Lectures 3 & 4 Questions Flashcards
(80 cards)
1
Q
Name the 3 body fluid compartments, differential b/t the 3.
A
- EXTRAcellular fluid (ECF) outside the cells &
- INTRAcellular fluid (ICF) within the cells
- dividing wall b/t ECF & ICF is the cell membrane
- the extracellular fluid SUBDIVIDES further into PLASMA, the fluid portion of the blood, & INTERstitial fluid, which surrounds most cells of the body
2
Q
give the ionic concentration of intracellular fluid, interstitial fluid (note that this is
usually called extracellular fluid in this course)
A
extracellular:
K+ 5mM Na+ 145mM Cl- 108mM Ca2+ 1mM ~290 mOsM
intracellular:
K+ 150mM Na+ 15mM Cl- 5mM Ca2+ 0.0001mM ~290 mOsM
3
Q
list and explain the functions of the cell membrane
A
- Physical barrier
- separates intracellular fluid from extracellular fluid - Gateway for exchange
- controls movement of solutes: allows some to cross, prevents others from crossing (semipermeable) - Communication
- home to receptors that detect physical & chemical stimuli & starts cascade of response to stimuli - Cell Structure
- some membrane proteins hold cytoskeleton proteins to give cell structure
- may also form specialized junctions
4
Q
what’s a cell wall?
A
a rigid layer of polysaccharides lying outside the plasma membrane of the cells of plants, fungi, and bacteria. ?
5
Q
state the difference between the butter sandwich and fluid mosaic models of
membrane structure
A
early model of the cell membrane structure was a “Butter sandwich”
- a clear layer of lipids sandwiched b/t 2 dark layers of proteins
- NOT accurate as it implies that it is homogenous
present day model of the cell membrane structure is “Fluid mosaic”
- proteins are afloat on a sea of lipid
6
Q
draw a typical cell membrane, including phospholipids, glycolipids, glycoproteins,
integral proteins, peripheral proteins, lipid anchored proteins, cytoskeleton and
extracellular matrix
A
page 63 or slide 8
7
Q
what is a lipid raft
A
sphingolipids tend to aggregate together (with themselves rather than with the phospholipids) = lipid rafts
8
Q
why is it the lipid raft an important structure?
A
- rafts have a high density of cholesterol (as cholesterol prefers to associate with sphingolipids over phospholipids)
- some proteins associate ONLY with lipid rafts, leading to areas of SPECIALIZATION on cell membrane for ex, some G-protein coupled receptors (that are only associated with lipid rafts)
9
Q
how many amino acids span the membrane in a membrane spanning domain?
A
approximately 20-25 hydrophobic amino acids to span the cell membrane ???
10
Q
what kind of amino acids are they (acidic, basic, hydrophobic, hydrophilic)?
A
hydrophobic (non-polar) ??
- (this allows those amino acids to create strong noncovalent interactions with the lipid tails of the membrane phospholipids, holding them tightly in place)
11
Q
What are the ways a molecule gets from one side of a cell membrane to the other?
A
- diffusion/osmosis
- protein mediated transport
- transport thru channel proteins
- transport via carrier proteins which include:
- facilitated diffusion
- primary active transport
- secondary active transport - vesicular transport
12
Q
what are the factors that influence diffusion?
A
- Fast over short distances, slow over long distances
- time taken to get from A to B is a “distance squared” relationship: if distance doubles from 1 to 2, time increases from 1 to 4 (=2 squared) - Rate of diffusion is:
- faster at high temp
- faster for small molecules
- slower across a membrane
13
Q
what are the factors that influence diffusion across a cell membrane?
A
1) Permeability (of solute) across cell membrane
- size
- lipid solubility: polar or non-polar or VERY non-polar
2) Concentration gradient
3) Surface area
4) Temp
5) Composition of membrane
- simple lipid bilayer vs membrane with many proteins & extracellular matrix
- types of phospholipids & sphingolipids
- presence of cholesterol
14
Q
define selective permeability
A
cell membranes are selectively permeable, which means that some molecules can cross them but others cannot
15
Q
is a cell membrane selective permeable?
