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AND F UNCTION
The sandwich model and its associated unit membrane model
preceded the currently accepted f luid-mosaic model of
membrane structure. According to the fluid-mosaic model,
phospholipid molecules provide a fluid, lipid bilayer with
their polar heads at the membrane surfaces. Hydrophobic portions of protein molecules are in the lipid bilayer;
their hydrophilic portions are at the surfaces. The proteins
form channels and function as receptors, enzymes, and
Molecules move across a membrane in several ways.
By d iffusion, molecules move down their concentration
gradient. O smosis is the diffusion of water through a differentially permeable membrane. When cells are in a h ypotonic solution, they gain water; when they are in a
hypertonic solution, they lose water. In i sotonic solutions, cells neither gain nor lose water. Both diffusion and osmosis are passive processes that do not require energy.
Facilitated transport is also passive and involves carrier
molecules moving substances from higher to lower concentrations. Active transport moves substances in the opposite direction, with the function of a carrier molecule
and energy. Larger substances pass through cells by e ndocytosis and e xocytosis.
An extracellular matrix is a meshwork of insoluble proteins and carbohydrates that is now known to influence animal cell differentiation and cellular
metabolism. Plant cells are bounded by a cell wall that
is external to the plasma membrane. This boundary is
freely permeable. Several types of junctions exist between
animal cells: desmosomes, gap junctions, and tight junctions. Between plant cells, plasmodesmata perform this
function. S T U DY E X E R C I S E S
Study the text section by section as you answer the questions that follow. 4.1 PLASMA MEMBRANE STRUCTURE AND FUNCTION (P 68)
. • The plasma membrane regulates the passage of molecules into and out of the cell.
• The membrane contains lipids and proteins. Each protein has a specific function.
1 . Label this diagram of the plasma membrane with the following terms:
protein molecule c. d.
f. | L L 30 | e-Text Main Menu | Textbook Table of Contents | Study Guide Table of Contents 2. The two components of the fluid-mosaic model of membrane structure are
3. b. ______________. a. ______________ and the
lipid a. ______________ form a bilayer, in which the c. ______________ d. ______________, b. ______________ heads are at the surfaces of the membranes, tails face each other, making up the interior of the membrane. The which is also in the membrane, e. ______________ the membrane’s permeability. 4. Complete the sentences, using the terms h ydrophilic a nd/or h ydrophobic: T ransmembrane proteins are found
within the plasma membrane. a. ______________ regions are embedded within the membrane, and b. ______________ regions project from both surfaces of the bilayer.
5. Both glycolipids and glycoproteins have a(n) a. ______________ chain and are active in cell
to b. ______________ recognition. 6. Label the diagrams of proteins found in the membrane and state a function on the lines provided:
cell recognition protein
receptor protein a. ______________ b. ______________ c. ______________ ________________ ________________ ________________ d. _______________ e. _______________ _________________ _________________ | L L 31 | e-Text Main Menu | Textbook Table of Contents | Study Guide Table of Contents 4.2 THE PERMEABILITY OF THE PLASMA MEMBRANE (P 72)
. • Some substances, particularly small, noncharged molecules, pass freely across the plasma membrane. Ions
and other types of molecules need assistance to cross the membrane.
7. In the following diagram, assume that glucose and water can cross the membrane and that protein cannot. Will the amount of water on side A stay the same, or increase or decrease with time? a. ______________ Will the amount of protein on side A stay the same, or increase or decrease with time?
Will glucose cross the membrane toward side A or toward side B? b. ______________ c. ______________ What will happen to the level of solution on each side of the membrane? d. _______________________________ _________________________________________________________________________________________________ 4.3 DIFFUSION AND OSMOSIS (P 73)
. • Molecules spontaneously diffuse (Move from an area of higher concentration to an area of lower
concentration), and some can diffuse across a plasma membrane.
• Water diffuses across the plasma membrane, and this can affect cell size and shape.
8. Label each of the following as diffusion (D) or osmosis (O):
a. Algae in a pond become dehydrated.
b. A hypertonic solution draws water out of a cell.
c. A red blood cell bursts in a test tube.
d. Dye crystals spread out in a beaker of water.
e. Gases move across the plasma membrane.
f. Perfume is sensed from the other side of a room.
9. Complete this diagram to describe the effect of tonicity on red blood cells. | L L 32 | e-Text Main Menu | Textbook Table of Contents | Study Guide Table of Contents 10. Complete this diagram to describe the effect of tonicity on plant cells. 11. If a solution is 8% solute, it is a. _____________% If a solution is 99.5% solvent, it is b . __________% solvent.
solute. If solution A is 2% solute and solution B is 3% solute, then solution A is
is c. ______________ to solution B which d.______________ to solution A. Compared to solution A, a solution with 2% solute is 4.4 TRANSPORT BY e. ______________. CARRIER PROTEINS (P 76)
. • Carrier proteins assist the transport of some ions and molecules across the plasma membrane.
