1. Amphipathic means that the phospholipid has both a hydrophilic region and a hydrophobic
region.
2. In this model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of
phospholipids.
3. A membrane is held together primarily by hydrophobic interactions, which are much weaker
than covalent bonds. Most of the lipids and some of the proteins can shift about laterally—that is,
in the plane of the membrane. The lateral movement of phospholipids within the membrane is
rapid, occurring about 107 times per second.
4. A membrane remains fluid as temperature decreases until finally the phospholipids settle into
a closely packed arrangement and the membrane solidifies. The temperature at which a
membrane solidifies depends on the types of lipids it is made of.
b. The membrane remains fluid to a lower temperature if it is rich in phospholipids with
unsaturated hydrocarbon tails. Because of kinks in the tails where double bonds are located,
unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, and
this makes the membrane more fluid.
c. At relatively high temperatures—at 37°C, the body temperature of humans, for example—
cholesterol makes the membrane less fluid by restraining phospholipid movement.
d. Saturated hydrocarbon tails pack together, increasing membrane viscosity and decreasing
fluidity.
5. integral proteins: Penetrate the hydrophobic interior of the lipid bilayer.
peripheral proteins: Appendages loosely bound to the surface of the membrane, often to
exposed parts of integral proteins.
Study Figure 7.7 in your text. Use it to briefly describe the following major functions of
membraneproteins.
Transport A protein that spans the membrane may provide a hydrophilic channel across
the membrane that is selective for a particular solute. Other transport proteins shuttle a substance
from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an
energy source to pump substances across the membrane.
Enzymatic activity A protein built into the membrane may be an enzyme with its active site
exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane
are organized as a team that carries out sequential steps of a metabolic pathway.
Signal transduction A membrane protein (receptor) may have a binding site with a specific shape
that fits the shape of a chemical messenger, such as a hormone. The external messenger
(signaling molecule) may cause the protein to change shape, allowing it to relay the message to
the inside of the cell, usually by binding to a cytoplasmic protein.
Cell-cell recognition Some glycoproteins serve as identification tags that are specifically
recognized by membrane proteins of other cells.
Intercellular joining Membrane proteins of adjacent cells may hook together in various kinds of
junctions, such as gap junctions or tight junctions.
Attachment to cytoskeleton and ECM Microfilaments or other elements of the cytoskeleton may
be noncovalently bound to membrane proteins, a function that helps maintain cell shape and
stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules
can coordinate extracellular and intracellular changes.
7. Two examples include the sorting of cells into tissues and organs in an animal embryo, and the
rejection of foreign cells by the immune system.
8. Glycolipids: Membrane carbohydrates covalently bonded to lipids.
Glycoproteins: Membrane carbohydrates covalently bonded to proteins.
9. See page 125 in your text for the labeled figure.
Concept 7.2 Membrane structure results in selective permeability
10. Channel proteins function by having a hydrophilic channel that certain molecules or atomic
ions use as a tunnel through the membrane. Carrier proteins hold on to their passengers and
change shape in a way that shuttles them across the membrane.
11. Yes. A transport protein is specific for the substance it translocates, allowing only a certain
substance to cross the membrane. For example, a specific water carrier protein in the plasma
membrane of red blood cells transports glucose across the membrane.
12. Aquaporins are channel proteins that facilitate the passage of water molecules through the
membranes of certain cells.
13. Material Method
CO2 simple diffusion
Glucose transport proteins
H+ transport proteins
O2 simple diffusion
H2O simple diffusion and protein channels (aquaporins)
Concept 7.3 Passive transport is diffusion of a substance across a membrane with no energy
investment
14.
diffusion: The movement of molecules of any substance so that they spread out evenly into the
available space.
concentration gradient: The region along which the density of a chemical substance increases
or decreases.
passive transport: Diffusion of a substance across a biological membrane; a cell does not have
to expend energy to make it happen.
osmosis: The diffusion of water across a selectively permeable membrane.
isotonic: A solution with the same concentration of solutes as the cell it surrounds. There will be
no net movement of water across the plasma membrane. In an isotonic environment, the volume
of the animal cell is stable.
hypertonic: A solution with more solutes than the cell it surrounds. The cell will lose water,
shrivel, and probably die.
hypotonic: A solution with less solutes than the cell it surrounds. The cell will swell and lyse
(burst).
turgid: Very firm.
flaccid: Limp.
plasmolysis: Phenomenon during which the plant cell shrivels, and its plasma membrane pulls
away from the wall.
15. The cells of the carrot contain more water than the surrounding air, and therefore water will
leave the carrot cells. The cells are hypotonic to the surrounding air. As water leaves the carrot
cells, the cells will become flaccid as plasmolysis occurs. The water is leaving the cells by
osmosis; the solutes remain in the cells.
16. Facilitated diffusion is the phenomenon during which polar molecules and ions impeded by
the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span
the membrane.
Facilitated diffusion is considered passive transport because the solute is moving down its
concentration gradient, a process that requires no energy. Examples include the movement of
water through aquaporins, movement of sugars through protein channels, sodium ion gated
channels in nerve cells, and any other example that requires a transport protein.
17. See page 132 in your text for the labeled figure.
18. Plant cells have a cell wall and animal cells do not. Plant cells are turgid (firm) and generally
healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the
wall pushing back on the cell.
19. Active transport is a type of membrane traffic during which the cell must expend energy. The
transport proteins involved are called carrier proteins. ATP supplies the energy for most active
transport.
20. See page 135 in your text for the labeled figure.
Summary
1. Cytoplasmic Na+ binds to the sodium-potassium pump.
2. This stimulates phosphorylation of the pump by ATP.
3. The pump changes shape and releases Na+ to the outside.
4. The new pump shape now has an affinity for K+, and binds them. This triggers the
dephosphorylation of the pump.
5. Loss of the phosphate group restores the protein’s original shape , which has a lower affinity
for K+, releasing it to the other side of the membrane.
6. In the new shape, the pump will now bind Na+ again.
21. See page 135 in your text for the labeled figure.
Examples
Facilitated diffusion with a carrier protein: glucose through glucose transporters
Facilitated diffusion with a channel protein: aquaporins transporting water
Active transport with a carrier protein: sodium-potassium pump
Simple diffusion: movement of oxygen
Concept 7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
25. Define each of the following, and give a specific cellular example.
exocytosis: The cellular secretion of biological molecules by fusion of vesicles containing them
with the plasma membrane.
endocytosis: Cellular uptake of biological molecules and particulate matter via formation of
vesicles from the plasma membrane.
receptor-mediated endocytosis: The movement of specific molecules into a cell by the inward
budding of vesicles containing proteins with receptor sites specific to the molecules being taken
in; enables a cell to acquire bulk quantities of specific substances.
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 certain immune cells of animals.
pinocytosis: A type of endocytosis in which the cell ingests extracellular fluid and its dissolved
solutes.
26. A ligand is a molecule that binds specifically to another molecule, usually a larger one.
Human cells use receptor-mediated endocytosis to take in cholesterol for membrane synthesis
and the synthesis of other steroids. Cholesterol travels in the blood in particles called low-density
lipoproteins (LDLs), each a complex of lipids and a protein. LDLs bind to the LDL receptors on
plasma membranes and then enter the cell by endocytosis. The LDLs thus act as ligands.