Cell Membrane Structure and Transport PDF

28/09/2023 Plasma Membrane Fluid mosaic model boundary that separates the living cells from the environment; selective permeability...

28/09/2023 Plasma Membrane Fluid mosaic model boundary that separates the living cells from the environment; selective permeability Phospholipid Structure bilayer Phospholipids Amphipathic Hydrophilic Hydrophobic regions Hydrophilic hydrophobic of protein regions of protein Urry et al 2017. Essential Biology 11th Edition 1 2 Fluidity of the membranes Most of the lipids & some proteins drift laterally Fluid Viscous Rarely does the molecule flip flop transversely Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails Lateral Flip-flop movement ( once per (b) Membrane fluidity (107 times per month) second) (a) Movement of phospholipids 3 4 1  28/09/2023 Membrane proteins & functions Proteins determine the membrane’s specific functions Cholesterol (c) Cholesterol within the animal cell membrane Warm temperatures (37°C), restrains movement of phospholipids, reduces membrane fluidity Cool temperatures, maintains fluidity & hinders solidification by preventing tight packing of phospholipids Urry et al 2017. Essential Biology 11th Edition 5 6 1. PERIPHERAL PROTEINS – bound to the surface of the membrane Functions of membrane proteins 2. INTEGRAL PROTEINS – penetrate the hydrophobic core Signaling molecule Transmembrane proteins – integral proteins that span the Enzymes Receptor membrane Hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices EXTRACELLULAR ATP N-terminus SIDE Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Glyco- protein C-terminus CYTOPLASMIC  Helix SIDE (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton & (ECM) 7 8 2  28/09/2023 Membrane structure results in selective permeability Transport proteins allow passage of hydrophilic substances across the membrane A cell must exchange materials with its surroundings channel proteins carrier proteins Selectively permeable, regulating the cell’s molecular have a hydrophilic bind to molecules and traffic channel that certain change shape to nonpolar/hydrophobic molecules (hydrocarbons, CO2, O2), molecules or ions can shuttle them across can dissolve in the lipid bilayer and pass through the use as a tunnel the membrane membrane rapidly polar/hydrophilic molecules (sugars, water) do not cross the A transport protein is specific for the substance it moves membrane easily – TRANSPORT PROTEINS AQUAPORINS - passage of water Glucose transporter – specific for glucose 9 10 Passive transport Active transport Passive transport : Diffusion Molecules of dye Membrane (cross section) WATER ATP Net diffusion Net diffusion Equilibrium Diffusion Facilitated diffusion (a) Diffusion of one solute diffusion of a substance across a membrane with no energy investment; from an area of high concentration to low 11 12 3  28/09/2023 Lower Higher Same concentration Passive transport concentration concentration of sugar of solute (sugar) of sugar Osmosis Water diffuses H2 O across a Selectively membrane permeable membrane Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium ↓ LOW solute concentra on (b) Diffusion of two solutes TO ↑HIGH solute concentra on substances diffuse down their concentration gradient Osmosis 13 14 Hypotonic solution Isotonic solution Hypertonic solution Water balance of cells H2 O H2 O H2 O H2 O Tonicity – ability of a solution to cause a cell to gain (a) Animal or lose water cell Isotonic Solute concentration is same as Lysed Normal Shriveled solution that inside the cell H2 