A
yes - cell membranes are selectively permeable, which means that some molecules can cross them but others cannot
16
Q
what kinds of molecules can diffuse across a cell membrane ? what kinds cannot?
A
can:
- hydrophobic, non-polar molecules, such as:
- O2, CO2,
- lipids
- steroids
- fat soluble molecules
- small uncharged polar molecules, such as:
- urea
- H20 ????
cannot:
- large uncharged polar molecules, such as:
- glucose, proteins, amino acids
- charged molecules, such as:
- ions
17
Q
define osmosis
A
is the diffusion of water
18
Q
define concentration gradient in terms of water
A
water can have a concentration gradient
water will “diffuse down its concentration gradient”
- pure water has the “highest concentration of water”
- solutes lower the concentration of water
movement of water can cause pressure
19
Q
in a U-tube system with a selectively permeable membrane, identify which solution is
hyperosmolar, and which way water or solutes will move.
A
slides 40-42 or fig. 4.2 on page 124
- 2 compartments are separated by a membrane that is permeable to water but not glucose. Solution B is more concentrated than solution A
- Water moves by osmosis into the more concentrated solution. Osmosis stops when concentrations are equal.
- Compartment A is pure water, & compartment B is a glucose solution. Osmotic pressure is the pressure that must be applied to oppose osmosis.
20
Q
define hypertonic
A
a solution that causes net movement of water out of a cell (cell shrink)
21
Q
a cell is placed in a solution of 150 mM NaCl. What happens?
What about 150 mM glucose?
What about 150 mM maltose (a dimer of glucose)?
150 mM urea?
A
? all hyposmotic - non-pen so it would be hypertonic? - non-pen so it would be hypertonic? - non-pen so it would be hypertonic? - pen so it would be hypotonic?
22
Q
what’s the difference between a channel and a carrier protein?
A
channel proteins create water-filled passageways that directly link the intracellular & extracellular compartments
carrier proteins, also just called transporters, bind to the substrates that they carry but NEVER form a direct connection b/t the intracellular fluid & extracellular fluid
carriers are open to one side of the membrane or the other, but not to both at once the way channel proteins are
23
Q
differentiate between facilitated diffusion, primary active transport and secondary
active transport. Give an example of each. ATP is directly used in which processes.
A
Facilitated Diffusion: is defined as moving a molecule across the cell membrane via a carrier protein, & the transport does not require energy other than the concentration gradient
- does NOT require ATP, or other solutes
- AKA passive transport
- this process along cannot accumulate solute against a concentration gradient
EX: glucose transporter: GLUT protein
Primary Active Transport:
- uses ATP
- establishes gradients
- sometimes called pumps
- Na+/K+/ATPase is the most widely known ex, but there are others:
- Ca2+ ATPase
- H+ ATPase
- H+/K+ ATPase
Secondary Active Transport:
- does NOT directly utilize ATP as a source of energy
- instead, uses the concentration gradient of one molecule/ion to move another against its gradient (acts as energy source)
- Na+- glucose secondary active transporter is a good ex: SGLT-protein
24
Q
explain how glucose moves across gut epithelia into the blood.
A
- Na+/K+ ATPase - establishes & maintains a Na+ gradient. Primary active transport
- Using the Na+ gradient, glucose is transported into the cell via the Na+ glucose co-transporter. Secondary active transport
- Glucose is transported across the basal membrane by the GLUT transporter. Facilitated diffusion
- & then moves the glucose into the bloodstream & then circulated around your body for general use
25
what would happen if the Na+/K+ ATPase was blocked with the poison oubain?
