Transport Mechanisms Flashcards
1
Q
What is the cell membrane highly permeable to
A
Water, lipid-soluble substances, dissolved gases (O2 and CO2) and small uncharged molecules
2
Q
What is the cell membrane less permeable to
A
Larger molecules and charged molecules
3
Q
What is the cell membrane impermeable to
A
Very large molecules (macromolecules)
4
Q
Describe the structure of the cell membrane
A
Made up of phospholipids (phospholipid belayer) with polar hydrophilic heads and non-polar hydrophobic tails making the membrane amphipathic
5
Q
Function of cholesterol inserted into the phospholipid bilayer
A
Reduces packing of fatty acid tails and increases membrane fluidity keeping fatty acids apart. May also be involved in the formation of vesicles that pinch off the plasma membrane. Slightly amphipathic
6
Q
Function of cholesterol in cell membrane at high temperatures
A
Acts to stabilize cell membrane and adds firmness
7
Q
Function of cholesterol in cell membrane at low temperatures
A
Inserts phospholipids and prevents them from interfering with each other to avoid aggregation
8
Q
What are the two types of proteins and what are their function
A
Integral: not easy to extract, closely associated with phospholipids, mostly cross the membrane
Peripheral: more loosely associated, mostly on cytoplasmic side, easier to extract
9
Q
What is glycocalyx
A
Layer of carbohydrate formed by a chain of monosaccharides that extend from the extracellular surface of the cell membrane bound to proteins
10
Q
Function of glycocalyx
A
Provides protection from infection, enables cells to identify each other and interact
11
Q
Is the fluid mosaic model of a cell membrane a stable situation
A
No, cell membrane always being replenished, proteins are not fixed and can move around
12
Q
What are the functions of plasma membrane proteins
A
Channels and transporters:
- Transport and diffusion of specific molecules in and out of cell
- Act as enzymes that catalyze membrane associated reactions
- Serve as receptors for receiving and transduction chemical signals from the cell environment
- Cell surface identity markers
- Cell-cell adhesion
- Attachment to cytoskeleton
13
Q
Examples of how a protein acts as an enzyme
A
Amino acid transport, Na-K pump
14
Q
Example of how a protein can act as a cell surface receptor
A
G-protein coupled receptors and insulin receptor
15
Q
What are the two transmembrane transport pathways
A
- Phospholipid bilayer (small uncharged molecules)
- Interaction with a protein cluster (carrier or channel) often with conformational changes
16
Q
Difference between active and passive transport
A
Passive = energy independent
Active = energy dependent
17
Q
Types of passive transport
A
- Diffusion
- Carrier-mediated facilitated diffusion (protein channel)
- Osmosis
18
Q
Types of active transport
A
- carrier mediated active transport (primary and secondary)
- Pino/phagocytosis
19
Q
Simple diffusion is the movement of molecules from one location to another as a result of what
A
Random thermal motion
20
Q
What is flux
A
Amount of particles crossing a surface per unit of time
21
Q
What is net flux
A
From high concentration to lower concentration
22
Q
At equilibrium, what are the diffusion fluxes and net flux
A
Diffusion fluxes are equal and net flux is 0
23
Q
What is fick’s law of diffusion
A
J = PA (C0-Ci)
J = net flux (rate of diffusion)
P = permeability coefficient (constant based on easy of movement of molecule through a membrane)
A = surface area of membrane
C0-Ci = Concentration gradient of the diffusing molecule across membrane
24
Q
Diffusion time increases in proportion to what
A
Square of the distance travelled by the solute molecules
25
Why are capillaries great for diffusion
Diffusion is an effective transport process only over short distances, capillaries are thin and close to cells
26
What are the factors that affect diffusion across the cell membrane
1. Mass of the molecule
2. Concentration gradient across the cell membrane
3. Lipid solubility
4. Electrical charge
5. Availability of selective ion channels or membrane carriers
27
Diffusion particles may penetrate either by
1. Dissolving in the lipid component (for non polar molecules)
2. Diffusing through channels (for ions)
28
What does movement of particles across a membrane depend upon
The existence of a concentration gradient and an electrical gradient
29
What do ion channels consist of
A single protein or more often a cluster of proteins
30
What do ion channels show selectivity based on
Diameter as well as on the distribution of charges lining he channel
31
What is the simultaneous existence of an electrical and concentration gradient for a particular ion known as
An electrochemical gradient
32
What is the charge inside of cells
Always negative
33
Ion channels can exist in what two states as they undergo conformational changes and what is it known as
Open or closed, known as gating
34
How chan channels be gated
1. Ligand gated (neurotransmitter binding to receptor)
2. Voltage gated (cell goes through action potential)
3. Mechanically gated (transform mechanical into electrical signal)
35
What are the types of voltage gated ion channels
Na+, K+, Ca+, Cl-
36
The total number of ions that flow through voltage gated ion channels generating ionic current depend on
1. The channel conductance
2. How often the channel opens
3. How long the channel stays open
37
What is mediated transport
The movement of ions and other molecules by integral membrane proteins called transports
38
What is slower ion movement across membranes via transporters or ion channels
Transporters
39
What are the characteristics of mediated-transport systems
1. Specificity
2. Saturation
3. Competition
40
What does specificity mean in terms of mediated-transport systems
System usually transports one particular type of molecule only
41
What does saturation mean in terms of characteristics of mediated-transport systems
Rate of transport reaches a maximum when all binding sites on all transporters are occupied.
