Study Guide

Cells Terms

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Cells Terms


Adenosine triphosphate. An adenine molecule, or a nucleotide, attached to three linearly connected phosphate groups (–H2PO4R, where R is a functional group). The breaking of chemical bonds between the 2nd and 3rd phosphate groups provides most of the chemical energy used by a cell. Most of the ATP in a cell is made in the mitochondria, the cell's powerhouse. ATP is a coenzyme (the -ase in the name gives it away) and a strong reducing agent, or electron donor, that acts as the principal energy carrier in the cell. Donating the terminal phosphate group, or the phosphate group on the end, from ATP causes the release of a large amount of energy. ATP basically shuffles energy around to support metabolism and a bunch of super important cellular processes, like photosynthesis.

Cell theory

The scientifically supported idea that the basic structural unit of life is the cell and that all cells arise from other cells. Remember that a "theory" in biology is equivalent to a "law" in physics: neither of which are very likely to be refuted by further observations or experiments. This theory is here to stay.

Cell wall

A rigid, but often flexible, layer containing cellulose or chitin, pectin, and other polymers. The cell wall is the outermost structure of plant, algal, fungal, and some prokaryotic cells.


A tubular structure that is made of protein and found only in animal cells. It is involved in cell division and the formation of flagella and cilia (types of pointy things that assist cells in what they do).


The organelle (see definition; think "mini organ") in plant cells, and a few other eukaryotic cells, that contains chlorophyll, the magical green pigment, and carries out the process of photosynthesis, aka the conversion of sunlight into food.


A carbon-based steroid molecule that provides permeability and fluidity to plasma membranes. Cholesterol gets a bad rap for clogging arteries and causing heart attacks when present in the body in excessive amounts.


A single piece of wound (as in, "twisted," not "injury") DNA bound to other "stuff" like proteins and nucleotides.


A condensed package of DNA and proteins in the nucleus of a cell.

Cilium (plural cilia)

A projection of a cell membrane sticking out from the main cell body. Cilia are used for locomotion (choo choo) in some unicellular, aka one-celled, organisms, and for movement of matter past a group of cells in some multicellular, aka more-than-one-celled, organisms. For example, cilia move the mucus and debris past the mucosal cells lining the tracheae in some animals.


The cytosol (fluid inside cells), organelles (except the nucleus), and other particles enclosed within the cell membrane. The cytoplasm is the site of most cellular activities, including glycolysis, aka the production of energy from carbohydrates, and cell division, or the way cells reproduce.


The process by which cells bring in or release very large particles and quantities of fluid. Exocytosis is the general term for cell release, endocytosis is the general term for cell consumption, and pinocytosis and phagocytosis are specific types of endocytosis. Enough -cytosis for you? Pinocytosis refers to the cellular uptake of fluid, or "cell drinking," while phagocytosis refers to the cellular uptake of solid matter, or "cell eating." Not like cookie eating.


The complex structure of protein filaments within the cytosol (fluid inside cells) that maintains cell shape and structure, controls cellular locomotion and cytosis (see the definition before this one), provides scaffolding for intracellular transport, and controls cell division. A cytoskeleton is found in all types of both eukaryotic and prokaryotic cells.


The fluid component of the cytoplasm (collective name for the stuff within the boundaries of the cell membrane) composed of cytoskeleton filaments, dissolved molecules, and water. The cytosol is the part of the cytoplasm between the cell membrane and organelle membranes. Membrane = thin encasing layer.


Deoxyribonucleic acid. DNA is a macromolecule ("macro" = big) also known as a nucleic acid that is composed of phosphate groups, deoxyribose sugar groups, and the nucleotides adenine, guanine, cytosine, and thymine. DNA contains the genetic code needed by all cells to produce proteins and other molecules necessary to sustain life. He seems to make into every one of Shmoop's Biology glossaries.


The condition of living within the body or cells of another organism. Sounds weird, and it is. There is evidence that millions of years ago, the ancestors of mitochondria and chloroplasts, two organelles ("mini organs") were actually prokaryotic organisms that entered into endosymbiotic relationships with eukaryotic cells. We know; that is some wild and crazy talk right there.