The removal of Na+ from the cell is essential if glucose is to continue to be absorbed from the lumen. The potential energy to run the SGLT symporter comes from the sodium concentration gradient, which depends on low intracellular concentrations of Na+.
If the basolateral Na+-K+-ATPase is poisoned with oubain, Na+ that enters the cell cannot be pumped out. The Na+ concentration inside the cell gradually increases until it is equal to that in the lumen. With out a sodium gradient, there is no energy source to run the SGLT symporter, & the absorption of glucose across the epithelium stops.
26
Assume that a hypothetical cell X can only takes up glucose by facilitated diffusion
(passive and does not move molecules against concentration gradients). How can this
cell accumulate glucose at a concentration higher than found in the extracellular
fluid?
s
27
differentiate between Phagocytosis, Endocytosis, Exocytosis
phagocytosis: the process by which a cell engulfs a particle into a vesicle by using the cytoskeleton to push the membrane around the particle
endocytosis: by which large molecules or particles move into cells
exocytosis: process in which intracellular vesicles fuse with the cell membrane & release their contents into the extracellular fluid
28
Discuss the use of liposomes for drug delivery
- some drugs may have low "bioavailability" due to poor solubility
- some drugs may be toxic at useful doses, & must be TARGETED TO A SPECIFIC CELL TYPE
- liposomal drug delivery is an emerging technology that may help address these issues
29
What is osmotic equilibrium?
water is essentially the only molecule that moves freely b/t cells & the extracellular fluid
- b/c of this free movement of water, the extracellular & intracellular compartments reach a state of OSMOTIC EQUILIBRIUM, in which the fluid concentrations are equal on the 2 sides of the cell membrane
30
What is chemical disequilibrium? Give some examples of specific solutes
that exist in a state of chemical disequilibrium in our body. (Fig. 5.1d)
although the overall concentrations of the ECF & intracellular fluid (ICF) are equal, some solutes are more concentrated in one of the two body compartments than in the other - this means the body is in a state of CHEMICAL DISEQUILIBRIUM
- fig. 5.2d shows the uneven distribution of major solutes among the body fluid compartments
- for ex: sodium, chloride, & bicarbonate (HCO3-) ions are more concentrated in extracellular fluid than in intracellular fluid
- potassium ions are more concentrated in the extracellular fluid than in the cytosol, although many cells store Ca2+ inside organelles such as the endoplasmic reticulum & mitochondria
31
How does chemical disequilibrium in the body give rise to electrical disequilibrium? What is the result of electrical disequilibrium?
The concentration differences of chemical disequilibrium are a hallmark of a living organism, as only the continual input of energy keeps the body in this state. If solutes leak across the cell membranes dividing the intracellular & extracellular compartments, energy is required to return them to the compartments they left.
Ex: K+ ions that leak out of the cell & Na+ ions that leak into the cell are returned to their original compartments by an energy-utilizing enzyme known as the Na+-K+-ATPase, or the sodium-potassium pump. When cells die & cannot use energy, they obey the 2nd law of thermodynamics & return to a state of randomness that is marked by loss of chemical disequilibrium.
The body as a whole is electrically neutral, but a few extra negative ions are found in the intracellular fluid, while their matching positive ions are located in the extracellular fluid. As a result, the inside of cells is slightly negative relative to the extracellular fluid. This ionic imbalance results in a state of electrical disequilibrium
32
What is the physiological significance of the “70-kg man”? What is his total
body water volume?
people ask how much water is in the human body but b/c 1 individual differs from the next, there is no single answer
- however, in human physiology we often speak of standard values for physiological functions based on the "the 70-kg man"
- these standard values are derived from data published in the mid-twentieth century by the International Commission on Radiological Protection (ICRP)
- the ICRP was setting guidelines for permissible radiation exposure, & they selected a young (age 20-30) white European male who weighed 70 kilograms (kg) or 154 pounds as their "reference man," or "standard man"
*The 70-kg Reference Man has 60% of his total body weight, or 42 kg (92.4 lb), in the form of water. Each kilogram of water has a volume of 1 liter, so his TOTAL BODY WATER is 42 liters. This is equivalent of 21 two-liter soft drink bottles!
