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Cell theory: the theory stating that cells are the basic unit of life, that all organisms are made of one or more cells, and that cells arise from existing cells.

Cell membrane: a thin layer of lipid and protein separating the cell's contents from the outside environment.

Phospholipid: a structural substance found in cell membranes.

Cytoplasm: a jellylike substance made up primarily of water and the organelles suspended in it; the center for most of the cell's metabolic activity.

Cytoskeleton: a network of fibers and tiny tubes throughout the cytoplasm; gives a cell support and helps it to maintain or change its shape.

Nucleus: the control center of a cell.

Chromosomes: a structure within a cell that contains a cell's genetic information.

Nucleolus: a specialized organelle in the nucleus of cells; the site of ribosome production.

Prokaryotes: an organism whoe cells do not contain a nucleus or membrane-bound organelles.

Eukaryotes: an organism whoe cells have a nucleus surrounded by a nuclear membrane.

Ribosomes: an organelle that helps to assemble proteins from amino acids.

Endoplasmic reticulum: an extensive network of membranes that transports materials through a cell.

Golgi apparatus: a membrane-bound organelle that packages and secretes products of the endoplasmic reticulum from the cell.

Mitochondria: an organelle that changes energy found in food compounds into energy, or ATP, necessary to power the cell's functions; the site of cellular respiration.

Lysosomes: a small saclike organelle that contains digestive enzymes that help the cell break down large molecules of carohydrates, proteins, and lipids.

Cilia: short, hairlike projections that usually occur in large numbers on the surface of certain cells.

Flagella: long, tail-like projections.

Cell wall: a tough, rigid outer convering that provides a cell with protection and maintenance of its shape.

Chloroplasts: an organelle common in plants and algae that converts sunlight, carbon dioxide, and water into sugars during the process of photosynthesis; contains chlorophyll.

Vacuole: a membrane-bound compartment that absorbs water, store proteins, ions, and waste products of metabolism.

Semipermeable: allowing only certain molecules to pass through membrane.

Passive transport: the movement of a substance across a cell membrane without the use of ATP energy.

Active transport: transport of a substance across a cell membrane requiring the use of enegrgy; usually occurs against a concentration gradient.

Diffusion: the random movement of molecules from an area of higher concentration (more molecules) to an area of lower concentration (fewer molecules).

Osmosis: the diffusion of water molecules across a semipermeable membrane.

Endocytosis: a form of active transport in which a portion of the cell membrane surrounds outside particles and moves them into the cell.

Exocytosis: a process in which wastes and cell products are secreted out of a cell through the cell membrane.