Cell Phys Exam 2

LECTURE 1 (Biological membranes and Transporters Lec 5)

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What are the main functions/characteristics of the phospholipid membrane?

• Highly selective boundaries

• Between a cell and its environment

• Within the cell

• Allow [and maintain] the establishment of gradients

• Chemical (small solutes; ions, sugars, etc.)

• Electrical (e.g., membrane potential)

The phospholipid membrane participates ________ in cell signaling

chemically in cell signaling

Provide substrates

• e.g., phosphatidylinositol, diacylglycerol, cholesterol

The plasma membrane allows the creation of two major

compartments, —————— & __________, that differ in

composition

intracellular & extracellular

Recite table 12.1

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Cation Na+

Intracellular Concentration

5-15mEq/

5-15 mM

Extracellular Concentration

145 mEq/l

145 mM

Cation K+

Intracellular Concentration

140 mEq/l

140 mM

Extracellular Concentration

5 mEq/l

5 mM

Cation Mg2+

Intracellular Concentration

1.0 mEq/l

0.5** mM

Extracellular Concentration

2-4 mEq/l

1-2 mM

Cation Ca2+

Intracellular Concentration

2x10^-4 mEq/l

10^-4 mM

Extracellular Concentration

2-4 mEq/l

1-2 mM

Cation H+

Intracellular Concentration

7*10^-5 mM

{10^-7.2 M or pH 7.2}

Extracellular Concentration

4x10-5 mM

{10-7.4 M or pH 7.4}

Anion Cl-

Intracellular Concentration

5-15 mEq/l

5-15 mM

Extracellular Concentration

110 mEq/l

110 mM

Phospholipid membranes permeation depends on ____________

lipid solubility (i.e., log P)

__________ are perimiable

_________ are modest but detectable permeability

_________ are virtually impermeable

Permeable: Small nonpolar molecules

Modest but detectable permeability: Small uncharged polar molecules

Virtually impermeable: Large uncharged polar molecules or Ion

______________ proteins provide pathways for otherwise impermeant substances to cross selected membranes

Integral transmembrane

proteins provide pathways for otherwise

impermeant substances to cross

selected membranes

How do Integral transmembrane proteins

provide pathways for otherwise

impermeant substances to cross

selected membranes?

by channels, carriers and pumps (facilitated diffusion)

• *Pharmaceutical ionophores can form

artificial channels and carriers

• Regulated, selective mechanisms

that permit or cause movement

between cellular compartments

Simple diffusion is _________ as it is gradient driven and dissipates gradient

‘Simple’ diffusion is ‘passive’

– Potential energy is expended

– Gradient driven, dissipates gradient

– Includes partitioning / extraction

– Not saturable

Facilitated diffusion is passive, but _________

Facilitated diffusion is passive,

but regulated

– Potential energy is expended

– Gradient driven, dissipates gradient; saturable

Active transport is _________

– Both ________and _______active transport

Active transport is regulated

– Both 1º (ATP-ase) and 2º (Na+ solute concentration) active transport

– Generates or overcomes gradient; saturable

– Potential energy is required and expended – e.g., chemical bonds

Simple diffusion is ____________ Rate of net flux will always be __________ to the magnitude of the gradient

Simple diffusion is unsaturable Rate of net flux will always be proportional to the magnitude of the gradient

Facilitated diffusion / transport is __________ at some substrate concentration all available binding sites will be occupied and turn-over will be at a ___________

Facilitated diffusion / transport is saturable at some substrate concentration all available binding sites will be occupied and turn-over will be at a maximal rate

Facilitated diffusion / transport is _________

– Structurally related compounds can

compete for transport (competitive

inhibition)

inhibitable

Energy is ________ when it moves down the concentration gradient in facilitated difussion

lost

What allows a solute to undergo a conformational change,

and then release the solute on the other side of the membrane?