33
How does a woman’s total body water content compare to a man’s? (Tbl.
5.1)
adult women have less water per kilogram of body mass than men because women have more adipose tissue
- large fat droplets in adipose tissue occupy most of the cell's volume, displacing the more aqueous cytoplasm.
34
Describe the distribution of water among the body compartments. (Fig. 5.1b,
c)
when we look at the relative volumes of the body compartments, the INTRAcellular compartment contains about 2/3 (67%) of the body's water
- the remaining 1/3 (33%) is split b/t the INTERstitial fluid (which contains about 75% of the extracellular water) & the plasma (which contains abut 25% of the extracellular water)
35
Define and describe osmosis. (Fig. 5.2)
osmosis is the diffusion of water
water can have a concentration gradient
water will "diffuse down its concentration gradient"
- pure water has the "highest concentration of water"
- solutes lower the concentration of water
movement of water can cause pressure
36
What is osmotic pressure? (Fig. 5.2)
the pressure that exactly opposes a given concentration gradient
- the pressure on the piston that exactly opposes the osmotic movement of water into compartment B is known as the osmotic pressure of solution B
37
How does osmolarity differ from molarity? What equation converts molarity to osmolarity?
the important factor for osmosis is the # of osmotically active particles in a given volume of solution, not the number of molecules
- b/c some molecules dissociate into ions when they dissolve in a solution, the # of particles in solution is not always the same as the # of molecules
38
Contrast osmolarity and osmolality.
osmolarity is the diffusion of water
a term related to osmolarity is osmolality
- osmolality is concentration expressed as osmoles of solute per kilogram of water
- b/c biological solutions are dilute & little of their weight comes from solute, physiologists often use the terms osmolarity & osmolality interchangeably
- osmolality is usually used in clinical solutions b/c it is easy to estimate people's body water conent by weighing them
39
Distinguish between the terms isosmotic, hyposmotic, and hyperosmotic.
If 2 solutions contain the same # of solute particles per unit volume, we say that the solutions are ISOSmotic (equal)
If solution A has a higher osmolarity (contains more particles per unit volume, is more concentrated) than solution B, we say that solution A is HYPERosmotic to solution B.
Solution B, with fewer osmoles per unit volume, is HYPOsmotic to solution A
40
If solution A has more particles per liter than solution B, then solution A is
said to be ________________ osmotic to solution B, while solution B is said
to be ________________ osmotic to solution A. (Tbl. 5.2)
HYPER
HYPO
41
Contrast osmolarity and tonicity.
OSMOLARITY:
- describes only the # of solute molecules in a cell (units of Osm)
- can compare any 2 solutions
- does NOT always tell if a cell swells or shrinks
TONICITY:
- is a comparative term describes whether a cell changes volume (has NO units)
- compares a solution to a cell's intracellular solution (for our purposes 290 mOsm)
- specifically TELLS if a cell swells or shrinks
42
Distinguish between the terms isotonic, hypotonic, and hypertonic. (Tbl. 5.3)
- if a cell placed in the solution gains water at equilibrium & swells, we say that the solution is HYPOtonic to the cell
- if the cell loses water & shrinks at equilibrium, the solution is said to be HYPERtonic
- if the cell in the solution does not change size at equilibrium, the solution is ISOtonic
43
Contrast penetrating solutes and nonpenetrating solutes. What does it mean
when a solute is considered “functionally nonpenetrating”?
