《分子细胞生物学》 Chapter 5 A The Movement of Substances Across Cell Membranes

Chapter 5 A. The Movement of Substances Across cell membranes Learning Obiectives: 1. Principles of membrane transport; 2. Passive transport and active transport; 3. Two main classes of membrane transport proteins: Carriers and Channels: The ion transport systems; 5. Endocytosis and Phagocytosis: cellular uptake of macromolecules and particles

Chapter 5 Learning Objectives: 1. Principles of membrane transport; 2. Passive transport and active transport; 3. Two main classes of membrane transport proteins: Carriers and Channels; 4. The ion transport systems; 5. Endocytosis and Phagocytosis: cellular uptake of macromolecules and particles. A. The Movement of Substances Across Cell Membranes

A motor neuron cell body in the spinal cord. (A) Many thousands of nerve terminals synapse on the cell body and dendrites. These deliver signals from other parts of the organism to control the firing of action potentials along the single axon of this large cell. (B)Micrograph showing a nerve cell body and its dendrites stained with a fluorescent antibody that recognizes a cytoskeletal protein green) Thousands of axon terminals(red) from other nerve cells (not visible) make synapses on the cell body and dendrites they are stained with a fluorescent antibody that recognizes a protein in synaptic vesicles

A motor neuron cell body in the spinal cord. (A) Many thousands of nerve terminals synapse on the cell body and dendrites. These deliver signals from other parts of the organism to control the firing of action potentials along the single axon of this large cell. (B) Micrograph showing a nerve cell body and its dendrites stained with a fluorescent antibody that recognizes a cytoskeletal protein (green). Thousands of axon terminals (red) from other nerve cells (not visible) make synapses on the cell body and dendrites; they are stained with a fluorescent antibody that recognizes a protein in synaptic vesicles

1. Principles of membrane transport A. The plasma membrane is a selectively permeable barrier. It allows for separation and exchange of materials across the plasma membrane

1. Principles of membrane transport A. The plasma membrane is a selectively permeable barrier. It allows for separation and exchange of materials across the plasma membrane

B. The protein-free lipid bilayers are highly impermeable to ions HYDROPHOBIC COz o If uncharged solutes are small enough, MOLECULES N2 they can move down their concentration benzene gradients directly across the lipid UNCHARGED H20 bilayer by simple diffusion. POLAR urea MOLECULES lye erol Most solutes can cross the membrane LARGE only if there is a membrane transport UNCHARGED glucose POLAR sucrose protein to transfer them. MOLECULES H Na x Passive transport, in the same IONS HCO, K Ca C direction as a concentration gradient. M Active transport, is mediated by Diffusion of small molecules across carrier proteins, against a concentration synthetic phospholipid lipid gradient, require an input of energy. bilayer bilayers

Figure 11-1 The relative permeability of a synthetic lipid bilayer to different classes of molecules. The smaller the molecule and, more important, the fewer hydrogen bonds it makes with water, the more rapidly the molecule diffuses across the bilayer. B. The protein-free lipid bilayers are highly impermeable to ions. ❖If uncharged solutes are small enough, they can move down their concentration gradients directly across the lipid bilayer by simple diffusion. ❖Most solutes can cross the membrane only if there is a membrane transport protein to transfer them. ❖Passive transport, in the same direction as a concentration gradient. ❖ Active transport, is mediated by carrier proteins, against a concentration gradient, require an input of energy. Diffusion of small molecules across phospholipid bilayers

high permeability Figure 11-2 Permeability coefficients (cm/sec)for the passage of various H2O molecules through synthetic lipid bilayers. The rate of flow of a solute across the bilayer is directly proportional to the difference in its concentration on the urea glycerol two sides of the membrane. Multiplying this concentration difference(in mol/cm) tryptophan glucose by the permeability coefficient(cm/sec) gives the flow of solute in moles per second per square centimeter of membrane. a concentration difference of tryptophan of 10-4 mol/cms(10-4/10-L 0.1 M), for example would cause a flow of 10 10-4 mol/cm3x 10-7 cm/sec=10-11 mol/sec through 1 cm2 of membrane or6x 104 10 molecules/sec through 1 microns 2 of low permeability membrane

