Cell

The cells of eukaryotes (left) and prokaryotes (right)

Endothelial cell: nuclei stained blue, mitochondria stained red, and F-actin, a component of microfilaments, stained green. Cell imaged on a fluorescent microscope.

In biology, the cell is the basic structure of organisms. All cells are made by the division of other cells.

The cell is the fundamental unit of life. It is the smallest structure of the body capable of performing all of the processes that define life. Each of the organs in the body, such as the lung, breast, colon, and brain, consists of specialized cells that carry out the organ's functions such as the transportation of oxygen, digestion of nutrients, excretion of waste materials, locomotion, reproduction, thinking, etc.

Kinds of cells

There are two basic kinds of cells: prokaryotic cells and eukaryotic cells. Prokaryotes, bacteria and archaea, are simple cells that have no cell nucleus. They do have bacterial microcompartments.

Eukaryotes are complex cells with many organelles and other structures in the cell. They are larger than prokaryote cells: they can be as much as 1000 times greater in volume. Eukaryotes store their genetic information (DNA) on chromosomes in the cell nucleus. Organisms (living things) which are made up of multiple cells are eukaryotes.

Kinds of prokaryotic organisms

The only kinds of prokaryotic organisms alive at present are bacteria and archaea. Prokaryotic organisms evolved before eukaryotic organisms, so at one point the world consisted of nothing but prokaryotic organisms. There are also viruses, which are hard to classify, but cause some important diseases. Viruses are made of RNA, or DNA, and protein, and they reproduce themselves inside the cells of bacteria or eukaryotes.

Kinds of eukaryotic organisms

Unicellular

A Paramecium, a single-celled organism

A simple diagram of an animal cell

A simple diagram of a plant cell

Unicellular organisms are made up of one cell. Examples of unicellular organisms are:

• Amoeba
• Paramecium

Unicellular organisms need to:

• eat
• respire (most use oxygen to make sugar into energy)

All unicellular organisms must:

• get rid of waste (discard)
• reproduce (make more of itself)
• grow

Some may:

• move
• sense their environment
• get their energy from the sun (e.g., cyanobacteria)
• ferment (e.g., yeasts)
• use anaerobic respiration (e.g. Clostridium botulinum)

Multicellular

Multicellular organisms are made from many cells. They are complex organisms. This can be a small number of cells, or millions or trillions of cells. All plants and animals are multicellular organisms. The cells of a multicellular organism are not all the same. They have different shapes and sizes, and do different work in the organism. The cells are specialized. This means they do only some kinds of work. By themselves, they cannot do everything that the organism needs to live. They need other cells to do other work. They live together, but cannot live alone.

To assure the proper performance of each organ, worn out or injured cells must be replaced, and particular types of cells must increase in response to environmental changes. For example, the bone marrow increases its production of oxygen-carrying red blood cells sevenfold or greater in response to bleeding or high altitude. Certain white blood cells are produced more rapidly during an infection. Similarly, the liver or endocrine organs frequently respond to injury by regenerating damaged cells.

Cell reproduction

The body cells of metazoans divide by simple mitotic cell division. Sexual reproduction is ancestral in eukaryotes, and in metazoa it is carried out by specialized sex cells. They are produced by a process called meiosis.

Prokaryotic cells reproduce using binary fission, where the cell simply splits in half. For both mitosis and binary fission the cell must replicate (copy) all its genetic information (DNA) so that each new cell will have a copy.

Cells have long been recognized as the simplest units of living matter that can maintain life and reproduce themselves. The human body, which is made up of numerous cells, begins as a single, newly fertilized cell. Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves.

Cell organelles

Cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell. Human cells contain the following major parts, listed in alphabetical order:

Cytoplasm

Within cells, the cytoplasm is made up of a jelly-like fluid (called the cytosol) and other structures that surround the nucleus.

Cytoskeleton

The cytoskeleton is a network of long fibers that make up the cell’s structural framework. The cytoskeleton has several critical functions, including determining cell shape, participating in cell division, and allowing cells to move. It also provides a track-like system that directs the movement of organelles and other substances within cells.

Endoplasmic reticulum (ER)

This organelle helps process molecules created by the cell. The endoplasmic reticulum also transports these molecules to their specific destinations either inside or outside the cell.

Golgi apparatus

The Golgi apparatus packages molecules processed by the endoplasmic reticulum to be transported out of the cell.

Lysosomes and peroxisomes

These organelles are the recycling center of the cell. They digest foreign bacteria that invade the cell, rid the cell of toxic substances, and recycle worn-out cell components. Mitochondria

Mitochondria are complex organelles that convert energy from food into a form that the cell can use. They have their own genetic material, separate from the DNA in the nucleus, and can make copies of themselves.

Nucleus

The nucleus serves as the cell’s command center, sending directions to the cell to grow, mature, divide, or die. It also houses DNA (deoxyribonucleic acid), the cell’s hereditary material. The nucleus is surrounded by a membrane called the nuclear envelope, which protects the DNA and separates the nucleus from the rest of the cell.

Plasma membrane

The plasma membrane is the outer lining of the cell. It separates the cell from its environment and allows materials to enter and leave the cell.

Ribosomes

Ribosomes are organelles that process the cell’s genetic instructions to create proteins. These organelles can float freely in the cytoplasm or be connected to the endoplasmic reticulum (see above).

Also see

• Cellular metabolism
• Cell cortex
• Cell culture
• Cellular model
• Cytoneme
• Cytorrhysis
• Cytotoxicity
• Cell death

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