[BIOCHEM] biological membranes

What are the main functions of biological membranes?

Barrier to toxic molecules, transport and accumulation of nutrients, site of important cellular reactions, signal transduction and cell-to-cell interactions, facilitation of cell motion, and role in reproduction.

What role do membrane receptors and antigens play?

They play a role in signal transduction and cell recognition.

What is the protein composition range in biological membranes?

Biological membranes may contain as much as 75% to 80% protein or as little as 15-20% protein.

How does the protein content relate to the function of membranes?

Membranes that carry out many enzyme-catalyzed reactions and transport activities are richer in protein, while those with fewer functions, like myelin sheaths, are richer in lipid.

What drives the self-association of lipids in water?

The hydrophobic effect and the preference of water for polar interactions promote the self-association of lipids.

What are the characteristics of biological membranes?

They exhibit selective permeability, are not rigid, may change shape and size, are self-sealing, relatively stable, and exhibit lateral membrane asymmetry.

What is the Fluid Mosaic Model?

Proposed by S. J. Singer and G. L. Nicolson in 1972, it describes the phospholipid bilayer as a fluid matrix where lipids and proteins can move laterally and rotate.

What are the two classes of proteins found in biological membranes?

Peripheral proteins (extrinsic proteins) and integral proteins (intrinsic proteins).

How do the two monolayers of the bilayer differ?

They have different lipid compositions and different protein complements.

What is the significance of lateral heterogeneity and transverse asymmetry in membranes?

It indicates that both lipids and proteins are distributed unevenly across the membrane, affecting their functions.

What is the behavior of lipid chains in biological membranes?

Lipid chains can bend, tilt, and rotate, allowing for mobility and diffusion in the membrane plane.

What is the difference between lateral and transverse motion in membranes?

Lateral motion is faster, while transverse motion is much slower.

What is the structure of micelles?

Micelles bury the nonpolar tails in the center of a spherical structure.

What happens to amphipathic structures in nonpolar solvents?

Micelles reverse in nonpolar solvents.

What promotes the formation of ordered structures by lipids in water?

The hydrophobic effect and the preference of water for polar interactions.

What is the role of the hydrophobic effect in membrane formation?

It drives amphiphilic lipids to form membranes by maximizing entropy.

What does it mean for membranes to be self-sealing?

Membranes can spontaneously reseal themselves after being disrupted, maintaining their integrity.

How does the composition of membranes affect their function?

Different compositions of lipids and proteins influence the membrane's selective permeability and the types of reactions it can facilitate.

What is the significance of membrane asymmetry?

Asymmetry in membranes is crucial for their functionality, affecting how they interact with the environment and other cells.

What is the importance of lipid clusters and lipid-protein aggregates in membranes?

They contribute to the functional diversity and dynamic behavior of membranes.

What proteins are responsible for moving lipids between monolayers in a membrane?

Flippase and floppase proteins.

Do all flippases and floppases require energy to operate?

No, some operate passively and do not require an energy source, while others require the energy of ATP hydrolysis.

What is the gel phase in membrane fluidity?

At low temperatures, bilayer lipids are highly ordered, forming a gel phase where acyl chains are nearly perpendicular to the membrane plane.

What is the solid-ordered state (So) in membrane fluidity?

It refers to the state where lipid chains are tightly packed and undergo relatively little motion.

What happens to lipid chains in the liquid crystalline phase?

At higher temperatures, acyl chains undergo more motion, including rotations and bending, resulting in a liquid-disordered state (Ld).

What is the transition temperature (Tm) in membrane lipids?

The temperature at which the transition from the gel phase to the liquid crystalline phase occurs.

What characterizes the transition temperature (Tm) of lipids in a membrane?

It is characteristic of the specific lipids in the membrane and is well-defined in pure lipid systems.

What happens to membrane lipids above the transition temperature (Tm)?

They exist in the liquid crystalline phase and are fluid.

What happens to membrane lipids below the transition temperature (Tm)?

They are in a gel phase and are rigid.

How do warm-blooded animals regulate membrane fluidity?

regulate membrane fluidity primarily through tissue-specific lipid composition and cholesterol content, rather than relying on drastic changes in environmental temperature like ectotherms.

What factors affect the fluidity of the membrane?

Fatty acyl chain length, fatty acyl unsaturation, and temperature.

How does chain length affect the melting temperature (Tm) of membrane lipids?

Increasing chain length raises the Tm, making it harder to melt.

How does unsaturation in fatty acyl chains affect the melting temperature (Tm)?

More unsaturation (more double bonds) lowers the Tm, making it easier to melt.

What is the effect of temperature on membrane fluidity?

Temperature influences the phase transition between gel and liquid crystalline states.

What is the relationship between acyl chain length and membrane fluidity?

Longer acyl chains increase Tm and make the membrane harder to melt.

