BIOL2120 Cell Biology Transport Across Membranes PDF

BIOL2120 CELL BIOLOGY Chapter 8 3. Transport across Membranes: Overcoming the Permeability Barrier Membranes Are Selectively Permeable Overcoming the permeability barrier of cell membran...

BIOL2120 CELL BIOLOGY Chapter 8 3. Transport across Membranes: Overcoming the Permeability Barrier Membranes Are Selectively Permeable Overcoming the permeability barrier of cell membranes is crucial to proper functioning of the cell Specific molecules and ions need to be selectively moved into and out of the cell or organelle 2 Cells and Transport Processes Cells and cellular compartments are able to accumulate a variety of substances in concentrations that are very different from those of the surroundings Most of the substances that move across membranes are dissolved gases, ions, and small organic molecules (solutes) 3 4 The Movement of a Solute Across a Membrane Is Determined by Its Concentration Gradient or Its Electrochemical Potential The movement of a molecule that has no net charge is determined by its concentration gradient Simple or facilitated diffusion involves exergonic movement “down” the concentration gradient (negative DG) The movement of an ion is determined by its electrochemical potential (the combined effect of both concentration gradient and charge gradient) 5 Simple Diffusion: Unassisted Movement Down the Gradient The most straightforward way for a solute to cross a membrane is through simple diffusion (the unassisted net movement of a solute from high to lower concentration) Typically this is only possible for gases, nonpolar molecules, or small polar molecules such as water, glycerol, or ethanol 6 Diffusion Always Moves Solutes Toward Equilibrium Solutes will move toward regions of lower concentration until the concentrations are equal Diffusion always tends to create a random solution in which the concentration is the same everywhere Video for demonstration https://www.youtube.com/watch?v=6yMPpDeNwqQ 7 Solute Size In general, lipid bilayers are more permeable to small molecules (water, oxygen, carbon dioxide) than larger ones But without a transport protein even these small molecules move more slowly than in the absence of a membrane 8 Solute Polarity Lipid bilayers are more permeable to nonpolar substances than to polar ones Nonpolar substances dissolve Estrogen readily into the hydrophobic region of the bilayer Large nonpolar molecules such as estrogen and testosterone cross membranes easily, despite Testosterone their large size 9 Solute Charge The relative impermeability of polar substances, especially ions, is due to their association with water molecules The molecules of water form a shell of hydration around polar substances https://www.quora.com/Why-does- water-dissociate-to-a-small-degree- into-OH-and-H-ions 10 Solute charge – relevance to cell function Every cell must maintain an electrochemical potential across its plasma membrane in order to function In most cases this potential is a gradient of either sodium ions (animal cells) or protons (other cells) Membranes must still be able to allow ions to cross the bilayer in a controlled manner 11 Facilitated Diffusion: Protein-Mediated Movement Down the Gradient Most substances in the cell are too large or too polar to cross membranes by simple diffusion These can only move in and out of cells with the assistance of transport proteins If the process is exergonic, it is called facilitated diffusion; the solute diffuses as dictated by its concentration gradient 12 Carrier Proteins and Channel Proteins Facilitate Diffusion by Different Mechanisms Transport proteins are large, integral membrane proteins with multiple transmembrane segments Carrier proteins bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane Channel proteins form hydrophilic channels through the membrane to provide a passage route for solutes 13 Carrier Proteins Alternate Between Two Conformational States The alternating conformation model states that a carrier protein is allosteric protein and alternates between two conformational states In one state the solute binding site of the protein is accessible on one side of the membrane The protein shifts to the alternate conformation, with the solute binding site on the other side of the membrane, triggering solute release 14 15 The Glucose Transporter: A Uniport Carrier The erythrocyte is capable of glucose uptake by facilitated diffusion because the level of blood glucose is much higher than that inside the cell Glucose is transported inward by a glucose transporter (GLUT) GLUT1 is an integral membrane protein with 12 transmembrane segments, which form a cavity with hydrophilic side chains 16 Video for demonstration https://vimeo.com/134016329 17 Transport by GLUT1 is reversible A carrier protein can facilitate transport in either direction The direction of transport is dictated