Cell (biology)

The cell is the basic structural and functional unit of all forms of life or organisms. The term comes from the Latin word cellula meaning 'small room'. A biological cell basically consists of a semipermeable cell membrane enclosing cytoplasm that contains genetic material. Most cells are only visible under a microscope.

All organisms are grouped into prokaryotes, and eukaryotes. Prokaryotes are single-celled and include archaea and bacteria. Eukaryotes can be single-celled or multicellular, and include protists, plants, animals, most species of fungi, and some species of algae.

Cells were discovered by Robert Hooke in 1665, who named them after their resemblance to cells in a monastery. Cell theory, developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all organisms, and that all cells come from pre-existing cells.

Types

Organisms are grouped into two main types: eukaryotes and prokaryotes. Eukaryotic cells have a membrane-bound nucleus, while prokaryotic cells do not have a nucleus but have a nucleoid region instead. Prokaryotes are always single-celled, such as bacteria and archaea, while eukaryotes can be single-celled, like microalgae including diatoms, or multicellular, like animals, plants, and most fungi.

Prokaryotes, including bacteria and archaea, were likely the first life forms on Earth. They are simpler and smaller than eukaryotic cells, lacking a nucleus and most membrane-bound organelles. Eukaryotic cells are larger and more complex, containing a nucleus and various organelles such as mitochondria and, in plants, chloroplasts. These cells can form multicellular organisms made up of many different types of cells.

Animal cells

Further information: Animal embryonic development and Cell types

All cells in an animal body come from one special cell called a zygote. As the animal grows, these cells change and form different parts like tissues and organs. Some animals have just two layers of cells, while more advanced animals, including vertebrates, have three layers.

Animal cells have a cell membrane that holds everything inside. Inside, there is a gel-like substance called cytoplasm that contains important parts like the nucleus, which holds the cell's DNA, and mitochondria that give the cell energy. The cell membrane helps control what goes in and out of the cell.

The cytoplasm has a network of fibers called the cytoskeleton that helps the cell keep its shape and move things around. There are many small parts inside the cell called organelles, each with its own job. For example, lysosomes help clean up waste, and the Golgi apparatus packages things the cell makes to send out or use elsewhere.

Plant cells

Main article: Plant cell

Plant cells have special outer walls made of materials like cellulose that give them shape and help them connect with other cells. These cells also contain important parts called organelles, such as chloroplasts that capture sunlight to make food for the plant through a process called photosynthesis. Vacuoles in plant cells are bigger than in animal cells and help store water and nutrients.

Plant cells use tiny structures called microtubules to help them move and organize their contents. They also produce hormones that act as signals to control growth and protect the plant from harm.

Algal cells

Further information: Eukaryotic algae

Algae are special living things that can make their own food using a process called photosynthesis. They do this with help from tiny parts inside their cells called chloroplasts, which contain something called plastids. Some of these algae are known as red algae.

Algae also have a substance called alginate in their cell walls. This alginate is very useful and is used in food and medicine products. Brown algae, for example, are a type of algae that contains this alginate.

Fungal cells

Main article: Fungus

The cells of fungi have special parts that help them grow. One of these parts is called a spitzenkörper, which helps the tips of fungal threads grow by moving forward.

Fungal cell walls are made of a unique mix called a chitin-glucan complex.

Protist cells

Further information: Protist § Common types

The cells of protists can have just a cell membrane, or they might also have a cell wall, a pellicle (in ciliates), a test (in testate amoebae), or a frustule (in diatoms). Some protists, like amoebae, can eat other tiny creatures by pulling them inside using a process called phagocytosis. Others make their own food using sunlight through a process called photosynthesis. Many single-celled protists can move around using structures like cilia, flagella, or pseudopodia.

Ciliates are special because they have two kinds of nuclei: a small diploid micronucleus that helps them reproduce, and a large ampliploid macronucleus that helps manage everyday cell activities.

Cellular processes

See also: Cell cycle and Cell physiology

Replication

Main article: Cell division

During cell division, a single cell, the mother cell, divides into two daughter cells. This leads to the growth of tissue in multicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis.

Signaling

Main article: Cell signaling

Cell signaling is how a cell talks to itself, other cells, and its surroundings. It usually involves three parts: a message, a receiver, and the signal itself. Most cell signaling is chemical and can happen with nearby cells or faraway ones. Signal receivers are special proteins that help the cell respond to messages.

Protein targeting

Protein targeting is how proteins find their right places inside or outside the cell. Proteins can go to different parts of the cell, such as organelles, membranes, or outside the cell through secretion. This process is important for the cell to work properly.

DNA repair

Main article: DNA repair

All cells have ways to fix DNA damage. Fixing DNA is important to keep the cell working and to stop mistakes that could cause problems. There are many ways cells can fix DNA, and these processes help keep the cell healthy.

Growth and metabolism

Main articles: Cell growth, Metabolism, and Photosynthesis

Between dividing, cells grow by using energy from food. This is called metabolism. It has two parts: breaking down food to get energy and using that energy to build new things. In plants, special parts called chloroplasts make food using sunlight in a process called photosynthesis.

Protein synthesis

Main article: Protein biosynthesis

Cells can make new proteins from small building blocks called amino acids. This process has two steps: making a copy of the DNA instructions as RNA, and then using that RNA to build the protein. This helps the cell do its many jobs.

Motility

Main article: Motility

Some cells can move to find food or avoid danger. Ways cells move include using flagella, cilia, or changing shape like amoeboid movement. In animals, cells can move to heal wounds or fight infections.

Cell death

Main article: Cell death

Cell death happens when a cell stops working, either because it is old or because it is hurt. Sometimes cells die in a planned way to make space for new cells. This is a natural part of keeping the body healthy.

Origins

Main article: History of life

Further information: Abiogenesis and Evolution of cells

The origin of cells is connected to the beginning of life on Earth. Small molecules important for life might have arrived on meteorites, formed near deep-sea vents, or been created by lightning. RNA may have been one of the first molecules able to copy itself and help with chemical reactions.

Cells first appeared around 4 billion years ago. The earliest cells were likely heterotrophs, meaning they got their energy by consuming other organisms. Eukaryotic cells, which have a nucleus and other complex parts, formed about 2.2 billion years ago through a process where two different types of simple cells merged. These eukaryotic cells later evolved into many types of organisms, including the ancestors of animals, fungi, and plants. Green plants appeared around 1.6 billion years ago when a special cell gained structures called chloroplasts.

Multicellularity, where many cells work together, began with simple microbes forming groups. The first signs of this happened over 3 billion years ago with cyanobacteria, which showed early forms of working together. The development of materials outside cells helped this process, allowing cells to stick together and form more complex living things.

History of research

Main article: Cell theory § Discovery of cells

In 1665, Robert Hooke looked at a piece of cork under a microscope and saw tiny boxes, which he called "cells" because they looked like small rooms. Later, scientists like Matthias Schleiden and Theodor Schwann studied cells in both plants and animals and found that cells are the basic building blocks of all living things.

Important milestones include Antonie van Leeuwenhoek making his own lenses to see tiny creatures in water and Rudolf Virchow discovering that new cells come from existing cells. In 1931, Ernst Ruska created a powerful microscope that showed even more details inside cells.