M3L3 - Biomembranes and Cell Architecture

Biomembranes Importance

• They define boundaries in a cell by seperating interior and exterior

  • Creatures a unique internal env

• They also define internal microenvs by surrounding organelles

Biomembranes Characteristics / Functions

• Selectively permeable: Few molecules can freely travel across 

• They contain proteins for cell signaling (receptors) and adhesion to other cells or the environment.

• Membranes are flexible and dynamic, letting them change shape for processes like movement and cell division. 

Biomembrane Terminology: Cells and Organelles

• Cells

  • Exoplasmic Face (Faces outside of cell) 

  • Cytosolic Face (Faces inside the cell) 

• Organelles

  • Cytosolic Face (Faces the cytosol of cell)

  • Lumenal Face (Faces inside the organelle) 

  • Intermembrane space

    • Space between 2 membranes inside organelle

    • Mitochondria has this 

Bilayer Structure of Biomembranes 

• Visualized using TEM 

• 2 thin parallel lines seen at cell surface indicates bilayer structure 

  • Row of polar groups facing inside/outside

• Hydrophobic tail (np) and polar headgroup (hydrophilic)

  • Hydrophobic core as tails face eachother in (aq) cell env

Phospholipids

• Amphipathic molecule (hydrophilic/phobic) 

• Spontaneously arrange in (aq) solution to form a micelle

  • bubble-like structure

  • Produced when single sheet of phopholipids assemble

  • Hydrophilic wall and hydrophobic core

• At higher [phospholipids], they spontaneously assemble to form bilayer

Chemical Makeup of Phospholipid

• A diglyceride has two fatty acids (long hydrocarbon chains with a carboxyl group) linked to glycerol.

• A phosphate group attaches to the third –OH of glycerol, forming a phospholipid, which often has additional charged groups on the phosphate.

• The fatty acid tails are hydrophobic (water-insoluble), while the phosphate head is hydrophilic (water-soluble).

• Because of this, phospholipids spontaneously form bilayers in water — their hydrophobic cores face inward and hydrophilic surfaces face outward, creating the lowest free-energy configuration.

Proteins associated with bilayer

• Integral membrane proteins (embedded in hydrophobic core)

• Lipid-anchored and peripheral membrane proteins

  • Associated with one surface of bilayer 

• The proteins associated determine the functinos of a membrane 

• Some membranes are dense w proteins 

  • Inner mitochondrial membrane with 76% protein composition 

• Some have very few proteins 

  • Myeline membrane with 18% protein composition 

Why is it good that the membrane is dynamic and fluid?

• Allows biomembranes to

  • fuse with one another 

  • deform without tearing 

  • change in shape to accompany any cell movement (ex cell division) 

• Proteins and phospholipids can move laterally through the membrane 

  • Proteins can cluster in membrane areas called microdomains

  • Helps perform specific cellular activity 

  • They can disperse after cell activity is complete

Fluid Mosaic Model

• Proposed by SJ singer and G nicolson

• Describes structural features of biomembranes

• Defined as fluid bc

  • membrane components more laterally or sidewars throughout 

  • Not solid, more fluid 

• Defined as mosaic bc

  • made of many different kinds of macromolecules 

• Fluidity is seen in both the outer and inner leaflets of membrane bilayer 

FRAP in determining membrane fluidity

• Microscopy labelling technique: Fluorescence recovery after photobleaching 

• Allows to track and measyre the fluidity of proteins in membrane 

Mechanism

• Protein can be tagged by adding an antibody or fusion to GFP

• The fluorescent molecule can be damaged by exposure to too much light

• They then become bleached and no longer fluoresce

Experiment

• At start, the proteins are evenly placed in membrane so fluoresce evenly

• A small patch is then bleached by a laser

• Depending on membrane fluididt, the patch can be recovered

  • The molecules dont regain ability to fluoresce but they just moved to disperse

FRAP: Calculating Fluidity

• y-axis: Fluorescence in arbitrary units

• x-axis: Time

• Initial fluorescence patch is 3000 units 

• Bleaching step reduces to 1000 units 

• Overtime, fluorescence inc as fluorescennt molecules move in while bleached ones move out 