by the relative solute concentrations outside and inside the cell Glucose concentration is kept low inside most animal cells 18 Phosphorylation of glucose The immediate phosphorylation of glucose upon entry into the cell keeps the concentration of glucose low Once phosphorylated, glucose cannot bind the carrier protein any longer, and is effectively locked into the cell 19 The Erythrocyte Anion Exchange Protein: An Antiport Carrier The anion exchange protein (also called the chloride-bicarbonate exchanger Band 3) facilitates reciprocal exchange of Cl– and HCO3– ions only Exchange will stop if either anion is absent The ions are exchanged in a strict 1:1 ratio 20 21 22 Channel Proteins Facilitate Diffusion by Forming Hydrophilic Transmembrane Channels Channel proteins form hydrophilic transmembrane channels that allow specific solutes to cross the membrane directly There are three types of channels: ion channels, porins, and aquaporins 23 Ion Channels: Transmembrane Proteins That Allow Rapid Passage of Specific Ions Tiny pores lined with hydrophilic atoms Because most allow passage of just one ion, there are separate channels needed to transport Na+, K+, Ca2+, and Cl–, etc. Selectivity is based on both binding sites involving amino acid side chains, and a size filter 24 Gated channels Most ion channels are gated, meaning that they open and close in response to some stimuli - Voltage-gated channels open and close in response to changes in membrane potential - Ligand-gated channels are triggered by the binding of certain substances to the channel protein - Mechanosensitive channels respond to mechanical forces acting on the membrane 25 Porins: Transmembrane Proteins That Allow Rapid Passage of Various Solutes Pores on outer membranes of bacteria, mitochondria and chloroplasts are larger and less specific than ion channels The pores are formed by multipass transmembrane proteins called porins The transmembrane segments of porins cross the membrane as b barrels 26 27 Structure of porins The b barrel has a water-filled pore at its center Polar side chains line the inside of the pore, allowing passage of many hydrophilic solutes The outside of the barrel contains many nonpolar side chains that interact with the hydrophobic interior of the membrane 28 Aquaporins: Transmembrane Channels That Allow Rapid Passage of Water Movement of water across cell membranes in some tissues is faster than expected given the polarity of the water molecule Aquaporins allow rapid passage of water through membranes of erythrocytes and kidney cells in animals, and root cells and vacuolar membranes in plants 29 30 Aquaporin structure The transmembrane segments oriented to form four central channels The channels, lined with hydrophilic side chains, are just large enough for water molecules to pass through one at a time 31 Active Transport: Protein-Mediated Movement Up the Gradient Facilitated diffusion is important, but only accounts for movement of molecules down a concentration gradient, toward equilibrium Sometimes a substance must be transported against a concentration gradient Couples endergonic transport (positive DG) to an exergonic process, usually ATP hydrolysis 32 Functions of active transport Three important cellular functions - Uptake of essential nutrients - Removal of wastes - Maintenance of nonequilibrium concentrations of certain ions Active transport has intrinsic directionality 33 The Coupling of Active Transport to an Energy Source May Be Direct or Indirect Active transport mechanisms can be divided based on the sources of energy and whether or not two solutes are transported at the same time Active transport is categorized as direct or indirect 34 Direct active transport Primary active transport - the accumulation of solute molecules on one side of the membrane is coupled directly to an exergonic chemical reaction This is usually hydrolysis of ATP Transport proteins driven by ATP hydrolysis are called transport ATPases (ATPase pumps) 35 36 Indirect active transport Secondary active transport depends on the simultaneous transport of two solutes Movement of one solute down its gradient drives movement of another solute up its gradient This can be a symport or an antiport, depending on whether the two molecules are transported in the same or different directions 37 38 Direct Active Transport Depends on Four Types of Transport ATPases Four types of transport ATPases have been identified - P-type - V-type - F-type - ABC-type They differ in structure, mechanism, location, and roles 39 P-type ATPases Members of a large family and are reversibly phosphorylated by ATP on a specific aspartic acid residue They have transmembrane segments forming a http://classconnection.s3.amazonaws.com/664/fl hydrophilic channel ashcards/1813664/jpg/81347412944026.jpg 40 P-type ATPase subfamilies P-type ATPases fall into five subfamilies - P1-ATPases are