unit 1 topic 3: membrane structure & function

membrane composition: membrane lipids

·       Membrane lipids are amphipathic and insulate electrical charge

-        Hydrophobic core creates barrier on sides of water environment and ionic charges

-        Good barrier

-        Hydrophobic core represented by FA chains

-        Hydrophilic head toward water environment

·       smooth ER produces membrane lipids.

-nothing codes for FA, made in smooth ER.

-main lipid involved in membrane lipids are phospholipids

·       Phospholipids have phosphate, glycerol, & 2FAs ( nonpolar, hydrophobic)_.

·       Longer chains = more rigid structure, high SA, more durable

o   Spontaneously forms barriers.

o   Saturated fatty acids = more tightly packed

o   Unsaturated fatty acids = more loosely packed

-kinks prevent other phospholipids from packing as orderly

membrane composition: sphingolipids

∙         Sphingolipids have sphingosine, 1 FA, and something else

∙         Insulates better than most phospholipids so enriched in neurons

∙         More sturdy and rigid compared to phospholipids, good for less permeability

∙         Higher compactness in membrane

∙         Ex: in nervous tissue and things that insulate neurons

∙         Insulates cell

∙         Cholesterol is a multifunctional sterol (4 fused rings).

∙         Increased cholesterol = increased stability at warm temperatures and increased fluidity at low temperatures

Cholesterol is like a "fluidity buffer" in membranes — it helps keep the membrane stable but flexible across different temperatures.

• At warm (high) temperatures: Cholesterol stabilizes the membrane by restraining movement of phospholipids, preventing the membrane from becoming too fluid or leaky.

• At cool (low) temperatures: Cholesterol prevents tight packing of phospholipids, so it keeps the membrane from becoming too rigid or solid.

∙         Only sterol in membranes

∙         At warm temperatures -> stabilizes membrane to not be too lose or permeable

∙         At low temperatures -> make less rigid and more fluid

∙         Has hydrophilic end -OH ; polar

membrane composition: membrane proteins

∙         can perform a number of functions and occupy multiple locations in the membrane.

∙         -integral proteins (imbedded in membrane, transmembrane span top to bottom)

∙         Whatever part imbedded in membrane is np amino acids

∙         Intermembrane area composed of np membranes

∙         Polar at hydrophilic ends on outside

∙         -peripheral protein on the outside of membrane ( not entirely)

ER will attach lipid to protein ( lipoprotein), FA chain act as anchor

-glycolipids; good for identifying surface antigens

∙         Integral and transmembrane use nonpolar amino acids in a-helices and b-sheets to embed in membrane interior.

∙         Peripheral proteins anchored by lipid chains form lipoproteins

disrupting membrane proteins

∙         Phospholipases, common in snake venom, digest membranes.

-snakes put enzymes in prey before they eat them

-digestive enzymes; phospholipase is enzyme that works on phospholipids

∙         Tay Sachs Disease occurs from mutation in hexosaminidase A (Hex-A) that allows sphingolipid to accumulate to deadly levels in nervous tissue.

-usually help to break sphingolipids ( which generally accumulate in nervous tissue, harder to penetrate) is mutated

Membrane function

∙         Fluid Mosaic Model emphasizes the consistency and composition of membranes

∙         Membrane fluidity affects permeability

∙         Membrane composition affects function and shape of membrane

∙         Mosaic : composed of many things

∙         More permeable = more fluid for gases, ions, etc.

∙         To generate control of whats coming in/out and at what rates, inside, and outer top/bottom layer all look different

∙         Membranes allow internal and external environments to vary.

∙         Faces or leaflets of membrane are asymmetrical

∙         Proteins are often localized whereas lipids are much more mobile

∙         Lateral movement is easy for lipids but flipping is rare.  Maintains shape. ( not usually go from top to bottom)

∙         Proteins connected to cytoskeleton ( if internal side of membrane ) or extracellular matrix ( if on outside of membrane)

∙         Proteins do NOT move as much as lipids; create top and bottom orientations of membrane

∙         Most proteins attached to membrane

disrupting membrane function

∙         Hereditary spherocytosis caused by mutation in cytoskeletal proteins and/or peripheral proteins. ( red blood cells become beach-ball shaped, not good for cell.)

∙         Improper binding between cytoskeleton and membrane proteins disrupts red blood cell shape and increases fragility.

∙         Less gas exchange

∙         More tension on membrane; easy to pop

∙         Disrupt function of RBCs

∙         Can cause infection + sepsis

∙         Treatment : remove spleen and blood transfusions

∙         Melittin, a component of bee venom, creates pores in membranes.

-forms holes in membrane : make it more permeable , which can cause cell lysis

-don’t break phsopholipids for digestion but to kill