Membranes Chapter 5

Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond

Cellular membranes have 4 components Phospholipid bilayer Flexible matrix, barrier to permeability Transmembrane proteins Integral membrane proteins Interior protein network Peripheral membrane proteins Cell surface markers Glycoproteins and glycolipids

One method to embed specimen in resin Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes One method to embed specimen in resin 1µm shavings TEM shows layers

Freeze-fracture visualizes inside of membrane

Phospholipids Structure consists of Spontaneously forms a bilayer Glycerol – a 3-carbon polyalcohol 2 fatty acids attached to the glycerol Nonpolar and hydrophobic (“water-fearing”) Phosphate group attached to the glycerol Polar and hydrophilic (“water-loving”) Spontaneously forms a bilayer Fatty acids are on the inside Phosphate groups are on both surfaces

Bilayers are fluid Hydrogen bonding of water holds the 2 layers together Individual phospholipids and unanchored proteins can move through the membrane

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: Transporters Enzymes Cell-surface receptors Cell-surface identity markers Cell-to-cell adhesion proteins Attachments to the cytoskeleton

Structure relates to function Diverse functions arise from the diverse structures of membrane proteins Have common structural features related to their role as membrane proteins Peripheral proteins Anchoring molecules attach membrane protein to surface

Anchoring molecules are modified lipids with Nonpolar regions that insert into the internal portion of the lipid bilayer Chemical bonding domains that link directly to proteins

Integral membrane proteins Span the lipid bilayer (transmembrane proteins) Nonpolar regions of the protein are embedded in the interior of the bilayer Polar regions of the protein protrude from both sides of the bilayer Transmembrane domain Spans the lipid bilayer Hydrophobic amino acids arranged in α helices

Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain

Bacteriorhodopsin has 7 transmembrane domains forming a structure within the membrane through which protons pass during the light-driven pumping of protons

Membrane Proteins Pores Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane Cylinder of  sheets in the protein secondary structure called a b-barrel Interior is polar and allows water and small polar molecules to pass through the membrane

Passive Transport Passive transport is movement of molecules through the membrane in which No energy is required Molecules move in response to a concentration gradient Diffusion is movement of molecules from high concentration to low concentration Will continue until the concentration is the same in all regions

Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules Nonpolar molecules will move until the concentration is equal on both sides Limited permeability to small polar molecules Very limited permeability to larger polar molecules and ions

Facilitated diffusion Molecules that cannot cross membrane easily may move through proteins Move from higher to lower concentration Channel proteins Hydrophilic channel when open Carrier proteins Bind specifically to molecules they assist Membrane is selectively permeable

Channel proteins Ion channels Allow the passage of ions Gated channels – open or close in response to stimulus (chemical or electrical) 3 conditions determine direction Relative concentration on either side of membrane Voltage differences across membrane Gated channels – channel open or closed

Carrier proteins Can help transport both ions and other solutes, such as some sugars and amino acids Requires a concentration difference across the membrane Must bind to the molecule they transport Saturation – rate of transport limited by number of transporters

Osmosis Cytoplasm of the cell is an aqueous solution Water is solvent Dissolved substances are solutes Osmosis – net diffusion of water across a membrane toward a higher solute concentration

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Osmotic concentration When 2 solutions have different osmotic concentrations Hypertonic solution has a higher solute concentration Hypotonic solution has a lower solute concentration When two solutions have the same osmotic concentration, the solutions are isotonic Aquaporins facilitate osmosis

Osmotic pressure Force needed to stop osmotic flow 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 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 concentration 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 is used directly or indirectly to fuel active transport Moves substances from low to high concentration Requires the use of highly selective carrier proteins

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 Terms can also be used to describe facilitated diffusion carriers

Sodium–potassium (Na+–K+) pump Direct use of ATP for active transport Uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell Against their concentration 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 concentration gradient

Bulk Transport Exocytosis Endocytosis Movement of substances into the cell Phagosytosis – 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 Exocytosis Movement of substances out of cell Requires energy

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.

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