General Physiology

CELL MEMBRANES AND TRANSPORT ACROSS MEMBRANES

What are the components of a cell membrane?

1. Cholesterol
2. Phospholipids
3. Sphingolipids
4. Glycolipids
5. Proteins

Which cell membrane component contributes the most to stability?

Cholesterol

What are the two molecular components of cell membrane phospholipids?

Glycerol backbone (hydrophilic) and fatty acid chains (hydrophobic)

How are integral and peripheral proteins different?

Integral: span entire membrane

Peripheral: located on either side of the membrane

How are integral proteins attached to the membrane?

Hydrophobic interactions with the phospholipid bilayer

Give some examples of integral and peripheral proteins:

Integral: ion channels, transport proteins

Peripheral: spectrin, ferrochelatase (an example of a membrane-linked enzyme)

How do lipid-soluble substances move across cell membranes?

Simple diffusion across the hydrophobic lipid bilayer

How do water-soluble substances move across cell membranes?

Cross through water-filled channels or transported by carriers, as they cannot dissolve in the lipid bilayer

What are the different types of transport across a cell membrane?

Simple diffusion, facilitated diffusion, primary active transport, and secondary active transport

Which types of cellular transport are carrier mediated?

Facilitated diffusion, primary active transport, and secondary active transport

Which types of cellular transport require metabolic energy?

Primary active transport and secondary active transport

What is an example of a primary active transporter?

Any ion-translocating ATPase

What are the two subtypes of secondary active transporters?

1. Cotransport
2. Countertransport

What is cotransport (symport)?

Mediated transfer of two or more solutes in the same direction across a cell membrane

What is countertransport (antiport)?

Mediated transfer of two or more solutes in opposite directions across a cell membrane

Explain secondary active transport:

It couples the transfer of two or more solutes across a membrane; one solute moves down its electrochemical gradient providing the motive force for movement of the other solute(s) against its electrochemical gradient

Which ion most often functions as the motive force for secondary active transport?

Na⁺; it has a favorable inward gradient across almost all cell membranes

What is the direction of solute movement relative to the electrochemical gradient in the following types of transport?

Simple diffusion

Downhill

Facilitated diffusion

Downhill

Primary active transport

Uphill

Secondary active: cotransport and countertransport

For both types of transport one solute moves down its gradient providing energy for the other solutes to move against their gradients. In cotransport, solutes move in the same direction. In countertransport, they move in opposite directions.

What does permeability describe?

Ease with which a solute is able to diffuse across a membrane

What factors can increase membrane permeability?

1. Increased oil/water partition coefficient
2. Decreased size of solute
3. Decreased membrane thickness

What are the characteristics that are important in carrier-mediated transport?

Stereospecificity, saturation, and inhibition (competitive, noncompetitive)

What is the transport rate when the carriers are saturated?

Transport maximum (T_m)

Which is faster, diffusion or facilitated diffusion of a hydrophilic solute?

Facilitated diffusion, as it is carrier mediated

Which has unlimited (theoretically) capacity for cellular transport?

Diffusion, it is limited only by its gradient and its permeability characteristics

Na⁺-K⁺ PUMP

What is the Na⁺-K⁺ pump?

An integral membrane protein mediating ATP hydrolysis as energy for Na⁺-K⁺ exchange

What does the Na⁺-K⁺ pump do?

Extrudes 3 Na⁺ from within the cell and takes in 2 K⁺ from outside the cell

Is the Na -K pump electroneutral?

No; the Na -K pump is electrogenic, mediating net efflux of positive charge creating a negatively charged interior of the cell.

What provides the energy for the Na⁺-K⁺ pump?

Hydrolysis of adenosine triphosphate (ATP)

What example substances inhibit activity of the Na⁺-K⁺ ATPase pump?

Ouabain and digitalis glycosides

INTERCELLULAR CONNECTIONS

What are some examples of intercellular connections?

Tight junctions, gap junctions, and desmosomes

What is the role of tight junctions? (Histology often calls these zonula occludens.)

Circumferential bands of proteins that impede the movement of solute paracellularly

Tight junctions have been classically divided into tight and leaky epithelia. Explain the difference and give examples:

Tight: many bands of protein blocking all (most) paracellular movement of water and solute; found in the ascending limb of the loop of Henle

Leaky: few bands of protein, allows paracellular movement of water; found in the proximal tubule and throughout the gut

What is the role of desmosomes? (Histology often calls these zonula adherens.)

These are regions of connections between adjacent cells. They permit paracellular movement, but do not allow for cell-to-cell flow.

What is the role of gap junctions?

Protein bridges from one cell to an adjacent cell. These allow for solute flow between adjacent cells.

What important tissue relies on gap junctions for proper function?

Myocardium, allow for rapid conduction of electrical impulses

OSMOSIS

Define osmolarity:

Concentration of osmotically active particles in a solution

Define osmosis:

Flow of water across a semipermeable membrane from a low osmolarity compartment to a compartment with a higher osmolarity

Define osmotic pressure:

The hydrostatic pressure necessary to counter osmosis between two compartments

How is osmotic pressure calculated?