Transporters

Channels tend to have high _________ & high

_________ while transporters tend to be saturated at low

concentration (low KM)

Channels tend to have high permeation (‘throughput’) & high saturation concentration (high KM) while transporters tend to be saturated at low concentration (low KM)

A ________ is a regulated selective pathway

Channel

ex. sodium gradient used for movement

Permeation via conformational change is typically through a _____________

Transporter

Uniporter:

Symporter:

Antiporter:

• Uniporter – transports one molecule (ex. Glut 4)

• Symporter – transports at least two

molecules at the same time in the same direction

• Antiporter – transports at least two molecules

at the same time in opposite directions (ex. na+ Ca2+ exchanger)

A _______ uses ATP (or other energy source) to move

solute against a gradient

Pump

Ca2+ channels (cell membrane & SR) is an example of ___________

Facilitated diffusion for Ca2+

What type of transporter is the Na+/Ca2+ exchanger?

Antiporter

• 2º active transport with respect to Ca2+

• Facilitated diffusion with respect to Na+

What type of transporter is the Ca2+ ATPase exchanger?

Uniporter

• 1º active transport with direct coupling to

ATP hydrolysis

The 1° Active Transport of Na+/K+ ATPase pumps both Na+ and K+ against___________ gradients.

Which creates what type of imbalance?

against electro-chemical gradients

– Creates charge imbalance across the membrane

• Pumps 3 Na+ out for 2 K+ in

• Electrogenic

• Accounts for 5-30% of the resting membrane potential

1° Active Transport of Na+/K+ ATPase Generates gradient used for ________ contributing to osmotic regulation

2º active transport

List some examples of Na+ gradient-driven symporters and antiporters:

SGLT, EAAT, GlyT, NHE, Na+Ca2+ exchanger, NBC, etc.

Combinations of transport proteins account for vectoral

movement of solutes and water across a variety of

epithelia Sweat duct, salivary gland, mammary gland, nephron, gut, pancreatic duct, vas deferens, endometrium, etc.

Glucose absorption from Gut utilizes Na+/glucose co-transporter in the _______ membrane (SGLT1)

apical

– Glucose facilitated diffusion carrier in the

basolateral membrane (Glut2)

– Na+/K+ ATPase in the basolateral membrane

LECTURE 2 (Osmosis, Osmotic pressure, and Donna equilibrium LEC 6)

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Sterile saline has ____% saline and _____mM Nacl, with an osmolarity of _______ (also the osmolarity of plasma)

– 0.9% saline, 154 mM NaCl

(Added NaCl, 9 g/l to water)

osmolarity 286 (mOsmolar)

Concentration of water

55.56 moles/kg solvent (Molal)

In bulk solution, the water flux is often ____ compared to flux of solute because the gradient is quite small

small

Osmosis is defined as

Water flow (or flux) across a semi-permeable membrane from a compartment in which the total solute concentration is lower when compared to one in which the total solute concentration is higher. (or opposite)

Osmotic Pressure is caused by

the diffusive flow of water down its concentration gradient

• Proportional to the number of particles in

  solution

Osmotic Pressure has colligative property meaning_______

Solutions containing mixtures of molecular species (i.e.,

different solutes) have an osmotic pressure that is the

sum of the individual pressures

Define the components of the osmotic pressure equation

R = ideal gas constant (0.082 L•atm/(K•mole))

T = absolute temperature (K)

Φ = osmotic coefficient (no units)

i = number of ions formed by the molecule in solution

c = molar concentration of the solute (moles/L)

______________ takes into account that

solutions are not ideal.

Φ (the osmotic coefficient) takes into account that

solutions are not ideal.

