Membranes and Cell Structure

Becker's 10th Edition: The Cell - Chapter 7: Membrane

Functions of Membranes

  1. Define Cell Boundaries:

    • Membranes delineate the cell's outer limits and the boundaries of organelles.

  2. Semi-permeable Nature:

    • Membranes possess transport proteins that regulate the movement of substances into and out of cells and organelles.

  3. Receptors:

    • Membranes have receptors to detect external signals and mediate cellular responses.

  4. Plasma Membrane:

    • All cells possess a plasma membrane.

Structure of Membranes

  • In the 1890s, Charles Overton observed that lipid-soluble substances could easily penetrate cells, leading to the conclusion that the cell surface has a lipid "coat".

  • Irving Langmuir determined that phospholipids are amphipathic, arranging themselves in a way where hydrophobic tails face away from water.

  • Understanding membrane structure should come from observation rather than rote memorization.

Gorter and Grendel: Basis of Membrane Structure

  • Year: 1925

  • Extracted lipids from red blood cells and spread them in a monolayer on water.

  • Observed that the film's area was twice that of the blood cells' surface area, suggesting a lipid bilayer structure with nonpolar regions facing inward.

Davson and Danielli: Membrane Proteins Present

  • Key Contributions:

    • Argued that solely a lipid bilayer could not account for all membrane properties, including:

    • Surface tension

    • Solute permeability

    • Electrical resistance

  • Proposed a protein-lipid-protein sandwich model, indicating proteins coat the lipids.

Shortcomings of the Davson-Danielli Model

  • Electron microscopy showed insufficient space for protein layers next to the bilayer.

  • Model didn't account for chemical diversity in membranes, particularly the protein/lipid ratio.

  • Membranes were found susceptible to digestion by phospholipases, indicating lipid exposure.

  • Isolation of "surface" proteins required detergents or organic solvents.

  • The fluid mosaic model incorporates inconsistencies by describing:

    • A fluid lipid bilayer

    • A mosaic of proteins attached or embedded within it.

Membrane Proteins

Characteristics of Membrane Proteins

  • Most integral membrane proteins contain transmembrane segments that span the lipid bilayer and anchor the protein.

  • Membranes are heterogeneous, comprising ordered dynamic microdomains known as lipid rafts.

  • Cellular functions involving membranes rely on specific complexes of lipids and proteins.

Fluidity and Organization of Membranes

Membrane Lipid Classes

  • Key classes include:

    • Phospholipids

    • Glycolipids

    • Sterols

Phospholipids

  • Definition: Any amphipathic lipid with a phosphate-based head group and a two-pronged hydrophobic tail.

  • Types of Phosphoglycerolipids (Phosphoglycerides):

    • Phosphatidylcholine

    • Phosphatidylethanolamine

    • Phosphatidylserine

    • and others

  • All phosphoglycerides are phospholipids but not all sphingolipids are phospholipids.

Glycolipids

  • Formed by adding carbohydrates to lipids, they can be glycerol based (glycoglycerolipids) or sphingosine based (glycosphingolipids).

Sterols

  • Cholesterol: The primary sterol in animal cell membranes needed to stabilize and maintain membrane integrity.

  • Plant cells feature phytosterols; fungal cells contain ergosterol, which resembles cholesterol.

Membrane Asymmetry

  • Lipid composition differs between exoplasmic and cytosolic leaflets, affecting membrane function.

  • This asymmetry reflects lipid synthesis locations (ER and Golgi).

Lipid Mobility

  • Lipids move freely within their monolayer via rotation and lateral diffusion.

  • Phospholipid flip-flop occurs rarely; flippases (translocators) can enhance this process in some membranes.

Quantifying Membrane Protein and Lipid Movement

  • Fluorescence Recovery After Photobleaching (FRAP):

    • Measures protein and lipid mobility in membranes by observing fluorescence recovery in bleached areas.

Membrane Fluidity and Temperature

  • Proper membrane function relies on a fluid state, which varies with temperature:

    • Fluidity decreases as temperature lowers and increases when temperatures rise.

  • Fatty acid chain length and saturation influence fluidity:

    • Longer chains reduce fluidity; higher saturation also reduces fluidity.

  • Cholesterol impacts fluidity—decreasing it at high temperatures and increasing it at low temperatures.

Membrane Proteins

Types of Membrane Proteins

  1. Integral Membrane Proteins: Embedded within lipid bilayer due to their hydrophobic regions.

  2. Peripheral Proteins: Hydrophilic and found on the surface of the bilayer.

  3. Lipid-anchored Proteins: Hydrophilic proteins covalently attached to lipid molecules embedded in the bilayer.

Functions of Membrane Proteins

  • Enzymatic activity

  • Transport facilitation (Transport proteins, Channel proteins)

  • Energy transport (Transport ATPases)

  • Signal reception (Hormones, neurotransmitters, growth factors)

Transmembrane Segments

  • Hydrophobic segments of transmembrane proteins that cross the lipid bilayer typically exhibit an α-helical structure, containing about 20-30 amino acids, generally with hydrophobic R groups.

Glycosylation of Membrane Proteins

  • Glycoproteins: Proteins with carbohydrate chains covalently linked to side chains, aiding in cell-cell recognition.

    • Glycoproteins are prominent in plasma membranes, with carbohydrates protruding outward.

Blood Type Determination

  • Determined by specific antigens on red blood cell surfaces.

  • ABO Blood Group: Antigens are oligosaccharide chains on glycolipids or glycoproteins:

    • Type O: Universal donor (lacks A and B antigens)

    • Type AB: Universal recipient (has A and B antigens)

    • Type A and B: Contain respective synthesizing enzymes for their specific antigens.

SDS-PAGE: Protein Analysis Technique

  • Separation of proteins by size through a polyacrylamide gel after denaturation by sodium dodecyl sulfate (SDS).

  • Proteins charged negatively, migrate towards the positive end of the gel; smaller proteins move faster than larger ones.

Cell Walls in Comparison

  • Human cells lack peptidoglycan, making it an antibiotic target.

  • Overview of other organisms:

    • Plant cell wall: Cellulose

    • Fungal cell wall: Chitin

    • Bacterial cell wall: Peptidoglycan