Figure 11-2 Permeability coefficients (cm/sec) for the passage of various molecules through synthetic lipid bilayers. The rate of flow of a solute across the bilayer is directly proportional to the difference in its concentration on the two sides of the membrane. Multiplying this concentration difference (in mol/cm3 ) by the permeability coefficient (cm/sec) gives the flow of solute in moles per second per square centimeter of membrane. A concentration difference of tryptophan of 10-4 mol/cm3 (10-4 /10-3 L = 0.1 M), for example, would cause a flow of 10-4 mol/cm3 x 10-7 cm/sec = 10-11 mol/sec through 1 cm2 of membrane, or 6 x 104 molecules/sec through 1 microns2 of membrane

C. The energetics of solute movement Diffusion is the spontaneous movement of material from a region of high concentration to a region of low concentration %The free-energy change during diffusion of nonelectrolytes depends on the concentration grdient The free-energy change during diffusion of electrolytes depends on the electrochemical grdient

C. The energetics of solute movement: ❖Diffusion is the spontaneous movement of material from a region of high concentration to a region of low concentration. ❖The free-energy change during diffusion of nonelectrolytes depends on the concentration grdient. ❖The free-energy change during diffusion of electrolytes depends on the electrochemical grdient

D. Transport processes within an eu karyotic cell Carner goren Meochoncion L Fatty acids dIP Ttiosephospra学s <3-phosphogycerao Sene acids acds

D. Transport processes within an eukaryotic cell

2. Passive transport and active transport A. Comparison of two classes of transport. Table 8-2 Properties of Passive and Active Transport Diffusion (Passive Transport Properties Simple Diffusion Facilitated Diffusion Active Transport Solutes transported Examples Small nonpolar Oxygen Large nonpolar Fatty acids No Small polar Water No Large polar Glucose N Yes lons N+K+ cac- No Yes Thermodynamic properties Direction relative to electrochemical gradient Down D Effect on entropy Increased Increased Decreased Metabolic energy required No No Intrinsic directionality No Kinetic properties Carrier-mediated Yes Yes(pump) Michaelis-Menten kinetics No Yes Competitive inhibition N Yes Yes

2. Passive transport and active transport A. Comparison of two classes of transport

carrier- meditated diffusion 百 1 Vmax simple diffusion KM concentration of→ transported molecule Figure 11-7 Kinetics of sim ple diffusion com pared to carrier-mediated diffusion. Whereas the rate of the former is always proportional to the solute concentration, the rate of the latter reaches a maximum( Vmax when the carrier protein is saturated. The solute concentration when transport is at half its maximal value approximates the binding constant (KM) of the carrier for the solute and is analogous to the Km of an enzyme for its substrate. The graph applies to a carrier transporting a single solute; the kinetics of coupled transport of two or more solutes(see text) are more complex but show basically similar phenomena

Figure 11-7 Kinetics of simple diffusion compared to carrier-mediated diffusion. Whereas the rate of the former is always proportional to the solute concentration, the rate of the latter reaches a maximum (Vmax) when the carrier protein is saturated. The solute concentration when transport is at half its maximal value approximates the binding constant (KM) of the carrier for the solute and is analogous to the KM of an enzyme for its substrate. The graph applies to a carrier transporting a single solute; the kinetics of coupled transport of two or more solutes (see text) are more complex but show basically similar phenomena

B Two classes of membrane transport proteins Carrier proteins are responsible for both the passive and the active transport. Channel proteins are only responsible for passive transport transported molecule channel○ carrier prote proter 8 lipid electrochemical bilayer gradient mple channel. cartier. diffusion mediated mediated diffusion diffusion PASSIVE TRANSPORT ACTIVE TRANSPORT (FACILITATED DIFFUSION)

B. Two classes of membrane transport proteins ❖Carrier proteins are responsible for both the passive and the active transport. ❖Channel proteins are only responsible for passive transport