Φ is less than ____for physiological

ions, and approaches ___ as solutions become infinitely dilute

1

For osmolarity:

Molarity = Osmolarity =

Molality = Osmolality =

List some Osmotic Coefficients, Φ

NaCl:

KCl:

NaHCO3:

KH2PO4:

CaCl2:

NaCl: 0.93

KCl: 0.92

NaHCO3: 0.96

KH2PO4: 0.87

CaCl2: 0.86

Calculate the Osmolarity of Saline 0.9% w/vol or 154 mM NaCl

• Isotonic =

• Hypertonic =

• Hypotonic =

• Isotonic = equal osmolarity w/ RBC

An RBC dropped in an isotonic

solution will not change size

solution will not change size

• Hypertonic = greater osmolarity

An RBC dropped in a hypertonic

solution will tend to shrink

• Hypotonic = lesser osmolarity

An RBC dropped in a hypotonic

solution will tend to swell

What happens when a cell contains large, negatively charged molecules that cannot cross the membrane? (Donnan Effect)

These impermeant negative ions (Y⁻) attract positive ions like K⁺ and Na⁺. This changes where permeant ions are distributed across the membrane and creates an osmotic gradient that tends to pull water into the cell.

Think: 🚫 Y⁻ can't leave → ➕ ions are attracted to it → 💧 water follows → cell tends to swell.

What does electroneutrality mean inside a fluid compartment?

The total amount of _____________ charge must equal the total amount of __________ charge.

The total amount of positive charge must equal the total amount of negative charge.

Example:
If there are 100 mEq/L of negative charge, there must be about 100 mEq/L of positive charge.

What happens to permeant ions when a cell reaches Gibbs-Donnan equilibrium?

The ions redistribute until:

• Electroneutrality is maintained.

• The product of permeant cation × anion concentrations is equal on both sides.

• Ion gradients develop (such as K⁺ and Cl⁻ gradients).

• An osmotic gradient remains, which tends to make the cell swell.

• Big idea: The impermeant Y⁻ inside the cell forces the permeant ions to redistribute.

What does the Na⁺/K⁺ ATPase pump, and in which direction?

A: Using ATP energy, it pumps:

3 Na⁺ OUT ⬅
2 K⁺ IN ➡

This happens against their electrochemical gradients.

Why it matters:

• Keeps Na⁺ high outside and K⁺ high inside.

• Creates an inward Na⁺ gradient that powers secondary active transport.

• Helps control the cell's osmotic balance.

• Makes the inside of the cell relatively more negative because 3 positive charges leave for every 2 that enter.

Memory trick: 3 Na⁺ out, 2 K⁺ in = Na⁺ leaves the cell.

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Why does the Na⁺/K⁺ pump affect water movement?

Q: How does the Na⁺/K⁺ ATPase help prevent excessive cell swelling?

A: The pump moves 3 Na⁺ out for every 2 K⁺ in, helping move positive solute particles outward overall. This contributes to an outward osmotic gradient, so water tends to move from the cytosol → ECF.

Think:
Na⁺/K⁺ pump → solute moved outward → 💧 water tends to follow outward → helps prevent swelling.

Also remember: The pump is electrogenic because it moves unequal numbers of positive charges.

LECTURE 3 (Osmosis, Osmotic pressure, and Donna equilibrium LEC 6)

What is flux?

The net movement of particles (or charge) per unit time.

What does Fick’s Law describe?

Chemical/diffusion flux caused by a concentration gradient.

Factors that increase diffusion according to Fick’s Law are:

Greater diffusion coefficient, larger membrane/cross-sectional area, and a larger concentration difference; greater diffusion distance decreases flux

Ohms Law (electrical flux) =

voltage x conductance

(Electrical flux/current through a conductive pathway.)

equation for electrical current: I = Vg

ELECTRIC CHEMICAL POTENTIAL

The ___________ gradient is directed from left to right w/ barrier

A _____ selective pathway is opened

_______diffuses down its chemical gradient

______ diffuses until the right compartment becomes sufficiently electro-_________; No further ______ occurs

The Chemical (concentration) gradient

directed from left to right w/ barrier

A K+ selective pathway is opened

K+ diffuses down its chemical gradient

K+ diffuses until the right compartment

becomes sufficiently electro-positive;

No further net flux occurs ( stops having net movement bc K⁺ movement creates an electrical gradient opposite the chemical gradient until the two forces are equal and opposite)

What two forces act on K+ in the electrochemical potential pathway?

Two forces act on K+:

chemical gradient from left to right &

electrical gradient from right to left

What is an electrochemical gradient?

The combined effect of an ion's chemical gradient + electrical gradient.

