A cell is the smallest structural and functional unit of living organisms. It is a small compartment wrapped in a membrane. Inside, it keeps its own genetic material and the molecular machinery needed to turn energy into work, build its parts and make copies of itself. All living organisms are composed of one or more cells. Some organisms are a single free-living cell, such as most microorganisms. Others, such as plants and animals, are made of trillions of cells that have taken on specialized jobs. The study of cells, called cell biology or cytology, is a central part of biology. It overlaps heavily with biochemistry, genetics and molecular biology.
History of discovery
The cell was first seen because of the microscope. In 1665 the English natural philosopher Robert Hooke looked at thin slices of cork and saw tiny empty compartments. He named them "cells," from the Latin for "small room," because they reminded him of the cells in a honeycomb. In the 1670s Antonie van Leeuwenhoek used single-lens microscopes he built himself to describe living single-celled organisms, including bacteria and protists.
In the nineteenth century these observations were brought together as cell theory. In 1838–1839 the botanist Matthias Schleiden and the zoologist Theodor Schwann argued that plants and animals are both built from cells. In 1855 Rudolf Virchow added the principle omnis cellula e cellula, meaning that every cell comes from an existing cell. Cell theory is usually stated in three parts: all living organisms are composed of cells; the cell is the basic unit of life; and all cells arise from pre-existing cells. In the twentieth century, electron microscopy, cell fractionation and molecular techniques showed the cell's internal structure at the scale of individual molecules.
Basic structure
Every cell has a few features in common. The cell membrane (plasma membrane) is a lipid bilayer studded with proteins. It separates the molecular machinery of life from the outside environment, while allowing for transport of nutrients, wastes and ions. Inside is the cytoplasm, a crowded water-based solution of enzymes, metabolites and structural filaments. Genetic information is stored in DNA. Its genes are transcribed into RNA and then translated into proteins by ribosomes, which every cell has. Many cells also have a rigid cell wall outside the membrane. In plants this wall is made mostly of cellulose. In fungi it is made of chitin, and in bacteria of peptidoglycan.
Prokaryotic and eukaryotic cells
Cells come in two broad organizational types. Prokaryotic cells include the domains Bacteria and Archaea, while eukaryotic cells belong to Eukarya. Prokaryotes have no nucleus. Their DNA is present in a non-membrane-bound region called the nucleoid, and they lack membrane-bound organelles. They are usually a few micrometres across. Their ribosomes are the smaller 70S type, and they reproduce mainly by binary fission. Bacteria and archaea look similar under the microscope, but they differ deeply in their membrane chemistry and molecular machinery. In 1977 Carl Woese used ribosomal RNA sequences to show that they are separate lineages.
Eukaryotic cells are typically 10–100 micrometres across. They make up animals, plants, fungi and protists. Their DNA is held in a nucleus surrounded by a double membrane. Inside the cell, internal membranes divide the space into organelles. The endoplasmic reticulum and Golgi apparatus make, modify and sort proteins and lipids. Lysosomes and vacuoles break down materials or store them. Mitochondria carry out most of the cell's aerobic cellular respiration, producing ATP. In plants and algae, chloroplasts carry out photosynthesis. A cytoskeleton made of actin filaments, microtubules and intermediate filaments gives the cell its shape, moves cargo around inside it and drives cell movement.
Origin and evolution
All living cells are thought to come from a single population of early cells, often called the last universal common ancestor. Fossil and geochemical evidence suggests that microbial life was present on Earth more than 3.5 billion years ago. Eukaryotic cells appeared much later. According to the endosymbiotic theory, developed mainly by Lynn Margulis, mitochondria and chloroplasts, major organelles of eukaryotic cells, are descendants of once free-living bacterial species. Mitochondria trace back to an alphaproteobacterium and chloroplasts to a cyanobacterium. The supporting evidence includes the organelles' own circular genomes, their double membranes, and the way they divide much like bacteria do. Margulis's work was at first met with skepticism, but now the endosymbiotic theory is widely accepted in the scientific community. The rise of eukaryotic cells eventually made several independent origins of multicellularity possible, and with them much of the complexity produced by evolution.
Cell division and the cell cycle
Cells multiply by cell division. In eukaryotes, the cell cycle goes through growth phases, DNA replication (S phase) and mitosis, in which the copied chromosomes are split evenly between two daughter cells. Meiosis is a specialized type of division that produces gametes with half the chromosome number and shuffles genetic variation. Checkpoint proteins control progress through the cycle. When this control fails, cells can divide without limit, which is a defining feature of cancer. Cells can also destroy themselves in an orderly way through programmed cell death (apoptosis), which helps shape tissues during development.
Cells in multicellular organisms
In multicellular organisms, cells become specialized (differentiate) and are organized into tissues and organs. A 2013 estimate puts the number of human cells in an average adult body at about 37.2 trillion. Most of these are red blood cells, which make up over 80 percent of the body's cells by number but only around 4 percent of its mass. Scientists commonly estimate that there are around 200 human cell types, from neurons in the brain to cells of the immune system. All of them develop from one fertilized egg. Stem cells keep the ability to renew themselves and to give rise to specialized cells, and they are important for development and tissue repair. Cells communicate through chemical signals, electrical signals and direct contact. This lets the whole organism maintain homeostasis.
Boundaries of the concept
Viruses are not usually counted as cells. They have no metabolism of their own and can replicate only inside a host cell. Cell size varies enormously. The smallest bacteria, such as mycoplasmas, are a few hundred nanometres across. Some neurons, by contrast, extend fibres longer than a metre. Large international projects such as the Human Cell Atlas now use single-cell RNA sequencing to catalogue cell types and states in whole organisms.
References
- 1.2: Cellular Organization - Prokaryotic and Eukaryotic Cells - Biology LibreTextsbio.libretexts.org
- Difference between Prokaryotic and Eukaryotic Cellsbyjus.com
- Prokaryotic & Eukaryotic Cells Quiz #7 Flashcardspearson.com
- Prokaryotic vs Eukaryotic Cells - Similarities and Differencessciencenotes.org
- Prokaryotic & Eukaryotic Cells Explained: Definition, Examples, Practice & Video Lessonspearson.com
- Prokaryotic and Eukaryotic Cells - Types, Examples and Definitionflexbooks.ck12.org
- Lynn Margulisbritannica.com
- Endosymbiosis theory (video)khanacademy.org
- How many cells are in the human body?medicalnewstoday.com
- How Many Cells Are in the Human Body? |biologydictionary.net
- The human body contains some 37 trillion cells. Scientists are working to map every one of them - MaRS Discovery Districtmarsdd.com