Ch 5: Membrane Transport and Cell Signaling

Question 1

Some solutes are able to pass directly through the lipid bilayer of a plasma membrane, whereas other solutes require a transport protein or other mechanism to cross between the inside and the outside of a cell. The fact that the plasma membrane is permeable to some solutes but not others is what is referred to as selective permeability.
Which of the following molecules can cross the lipid bilayer of a membrane directly, without a transport protein or other mechanism? Select all that apply.

proteins
lipids
ions
sucrose
oxygen
carbon dioxide
water

Answer 1

lipids, oxygen, carbon dioxide, water

Question 2

Carrier proteins and channels are both transport proteins involved in ___________ diffusion, the passive transport of solutes across a membrane down their concentration or electrochemical gradient. As integral membrane proteins, both carriers and channels protect polar or charged solutes from coming into contact with the hydrophobic interior of the lipid bilayer. Furthermore, all transport proteins are specific for the solutes they transport, owing to the specificity of the interactions between the solute and the transport protein.
Channels are protein-lined pores across the membrane. A channel may be open at all times (non-gated), or may be gated such that the channel opens and closes under specific conditions. Channels transport _________ ____ or water.

Answer 2

facilitated
inorganic ions

Question 3

Some solutes pass readily through the lipid bilayer of a cell membrane, whereas others pass through much more slowly, or not at all.
Small nonpolar (hydrophobic) molecules, such as dissolved gases (O2, CO2, N2) and small lipids, can pass \_\_\_\_\_\_\_\_ through the membrane. They do so by interacting directly with the hydrophobic interior of the lipid bilayer.

Answer 3

directly

Question 4

Very small polar molecules such as water and glycerol can pass directly through the membrane, but much more ______ than small nonpolar molecules. The mechanism that permits small polar molecules to cross the hydrophobic interior of the lipid bilayer is not completely understood, but it must involve the molecules squeezing between the hydrophobic tails of the lipids that make up the bilayer.

Polar molecules such as glucose and sucrose have very limited permeability.
Large molecules such as proteins cannot pass through the lipid bilayer.
Ions and charged molecules of any size are essentially impermeable to the lipid bilayer because they are much more soluble in water than in the interior of the membrane.

Answer 4

slowly

Question 5

Active transport by the sodium-potassium pump follows this cycle.

1. Three Na+ ions from the cytosol bind to the pump.
2. The binding of Na+ stimulates the phosphorylation of the pump protein by ATP.
3. Phosphorylation causes a conformational change in the pump that moves the three Na+ ions against their concentration gradient and releases them outside the cell.
4. The release of the Na+ ions permits two K+ ions from outside the cell to bind to the pump (remember PumpK+in), and the phosphate group is released.
5. Release of the phosphate group causes another conformational change in the pump.
6. The conformational change in the pump moves the two K+ ions against their concentration gradient and releases them into the cytosol.

Answer 5

***

Question 6

Together, the concentration (chemical) gradient and the charge difference (electrical gradient) across the plasma membrane make up the electrochemical gradient.

Question 7

Which of the following statements correctly describe(s) the driving forces for diffusion of Na+ and K+ ions through their respective channels? Select all that apply.

The diffusion of Na+ ions into the cell is facilitated by the Na+ concentration gradient across the plasma membrane.

The diffusion of Na+ ions into the cell is impeded by the electrical gradient across the plasma membrane.

The diffusion of K+ ions out of the cell is impeded by the K+ concentration gradient across the plasma membrane.

The diffusion of K+ ions out of the cell is impeded by the electrical gradient across the plasma membrane.

The electrochemical gradient is larger for Na+ than for K+.

Answer 7

The diffusion of Na+ ions into the cell is facilitated by the Na+ concentration gradient across the plasma membrane.

The diffusion of K+ ions out of the cell is impeded by the electrical gradient across the plasma membrane.

The electrochemical gradient is larger for Na+ than for K+.

