Cell Bio - Chapter 7

barriers between aqueous compartments

membranes

spontaneously form bilayers with hydrophilic faces and a hydrophobic core

amphipathic phospholipids

vary in lipid composition

biological membranes

impermeable to water soluble molecules and ions

biological membranes

have a viscous consistency with fluidlike properties

biological membranes

the outer boundary of the cell that separates it from the world is a thin, fragile structure about 5-10 nm thick

plasma membrane

not detectable with light microscope, need electron microscope

plasma membrane

the 2 dark staining layers in the electron micrographs correspond primarily to the inner and outer polar surfaces of the bilayer

plasma membrane

whether from plants, animals, or microorganisms, have the same ultrastructure

plasma membrane

membranes form continuous sheets that enclose intracellular compartments

compartmentalization

membranes provide a framework that organizes enzymes for effective interaction

scaffold for biochemical activities

Membranes allow regulated exchange of substances between compartments.

selectively permeable barrier

•Membrane proteins facilitate the movement of substances between compartments.

**Transporting solutes**

•Membrane receptors transduce signals from outside the cell in response to specific ligands.

**Responding to external signals**

•Membranes mediate recognition and interaction between adjacent cells.

intracellular interaction

•Membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy.

energy transduction

defines the cell and separates the cytosol from the extracellular environment

plasma membrane

Noncovalent interactions between phospholipids, and between phospholipids and proteins provide membrane

integrity and resilience

•Individual phospholipids spin and diffuse ___ within the plane of the membrane.

laterally

hydrophobic core prevents unassisted movement of water-soluble substances from one side to the other.

barrier

membrane proteins provide each cellular membrane its unique set of functions.

proteins

span the bilayer and often form dimers and higher-order oligomers.

**Integral membrane proteins**

•tethered to one leaflet by a covalently attached hydrocarbon chain

lipid-anchored proteins

•associate primarily by specific noncovalent interactions with integral membrane proteins or membrane lipids \[include cytoskeletal proteins\]

peripheral proteins

•Enables organelles to assume their typical shapes

membrane fluidity and flexibility

•Provides dynamic property that enables membrane budding and fusion

membrane fluidity and flexibility

ends have different chemical properties

amphipathic molecules

(“water fearing”) fatty acid-based (fatty acyl) hydrocarbon “tail” – partitions away from water

hydrophobic

(“water loving”) polar “head group” – interacts with water molecules

hydrophillic

The most energetically favored orientation for polar head groups is

facing the aqueous compartments outside of the bilayer

Lipid composition can influence the activity of membrane proteins and determine

the physical state of the membrane

The cohesion of bilayers to form a continuous sheet makes cells deformable and facilitates

splitting and fusion of membranes.

Phospholipid structures form spontaneously, driven by

behavior of hydrophilic and hydrophobic end exposure to water:

close packing stabilized by van der Waals and hydrophobic effects interactions between the hydrocarbon chains

nonpolar tails

ionic and hydrogen bonds stabilize  interactions with each other and with water

polar head groups

Polar groups face outward to shield the hydrophobic fatty acyl tails from

water

Hydrophobic effect and van der Waals interactions between the fatty acyl tails drive the assembly of the

bilayer

High concentrations of fatty acids in water form

micelles

hydrophobic interior composed entirely of fatty acyl chains – will not accommodate biomembrane phospholipids with two tails

micelles

High concentrations of phospholipids in water form

bilayers

cannot spontaneously form into the lipid bilayer

triglycerides

Although triglycerides are weakly amphipathic the glycerol backbone is not

ionized

carry a charged phosphate and some polar head groups can also carry a full charge

phosphoglycerides

very amphipathic and are stabilized by the energy gained from burying hydrophobic groups out of contact with the water.

phosphoglycerides

a single bilayer that encloses the cell

plasma membrane

•Bounded by single membranes.

•Internal aqueous space – topologically equivalent to the outside of the cell

vesicles and some organelles

•Enclosed by two membranes separated by a small intermembrane space

•Exoplasmic faces of the inner and outer membranes border the intermembrane space

nucleus, mitochondrion, and chloroplast organelles

•Plasma membrane segment – buds inward toward the cytosol and eventually pinches off a separate vesicle

endocytosis

remains facing the cytosol

remains facing they cytosol

•– faces vesicle lumen

exoplasmic face

An intracellular vesicle fuses with the plasma membrane

exocytosis

Vesicle lumen connects with the

extracellular medium

Cytoplasmic face remains facing

Cytoplasm

Membrane-spanning proteins retain during vesicle budding and fusion; same protein segment(s) always faces the cytosol.

asymmetric orientation

The lipid and protein components are bound together by

non covalent bonds

Membranes also contain

carbohydrates

membrane lipids are

amphipathic

\-diacylglycerides with small functional head groups linked to the glycerol backbone by phosphate ester bonds.

phosphoglycerides

**:** ceramides formed by the attachment of sphingosine to fatty acids

sphingolipids

smaller and less amphipathic lipid that is only found in animals

sterols

•Glycerol backbone.