A protein that catalyzes, or increases the rate of, a chemical reaction in a cell. Most enzyme-catalyzed reactions in the cell take place within the cytosol (fluid inside cells), but many others take place within the nucleus and within certain organelles ("mini organs") like lysosomes, mitochondria, and chloroplasts (the green organelle in plants).


An organism whose cells contain a membrane-bound nucleus. Many eukaryotic cells also possess other membrane-bound organelles. All eukaryotes are more closely related to each other than they are to prokaryotes. Apples and oranges you might say, except apples and oranges are both eukaryotes so…that analogy doesn't work. Scratch that.


A protrusion of the cell membrane in some eukaryotic and prokaryotic cells that spins or lashes back and forth to aid in cellular locomotion. We say whip it; whip it good! There are significant differences among bacterial, Archaean, and eukaryotic flagella structure and movement.


All of an organism's heredity information encoded in either DNA or RNA. You may have heard of the Human Genome Project.

Golgi body

An organelle ("mini organ") in eukaryotic cells containing between three and seven flattened membrane disks called cisternae. The Golgi body packages and processes proteins and lipids, and is also called the "Golgi apparatus." Stack of pancakes, anyone?


Large protein complexes that control the messages sent from the DNA to the rest of the cell.


The physical property of being attracted to water, or "water-loving." Phosphate ions (PO43-) and other ions are notoriously hydrophilic. They have an unhealthy obsession with water.


The physical property of being repelled by water, or "water-fearing." Lipids (fatty fat fats) are notoriously hydrophobic. Living in fear is no fun, so hydrophobic things like to cling to each other like white on rice when water is nearby.


A spherical, membrane-bound organelle ("mini organ") in eukaryotic cells (membrane-bound-nucleus-containing) that contains enzymes (catalysts) and other proteins that digest, or break down, substances that have been taken into a cell by phagocytosis (swallowing them up).

Membrane protein

Any of several types of proteins embedded in the plasma membrane (the lipid bilayer that encapsulates the cell). Here are the heavier details:

  • Receptor proteins perform cell communication.
  • Transport proteins move materials into and out of the cell.
  • Surface (or peripheral) proteins do not penetrate the membrane but can be found on the inside or outside of the cell and are usually involved in cell communication.
  • Transmembrane (or integral) proteins completely traverse the lipid bilayer and are usually involved in transport of materials into and out of the cell, and they are also sometimes in cell communication. Phew.

Mitochondrion (plural mitochondria)

A membrane-bound organelle ("mini organ") in eukaryotic cells, aka those cells that contain a membrane-bound nucleus. Mitochondria have their own inner and outer membranes that provide the structure needed to make large quantities of usable energy (in the form of ATP) from lipids, sugars, and proteins in a process known as cellular respiration. The mitochondrion is the cell's powerhouse.

Nucleus (plural nuclei)

The membrane-bound organelle ("mini organ") in eukaryotic cells, aka those cells that contain a membrane-bound nucleus, that is the "control center" of the cell. The nucleus houses the genetic instructions necessary to synthesize proteins and other molecules needed for cell survival, growth, and reproduction. The nucleus also contains the nucleolus (see definition) where ribosomal RNA (rRNA) is transcribed and assembled. Think of the nucleus as the control center in some far away space station. You may have noticed that the word nucleus gets thrown around a lot. Just don't confuse the nucleus of an atom with the nucleus of a cell—they are not the same thing!


The general name given to the membrane-bound structures within a eukaryotic cell (the cells that have nuclei) that perform specific functions necessary for cell survival, growth, or reproduction. Examples include Golgi bodies, lysosomes, mitochondria, chloroplasts, endoplasmic reticulum, vacuoles, and vesicles. We at Shmoop like to call them "mini organs."


A macromolecule ("macro" = big) composed of two parts: a lipid "tail" and a phosphate "head." The lipid "tail" is usually a diglyceride, or two fatty acid chains and a glycerol, and is hydrophobic ("water-fearing"). The phosphate "head" group has a chemical structure of –H2PO4R, where R is a functional group, and is hydrophilic ("water-loving"). In the plasma membrane (see next definition), phospholipids form a bilayer with the lipid "tails" facing each other on the inside of the bilayer and the phosphate "heads" facing outward, toward both the outside and inside of the cell. Take deep breaths. We have fun pictures to help.

