Cell (biology)

The cell is the basic building block of all forms of life or organisms. The word comes from the Latin word cellula, meaning 'small room'. A cell is made up of a semipermeable cell membrane that holds cytoplasm with genetic material inside. Most cells can only be seen with a microscope.

All living things are either prokaryotes or eukaryotes. Prokaryotes are single-celled and include archaea and bacteria. Eukaryotes can be single-celled or multicellular. They include protists, plants, animals, most fungi, and some algae.

Cells were first seen by Robert Hooke in 1665. He named them because they looked like rooms in a monastery. Cell theory was created in 1839 by Matthias Jakob Schleiden and Theodor Schwann. This theory tells us that all living things are made of one or more cells, that cells are the basic unit of life, and that new cells come from old cells.

Types

Organisms are grouped into two main types: eukaryotes and prokaryotes. Eukaryotic cells have a membrane-bound nucleus. Prokaryotic cells do not have a nucleus but have a nucleoid region instead. Prokaryotes are always single-celled, such as bacteria and archaea. 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. Eukaryotic cells are larger and more complex, containing a nucleus and various parts such as mitochondria. 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 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 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. The cytoplasm 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. These walls give the cells their shape and help them stay connected.

Inside plant cells are important parts called organelles. One of these is called a chloroplast, which captures sunlight to make food for the plant in a process called photosynthesis. Plant cells also have big spaces called vacuoles that store water and nutrients.

These cells use tiny structures to help move things inside them. They also make special chemicals called hormones that help control growth and keep the plant safe.

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. 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.

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 a cell membrane or a cell wall. Some also have a pellicle, a test, or a frustule.

Some protists, like amoebae, can eat tiny creatures by pulling them inside. Others make their own food using sunlight. Many can move around using structures like cilia, flagella, or pseudopodia.

Ciliates have two kinds of nuclei: a small micronucleus that helps them reproduce, and a large macronucleus that helps with everyday activities.

Cellular processes

See also: Cell cycle and Cell physiology

Replication

Main article: Cell division

During cell division, a cell splits into two new cells. This helps living things grow. Prokaryotic cells divide in a simple way, while eukaryotic cells use a process called mitosis before they split.

Signaling

Main article: Cell signaling

Cell signaling is how cells send and receive messages. It involves a message, a receiver, and the signal. Most messages are chemicals and can reach nearby or distant cells. Special proteins help the cell respond to these messages.

Protein targeting

Protein targeting is how proteins move to their right places inside or outside the cell. Proteins may go to different parts of the cell, such as organelles, membranes, or outside the cell through secretion. This keeps the cell working well.

DNA repair

Main article: DNA repair

All cells can fix DNA damage. Fixing DNA keeps the cell healthy and helps prevent problems.

Growth and metabolism

Main articles: Cell growth, Metabolism, and Photosynthesis

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

Protein synthesis

Main article: Protein biosynthesis

Cells make new proteins from small pieces called amino acids. This happens in two steps: making a copy of DNA as RNA, and then using that RNA to build the protein. This helps the cell do its many tasks.

Motility

Main article: Motility

Some cells can move to find food or stay safe. Cells can move using flagella, cilia, or by changing shape like amoeboid movement. In animals, cells move to help heal wounds or fight infections.

Cell death

Main article: Cell death

Cell death occurs when a cell stops working. This can happen because the cell is old or damaged. Sometimes cells die on purpose to make room for new cells. This is a normal part of staying healthy.

Origins

Main article: History of life

Further information: Abiogenesis and Evolution of cells

The origin of cells is tied to the start of life on Earth. Small molecules needed for life may have come from meteorites, formed near deep-sea vents, or been made by lightning. RNA might have been one of the first molecules that could copy itself and help with chemical reactions.

Cells first appeared around 4 billion years ago. The earliest cells likely got their energy by eating other tiny organisms. Eukaryotic cells, which have a nucleus and other complex parts, formed about 2.2 billion years ago when two different simple cells joined together. These eukaryotic cells later evolved into many kinds 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 ways 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. He called them "cells" because they looked like small rooms. Later, scientists like Matthias Schleiden and Theodor Schwann studied cells in plants and animals. They found that cells are the basic building blocks of all living things.

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