3.1 - Membrane Transport

What does selective permeability allow?

allows for separation and exchange of materials across the plasma membrane

Diffusion

• Spontaneous movement of material from region of high concentration to region of low concentration.

• For uncharged molecules, diffusion depends on the concentration gradient.

• For charged molecules, diffusion depends on both the concentration & the electrochemical gradients.

What does diffusion directly through the membrane require?

requires both a concentration gradient and membrane permeability

What is lipid permeability determined by?

determined by molecular size and polarity

Osmosis

Diffusion of water from area of low solute concentration to area of high solute concentration across a semi-permeable membrane

Hypotonic solution

• the cell is going to rapidly gain water by osmosis and will swell

• net water gain (cell swells)

• hypo- hippo

Hypertonic solution

• water going from low solute concentration within cell goes out into areas with higher solution concentration

• net water loos (cell shrinks)

Isotonic solution

• no net loss or gain

Aquaporins

specialized protein channels that allow passive movement of water across the membrane

Description of Aquaporins

• each aquaporin subunit in a 4-unit protein is highly specific for water molecules

• pass through single file and over a billion molecules can pass through channel in 1 sec

Where are aquaporins prevalent?

Prevalent in the kidneys

• VERY prevalent in cells where the passage of water plays a crucial role in the activity in the tissue it is a part of

• kidneys are constantly filtering blood and everything so we can filter bad things out and keep good things in

Which of the following compounds can diffuse easily across a phospholipid bilayer?

a) Ca^2+

b) O2

c) Glucose

d) Cl^–

e) All of these

b) O2

Steroid hormones would cross the membrane by…

A. Simple diffusion

B. Facilitated diffusion

C. Osmosis

D. Active transport

E. Endocytosis

A. Simple diffusion

Paramecium have contractile vesicles that continuously expel water from the cell. This suggests that they live in a(n) __________ environment.

A. Hypotonic

B. Isotonic

C. Hypertonic

A. Hypotonic

Diffusion of ions through membrane

• Lipid bilayer is highly impermeable to charged substances

• Ions such as K^+, Na^+, Ca^++, and Cl^– cannot cross the lipid bilayer

• Ions cross membranes through ion channels

• Ion channels are selective and bidirectional

• Diffusion in the direction of the electrochemical gradient

Ion channels

• leak channels

• voltage-gated

• ligand-gated

• mechano-gated

“Leak” Channels

always open

Voltage-gated

state depends on difference in charge (ions) between the inside and the outside

Ligand-gated

conformation depends on binding to a specific molecule (ligand)

Mechano-gated

conformation depends on mechanical forces (e.g. stretch)

How can gated channels exist?

can exist in either an open or a closed conformation

Ionic concentrations of ‘typical’ mammalian cell

• Outside (+) > (-) = positive charges outweigh negative

• Cytoplasm (+) < (-) = negative charges outweigh positive

What is the efflux of K^+ driven by and opposed by?

• driven by concentration gradient, but opposed by electrical gradient

• because K^+ is in a positively charged area and is moving to space that is more positive, it is NOT moving with electrical gradient

Voltage

difference in charge, potential energy

Influx of Na+ would be…

A. Driven by concentration gradient, opposed by electrical gradient

B. Driven by concentration gradient and by electrical gradient

C. Opposed by concentration gradient, driven by electrical gradient

D. Opposed by concentration gradient and by electrical gradient

B. Driven by concentration gradient and by electrical gradient

Voltage-gated eukaryotic K^+ channels

• 6 membrane-associated helices (S1-S6)

• 2 functionally distinct domains:

• Pore domain

• Voltage-sensing domain

Pore domain

selectively filters, and then permits the selective passage of K^+ ions (not everything can get through, ONLY potassium)

Voltage-sensing domain

• consists of helices S1-S4 that senses the voltage across the plasma membrane

• a change in the relative charge between the inside and outside of cell (depolarization) causes a conformational change in the S4 region, which causes the channel to open

How many potassium ions can pass through voltage-gated eukaryotic K^+ channels?

