Dr. Ketki Assistant Professor Department of Biochemistry Heritage IMS, Varanasi

Transcription

1 TRANSPORT MECHANISMS Dr. Ketki Assistant Professor Department of Biochemistry Heritage IMS, Varanasi

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3 Membrane selectivity allows adjustments of cell composition and function If plasma membrane is relatively impermeable, how do most molecule enter a cell? How is selectivity of this movement established? Permeability of substance across membrane, depends upon their solubility in lipids, not on their molecular size. Important function of membrane is to withhold unwanted molecules, while permitting entry of molecules necessary for cellular metabolism.

4 Factors affecting net diffusion of a substance 1) Concentration gradient across membrane 2) Electrical potential across membrane 3) Permeability coefficient of substance for the membrane 4) Hydrostatic pressure gradient across membrane 5) Temperature

5 Classification of transport mechanism

6 Simple diffusion Passive Transport Passive flow of solute from higher to lower concentration Does not require energy Very slow process Example 1) Diffusion of gas 2) Lipophilic substances

7 Facilitated Diffusion Carrier mediated process Carrier mechanism can be saturated Can operate bidirectionally More rapid process Does not require energy Ping-pong mechanism explains facilitated diffusion

8 Carrier protein exists in two principal conformations In ping state, active site of carrier protein is exposed to higher concentration of solute(exterior) Solute bind to this specific site on carrier protein Then,there is conformational change in carrier protein In the pong state, active sites are facing interior of the cell( where concentration of solute is minimal) Results in transport of solute across membrane. process is completely reversible. (net flux depends upon concentration gradient)

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10 Hormones regulate facilitated diffusion by changing no. of transporters available. Examples 1) Insulin: glucose transport in fat and muscles Amino acid transport in liver 2) Gluco-corticoids: Amino acid transport in liver 3) Growth Hormones: Amino acid transport in all cells 4) Estrogen: Amino acid transport in uterus

11 Water channels Aquaporins Family of membrane channel protein that serve as selective pores through which water crosses plasma membrane of cells 11 aquaporin channels in mammals,of which 10 are known in humans Most significant location of these channels is kidney c/a: reduced expression of AQP2 is associated with nephrogenic diabetes insipidus, acquired hypokalemia & hyperkalemia

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13 Facilitated diffusion: via ion channels Salient features: Membranes have specific devices called ion channels(which span the membrane) Ion channels are trans-membrane proteins that allows selective entry of various ions Regulation of activity is done by Ligand gated Voltage gated Very quick process

14 Ligand gated channels Binding of effectors (Ligand) to channel, Results in opening/closing of channel Ligand can be extracellular signalling molecule/ Intracellular messanger Examples: 1) Acetylcholine receptor 2) Calcium channels

15 Acetylcholine receptor Present in post synaptic membrane Complex of 5 subunits: Consists of acetylcholine binding site and ion channel

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17 Release of acetylcholine from pre synaptic region Binds to receptor on postsynaptic region Opening of channel, influx of sodium Generation of action potential in post synaptic nerve

18 Calcium channels Upon stimuli, opening of calcium channels in sarcoplasmic reticulum membrane Calcium in cytosol of muscle cells c/a: Calcium channel blockers used in management of hypertension

19 Voltage gated channels Opened by membrane depolarization (channel is usually closed in ground state) Voltage difference switches the ion channels to open, lasting < 25 ms Examples: Sodium channel, Potassium channel

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21 Sodium channel It has 4 subunits I,II III IV Each subunit consists of six α helical transmembrane domains The actual pore through which ion pass, is still unknown, but is formed by apposition of 5 and 6 transmembrane α helices of domain I IV(colored yellow) Four blue shaded subunits in the different domains represent voltage sensing portion of α subunit

22 Potassium channel Integral membrane protein composed of four identical subunits, each with two transmembrane segments creating an inverted V like structure Part of channel that confers ion selectivity (selective filter) measures 12 A ⁰ long( a relatively short length of membrane) and is situated at wide angle of inverted V.