PENEtrating (easily cross cell membranes)
- small polar, & non-polar molecules
- ex's: urea, glycerol, ethanol
NON-penetrating (do NOT cross cell membranes)
- ions & large polar molecules
- ex's: Na+, glucose, amino acids
- the most imp. nonpenetrating solute in physiology is Nacl
- if a cell is placed in a solution of NaCl a nonpentrating solute (in reality, a few Na+ ions may leak across, but they are immediately transported back to the extracellular fluid by the Na+-K+-ATPase
- for this reason, NaCl is considered a FUNCTIONALLY nonpenetrating solute
44
What are the rules for predicting tonicity? (Tbl. 5.4)
1. Assume that all intracellular solutes are nonpenetrating
2. Compare osmolarities before the cell is exposed to the solution (at equilibrium, the cell & solution are always isosmotic)
3. Tonicity of a solution describes the volume change of a cell at equilibrium
4. Determine tonicity by comparing nonpenetrating solute concentrations in the cell & the solution. Net water movement is into the compartment with the higher concentration of non-penetrating solutes
5. Hyposmotic solutions are always hypotonic
45
How can a solution be isosmotic and also hypotonic? (Fig. 5.3)
- it can never be hypertonic because it can never have a higher concentration of nonpenetrating solutes than the cell
- if all solutes in the isosmotic solution are nonpenetrating, then the solution is also isotonic
- if there are any penetrating solutes in the isosmotic solution, the solution will be hypotonic
46
What properties of a cell membrane contribute to its permeability? Give examples of substances that typically are permeable and impermeable to human cell membranes.
permeability across cell membrane:
1. size
- very small molecules & those that are lipid soluble can cross directly through the phospholipid bilayer
- larger & less lipid-soluble molecules usually do not enter or leave a cell unless the cell has specific membrane proteins to transport these molecules across the lipid bilayer
2. lipid solubility: polar or non-polar or VERY non polar
- very large lipophobic molecules cannot be transported on proteins & must enter & leave cells in vesicles
Some molecules, such as oxygen, CO2, & lipids, move easily across most membranes
- on the other hand, ions, most polar molecules, & very large molecules (such as proteins), enter cells with more difficulty or may not enter at all
?
47
What two properties of a molecule determine whether it can diffuse across a
membrane?
2 properties of a molecule influence its movement across cell membranes:
1. the size of the molecule
2. its lipid solubility
- very small molecules & those that are lipid soluble can cross directly through the phospholipid bilayer
- larger & less lipid-soluble molecules usually do not enter or leave a cell unless the cell has specific membrane proteins to transport these molecules across the lipid bilayer
- very large lipophobic molecules cannot be transported on proteins & must enter & leave cells in vesicles
?
48
What is the difference between active transport and passive transport?
passive transport: does not require the input of energy other than the potential energy stored in a concentration gradient
active transport: requires the input of energy from some outside source, such as the high-energy phosphate bond of ATP
49
What are the different ways a molecule can move across a membrane? (Fig.
5.5)
```
Transport across membranes:
Active:
Vesicular transport (ATP)
- Exocytosis
- Endocytosis
- Phagocytosis
```
Protein-Mediated:
Active:
- Direct or primary active transport (ATP)
- Indirect or secondary active transport (concentration gradient created by ATP)
Passive:
- Facilitated diffusion (concentration gradient)
- Ion channel (electrochemical gradient)
- Aquaporin channel (osmosis)
Passive:
- Simple Diffusion (concentration gradient)
50
Define diffusion. (Fig. 5.6)
process of moving solutes molecule away from an area of high concentration towards area of low concentration
51
List seven properties of diffusion. (Tbl. 5.6)
1. diffusion is a passive process
2. molecules move from an area of higher concentration to an area of lower concentration
3. net movement of molecules occurs until the concentration is equal everywhere
4. diffusion is rapid over short distances but much slower over long distances
5. diffusion is directly related to temp
6. diffusion rate is inversely related to molecular weight & size
7. diffusion can take place in an open system or across a partition that separates 2 compartments
52
Describe seven general properties of diffusion. (Tbl. 5.6)
1. Diffusion is a passive process
2. Molecules move from an area of higher concentration to an area of lower concentration
3. New movement of molecules occurs until the concentration is equal everywhere
4. Diffusion is rapid over short distances but much slower over long distances
5. Diffusion is directly related to temp (faster at high temp)
6. Diffusion rate is inversely related to molecular weight & size (faster for small molecules)
7. Diffusion can take place in an open system or across a partition that separates 2 compartments
53
Fick’s law of diffusion describes simple diffusion across a membrane. (Fig.
