cell physiology

Question 1

ion channels

Answer 1

ion channels are:
* Selective.
* Conductance(measured in picosiemens (pS)).
* The single-channel conductance of typical ion channels ranges from 0.1 to 100 pS.
* Gating = Fluctuation between open and closed states.

controlling ion channels:
membrane voltage (e.g. depolarisation)
extracellular agonists or antagonists (e.g. lig and gated)
intracellular messengers (e.g. Ca2+, ATP, cGMP).
mechanical stretch of the plasma membrane (physical stimulation).

Question 2

Describe different types of ion channels and explain the term “channel gating”. —
ionotropic receptors

Answer 2

Ligand-gated ion channels made of 3, 4, or 5 protein subunits that together form an ion-conducting pore in the center of the receptor.
Activation of receptor causes a pore to open through which ions can pass.

Examples include:
- Ligand gated sodium channels (eg. AChR nicotinic not muscarinic).
- TRPV1 receptors (receptor for capsaicin found in hot chilli peppers).

Fast acting
“On” or “Off” - All or None
Receptor is made up of multiple subunits
Example of function is in triggering an action potential.

Question 3

metabotropic receptors

Answer 3

AKA G-protein coupled receptors
Activation of receptor initiates an intracellular signalling mechanism (ions do not pass through the receptor protein).

Examples include:
– Metabotropic glutamate receptors (mGluRs).
– Adrenergic receptors of the autonomic nervous system (e.g. beta-adrenergic receptors in heart).

Slower prolonged response compared to ionotropic receptors.
Can amplify or dampen signals:
Gs = stimulatory.
Gi = inhibitory.
Receptor proteins are monomers.

Question 4

membranes and movement of water

Answer 4

Hydrophobic core.
Effective barrier to movement of virtually all biologically important solutes.
Intracellularandextracellularfluidsareprimarilywater(in which solutes such as ions, glucose and amino acids are dissolved).
Gases(e.g.,O2 and CO2)and ethanol can diffus eacross lipid bilayers.
Movement of water and solutes is restricted.

Membranes are not very permeable to water
* Water channels = Aquaporins (AQPs) image is human AQP1.
* Widely distributed throughout the body, especially in kidney.
* Different isoforms found in different cell types.
* Amount of H2O influx/efflux can be regulated: – altering number of AQPs in the membrane
(membrane protein trafficking)
– changing their permeability (i.e. gating) e.g. by pH

Question 5

Outline the general properties of carrier mediated transport systems.

Answer 5

solute carriers
3 major functional groups:
1. Uniporters (transport one substance).
2. Symporters (transport more than one substance in the same direction).
3. Antiporters (transport substance in different directions).

uniporters:
Transport a single molecule across membrane.
Example: GLUT2 (brings glucose into the cell).
Mutations can cause diabetes.

symporters:
Couple the movement of two or more molecules/ions across the membrane.
molecules transported in the same direction (co-transport).
Example: NKCC2 found in the kidneys
1Na+,1K+,2Cl- symporter.
critically important for diluting and concentrating urine.

antiporters:
Couple movement of two or more molecules/ions across the membrane in opposite directions
Also called “exchangers” and “counter transporters”
Example: Na-H exchanger
– Na+-H+ antiporter
– found in all cells
– important role in regulating intracellular pH

Question 6

other examples of carrier mediated transport systems.

Answer 6

H+ -ATPase
Vacuolar H+-ATPase: found in membranes of many intracellular organelles (e.g. lysosomes)
Plasma membrane H+- ATPase: important role in urinary acidification

ATP dependant ion transporters:
Example: Na+, K+ATPase (also called Na+K+pump)
Found in all cells
Three subunits (α, β, and γ) α subunit has binding sites for: Na+, K+, ATP and Ouabain (inhibitory – used to treat hypotension)

ATP binding cassette(ABC) transporters:
An example of an ABC transporter. Cystic fibrosis transmembrane regulator (CFTR).

Question 7

Explain primary and secondary active transport mechanisms with appropriate examples.

Answer 7

• transport is directly coupled to ATP hydrolysis (to move substances against their concentration gradient).

