Cell (biology)

• 1. Introduction

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  The cell takes its name from a Latin word, cellula, meaning small room. That borrowed term hides one of the largest claims in all of science. The cell is the basic structural and functional unit of every form of life. Most are too small to see without a microscope, yet each one carries the genetic material that makes life possible. They emerged on Earth about four billion years ago. A semipermeable membrane wraps around cytoplasm, and inside that cytoplasm sits the information a cell needs to copy itself and build proteins. Some cells can move. Most can replicate. A few, like red blood cells and gametes, are so specialized they give up that ability. From a thin slice of cork to thirty trillion cells in a single human body, the question that follows is simple to ask and hard to answer. How did something so small come to organize all of life, and how do its parts actually work?

• 2. Two Kingdoms Of Cells

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  Prokaryotes lack a membrane-bound nucleus and keep their DNA in a region called the nucleoid instead. This single difference splits all life into two great groups. Eukaryotes enclose their nucleus inside a nuclear membrane, and the word eukaryote means true nut or true kernel, where nut refers to that nucleus. Prokaryotes are always single-celled and include the archaea and bacteria. Eukaryotes can be single-celled or multicellular, covering protists, plants, animals, most fungi, and some algae. A eukaryotic cell can be 2 to 100 times larger in diameter than a typical prokaryotic one. Beyond the nucleus, eukaryotic cells carry membrane-bound organelles that prokaryotes mostly do without. Mitochondria supply energy for cell functions. In plants, chloroplasts build sugars through photosynthesis. The archaea hold a further surprise: their cell membranes are built from ether-linked lipids, unlike the ester-linked lipids of bacteria and eukaryotes. Archaea are also noted for extremophiles that thrive in extreme heat, cold, acid, alkali, or high salt, and there are no known archaean pathogens.

• 3. Inside The Prokaryote

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  Most prokaryotes range from 0.1 to 5.0 micrometers in diameter, making them the smallest of all organisms. The largest bacterium known, Thiomargarita magnifica, breaks that rule and is visible to the naked eye. Bacteria sit inside a cell envelope that filters the interior from the exterior. A plasma membrane is wrapped in a cell wall of peptidoglycan, and in some bacteria a third gelatinous layer called a bacterial capsule. That capsule may be polysaccharide as in pneumococci and meningococci, polypeptide as in Bacillus anthracis, or hyaluronic acid as in streptococci. The wall stops the cell expanding and bursting from osmotic pressure in a hypotonic environment. A bacterium's DNA usually forms a single circular chromosome in direct contact with the cytoplasm. Extra genes ride on plasmids, including those for antibiotic resistance. Linear plasmids appear in spirochete bacteria such as Borrelia, which causes Lyme disease. Certain membrane-bound prokaryotic organelles do exist: the magnetosome of magnetotactic bacteria, and the anammoxosome of anammox bacteria. Cell-surface appendages like flagella and pili drive movement and communication, with fimbriae made of a protein called pilin that fastens bacteria to receptors on host cells.

• 4. The Eukaryotic Interior

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  The nucleus is the largest membrane-bound organelle in the eukaryotic cell, and it is where almost all DNA replication and RNA synthesis occur. A double-membraned nuclear envelope separates it from the cytoplasm, protecting the cell's DNA from molecules that could damage it. Inside, DNA is transcribed into messenger RNA, which travels out of the nucleus to be translated into protein. In humans, the nuclear genome is divided into 46 linear chromosomes, including 22 homologous pairs and a pair of sex chromosomes. The endomembrane system ties many of these compartments together, covering the cell membrane and the membranes of every organelle except the mitochondria. Mitochondria stand apart for a reason. They are descended from bacteria that formed an endosymbiotic relationship with ancient prokaryotes. They carry their own DNA in small circular chromosomes, coding for 13 proteins involved in energy production, and even their own ribosomes, called mitoribosomes. Other organelles each do one job. A lysosome is the most acidic compartment, holding over 60 hydrolytic enzymes that digest worn-out parts and engulfed invaders. Vaults remain a puzzle: most human cells hold around 10,000 of them, with up to 100,000 in some immune cells, yet their functions are purely speculative.

