Chapter 3: Cells

Prokaryotes

An organism characterized by prokaryotic cells.

Cytoplasm

Everything inside a eukaryotic cell between the plasma membrane and the nucleus; consists of a semifluid medium and organelles; can also refer to the interior of a prokaryotic cell.

Plasma Membrane

The thin layer of lipids and proteins that sets a cell off from its surroundings and acts as a selective barrier to the passage of ions and molecules into and out of the cell; consists of a phospholipid bilayer in which proteins are embedded.

Cytosol

The fluid part of the cytoplasm, in which organelles are suspended.

Extracellular Fluid

The liquid in the environment that surrounds the outside of a cell.

Fluid Mosaic

A description of membrane structure, depicting a cellular membrane as a mosaic of diverse protein molecules embedded in a fluid bilayer of phospholipid molecules.

Active Transport

The movement of a substance across a biological membrane against its concentration gradient, aided by specific transport proteins and requiring the input of energy (often as ATP).

Diffusion

The spontaneous movement of particles of any kind down a concentration gradient; that is, movement of particles from where they are more concentrated to where they are less concentrated.

Endocytosis

The movement of materials from the external environment into the cytoplasm of a cell via vesicles or vacuoles.

Exocytosis

The movement of materials out of the cytoplasm of a cell via membranous vesicles or vacuoles.

Facilitated Diffusion

The passage of a substance across a biological membrane down its concentration gradient, aided by specific transport proteins.

Osmosis

The diffusion of water across a selectively permeable membrane.

Chromatin

The combination of DNA and proteins that constitutes chromosomes; often used to refer to the diffuse, very extended form taken by the chromosomes when a eukaryotic cell is not dividing.

Chromosomes

A gene-carrying structure found in the nucleus of a eukaryotic cell and most visible during mitosis and meiosis; also, the main gene-carrying structure of a prokaryotic cell. Each chromosome consists of one very long threadlike DNA molecule and associated proteins. See also chromatin.

Nuclear Envelope

A double membrane, perforated with pores, that encloses the nucleus and separates it from the rest of the eukaryotic cell.

Nuclear Pores

A protein-lined opening in the nuclear envelope through which materials (such as messenger RNA) can pass between the nucleus and the cytoplasm.

Endoplasmic Reticulum

(ER) An extensive membranous network in a eukaryotic cell, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions. See also rough ER; smooth ER.

Golgi Apparatus

An organelle in eukaryotic cells consisting of stacks of membranous sacs that modify, store, and ship products of the endoplasmic reticulum.

Lysosomes

A digestive organelle in eukaryotic cells; contains enzymes that digest the cell’s food and wastes.

Ribosomes

A cellular structure consisting of RNA and protein organized into two subunits and functioning as the site of protein synthesis in the cytoplasm. The ribosomal subunits are constructed in the nucleolus.

Rough ER

(rough endoplasmic reticulum) A network of interconnected membranous sacs in a eukaryotic cell’s cytoplasm. Rough ER membranes are studded with ribosomes that make membrane proteins and secretory proteins. The rough ER constructs membrane from phospholipids and proteins.

Smooth ER

(smooth endoplasmic reticulum) A network of interconnected membranous tubules in a eukaryotic cell’s cytoplasm. Smooth ER lacks ribosomes. Enzymes embedded in the smooth ER membrane function in the synthesis of certain kinds of molecules, such as lipids.

Transcription

The synthesis of RNA on a DNA template.

Translation

The synthesis of a polypeptide using the genetic information encoded in an mRNA molecule. There is a change of “language” from nucleotides to amino acids.

Vesicles

Within a eukaryotic cell, a hollow sac or bubble formed by membrane that is used to transport materials.

ATP

(adenosine triphosphate) A molecule composed of adenosine and three phosphate groups; the main energy source for cells. A molecule of ATP can be broken down to a molecule of ADP (adenosine diphosphate) and a free phosphate; this reaction releases energy that can be used for cellular work.

Cellular Respiration

The aerobic harvesting of energy from food molecules; the energy-releasing chemical breakdown of food molecules, such as glucose, and the storage of potential energy in a form that cells can use to perform work; involves glycolysis, the citric acid cycle, the electron transport chain, and chemiosmosis.

Chloroplasts

An organelle found in plants and photosynthetic protists. Enclosed by two concentric membranes, a chloroplast absorbs sunlight and uses it to power the synthesis of organic food molecules (sugars).

Mitochondria

(singular, mitochondrion) An organelle in eukaryotic cells where cellular respiration occurs. Enclosed by two concentric membranes, it is where most of the cell’s ATP is made.

Photosynthesis

The process by which plants, algae, and some bacteria transform light energy to chemical energy stored in the bonds of sugars. This process requires an input of carbon dioxide (CO2) and water (H2O) and produces oxygen gas (O2) as a waste product.

Cell Wall

A protective layer external to the plasma membrane in plant cells, bacteria, fungi, and some protists; protects the cell and helps maintain its shape.

Cilia

(singular, cilium) A short appendage that propels some protists through the water and moves fluids across the surface of many tissue cells in animals.

Cytoskeleton

A meshwork of fine fibers in the cytoplasm of a eukaryotic cell; includes microfilaments, intermediate filaments, and microtubules.

Extracellular Matrix

The meshwork that surrounds animal cells, consisting of a web of protein and polysaccharide fibers embedded in a liquid, jelly, or solid.

Flagellum

A long appendage that propels protists through the water and moves fluids across the surface of many tissue cells in animals. A cell may have one or more flagella.

Vacuoles

A membrane-enclosed sac, part of the endomembrane system of a eukaryotic cell, having diverse functions.