5.7) Using words, state Fick’s law.
fick's law says that the diffusion rate increases with an increase in surface area, the concentration gradient, or the membrane permeability
54
What factors influence membrane permeability?
1. the size (& shape, for large molecules) of the diffusing molecule
- as molecular size increases, membrane permeability decreases
2. the lipid-solubility of the molecule
- as lipid solubility of the diffusing molecule increases, membrane permeability to the molecule increases
3. the composition of the lipid bilayer across which it is diffusing
- alterations in lipid composition of the membrane change how easily diffusing molecules can slip b/t the individual phospholipids
55
Why is simple diffusion not an option for most molecules in our body?
in the body, simple diffusion across membranes is limited to lipophilic molecules
- the vast majority of molecules in the body are either lipophobic or electrically charged & therefore cannot cross membranes by simple diffusion
56
Differentiate between protein-mediated transport, facilitated diffusion, and
active transport.
protein-mediated transport: the vast majority of solutes cross membranes with the help of membrane proteins
if mediated transport is passive & moves molecules down their concentration gradient, & if net transport stops when concentrations are equal on both sides of the membrane, the process is facilitated diffusion
if protein-mediated transport requires energy from ATP or another outside source & moves a substance against its concentration gradient, the process is known as active transport
57
What are the three major roles of structural proteins? (Fig. 5.8)
1. they help create cell junctions that hold tissues together, such as tight junctions & gap junctions
2. they connect the membrane to the cytoskeleton to maintain the shape of the cell
- the microvilli of transporting epithelia are one ex of membrane shaping by the cytoskeleton
3. they attach cells to the extracellular matrix by linking cytoskeleton fibers to extracellular collagen & other protein fibers
58
What are membrane enzymes? How are they different in function from
intracellular enzymes?
membrane enzymes catalyze chemical rxns that take place either on the cell's external surface or just inside the cell
enzymes attached to the intracellular surface of many cell membranes play an important role in transferring signals from the extracellular environment to the cytoplasm
59
What is the role of membrane receptor proteins? (Fig. 5.9)
are part of the body's chemical signaling system
| - the binding of a receptor with its ligand usually triggers another event at the membrane
60
Compare channel proteins and carrier proteins.
a channel protein is a water filled pore
- can open to both sides
carrier proteins NEVER form an open channel b/t the 2 sides of the membrane
61
Describe a typical channel protein. What types of molecules pass through
channel proteins? (Figs. 5.10a, 5.11)
is a water filled pore that can open to both sides
- most cells have water channels made from a protein called aquaporin
- ion channels may be specific for one ion or may allow ions of similar size & charge to pass
- for ex: there are Na+ channels, K+ channels, & nonspecific monovalent cation channels that transport Nat+, K+, & lithium ions Li+
- other ion channels are Ca2+ channels & Cl- channels
62
Distinguish between an open channel and a gated channel. (Fig. 5.10a)
open channels open & close in response to signal
gated channels or pores are usually open
63
List three types of gated channels.