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24 Clinical application Channelopathies : result from abnormality in proteins forming ion channels. It can be congenital or acquired Sodium channel Procaine Point mutation Liddle s disease Potassium channel Long QT syndrome Bartter syndrome

25 Chloride channel Inhibitory neurotransmitters Cystic fibrosis Calcium channel Calcium channel blocker

26 Ionophores Membrane shuttles for specific ions It increases permeability of ions by acting as channel formers Two types - Channel formers (Gramicidin) - Mobile ion carriers (Valinomycin) They are produced by certain micro-organisms and are used as antibiotics

27 When cells of higher organisms are exposed to ionophores, ion gradient is dissipated Valinomycin allows potassium to permeate mitochondria, so it dissipates the protein gradient, and acts as an uncoupler of ETC

28 Active transport Salient features Requires energy, transport against concentration gradient, energy comes from hydrolysis of ATP, electron transport or light Unidirectional Requires specialized integral proteins called transporters Transport system is saturated at higher concentration of solutes Transporters are susceptible to inhibition by specific compounds Example : sodium pump, calcium pump

29 Sodium Pump

30 Cells has low intracellular sodium, but concentration of potassium inside the cell is very high This is maintained by sodium potassium activated ATPase generally called as sodium pump It has binding site for ATP and sodium on inner side Potassium binding site is located outside the membrane It is made up of two subunits α2β2,both span whole thickness of membrane

31 Four different types of ATPase Types Transports A type ATPase P type ATPase F type ATPase V type ATPase Anions H+, K+, Na+, Ca++ Functions in translocation of H+ in mitochondria during oxidative phosphorylation Similar to F type ATPase

32 Clinical application

33 Calcium Pump: ATP dependent Ca pump function to regulate muscle contraction In resting muscle, concentration of calcium around muscle fibers is low Upon stimulation by nerve impulse, sudden release of large amount of calcium Trigger muscle contraction

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35 Uniport Co-transport (symport, antiport) Uniport carries single solute across the membrane Example: Glucose transport in cells, Calcium pump Co-transport Transfer of one molecule depends upon sequential transfer of another molecule May be coupled with energy indirectly

36 Symport Transporter carries two solutes in same direction across the membrane Examples : 1) SGLUT in intestinal cells, kidney c/a : Phlorhizin- inhibits SGLUT2 in PCT of kidney, produces renal damage, renal glycosuria 2) Aminoacid transport in all cells

37 Antiport Carries two solutes/ions in opposite direction Examples : sodium pump, chloride bicarbonate exchange in RBC C/a of these system: 1)Hartnup s disease: Defective aminoacid transport in intestine and renal tubules 2) Cystinuria: Renal reabsorption of cysteine is abnormal

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41 Transport of large molecules Endocytosis Exocytosis Pinocytosis Phagocytosis Non-selective Pinocytosis Selective Pinocytosis

42 Endocytosis Mechanism by which cells internalize extracellular molecules to form endocytic vesicle Plasma membrane is invaginated, enclosing the matter. This forms endocytic vesicle Types: pinocytosis, phagocytosis Pinocytosis: Drinking by the cell, 1) Non-selective pinocytosis 2) Selective/adsorptive pinocytosis

43 Selective/Adsorptive Pinocytosis Receptor mediated endocytosis Example: LDL binds to LDL receptor, complex is internalized. Cytoplasmic side is coated with filaments mainly composed of clathrin forming clathrin coated pits. Cholesterol is absorbed by clathrin coated pits After complex is internalized,receptor molecules are released back to cell surface

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45 Phagocytosis Derived from greek word phagein which means to eat Engulfment of large particles such as bacteria by macrophages and granulocytes They extend pseudopodia and surround the particles to form phagosomes Phagosomes fuses with lysosomes to form phagolysosomes, inside which particles are digested.

46 Exocytosis Under appropriate stimuli, secretory vesicles move towards and fuse with plasma membrane This movement is created by cytoplasmic contractile elements Inner membrane of vesicle fuses with outer plasma membrane, while cytoplasmic side of vesicle fuses with cytoplasmic side of plasma membrane Thus the contents of vesicles are externalized This process is called as exocytosis/reverse pinocytosis

47 Examples of Exocytosis 1) Release of insulin by β cells of Langerhans 2) Release of trypsinogen by pancreatic acinar cells 3) Release of acetylcholine by presynaptic cholinergic nerves

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50 THANK YOU