Membrane TransportXia Qiang, PhDDepartment of PhysiologyRoom C518, Block C, Research Building, School of MedicineTel: 88208252Email: xiaqiang@zju.edu.cn

Sizes, on a log scale

Organelles have their own membranes

Cell Membrane (plasma membrane)

Electron micrograph and sketch of plasma membranesurrounding a human red blood cell

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Schematic cartoon of a transmembrane protein

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Structure of cell membrane: Fluid Mosaic Model (Singer & Nicholson, 1972)

Composition of cell membrane:

▫Lipids

▫Proteins

▫Carbohydrates

Lipid BilayerPhospholipid

Phosphatidylcholine

Phosphatidylserine

Phosphatidylethanolamine

Phosphatidylinositol

Cholesterol

Sphingolipid

Lipid mobility

enhancing membrane fluidityreducing membrane fluidity

Rotation

Membrane proteins

Integral (intrinsic) proteinsPeripheral (extrinsic) proteins

Integral protein Peripheral

protein

Integral proteins

Functions of membrane proteins

Adhesion Some glycoproteins attach to the cytoskeleton and extracellular matrix.

CarbohydratesGlycoprotein Glycolipid

Membrane TransportLipid Bilayer -- primary barrier, selectively permeable

Membrane Transport

▫Simple Diffusion（单纯扩散）▫Facilitated Diffusion（易化扩散）▫Active Transport（主动转运）▫Endocytosis and Exocytosis（出胞与入胞）

Over time, solute moleculesplaced in a solventwill evenly distribute themselves.

START: Initially higher concentration of molecules randomly

move toward lower concentration.

Diffusional equilibrium is the result (Part b).

At time B, some glucose has crossed into side 2 as some cross into side 1.

Note: the partition between the two compartments is a membrane that allows this solute to move through it.

Net flux accounts for solute movements in both directions.

Simple Diffusion

Relative to the concentration gradient movement is DOWN the concentration gradient ONLY (higher concentration to lower concentration)

Rate of diffusion depends on• The concentration

gradient • Charge on the molecule • Size• Lipid solubility

Facilitated Diffusion

▫Carrier-mediated

The solute acts as a ligand that binds to the transporter protein….

A cartoon model of carrier-mediated transport.

… and then a subsequent shape change in the protein releases the solute on the other side of the membrane.

In simple diffusion,flux rate is limited only by the concentration gradient.

In carrier-mediated transport, the number of available carriersplaces an upper limit on the flux rate.

Characteristics of carrier-mediated diffusion

net movement always depends on the concentration gradient  

▫Specificity

▫Saturation

▫Competition

– Channel-mediated

3 cartoon models of integral membraneproteins that functionas ion channels; theregulated opening and closing of these channels is the basis of howneurons function.

The opening and closing of ion channels results from conformational changes in integral proteins.Discovering the factors that cause these changes is key to understanding excitable cells.

Characteristics of ion channels

▫Specificity

▫Gating（门控）

Three types of passive, non-coupled transport through integral membrane proteins

Outside

Inside

NH2CO2H

I II III IV

+++

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Voltage-gated Channel

– e.g. Voltage-dependent Na+ channel

Na+ channel

Na+ channel

Balloonfish or fugu

Closed Activated Inactivated

Na+ channel conformation– Open-state

– Closed-state

Ligand-gated Channel

– e.g. N2-ACh receptor channel

Aquaporin

•Aquaporins are water channels that exclude ions

•Aquaporins are found in essentially all organisms, and have major biological and medical importance

The Nobel Prize in Chemistry 2003"for discoveries concerning channels in cell membranes"

Peter Agre Roderick MacKinnon

"for the discovery of water channels"

"for structural and mechanistic studies of ion channels"

The dividing wall between the cell and the outside world – including other cells – is far from being an impervious shell. On the contrary, it is perforated by various channels. Many of these are specially adapted to one specific ion or molecule and do not permit any other type to pass. Here to the left we see a water channel and to the right an ion channel.

Peter Agre’s experiment with cells containing or lacking aquaporin. The aquaporin is necessary for making the cell absorb water and swell

Passage of water molecules through the aquaporin AQP1. Because of the positive charge at the center of the channel, positively charged ions such as H3O+, are deflected. This prevents proton leakage through the channel.

