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1 LIU Chuan Yong ??? Institute of Physiology Medical School of SDU Tel 88381175 (lab) 88382098 (office) Email: liucy@sdu.edu.cn Website: www.physiology.sdu.edu.cn

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2 Chapter 2 Functions of The Cell

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3 The Cell is The basic unit of the body to carry out and control the functional processes of life. Contained within a limiting membrane consists of various organelles suspended in cytoplasm.

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5 General Subdivisions of a Cell A. Nucleus (regulatory center of the cell) B. Plasma Membrane (selectively permeable boundary between the cell and the environment) C. Cytoplasm (everything between the plasma membrane and the nuclear compartment) Organelles are individual compartments in the cytoplasm

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6 Basic Physiological Function of the Cells Transport across the cell membrane Bioelectrical phenomena of the cell Contraction of muscle

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7 Section 1 Transport of Ions and Molecules through the Cell Membrane

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8 I Review Structure of the Cell Membrane

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9 What do membranes do? Act as a barrier AND… Receive information Import/export molecules Move/expand Membranes are Active Dynamic !

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10 The Cell Membrane System Membranes surrounding the cell Membrane systems inside the cell The nucleus, endoplasmic reticulum, golgi apparatus, Endosomes (???) and lysosomes form the endomembrane system

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11 Composition of the cell membrane Protein 55% Phospholipids 25% Cholesterol 13% Other lipids 4% Carbohydrates 3%

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12 Lipids Amphipathic Spontaneously form lipid bilayers

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13 Lipids are amphipathic Polar Nonpolar

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14 Lipids spontaneously form structures A lipid bilayer is a stable, low energy structure Self sealing structure/eliminate free edge What drives this structured association? Exclusion of Lipids from Water… not lipid association

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15 Lipid bilayers will form closed structures compartments behave as 2D fluids Self seal if disrupted

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16 Lipids are effective barriers to some compounds hydrophobic compounds can reach equilibrium quickly “unfavored” compounds can be brought across by transport proteins Need for …..Transport Mechanisms

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17 Proteins in Membrane Bilayer Types: Integral - Transmembrane: ionic channel, ionic pump, carrier, controller (G protein)

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18 Integral proteins

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19 Proteins in Membrane Bilayer Types: Peripheral – Lipid anchored and Protein anchored, located mainly at the inside of membrane surface: enzymes, controllers

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20 Peripheral proteins associated by weak electrostatic bonds to membrane proteins or lipids, can be solubilized in high salt concentrations associations with membrane or protein may be dynamic: transient, and regulated arg p.ser. --- +++ +++ +++ +++ --- --- ---

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21 Membrane Carbohydrates: Small amounts located at the extracellular surface. in combination with membrane proteins or lipids glycoproteins or glycolipids. Functions: Negatively charged, let the cell to repel negative objects Attach cells one to another Acts as receptor substance for binding hormone such as insulin Participate in immune reaction as antigen

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23 II Transport Through the Cell Membrane

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24 Categories of Transport Across the Plasma Membrane Cell membrane is selectively permeable to some molecules and ions. Mechanisms to transport molecules and ions through the cell membrane: Non-carrier mediated transport. Simple Diffusion. Facilitated Diffusion: Via Carrier Channel Voltage, Chemical and Mechanical gating channel Active Transport

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25 Categories of Transport Across the Plasma Membrane May also be categorized by their energy requirements: Passive transport: Net movement down a concentration gradient does not need ATP Active transport: Net movement against a concentration gradient needs ATP

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26 1. Simple Diffusion Molecules/ions are in constant state of random motion due to their thermal energy. Simple diffusion occurs whenever there is a concentration difference across the membrane the membrane is permeable to the diffusing substance.

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27 Simple Diffusion Through Plasma Membrane Cell membrane is permeable to: Non-polar molecules (02). Lipid soluble molecules (steroids). Small polar covalent bonds (C02). H20 (small size, lack charge). Cell membrane impermeable to: Large polar molecules (glucose). Charged inorganic ions (Na+).

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28 Rate of Diffusion Speed at which diffusion occurs. Dependent upon: The magnitude of concentration gradient. Driving force of diffusion. Permeability of the membrane. Neuronal plasma membrane 20 x more permeable to K+ than Na+. Temperature. Higher temperature, faster diffusion rate. Surface area of the membrane. Microvilli increase surface area.

