Physiology Flashcards
What are the functional differences between cell types are due to?
Variations in the composition of their plasma membranes - Different cells interact in different ways with essentially the same ECF
Two main components of membranes?
Lipids and proteins
Describe the glycerol background of phospholipids?
Two hydroxyl groups of which are esterified to various fatty acids (maybe saturated or unsaturated)
The third glycerol hydroxyl group is esterified
to a phosphate group which is attached to a head
group
What determines the thickness of the bilayer?
Length of the fatty acid chains (2-3nm).
What does the head group determine?
How densely packed adjacent phospholipid molecules are
What can dissolve phospholipids?
Detergents
What does cholesterol do?
Aids in rigidity
Constantly flip flops
T or F: The phospholipid composition of the two leaflets of the plasma membrane are identical
False
What is permeable to phospholipid layers?
Small uncharged polar molecules
E.g. O2, CO2, NH3, water
What is impermeable to phospholipid layers?
Almost any water-soluble substance
E.g. ions, proteins, and sugars
T or F: phospholipid layers is responsible for the fluidity of the membrane
True
Eg: RBC, skeletal muscle cell
T or F: phospholipids contain more lipids than proteins
False
Lipids
What are peripheral membrane proteins?
Not embedded within the membrane, instead they adhere tightly to the cytoplasmic or extracellular surfaces of the PM
What are integral membrane proteins?
Intimately associated with the lipid bilayer
What are the three different ways integral membrane proteins present themselves?
Span the lipid bilayer once or several times: transmembrane proteins
Some are embedded but do not cross the bilayer
Some are linked to a lipid component of the membrane or a fatty-acid derivative that intercalates into the membrane
Functions of integral membrane proteins
Ligand-binding receptors - hormones
Adhesion molecules - cadherins, integrals
Pores and channels - nerve cell (gated/leak channel)
Carriers
Pumps
Enzymes - carbonic anhydrase
Intracellular signalling - GTP binding proteins, kinases
What are dock-marker receptors?
Located on the inner membrane surface
Interact with secretory vesicles leading to exocytosis of the vesicle contents
Glycoproteins + glycolipids = ?
Glycocalyx
Cancer is an example of..
Abnormal surface marker
Role of carbohydrates?
Self-recognition
Cell-to-cell interactions
Two properties that influence whether a particle can
permeate the plasma membrane without assistance?
Solubility of the particle in lipid
Size of the particle
T or F: A Pathway and a driving force is required for movement across a membrane (applies for assisted or unassisted)
True
T or F: If impermeable diffusion will still takes place across the membrane even if a concentration gradient for that substance exists
False
Law’s of Fick’s diffusion
The magnitude of the concentration gradient
The larger the SA the greater the rate of diffusion it can
accommodate
The greater the lipid solubility the more rapidly the substance can diffuse through the membrane bilayer
down its concentration gradient
As molecular weight increases, the rate of diffusion will decrease
The greater the distance, the slower the diffusion
What is an example of a channel where diffusion takes place across?
Ion-specific channel proteins: Leak or gated
Define electrochemical gradient
The net effect of simultaneous electrical and
concentration gradients on a ion
Water moves by osmosis to the area of…
Higher solute concentration
Define Osmolarity and state its units
Concentration of osmotically active particles present in a solution (Osm/l)
Body fluids: ~300 mOsm/l
Define Tonicity
Effect a solution has on cell volume
Isotonic vs hypotonic vs hypertonic
Iso: No net movement
Hypo: Water diffuses into cell = swelling
Hyper: Water diffuses out of cell = shrinking
Passive methods of transport
Ion channels: Movement along electrical gradients
Simple diffusion: Diffusion down concentration gradients
Osmosis
What is carrier mediated transport?
Substance binds onto a specific carrier which undergoes a conformational change (shape change) which transports the substance
What determines carrier mediated transport?
Specificity
Saturation (transport maximum eg renal glucose re-absorption)
Competition
Two forms of carrier mediated transport?
Facilitated diffusion eg GLUT4 glucose transporter
Active transport
Two forms of active transport?
Primary active transport - energy is directly required to move a substance against its concentration gradient
Secondary active transport - energy is required, but it is not used directly to produce ‘uphill’ movement. The carrier does not split ATP – instead it uses secondhand energy stored in the form of an ion concentration gradient
What is need to induce a conformational change in active transport?
ATP splits - P binds to carrier
Change in shape reduces affinity to P
When carrier is released, so is P
Na+ -K+ ATPase pump transports 3x___ out of the cell for every 2x___
3x Na+ out of the cell for every 2x K+ in
Mechanisms of which secondary active transport occur?
Symport: move in the same direction.
E.g. glucose absorption at the apical membrane of
enterocytes
Antiport: move in opposite directions
E.g. Na+ -K+ ATPase pump
Main intracellular ion is..
Potassium
Main extracellular ion is..
Sodium
Define membrane potential
Difference in charge between the thin layers of ECF and ICF located next to the outside and inside of the membrane
Extracellular and intracellular conc. of K
E: 5
I: 150
Extracellular and intracellular conc. of Na
E: 150
I: 15
Extracellular and intracellular conc. of Cl-
E: 110
I: 7
T or F: At resting potential the membrane is 100x more permeable to Na+ than K+
False
More permeable to K+ than Na+
Two opposing forces acting on K+
The concentration gradient (tending to move K+ out
of the cell)
The electrical gradient (tending to move K+ into the
cell)
The membrane potential at EK
is..
-90mV
Measured Em for a typical nerve cell at rest is..
-70mV
Much closer to EK than ENa
T or F: Passive movement an ion through an ion channel is driven by the concentration gradient for that ion
False
Electrochemical
The driving force for Na+ influx
-130mV
The driving force for K+ efflux
+20
The ion channels responsible for the action
potential in neurones
Voltage-activated Na+ channels
Voltage-activated K+ channels
K+ flows outwardly generating an outward current because…
The concentration gradient is outward and has an energy which exceeds that of the electrical gradient, which is inward
What are action potentials
Brief electrical signals in which the polarity of the nerve cell membrane is momentarily (about 2 msec) reverse
The activation of Na+ channels is..
Self-reinforcing – the opening of a few channels causes further channels to open, causes further depolarization. This is positive feedback
The activation of K+ channels is..
Self-limiting – outward movement of K+ causes repolarisation which turns off the stimulus for
opening. This is negative feedback
What is the state of voltage-activated N+ during maintained depolarization?
Inactivated stat
Define Relative refractory period
A stronger than normal stimulus may elicit a second action potential
What is passive conduction?
A factor in the propagation of the action potential (AP)
T or F: The less ‘leaky’ the axon the greater the local current spread
True
Why does backward current flow does not re-excite previously active area?
Because it’s in its refractory period
How is current speed increased?
Increase axon diameter
Decrease leak of current across the axon (possible by adding an insulating material – myelin)
What is upstroke, downstroke and overshoot due to?
U: Mediated by the opening of voltage-activated Na+
channels
D: Opening of voltage-activated K+ channels and the inactivation of voltage-activated Na+ channel
O: Delayed closure of voltage-activated K+ channels