Plasma membrane

A phospholipid (see previous definition) bilayer separating the interior of a cell from the surrounding environment. The plasma membrane is composed primarily of phospholipids, protein complexes, and cholesterol molecules. The membrane does lots of stuff, including protecting the cell, transporting materials into and out of the cell, and helping the cell communicate to other cells.


An organism whose cells lack nuclei (a control center) and membrane-bound organelles ("mini organs"). Prokaryotes are generally single-celled. One is the loneliest number. Too 60s? We thought so, too.


A chain, or chains, of amino acids specifically folded to take on a certain shape, one that determines the protein’s function. All enzymes are proteins, but not all proteins are enzymes. Tricky how that works.


A complex structure made of proteins and ribosomal RNA, or rRNA. Ribosomes are found in all cells, both prokaryotic and eukaryotic. Together with messenger RNA (mRNA) and transfer RNA (tRNA), ribosomes synthesize proteins from amino acids (the building blocks of proteins). Ribosomes are not generally considered organelles because they are not membrane-bound.


Ribonucleic acid. RNA is a macromolecule ("macro" = big) composed of phosphate groups (aka –H2PO4R, where R is a functional group), ribose sugars, and the nucleotides adenine, guanine, cytosine, and uracil. It functions as a go-between for DNA and proteins as messenger RNA, or mRNA; as a key player in protein synthesis as ribosomal RNA, or rRNA, and transfer RNA, or tRNA; and in gene regulation as small interfering RNA, or siRNA, and microRNA, or miRNA. Why do there need to be so many different kinds of RNA? We were wondering the same thing. Don’t worry about all the different types for now.

Rough endoplasmic reticulum (RER)

A highly membranous organelle ("mini organ") in eukaryotic cells, aka cells with a membrane-bound nucleus, dotted with ribosomes (see definition). The dots inspired the "rough" part of the name. The RER is the major site of protein synthesis, and is continuous with the nuclear membrane and smooth endoplasmic reticulum, or SER. The RER and the SER are both tubular in structure, but not like the 80s adjective.

Smooth endoplasmic reticulum (SER)

A highly membranous organelle in eukaryotic cells that is not associated with ribosomes. The SER is the major site of lipid and steroid synthesis, and is continuous with the nuclear membrane and rough endoplasmic reticulum, or RER. The RER and the SER are both tubular in structure. Random fact: Reticulum means "twisted net."


A large, membrane-bound organelle ("mini organ") found in most plant and fungal cells, as well as some animal and bacterial cells. The vacuole's job varies with cell type, but in many cases, the vacuole is involved in water regulation, waste removal, and pH balance within the cell. It is sometimes called the central vacuole. In appearance, the vacuole of the cell looks like a round blob.


A small, sac-like organelle ("mini organ") involved in the transport and storage of cellular substances, especially proteins marked for secretion from the cell, aka cell exit. Golgi bodies form the vesicles (stack of pancakes...we mean, see definition).

Roots of Common Terms in Cells

(L=Latin, G=Greek, F=French)


cell = small room (L)


chloro = pale green (G); plast = formed, molded (G)


cyto = cell (G); plasma = form, shape (L)


cyto = cell (G); skeleton = framework (L)

Endoplasmic reticulum

endo = within, inside (G); plasma = form, shape (L); reticulum = little twisted net (L)


endo = within, inside (G); sym = together (G); bios = life (G)


eu = true, well (G); karyo = nut, kernel (G)


hydro = water (G); phil = love (G, L)


hydro = water (G); phob = fear (G)


lyse = loosening, breaking (G); some = body (G)


mito = thread (G); chondrion = little granule (G)


nucleus = kernel (L)


phago = to eat (G); cyto = cell (G)


pino = to drink (G); cyto = cell (G)


pro = before (G); karyo = nut, kernel (G)


ribo = nucleic acid; some = body (G)

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