MOVEMENT OF MOLECULES ACROSS CELL MEMBRANES 物质的跨膜转运 XIA Qiang, MD & PhD Department of Physiology Room 518, Block C, Research Building School of Medicine, Zijingang Campus Tel: (Undergraduate school), (Medical school)

Outline  Cell structure  Simple diffusion  Facilitated diffusion  Active transport  Endocytosis and exocytosis

Sizes, on a log scale

Electron Micrograph of organelles in a hepatocyte (liver cell)

Organelles have their own membranes

Cell Membrane (plasma membrane) 细胞膜

Phospholipid bilayer

Circles represent amino acids in the linear sequence of the protein Schematic cartoon of a transmembrane protein The amino acids along the membrane section are likely to have non-polar side chains

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

Drawing of the fluid-mosaic model of membranes, showing the phospholipid bilayer and imbedded proteins

Composition of cell membrane:  Lipids 脂类  Proteins 蛋白质  Carbohydrates 糖类

Lipid Bilayer Phospholipid Phosphatidylcholine Phosphatidylserine Phosphatidylethanolamine Phosphatidylinositol Cholesterol Sphingolipid

Lipid mobility enhancing membrane fluidityreducing membrane fluidity Rotation

Membrane proteins Integral (intrinsic) proteins Peripheral (extrinsic) proteins Integral protein Peripheral protein

Integral proteins

Functions of membrane proteins Adhesion Some glycoproteins attach to the cytoskeleton and extracellular matrix.

Carbohydrates GlycoproteinGlycolipid

Membrane Transport 跨膜转运 Lipid Bilayer -- primary barrier, selectively permeable

 Simple Diffusion  Facilitated Diffusion  Active Transport  Endocytosis and Exocytosis Primary Active Transport Secondary Active Transport Membrane Transport

Importance of pumps, transporters & channels  Basis of physiologic processes  Growth  Metabolic activities  Sensory perception  Basis of disease  Defective transporters (cystic fibrosis)  Defective channels (long QT syndrome, paralysis)  Basis of pharmacological therapies  Hypertension (diuretics)  Stomach ulcers (proton pump inhibitors)

Over time, solute molecules placed in a solvent will 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 单纯扩散 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 Temperature

Simple diffusion & gap junctions

Facilitated Diffusion 易化扩散  Carrier-mediated diffusion 载体中介的扩散  Channel-mediated diffusion 通道中介的扩散

The solute acts as a ligand that binds to the transporter protein…. A cartoon model of carrier-mediated diffusion … 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 carriers places 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 diffusion 3 cartoon models of integral membrane proteins that function as ion channels; the regulated opening and closing of these channels is the basis of how neurons function.

A thin shell of positive (outside) and negative (inside) charge provides the electrical gradient that drives ion movement across the membranes of excitable cells.

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

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

Outside Inside NH 2 CO 2 H IIIIIIIV Voltage-gated Channel –e.g. Voltage-dependent Na + channel

Na + channel

Balloonfish or fugu

Closed Activated Inactivated Na + channel conformation –Open-state –Closed-state

Ligand-gated Channel –e.g. N 2 -ACh receptor channel

Stretch-sensitive Channel Closed Open Stretch

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 AgreRoderick 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 the direction predicted by the concentration gradient. membrane

Active transport 主动转运  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

The transported solute binds to the protein as it is phosphorylated (ATP expense).

Concentration gradient of Na + and K + Extracelluar (mmol/L) Intracellular (mmol/L) Na K

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.

electrogenic pump 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 the ion and the second solute cross the membrane in the same direction (e.g. Na + -glucose, Na + -amino acid cotransport) Countertransport the ion and the second solute move in opposite directions (e.g. Na + -Ca 2+, 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 between the compartments is 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 between the compartments is permeable to water only. After diffusional equilibrium has occurred: Movement of water only has equalized solute concentration.

Role of Na-K pump in maintaining cell volume

Response to cell shrinking

Response to cell swelling

Endocytosis and Exocytosis 入胞与出胞

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

Epithelial Transport

Glands

Summary Diffusion: solute moves down its concentration gradient: simple diffusion: small (e.g., oxygen, carbon dioxide) lipid soluble (e.g., steroids) facilitated diffusion: requires transporter (e.g., glucose)

Active transport: solute moves against its concentration gradient: primary active transport: ATP directly consumed (e.g., Na+ -K+ ATPase) secondary active transport: energy of ion gradient (usually Na+) used to move second solute (e.g., nutrient absorption in gut) Exo- and endo- cytosis: large scale movements of molecules

Thank you for your attention!