Cell (biology)

All living things are made of cells.

Cells are like tiny rooms. Most are too small to see. You need a microscope to find them.

Cells have a thin skin. This skin holds everything inside. It keeps the cell safe.

Inside, there is a jelly. This jelly holds many parts. Some parts make food. Other parts give the cell energy.

Some cells are very simple. They are just one tiny unit. Other living things have many cells. This is how plants and animals grow.

Cells can make more cells. This helps living things stay alive. They have been on Earth for a long time.

All living things are made of cells.

A cell is the basic unit of life. The name comes from a Latin word for "small room." Most cells are too small to see. You need a microscope to find them.

There are two main groups of cells. The first group is called prokaryotes. These are simple, single-celled life forms like bacteria. They do not have a nucleus. A nucleus is a part that holds genetic material.

Prokaryotes are usually very small.

The second group is called eukaryotes. These cells have a nucleus. They can be single-celled or make up big living things. Plants and animals are made of many eukaryotic cells.

Inside a cell, there is a jelly called cytoplasm. This jelly holds many tiny parts. Some parts are called organelles. Mitochondria are organelles that give the cell power. In plants, chloroplasts make sugar from light.

A thin skin called a cell membrane holds everything in. It lets some things in and keeps others out.

Every living thing on Earth is made of cells. A cell is the basic unit of life and structure.

The word comes from the Latin word cellula, which means "small room." Most cells are too tiny to see with your eyes. You must use a microscope to find them. Cells first appeared on Earth about four billion years ago. They are the building blocks for everything from tiny bacteria to huge animals.

Cells work in different ways depending on their type. A cell membrane acts like a thin skin. It is a semipermeable barrier, meaning it chooses what enters and leaves.

Inside this skin is a jelly-like substance called cytoplasm. This cytoplasm holds the genetic material and many tiny parts called organelles. In plant cells, chloroplasts use light to make sugars. In other cells, mitochondria provide the energy needed for the cell to work.

Scientists group cells into two main categories: prokaryotes and eukaryotes.

Prokaryotes are simple, single-celled life forms like bacteria and archaea. They do not have a nucleus to hold their DNA. Instead, their genetic material sits in a region called a nucleoid. Eukaryotes are more complex and have a nucleus. Eukaryotes can be single-celled, like tiny algae, or multicellular, like plants and animals.

We have learned a lot about cells through history. Robert Hooke discovered cells in 1665.

He gave them their name because they looked like small rooms in a monastery. Later, in 1839, Matthias Jakob Schleiden and Theodor Schwann developed cell theory. This theory says all living things are made of cells. It also says that all cells come from other cells that already existed.

Cells connect to many things you see every day. For example, your own body is made of trillions of cells. An adult human male has about 36 trillion cells.

An adult female has about 28 trillion cells. All these cells work together to make your skin, bones, and organs. Even the smallest bacterium, like the one that causes Lyme disease, follows these rules of life. Every living thing follows this amazing tiny blueprint.

The cell is the fundamental structural and functional unit of all living organisms. The term originates from the Latin word *cellula*, meaning "small room."

Cells serve as the basic building blocks for every form of life on Earth. Most cells are microscopic and require magnification to be seen. They emerged on our planet approximately four billion years ago. Almost all cells are capable of protein synthesis and replication. Only a few highly-differentiated cell types, such as red blood cells or gametes, lack these abilities.

Biologists categorize all life into two primary groups: prokaryotes and eukaryotes.

Prokaryotes are simple, single-celled organisms. This group includes the domains of bacteria and archaea. Eukaryotes are more complex and can be either single-celled or multicellular. Examples of eukaryotes include protists, plants, animals, and most fungi. All multicellular organisms are composed of many different specialized cell types. In plants and animals, the diploid cells that form the body are called somatic cells. This term excludes the haploid gametes used for reproduction.

Prokaryotic cells are characterized by their lack of a membrane-bound nucleus. Instead, their genetic material is located in an irregular region called a nucleoid. Prokaryotes are generally smaller and simpler than eukaryotes. Most prokaryotes range from 0.5 to 2.0 micrometers in diameter. However, the largest known bacterium, *Thiomargarita magnifica*, can reach lengths of 1 to 2 centimeters. While they lack most membrane-bound organelles, some bacteria possess protein-based microcompartments. These include gas vesicles, carboxysomes, and encapsulin nanocompartments. Some specialized bacteria even possess unique organelles like magnetosomes or anammoxosomes.

Bacteria possess a complex cell envelope to protect their internal environment. This envelope typically consists of a plasma membrane and a cell wall made of peptidoglycan.

Some bacteria also have a third gelatinous layer called a capsule. This capsule may be composed of polysaccharides or polypeptides. The cell wall provides rigidity and prevents the cell from bursting due to osmotic pressure. Inside the bacterium, the DNA usually exists as a single circular chromosome. Some bacteria also carry plasmids, which are small, circular, extrachromosomal DNA molecules. These plasmids often encode helpful genes, such as those providing antibiotic resistance.

Eukaryotic cells are distinguished by a membrane-bound nucleus that houses their genetic material. The name eukaryote translates to "true nut" or "true kernel."

These cells can be 2 to 100 times larger than prokaryotic cells. The interior of the cell is filled with a gel-like cytoplasm. This cytoplasm contains various specialized organelles. For example, mitochondria provide necessary energy for cellular functions. In plants, chloroplasts perform photosynthesis to create sugars.

Eukaryotes also utilize an endomembrane system to modify, package, and transport proteins and lipids.

Animal cells follow specific developmental pathways to create complex bodies. All animal cells originate from a single, totipotent diploid cell called a zygote. During embryonic development, these cells undergo differentiation to become specialized tissues. This process is driven by intrinsic molecular distributions and environmental cues. Animals are often classified by the number of germ layers they possess. Sponges have only one layer, while diploblasts have two: the ectoderm and endoderm. More advanced animals are triploblastic, meaning they possess a third middle layer called the mesoderm.

The ectoderm eventually gives rise to the skin, glands, and nervous tissue.

Understanding the cell has a rich history of scientific discovery. Robert Hooke first discovered cells in 1665. He chose the name because the structures reminded him of monastery cells.

In 1839, scientists Matthias Jakob Schleiden and Theodor Schwann developed cell theory. This theory established three vital principles. First, all organisms are composed of one or more cells. Second, the cell is the fundamental unit of structure and function. Third, all cells arise from pre-existing cells. This discovery changed our entire understanding of biology and life.

Cells are incredibly numerous within complex organisms. For instance, the human body contains a staggering number of individual cells. An estimated 30 trillion cells make up a typical adult human. Specific estimates suggest approximately 36 trillion cells in an adult male and 28 trillion in an adult female. These cells work in harmony to maintain the functions of the entire organism. From the smallest bacterium to the largest human, the cell remains the essential unit of life.