CELLULAR MEMBRANES Feb 11, 2015

Membrane Structure Membranes are composed of phospholipids Arranged in a bi-layer Proteins inserted in the lipid bilayer Fluid mosiac model – proteins float in or on the fluid lipid bi-layer like boats on a pond Fluid Mosaic Model

Four Components of Cellular Membranes Phospholipid bilayer Flexible matrix, barrier to permeability Transmembrane proteins (span the membrane) Integral membrane proteins Interior protein network (inner surface of the membrane) Peripheral membrane proteins Cell surface markers Glycoproteins and glycolipids

Phospholipids Components of phospholipids 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 (tails) are on the inside Phosphate groups (heads) are on both surfaces

Environment Influences on the Membrane 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 colder temperatures

Membrane Protein Functions Transporters Enzymes Cell-surface receptors Cell-surface identity markers Cell-to-cell adhesion proteins Attachments to the cytoskeleton

Integral Membrane Proteins Transmembrane Proteins Span the lipid bilayer Non-polar 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 or β-sheets

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

PORES Extensive non-polar 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 where 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

Hydrophobic interior (fatty acids) of membrane repels polar molecules Non-polar 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. Molecules pass through without binding Carrier proteins Bind molecules that pass through the pore

Channel Proteins Ion channels Allow the passage of ions (cations and anions) Gated channels – open or close in response to stimulus (chemical or electrical) 3 conditions determine direction of flow 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 Higher concentration to lower concentration Must bind to the molecule they transport Saturation – rate of transport limited by number of transporters

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

Osmotic Concentration When two solutions have different osmotic concentrations, one solution is hypertonic and the other solution is hypotonic Hypertonic solutions have a higher solute concentration Hypotonic solutions have a lower solute concentration When two solutions have the same osmotic concentration, the solutions are isotonic Aquaporins—proteins that transport water across the membrane. Facilitates osmosis.

Osmotic Pressure Force needed to stop osmotic flow Cell in a hypotonic solution gains water causing cell to swell and creates pressure on cell membrane If membrane strong enough, cell reaches counterbalance of osmotic pressure Cell wall of prokaryotes, fungi, plants, protists (turgor pressure) aids in pressure counterbalance If membrane is not strong the cell will burst (cytolysis) Animal cells must be in isotonic environments

Plasmolysis Cytolysis

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 areas of low concentration to areas of high concentration “Against concentration gradient” 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

Coupled Transport Uses ATP (energy) indirectly Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule Symporter is used

Bulk Transport Exocytosis Both mechanisms require energy Endocytosis Movement of substances into the cell Phagocytosis – cell takes in particulate matter (cell eating) Pinocytosis – cell takes in only fluid (cell drinking) Receptor-mediated endocytosis – specific molecules are taken in after they bind to their receptor on cell surface Exocytosis Movement of substances out of cell Both mechanisms require energy

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