Model for water permeation through aquaporin

In both simple and facilitated diffusion, solutes move in the direction predicted by the concentration gradient.

In active transport, solutes move opposite to thedirection predicted by the concentration gradient.

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Active transport

•Primary active transport（原发性主动转运）•Secondary active transport（继发性主动转运）

Primary Active Transport

making direct use of

energy derived from

ATP to transport the

ions across the cell

membrane

Concentration gradient of Na+ and K+

Extracelluar (mmol/L) Intracellular

(mmol/L)

Na+ 140.0 15.0

K+ 4.0 150.0

Here, in the operation of the Na+-K+-ATPase, also known as the “sodium pump,” each ATP hydrolysis moves three sodium ions out of, and two potassium ions into, the cell.

Na-K Pump

electrogenic pump（生电性泵）

Na+-K+ pump (Na+ pump, Na+-K+ ATPase)

Physiological role of Na+-K+ pump

–Maintaining the Na+ and K+ gradients across the

cell membrane

–Partly responsible for establishing a negative

electrical potential inside the cell

–Controlling cell volume

–Providing energy for secondary active transport

Other primary active transport

•Primary active transport of calcium•Primary active transport of hydrogen

ions•etc.

Secondary Active Transport

The ion gradients

established by primary

active transport permits

the transport of other

substances against their

concentration gradients

Secondary active transport uses the energy in an ion gradient to move a second solute.

Cotransport（同向转运）the ion and the second solute cross

the membrane in the same direction

(e.g. Na+-glucose, Na+-amino acid cotransport)

Countertransport（逆向转运）

the ion and the second solute move in opposite directions(e.g. Na+-Ca2+, Na+-H+ exchange)

Cotransporters

Exchangers

Ion gradients, channels, and transporters in a typical cell

Here, water is the solvent. The addition of solute lowers the water concentration. Addition of more solute would increase the solute concentration and further reduce the water concentration.

Solvent + Solute = Solution

Osmosis（渗透）

Begin: The partition betweenthe compartmentsis permeable to water and to the solute.

After diffusional equilibrium has occurred: Movement of water and solutes has equalized solute and water concentrations on both sides of the partition.

Begin: The partition betweenthe compartmentsis permeable to water only.

After diffusional equilibrium has occurred: Movement of water onlyhas equalized solute concentration.

Role of Na-K pump in maintaining cell volume

Response to cell shrinking

Response to cell swelling

Endocytosis and Exocytosis

Alternative functions of endocytosis:

1. Transcellular transport

2. Endosomal processing

3. Recycling the membrane

4. Destroying engulfed materials

Endocytosis

Exocytosis

Two pathways of exocytosis

•Constitutive exocytosis pathway -- Many soluble proteins are continually secreted from the cell by the constitutive secretory pathway

•Regulated exocytosis pathway -- Selected proteins in the trans Golgi network are diverted into secretory vesicles, where the proteins are concentrated and stored until an extracellular signal stimulates their secretion

Steps to exocytosis

• Vesicle trafficking: In this first step, the vesicle containing the waste product or chemical transmitter is transported through the cytoplasm towards the part of the cell from which it will be eliminated

• Vesicle tethering: As the vesicle approaches the cell membrane, it is secured and pulled towards the part of the cell from which it will be eliminated

• Vesicle docking: In this step, the vesicle comes in contact with the cell membrane, where it begins to chemical and physically merge with the proteins in the cell membrane

• Vesicle priming: In those cells where chemical transmitters are being released, this step involves the chemical preparations for the last step of exocytosis

• Vesicle fusion: In this last step, the proteins forming the walls of the vesicle merge with the cell membrane and breach, pushing the vesicle contents (waste products or chemical transmitters) out of the cell. This step is the primary mechanism for the increase in size of the cell's plasma membrane

Epithelial Transport（上皮转运）

Measurement of voltage in an epithelium

Models of epithelial solute transport

Mechanisms of intestinal glucose absorption

Models of “isotonic” water transport in a leaky epithelium

Glands