Question 8

In cotransport, the energy required to move one solute _______ its concentration or electrochemical gradient is provided by an ion moving into the cell down its electrochemical gradient. The ion that moves into the cell down its gradient is usually the same ion that is pumped out of the cell by an active transport pump: for example, Na+ in animal cells using the sodium-potassium pump, or H+ in plants and prokaryotes using the proton pump.
In the case of the glucose-sodium cotransporter in animals, Na+ moves back into the cell down its electrochemical gradient, providing the energy for glucose to move into the cell against its concentration gradient. The energy for glucose transport into the cell is supplied indirectly by the sodium-potassium pump’s hydrolysis of ATP, and directly by the Na+ electrochemical ________ created by the pump.

Answer 8

AGAINST
gradient

Question 9

What is an amphipathic molecule?

Answer 9

One which has both a hydrophilic portion and a hydrophobic portion.

Question 10

Cells recognize other cells by _______ to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane.
These membrane carbohydrates function as markers that distinguish one cell from another. An important example is blood typing. The blood types designated A, B, AB, and O reflect variation in the carbohydrate part of _____________ on the surface of red blood cells.

Answer 10

binding

glycoproteins

Question 11

Membranes have distinct inside and outside faces. The two lipid layers may differ in composition and each embedded protein has directional __________ in the membrane.

Answer 11

orientation

Question 12

________ molecules such as hydrocarbons, CO2 and O2, are hydrophobic, as are lipids. They can therefore, dissolve in the lipid bilayer of the membrane and cross it easily, without the help of membrane proteins.

Answer 12

Nonpolar

Question 13

Polar molecules such as glucose pass only slowly through the bilayer, and even water, a very small polar molecule, does not cross rapidly, relative to nonpolar molecules.
A _______ atom or molecule and its surrounding shell of water is even less likely to penetrate the hydrophobic interior of the membrane.

Answer 13

charged

Question 14

What is the definition of tonicity?

Answer 14

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

Question 15

When a plant cell is immersed in a hypotonic solution, (such as rainwater), the cell wall helps it to maintain wall balance. The plant cell will swell as water enters it by osmosis, however, the relatively _________ cell wall will expand only so much before it exerts a back pressure on the cell called ______ ________, which opposes further water uptake.

Answer 15

inelastic
turgor pressure

Question 16

A cell wall is of no advantage if a cell is placed in a hypertonic environment. In this case, it will lose water to its surroundings and shrink. As it does this, the cell membrane pulls away from the cell wall in multiple places. This phenomenon is called ___________; this causes the plant to wilt and possibly die.

Answer 16

plasmolysis

Question 17

The walled cells of bacteria and fungi also plasmolyze in __________ environments.

Answer 17

hypertonic

Question 18

What are the two types of transport proteins?

Answer 18

channel proteins and carrier proteins

Question 19

Certain kidney cells have many aquaporin molecules, which allow them to reclaim water from urine before it is excreted. If the kidneys did not perform this function, you would excrete about ___ L of urine every day – and have to drink an equal amount of water to stay alive!

Answer 19

180

Question 20

A carrier protein __________ between two shapes, moving a solute across the membrane during the shape change. In both the case of a carrier protein and a channel protein, the protein can transport the solute in ______ direction, but the net movement is down the concentration gradient of the solute.

Answer 20

alternates
either

Question 21

Channel proteins that transport ions are called ion channels. Many ion channels function as _____ channels, which open or close in response to a stimulus. The stimulus may be ________ or ________.

Answer 21

gated
electrical or chemical

Question 22

True or False

Facilitated diffusion is a type of active transport, which requires ATP.

Answer 22

FALSE
facilitated diffusion is passive, and thus, does not require ATP

Question 23

Transport of a solute against a concentration gradient requires ______ ________.

Answer 23

active transport

Question 24

The transport proteins that move solutes against their concentration gradients are all _______ proteins rather than channel proteins.

Answer 24

carrier

Question 25

As in other types of cellular work, ATP __________ supplies the energy for most active transport.