O H2O H2 O H2 O Hypertonic Solute concentration is greater solution than that inside the cell (b) Plant cell Hypotonic Solute concentration is less solution than that inside the cell Turgid (normal) Flaccid Plasmolyzed 15 16 4  28/09/2023 Osmoregulation Filling vacuole 50 µm Control of water Facilitated Diffusion balance e.g. Paramecium Transport proteins – speed up passive movement of molecules (a) A contractile vacuole filled with fluid that enters from a system of canals, radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell. 17 18 Channel proteins – provide corridors that allow a Carrier proteins – undergo a subtle change in shape specific molecule or ion to cross the membrane that translocates the solute-binding site across the Aquaporins – water membrane Ion channels – open or close in response to stimulus (gated channels) Urry et al 2017. Essential Biology 11th Edition Urry et al 2017. Essential Biology 11th Edition 19 20 5  28/09/2023 EXTRACELLULAR [Na+] high Na+ FLUID [K+] low Na+ Active Transport Na+ Na+ Na+ Na+ Na+ Na+ Moves substances against their concentration gradient Na+ [Na+] low [K+] high P ATP P CYTOPLASM ADP 1 Cytoplasmic Na+ binds to 2 Na+ binding stimulates 3 Phosphorylation causes protein the sodium-potassium pump. phosphorylation by ATP. to change shape. Na+ is expelled Requires energy, usually in the form of ATP to the outside. Performed by proteins embedded in the membranes The sodium potassium pump is one type of active transport system P P 6 K+ is released, and the 5 Loss of the phosphate 4 K+ binds on the extracellular side cycle repeats. restores the protein’s original and triggers release of PO4 shape. group. 21 22 Electrogenic pump is a transport protein that How Ion Pumps Maintain Membrane Potential generates voltage across a membrane 1. sodium-potassium pump - animal cells Voltage is created by differences in the distribution of positive and negative ions. 2. proton pump - plants, fungi, bacteria Membrane potential is the voltage difference across a membrane. Electrochemical gradient, drives the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) an electrical force (the effect of the membrane potential, i.e., voltage difference, on the ion’s movement) Urry et al 2017. Essential Biology 11th Edition 23 24 6  28/09/2023 Cotransport: Coupled transport by a membrane protein Bulk transport : exocytosis & endocytosis occurs when active transport of a solute indirectly drives transport of another solute plants use the gradient of H+ ions generated by proton pumps to small molecules & water enter or leave the cell drive active transport of nutrients (sugar) into the cell across the lipid bilayer or by transport proteins. large molecules (e.g. polysaccharides & proteins) cross the membrane in bulk via vesicles; requires energy Urry et al 2017. Essential Biology 11th Edition 25 26 Phagocytosis EXOCYTOSIS ENDOCYTOSIS transport vesicles the cell takes in a cell engulfs a the vacuole fuses migrate to the macromolecules by particle in a with a lysosome to vacuole digest the particle membrane, fuse with forming vesicles from it, & release or take the plasma membrane out their contents Three types: used by many phagocytosis (“cellular secretory cells such as eating”) pancreas to export pinocytosis (“cellular their products drinking”) receptor-mediated endocytosis Urry et al 2017. Essential Biology 11th Edition 27 28 7  28/09/2023 Receptor-mediated Endocytosis Pinocytosis molecules are taken up when extracellular fluid is “gulped” into tiny vesicles binding of ligands to PINOCYTOSIS receptors