolus, nuclear lamina, nucleoskeleton, microtubule, microfilament, intermediate filament, nuclear matrix, and cytoplasm.

Biological membrane or biomembrane is the general term for cell membrane and intracellular membranes.

Cell biology has a close relationship with medicine. Biologist Edmund Beecher Wilson once declared that “the key to every biological problem must finally be sought in the cell”. Defects in cell function may cause diseases, such as cancer.

The key feature of cancer cells is malignant proliferation and the ability to invade other tissues. Cancer cells have some totipotency that they can differentiate into different cell types that cooperate to maximize the growth of the cancer. Taking the body's nutrition for a malignant growth, producing toxins and destroying the normal tissue nearby, the tumor will eventually drain the body. Tumorigenesis normally rise from activation of the oncogenes or loss function of anti-oncogenes.

Apoptosis is a process of programmed cell death (PCD) that may occur in multicellular organisms. The process starts when apoptosis genes are activated and the protein enzymes are synthesized to destroy the cellular structure. These changes include endonuclease cutting of DNA, organelle concentrating, cell membrane shrinking and nuclear fragmenting into apoptosis body. The apoptotic cell was finally ingested by phagocyte. Keeping apoptosis in balance is essential for maintaining the homeostasis of the body. Less apoptosis may incur tumor. For example, obstacle apoptosis of the mature white cell, which should live for a few days only, may cause leukemia. Meanwhile, too much cell apoptosis may trigger senile dementia. Some degenerative diseases of nervous system, such as Parkinson disease and Alzheimer disease, has also have close relation with neuronal apoptosis.

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