A cell is the most fundamental unit of biological life. All known life, except for viruses, is made up of cells. Cells are also the smallest metabolically functional unit of life, meaning the smallest unit that can take in nutrients from the bloodstream, convert them into energy, perform useful functions, and excrete waste. There are two primary types of cells in the kingdom of life - prokaryotic cells, smaller bacterial cells without a nucleus, and eukaryotic cells, larger plant and animal cells with a true nucleus.
Cells are quite small. Prokaryotic cells are typically 1-10 µm (micrometers, or millions of a meter) across, while eukaryotic cells are 10-100 µm. Eggs are large single cells, and the largest known cell today is the egg of the ostrich, although prehistoric birds and some dinosaurs had eggs almost a foot in length. Every cell is produced from another cell, and each contains special genetic programming to manufacture proteins to replace things when they break down, divide, and perform the functions of life.
An aggregation of cells is known as a multicellular organism, humans being one example. These cells are so tiny and numerous, and work together so smoothly and uniformly that it took until 1839 for us to figure out that all life is made of cells. This "cell theory" is attributed to Matthias Jakob Schleiden and Theodor Schwann, German botanists who observed cells under a microscope. Soon after, Robert Hooke, the English scientist, named these little structures cells, after the Latin cellula, meaning a small room.
Another difference between prokaryotic and eukaryotic cells is the presence of intracellular machinery, or organelles. Prokaryotic organelles are quite minimal, with a plasma membrane (phospholipid bilayer) that does most of the work done by specialized organelles in eukaryotes, such as serving as the power plant of the cell and packaging macromolecules synthesized by the ribosomes. Aside from the ribosomes, cytoplasm (cell fluid), and the plasma membrane, prokaryotic cells may have another additional organelle called the mesosomes, but recent research suggests that these may merely be artifacts formed during the process of chemical fixation for electron microscopy and thus not even natural.
For some organelles in the more complex eukaryotic cells, see the article "What are some Organelles in the Cell?"
Frequently Asked Questions
What is a cell?
The smallest unit of life is a cell. Of all living things, it is the smallest structural and functional unit. Cells perform various essential functions, such as metabolism, reproduction, and communication with other cells.
What are the types of cells?
Prokaryotic cells and eukaryotic cells are the two primary categories of cells. Unlike eukaryotic cells, which have a more complicated structure with a nucleus, prokaryotic cells are smaller and lack a nucleus.
What is the function of the cell membrane?
A thin covering that encircles each cell and divides its internal environment from its external environment is known as the cell membrane. It controls the flow of chemicals into and out of the cell, offering protection and preserving the shape of the cell.
What is the role of the nucleus?
The nucleus is the control center of the cell. It contains the cell's genetic material, including DNA and RNA. Gene expression, cell division, and the creation of proteins necessary for cellular function are all controlled by the nucleus.
What is the mitochondria's function?
The heart of the cell's power is its mitochondria. They carry out a process known as cellular respiration to produce energy in the form of ATP. Mitochondria are crucial for numerous cellular functions, including cell division, proliferation, differentiation, and energy metabolism.
What are the differences between plant and animal cells?
Animal cells do not have a cell wall, chloroplasts, or a large central vacuole like in plants. Animal cells are shaped erratically, but plant cells are uniformly shaped. Plant cells perform photosynthesis, while animal cells do not.
What is the cytoplasm?
A gel-like material called cytoplasm fills the cell between the nucleus and the cell membrane. It is the location of numerous cellular functions, including metabolism and protein synthesis, and contains a variety of organelles, such as mitochondria, ribosomes, and the endoplasmic reticulum.