Membrane Structure and Function

Overview and Learning Objectives

This topic covers the structure and function of biological membranes, focusing on the plasma membrane. Students should understand the composition, formation, and permeability of membranes, as well as the mechanisms of transport across them.

• Describe the structure of a membrane

• Understand the roles of lipids, proteins, and carbohydrates in membranes

• Explain membrane formation and factors influencing permeability

• Distinguish between passive and active transport

• Discuss processes such as diffusion, osmosis, facilitated diffusion, pumps, co-transport, exocytosis, and endocytosis

What is a Membrane?

Definition and Function

Membranes are essential boundaries within and around cells, allowing for compartmentalization and maintenance of distinct cellular environments.

• Membranes separate different regions of a cell, enabling specialized functions.

• The plasma membrane surrounds the entire cell, separating the internal cytoplasm from the external environment.

• Internal membranes create specialized compartments (organelles) within eukaryotic cells.

Membrane Structure

Major Components

Biological membranes are primarily composed of lipids, proteins, and carbohydrates, each contributing to membrane structure and function.

• Lipids: Mainly phospholipids, which form a bilayer due to their amphipathic nature (having both hydrophilic and hydrophobic regions).

• Proteins: Embedded within or attached to the lipid bilayer, serving various functions such as transport, signaling, and structural support.

• Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins) on the extracellular surface, important for cell recognition and signaling.

Example: The classic "fluid mosaic model" describes the membrane as a mosaic of proteins floating in a fluid lipid bilayer.

Phospholipid Bilayer

Phospholipids spontaneously arrange into a bilayer in aqueous environments, forming the fundamental structure of membranes.

• Hydrophilic heads face outward toward water, while hydrophobic tails face inward, away from water.

• This arrangement creates a semi-permeable barrier, allowing selective passage of substances.

• Proteins may be integral (spanning the bilayer) or peripheral (attached to the surface).

Additional info: Amphipathic molecules have both polar and nonpolar regions, which drives bilayer formation.

Membrane Fluidity

Membrane fluidity is crucial for function, affecting permeability and the movement of proteins and lipids within the bilayer.

• Phospholipids can move laterally within the layer; "flip-flop" between layers is rare.

• Fluidity is influenced by lipid composition (e.g., presence of unsaturated fatty acids increases fluidity) and cholesterol content.

• Fluidity affects how easily substances can cross the membrane.

Membrane Proteins and Carbohydrates

Types and Functions of Membrane Proteins

Membrane proteins are diverse and perform many essential roles.

• Transport: Move substances across the membrane (channels, carriers, pumps).

• Enzymatic activity: Catalyze reactions at the membrane surface.

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

• Cell-cell recognition: Allow cells to identify each other (often via glycoproteins).

• Intercellular joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

Membrane Carbohydrates

Carbohydrates on the cell surface play a key role in cell recognition and communication.

• Usually found on the extracellular side of the membrane.

• Form glycolipids and glycoproteins.

• Enable immune recognition and tissue organization.

Membrane Permeability and Transport

Selective Permeability

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) cross easily.

• Small, uncharged polar molecules (e.g., H2O) cross to some extent.

• Large polar molecules and ions require transport proteins.

Types of Transport Mechanisms

Transport across membranes can be passive or active, depending on energy requirements and direction relative to concentration gradients.

• Passive Transport: No energy required; moves substances down their concentration gradient.

• Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient.

Comparison of Passive and Active Transport

Passive Transport

• Simple Diffusion: Movement of molecules from high to low concentration directly through the lipid bilayer.

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

• Facilitated Diffusion: Movement of substances via specific transport proteins (channels or carriers).

Osmosis and Tonicity

• Osmosis: Water moves toward the side with higher solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

  • Hypertonic: Solution has higher solute concentration; cell loses water.

  • Hypotonic: Solution has lower solute concentration; cell gains water.

  • Isotonic: Equal solute concentration; no net water movement.

Example: Animal cells in a hypotonic solution may lyse (burst); in a hypertonic solution, they may crenate (shrink).

Facilitated Diffusion

• Uses channel proteins (form pores) or carrier proteins (change shape to move molecules).

• Still passive; moves substances down their concentration gradient.

• Can be regulated by chemical or electrical signals (gated channels).

Active Transport

• Requires energy input (often from ATP hydrolysis).

• Uses carrier proteins (pumps) to move substances against their gradient.

• Maintains concentration differences essential for cell function.

Types of Active Transport

• Pumps: e.g., Sodium-potassium pump ( out, in per ATP hydrolyzed).

• Electrogenic pumps: Create voltage differences across membranes (membrane potential).

• Cotransport (Secondary Active Transport): One substance moves down its gradient, driving another against its gradient via the same protein.

Electrochemical Gradient

• Combination of concentration gradient and electrical potential across the membrane.

• Drives the movement of ions.

Bulk Transport: Exocytosis and Endocytosis

Large molecules and particles are transported via vesicles in processes requiring energy.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: Plasma membrane engulfs material, forming a vesicle inside the cell.

  • Phagocytosis: "Cell eating"; ingestion of large particles.

  • Pinocytosis: "Cell drinking"; ingestion of fluid and dissolved solutes.

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins.

Summary Table: Membrane Transport Mechanisms