Membrane Structure and Function

Basic Functions of Membranes

Cell membranes are essential for maintaining cellular integrity and regulating interactions with the environment. They serve as selective barriers, compartmentalize cellular processes, and facilitate communication and transport.

• Selective Permeability: Membranes control the entry and exit of substances, maintaining homeostasis.

• Compartmentalization: Membranes create distinct cellular compartments, enabling specialized functions.

• Communication: Membranes contain receptors for signal transduction and cell recognition.

• Energy Transduction: Membranes participate in processes such as ATP synthesis and photosynthesis.

• Anchoring: Membranes anchor the cytoskeleton and extracellular matrix components.

Fluid-Mosaic Model of Membrane Structure

The fluid-mosaic model describes membranes as dynamic structures composed of a lipid bilayer with embedded proteins. Lipids provide fluidity, while proteins perform specialized functions.

• Lipid Bilayer: Composed mainly of phospholipids, providing a flexible matrix.

• Proteins: Integral, peripheral, and lipid-anchored proteins are interspersed, contributing to diverse functions.

• Fluidity: Lipids and some proteins can move laterally within the membrane.

• Mosaic: The arrangement of proteins and lipids is not uniform, creating a mosaic pattern.

Biomolecule Content of Membranes

Table 7-1: This table compares the proportions of lipids, proteins, and carbohydrates in various biological membranes. The major conclusion is that membrane composition varies by cell type and function, affecting properties such as fluidity and permeability.

Membrane Lipids

Phospholipids and Sterols

Membrane lipids are primarily phospholipids and sterols, each with distinct structures and functions.

• Phospholipids: Amphipathic molecules with a hydrophilic head (phosphate group) and hydrophobic tails (fatty acids). They form the bilayer structure.

• Sterols: Such as cholesterol in animal cells, have a rigid ring structure that modulates membrane fluidity and stability.

• Identification: Phospholipids have two fatty acid tails; sterols have a four-ring core structure.

Major Point of Figure 7-7

Figure 7-7 illustrates the amphipathic nature of membrane lipids, emphasizing how their structure enables bilayer formation and selective permeability.

• Amphipathic Structure: Drives self-assembly into bilayers.

• Barrier Function: Hydrophobic core prevents passage of polar molecules.

Lipid Movement in Membranes

Membrane lipids exhibit several types of movement, contributing to membrane fluidity and function.

• Lateral Diffusion: Lipids move side-to-side within the same leaflet; occurs readily.

• Rotation: Lipids rotate around their axis; occurs readily.

• Flip-Flop (Transverse Diffusion): Lipids move from one leaflet to the other; occurs rarely without enzyme assistance (flippases).

Fatty Acids and Membrane Fluidity

Saturated vs. Unsaturated Fatty Acids

The degree of saturation and length of fatty acids in phospholipids affect membrane fluidity.

• Saturated Fatty Acids: No double bonds; straight chains pack tightly, decreasing fluidity.

• Unsaturated Fatty Acids: One or more double bonds; kinked chains prevent tight packing, increasing fluidity.

• Example: Membranes with more unsaturated fatty acids are more fluid, especially at lower temperatures.

Fatty Acid Length

The number of carbons in fatty acid chains also influences fluidity.

• Shorter Chains: Increase fluidity due to less van der Waals interactions.

• Longer Chains: Decrease fluidity due to more interactions.

Membrane Proteins

Types of Membrane Proteins

Membrane proteins are classified based on their association with the lipid bilayer.

• Integral Proteins: Span the bilayer; have hydrophobic regions embedded in the membrane.

• Peripheral Proteins: Associate with membrane surfaces via non-covalent interactions; do not penetrate the bilayer.

• Lipid-Anchored Proteins: Covalently attached to lipids within the bilayer; anchor the protein to the membrane.

• Mobility: Proteins can move laterally unless anchored to internal (cytoskeleton) or external structures.

Hydropathy Plots

Hydropathy plots are graphical representations used to predict transmembrane regions in proteins based on amino acid hydrophobicity.

• Interpretation: Peaks in the plot indicate hydrophobic stretches likely to span the membrane.

• Utility: Useful for identifying integral membrane proteins and their topology.

Membrane Carbohydrates

Carbohydrate Linkages

Membrane carbohydrates are typically covalently attached to proteins (glycoproteins) or lipids (glycolipids), playing roles in cell recognition and signaling.

• Glycoproteins: Proteins with carbohydrate chains attached; important for cell-cell interactions.

• Glycolipids: Lipids with carbohydrate chains; contribute to membrane stability and recognition.

• Location: Carbohydrate chains are usually found on the extracellular surface of the membrane.

Additional info: Protein glycosylation will be discussed in detail in later chapters.