Van’t Hoff’s law:

π = gCRT

π = osmotic pressure (mm Hg),

g = number of particles in solution (osm/mol),

C= concentration (mol/L),

R = gas constant (0.082 L . atm/mol . K),

T= absolute temperature (K)

What is oncotic pressure?

Osmotic pressure created by proteins (e.g., colloid osmotic pressure)

What is tonicity?

The relative difference in osmolarity in different compartments

What are hypotonic and hypertonic solutions?

These terms refer to the relationship between two compartments. Hypotonicity refers to a relatively lower osmolarity of one compartment to another. Hypertonicity, likewise, refers to a solution having higher osmolarity than another.

How does infusion of hypo/hypertonic solutions influence fluid compartments?

Infusion of a hypotonic solution into the intravascular space will initially decrease osmolarity, and water will tend to move into the interstitium, increasing edema. Infusion of hypertonic fluids will lead to the opposite effect.

MEMBRANE POTENTIALS

Define steady state:

The stable condition between biologic compartments when the net ionic flow is zero

Define diffusion potential:

Potential difference across a membrane due to the separation of positive and negative charge across a membrane

What determines the size of the diffusion potential?

The magnitude of the ionic charge separation

Can a diffusion potential be generated if the membrane is not permeable to the ion?

No

What determines the sign of the diffusion potential?

The direction of the positive and negative charge separation across the membrane

True or False? Creation of the diffusion potential requires the movement of a significant number of ions.

False. The separation of very few ions creates a diffusion potential.

What facilitates the movement of ions across the membrane?

Typically, ion channels, but also pumps and carrier transporters

What controls the opening and closing of ion channels?

A molecular gating mechanism

What are the three general components of an ion channel?

1. The pore
2. The selectivity filter
3. The gating mechanism

What is the selectivity filter?

Grants specificity to the various channels, allowing only one type of ion through the channel

What does the conductance of an ion channel depend on?

The probability that the channel is open and its capacity to mediate translocation through the pore

What types of ion channel gating mechanisms are there?

Voltage-sensitive, ligand-induced, and mechanical gates.

What controls the opening of voltage-gated channels?

Membrane potential

What controls the opening and closing of ligand-gated channels?

Hormones

Second messengers

Neurotransmitters

What controls the opening of mechanical gates?

Membrane stretch and manipulation. Commonly seen in epithelial layers.

Define equilibrium potential (£):

Electrical potential that exactly opposes diffusion caused by an ionic concentration difference. (In other words: how much energy do we have to use to stop diffusion)

What is it called when the electrical and chemical driving forces of an ion are equally opposed?

Electrochemical equilibrium

What equation is used to calculate equilibrium potentials?

Nernst equation:

E = equilibrium potential (mV),

2.3 RT/F = constants (60 mV at 37°C)

z = charge on the ion

C_i = intracellular concentration (mM)

C_e = extracellular concentration (mM)

What are the approximate values of the equilibrium potential for the following ions in nerves and muscles?

Na⁺

+65 mV

K⁺

−85 mV

Ca²⁺

+120 mV

Cl^–

−85 mV

Define the resting membrane potential.

Electrical potential across a semipermeable membrane when the cell is at steady state

Is energy consumed to generate the resting membrane potential?

Yes, the maintenance of the ionic gradients requires constant energy input

Define depolarization:

Membrane potential becomes less negative (i.e., −60 mV → −40 mV)

Define hyperpolarization:

Membrane potential becomes more negative (i.e., −60 mV → −90 mV)

What is a cell called if it is capable of producing an action potential?

Excitable

What do action potentials consist of?

Rapid depolarization and repolarization

What are the unique characteristics of action potentials?

All-or-none events, propagation, and stereotypical size and shape (differs for different cells, e.g., cardiac versus neurons but always reproducible in each cell type)

What is the point after which an action potential is inevitable?

Threshold

What does inward or outward current charge refer to?

Movement of positive charge into (inward) and out of (outward) the cell

What is the resting membrane potential for nerve cells?

−70 mV

What ion is primarily responsible for the resting membrane potential of nerve cells?

K⁺

Why does K⁺ contribute the most to the resting potential?

At rest, K⁺ has the greatest ionic conductance

What signals the activation gates of Na⁺ channels?

Depolarization of the cell membrane; these are voltage-gated channels

What ion is responsible for the upstroke of the action potential in nerve cells?

Na⁺

Define overshoot:

Peak of the action potential at which the membrane potential is positive

What signals the inactivation gates of Na⁺ channels?

Depolarization of the cell membrane

How can depolarization cause both activation and deactivation of Na⁺ channel gates?

Inactivation occurs more slowly than activation

What causes repolarization of the cell membrane following an action potential?

As the inward Na⁺ conductance falls due to channel inactivation, outward K⁺ conductance leads to repolarization

What signals the opening of K⁺ channels during an action potential?

Depolarization of the cell membrane

What is the hyperpolarization of the afterpotential called?

Undershoot

What causes undershoot?

K⁺ channels stay open after the Na⁺ channels close

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