1. chemically gated channels
2. voltage-gated channels
3. mechanically gated channels
64
Define the following terms: uniport carriers, cotransporter, symport carriers,
and antiport carriers. (Fig. 5.10b)
uniport carriers: allows a single molecule to move across a membrane (in 1 direction)
cotransporter: a carrier that moves more than 1 kind of molecule at one time (branches into symport & antiport)
symport carriers: 2 molecules are transported across the membrane simultaneously across the cell membrane in the same direction
antiport carriers: moving more than 1 solute molecules, but is moving them in opposite directions
- using ATP
65
Diagram the general mechanism by which carrier proteins move molecules
across a membrane. (Fig. 5.12)
- the molecule being transported binds to the carrier on one side of the membrane
- this binding changes the conformation of the carrier protein so that the opening closes
- after a brief transition in which both sides are closed, the opposite side of the carrier opens to the other side of the membrane
- the carrier then releases the transported molecule into the opposite compartment, having brought it through the membrane without creating a continuous connection b/t the extracellular & intracellular compartments
66
Compare and contrast facilitated diffusion and simple diffusion.
Facilitated Diffusion: is defined as moving a molecule across the cell membrane via a carrier protein, & the transport does not require energy other than the concentration gradient
- does NOT require ATP, or other solutes
- AKA passive transport
- this process alone cannot accumulate solute against a concentration gradient
Simple Diffusion: diffusion directly across the phospholipid bilayer of a membrane
facilitated diffusion has the same properties as simple diffusion
- the transported molecules move down their concentration gradient, the process requires no input of outside energy, & net movement stops at equilibrium, when the concentration inside the cell equals the concentration outside the cell
67
Give examples of molecules that might cross a membrane by facilitated
diffusion.
example is glucose transporter: GLUT protein
68
What is active transport? Why does it require the input of energy?
is a process that moves molecules AGAINST their concentration gradient - that is, from areas of lower concentration to areas of higher concentration
- rather than creating an equilibrium state, where the concentration of the molecule is equal throughout the system, active transport creates a state of disequilibrium by making concentration differences more pronounced
- moving molecules against their concentration gradient requires the input of outside energy, just as pushing a ball up a hill requires energy
- the energy for active transport comes either directly or indirectly from the high-energy phosphate bond of ATP
69
Distinguish between primary (direct) active transport and secondary
(indirect) active transport.
primary active transport:
1. uses ATP
2. establishes gradients
3. sometimes called pumps
4. Na+/K+/ATPase is the most widely known ex, but there are others
- Ca2+ ATPase
- H+ ATPase
- H+/K+ ATPase
secondary active transport:
- does NOT directly utilize ATP as a source of energy
- instead, uses the concentration gradient of one molecule/ion to move another against its gradient (acts as energy source)
- Na+-glucose secondary active transporter is a good ex: SGLT-protein
70
Diagram the structure and mechanism of the Na+-K+-ATPase as an example
of primary active transport. (Figs. 5.14, 5.15; Tbl. 5.7)
- pumps 2K+ ions into cell, removes Na+ ions
In order to do this:
1. Hydrolyses ATP (uses energy from ATP to pump Na+ out of the cell & K+ into the cell)
2. Several conformational changes (b/c it's not a channel)
71
How is the relatively high extracellular [Na+
] used to drive transport of other
molecules against their concentration gradient across a membrane? Give
some specific examples. (Tbl. 5.8)
f
72
Describe the action of the Na+
-glucose secondary active transporter (SGLT)
as a representative example of secondary active transport. How does an
SGLT transporter compare with a GLUT transporter? (Fig. 5.16)
The mechanism of the Na+-glucose secondary active transporter (SGLT)
1. Na+ binds to carrier
2. Na+ binding creates a high-affinity site for glucose
3. Glucose binding changes carrier conformation so that binding sites now face the ICF
4. Na+ is released into cytosol where [Na+] is low. Release changes glucose-binding site to low affinity. Glucose is released
In contrast, GLUT transporters are reversible & can move glucose into or out of cells depending on the concentration gradient
73
Give examples of how specificity applies to carrier-mediated transport. (Fig.
