Chapter 5: Membrane Transport and Cell Signaling

Question 1

selective permeability

Answer 1

a property of biological membranes that allows them to regulate the passage of substances across them

Question 2

amphipathic

Answer 2

having both a hydrophilic region and a hydrophobic region

Question 3

fluid mosaic model

Answer 3

the currently accepted model of cell membrane structure, which envisions the membrane as as mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids

Question 4

integral proteins

Answer 4

a transmembrane protein with hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on one or both sides of the membrane (or lining the channel in the case of a channel protein).

Question 5

peripheral proteins

Answer 5

a protein loosely bound to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer

Question 6

glycolipids

Answer 6

a lipid with one or more covalently attached carbohydrates

Question 7

glycoproteins

Answer 7

a protein with one or more covalently attached carbohydrates

Question 8

transport proteins

Answer 8

a transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane

Question 9

aquaporins

Answer 9

a channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of free water across the membrane

Question 10

diffusion

Answer 10

the random thermal motion of particles of liquids, gases, or solids. in the presence of a concentration or electrochemical gradient, diffusion results in the net movement of a substance from a region where it is more concentrated to a region where it is less concentrated.

Question 11

concentration gradient

Answer 11

a region along which the density of a chemical substance increases or decreases

Question 12

passive transport

Answer 12

the diffusion of a substance across a biological membrane with no expenditure of energy

Question 13

osmosis

Answer 13

the diffusion of free water molecules across a selectively permeable membrane

Question 14

tonicity

Answer 14

the ability of a solution surrounding a cell to cause that cell to gain or lose water

Question 15

isotonic

Answer 15

referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell

Question 16

hypertonic

Answer 16

referring to a solution that, when surrounding a cell, will cause the cell to lose water

Question 17

hypotonic

Answer 17

referring to a solution that, when surrounding a cell, will cause the cell to take up water

Question 18

osmoregulation

Answer 18

regulation of solute concentrations and water balance by a cell or organism

Question 19

turgid

Answer 19

swollen or distended, as in plant cells. (a walled cell becomes turgid if it has a lower water potential than its surroundings, resulting in an entry of water.)

Question 20

flaccid

Answer 20

limp. lacking turgor (stiffness or firmness), as in a plant cell in surroundings where there is a tendency for water to leave the cell. (a walled cell becomes flaccid if it has a higher water potential than its surroundings, resulting in the loss of water.)

Question 21

plasmolysis

Answer 21

a phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall; occurs when the cell loses water to a hypertonic environment.

Question 22

facilitated diffusion

Answer 22

the passage of molecules or ins down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure

Question 23

ion channels

Answer 23

a transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient

Question 24

gated channels

Answer 24

a transmembrane protein channel that opens or closes in response to a particular stimulus

Question 25

active transport

Answer 25

the movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy
as in other types of cellular work, ATP hydrolysis supplies the energy for most active transport.

Question 26

sodium-potassium pump

Answer 26

a transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell

Question 27

membrane potential

Answer 27

the difference in electrical charge (voltage) across a cell’s plasma membrane due to the differential distribution of ions. membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances

Question 28

electrochemical gradient

Answer 28

the diffusion of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion’s tendency to move relative to the membrane potential (an electrical force).

Question 29

electrogenic pump

Answer 29

an active transport protein that generates voltage across a membrane while pumping ions

Question 30

proton pump

Answer 30

an active transport protein in a cell membrane that uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process

Question 31

exocytosis

Answer 31

the cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane

Question 32

endocytosis

Answer 32

cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane

Question 33

phagocytosis

Answer 33

a cell engulfs a particle by extending pseudopodia around it and packaging it within a membranous sac called a food vacuole. the particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes.

Question 34

pinocytosis

Answer 34

a cell continually “gulps” droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane. in this way, the cell obtains molecules dissolved in the droplets. because any and all solutes are taken into the cell, pinocytosis is nonspecific for the substances it transports. in many cases, the parts of the plasma membrane that form vesicles are lined on their cytoplasmic side by a fuzzy layer of coat protein; the “pits” and resulting vesicles are called coated pits.

Question 35

receptor-mediated endocytosis

Answer 35

a specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. embedded in the plasma membrane are proteins with receptor sites exposed to the extracellular fluid. specific solutes bind to the receptors. the receptor proteins then cluster in coated pits, and each coated pit forms a vesicle containing the bound molecules. after the ingested material is liberated from the vesicle, the emptied receptors are recycled to the plasma membrane by the same vesicle.

Question 36

local regulators

Answer 36

a secreted molecule that influences cells near where it i secreted

Question 37

synaptic signaling

Answer 37

a nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell, such as a muscle or another nerve cell.

Question 38

endocrine (hormonal) signaling

Answer 38

specialized endocrine cells secrete hormones into body fluids, often blood. hormones reach most body cells, but are bound by and affect only some cells.

Question 39

hormones

Answer 39

in multicellular organisms, one of many types of secreted chemicals that are formed in specialized cells, travel in body fluids, and act on specific target cells in other parts of the body, changing the target cells’ functioning. hormones are thus important in long-distance signaling.

Question 40

reception

Answer 40

the binding of a signaling molecule to a receptor protein, activating the receptor by causing it to change shape

Question 41

signal transduction pathway

Answer 41

a series of steps linking a mechanical, chemical, or electrical stimulus to a specific cellular response

Question 42

response

Answer 42

in cellular communication, the change in a specific cellular activity brought about by a transduced signal from outside the cell

Question 43

ligand

Answer 43

a molecule that binds specifically to another molecule, usually a larger one

Question 44

G protein-coupled receptor

Answer 44

a signal receptor protein in the plasma membrane that responds to the binding of a signaling molecule by activating a G protein. also called a G protein-linked receptor.

Question 45

G protein

Answer 45

a GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G protein-coupled receptor, to other signal transduction proteins inside the cell

Question 46

ligand-gated ion channel

Answer 46

a transmembrane protein containing a pore that opens or closes as it changes shape in response to a signaling molecule (ligand), allowing or blocking the flow of specific ions; also called an ionotropic receptor.

Question 47

protein kinase

Answer 47

an enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein

Question 48

phosphorylation cascade

Answer 48

a series of chemical reactions during cell signaling mediated by enzymes (kinases), in which each kinase in turn phosphorylates and activates another, ultimately leading to phosphorylation of many proteins

Question 49

protein phosphatases

Answer 49

an enzyme that removes phosphate groups from (dephosphorylates) proteins, often functioning to reverse the effect of a protein kinase

Question 50

second messengers

Answer 50

a small, nonprotein, water-soluble molecule or ion, such as a calcium ion or cyclic AMP, that relays a signal to a cell’s interior in response to a signaling molecule bound by a signal receptor protein

Question 51

cyclic AMP (cAMP)

Answer 51

cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic cells. it is also a regulator of some bacterial operons.