What are the conditions at electrochemical equilibrium?

The chemical and electrical gradients are equal in magnitude and opposite in direction, producing no net flux.

key idea behind electrochemical equilibrium:

Two forces are acting on the ion, and they cancel each other out.

What does the Nernst equation determine?

The equilibrium potential (Nernst potential/ no net flux) for a particular ion.

• determining the direction an ion will flow in non-equilibrium conditions

• determines membrane potential (Vmem)

  • if the membrane is permeant to only one ion

Nernst potential represents: The membrane voltage at which there is no net flux of that particular ion.

What is the nernst equation?

• RT are constant 37°C = 60mv

• Outside (Reference electrode) concentration of cell goes ON TOP

• Inside (sensing electrode) concentration of cell goes on the BOTTOM

• Z is the charge of the Ion

What are the minimum requirements for establishing a resting membrane potential?

• An ion concentration gradient

• A charge carrier

• A conductive/selective pathway

• A power source to establish/maintain the gradient.

  • (Na⁺/K⁺ ATPase) specifically 3 Na⁺/ 2 K⁺ ATPase

conductive pathway necessary bc: Ions need a pathway through which they can move; conductance is required for voltage to be measured.

The equation for determining the driving force of net flux is:

Membrane potential (vMem) - Nernst potential (mV)= Direction of net flux (towards the compartment of lower concentration)

If the membrane potential is more positive than the nernst potential for a cation then that cation will tend to _______ the cell

Leave

If the membrane potential is more positive than the nernst potential for a anion then that anion will tend to _______ the cell

Enter

If the membrane potential is more Negative than the nernst potential for a anion then that anion will tend to _______ the cell

Leave

If the membrane potential is more Negative than the nernst potential for a Cation then that Cation will tend to _______ the cell

Enter

The nernst equation defines equilibrium conditions for a _______ ion species

while the Goldman-Hodgkin-Katz (GHK; Constant-Field) equation defines the equilibrium conditions for ______ ion species

nerst- single

Goldman-Hodgkin-Katz (GHK; Constant-Field) equation= mutiple

What is the Goldman-Hodgkin-Katz (GHK; Constant-Field) equation= mutiple equation?

‘Px’ is defined as the permeability of ion ‘x’ relative to a benchmark ion (e.g.,PK+ is defined as 1)

Calculate Vmem if PNa = 0.03 and PCl = 0.1

A change in conduction = change in __________

membrane potential

True or false

Ca²⁺ contributes significantly to the membrane potential in the same way as K⁺ and Na⁺

False

Ca²⁺ does not contribute as much under resting conditions, although it can contribute when its permeability changes.

What happens when PNa increases dramatically?

Vmem moves toward the Na⁺ Nernst potential, causing depolarization.

During repolarization, what happens to Na⁺ and K⁺ permeability?

Na⁺ permeability decreases while K⁺ permeability increases.

• Increased K⁺ permeability tends to move Vmem toward the K⁺ Nernst potential, making the membrane more negative.

What does hyperpolarization mean?

The membrane potential becomes more negative.

What does depolarization mean?

The membrane potential becomes more positive.

What does repolarization mean?

The membrane potential returns toward its resting/negative value after depolarization.

SLIDE DECK 4 ( Cytoskeleton 9- basal)

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What are the 3 categories of structural proteins?

• Microtubules

• Microfilaments

• Intermediate Filaments

What is the largest component in microtubules? How is this component structured?

Largest compoent:

Hollow tubes, ~24 nm diameter

• Often centered around nucleus radiating outward

Structure

• alpha and Beta -tubulin polarized tubes

• - end = alpha exposed

• + end = Beta exposed

+/- are NOT charges!

• - = regress

• + = grow

What are microtubules found in?

• General cytoplasmic organization

• Mitotic spindle

• Axons and dendrites

• Cilia

What are the primary functions of microtubules?

• Resists compression**** (major function)

• Cell scaffolding and polarization

• Polarized movement of organelles, proteins, and DNA

Microtubules polarized tubules allow polarized movement

What are the two major families of microtubules motor proteins?