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29 2 Facilitated Diffusion Definition: the diffusion of lipid insoluble or water soluble substance across the membrane down their concentration gradients by aid of membrane proteins (carrier or channel) Substances: K+, Na+, Ca2+, glucose, amino acid, urea etc.

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30 2. Facilitated Diffusion 2.1 Facilitated diffusion via carrier 2.2 Facilitated diffusion through channel 2.2.1 Voltage-gated ion channel 2.2.2 Chemically-gated ion channel 2.2.3 Mechanically-gated ion channel 2.2.4 Water channel

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31 2.1 Facilitated Diffusion via carrier Concept: Diffusion carried out by carrier protein Substance: glucose, amino acid Mechanism: a “ferry” or “shuttle” process

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32 Facilitated Diffusion via Carrier Characteristics of carrier mediated diffusion: Down concentration Gradient Chemical Specificity: Carrier interact with specific molecule only. Competitive inhibition: Molecules with similar chemical structures compete for carrier site. Saturation: Vmax (transport maximum): Carrier sites have become saturated.

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33 2.2 Facilitated diffusion through channels Definition Some transport proteins have watery spaces all the way through the molecule allow free movement of certain ions or molecules. They are called channel proteins. Diffusion carried out by protein channel is termed channel mediated diffusion.

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34 Two important characteristics of the channels: selectively permeable to specific substances opened or closed by gates Facilitated diffusion through channels

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35 Channel: aqueous pathways through the interstices of the protein molecules. Each channel molecule is a protein complex. through which the ions can diffuse across the membrane. Facilitated diffusion through channels

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36 According to the factors that alter the conformational change of the protein channel, the channels are divided into 3 types: Voltage gated channel Chemically gated channel Mechanically gated channel

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37 2. Facilitated Diffusion 2.1 Facilitated diffusion via carrier 2.2 Facilitated diffusion through channel 2.2.1 Voltage-gated ion channel 2.2.2 Chemically-gated ion channel 2.2.3 Mechanically-gated ion channel 2.2.4 Water channel

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38 2.2.1 Voltage-gated Channel The molecular conformation of the gate responds to the electrical potential across the cell membrane this type of channel is called voltage gate (or dependent) channel.

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39 Voltage-gated Na+ Channels Many flavors nerves, glia, heart, skeletal muscle Primary role is action potential initiation Multi-subunit channels (~300 kDa) Skeletal Na+ Channel: a1 (260 kDa) and b1 (36kDa) Nerve Na+ Channel: a1, b1, b2 (33 kDa) gating/permeation machinery in a1 subunits Three types of conformational states (close, open or activation, inactivation) - each controlled by membrane voltage

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41 The modern interpretation of the Hodgkin-Huxley model Voltage gated ion channels are transmembrane protein complexes containing a pore permeable to one or more ions, gated in an all-or-none fashion, according to the membrane potential The n, m and h particles in the H-H model are now popularly re-interpreted as voltage and time dependent, two-state ion channel components driving the workings of two-state gates in the channel pore.

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46 How these voltage-gated ion channels work movement of the voltage sensor generates a gating current S4 transmembrane segment may be voltage sensor pore formed by a nonhelical region between helix 5 and 6 (postulated to form b sheets) inactivation gate is in the cytoplasm

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47 The voltage sensor

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48 Na+ Channel Conformations IFM IFM IFM Closed Open Inactivated Outside Inside

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49 Tetrodotoxin (TTX) selectively block the voltage-gated Na+ channel

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50 2.2.2 Chemically-Gated Ion Channel Some protein channel gates are opened by the binding of another molecule with the protein; this causes a conformational change in the protein molecule that opens or closes the gate. This is called chemical gating. This type of channel is called chemically-gated (or dependent) channel

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51 Ligand-Operated ACh Channels

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52 Ion channel runs through receptor. Receptor has 5 polypeptide subunits that enclose ion channel. 2 subunits contain ACh binding sites. Ligand-Operated ACh Channels

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53 Channel opens when both sites bind to ACh. Permits diffusion of Na+ into and K+ out of postsynaptic cell. Inward flow of Na+ dominates . Produces EPSPs. Ligand-Operated ACh Channels

