Skip Over Navigation Links

Inside the Cell


Have you ever wondered why we age? What exactly is happening inside our bodies to bring on the wrinkles, gray hair, and the other changes seen in older people? Considering the universality of the process, you might be surprised to know that there remain many unanswered questions about how aging happens at the cellular level. However, theories abound, and the roles played by various suspects in the aging process are beginning to take shape.

Cell death, on the other hand, is an area in which scientists have made great leaps in understanding in recent years. Far from being strictly harmful, scientists have found that cell death, when carefully controlled, is critical to life as we know it. Without it, you wouldn't have your fingers and toes or the proper brain cell connections to be able to read the words on this page.

If you'd like to know more about these fascinating processes, read on. And thank cell death for it!

A | B | C | D | E | F | G | H | I | L
M | N | O | P | R | S | T | U | V | Z

Actin filament (AK-tin) Part of the cytoskeleton. Actin filaments contract or lengthen to give cells the flexibility to move and change shape. Together with myosin, actin filaments are responsible for muscle contraction.

Adult stem cells Cells that can renew themselves and differentiate into a limited number of specialized cell types. They replace and renew damaged tissues.

Amino acid (uh-MEE-no) A chemical building block of proteins. There are 20 standard amino acids. A protein consists of a specific sequence of amino acids.

Anaphase (ANN-uh-faze) The fourth of six phases of cell division, following metaphase and preceding telophase. In anaphase, the chromosomes separate into two genetically identical groups and move to opposite ends of the spindle.

Aneuploidy (ANN-yoo-PLOY-dee) The condition of having an abnormal number of chromosomes. See Down syndrome.

Antibody A protein produced by the immune system in response to a foreign substance such as a virus or bacterium.

Antioxidant (ANN-tee-AWK-si-dunt) A substance that can neutralize dangerous compounds called reactive oxygen species. Antioxidants are found naturally in our bodies and in foods such as fruits and vegetables.

Apoptosis (ay-PAH-TOE-sis) Programmed cell death, a normal process in which cells die in a controlled and predictable way. See necrosis.

ATP, adenosine triphosphate (ah-DEH-no-seen try-FOSS-fate) The major source of energy for biochemical reactions in all organisms.

Bacterium (plural: bacteria) A one-celled microorganism that contains no nucleus. Some bacteria are helpful, such as those in the intestines that help digest food, while others cause disease. Bacteria are frequently used as model organisms to study basic biological processes. See prokaryotic cell and model organism.

Carbohydrate A molecule made up of one or more sugars. In the body, carbohydrates can exist independently or be attached to proteins or lipids.

Cell The basic subunit of any living organism; the simplest unit capable of independent life. Although there are some single-celled organisms, such as bacteria, most organisms consist of many cells that are specialized for particular functions. See prokaryotic cell and eukaryotic cell.

Cell cycle The sequence of events by which a cell duplicates its contents and divides in two.

Channel protein A hollow or pore-containing protein that spans a cell membrane and acts as a conduit for small molecules, such as charged particles (ions).

Checkpoint One of several points in the cell cycle where the cycle can pause if there is a problem such as incomplete DNA synthesis or damaged DNA. See cell cycle.

Chemotaxis (KEE-moh-TACK-sis) The movement of a cell toward or away from the source of a chemical.

Cholesterol A waxy lipid produced by animal cells that is a major component of cell membranes. Cholesterol is also used as a building block for some hormones.

Chromosome (KROH-muh-sohm) A cellular structure containing genes. Excluding sperm and egg cells, humans have 46 chromosomes (23 pairs) in each cell.

Cilium (SILL-ee-um) (plural: cilia) A hairlike projection from a cell surface. The rhythmic beating of cilia can move fluid or mucus over a cell or can propel single-celled organisms. Cilia are shorter than flagella.

Computational biology A field of science that uses computers to study complex biological processes that involve many molecular interactions.

Crossing over A process that occurs during meiosis in which chromosome partners, one inherited from each parent, physically swap sections with one another. This creates hybrid chromosomes that are a patchwork of the original pair. Crossing over occurs in species that reproduce sexually and increases the genetic variety of offspring.

Cytokinesis (SYE-toe-kin-EE-sis) The last of six phases of cell division. It occurs after the duplicated genetic material has segregated to opposite sides of the cell. During cytokinesis, the cell splits into two daughter cells.

Cytoplasm (SYE-toe-PLAZ-um) The material found between the cell membrane and the nuclear envelope. It includes the cytosol and all organelles except the nucleus. See cytosol.

Cytoskeleton (SYE-toe-SKEL-uh-tun) A collection of fibers that gives a cell shape and support and allows movement within the cell and, in some cases, by the cell as a whole. The three main types of cytoskeletal fibers are microtubules, actin filaments, and intermediate filaments.

Cytosol (SYE-tuh-sol) The semi-fluid portion of the cytoplasm, excluding the organelles. The cytosol is a concentrated solution of proteins, salts, and other molecules. See cytoplasm.

