structure of biological membranes (11)

fluid mosaic model

a dynamic structure where lipids and proteins move laterally

• membrane is a mosaic of phospholipds, cholestorl, proteins and carbohydrates

biological membranes

• 8nm thick

• fluid mosaic model

• dynamic strucutres - remodel constantly

• differ in molecular composition across organelles, structures, organisms

• asymmetrical - outer and inner layer differ in composition

what kind of chemical interactions hold the membrane proteins embedded in the membrane

hydrophobic interactions

what kind of chemical interactions keep the proteins on the surface of the cell

• ionic bonds

what do lipids do in the membrane

• physical integrity of the membrane

  • creates a barrier to the rapid passage of hydrophilic materials like polar molecules and ions

what does the phospholipid bilayer do

• serves as a lipid “lake” in which a variety of proteins “ float”

what does long saturated fatty acid do

• allows for tighter packing

what does unsaturated fatty acid do

• make membrane less dense and more fluid

what is the general amount of lipids in a eukaryotic cell

• 60% of phospholipids

• 10% of sphingolipids

• 0.1-40 % sterol lipids

  • depends on the type of cell

  • 20-50% in animal cells

what do the protein inside the cell do

• can restrict the movement of proteins within a membrane

how does the fragments of membrane move

• in form of vesicles

• the endoplasmic reticulum (ER) to the Golgi apparatus, and from the golgi appartaus to the cell membrane

how does secondary lysosomes form 

• when primary lysosomes from the Golgi apparatus fuse with phagosomes 

glycolipid

• outside of the cell

• consists of a carbohydrate covalently bonded to a lipid

• The carbohydrate may serve as a recognition signal for interactions between cells

example - the carbohydrates on some glycolipid change when cells become cancerous, may allow white blood cells to target cancer cells

glycoprotein

• consists of one or more short carbohydrate chains covalently bonded to a protein

• bound carohydrates are oligosaccaharides

• cell recognition and adhesion

• only on the outer side of the membrane

proteoglycan

• more heavily glycosylated protein

• more carbohydrate molecules attached to it

• longer carbohydrate chains

• cell recognition and adhesion

• only on the outer side of the membrane

monosaccharides

• single carbohydrates

• containing 5 or 6 carbons in a ring structure

• can bond with one another in various configurations

  • form linear or branched oligosaccharides

how do the hydrophobic and hydrophilic regions of phospholipids form a membrane bilayer

• hydrophilic “ heads” of fatty acids are the polar ends

• hydrophobic “tails” are the nonpolar region

• heads tend to associate with water molecules and tails away from water

integral membrane protein

• at least partially embedded in the phospholipid bilayer by hydrophobic interactions with the lipid interior

• must have amino acids with hydrophobic R groups to insert into the nonpolar fatty acid tail region 

• span across the entire membrane

• transport, signaling and structural anchoring

peripheral membrane protein

• proteins loosely attached to the membrane surface via non-covalent interactions

• lack surface hydrophobic group ( do not penetrate the phospholipid bilayer)

• have polar or charged regions that interact with exposed membrane proteins or polar heads of phospholipids

• both sides of the membrane ( different composition)

• signaling and cytoskeletal support

how does membrane and proteins mostly interact with each other

• non covalently

what affects membrane fluidity

• lipid composition - how tightly the lipids are packed ( shorter , unsaturated fatty acid = more fluid)

• temperature - fluidity decreases at reduced temperature( some organisms would change the lipid compositions, changing long chains with short ones,insulation)

anchored membrane proteins

• covalently attached to fatty acids or other lipid groups

• hydrophobic lipid groups insert into the phospholipid bilayer

• signaling or localization

example - phospholipase c - intracellular signaling

transmembrane proteins

an integral membrane protein that spans the phospholipid bilyaer

how does lipid composition vary across organelles

• mitochondria have low choleserol

• plasma memraben are rich in sterols

• ER favors phosphatidylcoline

• GA has sphinoglipds

how does lipid composition change with physicological state

• during starvation, yeast cells increase triacyglycerols’s (TAGs) to store energy and maintain viability

what lipid changes help cells adapt to low temperature

• increase unsaturated fatty acids and shorter tails to maintain fluidity

  • decrease cholesterol

what lipid changes help cells adapt to high temperature

• increase saturated fatty acids and longer tails for stability

• increase cholesterol to prevent excess fluidity

what is the cholesterol’s role in membrane fluidity

• acts as a bidirectional regulator

• stabilizes membranes at high temperatures

• prevents rigidity at low temperature

where is cholesterol most abundant in the cell

• over 90% is in the plasma membrane

• absent in bacterial membranes and low in mitochondria

  • plants have other sterols

what does cholesterol prefer to associate with

saturated fatty acids

amphipathic molecules

• hydroxyl group aligns with the polar heads of phospholipids

• four rings integrated within fatty acid chains of phospholipds

do membrane proteins move freely

• some diffuse laterally; others are anchored to cytoskeletal structures or localized to specific membrane domains

how do phospholipids move in the membrane

• they diffuse laterally and occasionally flip-flop between leaflets via enzymes

why is phospholipid mobility important

• enables membrane remodeling 

• vesicle formation

• receptor clustering during immune response

cell recognition

• one cell specifically recognizes and binds to another cell of a certain type

ex. sponges - species-specific identification

cell adhesion

• the binding of one cell to another

• often by noncovalent forces

• connection between the two cells is strengthened

what molecules are responsible for cell recognition and adhesion in sponges

proteoglycans

• often 80% of carbonhydrates that carries two kinds of carbo

what is the ratio of proteins to lipid

• differ among different cells and cellular organelles

• 1:25 typical cell membrane

• 1:15 inner mitochondrial membrane ( electron transport chain and ATp syntehsis)

• 1:70 in myelin ( some neurons, insulation, saltatory condution)

how id peripheral protein distributed

• only on one side of the membrane

• two sides of the membrane differ in composition of peripheral proteins

what is found only on the outer side of the membrane

• glycolipids, glycoproteins, proteoglycans

what is homotypic binding

• pertaining to the adhesion of cells of the same type

• Usually the binding of cells in a tissue, same molecules from both cells

• The same adhesion molecules bind

• skin tissue

what is heterotypic binding

• between different molecules on different cells 

• ex. sperm and egg

what are cell junctions

• link animal cells together

• specialized structures associated with the cell membranes of epithelial

• layers of cells that line boyd cavities or surfaces

what is tight junction

• prevents substances from moving through the spaces between cells

• no gaps

  • example - cell lining in the bladder so urine can’t leak

what is desmosomes

• hold neighboring cells firmly together, acting like spot welds or rivets

• Materials can still move around in the extracellular matrix

• provides mechanical stability for tissues like skin

what is gap junctions

• channels that run between membrane pores in adjacent cells, allowing substances to pass through cells

• example - heart, the gap junction allows rapid spread of electric current so the muscle cells beat in unison