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54 2.2.3 Mechanically-gated channel Some protein channel gates may be opened by the mechanical deformation of the cell membrane. mechanically-gated channel. It plays a very important role in the genesis of excitation of the hair cells

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55 Organ of Corti When sound waves move the basilar membrane it moves the hair cells that are connected to it, but the tips of the hair cells are connected to the tectorial membrane the hair cell get bent . There are little mechanical gates on each hair cell that open when they are bent. K+ goes into the cell and Depolarizes the hair cell. (concentration of K+ in the endolymph is very high)

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56 2.2.4 Water Channel The structure of aquaporin (AQP)

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57 Simple diffusion Ion channel Water channel Water transportation through the membrane:

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58 Characteristics of the channel High ionic selectivity Gating channel Functional states of channel Time dependence

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59 Short Review 2. Facilitated diffusion 2.1 Facilitated diffusion via carrier 2.2 Facilitated diffusion through channel 2.2.1 Voltage-gated ion channel 2.2.2 Chemically-gated ion channel 2.2.3 Mechanically-gated ion channel 2.2.4 Water channel

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60 3 Active transport When the cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called active transport 3.1 Primary active transport 3.2 Secondary active transport:

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61 3 Active transport 3.1 Primary active transport: the energy used to cause the transport is derived directly from the breakdown of ATP or some other high-energy phosphate compound 3.2 Secondary active transport: The energy is derived secondarily from energy has been stored in the form of ionic concentration differences between the two sides of the membrane created by primarily active transport

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62 Intracellular vs extracellular ion concentrations Ion Intracellular Extracellular Na+ 5-15 mM 145 mM K+ 140 mM 5 mM Mg2+ 0.5 mM 1-2 mM Ca2+ 10-7 mM 1-2 mM H+ 10-7.2 M (pH 7.2) 10-7.4 M (pH 7.4) Cl- 5-15 mM 110 mM Fixed anions high 0 mM

Slide 63 :

63 3.1 Primary Active Transport Hydrolysis of ATP directly required for the function of the carriers. Molecule or ion binds to “recognition site” on one side of carrier protein.

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64 3.1 Primary Active Transport Binding stimulates phosphorylation (breakdown of ATP) of carrier protein. Carrier protein undergoes conformational change. Hinge-like motion releases transported molecules to opposite side of membrane.

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65 Na+/K+ Pump

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66 A Model of the Pumping Cycle of the Na+/K+ ATPase

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67 Characteristics of the Transport by Na+ pump Directional transport Coupling process ATP is directly required Electrogenic process

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68 Importance of the Na+-K+ Pump Maintain high intracellular K+ concentration gradients across the membrane. Control cell volume and phase Maintain normal pH inside cell Develop and Maintain Na+ and K+ concentration gradients across the membrane Electrogenic action influences membrane potential Provides energy for secondary active transport

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69 3.2 Secondary Active Transport Coupled transport. Energy needed for “uphill” movement obtained from “downhill” transport of Na+. Hydrolysis of ATP by Na+/K+ pump required indirectly to maintain [Na+] gradient.

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70 Secondary active transport Na+ glucose Na+ H+ out in out in co-transport counter-transport (symport) (antiport) Co-transporters will move one moiety, e.g. glucose, in the same direction as the Na+. Counter-transporters will move one moiety, e.g. H+, in the opposite direction to the Na+.

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71 4. Bulk Transport (Endocytosis and Excytosis) Movement of many large molecules, that cannot be transported by carriers. Exocytosis: A process in which some large particles move from inside to outside of the cell by a specialized function of the cell membrane Endocytosis: Exocytosis in reverse. Specific molecules can be taken into the cell because of the interaction of the molecule and protein receptor.

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72 Exocytosis Vesicle containing the secretory protein fuses with plasma membrane, to remove contents from cell.

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73 Endocytosis Material enters the cell through the plasma membrane within vesicles.

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74 Types of Endocytosis Phagocytosis - (“cellular eating”) cell engulfs a particle and packages it with a food vacuole. Pinocytosis – (“cellular drinking”) cell gulps droplets of fluid by

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Peripheral – Lipid anchored and Protein anchored, located mainly at the inside ofassociated by weak more Peripheral – Lipid anchored and Protein anchored, located mainly at the inside ofassociated by weak electrostatic bonds to membrane proteins or lipids, … less