Chapter 5: Membrane Structure Flashcards

Chapter 5: Membrane Structure

Overview

• Membranes are crucial in biology and medicine.

• Computer programs aid in predicting the number of membrane proteins.

• Approximately 25% of all genes may encode membrane proteins across all domains of life (archaea, bacteria, and eukaryotes).

Key Concepts of Membrane Structure

• Membrane Structure: The arrangement of lipids, proteins, and carbohydrates in the cell membrane.

• Fluidity of Membranes: The ability of lipids and proteins to move relative to each other within the membrane.

Components of the Membrane

1. Phospholipids

• Form the framework of the membrane: the phospholipid bilayer.

• Amphipathic: Containing both polar (hydrophilic) and nonpolar (hydrophobic) regions.

  • Polar Head (hydrophilic): Attracted to water.

  • Nonpolar Tail (hydrophobic): Repelled by water; consists of fatty acid tails.

2. Proteins

• Membranes also contain proteins and carbohydrates.

• Determine most of the membrane’s specific functions.

• The membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids.

• Most membrane proteins are amphipathic, residing in the bilayer with their hydrophilic portions protruding.

Types of Membrane Proteins:

• Integral Proteins: Penetrate the hydrophobic interior of the lipid bilayer.

  • Transmembrane Proteins: Span the entire membrane.

  • Lipid-Anchored Proteins: An amino acid of the protein is covalently attached to a lipid.

• Peripheral Proteins: Loosely bound to the surface of the membrane.

  • Bound to integral membrane proteins projecting from the membrane.

  • Bound to polar head groups of phospholipids.

Functions of Membrane Proteins

• (a) Transport:

  • Provide a hydrophilic channel across the membrane for selective solutes.

  • Shuttle substances across the membrane by changing shape; some use ATP for active transport.

• (b) Enzymatic Activity:

  • Enzymes built into the membrane with active sites exposed to adjacent solutions.

  • Enzymes organized as a team to carry out sequential steps in a metabolic pathway.

• (c) Signal Transduction:

  • Receptor proteins with specific binding sites for chemical messengers (e.g., hormones).

  • Binding causes the protein to change shape, relaying the message to the cell interior.

• (d) Cell-Cell Recognition:

  • Glycoproteins serve as identification tags recognized by membrane proteins of other cells.

  • Short-lived binding compared to intercellular junctions.

• (e) Intercellular Joining:

  • Membrane proteins of adjacent cells hook together via gap junctions or tight junctions.

  • More long-lasting binding than cell-cell recognition.

• (f) Attachment to the Cytoskeleton and Extracellular Matrix (ECM):

  • Microfilaments or other cytoskeleton elements noncovalently bound to membrane proteins.

  • Helps maintain cell shape and stabilize the location of certain membrane proteins.

  • Proteins that bind to ECM molecules coordinate extracellular and intracellular changes.

3. Carbohydrates

• Membranes also contain proteins and carbohydrates.

Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often containing carbohydrates.

• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual.

• Glycosylation: Covalently bonding a carbohydrate to lipids (glycolipids) or proteins (glycoproteins).

4. Cholesterol

• Molecule produced by animal cells; amphipathic.

• Stabilizes membranes.

• Can fill the spaces left by kinks.

• Stiffens a membrane (less flexible).

• Less permeable.

Fluid-Mosaic Model

• Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules.

• Membrane resembles a fluid because lipids and proteins can move relative to each other within the membrane.

Fluidity of Membranes

• Membranes are semifluid.

• Most lipids can rotate freely around their long axes and move laterally within the membrane leaflet.

• "Flip-flop" of lipids from one leaflet to the opposite leaflet does not occur spontaneously.

• Flippase requires ATP to transport lipids between leaflets.

• The outside of the membrane has contact with the environment; this allows the cell to sense activity and respond to it.

• The inside and outside surface serve as a site for chemical reactions.

• The membrane controls what enters and exits the cell (selective permeability).

Factors Affecting Membrane Fluidity

1. Lipid Composition

• Length of Phospholipid Tails:

  • Shorter chains reduce the tendency of hydrocarbons to interact, leading to increased fluidity.

• Number of Double Bonds:

  • Double bonds create kinks in the tail, preventing tight packing, resulting in more fluid membranes.

• Cholesterol:

  • Stabilizes membranes.

2. Temperature

• As temperatures cool, membranes switch from a fluid state to a solid state.

• Membrane maintains fluidity by preventing tight packing.

• The temperature at which a membrane solidifies depends on the types of lipids it contains.

Role of Cholesterol at Different Temperatures:

• At warm temperatures (e.g., 37°C), cholesterol reduces membrane fluidity by restraining movement of phospholipids.

• At cool temperatures, cholesterol maintains fluidity by preventing tight packing.

Membrane Asymmetry

• The two leaflets (halves of bilayer) are asymmetrical, with different amounts of each component.

Restricted Movement of Membrane Proteins

• Depending on the cell type, 10 to 70% of membrane proteins may be restricted in their movement.

• Integral membrane proteins may be bound to components of the cytoskeleton, restricting lateral movement.

• Membrane proteins may also be attached to molecules outside the cell, such as the extracellular matrix.

Medical Importance

• Examples include how drugs like aspirin and ibuprofen interact with membrane proteins.

• Natural endorphins and morphine bind to endorphin receptors on brain cells. The boxed portion of the endorphin molecule binds to receptor molecules on target cells in the brain. The boxed portion of the morphine molecule is a close match. This is shown in the figure of the injection LSP.