• At 50s, the fluorescence of patch reaches 2000 units

  • about 50% of initial compared to when bleached

  • Suggests half proteins are mobile and able to move laterally but other half are immobile 

  • Other experiments, proteins may be less mobile or not at all (less fluid membrane)

  • If very fluid, it can go back up to 100%

Regulating Membrane Fluidity: Lipid Composition 

• Hydrophobic fatty acid tails can be saturated (no C=C bonds) 

  • Can pack together more closely, dec fluidity 

• They can be unsaturated (many C=C bonds) 

  • Kinked so more fluid 

• Long fatty acid chains (18C) 

  • Can pack together tightly to dec fluidity 

• Presence of cholesterol 

  • dec fluidity to maintain integrity 

  • without it, membrane would be too fluid and permeable 

  • At high [cholesterol], it helps seperate phospholipids so the fatty acid chains don’t come together and crystalize 

  • Good for hibernating animals 

  • Thus, it prevents extermes in mebrane fluidity (either too fluid or too gel-like)

Regulating Membrane Fluidity:  Temperature

• Low temp = dec fluidity

• high temp = more fluid

• Cells respond to temp by altering membrane makeup

  • High temp = add cholesterol to dec fluidity 

• Ex. Bacteria

  • Respond to low temp by cleaving fatty acid chains from 18C to 16C

  • OR 

  • They activate desaturase enzyme to introduce C=C

• Ex. Cold-tolerant plants

  • Have greater % of unsaturated fatty acid chains in phospholipids to prep for lower temp 

• Ex. Cold-blooded animals

  • incorporate more cholesterol into membrane in response to cold 

Lipid Rafts

• Demonstrates difference in fluidity within subregions of membrane 

• They’re a little taller than the rest of membrane due to longer fatty acid chain

• There’s higher [cholesterol] in this region

• These factors make the membrane in that area less fluid

  • The phospholipids/proteins are less mobile within this region 

  • The raft as a unit is mobile within the surroundings

  • It also doesn’t float on top of the membrane, but has 2 leaflets so it’s a part of it

Movement between leaflets

• Difficult to move proteins or phospholipids from one leaflet to another 

  • Phopholipid bc the hydrophilic head would have to go through the hydrophobic core 

• Thus proteins and phospholipds are places in correct orientation during synthesis 

• Enzyme flippases are capable of helping the flipping of phospholipids if needed

  • It’s possible but requires a lot of energy as well as the enzyme 

Integral Proteins: Single Pass 

• Leaves domains on both exterior and interior surfaces of membrane 

• Ex. glycophorin A found in human RBC 

  • It’s a homodimer 

  • (red) amino acids make up the hydrophobic alpha helix spanning the hydrophobic core

Integral Proteins: Multi Pass 

• Pass through membranes many different time

• Ex. Bacteriorhodopsin 

  • Uses 7 membrane spanning domain with 7 alpha helices 

  • They interact to form a transmembrane domain 

• Ion channels have this structure 

Integral Proteins: Beta Barrel  

• Ex. Formed from 16 beta strands

  • Exterior is hydrophobic so it can interact with hydrophobic membrane 

  • The interior is hydrophilic 

  • Forms hydrophilic pre through the hydrophobic membrance

• Ex. porins found in bacteria cells, chloroplasts and mitochondria 

Integral Proteins: Collection of Alpha Helices

• Shown in different colours in image 

• The exterior is hydrophobic while the interior is hydrophilic 

• This is an aquaporin that creates hydrophilic channel for water movement across cells during osmosis 

Lipid-Anchored Proteins

• Associated with one leaflet by covalently attached lipid modifications 

• During synthesis, they’re modified by acetylation 

  • 14C or 16C long acyl chains 

  • Attaches lipid anchor to the N-terminus

• And also prenylation 

  • 15C OR 20C long unsaturated chains 

  • Adds lipis anchor to C-terminus 

• Some lipid-anchored proteins have a structure called GPI anchor that forms hydrophobic anchor 

  • enables the association of a protein to membrane 

Peripheral Protein

• Peripheral proteins interact with membrane-embedded or anchored proteins, attaching indirectly to the membrane.

• Lipid-binding motifs let them bind to the polar head groups on the membrane surface.

• They often reversibly attach or detach through reversible modifications like phosphorylation or allosteric structural changes.