Chapter 7: Membrane Structure and Function

Learning Objectives

This chapter explores the structure and function of biological membranes, focusing on the fluid mosaic model, membrane permeability, and mechanisms of molecular transport. Students should be able to:

• Explain the fluid mosaic model and describe membrane components.

• Discuss how membrane structure leads to selective permeability.

• Use examples to demonstrate diffusion, osmosis, and facilitated diffusion.

• Describe the process of active transport.

Membrane Structure and Composition

Phospholipids and Amphipathic Nature

Biological membranes are primarily composed of phospholipids, which are amphipathic molecules containing both hydrophobic and hydrophilic regions.

• Hydrophobic tails: Nonpolar fatty acid chains that avoid water.

• Hydrophilic heads: Polar phosphate groups that interact with water.

Phospholipids spontaneously arrange into a bilayer in aqueous environments:

• Hydrophobic tails face inward, shielded from water.

• Hydrophilic heads face outward, exposed to water on both sides of the membrane.

Other Membrane Components

• Proteins: Embedded within or attached to the bilayer, responsible for most membrane functions.

• Carbohydrates: Often attached to lipids (glycolipids) or proteins (glycoproteins), playing roles in cell recognition.

Fluid Mosaic Model

Definition and Features

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

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

• Proteins form a mosaic within the phospholipid bilayer, either spanning the membrane or attached to its surfaces.

Membrane Proteins

• Most membrane proteins are amphipathic:

  • Hydrophilic regions: Exposed to cytosol and extracellular fluid.

  • Hydrophobic regions: Embedded within the bilayer.

Membrane Fluidity

Factors Affecting Fluidity

Membrane fluidity is essential for proper function and is influenced by several factors:

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

• Fatty acid composition:

  • More unsaturated fatty acids (with double bonds) increase fluidity due to kinks preventing tight packing.

  • More saturated fatty acids decrease fluidity.

• 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.

Types of Membrane Proteins

Integral Proteins

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

• Hydrophobic regions: Composed of nonpolar amino acids, often arranged in alpha helices.

• Hydrophilic regions: Exposed to aqueous environments on both sides of the membrane.

• Some integral proteins form hydrophilic channels for passage of ions and polar molecules.

Peripheral Proteins

Peripheral proteins are loosely bound to the membrane surface, often interacting with integral proteins.

• On the cytoplasmic side, some attach to the cytoskeleton for structural support.

• On the extracellular side, some attach to the extracellular matrix (ECM), aiding cell interactions.

Major Functions of Membrane Proteins

• Transport: Move specific solutes across the membrane.

• Enzymatic activity: Catalyze metabolic reactions.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Identify and interact with other cells.

• Intercellular joining: Form junctions between adjacent cells.

• Attachment: Connect to cytoskeleton and ECM for shape and stability.

Membrane Carbohydrates and Cell Recognition

Role in Cell-Cell Recognition

Cells recognize each other by binding to molecules on the membrane surface, many of which are carbohydrates attached to lipids or proteins.

• Glycolipids: Carbohydrates bonded to lipids.

• Glycoproteins: Carbohydrates bonded to proteins.

• Carbohydrate composition varies among species, individuals, and cell types.

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

Transport Across Membranes

Types of Transport

The plasma membrane regulates the movement of substances into and out of the cell through several mechanisms:

• Passive transport: Movement of small molecules without energy input; may involve transport proteins.

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

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

Exocytosis and Endocytosis

• Exocytosis: Large molecules are secreted when vesicles fuse with the plasma membrane.

• Endocytosis: Large molecules are taken in when the plasma membrane pinches inward, forming a vesicle.

Diffusion, Osmosis, and Facilitated Diffusion

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

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

• Facilitated diffusion: Passive transport of molecules via specific transport proteins.

Key Equations

• Fick's Law of Diffusion:

• Osmotic Pressure:

where is osmotic pressure, is the van 't Hoff factor, is molarity, is the gas constant, and is temperature.

Summary Table: Types of Membrane Transport

Additional info: The notes have been expanded to include definitions, examples, and equations for clarity and completeness. The summary table is inferred from standard textbook content for General Biology.