Differentiation The series of biochemical and structural changes by which an unspecialized cell becomes a specialized cell with a specific function. During development, embryonic stem cells differentiate into the many cell types that make up the human body.

Diploid (DIP-loyd) Having two sets of chromosomes, one inherited from each parent. All human cells except eggs and sperm are diploid and have 46 chromosomes, 23 from each parent.

DNA, deoxyribonucleic acid (dee-AW-ksee-RYE-bo-new-CLAY-ick) The substance of heredity. A long, helical, double-stranded molecule that carries the cell's genetic information. See chromosome.

Down syndrome An inherited condition caused by having an extra copy of chromosome 21. See aneuploidy.

Electron microscope A powerful microscope that uses beams of fast-moving electrons instead of light to magnify samples. Powerful magnets focus the electrons into an image.

Embryonic stem cell A cell found in early embryos that can renew itself and differentiate into the many cell types that are found in the human body.

Endocytosis (EN-doe-sye-TOE-sis) A process cells use to engulf particles or liquid from their surroundings. It occurs when the cell surface membrane puckers inward, encircling the material, then pinches off, producing a vesicle inside the cell.

Endoplasmic reticulum (ER) (EN-doe-PLAZ-mik reh-TIK-yoo-lum) An organelle made up of interconnected tubes and flattened sacs. There are two kinds of ER: rough (because it is dotted with ribosomes) ER, which processes newly made proteins, and smooth ER, which helps make lipid and neutralizes toxins.

Enzyme A protein that speeds up a specific chemical reaction without being permanently altered or consumed.

Eukaryotic cell (YOO-kare-ee-AW-tick) A cell that has a nucleus and other organelles not found in prokaryotes; includes all animal and most plant cells.

Exocytosis (EK-so-sye-TOE-sis) A process cells use to send substances outside their surface membrane via vesicles.

Extracellular matrix The material that surrounds and supports cells. It includes structural proteins such as collagen and elastin.

Flagellum (fluh-JELL-um) (plural: flagella) A long, taillike structure extending from a cell. Sperm and many microorganisms move using flagella.

G protein A protein located on the inside of the cell membrane that helps transmit molecular signals into cells.

Gene A unit of heredity; a segment of DNA that contains the code for making a specific protein or RNA molecule.

Genome (JEE-nome) All of an organism's genetic material.

Glial cell (GLEE-uhl) A kind of cell in the nervous system that provides nutrition and support to a nerve cell.

Glycosylation (glye-KAW-sil-AY-shun) The process of adding specialized chains of sugar molecules to proteins or lipids; occurs in the ER and Golgi.

Golgi (GOLE-jee) Also called the Golgi apparatus or Golgi complex; an organelle composed of membranous sacs in which many newly made proteins mature and become functional.

Haploid (HAP-loyd) Having a single set of chromosomes, as in egg or sperm cells. Haploid human cells have 23 chromosomes.

Hormone A molecule that stimulates specific cellular activity; made in one part of the body and transported via the bloodstream to tissues and organs. Examples include insulin, estrogen, and testosterone.

Intermediate filament Part of the cytoskeleton that provides strength. Some intermediate filaments form nails, hair, and the outer layer of skin. Others are found in nerves or other organs.

Interphase (IN-tur-faze) A period in a cell's life cycle when it is not undergoing mitosis.

Lipid (LIP-id) A fatty, waxy, or oily compound that will not dissolve in water. Lipids are a major part of biological membranes.

Lysosome (LYE-so-sohm) A bubble-like organelle that contains powerful enzymes that can digest a variety of biological materials.

Meiosis (my-OH-sis) The type of cell division that makes egg and sperm cells. Meiosis generates cells that are genetically different from one another and contain half the total number of chromosomes in the parent cell. See haploid.

Membrane A semi-fluid layer of lipids and proteins. Biological membranes enclose cells and organelles and control the passage of materials into and out of them.

Metaphase (MET-uh-faze) The third phase of cell division, following prometaphase and preceding anaphase. In metaphase, the copied chromosomes align in the middle of the spindle.

Micrometer (MY-kroh-MEE-tur) One micrometer is one millionth (10–6) of a meter or one thousandth of a millimeter. The micrometer is frequently used to measure cells and organelles.

Microtubule (MY-kroh-TOO-byool) Part of the cytoskeleton; a strong, hollow fiber that acts as a structural support for the cell. During cell division, microtubules form the spindle that directs chromosomes to the daughter cells. Microtubules also serve as tracks for transporting vesicles and give structure to flagella and cilia.

Mitochondrion (MITE-oh-KON-dree-un) (plural: mitochondria) The cell's power plant; the organelle that converts energy from food into ATP, fueling the cell. Mitochondria contain their own small genomes and appear to have descended from free-living bacteria.

Mitosis (my-TOE-sis) The type of cell division that eukaryotic cells use to make new body cells. Mitosis results in two daughter cells that are genetically identical to the parent cell.

Model system (or Model organism) A cell type or simple organism—such as a bacterium, yeast, plant, fruit fly, or mouse-used to answer basic questions about biology.

Mutation (myoo-TAY-shun) A change in a DNA sequence.