Answer 25

hydrolysis

Question 26

ATP hydrolysis powers the shape change by transferring a phosphate group to the transport protein (___________ the protein).

Answer 26

phosphorylating

Question 27

All cells have voltages across their plasma membranes. Voltage is __________ _________ ______—a separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides.

Answer 27

electrical potential energy

Question 28

Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of ______ into the cell and ______ out of the cell.

Answer 28

cations
anions

Question 29

A transport protein that generates voltage across a membrane is called an ___________ pump. The sodium-potassium pump appears to be the major one of animal cells.

Answer 29

electrogenic

Question 30

The main electrogenic pump of plants, fungi, and bacteria is a _______ pump, which actively transports protons (hydrogen ions, ) out of the cell. Most of these pumps are also powered by ___ __________.

Answer 30

proton
ATP hydrolysis

Question 31

In a mechanism called cotransport, a transport protein (a cotransporter) can couple the “________” diffusion of the solute to the “________” transport of a second substance against its own concentration (or electrochemical) gradient.

Answer 31

downhill
uphill

Question 32

A plant cell uses the gradient of H+ generated by its ATP-powered proton pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell. A cotransporter couples the return of H+ to the transport of sucrose into the cell. This protein can translocate _______ into the cell against its concentration gradient, but only if the sucrose molecule travels in the company of an H+. The H+ uses the transport protein as an avenue to diffuse down its own electrochemical gradient, which is maintained by the proton pump. Plants use H+/sucrose cotransport to load sucrose produced by photosynthesis into cells in the veins of leaves.

Answer 32

sucrose

Question 33

What we know about cotransport proteins in animal cells has helped us find more effective treatments for diarrhea, a serious problem in developing countries. Normally, sodium in waste is reabsorbed in the colon, maintaining constant levels in the body, but diarrhea expels waste so rapidly that reabsorption is not possible, and sodium levels fall precipitously. To treat this life-threatening condition, patients are given a solution to drink containing high concentrations of salt and glucose. The solutes are taken up by sodium-glucose cotransporters on the _______ of __________ cells and passed through the cells into the blood. This simple treatment has lowered infant mortality worldwide.

Answer 33

surface
intestinal

Question 34

Many secretory cells use __________ to export products. For example, the cells in the pancreas that make insulin secrete it into the extracellular fluid by this method.

Answer 34

exocytosis

Question 35

The 3 types of three types of endocytosis are: ____________ (“cellular eating”), _________ (“cellular drinking”), and ________-________ __________.

Answer 35

phagocytosis
pinocytosis
receptor-mediated endocytosis

Question 36

Human cells use receptor-mediated endocytosis to take in ___________ for membrane synthesis and the synthesis of other steroids.

Answer 36

cholesterol

Question 37

In the inherited disease familial hypercholesterolemia, characterized by a very high level of cholesterol in the blood, LDLs cannot enter cells because the LDL ________ ________ are defective or missing

Answer 37

receptor proteins

Question 38

Hormones vary widely in size and type. For instance, the plant hormone ethylene, a gas that promotes fruit ripening, is a hydrocarbon of only six atoms (molecular formula: C₂H₄), small enough to pass through cell walls. In contrast, the mammalian hormone insulin, which regulates sugar levels in the blood, is a protein with _________ of atoms.

Answer 38

hundreds

Question 39

What are the 3 stages of cell-to-cell signaling?

Answer 39

Reception, transduction, and response

Reception is the target cell’s detection of a signaling molecule coming from outside the cell. A chemical signal is “detected” when the signaling molecule binds to a receptor protein located at the cell’s surface or, in some cases, inside the cell.

Transduction is a step or series of steps that converts the signal to a form that can bring about a specific cellular response. Transduction usually requires a sequence of changes in a series of different molecules—a signal transduction pathway. The molecules in the pathway are often called relay molecules.