Secondary active transport (no ATP)
* Energy for the transport comes from the electrochemical gradient. The energy from one molecule is used to move another molecule(s) against its electrochemical gradient
e.g. 3Na+-Ca2+ antiporter

Question 8

Describe osmosis.

Answer 8

diffusion:
solute is added to a solvent, solute particles will move randomly (Brownian motion) from area of high concentration of solute particles to areas of low concentration of solute particles (down their concentration gradient) until equilibrium is reached

osmosis:
*Diffusion of water across a semi-permeable membrane
*Membrane is freely permeable to water but not permeable to solute (see image)
*Water diffuses from a high water concentration to low water concentration (note: high water concentration = low solute concentration)

Question 9

Define hydrostatic pressure.

Answer 9

*A difference in hydrostatic pressure between the 2 compartments is created, and it opposes osmosis.
*Hydrostatic pressure is the pressure exerted by a stationary fluid on an object – a “pushing force”.

Question 10

Define osmotic pressure.

Answer 10

*The osmotic pressure of a solution is a measure of the tendency for water to move into that solution because of its relative concentration of non-penetrating solutes and water – a “pulling force”.
*Net movement of water continues until the opposing hydrostatic pressure exactly counterbalances the osmotic pressure.

Question 11

Compare molarity and osmolarity.

Answer 11

Molarity
Number of molecules in a solution
Mol / L
(remember this useful equation: Mols = Mass / Mr)

Osmolarity = number of particles in a solution
Osm / L

The difference between molarity and osmolarity depends on the substance:
examples:

Glucose
Molecules DO NOT separate out in solution therefore molarity and osmolarity are the same.
Molar it’s = 5.5 mM/L
Osmolarity= 5.5mOsm/L

NaCl
Separates out into two different ions (Na+ & Cl-) therefore the osmolarity is double the molarity.
Molarity = 1 mol/L
Osmolarity = 2 osm/L

Question 12

Define tonicity.

Answer 12

Tonicity = the effect the osmotic pressure gradient has on cell volume
* Cells change shape due to the net movement of water (into or out of the cell).

Question 13

explain what would happen to a cell if it was placed in the following solutions:
Isotonic

Answer 13

Isotonic = the two solutions have the same solute concentration
therefore no net movement of water

Question 14

Explain what would happen to a cell if it was placed in the following solutions: Hypotonic

Answer 14

extracellular: 150mOsm/L
intracellular: 300mOsm/L

therefore as the solute concetration of particles is lower outside- therefore it is more dilute and there is more water outside the cell
water moves into the cell causing it to swell

Question 15

Explain what would happen to a cell if it was placed in the following solutions: Hypertonic.

Answer 15

extracellular: 600mOsm/L
intracellular: 300mOsm/L

hyperosmotic solution outside cell therefore water will move out causing the cell to shrink

Question 16

osmolality and water balance disorders

Answer 16

*Osm per kg of body weight (Osm/kg)
*Useful for human / medical studies
*Normally about the same as osmolarity
*Normal plasma values 280-295 mOsm/kg
Osmolarity = osmoles per volume
Osmolality = osmoles per weight (typically kg)

Extra cellular fluid can be:
hypotonic (positive water balance): cells swell then undergo regulatory volume decrease (RVD).
hypertonic (negative water balance): cells shrink then undergo regulatory volume increase (RVI).

Question 17

Describe the mechanism of contraction in skeletal muscle.

Answer 17

1. Action potential propagates into the skeletal muscle cell.
   2.Depolarisation of sarcolemma and T-tubules.
   3.Activation of DHP receptors and Ryanodine receptors.
   4.Calcium release from terminal cisternae of sarcoplasmic reticulum (SR).
   5.Contractile machinery activated.
   6.Calcium pumped back into SR (active process - requires ATP).
   7.Calcium diffuses to terminal cisternae of SR ready to be released again.

Question 18

Explain the sliding filament theory of muscle contraction, including the detailed sequence of events that occurs in a cross-bridge cycle.

Answer 18

when contracted:
z disk moves in
I band gets smaller
H zone gets smaller
A band stays same

Actin and myosin dissociate when ATP is bound by myosin.

