Chapter 7

facilitated diffusion

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

osmoregulation

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

passive transport

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

isotonic

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

ion channels

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

hypertonic

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

hypotonic

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

transport proteins

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

plasmolysis

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.

gated channels

A protein channel in a cell membrane that opens or closes in response to a particular stimulus.

Peripheral proteins

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

glycoproteins

A protein with one or more covalently attached carbohydrates.

concentration gradient

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

Integral proteins

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.)

sodium-potassium pump

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

aquaporins

A transport protein in the plasma membrane that specifically facilitates osmosis, the diffusion of free water across the membrane

How do membrane phospholipids interact with water? A. The polar heads interact with water; the nonpolar tails do not. B. Phospholipids do not interact with water because they are lipids, and thus are hydrophobic. C. The polar heads repel water and the nonpolar tails attract water. D. They have hydrophilic tails that face outward and are exposed to water and hydrophobic heads that face the center of the membrane and are shielded from water.

A. The polar heads interact with water; the nonpolar tails do not. A phospholipid is similar to a fat molecule but has only two fatty acids attached to glycerol rather than three. The third hydroxyl group of glycerol is joined to a phosphate group, which has a negative electrical charge in the cell. The fatty acids, referred to as the "tails" of the phospholipid, are hydrocarbons that are hydrophobic and therefore do not interact with water. The phosphate group and its attachments form a hydrophilic "head" that has an affinity for water.

Hydrophobic substances like salad oil are A. nonpolar molecules that repel water molecules. B. polar molecules that have an affinity for water because they contain many hydrogens that can form hydrogen bonds with water. C. nonpolar molecules that have an affinity for water because they contain many hydrogens that can form hydrogen bonds with water. D. polar molecules that repel water molecules.

A. nonpolar molecules that repel water molecules. Salad oil is predominantly made up of carbon and hydrogen atoms, which share electrons almost equally, forming nonpolar covalent bonds. Substances that are nonpolar due to their large number of nonpolar bonds do not have an affinity for water and are termed hydrophobic ("water-fearing"). Substances that contain polar bonds are hydrophilic ("water-loving") because they contain atoms with partial charges due to those polar bonds. For example, a hydrogen atom with a partial positive charge can form a hydrogen bond with a partial negative charge on the oxygen atom of a water molecule.

proton pump

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.

electrogenic pump

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

According to the fluid mosaic model, a membrane ________. A. is composed of a mosaic of fluid polysaccharides and amphipathic proteins B. is composed of a fluid bilayer of phospholipids with embedded amphipathic protein C. sis composed of a fluid bilayer of phospholipids between two layers of hydrophilic protein D. sis composed of a single layer of fluid phospholipids between two layers of hydrophilic proteins

B. is composed of a fluid bilayer of phospholipids with embedded amphipathic protein

The permeability of a biological membrane to a specific polar solute depends primarily on which of the following? See Concept 7.2 (Page 132) A. the amount of cholesterol in the membrane B. the types of transport proteins in the membrane C. the phospholipid composition of the membrane D. the presence of unsaturated fatty acids in the membrane E. the types of polysaccharides present in the membrane

B. the types of transport proteins in the membrane The lipid bilayer will be impermeable or very poorly permeable to polar or charged solutes. The presence of the correct transport protein will determine the permeability.

Which of the following statements about a typical plasma membrane is correct? A. Carbohydrates on the membrane surface are important in determining the overall bilayer structure. B. The plasma membrane is a covalently linked network of phospholipids and proteins that controls the movement of solutes into and out of a cell. C. The two sides of the plasma membrane have different lipid and protein composition. D. The hydrophilic interior of the membrane is composed primarily of the fatty acid tails of the phospholipids. E. Phospholipids are the primary component that determines which solutes can cross the plasma membrane.

C. The two sides of the plasma membrane have different lipid and protein composition. Because the membrane serves different functions on the cytoplasmic and exterior surfaces, the structure and composition of the surfaces must be different.

endocytosis

Cellular uptake of biological molecules and particulate matter via formation of new vesicles from the plasma membrane.

Active and passive transport of solutes across a membrane typically differ in which of the following ways? See Concept 7.4 (Page 136) A. Active transport is usually down the concentration gradient of the solute, whereas passive transport is always against the concentration gradient of the solute. B. Active transport uses protein carriers, whereas passive transport uses carbohydrate carriers. C. Active transport is always faster than passive transport. D. Active transport always involves the utilization of cellular energy, whereas passive transport does not require cellular energy. E. Active transport is used for ions, passive transport is used for uncharged solutes.

D. Active transport always involves the utilization of cellular energy, whereas passive transport does not require cellular energy. Active and passive transport can be distinguished by whether or not they use cellular energy.

Which of the following statements is true regarding potential energy? A. Water acquires potential energy as it runs downhill. B. Matter has a natural tendency to acquire more potential energy until a maximum is reached. C. Potential energy is the energy matter could have if it were in a different location or structure. D. Potential energy is the energy possessed by matter due to its location or structure.

D. Potential energy is the energy possessed by matter due to its location or structure. Energy is defined as the capacity to cause change — for instance, by doing work. Potential energy is the energy that matter possesses because of its location or structure. Matter has a natural tendency to move from higher states of potential energy toward the lowest possible state of potential energy, such as water running downhill from a dam. As water runs downhill, the energy released can be used to do work.

In facilitated diffusion, what is the role of the transport protein? See Concept 7.3 (Page 135) A. Transport proteins provide a protein site for ATP hydrolysis, which facilitates the movement of a solute across a membrane. B. Transport proteins organize the phospholipids to allow the solute to cross the membrane. C. Transport proteins provide a low-resistance channel for water molecules to cross the membrane. D. Transport proteins provide a hydrophilic route for the solute to cross the membrane. E. Transport proteins provide the energy for diffusion of the solute.

D. Transport proteins provide a hydrophilic route for the solute to cross the membrane. This is the most general description of facilitated diffusion by membrane transport proteins.

Which of the following correctly describes some aspect of exocytosis or endocytosis? See Concept 7.5 (Page 139) A. These two processes require the participation of mitochondria. B. Endocytosis and exocytosis involve passive transport. C. The inner surface of a transport vesicle that fuses with or buds from the plasma membrane is most closely related to the inner surface of the plasma membrane. D. Both processes provide a mechanism for exchanging membrane-impermeable molecules between the organelles and the cytosol. E. Exocytosis and endocytosis temporarily change the surface area of the plasma membrane.

E. Exocytosis and endocytosis temporarily change the surface area of the plasma membrane. The fusion or budding of transport vesicles at the plasma membrane either adds or removes proteins and phospholipids, thus temporarily changing the surface area.

turgid (very firm)

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

tonicity

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

exocytosis

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

cotransport

The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

fluid mosaic model

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

membrane potential

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.

electrochemical gradient

The diffusion gradient 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).

diffusion

The random thermal of particles of liquids, gasses, or solids. In the presence of a concentration or electrochemcial 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.

glycolipids

a lipid with one or more covalently attached carbohydrates

amphipathic

having both a hydrophilic region and a hydrophobic region

flaccid

limp, not firm; lacking vigor or effectiveness. As in a plant cell in surroundings where there is a tendency for water to leave the cells. A walled cell becomes flaccid if it has a higher water potential than its surroundings, resulting in loss of water.