Myelin (MY-eh-lin) A fatty covering that forms a protective sheath around nerve fibers and dramatically speeds the transmission of nerve signals.

Nanometer (NAN-oh-MEE-tur) One billionth (10–9) of a meter or one thousandth of a micrometer. The nanometer is frequently used to measure organelles and small structures within cells.

Necrosis (neh-CROH-sis) Unplanned cell death caused by outside circumstances, such as traumatic injury or infection. See apoptosis.

Neuron A cell in the nervous system that is specialized to carry information through electrical impulses and chemical messengers. Also called a nerve cell.

Neurotransmitter A chemical messenger that passes signals between nerve cells or between a nerve cell and another type of cell.

Nuclear envelope A barrier that encloses the nucleus and is made up of two membranes perforated by nuclear pores.

Nuclear pores An opening in the nuclear envelope that allows the passage of small molecules such as salts, small proteins, and RNA molecules.

Nucleus The organelle in eukaryotic cells that contains genetic material.

Oocyte (oh-oh-SITE) The developing female reproductive cell; an immature egg.

Organ A group of tissues that perform a particular job. Animals have more than a dozen organs, including the heart, brain, eye, liver, and lung.

Organelle (OR-gun-EL) A specialized, membrane-bounded structure that has a specific function in a cell. Examples include the nucleus, mitochondria, Golgi, ER, and lysosomes.

Prokaryotic cell (PRO-kare-ee-AW-tick) A cell that lacks a nucleus. Bacteria are prokaryotes. See eukaryotic cell.

Prometaphase (pro-MET-uh-faze) The second of six phases of cell division, following prophase and preceding metaphase. In prometaphase, the nuclear membrane breaks apart and the spindle starts to interact with the chromosomes.

Prophase (PRO-faze) The first of six phases of cell division. In prophase, chromosomes condense and become visible and the spindle forms.

Proteasome (PRO-tee-uh-some) A cellular machine that digests proteins that have been tagged with ubiquitin for destruction.

Protein A molecule composed of amino acids lined up in a precise order determined by a gene, then folded into a specific three-dimensional shape. Proteins are responsible for countless biological functions and come in a wide range of shapes and sizes.

Reactive oxygen species One of several types of small molecules containing oxygen with an unstable number of electrons. Reactive oxygen species can damage many kinds of biological molecules.

Ribosome (RYE-bo-sohm) A molecular complex in which proteins are made. In eukaryotic cells, ribosomes either are free in the cytoplasm or are attached to the rough endoplasmic reticulum.

RNA, ribonucleic acid (RYE-bo-new-CLAY-ick) A molecule very similar to DNA that plays a key role in making proteins. There are three main types: messenger RNA (mRNA) is an RNA version of a gene and serves as a template for making a protein, ribosomal RNA (rRNA) is a major component of ribosomes, and transfer RNA (tRNA) transports amino acids to the ribosome and helps position them properly during protein production.

RNAi (RNA interference) The process of using small pieces of double-stranded RNA to reduce the activity of specific genes. The process occurs naturally in many organisms and is now commonly used in basic research. It has the potential to be therapeutically useful.

RNA polymerase (puh-LIH-mer-ase) An enzyme that makes RNA using DNA as a template in a process called transcription.

Spindle A football-shaped array of fibers made of microtubules and associated proteins that forms before cells divide. Some of the fibers attach to the chromosomes and help draw them to opposite ends of the cell.

Telomerase (tee-LAW-mer-ase) An enzyme that adds telomeres to the ends of eukaryotic chromosomes, preventing the chromosome from shrinking during each cell division.

Telomere (TEE-lo-meer) A repetitive segment of DNA at the ends of eukaryotic chromosomes. Telomeres do not contain genes and, in the absence of telomerase, they shorten with each cell division.

Telophase (TEE-lo-faze) The fifth of six phases of cell division, following anaphase and preceding cytokinesis. In telophase, nuclear membranes form around each of the two sets of chromosomes, the chromosomes begin to spread out, and the spindle begins to break down.

Tissue A group of cells that act together to carry out a specific function in the body. Examples include muscle tissue, nervous system tissue (including the brain, spinal cord, and nerves), and connective tissue (including ligaments, tendons, bones, and fat). Organs are made up of tissues.

Transcription The process of copying information from genes (made of DNA) into messenger RNA.

Translation The process of making proteins based on genetic information encoded in messenger RNA. Translation occurs in ribosomes.

Ubiquitin (yoo-BIH-kwe-tin) A small protein that attaches to and marks other proteins for destruction by the proteasome.

Vesicle (VEH-sih-kle) A small, membrane-bounded sac that transports substances between organelles as well as to and from the cell membrane.

Virus An infectious agent composed of proteins and genetic material (either DNA or RNA) that requires a host cell, such as a plant, animal, or bacterium, in which to reproduce. A virus is neither a cell nor a living organism because it can not reproduce independently.

Zygote (ZYE-gote) A cell resulting from the fusion of an egg and a sperm.

This page last reviewed on April 22, 2011