9+Membrane+Structure

Membrane Structure

Cell Membranes

• Plasma Membrane (Prokaryotes and Eukaryotes)

  • Defines cellular space and boundaries.

  • Maintains biochemical and electrical differences.

  • Serves as a conduit for extracellular communication.

• Organelle Membranes (Eukaryotes only)

  • Define organelle lumens and boundaries.

• Membrane Composition

  • Made up of a lipid bilayer.

  • Contains various inserted and surface-associated proteins.

  • Includes other types of molecules.

Lipid Bilayer

• Membrane lipids are amphipathic molecules (contain both polar and non-polar bonds).

• They have a unique shape:

  • Hydrophilic (polar) "head" region.

  • Hydrophobic (non-polar) "tail" region.

• Lipids spontaneously form bilayers in aqueous solutions.

Spontaneous Bilayer Formation

• Lipids can arrange themselves into:

  • Lipid micelles: Energetically unfavorable with exposed edges to water.

  • Planar phospholipid bilayers: Energetically favorable, forming sealed compartments.

Phospholipids

• Most abundant lipids in cell membranes.

• Four major types of phospholipids are found in cell membranes.

Lipid Bilayer Fluidity

• Lipids can:

  • Diffuse laterally.

  • Rotate rapidly about their axis.

  • Possess extremely flexible nonpolar tails.

Cells Regulate Bilayer Fluidity

• Mechanisms of regulation:

  • Adjusting the proportion of lipids with cis-double bonds in fatty acid tails.

  • Addition of different types of molecules, helping maintain fluidity irrespective of temperature.

Fluidity Adjustments

• The number of lipids with cis-double bond tails can vary to maintain consistent fluidity despite temperature changes.

Effects of Non-Phospholipid Molecules on Fluidity

• Certain non-phospholipid amphipathic molecules, like cholesterol, can affect fluidity:

  • It interacts with phospholipids, increasing rigidity between tails near the head region while keeping the lower ends flexible.

  • Disrupts attractive interactions between phospholipids to prevent crystallization (freezing).

Lipid Rafts

• Weak associations between lipids that often include cholesterol and proteins.

• Drift as groups, forming thicker membrane regions, potentially anchoring membrane proteins.

Glycolipids

• Composed of lipid molecules with attached sugar molecules.

• Found exclusively on the non-cytoplasmic surface (extracellular and intra-organelle).

Glycolipid Functions

• Functions include:

  • Protection for the cell.

  • Concentrating ions at the cell surface due to charge.

  • Cell recognition.

  • Can unintentionally allow bacterial toxins to enter cells.

Membrane Proteins

• Define much of the membrane's function.

• The amount and type of proteins in any membrane region are highly variable.

• Often, proteins have oligosaccharide chains attached to their non-cytosolic domains, similar to glycolipids.

Mechanisms of Protein Association

• Transmembrane Proteins:

  • Always amphipathic; may be single or multi-pass.

  • Spanning segments are often in the form of α-helices or β-barrels.

• Surface Proteins:

  • Usually located in the cytosolic region, sometimes extracellular.

  • Attachment mechanisms can vary, including:

    • α-helix association with the lipid bilayer.

    • Covalent attachments via oligosaccharide chains.

    • Noncovalent attachments to transmembrane proteins.

Peripheral vs Integral Proteins

• Peripheral Proteins:

  • Weak noncovalent surface attachments.

• Integral Proteins:

  • Transmembrane or covalently attached to the surface.

  • All peripheral proteins are surface proteins, but not all surface proteins are peripheral.

  • All transmembrane proteins are integral, but not all integral proteins are transmembrane.

Examples of Membrane Proteins

• Spectrin:

  • Cytosolic peripheral protein in red blood cells.

  • Associates with the cytoskeleton to maintain cell shape.

• Glycophorin:

  • Transmembrane protein in red blood cells with extensive extracellular domains and glycosylation, aiding motility.

• Bacteriorhodopsin:

  • Multi-pass transmembrane protein functioning as a proton pump in archaea, utilizing retinal responsible for light absorption and subsequent conformational changes to pump protons.

Membrane Protein Complexes

• Include various subunits (e.g., M subunit, H subunit, L subunit).

  • Hydrophobic cores within the lipid bilayer.

Glycocalyx

• Extracellular surface covered in sugars, covalently attached to:

  • Membrane Proteins (glycoproteins)

  • Lipids (glycolipids)

• Provides protection against mechanical damage and plays a role in cell recognition.