Sample Lecture Bio 361

8/10/2019 Sample Lecture Bio 361

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Phospholipid structure

Polar headCholine+

Phosphate-

Glycerol

Non-polarfatty acidchains

Nonpolar tails

Hydrophilic

Hydrophobic

Many positively chargedgroups can occupy this

position.

Bends symbolize doublebonds less densely

packed lipids.

Water

Micelle

Phospholipidbilayer

Hydrophobictails

Hydrophillicheads

Phospholipid behavior in water


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Fluidity of membranes

Frequent lateral movement oflipids; infrequent flip-flopsacross membrane leaflets.

Flip-flops(rare)

LateralMovement(frequent)

(a)

(b)

Fluid Viscous

Unsaturatedfatty acidtails with kinks

Saturatedfatty acidtails

Degree of unsaturated/saturatedfatty acid tails (hydrocarbon)determines membrane fluidity.

Phospholipid composition and habitat temperature

Phosphatidyl-ethanolamines

Phosphatidyl-cholines

Adaptation temperature (C)%o

fhydrocarbontailsunsaturated

Fatty acid chains vary in chemical unsaturation =

double bonds that produce a bend in the chain.Unsaturated phospholipids are less densely packed

and increase membrane fluidity. To maintainmembrane fluidity at low temperatures, cold adaptedspecies incorporate more unsaturated lipids into their

membrane.

after Logue et al. (2000), J Exp Biol 203, 2105-2115


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PASSIVE: Diffusion as passive solute transport

Diffusion of glucose toward equilibrium

High Low

glucose concentration

Membrane

Due to molecular movement glucose passes through the membrane passively.Because there is more glucose on the left side, on average and by chance moremolecules pass from left to right. This leads to equal concentrations on bothsides the equilibrium for this system.

PASSIVE: Solute diffusion

High C1 Low C2solute concentration

J=net rate of diffusion

X=length

J= D A C1 C2X

J ~ C1 C2: Diffusion rate is proportionalto concentration gradient

Ficks diffusion equation(Adolf Fick, 1829-1901)

J ~ 1/X: Diffusion rate is greater whendiffusion distance is shorter

JY ~ C solute Y : Diffusion rate for solute Ydepends on concentration gradientof the solute Y

J ~ D: Diffusion rate is proportional

to the diffusion coefficient

D is a measure of the ease with which asolute moves through a medium separatingthe two concentrations; temperaturedependent; defines permeability.

How do membranes affect diffusion?

J ~ A: Diffusion rate is greater whendiffusion area (surface) is greater


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Outward diffusion from the animal orcell increases the concentration in the

environmental solution next to theouter surface. The boundary layer

thus created may be very thin yet stillhave a substantial impact on the rate

of diffusion.

Boundary layers and diffusion


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PASSIVE: Permeability of a membrane for a solute

Diffusion of glucose

toward equilibrium

High Low

glucose concentration

Membrane

J= P A C1

C2

P = Dm

KX

The permeability of a membrane to a solute isthe ease with which it can move through themembrane by simple diffusion.

Membrane permeability is based on

Dm, the diffusion coefficient:rate of diffusion of a substance through a

membrane

K, the partition coefficient:

an indicator for the ratio between dissolvedand undissolved solute in the membrane

PASSIVE: Membranes and the diffusion of solutes

The diffusion of a solute across membranes depends on thehydrophobic or hydrophilic nature of the solute.

1. Lipid solutes (steroids and fatty acids): enter the interior ofthe cell membrane because of their hydrophobic nature.

2. Molecular oxygen (O2): enter interior of membrane because of theirsmall and nonpolar nature.

3. Inorganic ions: low permeability across membrane, but can diffusepassively through cell membranes at rapid rates due to ion channels.

Ion channels allow only passive transport across membranes.They do not bind the ions that pass through them.They are selective in determining which ion can pass.Types include: voltage-gated, stretch-gated, phosphorylation-gated

and ligand-gated channels.


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PASSIVE: Selectively permeable membranescan create electrical gradients

000

000

K+

A1-

A2-

Na+

Membrane isonly permeable

to K+.

+++

---

A1-

A2-

Na+K+

K+

Cross membrane diffusion of K+

creates positive charges on theright side, preventing K+ from

further diffusing.

At the beginningthere is zerocharge across

the membrane.

Donnanequilibrium

PASSIVE: Selectively permeable membranes can

create electrical gradients: Nernst equation

Ex = RT ln CoutzF Cin

R: Gas constantT: absolute temp in Kelvin (18C)F: Faraday constantz: charge on the permeant ion

Ex = 0.058 log Coutz Cin

Nernstequation

electrical gradient = chemical gradient

Substituting the constants results in:

Consequences:

Unequal concentrations of ions across a membranecan create a membrane potential = resting potential.

A membrane potential can affect the concentrationgradient across a membrane.


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PASSIVE: Diffusion of ions is affected bya chemical and an electrical gradient

Electricalgradient

moves ions insame

direction asconc.

gradient =fast

diffusion.

Electricalgradient

opposes conc.gradient =

slowingdiffusion orreversingdirection.

High Low

Na+ concentration

Diffusion (slow)

Membrane

Concentration gradient

Electrical gradient

- +

+++

---

Diffusion (fast)

High Low

Na+ concentration

Membrane

Concentration gradient

Electrical gradient

+ -

+++

---

PASSIVE: An electrochemical view of a single cell

Cl-

K+

A-

Extracellularfluid

Na+

Cell membraneAnionicproteins

Intra-cellular

fluid

K+Cl-

Na+

(a) Ion concentration inside asingle animal cell

++

++

++

++

++

++

--

----

--

-

Cl- is near electrochemicalequilibrium because the

concentration gradient andelectrical effect oppose each

other.