42
How does competition work in terms of characteristics of mediated-transport systems
It occurs when structurally similar substances compete for the same binding site on a membrane carrier
43
What are factors that determine flux magnitude of mediated-transport systems
1. Solute concentration
2. Affinity of transporter for the solute
3. Numbers of transporters
4. Rate of transporter conformational change
44
Facilitated diffusion involves the presence of what
A transporter or carrier
45
The transporter or carrier molecule enables what
The solute to penetrate more readily than it would be expected to by simple diffusion
46
What are the steps of facilitated diffusion
1. Solute binds transporter
2. Transporter changes configuration
3. Solute is delivered to other side of membrane
4. Transporter resumes original configuration
47
What are the three main components of facilitated diffusion
1. Transporter (carrier) mediated
2. Passive (no energy)
3. Net flux from high to low concentration
48
What are the effects of hormones on facilitated diffusion
They may increase the number and/or affinity of transporters in some membranes
49
What are the principles of active transport
1. Transporter-mediated
2. Requires supply of chemical energy (usually derived from enzymatic hydrolysis of ATP)
3. Susceptible to metabolic inhibitors
4. Can transport solute against its concentration gradient
50
What does phosphorylation of the transporter change
The conformation of the transporter and its solute binding affinity
51
What are changes in the binding site affinity fro a transported solute produced by
Phosphorylation and dephosphorylation of the Na+/K+ - ATPase
52
What is the Ca2+ - ATPase active transporter responsible for
Maintaining low intracellular Ca2+ levels
53
What is the H + - ATPase active transporter responsible for
maintaining low lysosomal pH
54
What is H+/K+ - ATPase responsible for
Acidification of the stomach
55
How does secondary active transport work for Na+
Movement of Na+ down its concentration gradient is coupled to the transport of another solute molecule uphill against its concentration gradient.
56
How does secondary active transport move the Na+ and the transported solute
It uses the energy stored of the electrochemical gradient to move both the Na+ and the transported solute
57
What does the creation and maintenance of the electrochemical gradient depend on
Primary active transport
58
What are the steps for secondary active transport
1. Na+ binds to a transporter outside the cell allowing glucose or an amino acid to bind to the same carrier
2. Through a change in configuration, the transporter delivers both Na+ and glucose or amino acid into the cell
3. The transporter then reverts to its original configuration and the Na+ is extruded from the cell by the Na+/K+ - ATPase
59
Is Na+ concentration high inside or outside of the cell
Outside
60
What is cotransport
When solute x is transported in the same direction as Na+ (also known as symport)
61
What is countertransport
When solute X is transported in the opposite direction to Na+ (anti port)
62
What are two examples of anti port
Na+/H+ exchanger (controls pH concentration)
Na+/Ca2+ exchanger (needs to stay low)
63
What are 3 examples of symporters
Na+/HCO3-
Na+ - amino acid
Na+ - glucose symporter (SGLT)
64
Active transport mechanisms involve the participation of what
The cell membrane itself
65
What is endocytosis
The cell membrane invaginates and pinches off the form a vesicle
66
What is exocytosis
An intracellular fuses with the cell membrane and its contents are released into the ECF
67
What are the two types of exocytosis
1. Constitutive Exocytosis
2. Regulated exocytosis
68
What is constitutive exocytosis
Natural turnover, non-regulated. Functions to replace plasma membrane, deliver membrane proteins to the cell membrane and get rid of substances from the cell
69
What is regulated exocytosis
Tends to be triggered by extra cellular signals and the increase of cystolic Ca2+. Responsible for the secretion of hormones, digestive enzymes and neurotransmitters
70
What are the two types of endocytosis
1. Pinnocytosis
2. Phagocytosis
71
What is pinocytosis
Fluid endocytosis, involves an endocytotic vesicle that engulfs the extra cellular fluid including whatever solutes are present. Non-specific and constitutive. vesicles travel into the cytoplasm and fuse with other vesicles
72
What is phagocytosis
Process where cells bind and internalize particulate matter. Specific and triggered. Extensions of the cell membrane called pseudopodia fuse to form large vesicles (phagosomes) the pinch off membrane. Phagosomes migrate and fuse with lysosomes where the contents of the phagosomes are degraded
73
What is receptor mediated endocytosis
Molecules in the extra cellular fluid (ligands) bind with high affinity to specific protein receptors on the plasma membrane