In the third stage of cell signaling, the transduced signal finally triggers a specific cellular response. The response may be almost any imaginable cellular activity, such as catalysis by an enzyme (for example, glycogen phosphorylase), rearrangement of the cytoskeleton, or activation of specific genes in the nucleus.

Question 40

Most signal receptors are ______ ________ proteins. Their ligands are water-soluble and generally too large to pass freely through the plasma membrane. Other signal receptors, however, are located inside the cell.

Answer 40

plasma membrane

Question 41

Name 3 types of transmembrane receptors:

Answer 41

G protein-coupled receptors (GPCRs)

ligand-gated ion channels

receptor tyrosine kinase (RTKs)

Question 42

What is a G protein?

Answer 42

A protein that binds the energy-rich molecule GTP (guanosine triphosphate), which is similar to ATP.

Question 43

The acetylcholine receptor is an example of what kind of receptor?

Answer 43

a ligand-gated ion channel; it’s molecular structure has many alpha-helices

Question 44

The more than ___ GPCRs examined to date make up the largest family of cell-surface receptors in mammals.

Answer 44

800

Question 45

GPCR pathways are extremely diverse in their functions, which include roles in embryonic development and the senses of sight, smell, and taste. They are also involved in many human diseases. For example, cholera, _________ (whooping cough), and botulism are caused by bacterial toxins that interfere with G protein function.

Answer 45

pertussis

Question 46

Up to __% of all medicines used today exert their effects by influencing G protein pathways.

Answer 46

60

Question 47

Intracellular receptor proteins are found in either the ________ or _______ of target cells. To reach such a receptor, a signaling molecule passes through the target cell’s plasma membrane. A number of important signaling molecules can do this because they are hydrophobic enough to cross the hydrophobic interior of the membrane.

Answer 47

cytoplasm or nucleus

Question 48

How does the activated hormone-receptor complex turn on genes? Recall that the genes in a cell’s DNA function by being transcribed and processed into messenger RNA (mRNA), which leaves the nucleus and is translated into a specific protein by ribosomes in the cytoplasm. Special proteins called ______________ _______ control which genes are turned on—that is, which genes are transcribed into mRNA—in a particular cell at a particular time.

Answer 48

transcription factors

Question 49

An enzyme that transfers phosphate groups from ATP to a protein is known as a _______ ______.

Dephosphorylation, or removal of a phosphate group, is done by _______ _______________.

Answer 49

protein kinase

protein phosphatases

By dephosphorylating and thus inactivating protein kinases, phosphatases provide the mechanism for turning off the signal transduction pathway when the initial signal is no longer present.

Question 50

Not all components of signal transduction pathways are proteins. Many signaling pathways also involve small, nonprotein, water-soluble molecules or ions called second messengers. (The pathway’s “first messenger” is considered to be the extracellular signaling molecule that binds to the membrane receptor.) Because they are small, second messengers can readily spread throughout the cell by diffusion. The two most common second messengers are ______ ___ and _______ ions.

Answer 50

cyclic AMP

calcium

Question 51

Following activation of a receptor, which sequence below represents the correct order in which components will be involved in a signaling pathway that utilizes the second messenger cAMP?

Answer 51

G protein →→ adenyl cyclase →→ cAMP →→ protein kinase

Question 52

The secretion of a signal molecule by a cell into the local environment, followed by a response by a number of cells in the immediate vicinity, is an example of…

a. ) synaptic signaling.
b. ) endocrine signaling.
c. ) hormonal signaling.
d. ) paracrine signaling.

Answer 52

d.) paracrine signaling

Question 53

Membrane carbohydrates function primarily in ________ __________, and are usually short, branched chains of fewer than 15 sugar units.

Answer 53

cell-cell recognition

Question 54

Activated adenylyl cyclase is responsible for converting [multiple] ATP molecules to _____.

Answer 54

cAMP

Question 55

Nuclear responses to cell signaling (such as in response to a growth factor) can cause activation of a transcription factor which alters ____ ___________.

Answer 55

gene expression