ATP breakdown to ADP and Pi causes a change in the angle of the head region of the myosin molecule, this enables it to move relative to the thin filament.

This cycle is then repeated. This process is known as cross-bridge cycling.

Question 19

Describe the role of calcium in muscle contraction, including triggering of contraction, release and re-uptake.

Answer 19

Propagation of (action potential) AP down into T-tubules
Activation of dihydropyridine receptors (DHPR) (T-tubules; conformational coupling with ryanodine receptors (RyR)).
Release of calcium from sarcoplasmic reticulum (SR).
Binding of Ca to troponin (conformational change tropomyosin).
Cross Bridge formation (Actin and Myosin; ATP).
Cross bridge cycling (Power Stroke; release of ADP + Pi).
Ca2+ removed from troponin restoring tropomyosin and Ca?+ taken back up by SR

Question 20

rigor mortis

Answer 20

Rigor mortis is caused by depletion of ATP.

*ATP causes actin-myosin bridges to separate during the relaxation of the muscle.

*Without ATP to separate the cross-bridging skeletal muscles are locked in place.

*As part of the decomposition process, myosin heads are eventually degraded by cellular enzymes allowing release of the cross-bridges and the muscles to relax.

*Peak rigor mortis ~13hours after death
*Decomposition of myofilaments ~48-60hours after the peak of rigor mortis

Question 21

Plot a classical experimental length-tension curve for a single muscle fibre and interpret it in terms of the sliding filament theory.

Answer 21

experiments initially conducted on isolated frog muscle fibre
clamp both ends and stretch the muscle fibre

sarcomere length:
between 2-2.5 is relaxed/optimum

phases:
latent/latency period
contraction phase
relaxation phase

Question 22

single contraction: twitch

Answer 22

Skeletal muscles are classified as being fast or slow twitch based on speeds of sarcomere shortening.

*Morphological differences too:

Fast = white (lower myoglobin & capillary content)

Slow = red (high myoglobin & capillary content)

Question 23

tetanus

Answer 23

Tetanic fusion frequency
Tetanus: The prolonged contraction of a muscle, caused by a rapidly repeated stimuli

TFF = the frequency of action potentials that are needed to not see summation and produce a smooth graded contraction as seen in normal muscle contraction.

Question 24

Describe the ultrastructure of cardiac muscle.

Answer 24

10 μm diameter
100 μm length

Mechanical junctions:
fascia adherens
desmosomes
Electrical connections
gap junctions–
*Desmosome = mechanical junctions between adjacent muscle fibres.
*Gap Junctions = Electrical connectivity between adjacent muscle fibres.

*Highly organised contraction

*Pumps blood around cardiovascular systems

*Action potential initiated in Sinoatrial node (SAN) passes quickly through electrical syncytium – firing action potentials spontaneously

*Refilling of heart requires synchronised relaxation

Question 25

Describe the ionic basis of cardiac action potentials.

Answer 25

Fast response and slow response action potentials

Fast responce:
Stable Resting Membrane Potential (RMP).

•Phase 0 due to Na+ entry.

•Phase 1 initial repolarisation due to K+ efflux.

•Phase 2 due to Ca2+ entry (different in cardiac muscle) and sodium-calcium exchanger.

•Phase 3 more K+ efflux.

•Phase 4 RMP slightly more negative than RMP at the beginning

Slow responce:
Unstable Resting Membrane Potential (RMP) allows spontaneous depolarisation.
•No early repolarisation (Phases 1&2) as in the fast response cells.

•Phase 0 due to slow inward current of Na+ and Ca2+ influx causing depolarisation.

•Phases 1 and 2 are not present, as a result, phase 0 is followed by phase 3.

•Phase 3 repolarisation due to closing of calcium channels and efflux of K+.

•RMP (phase 4) is less negative, gradual depolarization.

Question 26

Explain the detailed mechanisms underlying contraction and relaxation of cardiac muscle.

Answer 26

Contraction:
•Calcium comes in through L-type calcium channels
•Ryanodine receptors (RyR)

Relaxation:
•SERCA or SR pump (pumping Ca2+ back into the SR)
•NCX (sodium-calcium exchanger) 3Na -1Ca
•Sarcolemma Ca++ ATPase

Question 27

 Explain how the force of contraction is regulated in cardiac muscle cells.