triggers vesicle formation 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) ligand - molecule that binds specifically to a receptor site of Vesicle another molecule Urry et al 2017. Essential Biology 11th Edition Urry et al 2017. Essential Biology 11th Edition 29 30 8  14/09/2023 Plants - Autotrophs Heterotrophs Photosynthesis CHLOROPLAST 1 2 CHLOROPLASTS Photosynthesis can be summarized as the Site of Photosynthesis in following: Plants CO2 enters & O2 exits the leaf (stomata) Leaves  mesophyll tissue 6 CO2 + 12 H2O + Light energy Mesophyll cell ~ 30-40 chloroplasts C6H12O6 + 6 O2 + 6 H2O Green color – Chlorophyll (in the membranes of thylakoids, stacked into grana) Chloroplasts also contain stroma, a dense fluid Chlorophyll absorbs light energy  synthesis of organic molecules in the chloroplast Urry et al. 2017. Essential Biology 11th Edition 3 4  14/09/2023 The splitting of water Photosynthesis as a redox process Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Water Oxidized 6 CO2 12 H2O Carbon Reactants: Reduced dioxide Products: C6H12O6 6 H2O 6 O2 5 6 2 Stages of Photosynthesis: A Preview Photo part – Light Reactions in the thylakoids light absorption, split water, release O2, produce ATP, and form NADPH Synthesis part – Calvin Cycle in the stroma - forms sugar from CO2, using ATP and NADPH begins with carbon fixation, incorporating CO2 into organic molecules Urry et al. 2017. Essential Biology 11th Edition 7 8  14/09/2023 The light reactions convert solar energy The Nature of to the chemical energy of ATP and NADPH Sunlight Chloroplasts – solar powered chemical factories Light - form of electromagnetic energy or electromagnetic radiation Thylakoids transform light energy into the Travels in rhythmic waves chemical energy of ATP & NADPH Wavelength = distance between crests of waves Electromagnetic spectrum - entire range of electromagnetic energy, or radiation Visible light consists of colors we can see, including wavelengths that drive photosynthesis Urry et al. 2017. Essential Biology 11th Edition 9 10 Photosynthetic Pigments: 3 Types of Pigments in Chloroplast The Light Receptors Chlorophyll a - main photosynthetic pigment Pigments are substances that absorb visible light Different pigments absorb different Chlorophyll b - Accessory pigments; broaden the wavelengths spectrum used for photosynthesis Wavelengths that are not absorbed are reflected or transmitted Xantophylls, carotenoids – Accessory pigments; Leaves appear green because chlorophyll absorb excessive light that would damage reflects and transmits green light chlorophyll Urry et al. 2017. Essential Biology 11th Edition 11 12  14/09/2023 Excitation of chlorophyll by light A Photosystem: A reaction center associated with light-harvesting complexes When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable photosystem consists of a reaction center surrounded When excited electrons fall back by light-harvesting to the ground state, photons are complexes (LHC) given off, an afterglow called fluorescence light-harvesting complexes If illuminated, an isolated (pigment molecules bound solution of chlorophyll will to proteins) funnel the fluoresce, giving off light and energy of photons to the heat reaction center 13 14 2 types of photosystem in thylakoid membrane 1st step of light reactions: Photosystem II functions first (the numbers primary electron reflect order of discovery); absorbs wavelength acceptor in the reaction of 680 nm center accepts an excited electron from Photosystem I - absorbs wavelength of 700 nm chlorophyll a The two photosystems