• Once opened, more than 10 million K^+ ions per second can pass through

• Transient, after the channel is open for a few milliseconds, the movement of K^+ ions is “automatically” stopped.

How many states can voltage-gated eukaryotic K^+ channels exist in?

Can exist in three different states:

1. open

2. inactivated

3. closed

Human perspective – defects in ion channels as a cause of genetic disease

• Several inherited disorders have been linked to mutations in genes encoding ion protein channels.

• Cystic fibrosis (CF) is a genetic disease characterized by abnormal fluid secretions from tissues and caused by a defective chloride channel (CFTR).

Acetylcholine receptor – a ligand-gated channel

• Structure determined by electron crystallography

• The ion channel consists of a pore lined by a wall of 5 inner (M2) a-helices, one from each surrounding subunit.

• The gate opens following the binding of two ACh molecules, one per a subunit.

• ACh important in neural signaling, important in muscle function (especially important in neuromuscular junction)

Facilitated Diffusion

• Large or hydrophilic

• Facilitative transporter protein

• Passive transport, moves substances down concentration gradient

• Can be reversible

• Selective and saturable

• Slower than channels

• Can be regulate

Facilitated diffusion is saturable: eventually adding more solute will have no effect on the rate of transport. Why?

A. Higher solute concentrations aggregate & precipitate

B. High concentrations become toxic to the cell

C. All transporter proteins are working at full capacity

D. Solute collisions prevent them from binding to transporter

C. All transporter proteins are working at full capacity

Facilitated diffusion is saturable

- kinetics of facilitated diffusion (saturable, there is a maximum number of protein transporters, once they are working at a max, there is NO further rate that is possible

Active transport

• Cells maintain an imbalance of ions across the plasma membrane –cannot occur by either simple or facilitated diffusion

• Gradients are generated by active transport

Active Transport Description

• Moves solute against its concentration gradient

• Requires coupled energy input

• ATP hydrolysis, absorbance of light, electron transport, or concentration gradients of other substance

• ENDERGONIC requires it to be coupled with an EXERGONIC process (ATP hydrolysis, transport of light, etc.)

Na^+/K^+ ATPase pump (animals)

• pump is an example of primary active transport where transport is coupled to ATP hydrolysis

• what it does: maintaining the gradient of a higher concentration of sodium extracellular and higher concentration of potassium intracellular

• responsible for maintaining gradient

• 3 to 2 ratio (higher sodium outside-higher potassium inside)

E1 conformation

• Open to cytoplasm

• High affinity for Na^+

• Binds 3 Na^+ ions

E2 conformation

• Open to outside

• Na^+ ions released

• Binds 2 K^+ ions

• E2 binding sides are accessible to outside of cell

E3 conformation

• Open to cytoplasm

• K^+ ions released

• Binds 3 Na^+ ions

H^+/K^+ - ATPase is another P-type pump

• secretes a solution of concentrated acid in the stomach

• food triggers the release of histamine, which binds to receptors

• this causes H^+/K^+ - ATPase-containing vesicles to fuse with plasma membrane, activating the pump

Active transport can also be powered by other energy sources

• Light Energy

• Bacteriorhodopsin protein (archaebacteria ) acts as a light-driven proton pump

• Absorbs light energy to transport protons out of the cell

• Uses prosthetic group (retinal)

• The proton gradient is then used to make ATP

Cotransport

Use potential energy of concentration gradient of one solute to drive movement of another

Symport

same direction

Antiport

opposite directions

Na^+/glucose cotransporter

• Potential energy stored in ionic gradients is utilized to perform work.

• Na^+ concentration is kept low by a Na^+/K^+ -ATPase pump.

• Diffusion of sodium ions down a concentration gradient drives the cotransport of glucose