Biology Quiz: Membrane Structures and Functions

Biochemistry I (CHM219)

• Course instructor: Dr. Esra Aydemir

Lipids, Membranes, and Cellular Transport

• Lipids are generally insoluble in water

• Lipids can form structures like micelles, vesicles, and bilayers to become water-soluble

• Amphipathic lipid molecules have both hydrophilic and hydrophobic regions

• A simplified representation of an amphipathic lipid molecule shows a polar head group (hydrophilic) and a hydrocarbon tail (hydrophobic)

The Molecular Structure and Behavior of Lipids

• Fatty acids are the simplest lipids
• Their structure consists of a hydrophilic carboxylate group at one end, and a hydrocarbon chain at the other (typically 12-24 carbons)
• Stearic acid (C18) is an example of a saturated fatty acid (all carbons saturated with hydrogen)
• Oleic acid (C18) is an example of an unsaturated fatty acid (contains double bonds)
• Many naturally occurring fatty acids are unsaturated, meaning they have one or more double bonds in their hydrocarbon chains

Structures of the Ionized Forms of Some Representative Fatty Acids

• The figures show the ionized forms of stearic and oleic acids
• Stearate and oleate ions, have a polar head group and a long hydrocarbon tail

Membrane Lipids

• Membrane lipids are amphipathic
• They tend to form monolayers, bilayers, micelles, or vesicles in contact with water
• Most naturally occurring fatty acids contain an even number of carbon atoms
• If double bonds (unsaturation) are present, they are usually cis

Some Biologically Important Fatty Acids

• Tables of saturated and unsaturated fatty acids display their systematic names, abbreviations, structures and melting points

Fats or Triacylglycerols

• Fats, or triacylglycerols, are triesters of fatty acids and glycerol
• They are major long-term energy storage molecules in many organisms

Fat Storage in Animals

• Fat storage in animals serves three distinct functions: Energy production, Heat production, and Insulation

Lipid Composition of Some Biological Membranes

• A table shows the percentage of different lipids (e.g., saturated, unsaturated) in various natural fats

Adipocytes

• Adipocytes, are animal fat storage cells
• The designations MFC and VSFC correspond to "mature fat cell" and "very small fat cell" respectively

Saponification

• When fats are hydrolyzed with strong bases (e.g., NaOH or KOH, wood ashes historically), soap is produced.
• This process is called saponification
• Fatty acids are released as sodium or potassium salts

Synthetic Detergents

• Synthetic detergents do not have the same defect as soaps
• Sodium dodecyl sulfate (SDS) is an example of this class, with salts of dodecyl sulfate being more soluble with divalent cations
• SDS is widely used in forming micelles around proteins for gel electrophoresis
• Triton X-100 is another example of synthetic nonionic detergent

Waxes

• Waxes are formed by the esterification of fatty acids and long-chain alcohols
• They are completely water insoluble and often serve as water repellents.

Classes of Membrane-Forming Lipids

• Four major classes of membrane-forming lipids exist:
  • Glycerophospholipids
  • Sphingolipids
  • Glycosphingolipids
  • Glycoglycerolipids

Phospholipids and Membrane Structure

• Fatty acids are wedge-shaped and tend to form spherical micelles
• Phospholipids are more cylindrical and pack together to form a bilayer structure

Stereochemistry of Glycerophospholipids

• Glycerol is a prochiral molecule
• Phosphorylation of one CH₂OH group or the other gives the R- or S- enantiomer of glycerol phosphate
• The same molecule can be called L-glycerol-3-phosphate or D-glycerol-1-phosphate

Glycerophospholipid Structure

• R₃ is a hydrophilic group

Common Glycerophospholipid Hydrophilic Groups

Lipid Composition of Some Biological Membranes

• A table shows the percentage of different lipids (e.g., phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol, phosphatidylserine, sphingomyelin, glycolipids, cholesterol) in various membranes (human erythrocyte plasma membrane, rat liver plasma membrane, pg platelet plasma membrane)

Sphingolipids

• Sphingolipids are another class of membrane lipids based on the amino alcohol sphingosine, rather than glycerol
• They have a long-chain hydrophobic tail, requiring the addition of only one fatty acid
• Ceramides are a class of sphingolipids linked via an amide bond to the NH2 group

Sphingomyelin

• An important example of sphingolipid is sphingomyelin, in which a phosphocholine group is attached to the C-3 hydroxyl group

Glycosphingolipids

• Glycosphingolipids result from sphingosine attaching to a sugar
• Cerebrosides and Gangliosides are examples of glycosphingolipids, important parts of brain cell membranes

Glycoglycerolipids

• Another less common class of lipids, particularly in plant and bacterial membranes, are glycoglycerolipids, especially the monogalactosyl diglycerides.

Cholesterol

• Cholesterol is a component of many animal membranes, influencing membrane fluidity with its bulky structure

Fluid Mosaic Model

• Membranes are a fluid mixture of lipids and proteins.

• Unsaturated phospholipids increase fluidity.