•Tails – two esterified hydrophobic fatty acyl chains

•vary in length (commonly 16 or 18 C)

•vary in saturation – saturated (no double bonds) or unsaturated (one, two, or three double bonds)

•Head – a polar group esterified to the phosphate

phosphoglycerides

•4 major head groups – phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI).

phosphoglycerides

one fatty acyl chain attached to glycerol by an ester linkage and one attached by an ether linkage and the same head groups as other phosphoglycerides

plasmalogens

•Derivatives of sphingosine (an amino alcohol with a long hydrocarbon chain)

•Various fatty acyl chains connected by an amide bond

•Sphingomyelins (SM) – contain a phosphocholine head group

•Some sphingolipids are glycolipids that contain a single sugar residue or branched oligosaccharide attached to the sphingosine backbone. Glucosylcerebroside (GlcCer) has a glucose head group.

sphingolipids

•Membrane components – animals (cholesterol), fungi (ergosterol), and plants (stigmasterol)

•Can be very prevalent in membranes (e.g., up to 50% of animal membrane lipids)

•Amphipathic structure:

•head group – single polar hydroxyl

•tail – conjugated four-ring hydrocarbon and short hydrocarbon chain

sterols

•Membrane lipids exist in _____ consistency depending on temperature, lipid composition and saturation in the presence of cholesterol.

gel or fluidlike

required (“liquid-crystal”) for optimal membrane performance

fluidity

•Depends on length and degree of unsaturation in fatty acyl chains

–Kinks and bends that prevent regular packing

results in lower transition temperature

•Van der Waals interactions and the hydrophobic effect cause the nonpolar tails of phospholipids to aggregate.

•Favored by longer, more saturated fatty acyl chains that pack tightly together.

gel like state

•Phospholipids with short fatty acyl chains form fewer van der Waals interactions.

Cis-unsaturation forms kinks in fatty acyl chains; kinked tails form fewer and less stable van der Waals interactions with other lipids

more fluid state

–Membrane fluidity makes it possible for proteins and lipid components to _ __in the membrane and for membranes to assemble and grow__ .

move

several micrometers per second. (Diffusion rates indicate bilayer is 100 times more viscous than water – \~olive oil viscosity.)

lateral diffusion

high energetic barrier to moving hydrophilic head through membrane hydrophobic core

flip from leaflet to leaflet rarely

–Organisms (other than birds and mammals) maintain membrane fluidity as temperature changes by altering the composition of

membrane lipids

Remodeling lipid bilayers involves \*\*\* of acyl chains and replacement of acyl chains by *phospholipases* or *acyltransferases*

saturation or desaturation

Increase or decrease the number of double bonds in fatty acyl chains

saturases

Reshuffle chains to give different packing, split the fatty acid from the glycerol backbone

phospholipases

transfer the fatty acids to different phospholipid

acyltransferases

–Prevents regular packing of saturated fatty acyl chains

cholesterol

•Tends to abolish sharp transition temperature

cholesterol

•In the absence of cholesterol, membranes would crystallize at physiological temperatures

cholesterol

–Enhances mechanical rigidity of the membrane while preserving the fluidity

cholesterol

Research on phagocytosis and cell signaling suggest that cells can adjust the fluidity in certain regions of the membrane

lipid rafts

•Decrease in saturated fatty acids at lower temps and increase in saturated fatty acids at higher temps

homeoviscous adaptation

give membranes the ability to fuse, form networks, and separate charge.

lipids

Phospholipid distribution asymmetry reflects where lipids are synthesized in the

endoplasmic reticulum and golgi

Sphingomyelin is synthesized on the \*(exoplasmic) face of the Golgi, which becomes the exoplasmic face of the plasma membrane

luminal

Phosphoglycerides are synthesized on the

ER cytosolic face

Inner and outer membrane leaflets have different

lipid compositions

–Provides different physico-chemical properties appropriate for different interactions

membrane lipids

move easily within a leaflet but only rarely “flip-flop”

membrane lipids

•Influences bilayer thickness – influences membrane protein distribution

lipid composition

•Thicker than phosphoglyceride (phosphatidylcholine) bilayer

pure sphingomyelin bilayers

•Cholesterol lipid-ordering effect increases phosphoglyceride bilayer thickness. \[Lipid rafts are thicker than other membrane regions.\]

pure sphingomyelin bilayer

–Outer leaflet of plasma membrane contains specialized regions, –Provide a favorable environment for cell-surface receptors and GPI-anchored proteins.

lipid rafts

–Cholesterol and sphingolipids tend to pack together to form highly ordered microdomains forming ***** that float within the more fluid and disordered environment.

lipid rafts

– forms essentially flat monolayers

PC cylindrical shape

– forms curved monolayers

PE conical shape

•Bilayer enriched with PC in the \* leaflet and with PE in the \*(as in many plasma membranes) – natural curvature

exoplasmic, cystolic face

•Biological membranes contain integral, lipid-anchored, and peripheral

proteins