What occurs during the phase transition of membrane lipids?

The transition from gel phase to liquid crystalline phase is a true phase transition.

What is the significance of the acyl chain's conformation in the gel phase?

In the gel phase, lipid chains are in their fully extended conformation, minimizing surface area per lipid.

What happens to bilayer thickness during the transition to the liquid crystalline phase?

Bilayer thickness decreases by 10-15% as the lipid chains become more bent.

What is the impact of temperature on bacterial membrane fluidity?

Bacteria may change the composition of fatty acyl chains in response to temperature changes.

What is the liquid-disordered state (Ld) in membrane fluidity?

It is the state where lipid chains are more likely to be bent, increasing surface area per lipid.

What are the main functions of membrane proteins?

Transport substances across membranes, act as receptor sites, and sites of enzyme catalysis.

How can peripheral proteins be removed from the membrane?

By raising the ionic strength.

What characterizes peripheral proteins in relation to the membrane?

They are not strongly bound to the membrane and are bound by electrostatic interactions and hydrogen bonds with the polar headgroup.

What distinguishes integral proteins from peripheral proteins?

Integral proteins are tightly bound to the membrane and are strongly embedded in the bilayer.

How can integral proteins be removed from the membrane?

By denaturing the membrane using organic solvents or strong detergents.

What structural features are common in the portions of integral proteins that contact the lipid bilayer?

They are dominated by α-helices and β-sheets.

What are the four types of lipid-anchored proteins?

Amide-linked myristoyl anchors, thioester-linked fatty acyl anchors, thioether-linked prenyl anchors, and glycosyl phosphatidylinositol anchors.

What is unique about Glycophorin A as an integral protein?

It spans the membrane of the human erythrocyte via a single α-helical transmembrane segment.

What is the orientation of the C-terminus and N-terminus of Glycophorin A?

The C-terminus faces the cytosol, while the N-terminus is extracellular.

What does the extracellular portion of Glycophorin A contain?

Oligosaccharides that constitute the ABO and MN blood group determinants.

What is Bacteriorhodopsin known for?

It is a paradigm for membrane proteins with 7 helical segments.

What is bound to the lysine residue in Bacteriorhodopsin?

Light-absorbing retinal.

What effect does temperature have on peripheral proteins?

Increased temperature leads to denaturation and a decrease in peripheral proteins.

What happens to integral proteins when the membrane is denatured?

Integral proteins can be removed from the membrane.

What type of interactions bind peripheral proteins to the membrane?

Ionic interactions and hydrogen bonds.

What are the characteristics of integral proteins regarding their membrane position?

They are often transmembrane but not necessarily so.

What is the significance of the transmembrane segment in Glycophorin A?

It consists of 19 hydrophobic amino acids and is α-helical.

What is the role of the lipid bilayer in relation to integral proteins?

Integral proteins are embedded in the lipid bilayer, interacting with its nonpolar core.

What organism is Halobacterium halobium known for containing purple patches?

Halobacterium halobium.

What is the structure of the membrane protein mentioned in the notes?

It consists of 7 transmembrane helical segments with short loops interconnecting the helices.

How can the topology of a membrane protein be revealed?

By using a hydropathy plot based on the amino acid sequence.

What does the topology of a membrane protein specify?

The number of transmembrane segments and their orientation across the membrane.

What are glycoproteins?

Glycoproteins are proteins that contain carbohydrate units covalently bonded to a polypeptide chain.

What are the two types of glycosylation in glycoproteins?

N-linked and O-linked.

How are N-linked saccharides attached to proteins?

They are attached via the amide nitrogens of asparagine residues.

How are O-linked saccharides attached to proteins?

They are attached to hydroxyl groups of serine, threonine, or hydroxylysine.

What role do glycoproteins play in the immune system?

Antibodies are glycoproteins that act as antigenic determinants.

What are the four blood types classified in the ABO system?

A, B, AB, and O.

What is the biochemical basis for blood type classification?

A group of relatively small membrane-bound carbohydrates.

What are the three types of N-linked glycoproteins?

High mannose, complex, and hybrid.

What is the significance of transport processes in cells?

They are vital for importing nutrients and exporting waste.

What do cells maintain across their membranes?

Concentration gradients of various metabolites.

What type of molecules do specific transport proteins move across membranes?

Polar, water-soluble molecules or ions.

What is passive transport?

Transport driven by a concentration gradient.

What are the two types of passive transport?

Simple diffusion and facilitated diffusion.

What is active transport?

Transport of a substance against a concentration gradient.

What is primary active transport linked to?

The hydrolysis of ATP or other high-energy molecules.

What drives secondary active transport?

The H+ gradient.

What does passive diffusion of an uncharged species across a membrane depend on?

The concentrations on the two sides of the membrane.

What is facilitated diffusion?

A process where a molecule or ion is carried across a membrane by a carrier or channel protein.