5.17a, b)
a. the GLUT transporter brings glucose across cell membranes
| b. maltose is a competitive inhibitor that binds to the GLUT transporter but is not itself carried across the membrane
74
How does competition relate to specificity?
the property of competition is closely related to specificity
- a transporter may move several members of a related group of substrates, but those substrates compete with one another for binding sites on the transporters
for ex: GLUT transporters move the family of hexose sugars, but each different GLUT transporter has a "preference" for one or more hexoses, based on its binding affinity
75
Describe how the principle of saturation applies to carrier-mediated
transport. Include a description of transport maximum. (Fig. 5.17c)
the rate of substrate transport depends on the substrate concentration & the # of carrier molecules, a property that is shared by enzymes & other binding proteins
- for a fixed # of carriers, however, as substrate concentration increases, the transport rate increases up to a maximum, the point at which all carrier binding sites are filled with substrate
- at saturation, the carriers are working at their maximum rate, & a further increase in substrate concentration has no effect
transport maximum (Tm) - the maximum transport rate that occurs when all carriers are saturated
76
How can cells increase their transport capacity and avoid saturation?
one way is to increase the # of carriers in the membrane
- this would be like opening more doors into the concert hall
under some circumstances, cells are able to insert additional carriers into their membranes
under other circumstances, a cell may withdraw carriers to decrease movement of a molecule into or out of the cell
77
Compare and contrast the apical membrane and the basolateral membrane of a transporting epithelial cell. What is the physiological significance of transporting epithelial cell polarization? (Fig. 5.20)
- the apical membrane & the basolateral membrane are the 2 poles of the cell
- polarized epithelia have different transport proteins on apical & basolateral membranes
- this allows selective directional transport across the epithelium
- transport from lumen to ECF is called absorption
- transport from ECF to lumen is called secretion
?
78
Contrast paracellular transport and transcellular transport. (Fig. 5.20)
movement across an epithelium, or epithelial transport, may take place either as paracellular transport through the junctions b/t adjacent cells or as transcellular transport through the epithelial cells themselves
?
79
Diagram the movement of glucose across a transporting epithelial cell.
Include all membrane proteins, ions, and directionality of transport. (Fig.
5.21)
absorbing glucose from intestinal or kidney tubule lumen involves indirect (secondary) active transport of glucose across the apical membrane & glucose diffusion across the basolateral membrane
1. Na+-glucose symporter brings glucose into cell against its gradient using energy stored in the Na+ concentration gradient
2. GLUT transporter transfers glucose to ECF by facilitated diffusion
3. Na+-K+-ATPase pumps Na+ out of the cell, keeping ICF Na+ concentration low
?
80
Diagram the process of insulin secretion in pancreatic beta cells as a key
example of integrated membrane function. (Fig. 5.26)
- the beta cell has 2 such channels that help control insulin release
- 1 is a voltage-gated Ca2+ channel
- this channel is closed a the cell's RMP
- the other is a K+ leak channel that closes when ATP binds to it
- it is called an ATP-gated K+ channel (Katp channel)
- in the resting cell, when glucose concentrations are low, the cell makes less
- there is a little ATP to bind to the Katp channel, & the channel remains open, allowing K+ to leak out of the cell
- at the RMP, the voltage-gated Ca2+ channels are closed, & there is no insulin secretion
- after a meal, plasma glucose levels increase as glucose is absorbed from the intestine
- glucose reaching the beta cell diffuses into the cell with the aid of a GLUT transporter
- increased glucose in the cell stimulates the metabolic pathways of glycolysis & the citric acid cycle, & ATP production increases
- when ATP binds to the Katp channel, the gate to the channel closes, preventing K+ from leaking out of the cell
- retention of K+ depolarizes the cell,
- which then causes the voltage-sensitive Ca2+ channels to open
- calcium ions enter the cell from the ECF, moving down their electrochemical gradient
- the Ca2+ ions bind to proteins that initiate exocytosis of the insulin-containing vesicles, & insulin is released into the extracellular space