12. Label each of the following as describing facilitated transport (F) only, active (A) transport only, or both
(F, A) processes:
a. Uses a carrier molecule.
b. Substances travel down a concentration gradient.
c. Substances travel against a concentration gradient.
d. Sodium-potassium pump.
e. Energy is not required. 4.5 EXOCYTOSIS AND ENDOCYTOSIS (P 78)
. • Vesicle formation takes other substances into the cell, and vesicle fusion with the plasma membrane
discharges substances from the cell.
13. Label each of the following as describing exocytosis (Ex) or endocytosis (En):
a. Vesicles formed by the Golgi apparatus fuse with the plasma membrane.
b. Materials leave the cell.
c. Phagocytosis is an example.
d. Pinocytosis is an example.
e. Occurs after receptors bind to a ligand. | L L 33 | e-Text Main Menu | Textbook Table of Contents | Study Guide Table of Contents CHAPTER TEST
Do not refer to the text when taking this test. In questions 1–7, match the following descriptions with each
a. small particle or liquid intake into a cell
b. requires vacuole formation
c. carrier molecule, no energy
d. carrier molecule, energy required
e. water enters a hypertonic solution from a
f. secretion from the cell
g. dye molecules spread through water
1. active transport
4. facilitated transport
8. Proteins form the nonactive matrix of the plasma
9. Which type of molecule forms a bilayer within
the plasma membrane?
d. nucleic acid
10. Hydrophilic ends of proteins are oriented toward
11. Lipid-soluble molecules pass through the plasma
a. active transport.
b. diffusing through it.
c. facilitated transport.
d. use of the sodium-potassium pump.
12. A 2% salt solution is ______________ to a 4%
13. Which molecule is directly required for operation of the sodium-potassium pump?
b. NAD +
d. water 14. In cells, which process moves materials opposite
to the direction of the other three?
15. Which of the following statements is not true
a. A differentially permeable membrane must
separate two solutions.
b. One side of the membrane must have more
water than the other side of the membrane.
c. The membrane must permit water to pass.
d. Over time, the side that initially had the larger
concentration of solute will become even more
16. A small lipid-soluble molecule passes easily
through the plasma membrane. Which of these
statements is the most likely explanation?
a. A carrier protein must be at work.
b. The plasma membrane is partially composed
of lipid molecules.
c. The cell is expending energy to do this.
d. Phagocytosis has enclosed this molecule in a
17. Which of these does NOT require an expenditure of energy?
c. facilitated transport
d. None of these require energy.
18. Which term refers to the bursting of an animal
d. turgor pressure
19. An animal cell always takes in water when placed
in a(n) ______________ solution.
20. Which of the following is actively transported across
a. carbon dioxide
d. sodium ions | L L 34 | e-Text Main Menu | Textbook Table of Contents | Study Guide Table of Contents THOUGHT QUESTIONS
A nswer in complete sentences.
21. Why is the plasma membrane considered so important to the cell? 22. What osmotic problem would plant cells experience if they lost their cell walls? Test Results: ______ Number right ÷ 22 = ______ × 100 = ______ % ANSWER KEY
CHAPTER TEST STUDY EXERCISES
1. a. glycoprotein b. carbohydrate chain c. glycolipid
d. protein molecule e. phospholipid bilayer f. cytosketon filaments g. c holesterol 2. a. lipids b. proteins
3. a. Phospholipids b. hydrophilic (polar) c. hydrophobic
d. cholesterol e. reduces 4. a. hydrophobic b. hydrophilic 5. a. carbohydrate b. cell 6. a. receptor
protein, shaped in such a way that a specific molecule
can bind to it. b. channel protein, allows molecules to pass
across the plasma membrane. c. cell recognition protein, functions in cell to cell recognition. d. enzymatic
protein, catalyzes a specific reaction. e. carrier protein,
allows selective passage of molecules across the plasma
membrane. 7. a increase b. stay the same c. toward
side B d. side A e. Side A will rise, and side B will fall.
8. a. O b. O c. O d. D e. D f. D 9. a. cell is same
size and shape b. Hypertonic Solution c. cell is bursting 10. a. Isotonic Solution b. Hypotonic Solution
c. vacuole is much smaller 11. a. 92 b. 0.5 c. hypotonic
d. hypertonic e. isotonic 12. a. F, A b. F c. A d. A e. F
13. a. Ex b. Ex c. En d. En e. En 1. d 2. g 3. f 4. c 5. e 6. b 7. a 8. b 9. c
10. a 11. b 12. b 13. a 14. b 15. d 16. b
17. d 18. c 19. d 20. d 21. The plasma membrane
is very important because it is the outer living boundary
of the cell; it provides for and regulates the passage of molecules into and out of the cell. 22. In a hypotonic environment, plant cells would continue to gain water until
they burst. | L L 35 | e-Text Main Menu | Textbook Table of Contents | Study Guide Table of Contents ...
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