found in all organisms and transport heavy metal ions - P2-ATPases are responsible for maintaining gradients of ions (Na+, K+, H+, Ca2+) across plasma membranes of eukaryotic cells - They play roles in muscle contraction and the acidification of gastric juices in the stomach 41 P-type ATPase subfamilies (continued) - P3-ATPases of plants and fungi pump protons out across the plasma membrane, acidifying the external medium - P4-ATPases (flippases) pump hydrophobic molecules, such as cholesterol and fatty acids, and do not transport them all the way across the bilayer - P5-ATPases are not well characterized, but some are known to transport cations 42 V-type ATPases Pump protons into organelles such as vacuoles, vesicles, lysosomes, endosomes, and the Golgi complex They have an integral component (Vo) embedded in the membrane and a peripheral component (V1) that juts out from the https://ars.els-cdn.com/content/image/1-s2.0- S000527281500119X-gr1_lrg.jpg membrane surface 43 F-type ATPases Found in bacteria, mitochondria and chloroplasts They transport protons and have two components: a transmembrane pore (Fo) and a peripheral membrane component (F1) that contains the ATP binding site http://classconnection.s3.amazonaws.com/664/flas hcards/1813664/jpg/81347412944026.jpg Video for demonstration https://www.youtube.com/watch?v=XI8m6o0gXDY 44 F-type ATPases also function in reverse In this case, ATP is synthesized, driven by the exergonic flow of protons down their gradients In the reverse direction, the ATPases are more accurately called ATP synthases 45 ABC-type ATPases Also called ABC (ATP binding cassette) transporters The term cassette describes the catalytic domain that binds ATP as part of the transport http://classconnection.s3.amazonaws.com/664/flashcards/18 13664/jpg/81347412944026.jpg process 46 Importers and exporters Most of the ABC transporters initially discovered from bacteria were importers, involved in uptake of nutrients But many are now known to be exporters 47 Structure of ABC-type ATPases ABC transporters typically have four protein domains, two of which are highly hydrophobic and are embedded and form a channel in the membrane The other two domains are peripheral and bind ATP, coupling its hydrolysis to http://classconnection.s3.amazonaws.com/664/flashcards/18 13664/jpg/81347412944026.jpg transport Video for demonstration https://youtu.be/T8dZwSPr8i8 48 Medical significance ABC transporters are medically important because some of them pump antibiotics or drugs out of cells, rendering the cell resistant to the drug Some human tumors are resistant to drugs that normally inhibit growth of tumors; the resistant cells have high concentrations of an ABC transporter called MDR (multidrug resistance) transport protein 49 Indirect Active Transport Is Driven by Ion Gradients Not directly powered by ATP hydrolysis The inward transport of molecules up their electrochemical gradients is often coupled to and driven by simultaneous inward movement of Na+ (animals) or protons (plant, fungi, bacteria) down their gradients 50 Symport mechanisms of indirect active transport Most cells continuously pump either sodium ions or protons out of the cell (e.g., the Na+/K+ pump in animals) The resulting high extracellular concentration of Na+ is a driving force for the uptake of sugars and amino acids This is indirectly related to ATP because the pump that maintains the sodium ion gradient is driven by ATP 51 52 Direct Active Transport: The Na+/K+ Pump Maintains Electrochemical Ion Gradients In a typical animal cell, [K+]inside/[K+]outside is about 35:1 and [Na+]inside/[Na+]outside is around 0.08:1 The electrochemical potentials for sodium and potassium are essential as a driving force for coupled transport 53 Requirement for energy The pumping of both Na+ and K+ ions against their gradients requires energy The pump that is responsible, the Na+/K+ ATPase (or pump), uses the exergonic hydrolysis of ATP to drive the transport of both ions 54 Structure of the Na+/K+ ATPase The pump comprises a transmembrane protein with a and b subunits The a subunits contain binding sites for sodium and ATP on the cytoplasmic side and for potassium on the external side Two alternative conformational states, E1 and E2 55 56 57 Indirect Active Transport: Sodium Symport Drives the Uptake of Glucose Although most glucose into and out of our cells occurs by facilitated diffusion, some cells use a Na+/glucose symporter For example, the cells lining the intestine take up glucose and some amino acids even when their concentrations are much lower outside than inside the cells 58 Uptake of glucose via sodium symport requires energy A steep Na+ gradient that is maintained across the plasma membrane (via the Na+/K+ pump) is used to provide the energy needed The proteins responsible for sodium symport are called sodium-dependent glucose transporters (SGLT) 59 60