• 5. Walls, Plastids And Hyphae

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  Plant cells build walls from cellulose, hemicelluloses, and pectin outside the cell membrane. In sclerenchyma tissue, lignin forms a secondary wall, while cutin on outer surfaces creates the plant cuticle on leaves and stems. Pores called plasmodesmata pass through the primary wall, letting the membrane and endoplasmic reticulum of neighboring cells run continuous. Chloroplasts capture the sun's energy to make carbohydrates, and large water-storage vacuoles dwarf those of animal cells. Peroxisomes in germinating seeds even earn a second name, the glyoxysome, for running the glyoxylate cycle that turns fatty acids into sugars. Algae are photoautotrophs that produce energy by photosynthesis, and alginate from the walls of brown algae finds uses in the food industry and pharmacology. Fungal cells take a different path. Their walls are made of a unique chitin-glucan complex, and chloroplasts are absent, so their pigments sit in the cell walls instead. Fungi also carry a spitzenkörper, a phase-dark body of membrane-bound vesicles that drives hyphal tip growth as it moves forward. Protist cells, by contrast, may wear a pellicle in ciliates, a test in testate amoebae, or a frustule in diatoms, and ciliates keep two kinds of nucleus, a generative micronucleus and a vegetative macronucleus.

• 6. How Cells Live And Divide

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  During cell division, a mother cell splits into two daughter cells, growing the tissue of multicellular organisms. Prokaryotic cells divide by binary fission. Eukaryotic cells usually undergo mitosis followed by cytokinesis. A diploid cell may instead undergo meiosis to produce haploid cells, usually four, which serve as gametes that fuse into new diploid cells. DNA replication happens during the S phase, and in meiosis the DNA copies only once while the cell divides twice. Cell signaling carries the conversation between a cell, its neighbors, and its environment, built from a first messenger, a receptor, and the signal itself. Errors in that signaling can cause cancer, autoimmunity, and diabetes. Metabolism keeps the cell alive between divisions, splitting into catabolism that breaks molecules down for energy and anabolism that builds them back up. Glucose is broken down to make adenosine triphosphate, the cell's ready energy. When a cell stops working through ageing or injury, death follows, and programmed routes like apoptosis and autophagy replace dead cells with new ones. In August 2020, scientists described how slime mold cells and mouse pancreatic cancer-derived cells navigate mazes by sensing junctions before reaching them, even around corners.

• 7. From Vents To Many Cells

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  The origin of cells is bound up with the origin of life itself. Small molecules needed for life may have arrived on meteorites, formed at deep-sea hydrothermal vents, or been synthesized by lightning in a reducing atmosphere. RNA may have been the earliest self-replicating molecule, since it can both store information and catalyze reactions. Early cell membranes were probably simpler and more permeable, carrying only a single fatty acid chain per lipid, and such lipids spontaneously form bilayered vesicles in water. The first cells were most likely heterotrophs. Eukaryotic cells arose some 2.2 billion years ago in a process called eukaryogenesis, widely agreed to involve symbiogenesis, where an archaean and a bacterium came together to form the first eukaryotic common ancestor. Green plants appeared around 1.6 billion years ago, when a second episode of symbiogenesis added chloroplasts derived from cyanobacteria. The earliest evidence of multicellularity comes from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago, with cell differentiation such as heterocysts for nitrogen fixation. The extracellular matrix made multicellular life possible, and its emergence coincided with the origin of multicellularity. Researchers have even replicated the evolution of multicellularity in the laboratory, using predation as the selective pressure.

• 8. Seeing The Invisible

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  In 1665, Robert Hooke examined a thin slice of cork under his microscope and saw a structure of small enclosures. He wrote that he could plainly perceive it to be all perforated and porous, much like a honeycomb, but that the pores were not regular. He coined the term cell because the spaces resembled the cells of a monastery. Between 1632 and 1723, Antonie van Leeuwenhoek taught himself to make lenses, built basic optical microscopes, and drew protozoa such as Vorticella from rainwater and bacteria from his own mouth. In 1839, Theodor Schwann and Matthias Jakob Schleiden established that plants and animals are made of cells, founding cell theory. In 1855, Rudolf Virchow added that new cells come from pre-existing cells by division, captured in the phrase omnis cellula ex cellula. In 1931, Ernst Ruska built the first transmission electron microscope at the University of Berlin, and by 1935 he had an instrument with twice the resolution of a light microscope, revealing organelles no one had been able to resolve. In 1981, Lynn Margulis published Symbiosis in Cell Evolution, detailing how eukaryotic cells were created by symbiogenesis, the same idea that explains why mitochondria still carry their own bacterial DNA today.