Human Physiology 人体生理学 Qiang XIA (夏强), MD & PhD Department of Physiology Room C518, Block C, Research Building, School of Medicine Tel: 88208252 Email: xiaqiang@zju.edu.cn

Course Structure Lectures: 80 academic hours Practicals: 64 a.h. 5 a.h./week 2 a.h. on Tue., 3 a.h. on Fri. Practicals: 64 a.h. 4 a.h./week

Evaluation Participation in practicals: 5% Practical reports: 15% Weekly assessments & midterm exam: 30% Final examination: 50%

Recommended textbook Widmaier EP, Raff H, Strang KT (2006) Vander’s Human Physiology: The Mechanisms of Body Function, Tenth Edition. McGraw-Hill.

Course website Medical School Course Center: Course website: http://10.71.121.158 Course website: http://10.71.121.158/G2S/Template/View.aspx?action=view&courseType=0&courseId=27123

MOVEMENT OF MOLECULES ACROSS CELL MEMBRANES 物质的跨膜转运 MOVEMENT OF MOLECULES ACROSS CELL MEMBRANES

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

Sizes, on a log scale

Ostrich egg

Electron Micrograph of organelles in a hepatocyte (liver cell)

Organelles have their own membranes

Cell Membrane (plasma membrane) 细胞膜

Electron micrograph and sketch of plasma membrane surrounding a human red blood cell

Phospholipid bilayer

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

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 Rotation reducing membrane fluidity enhancing membrane fluidity

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. Functions of membrane proteins

Carbohydrates Glycoprotein Glycolipid

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

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

START: Initially higher concentration of molecules randomly move toward lower concentration. Over time, solute molecules placed in a solvent will evenly distribute themselves. Diffusional equilibrium is the result (Part b).

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

Net flux accounts for solute movements in both directions. 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

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

A cartoon model of carrier-mediated diffusion The solute acts as a ligand that binds to the transporter protein…. … 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

Channel Types: Voltage-gated Channel Ligand-gated Channel Stretch-sensitive Channel Others

I II III IV Voltage-gated Channel e.g. Voltage-dependent Na+ channel Outside Inside NH2 CO2H I II III IV +

Na+ channel

Na+ channel Balloonfish or fugu

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

Ligand-gated Channel e.g. N2-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" "for the discovery of water channels" "for structural and mechanistic studies of ion channels" Peter Agre Roderick MacKinnon http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/public.html

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 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 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

facilitated diffusion, solutes move in the direction predicted membrane 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.

Which of the following would decrease the rate of diffusion? QUIZ Which of the following would decrease the rate of diffusion? a. increasing the size of the diffusing solute b. increasing the temperature c. increasing the concentration gradient d. increasing the surface area for diffusion e. All of the choices would increase the rate of diffusion

Which of the following would decrease the rate of diffusion? QUIZ Which of the following would decrease the rate of diffusion? a. increasing the size of the diffusing solute b. increasing the temperature c. increasing the concentration gradient d. increasing the surface area for diffusion e. All of the choices would increase the rate of diffusion

Carrier- mediated transport QUIZ Carrier- mediated transport a. involves a specific membrane protein that functions as a carrier molecule. b. always involves the movement of substances against a concentration gradient. c. always requires the direct expenditure of energy. d. Both involves a specific membrane protein that functions as a carrier molecule and always involves the movement of substances against a concentration gradient are correct. e. All of the choices are correct.

Carrier- mediated transport QUIZ Carrier- mediated transport a. involves a specific membrane protein that functions as a carrier molecule. b. always involves the movement of substances against a concentration gradient. c. always requires the direct expenditure of energy. d. Both involves a specific membrane protein that functions as a carrier molecule and always involves the movement of substances against a concentration gradient are correct. e. All of the choices are correct.

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).

Extracelluar (mmol/L) Intracellular (mmol/L) 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 (Na+ pump, Na+-K+ ATPase) electrogenic pump

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 Countertransport 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+-Ca2+, Na+-H+ exchange)

Which of the following statements about the Na,K pump is false? QUIZ Which of the following statements about the Na,K pump is false? a. It transports Na+ out of cells and K+ into cells. b. It binds to and hydrolyzes ATP. c. It is constantly active in all cells. d. Its activity requires the expenditure of metabolic energy. e. It transports Na+ and K+ in a 1:1 ratio.

Which of the following statements about the Na,K pump is false? QUIZ Which of the following statements about the Na,K pump is false? a. It transports Na+ out of cells and K+ into cells. b. It binds to and hydrolyzes ATP. c. It is constantly active in all cells. d. Its activity requires the expenditure of metabolic energy. e. It transports Na+ and K+ in a 1:1 ratio.

QUIZ If the ATP-generating mechanisms in a cell are poisoned and the cell has depleted its ATP reserves, a. primary active transport of molecules would cease immediately. b. secondary active transport of molecules would cease immediately. c. eventually there will be no potential difference across the membrane. d. primary active transport of molecules would cease immediately and secondary active transport of molecules would cease immediately are correct. e. primary active transport of molecules would cease immediately and eventually there will be no potential difference across the membrane are correct.

QUIZ If the ATP-generating mechanisms in a cell are poisoned and the cell has depleted its ATP reserves, a. primary active transport of molecules would cease immediately. b. secondary active transport of molecules would cease immediately. c. eventually there will be no potential difference across the membrane. d. primary active transport of molecules would cease immediately and secondary active transport of molecules would cease immediately are correct. e. primary active transport of molecules would cease immediately and eventually there will be no potential difference across the membrane are correct.

Endocytosis and Exocytosis 入胞与出胞

Alternative functions of endocytosis: 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!