Na+ is far fromelectrochemical equilibrium,because the conc. gradient

and electrical effect move itinto the cell.

K+: The conc. effect isgreater than the

electrical effect, but notas much as for Na+.


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PASSIVE: Facilitated diffusion

Polar organic solutes (glucose, aminoacids) are hydrophilic: diffuse through

membrane with the help of carrierproteins: In direction of electrochemical

gradient Facilitated because faster than simple

diffusion Reversible and non-covalent binding

PASSIVE SOLUTE TRANSPORT - SUMMARY

Passive transport moves towards the electrochemical equilibrium.

Simple diffusion depends on the concentration gradient for an unchargedsolute. In case of a charged solute, conc. gradients and electrical effectscontribute to diffusion.

Polar organic solutes (glucose, amino acids) move across membranes withthe help of transporter proteins in the direction of the electrochemicalequilibrium (facilitated diffusion).

The permeability of a membrane for a lipid solute depends on how readily thethe solute enters into and moves across the membrane lipid bilayer simplediffusion.

For inorganic ions, the permeability depends on the number of ion channels.


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Primary and secondary ACTIVE TRANSPORT

Na+-K+-ATPase is an example of a primary active transport mechanism:

it draws energy directly from ATP. An ATPase is a transporter and anenzyme.

Other important ATPases:Ca2+-ATPase in the sarcoplasmic reticulum of muscle cells.H+-K+-ATPase is the proton-pump that acidifies stomach content.vesicular or v-type H+-ATPase in gills and kidneys.

Secondary active transport draws energy from an electrochemicalgradient of a solute, and therefore indirectly from ATP.

ACTIVE TRANSPORT

Primary active-transportmechanisms.

Cl-

K+

A-

Extracellularfluid

Na+

Cell membraneAnionicproteins

Intra-cellular

fluid

K+Cl-

Na+


++

++

++

++

++

++

--

----

--

-

Bloodcapillary

Intestinalepithelial cells

Glucose frommeal

(c) Glucose transport across intestinalepithelium into the blood system

Cross section ofsmall intestine

Intestinallumen

Secondary active-transport mechanisms.

Blood plasmaPondwater

Gillepithelial

cell

Cl-

Na+

Na+

Cl-

(b) Ion concentration across gillepithelium of a freshwater fish

Primary and secondary active-transport mechanisms.


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ACTIVE: Primary active transport mechanisms

Cl-

K+

A-

Extracellularfluid

3Na+

Anionicproteins

Intra-cellular

fluid

2K+

Cl-

Na+


++

+

+

+

+

++

+

--

--

-

-

ATP

ADP

Active transportDiffusion

Cl- is close to electrochemicalequilibrium.

Electrochemical gradients favorsteady diffusion of Na+ and K+

through resting channels in thecell membranes.

The Na+-K+ pump maintains Na+

and K+ ions out of equilibrium byusing ATP-bond energy.

Anionic proteins are trappedinside the cell.

Bloodcapillary

Intestinalepithelial cells

Glucose frommeal

(c) Glucose transport across intestinalepithelium into the blood system

Cross section ofsmall intestine

Intestinallumen

Apical Basolateralmembrane

++

+

--

-

ACTIVE: Secondary active transport mechanisms

3Na+

2K+ATP

ADP

Na+ Na+

GluGlu

Glu

2 Na+

Cotransporter

Transport against andin direction of electrochem.gradient

The Na+-K+-ATPase in thebasolateral membrane

pumps Na+ outside the cell,

and thereby maintains theNa+ gradient across the

apical membrane.

A transporter carriesNa+ towards its EC

equilibrium, and thereby

forces the cotransportof glucose across themembrane.

Na+


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Apical Basolateral

membrane

++

++

--

--

3Na+

2K+ATP

ADP

Na+

ACTIVE: Ion movements in fresh water fish gills

Blood plasmaPondwater

Gillepithelial

cell

Cl-

Na+

Na+

Cl-

(b) Ion concentration across gillepithelium of a freshwater fish

H+ATP

ADP

Transport against andin direction of electrochem.gradient

Countertransporter

Channel

Na+

A H+-ATPase generates anegative charge inside Na+ follows towards its

EC equilibrium.

Cl-

HCO3-+H+HCO3

- CO2 CO2 metabolic waste

A gradient favors the

outward diffusion ofbicarbonate ions (HCO3

-),which drives the influx of Cl-

against the Cl- EC gradient.

ACTIVE SOLUTE TRANSPORT - SUMMARY

Solute transport is active if it can move solutes away from electrochemicalequilibrium.

Active transport is primary if the transporter is an ATPase (energy comesdirectly from ATP), secondary if the energy comes from a solute electro-chemical gradient. Organic solutes are pumped by secondary-activetransport mechanism.

The transport of ions can create voltage differences (electrogenic) or not(electroneutral).


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OSMOSIS An exampleIsotonic solution:

empirical evaluation if

cell loses or gains water

Transport across membranes summary II

The transport of water depends on the colligative properties of a solvent.

Water moves from solution of lower osmotic pressure (more abundant water)to the solution of higher osmotic pressure (less abundant water).

If two solutions have the same osmotic pressure they are called iso-osmotic.A solution with higher osmotic pressure is hyper-osmotic relative to one withlower pressure (hypo-osmotic).

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Sample Lecture Bio 361