Question 28

 Give some examples of the location and functions of different types of smooth muscle.

Answer 28

Location
*Internal organs (viscera).
*Walls of blood vessels.
*Around hollow organs (e.g. bladder).
*Layers around respiratory, circulatory, digestive and reproductive tracts.

Function
* Move food, urine and reproductive tract secretions.
* Control diameter of respiratory passageways.
* Regulate diameter or blood vessels.

Cells
* spindle-shaped
* Length ~ 100-300mm
* Width ~ 2-5 mm
* Single nucleus

Smooth muscle fibres
* often embedded in a matrix of connective tissue
* arranged in series and in parallel with one another

ultrastructure:
intermediate filament
thick filament
thin filament
membrane dense area
dense body
mechanical junction coupling cells
gap junction for electrical and chemical communication

Innervated by efferent fibres of the autonomic nervous system(ANS)
*Sympathetic / parasympathetic
*Excitatory – contraction
*Inhibitory – relaxation
*Lack of voluntary control – autonomic reflexes

• Presynaptic terminals (synaptic knobs)
• Very close to effector cells (smooth muscle)
  Release neurotransmitter

Question 29

Compare single and multi-unit smooth muscle.

Answer 29

Single Unit:
Gap junctions cause propagation and the whole muscle functions together to contract.
eg. gastrointestinal tract bladder

Multi Unit:
Different parts of the muscle can function independently
eg. iris, vasculatur(artery lining) airways(trachea)

Question 30

Describe tonic and phasic contraction.

Answer 30

phasic:
relatively quick contraction with short durability (easily fatigued)
eg. phasic which is usually relaxed: esophagus
phasic which cyclesc between contracted/relaxed

tonic:
relatively slow contraction with long durability (resistant to fatigue)
eg. tonic muscle usually contracted: sphincter that relaxes to allow material to pass
tonic muscle which contraction is varied as needed: vascular smooth muscle

Question 31

 Explain the roles of calcium-calmodulin myosin light chain kinase (MLCK) and myosin phosphatase (MP) in smooth muscle contraction.

Answer 31

1. Hormones or neurotransmitters:
   -open voltage or ligand gated Ca2+ channels in the sarcolemma, causing Ca2+ influx
   OR
   -bind to G-protein coupled receptors and induce generation of IP3
2. Extracellular Ca2+ (main source) or IP3 induced Ca2+ release from SR.
3. Ca2+ binds to calmodulin in the sarcoplasm
4. Ca2+-Calmodulin activates Myosin Light Chain Kinase (MLCK)
5. Active MLCK phosphorylates MLC heads, enabling muscle contraction
6. muscle relaxes as Ca2+ is pumped into SR for storage
7. Ca2+ is also exchanged with Na+ at the sarcolemma
8. The Na+-K+ ATPase restores Na+ gradient

Question 32

 Describe the ‘latch state’ of smooth muscle.

Answer 32

Latch state = an adaptation of smooth muscle which allows sustained muscle tone.

• Enables sustained smooth muscle tone with low rate of cross-bridge cycling (so low rate of ATP usage).
• Occurs when some of the cross-bridges attached to thin filaments become dephosphorylated (by myosin phosphatase).
• This greatly slows rate of cross-bridge detachment therefore maintaining tone.
• Filaments tend to remain “locked” together.

Question 33

 Compare and contrast skeletal, cardiac and smooth muscle structure, function, anatomy & physiology (all three muscle lectures).

Answer 33

Skeletal:
Long cylindrical fibres
Striated

Cardiac:
Branched cyclindrical
Striated

Smooth muscle:
Spindle shape
Non-striated

Similarities to skeletal and cardiac muscle:
Sliding filament theory and cross bridge cycling occurs (although regulation is different).
Calcium plays an important role in contraction.

Differences (compared with skeletal and cardiac muscle):
Smooth muscle contraction is thick filament regulated.
Contractions can be slow and sustained maintaining muscle tone due to the “latch state”.