Topic 2 - Learning Objectives - Membrane Transport & Potential

Define amphipathic and explain why phospholipids are amphipathic

• amphipathic - molecules that contains hydrophobic and hydrophilic regions

• a phospholipid contains:

  • a hydrophilic head - phosphate glycerol head

  • a hydrophobic tail - fatty acid tail

Explain why membranes are describes as fluid mosaic models

• model stats that a membrane is a fluid structure with various proteins embedded in it

List two classes of phospholipids in animal cell membranes

• phosphoglycerides - most abundant in animal cell membranes

• Sphingolipids

Describe and identify general chemical structure of a phosphoglyceride

    where a phosphate group links with a polar head group

• phosphatidylethanolamine - a phosphate group links to a ethanolamine

• phosphatidylcholine - phosphate group links to a choline

• phosphatidylserine - phosphate group links to a serine

• phosphatidylinositol - phosphate group links to inositol

Describe and identify general chemical structure of a sphingolipid

• ceramides

• sphingomyelin

• glycosphingolipids

Differentiate between general chemical structure of a phosphoglyceride and a sphingolipid

• sphingolipids are chemically different but have similar 3D shape to phosphoglycerides

• sphingolipids have:

  • sphingosine as backbone - instead of glycerol

    • sphingosine - an 18-carbon amino alcohol with an unsaturated hydrocarbon chain

    • one fatty acid chain - instead of two

    • fatty acid chain is bound to sphingospine with an amide bond - instead of an ester bond

    • highly enriched in cell membranes of the brain and nervous tissue

List (and identify the chemical structure of) the three types of sphingolipids

• ceramides (simplest type) - where one fatty acid chain is attached sphingosine with an amide bond

• sphingomyelin - ceramide bound with either phosphocholine/phosphoethanolamine

• glycosphingolipids - cereamides with one or more sugar residues

List (and identify the chemical structure of) the two common types of glycosphingolipids

• types of glycosphingolipids:

  • cerebrosides - single glucose/galactose as sugar residues

  • gangliosides - at least three sugar as residues (one sugar must be sialic acid)

List and describe factors that can increase and decrease membrane fluidity

• temperature - when temperature increases, membrane moves from a solid gel state to a more fluid state

• fatty acid chains - shorter fatty acid chains are more fluid than those with longer fatty acid chains

  • shorter fatty acid chains means less surface area to allow for stabilizing van der Waal’s/hydrophobic interactions that occur between fatty acid chains of neighbouring phospholipid molecules

• unsaturated fatty acid chains - membrane rich in unsaturated fatty acid chains are more fluid than those rich in saturated fatty acids

  • the kinds (bends) in unsaturated fatty acids prevent tight packing between phospholipids

Explain how cholesterol affects membrane fluidity

• cholesterol acts as a fluidity buffer against temperature extremes

  • helping to maintain membrane integrity by preventing it from becoming too rigid in cold temperatures and too fluid in warm temperatures.

    • at warm temperatures - cholesterol restrains movement of phospholipids (cholesterol acts to decrease membrane fluidity)

    • at cold temperatures - cholesterol prevents tight packing between phospholipids (cholesterol acts to increase membrane fluidity)

*an increase in cholesterol decreases the membrane permeability - cholesterol limits the phospholipid’s movement and interaction

Describe lipid rafts

• identify the components that are enriched in the structure of lipid rafts

  • cholesterol

  • sphingomyelin

  • gangliosides

  • phosphoglycerides with saturated fatty acid chains

• explain why lipid rafts have reduced fluidity

  • due to all the components in the structure, they are tightly packed together which minimizes movement = fluidity

• list examples of functions associated with lipid rafts

  • serves as an organizing centre for the assembly of signalling molecules

  • recruits actin cytoskeleton - allows for further stabilization of lipid raft

  • facilitates formation of transport vesicles

List the 6 major functions of membrane proteins

• transport

• enzymatic activity

• signal transduction

• cell-cell recognition

• intercellular joining

• attachment to the cytoskeleton and extracellular matrix

With respect to plasma membrane asymmetry

• identify primary location (inner or outer leaflet of glycolipids)

  • glycolipids/glycosphingolipids - outer leaflet

• identify primary location (inner/outer leaflet) of the 4 phosphoglycerides (PS,PI,PE,PC)

  • PS/PE/PI - inner leaflet

  • PC - outer leaflet (can also be found on inner leaflet, but are predominantly found on the outer leaflet)

• state the two that have a net negative charge at physiological pH

  • PS/PI have a net negative charge

• identify primary location of sphingomyelin

  • sphingomyelin - outer leaflet

List and explain important reasons for maintaining plasma membrane asymmetry

• to create surface potential difference across membrane

  • can interact with positively-charged amino acid residues of membrane-bound proteins - keeps the proteins anchored to the membrane