• Peripheral membrane proteins are associated with one side of the bilayer

• Integral membrane proteins are more deeply embedded in the bilayer

Structure of a Typical Cell Membrane

• Proteins are embedded in and on the phospholipid bilayer.
• Some are glycoproteins that carry oligosaccharide chains.
• The membrane is about 6 nm thick
• Most membranes are more densely packed with proteins than shown

Experimental Demonstration of Membrane Fluidity (FRAP)

• When cells with surface membrane protein marked by fluorescent tags are induced to fuse, the proteins gradually mix over the fused surface

Protein, Lipid, and Carbohydrate Content of Some Membranes

• A table shows the percentage by weight of proteins, lipids, and carbohydrates in different membrane types (myelin, human erythrocyte plasma membrane, bovine retinal rod, mitochondria outer membrane, amoeba plasma membrane, sarcoplasmic reticulum, chloroplast lamellae, gram-positive bacteria and mitochondria inner membrane.)

The Transition Temperature in a Synthetic Lipid Bilayer

• The transition temperature (Tm) depends on lipid composition
• Lipids with longer, saturated tails tend to increase Tm
• Those with more cis double bonds and/or shorter tails reduce Tm

Effects of Cholesterol on Synthetic Membrane Structure

• Cholesterol's influence on membrane structure in different C:PL ratios is described (membrane width and phase separation as membrane “saturates” with cholesterol)

Composition of Membrane Leaflets

• The two leaflets (outer and inner membrane layers) of a membrane typically differ in lipid composition

Integral Membrane Proteins

Bacteriorhodopsin

• Acts as a light-driven proton pump in certain bacteria
• Contains seven helices that holds a retinal pigment

Glycophorin A

• The first integral membrane protein to be sequenced
• Its external (N-terminal) domain carries 15 O-linked and one N-linked oligosaccharides; ~60% of the protein mass
• The single transmembrane helix is highly hydrophobic
• The cytosolic C-terminal domain is quite hydrophilic

Co-translational Insertion and Folding of Transmembrane Helices

Crystal Structures of Secy Complex Conformational Changes

The “Translocon” Function

• Acts as a protein channel for the insertion of integral membrane proteins into the membrane bilayer

The “Inside Positive” Rule

• Wild-type leader peptidase (Lep) orientation of its two transmembrane helices
• Addition of four Lys to the N-terminus yield a mutant Lep that has the opposite membrane topology

Membrane Rafts

• Rich in cholesterol, sphingolipids, and glycoproteins linked to glycosylphosphatidylinositol
• The bilayer is thicker in rafts than other regions of the cell membrane
• Proteins coalesce, forming nanometer-sized dynamic domains, stabilized by actin fibers
• Rafts can associate to form larger structures.

Transport Across Membranes

• Adaptation to hydrophobic mismatch in a membrane
• If the thickness of the bilayer core and the hydrophobic surface area do not match, either the protein will change conformation or the bilayer's composition will change

Permeability Coefficients

• A table shows permeability coefficients for ions (K+, Na+, Cl−) and molecules (glucose, water) across both synthetic and biological membranes

Facilitated Transport

• Three major mechanisms for facilitated transport exist.
  • Protein pores
  • Carrier molecules
  • Permeases

Valinomycin

• Acts as an ion carrier
• Cyclic peptide with hydrophobic outside
• Oxygens complexes a K+ ion surrounded by CH3 groups

Channel-Forming Hemolysin from S. aureus

• A heptamer with a β-barrel stem that penetrates the cell membrane

Gramicidin A

• Acts as an ion pore formed by two molecules that adopt a helical conformation
• Hydrophobic side chains contact lipid
• N-terminus in cytosol, C-terminus extracellular
• Inside of helix forms hydrophilic pore

Aquaporins

• Water-specific channels that rapidly transport water
• Function as tetramers of identical monomers, each with six membrane-spanning helices and two shorter helices (NPA motif)

Ion Selectivity

Selective Binding of Na⁺ and K⁺ in Ion Channels

• Two binding sites make up the selectivity filter of LeuT
• Four bind in the KcsA channel

Voltage-Gated Channels and Action Potentials

• The channel portion of the voltage-gated channel is homologous to the KcsA channel
• The arginine- and lysine-rich S4 helices are highlighted in blue
• Depth of these helices changes as a function of membrane potential

Action Potential

• The membrane first becomes permeable to Na+, followed by a decrease in Na+ permeability and an outward flow of K+
• Changes in membrane potential accompany these permeability changes, increasing to a positive value before decreasing to undershoot the resting potential

Transmission of the Action Potential

Neurotoxins

Techniques for Studying Membranes

• Freeze fracture technique
• Membrane isolation

Preparation of Vesicles and Bilayers

• Isolating and preparing membrane fragments

Reconstitution of the Ca²⁺ Pump

• Combining phospholipids with detergent-solubilized Ca²⁺ pump protein to form vesicles
• Ca²⁺ accumulates in vesicles

Preparation and Resealing of Erythrocyte Ghosts

• Isolating and preparing erythrocyte ghosts to study membrane function

Fluorescence Recovery After Photobleaching (FRAP)

• An experimental method to study membrane dynamics

Test your knowledge on key biological concepts including the structures of membranes, the function of aquaporins, and the characteristics of fatty acids. This quiz covers various aspects from cellular structures to energy storage in animals. Perfect for students studying cell biology or biochemistry.