Membrane Structure. Membranes

Transcription

1 Membrane Structure Membranes Chapter 5 Phospholipid bilayer Globular proteins inserted in the bilayer Fluid mosiac model proteins float in or on the fluid lipid bilayer like boats on a pond 2 3 4

2 Cellular membranes have 4 components 1. Phospholipid bilayer Flexible matrix, barrier to permeability 2. Transmembrane proteins Integral membrane proteins 3. Interior protein network Peripheral membrane proteins 4. Cell surface markers Glycoproteins and glycolipids Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes One method to embed specimen in resin 1!m shavings 5 6 Freeze-fracture visualizes inside of membrane Phospholipids Structure consists of Glycerol 2 fatty acids Nonpolar and hydrophobic ( water-fearing ) Phosphate group Polar and hydrophilic ( water-loving ) Spontaneously forms a bilayer Fatty acids are on the inside Phosphate groups are on both surfaces 7 8

3 Bilayers are fluid Individual phospholipids and unanchored proteins can move through the membrane 9 10 Environmental influences Saturated fatty acids make the membrane less fluid than unsaturated fatty acids Kinks introduced by the double bonds keep them from packing tightly Most membranes also contain sterols such as cholesterol, which can either increase or decrease membrane fluidity, depending on the temperature Warm temperatures make the membrane more fluid than cold temperatures Cold tolerance in bacteria due to fatty acid desaturases Membrane Proteins Various functions: 1. Transporters 2. Enzymes 3. Cell-surface receptors 4. Cell-surface identity markers 5. Cell-to-cell adhesion proteins 6. Attachments to the cytoskeleton 11 12

4 Membrane Proteins Integral membrane proteins Span the lipid bilayer (transmembrane proteins) Nonpolar regions embedded in the interior of the bilayer Polar regions protrude from both sides of the bilayer Transmembrane domain Spans the lipid bilayer Hydrophobic amino acids arranged in " helices Bacteriorhodopsin has 7 transmembrane domains forming a structure within the membrane through which protons pass during the light-driven pumping of protons Pores Membrane Proteins Nonpolar regions within a transmembrane protein can create a pore through the membrane Cylinder of! sheets in the protein secondary structure called a!-barrel Interior is polar and allows water and small polar molecules to pass through the membrane 15 16

5 Passive Transport Diffusion is movement of molecules from high to low Will continue until equilibrium is reached Passive transport is movement of molecules through the membrane in which No energy is required Molecules move in response to a gradient Selectively Permeable Membrane Barrier to crossing a biological membrane is the hydrophobic interior Nonpolar molecules will move until the is equal on both sides O 2, CO 2, steroid hormones, fat-soluble vitamins Limited permeability to polar molecules and ions Na +, K +, Cl -, amino acids, sugars 19 20

6 Selectively Permeable Membrane Facilitated diffusion Molecules may move through proteins From higher to lower Passive transport Channel proteins Hydrophilic channel when open Carrier proteins Bind specifically to molecules they assist Ion channels Channel proteins Allow the passage of ions Gated channels open or close in response to stimulus (chemical or electrical) Carrier proteins Can transport ions and other solutes, such as sugars and amino acids Requires a difference across the membrane Must bind to the molecule they transport Saturation rate of transport limited by number of transporters 23 24

7 Osmosis Cytoplasm of the cell is an aqueous solution Water is solvent Dissolved substances are solutes Osmosis diffusion of water across a membrane toward a higher solute Tonicity Isotonic solution has the same solute on both sides of the membrane Hypertonic solution has a higher solute Hypotonic solution has a lower solute Aquaporins facilitate osmosis 27 28

8 Osmotic pressure Cell in a hypotonic solution gains water causing cell to swell creates pressure If membrane strong enough, cell reaches counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out Cell wall of prokaryotes, fungi, plants, protists If membrane is not strong, may lyse (burst) Animal cells must be in isotonic environments Maintaining osmotic balance Some cells use extrusion in which water is ejected through contractile vacuoles Isosmotic regulation involves keeping cells isotonic with their environment Marine organisms adjust internal to match sea water Terrestrial animals circulate isotonic fluid Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid Active Transport Requires energy ATP Moves substances from low to high Requires the use of highly selective carrier proteins 31 32

9 Sodium potassium (Na + K + ) pump Carrier proteins used in active transport include Uniporters move one molecule at a time Symporters move two molecules in the same direction Antiporters move two molecules in opposite directions Uses an antiporter to move 3 Na + out of the cell and 2 K + into the cell Against their gradient ATP energy is used to change the conformation of the carrier protein Affinity of the carrier protein for either Na + or K + changes so the ions can be carried across the membrane Coupled transport Uses ATP indirectly Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule Symporter is used Glucose Na + symporter captures the energy from Na + diffusion to move glucose against a gradient 35 36

10 Bulk Transport Endocytosis Exocytosis 37 Movement of substances out of cell Requires energy Movement of substances into the cell Phagocytosis cell takes in particulate matter Pinocytosis cell takes in only fluid Receptor-mediated endocytosis specific molecules are taken in after they bind to a receptor In the human genetic disease familial hypercholesterolemia, the LDL receptors lack tails, so they are never fastened in the clathrin-coated pits and as a result, do not trigger vesicle formation. The cholesterol stays in the bloodstream of affected individuals, accumulating as plaques inside arteries and leading to heart attacks. 40

11 Requires no energy Passive transport Diffusion Facilitated diffusion Higher solute Osmosis Higher water Requires energy Active transport Higher solute Exocytosis Movement of materials out of the cell Used in plants to export cell wall material Used in animals to secrete hormones, neurotransmitters, digestive enzymes Lower solute Solute Water Lower water Lower solute 41