  • peripheral membrane proteins (ie. proteins for cell signalling) contain positively-charged residues that can associate with negatively charged PS/PI

• creates curvature within the membrane

• preservation of cell viability

From the following list (PC, PE, PI, PS and the Sphingomyelin) state which are cylindrical shaped phospholipids and which are conical-shaped phospholipids

• Cylindrical-shaped phospholipids:

  • Phosphatidylcholine (PC)

  • Sphingomyelin

  • Phosphatidylinositol (PI)

  • Phosphatidylserine (PS)

• Conical-shaped phospholipids:

  • Phosphatidylethanolamine (PE)

Define concentration gradient, electrical gradient and electrochemical gradient

• concentration gradients - differential concentrations of a substance across a space or a membrane

• electrical gradient - a difference of charge across plasma membrane

• electrochemical gradient - the combined gradients of concentration and electrical charge that affects an ion

List two types of passive transport and provide examples of molecules that use each type of transport

• simple diffusion - where no transport proteins are needed

  • examples:

    • gases

    • small uncharged polar molecules (urea, water, and ethanol)

• facilitated diffusion - transport proteins speed passive movement of molecules across the plasma membrane (ideal for polar molecules + charged ions)

List an define three main types of transport proteins that carry out facilitated diffusion

• porins

  • large / barrel shaped

  • nonselective

  • moves hydrophilic molecules across the membrane based on size rather than charge

• permeases

  • transports specific molecules across a membrane by binding to the molecule and undergoes conformational changes

  • permease can become saturated when all of the binding sites are occupied by molecules (leads to max transport rate)

• ion channels

  • forms smaller pores where only specific ions may pass based on size and charge

  • typically gated

  • channels are specific to one/somtimes two ions

List and define the 5 types of ion channel proteins

• gated ion channels

  • ligand-gated channels - open when specific regulatory molecules bind

  • voltage-gated channels - open/closes in response to different membrane potentials

  • mechanogated channels - regulated through interactions with subcellular proteins that make up the cytoskeleton

  • signal gated channels - opens/closes in response to a specific intracellular molecule

• non-gated ion channels

  • leak channels - no trigger is required for their opening/closing

    • intrinsic rate of switching between open and closed states

Differentiate between primary and secondary active transport

• primary active transport - involves permease carrier proteins that use an exergonic reaction (ex. ATP hydrolysis) to provide the energy to transport the molecule

• secondary active transport - involves permease carrier proteins that move one molecule down its electrochemical gradient to help move another molecule against its concentration gradient

  • doesn’t use ATP directly - instead uses electrochemical energy

List the types of primary active transporters (provide specific examples for each)

• P-type ATPases

  • Na+/K+ ATPases

  • Ca2+ ATPases

• V-type ATPases

  • H+ ATPases

• F-type ATPases

  • mitochondrial ATP synthase

• ABC (ATP-binding cassette)

  • multi-drug resistance proteins - p-glycoproteins

Define uniporter, antiporter (exchanger), symporter (co-transporter)

• uniporter (primary active transport) - a carrier that moves one specific ion/molecule

• antiporter (secondary active transport) - a carrier that involved moving two different ions/molecules in opposite directions

• symporter (secondary active transport) - a carrier that involves moving two different ions/molecules in the same directions

List examples of secondary active transporters that are symporters and antiporters

• symporter

  • sodium glucose contrasporter protein

• antiporter

  • sodium-calcium exchanger

Define Osmosis

• the passive transport of water across a selectively permeable membrane

Define osmolarity

• refers to number of particles in a solution

  • osmolarity takes into consideration of the TOTAL number of solutes (permeable and impermeable)

    • iso-osmotic - solutions are equal in osmolarity → no net osmosis occurs between them if they are separated by a selectively permeable membrane

    • hypo-osmotic - osmolarity in a solution is lower than that of another solution

    • hyper-osmotic - osmolarity solution is greater than that of another solution

Determine dissociation coefficient of a particular solute in a solution + calculate osmolarity

• equation - osmolarity = sum of each solute (molarity x n)

  • where n is the coefficient of dissociation

Define tonicity

• refers to what happens to a cell when placed in a solution

  • isotonic - solution with same concentration of non-penetrating solutes as that found in cells

  • hypertonic - solution with higher concentration of non-penetrating solutes than that found in cells

  • hypotonic - solution with lower concentration of non-penetrating solutes than that found in cells

Differentiate penetrating and non-penetrating solutes

• penetrating solutes - substances that are permeable to the cell → can pass through the membrane

• non-penetrating cells - substances that are not permeable to the cell → cannot pass through the membrane

Differentiate between osmolarity and tonicity

• osmolarity - takes in consideration the total number of solutes (both permeable and impermeable)

• tonicity - takes into account concentration only relating to impermeable solutes only

  • typically penetrating solutes will reach equilibrium → doesn’t effect tonicity