Chapter 4 Biology Now

Cell

The smallest and most basic unit of life -- a microscopic, self-contained unit enclosed by a water-repelling membrane. (page 60)

Genome

The complete set of genes of an organism. (page 60)

DNA

Deoxyribonucleic acid, the genetic code of life, consisting of two parallel strands of nucleotides twisted into a double helix. DNA is the genetic material that transfers information from parents to offspring. (page 60)

Gene

The basic unit of information, consisting of a stretch of DNA, that codes for a distinct genetic characteristic. (page 60)

Cell theory

One of the unifying principles of biology; a theory stating that every living organism is composed of one or more cells, and that all cells living today came from a preexisting cell. (page 61)

Virus

A small, infectious agent that can replicate only inside a living cell. (page 61)

Adenine (A)

One of the four nucleotides that make up DNA. The other three are thymine (T), guanine (G), and cytosine (C). (page 61)

Thymine (T)

One of the four nucleotides that make up DNA. The other three are adenine (A), guanine (G), and cytosine (C). (page 61)

Guanine (G)

One of the four nucleotides that make up DNA. The other three are adenine (A), thymine (T), and cytosine (C). (page 61)

Cytosine (C)

One of the four nucleotides that make up DNA. The other three are adenine (A), thymine (T), and guanine (G). (page 61)

Plasma membrane

A barrier consisting of a phospholipid bilayer that separates a cell from its external environment. (page 63)

Phospholipid

An organic molecule with a hydrophilic head and a hydrophobic tail. (page 63)

Phospholipid bilayer

A double layer of phospholipids in which the heads face out and the tails face in. Plasma membranes are phospholipid bilayers. (page 63)

Liposome

A sphere formed by a phospholipid bilayer. (page 63)

Transport protein

A protein that acts like a gate, channel, or pump that allows molecules to move into and out of a cell. (page 64)

Selective permeability

The quality of plasma membranes by which some substances are allowed to cross the membrane at all times, others are excluded at all times, and still others can pass through the membrane when they are aided by transport proteins. (page 64)

Active transport

The movement of a substance in response to an input of energy. Compare passive transport. (page 64)

Passive transport

The movement of a substance without the addition of energy. Compare active transport. (page 64)

Diffusion

The movement of a substance from a region of higher concentration to a region of lower concentration. (page 65)

Simple diffusion

Diffusion in which substances such as the small, uncharged molecules of water, oxygen, or carbon dioxide, slip between the large molecules in the phospholipid bilayer without much hindrance. Compare facilitated diffusion. (page 65)

Osmosis

A form of simple diffusion in which water moves in and out of cells (and compartments inside cells). (page 65)

Viral strain

Also called serotype. Any of the variant forms of a particular type of virus. (page 66)

Isotonic

Describing a fluid that has a solute concentration equal to that of the cell it surrounds. Compare hypertonic and hypotonic. (page 66)

Hypertonic

Describing a fluid that has a solute concentration higher than that of the cell it surrounds. Compare hypotonic and isotonic. (page 66)

Hypotonic

Describing a fluid that has a solute concentration lower than that of the cell it surrounds. Compare hypertonic and isotonic. (page 66)

Facilitated diffusion

Diffusion that requires transport proteins. Compare simple diffusion. (page 66)

Receptor protein

A site where a molecule from another cell can bind. (page 66)

Vesicle

A sac, formed by the bulging inward or outward of a section of the plasma membrane, that moves molecules from place to place inside a cell but also may transport substances into and out of the cell. (page 67)

Exocytosis

The process by which materials are exported out of a cell via vesicles. Compare endocytosis. (page 67)

Endocytosis

The process by which materials are transported into a cell via vesicles. Compare exocytosis. (page 67)

Receptor-mediated endocytosis

A form of specific endocytosis in which receptor proteins embedded in the membrane recognize specific surface characteristics of substances that will be incorporated into the cell. (page 68)

Phagocytosis

Literally, 'cellular eating.' A large-scale version of endocytosis in which particles considerably larger than biomolecules are ingested. Compare pinocytosis. (page 68)

Pinocytosis

Literally, 'cellular drinking.' A large-scale version of endocytosis in which fluids are ingested. Compare phagocytosis. (page 68)

Prokaryote

An organism that belongs to either the Bacteria or the Archaea. Compare eukaryote. (page 68)

Eukaryote

An organism that belongs to the Eukarya. Animals, plants, fungi, and protists are all eukaryotes. Compare prokaryote. (page 68)

Nucleus (pl. nuclei)

1. The dense core of an atom, which contains protons and neutrons. 2. The control center of the eukaryotic cell, containing all of the cell's DNA and occupying up to 10 percent of the space inside the cell. (page 68)

Organelle

Any of the membrane-enclosed subcellular compartments found in eukaryotic cells. (page 68)

Nuclear envelope

The boundary of a cell's nucleus, consisting of two concentric phospholipid bilayers. (page 68)

Nuclear pore

Any of many small openings in the nuclear envelope that allow chemical messages to enter and exit the nucleus. (page 68)

Endoplasmic reticulum (ER)

An extensive and interconnected network of sacs made of a single membrane that is continuous with the outer membrane of the nuclear envelope. See also rough ER and smooth ER. (page 72)

Smooth ER

A part of the endoplasmic reticulum, having a smooth appearance, where lipids and hormones are manufactured. Compare rough ER. (page 72)

Ribosome

The site of protein synthesis (translation) in the cytoplasm. Ribosomes are embedded in the rough endoplasmic reticulum. (page 72)

Rough ER

A part of the endoplasmic reticulum, having a knobby appearance because of embedded ribosomes, where proteins are assembled. Compare smooth ER. (page 72)

Golgi apparatus

A collection of flattened membranes that packages and directs proteins and lipids produced by the ER to their final destinations either inside or outside the cell. (page 72)

Lysosome

An organelle in animal cells that acts as a garbage or recycling center. Compare vacuole. (page 72)

Vacuole

An organelle in plant cells that acts as a garbage or recycling center and that stores water. Compare lysosome. (page 72)

Mitochondrion (pl. mitochondria)

An organelle that is a tiny power plant fueling cellular activities. Mitochondria are the main source of energy in eukaryotic cells. (page 72)

Chloroplast

An organelle of plant cells and some protist cells that captures energy from sunlight and uses it to manufacture food molecules via photosynthesis. (page 72)

Cytoskeleton

The network of protein cylinders and filaments that forms the framework of a cell. (page 72)