Lecture 9 Membrane transport: passive transport

Overcoming the “permeability barrier”:

• Transport into the cell

• Membranes are permeability barriers

• For ions and large polar molecules to enter, our cells need mechanisms for transport

Mechanisms for moving across the membrane

Unaided by cellular transport proteins: Simple diffusion

Aided by cellular transport proteins: Facilitated diffusion, Active transport

Active transport requires energy; diffusion does not

Concentration and electrochemical gradients

Conditions often differ on two sides of a membrane

▪ Solute concentrations ▪ Ions concentrations

▪ Diffusion: spontaneous movement of solutes and ions to achieve equilibrium

Gradients govern what?

Gradients govern movement of molecules

▪ No net charge: governed by concentration gradient

▪ Ions: governed by sum of electrical and chemical forces

Diffusion:

Movement of solute from an area of high concentration to an area of low concentration (down the gradient)

▪ Spontaneous movement toward equilibrium

Simple diffusion solutes transported

• small polar (H2O, Glycerol)

• Small nonpolar (O2, CO2)

• Large nonpolar (oils, steroids)

Facilitated diffusion solutes transported

• small polar (H2O, Glycerol)

• Large polar (glucose)

• Ions (NA+, K+ Ca2+)

Active transport solutes transported

• Large polar (glucose) Ions (Na+, K+ Ca2+)

Simple diffusion thermodynamic properties

• Down the electrochemical gradient (high to low)

• No metabolic energy required

• No intrinsic directionality

Facilitated diffusion thermodynamic properties

• Down the electrochemical gradient (high to low)

• No metabolic energy required

• No intrinsic directionality

Active transport thermodynamic properties

• Up the electrochemical gradient

• metabolic energy required

• intrinsic directionality

Simple diffusion Kinetic properties

• no membrane proteins required

• no saturation kinentics

• no competitive inhibition

Facilitated diffusion Kinetic properties

• membrane proteins required

• saturation kinentics

• competitive inhibition

Active transport Kinetic properties

• membrane proteins required

• saturation kinentics

• competitive inhibition

Diffusion across a membrane

▪ Some solutes can move-across a semi-permeable membrane until equilibrium is reached (e.g., O2 )

HOWEVER, biological membranes are usually only permeable to small nonpolar (and uncharged) molecules

Osmosis

• Water is more likely to diffuse across cell membranes than most solutes

• Osmosis: Movement of water across a membrane to the side with higher solute concentration

Water flow across cellular membranes

▪ Liquids surrounding cells have a relative “tonicity”

▪ Hypertonic : more (membrane impermeable) solutes than in cells

▪ Hypotonic: fewer solutes than in cells

▪ Isotonic: equal amounts of solutes to cells

▪ E.g., Saline for intravenous hydration

▪ Water moves by osmosis across the membrane based on solute levels of the surrounding solution

Osmosis in action: IV injection of water

Experimental injection of ~350 ml of water led to chills, fever, malaise (a general, non-specific feeling of discomfort), and hemoglobinuria(the presence of free hemoglobin in the urine)

Simple diffusion

• No cellular proteins necessary

• Mostly limited to small nonpolar molecules

• E.g., O2 and CO2

Facilitated diffusion

Large, polar, and charged molecules do not diffuse across membranes

▪ Transport proteins can mediate passage of the membrane

▪ Carrier proteins

▪ Channels

▪ No energy required

carrier protein

membrane protein that transports solutes across the membrane by binding to the solute on one side of the membrane and then undergoing a conformational change that transfers the solute to the other side of the membrane.

channel protein

membrane protein that forms a hydrophilic channel through which solutes can pass across the membrane without any change in the conformation of the channel protein.

Differences between carriers and channels for facilitated diffusion

▪ Both use transmembrane proteins

▪ Carrier proteins: Bind extracellular solutes; change shape to bring solute into the cell. Highly specific; relatively slow transport

Channel proteins: ▪ Create hydrophilic channels in the membrane ▪ Variable specificity; usually very rapid transport

glucose transporter (GLUT)

• membrane carrier protein responsible for the facilitated diffusion of glucose.

• Changes shape (conformation) when bound by glucose

• T1 and T2 conformations

• ▪ Increases diffusion rate into the cell by ~50,000-fold

glucose transporter conformations

T1 represents the outward-open state (facing the extracellular space) for binding glucose. T2 represents the inward-open state (facing the cytoplasm) for releasing glucose.

Facilitated diffusion

membrane protein–mediated movement of a substance across a membrane that does not require energy because the ion or molecule being transported is moving down an electrochemical gradient.

one or two solutes at a time

▪ glucose transporter is a “uniporter”

▪ Some carriers transport 2 solutes at a time

coupled transport

coordinated transport of two solutes across a membrane in such a way that transport of either stops if the other is stopped or interrupted; the two solutes may move in the same direction (symport) or in opposite directions (antiport).

Chloride carbonate exchanger

Red blood cells convert waste CO2 to HCO3 -

▪ As HCO3 - concentration increases, it is transported out

▪ Coupling with Cl- uptake prevents net charge imbalance

▪ Antiporter of chloride and carbonate (1:1 ratio)

▪ Transport stops if either ion is absent

Channel proteins

▪ Transmembrane proteins that create pores in the membrane

▪ Allows specific solutes to cross the membrane (often gated so movement can be regulated) ▪ Example: Porins

▪ Transmembrane segments form a β-barrel

▪ Pore lined with polar amino acids (hydrophilic)

▪ Exterior of the pore contains nonpolar amino acids (hydrophobic)

Porin

transmembrane protein that forms pores for the facilitated diffusion of small hydrophilic molecules; found in the outer membranes of mitochondria, chloroplasts, and many bacteria.

Aquaporins

• Transmembrane water channels

• Aquaporins allow rapid passage of water through membranes of specialized cells (e.g., kidney cells)

• Water passage is regulated

• Integral membrane protein creates a channel between outside and inside of the cell

• Homotetramer: four identical monomers form channels

• ▪ The channels allow water to pass, one H2O at a time

aquaporin (AQP)

any of a family of membrane channel proteins that facilitate the rapid movement of water molecules into or out of cells in tissues that require this capability, such as the proximal tubules of the kidneys.