74
What are the two types of receptor-mediated endocytosis
1. Clathrin-dependent receptor-mediated endocytosis
2. Photocytosis
75
What is clathrin-dependent receptor-mediated endocytosis
1. When the ligand binds the receptor undergoes conformational change and clathrin is recruited to the plasma membrane.
2. Adaptor proteins link the ligand-receptor to the clathrin
3. The complex forms a cage like structure that leads to the aggregation of ligand bound receptors.
4. A clathrin-coated pit is formed which then invaginates and forms a clathrin-coated vesicle
5. Once the vesicle pinches off it sheds the clathrin coat and vesicles can fuse with the membrane of cellular organelles
76
What is potocytosis
Process by which molecules are sequestered and transported by tiny vesicles (caveolae) that are clathrin-independent. Caveolae can deliver their contents directly into the cell cytoplasm as well as to the endoplasmic reticulum, other organelles and plasma membrane on opposite side of cells
77
How does diffusion work
Water moves freely across most cell membranes. This is facilitated by groups of proteins (aquaporins) that form water permeable channels (aquaporins distributed homogeneously everywhere)
78
What is osmosis
Net dissuasion of water across a semi-permeable membrane
79
What is a semipermeable membrane property
Permeable to solvent but not all solute
80
What is osmotic pressure
The pressure required to prevent movement of water across a semi-permeable membrane (equal to the difference in the hydrostatic pressures of two solutions)
81
What is osmotic pressure proportional to
The number of particles in solution/unit volume and not to their size, configuration or charge
82
What is osmolarity
Total solute concentration of a solution (1 osmol = 1 mol of solute particles, Osm = osmol/liter, osmotic pressure is proportional to osmolarity)
83
What does it mean for a solution to be isosomotic
Solutions with the same osmolarity as normal extracellular or intracellular solutions (300 mOsm)
84
What does it mean for a solution to be hypoosmotic
Solution with an osmolarity lower than 300 mOsm
85
What does it mean for a solution to be hyperosmotic
Solutions with an osmolarity greater than 300 mOsm
86
Does osmolarity have an effect on cells
No
87
How most particles behave in order to be effective in exerting a sustained osmotic pressure
Particles must not be able to cross the membrane (non penetrating)
88
Give an examples of a non-penetrating solute
Na+ because the Na+ that moves into the cell is pumped out by the Na-K ATPase
89
What happens if the solution has a concentration of 300 mOsm of non penetrating solute particles
No net shift of water, isotonic solution (no swelling or shrinking of cell)
90
What happens when the solution has a concentration of non penetrating solute particles less than 300 mOsm
Water will enter the cell and the cell will swell (hypotonic solution)
91
What happens if the solution has a concentration of non penetrating solute particles more than 300 mOsm
Water will leave the cell and the cell will shrink, hypertonic solution
92
Where does the main exchange of solutes and other stuff happen
At the level of the capillaries
93
What is the structure of the capillary wall
A single layer of flattened endothelial cells and a supporting basement membrane
94
What are the types of transport across a capillary wall
1. Diffusion
2. Transcytosis
3. Bulk flow
95
What is the most important means of transport across cell membranes and through water filled channels
Diffusion
96
What is transcytosis
Endocytosis on the luminal side followed migration of the vesicle across the cell and then exocytosis on the interstitial side
97
What is bulk flow
Distributes the extracellular fluid volume between the plasma and ISF
98
What is the magnitude of bulk flow proportional to
The hydrostatic pressure difference between the plasma and the ISF. Capillary wall acts as a filter that permits protein free plasma to move from capillaries to the ISF
