Chapter 7: Membrane Structure and Function
aquaporins
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.
glycolipids
A lipid with one or more covalently attached carbohydrates.
ligands
A molecule that binds specifically to another molecule, usually a larger one.
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.
Selective permeability
A property of biological membranes that allows them to regulate the passage of substances across them.
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.
ion channels
A transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient.
gated channels
A transmembrane protein channel that opens or closes in response to a particular stimulus.
transport proteins
A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.
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.
phagocytosis
A type of endocytosis in which large particulate substances or small organisms are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).
pinocytosis
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
receptor-mediated endocytosis
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
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.
How do aquaporins affect the permeability of a membrane?
Aquaporins are channel proteins that greatly increase the permeability of a membrane to water molecules, which are polar and therefore do not readily diffuse through the hydrophobic interior of the membrane.
How do you think a cell performing cellular respiration rids itself of the resulting CO2?
CO2 is a nonpolar molecule that can diffuse through the plasma membrane. As long as it diffuses away so that the concentration remains low outside the cell, it will continue to exit the cell in this way. (This is the opposite of the case for O2).
endocytosis
Cellular uptake of biological molecules and particulate matter via formation of vesicles from the plasma membrane.
As a cell grows, its plasma membrane expands. Does this involve endocytosis or exocytosis? Explain.
Exocytosis. When a transport vesicle fuses with the plasma membrane.
amphipathic
Having both a hydrophilic region and a hydrophobic region.
Which type of endocytosis involves ligands? What does this type of transport enable a cell to do?
In receptor-mediated endocytosis, specific molecules act as ligands when they bind to receptors on the plasma membrane. The cell can acquire bulk quantities of those molecules when a coated pit forms a vesicle and carries the bound molecules into the cell.
flaccid
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.)
Two molecules that can cross a lipid bilayer without help from membrane proteins are O2 and CO2. What property allows this to occur?
O2 and CO2 are both nonpolar molecules that can easily pass through the hydrophobic interior of a membrane.
ATP is not directly involved in the functioning of a cotransporter. Why, then, is cotrasport considered active transport?
One of the solutes moved by the cotransporter is actively transported against its concentration gradient. The energy for this transport comes from the concentration gradient of the other solute, which was established by an electrogenic pump that used energy to transport the other solute across the membrane.
In what ways are membranes crucial to life?
Plasma membranes define the cell by separating the cellular components from the external environment. This allows conditions inside cells to be controlled by membrane proteins, which regulate entry and exit of molecules and even cell function. The processes of life can be carried out inside the controlled environment of the cell, so membranes are crucial. In eukaryotes, membranes also function to subdivide the cytoplasm into different compartments where distinct processes can occur, even under differing conditions such as pH.
isotonic
Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell.
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.
osmoregulation
Regulation of solute concentrations and water balance by a cell or organism.
turgid
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 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).
Passive transport
The diffusion of a substance across a biological membrane with no expenditure of energy.
osmosis
The diffusion of free water across a selectively permeable membrane.
active transport
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.
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.
Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes. Do these pumps use ATP or produce ATP? Explain.
The pump uses ATP. To establish a voltage, ions have to be pumped against their gradients, which requires energy.
diffusion
The spontaneous movement of a substance down its concentration or electrochemical gradient, from a region where it is more concentrated to a region where it is less concentrated.
In the supermarket, produce is often sprayed with water. Explain why this makes vegetables look crisp.
The water is hypotonic to the plant cells, so the plant cells take up water. Thus, the cells of the vegetable remain turgid rather than plasmolyzing and the vegetable (for example, lettuce or spinach) remains crisp and not wilted.
What happens to a cell placed in a hypertonic solution? Describe the free water concentration inside and out.
There will be a net diffusion of water out of a cell into the solution (where water molecules are not free, but are clustered around the high concentration of solute particles.
The carbohydrates attached to some proteins and lipids of the plasma membrane are added as the membrane is made and refined in the ER and Golgi apparatus. The new membrane then forms transport vesicles that travel to the cell surface. On which side of the vesicle membrane are the carbohydrates?
They are on the inner side of the transport vesicle membrane.
Why is a transport protein needed to move water molecules rapidly and in large quantities across a membrane?
Water is a polar molecule, so it cannot pass very rapidly through the hydrophobic region in the middle of a phospholipid bilayer.
Which of the following factors would tend to increase membrane fluidity? a. a greater proportion of unsaturated phospholipids b. a greater proportion of saturated phospholipids c. a lower temperature d. a relatively high protein content in the membrane e. a greater proportion of relatively large glycolipids compared with lipids having smaller molecular masses.
a
In what way do the membranes of a eukaryotic cell vary? a. Phospholipids are found only in certain membranes. b. Certain proteins are unique to each membrane. c. Only certain membranes of the cell are selectively permeable. d. Only certain membranes are constructed from amphipathic molecules. e. Some membranes have hydropobic surfaces exposed to the cytoplasm, while others have hydrophilic surfaces facing the cytoplasm.
b
Which of these experimental treatments would increase the rate of sucrose transport into the cell? a. decreasing extracellular sucrose concentration b. decreasing extracellular pH c. decreasing cytoplasmic pH d. adding an inhibitor that blocks the regeneration of ATP e. adding a substance that makes the membrane more permeable to hydrogen ion.
b
According to the fluid mosaic model of membrane structure, proteins of the membrane are mostly a. spread in a continuous layer over the inner and outer surfaces of the membrane. b. confined to the hydrophobic interior of the membrane. c. embedded in a lipid bilayer. d. randomly oriented in the membrane, with no fixed inside-outside polarity. e. free to depart from the fluid membrane and dissolve in the surrounding solution.
c
Which of the following processes includes all others? a. osmosis b. diffusion of a solute across a membrane c. facilitated diffusion d. passive transport e. transport of an ion down its electrochemical gradient
d