work together to use light energy to generate ATP and NADPH 15 16  14/09/2023 Photochemical Reactions Linear Electron Flow During the light reactions, there are two possible (Photophosphorylation) routes for electron flow: Photophosphorylation – process of making ATP CYCLIC from ADP and Pi using energy derived from light NON CYCLIC or LINEAR (photon) Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH 17 18 H2O Noncyclic or Linear CO2 H2O CO2 Light Light NADP+ Electron Flow NADP+ ADP LIGHT REACTIONS ADP CALVIN CYCLE (Photophosphorylation) LIGHT REACTIONS ATP CALVIN CYCLE ATP NADPH 1. NADPH O2 [CH2O] (sugar) O2 [CH2O] (sugar) -- photon of light strikes a Primary pigment molecule in LHC Primary acceptor Primary acceptor acceptor Pq e– Fd e– -- is relayed to other pigment 2 H+ H 2O e– Cytochrome e– NADP+ e– molecules until it reaches one of + 2 H+ + complex NADP+ reductase the 2 P680 chl a molecules in Energy of electrons Energy of electrons 1/2 O2 NADPH e– Pc + H+ the PSII reaction center. e– P700 Light P680 Light Light P680 -- it excites one of the P680 electrons of chl a to a higher ATP energy state. Photosystem I (PS I) Photosystem II (PS II) Photosystem II (PS II) 19 20  14/09/2023 H2O CO2 H2O CO2 Light Light NADP+ NADP+ ADP ADP CALVIN CALVIN LIGHT LIGHT CYCLE CYCLE REACTIONS REACTIONS ATP ATP NADPH NADPH O2 [CH2O] (sugar) O2 [CH2O] (sugar) Primary Primary 3. Photolysis acceptor 2. acceptor -- splitting of H2O into 2H & O atom H2O e– The electron is captured by H2O e– 2 H+ + the primary electron acceptor. 2 H+ + -- supply of electrons one by one to Energy of electrons Energy of electrons 1/ 2 O 2 e– 1/ 2 O 2 e– P680, each replacing an e- lost to e– e– the primary e- acceptor Light Light P680 P680 -- O atom combines with another O atom, forming O2 Photosystem II Photosystem II (PS II) (PS II) 21 22 H2O CO2 4. ETC H2O CO2 5. Light -- passing of photoexcited e- from Light Exergonic ‘fall” of electrons to a NADP+ ADP the 10 e-acceptor of PS II to PS I NADP+ ADP lower energy level provides LIGHT REACTIONS CALVIN CYCLE via ETC (Plastoquinone (Pq), LIGHT REACTIONS CALVIN CYCLE energy for ATP synthesis. ATP cytochrome complex, ATP NADPH plastocyanin (Pc)) NADPH O2 [CH2O] (sugar) O2 [CH2O] (sugar) Primary Primary Primary acceptor acceptor acceptor Pq Pq e– H2O e– H2O e– 2 H+ Cytochrome 2 H+ Cytochrome complex complex + + Energy of electrons Energy of electrons 1/ 2 O 2 1/ 2 O 2 e– Pc e– Pc e– e– P700 Light Light P680 P680 Light ATP ATP Photosystem I Photosystem II Photosystem II (PS I) (PS II) (PS II) 23 24  14/09/2023 6. 7. ETC -- transfer of light energy via LHC to PS I, H2O CO2 -- passing of photoexcited e-s from H2O CO2 exciting an e- of one of the 2 P700 chl a Light PS I’s primary e- acceptor down a Light -- capture of photoexcited e- by PS I’s NADP+ ADP second ETC through the protein NADP+ ADP primary e- acceptor, creating an e- “hole” in LIGHT REACTIONS CALVIN CYCLE ferredoxin (Fd) LIGHT REACTIONS CALVIN CYCLE P700 ATP NADPH ATP NADPH -- filling the “hole” by an e- that reaches the bottom of the ETC from PS II. O2 [CH2O] (sugar) O2 [CH2O] (sugar) Primary Primary acceptor Primary acceptor acceptor Fd Primary Pq e– acceptor e– Fd H 2O e– e– NADP+ Pq e– Cytochrome e– 2 H+ NADP+ + 2 H+ H 2O e– e– NADP+ complex Cytochrome + reductase Energy of electrons 2 H+ NADP+ + 2 H+ 1/2 O2 NADPH complex Pc + reductase e– Energy of electrons + H+ 1/2 O2 NADPH Pc e– P700 e– + H+ Light P700 P680 Light e– Light P680 Light ATP ATP Photosystem I (PS I) Photosystem I Photosystem II (PS I) (PS II) Photosystem II (PS II) 25 26 H2O CO2 