Chapter 3: Cells

Mitochondrion

The organelle in eukaryotic cells that converts the energy stored in food in the chemical bonds of carbohydrate, fat, and protein molecules into a form usable by the cell for all its functions and activities.

Chloroplast

The organelle in plant cells in which photosynthesis occurs.

Prokaryotic Cell

A cell bound by a plasma membrane enclosing the cell contents (cytoplasm, DNA, and ribosomes); there is no nucleus or other organelles.

Eukaryotic Cell

A cell with a membrane-surrounded nucleus containing DNA, membrane-surrounded organelles and internal structures organized into compartments.

Plasma Membrane

A complex, thin, two-layered membrane that encloses the cytoplasm of the cell, holding contents in place, and regulating what enters and leaves the cell; also called the cell membrane.

Tight Junction

A continuous, water-tight connection between adjacent animal cells. Tight junctions are particularly important in the small intestine, where digestion occurs, to ensure that nutrients do not leak between cells into the body cavity and so become lost as a source of energy

Gap Junction

A junction between adjacent animal cells in the form of a pore in each of the plasma membranes surrounded by a protein that links the two cells and acts like a channel between them, allowing materials to pass between cells.

Chromatin

A mass of long, thin fibers consisting of DNA and proteins in the nucleus of the cell.

Nuclear Membrane

A membrane that surrounds the nucleus of a cell, separating it from the cytoplasm, consisting of two bilayers and perforated by pores enclosed in embedded proteins that allow the passage of large molecules from nucleus to cytoplasm and from cytoplasm to nucleus.

Nucleus

A membrane-enclosed structure in the eukaryotic cells that contains the organism's genetic information as linear strands of DNA in the form of chromosomes.

Vacuole (Central)

A membrane-enclosed, fluid-filled, multipurpose organelle prominent in most plant cells (but also present in some protists, fungi, and animals); functions vary but can include storing nutrients, retaining and degrading waste products, accumulating poisonous materials containing pigments, and providing physical support.

Cytoskeleton

A network of protein structures in the cytoplasm of eukaryotes (and, to a lesser extent, prokaryotes) that serves as scaffolding, adding support, and in some cases, giving the animal cells of different types of their characteristic shapes. The cytoskeleton serves as tracks guiding the intercellular traffic flow and, because it is flexible and can generate force, gives cells some ability to control their movement.

Cell Wall

A rigid structure, outside the cell membrane, that protects and gives shape to the cell; found in many prokaryotes and plants.

Lysosome

A round, membrane-enclosed, enzyme and acid-filled vesicle in the cell that digests and recycles cellular waste products and consumed material.

Endomembrane System

A system of organelles (the rough endoplasmic reticulum, the smooth endoplasmic reticulum, and the Golgi apparatus) that surrounds the nucleus; it produces and modifies necessary molecules, breaks down toxic chemicals and cellular by-products and is thus responsible for many of the fundamental functions of the cell.

Pilus

A thin, hair-like projection that helps a prokaryote attach to surfaces.

Cell Theory

A unifying and universally accepted theory in biology that holds that all living organisms are made up of one or more cells, and that all cells arise from preexisting cells.

Nucleolus

An area near the center of the nucleus where subunits of the ribosomes are assembled.

Golgi Apparatus

An organelle, part of the endomembrane system, structurally like a flattened sack of unconnected membranes, each known as a Golgi body. The Golgi apparatus processes molecules synthesized in the cell and packages those molecules that are destined for use elsewhere in the body.

Smooth Endoplasmic Reticulum

An organelle, part of the endomembrane system, structurally like a series of branched tubes, called "smooth" because its surface has no ribosomes. Smooth endoplasmic reticulum synthesizes lipids such as fatty acids,phospholipids, and steroids.

Rough Endoplasmic Reticulum

An organelle, part of the endomembrane system, structurally like a series of interconnected flattened sacs connected to the nuclear envelope; called the "rough" because its surface is studded with ribosomes.

Eukaryote

An organism composed of eukaryotic cells.

Prokaryote

An organism consisting of a prokaryotic cell (all prokaryotes are one-celled organisms).

Desmosomes

Irregularly spaced connections between adjacent animal cells that, in the manner of Velcro, hold cells together by attachments, but are not water-tight. They provide mechanical strength and are found in muscle tissue that lines the cavities of animal bodies.

Ribosomes

Granular bodies in the cytoplasm, released from their initial positions on the rough endoplasmic reticulum, that copy the information in segments of DNA to provide instructions for the concentration of proteins.

Intermembrane Space

In a mitochondrion, the region between the inner and outer membranes.

Plasmodesmata

In plants, microscopic tubelike channels connecting the cells and enabling communication and transport between them.

Turgor Pressure

In plants, the pressure of the contents of the cell against the cell wall, which is maintained by osmosis as water rushes into the cell when it contains high concentrations of dissolved substances. Turgor pressure allows non-woody plants to stand upright, and its loss causes wilting.

Stroma

In the leaf of a green plant, the fluid in the inner compartment of a chloroplast, which contains DNA and protein-making machinery.

Thylakoids

Interconnected membranous structures in the stroma of a chloroplast, where light energy is collected and the conversation of light energy to chemical energy in photosynthesis takes place.

Flagellum

Long, thin, whip-like projection from the cell body of a prokaryote that aids in cell movement through the medium in which the organism lives; in animals, the only cell with a flagellum is the sperm cell.

Microfilaments

One of the three types of protein fibers that make up the eukaryotic cytoskeleton, providing it with structure and shape. These are the thinnest elements in the cytoskeleton; long, solid, rod-like fibers that help generate forces, including those important in cell contraction and cell division.

Intermediate Filaments

One of there types of protein fibers that make up the eukaryotic cytoskeleton, providing it with structure and shape; a durable, rope-like system of numerous different overlapping proteins.

Microtubules

One of three types of protein fibers that make up the eukaryotic cytoskeleton, providing it with structure and shape. These are the thickest elements in the cytoskeleton; they resemble rigid, hollow tubes, functioning as tracks to which molecules and organelles within the cell may attach and be moved along; also help pull chromosomes apart during cell division.

Cilia

Short projections from the cell surface, often occurring in large numbers on a single cell, that beat against the intercellular fluid to move the fluid past the cell.

Organelles

Specialized structures in the cytoplasm of eukaryotic cells with specific functions, such as the rough and smooth endoplasmic reticulum, Golgi apparatus, and mitochondria.

Cytoplasm

The jelly-like fluid that fills the inside of the cell; in eukaryotes, the cytoplasm contains the organelles.

Cell

The smallest unit of life that can function independently; a three-dimensional structure surrounded by a membrane and, in the case of prokaryotes and most plants, a cell wall, in which many of the essential chemical reactions of the life of an organism take place.

Matrix (Mitochondrial)

The space within the inner membrane, where the carriers NADH and FADH2 begin carrying high-energy electrons to molecules embedded in the inner membrane.

Endosymbiosis Theory

Theory of the origin of eukaryotes that hold that, in the past, two different types of prokaryotes engaged in a close partnership and eventually one, capable of performing photosynthesis, was subsumed into the other, a larger prokaryote. The smaller prokaryote made some of its photosynthetic energy available to the host and, over time, the two became symbiotic and eventually became a single, more complex organism in which the smaller prokaryote had evolved into the chloroplast of the new organism. A similar scenario can be developed for the evolution of mitochondria.