• Kinesin (toward + end)

• Dynein (toward – end)

• Bind to cargo and ‘walk’ down the tubules

Kinesin moves from _________ to aid in ________

Kinesin moves from

• - alpha end to + beta end to aid in exocytosis

Dynein moves from _________ to aid in ________

• + beta end to - alpha end to aid in Phagocytosis

Microtubules are slow to assemble so Introducing a _______ drastically increases MT assembly

• template

What are two major nucleation sites for microtubule regation?

• Centrosomes

• Basal bodies

• Both contain gamma-tubulin

A nucleation site provides a template that makes it much easier for the first tubulins to assemble.

The important protein is γ-tubulin, which is found at MTOCs (microtubule-organizing centers).

Location and number of centrosomes determines polarization of MT

How do nucleation sites REGULATE microtubules?

• Nucleation polarizes microtubule assembly

Once a microtubule is nucleated (anchored) at a centrosome:

Centrosome
↓
− end stays anchored
+ end extends outward → toward cell periphery

So instead of having microtubules randomly growing in every direction, the cell can establish an organized MT network.

Take home: Nucleation regulates MTs by providing γ-tubulin templates that rapidly start MT assembly and establish the direction of MT growth.

What cellular process is regulation of microtubules extremely important in?

Mitosis

Explain dynamic instability

• Tubulin dimers must be bound to GTP to bind to the + end of MT

-tubulin slowly hydrolyzes GTP to GDP

• High concentrations of GTP-dimers in solution promote MT growth, stability

• Low concentration of GTP-bound dimers in solution promote MT catastrophe and

collapse

• Dynamic instability uses up energy, but allows rapid regulation of MT network

What is the general function of MAPS?

MAPs are proteins that attach to microtubules and control what the microtubules do.

• Stabilize

• destabilize

• branch

• bundle

• connect

MAPs = "Microtubule Accessory Proteins" that modify, organize, connect, or use microtubules.

Think of the microtubule as a railroad track. MAPs are like different workers that can modify or use the track.

what is the structure and function of microfilaments?

Structure:

Double helical fibers (~7nm)

Found throughout the cell, but especially toward periphery

Only one subunit (g-actin)

Function:

Actin assembles (f-actin) into apolarized strand

• Resists stretching**** (has good tensile strength compared unlike microtubules)

• Cell membrane organization

• Ameboid movement

• Muscle contractions

• Cytokinesis

What structures are microfilaments found in?

• Cell cortex

• Microvilli and stereocilia

• Lamellipodia, filopodia

• Muscle fibers

How are microfilaments uses for contraction and transport?

Transport

• Actin filaments are polarized, meaning they have a + end and − end.

• This polarization gives myosin a specific direction of movement along the actin filament.

• Myosin is the motor protein.

• The myosin head binds to actin and hydrolyzes ATP.

• ATP hydrolysis changes the shape of myosin, causing it to move along the actin filament (microfilament).

• Myosin can carry things like vesicles along the actin "track."

Contreaction

The same actin–myosin interaction can produce force instead of simply transporting cargo.

When many actin filaments and myosin molecules interact:

Myosin pulls on actin → actin filaments slide → cell contracts

This is especially important in muscle cells, where actin and myosin are densely and highly organized.

How are microfilaments regulated?

Actin binding proteins!

• Stabilize Microfilament

actin-binding proteins that control whether actin filaments grow, shrink, stay stable, how they are organized, and how they generate force.

Dynamic Instability/Treadmilling

• Similar to MT, but with ATP (one side growing one side falling off)

Nucleation

• Partially similar to MT, but many nucleation events can occur, usually near membrane (which stabilize the proteins)

What regulates microtubules vs what regulates microfilaments?

Microtubules → MAPs regulate them
Microfilaments → Actin-binding proteins regulate them

What is the structure of intermediate filaments?

Structure:

• intermediate filaments are made up of fibrous (non- globular) proteins

• The long fibrous proteins bundle and twist together into rope like structures