8. -- transfer of electrons from Fd to Light NADP+ ADP NADP+ by NADP+ reductase -- 2 electrons required for its reduction CALVIN LIGHT CYCLE REACTIONS ATP NADPH to NADPH O2 [CH2O] (sugar) Primary Primary acceptor acceptor Fd Pq e– e– H 2O e– e– NADP+ 2 H+ Cytochrome NADP+ + 2 H+ complex + reductase Energy of electrons 1/2 O2 NADPH e– Pc + H+ e– P700 Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II) Urry et al. 2017. Essential Biology 11th Edition 27 28  14/09/2023 Cyclic Electron Flow: A Second Cyclic Electron Flow Photophosphorylation Sequence Cyclic electron flow uses only PS I and produces only ATP No NADPH is produced Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle 29 30 Light reactions Calvin cycle H2 O CO2 3 Phases: Calvin Cycle (C3 Pathway) Light NADP+ ADP 1. Carbon fixation (catalyzed by rubisco) + Pi Photosystem II RuBP 3-Phosphoglycerate 2. Reduction Electron transport chain Photosystem I 3. Regeneration of the CO2 acceptor (RuBP) ATP G3P Starch NADPH (storage) Chloroplast Amino acids The function of the pathway is to produce a single Fatty acids molecule of glucose O2 Sucrose (export) 31 32  14/09/2023 H2O CO2 Input 1. Carbon Fixation Light NADP+ 3 (Entering one ADP CO2 at a time) LIGHT CALVIN CYCLE -- involves carboxylation: CO2 combines with RuBP to produce PGA REACTIONS ATP Phase 1: Carbon fixation -- RuBP carboxylase (rubisco) catalyzes the merging of CO2 & RuBP NADPH Rubisco O2 [CH2O] (sugar) 3 P P (1 turn - 3 CO2 combine with 3 RuBP to produce 6 PGA) Short-lived intermediate 3 P P 6 P Ribulose bisphosphate 3-Phosphoglycerate (RuBP) 6 ATP H2O CO2 2. Reduction 6 ADP Input Light 3 (Entering one CALVIN NADP+ ADP CO2 at a time) -- ATP & NADPH are CYCLE 6 P P LIGHT REACTIONS CALVIN CYCLE incorporated into G3P, making it 1,3-Bisphosphoglycerate 6 NADPH ATP NADPH Phase 1: Carbon fixation very energy-rich 6 NADP+ -- ADP, Pi, NADP+ are released & 6 Pi Rubisco O2 [CH2O] (sugar) re-energized in noncyclic photo- 6 P 3 P P Short-lived phosphorylation Glyceraldehyde-3-phosphate Phase 2: (G3P) Reduction intermediate 6 P 3 P Ribulose bisphosphate P 3-Phosphoglycerate (6 ATP & 6 NADPH are used to (RuBP) 6 ATP convert 6 PGA to 6 G3P.) 6 ADP 1 P G3P Glucose and (a sugar) other organic CALVIN Output compounds CYCLE 33 34 H2O CO2 Light Input Summary of Calvin Cycle NADP+ 3 (Entering one ADP CO2 at a time) LIGHT CALVIN REACTIONS CYCLE ATP NADPH Phase 1: Carbon fixation Rubisco The cycle takes CO2 from the atmosphere & the O2 [CH2O] (sugar) 3 P P Short-lived energy in ATP & NADPH to create 1 glucose intermediate 3. Regeneration 3 P P 6 P Ribulose bisphosphate 3-Phosphoglycerate -- regenerating the 3 (RuBP) 6 ADP 6 ATP molecule (2 TURNS). RuBP originally used to 3 ADP CALVIN combine with 3 CO2 CYCLE 6 P P 6 CO2 + 18 ATP + 12 NADPH  1 glucose + 18 ADP + 18Pi + 12 NADP+ + 3 ATP -- allows the cycle to 1,3-Bisphosphoglycerate 6 NADPH repeat Phase 3: Regeneration of 6NADP+ 12 H+ the CO2 acceptor 6 Pi (RuBP) (3 ATP are used to convert 5 G3P P 6 P 5 G3P to 3 RuBP) Glyceraldehyde-3-phosphate Phase 2: (G3P) Reduction Calvin cycle (like citric acid cycle), regenerates its starting material after molecules enter and leave the cycle 1 G3P P Glucose and (a sugar) other organic Output compounds 35 36  14/09/2023 SUMMARY Light reactions Calvin cycle H2 O CO2 Light Alternative mechanisms of carbon fixation have evolved in NADP+ ADP + Pi Photosystem II RuBP 3-Phosphoglycerate hot, arid climates Electron transport chain Photosystem I ATP G3P Starch NADPH (storage) Chloroplast Amino acids Fatty acids O2 Sucrose (export) 37 38 C4 Plants