Chapter 7: Membrane Structure and Function

Learning Objectives

• Explain the fluid mosaic model, describing the components of the membrane.

• Discuss how membrane structure results in selective permeability.

• Use examples to demonstrate the processes of diffusion, osmosis, and facilitated diffusion.

• Describe the process of active transport.

Membrane Structure and the Fluid Mosaic Model

Membrane Composition

Biological membranes are primarily composed of phospholipids, with proteins and carbohydrates also playing important roles. The unique arrangement of these molecules gives the membrane its characteristic properties.

• Phospholipids: Amphipathic molecules with hydrophilic ("water-loving") heads and hydrophobic ("water-fearing") tails.

• Proteins: Embedded within or attached to the lipid bilayer, contributing to membrane function.

• Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids), important for cell recognition.

Structure of Phospholipids and Bilayer Formation

Phospholipids spontaneously arrange into a bilayer in aqueous environments:

• Hydrophobic tails face inward, away from water.

• Hydrophilic heads face outward, toward the aqueous environment inside and outside the cell.

This arrangement forms the basic structure of all biological membranes.

Fluid Mosaic Model

The fluid mosaic model describes the dynamic nature of biological membranes:

• The membrane is a fluid structure where lipids and proteins can move laterally within the layer.

• Proteins are distributed in a "mosaic" pattern, either embedded within or attached to the bilayer of phospholipids.

Example: The plasma membrane of a cell, where proteins and lipids are not fixed in place but can move, allowing the membrane to be flexible and self-healing.

Membrane Fluidity and Factors Affecting It

Membrane Stability and Movement

Membranes are held together by weak hydrophobic interactions, allowing for flexibility and movement:

• Lateral movement: Lipids and some proteins can move sideways within the membrane.

• Flip-flop movement: Rare movement of lipids from one leaflet of the bilayer to the other.

• Protein movement: Some proteins drift, while others are anchored to the cytoskeleton or extracellular matrix (ECM).

Factors Influencing Membrane Fluidity

• Temperature: Lower temperatures decrease fluidity; higher temperatures increase fluidity.

• Fatty acid composition:

  • More unsaturated fatty acids (with double bonds) increase fluidity due to kinks in the tails.

  • More saturated fatty acids (no double bonds) decrease fluidity due to tighter packing.

• Cholesterol: Acts as a "fluidity buffer" in animal cell membranes.

  • At high temperatures, cholesterol restrains movement, reducing fluidity.

  • At low temperatures, cholesterol prevents tight packing, maintaining fluidity.

Example: The presence of cholesterol in the plasma membrane of animal cells helps maintain optimal fluidity across temperature changes.

Types and Functions of Membrane Proteins

Integral and Peripheral Proteins

• Integral proteins: Penetrate the hydrophobic core of the lipid bilayer. Some span the entire membrane (transmembrane proteins), while others only partially penetrate.

• Transmembrane proteins: Integral proteins that span the whole membrane, with hydrophilic regions exposed to both sides and hydrophobic regions embedded in the core.

• Peripheral proteins: Loosely bound to the membrane surface, often attached to integral proteins or to the cytoskeleton/ECM.

Structure: Hydrophobic regions of integral proteins are typically composed of nonpolar amino acids, often arranged in alpha helices, anchoring them within the membrane.

Major Functions of Membrane Proteins

• Transport: Move specific solutes across the membrane (e.g., channels, carriers).

• Enzymatic activity: Catalyze metabolic reactions at the membrane surface.

• Signal transduction: Relay signals from outside to inside the cell (e.g., hormone receptors).

• Cell-cell recognition: Allow cells to identify each other (e.g., immune response).

• Intercellular joining: Connect adjacent cells (e.g., gap junctions, tight junctions).

• Attachment to cytoskeleton and ECM: Maintain cell shape and stabilize protein location.

Example: Channel proteins facilitate the passage of ions like Na+ and K+ across the membrane.

Membrane Carbohydrates and Cell-Cell Recognition

Role of Membrane Carbohydrates

Carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface of the membrane are crucial for cell-cell recognition.

• Carbohydrate chains are short and branched, varying among species, individuals, and cell types.

• These variations are the basis for phenomena such as blood type compatibility.

Example: The four human blood groups (A, B, AB, O) differ in the carbohydrate part of glycoproteins on red blood cells, determining blood transfusion compatibility.

Transport Across Membranes

Types of Transport

• Passive transport: Movement of small molecules across the membrane without energy input. Includes diffusion, osmosis, and facilitated diffusion.

• Active transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP) and a transport protein.

• Bulk transport: Movement of large molecules via vesicles (endocytosis and exocytosis).

Passive Transport

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated diffusion: Movement of molecules via transport proteins, still down their concentration gradient.

Active Transport

• Requires energy (ATP) to move substances against their concentration gradient.

• Involves specific transport proteins (e.g., sodium-potassium pump).

Bulk Transport

• Exocytosis: Secretion of large molecules when vesicles fuse with the plasma membrane.

• Endocytosis: Uptake of large molecules when the plasma membrane pinches inward, forming a vesicle.

Summary Table: Types of Membrane Proteins and Their Functions

Key Terms

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions.

• Selective permeability: Property of membranes that allows some substances to cross more easily than others.

• Glycoprotein: Protein with carbohydrate attached, important for cell recognition.

• Glycolipid: Lipid with carbohydrate attached, also involved in cell recognition.

• Cholesterol: Steroid molecule that modulates membrane fluidity in animal cells.