Chapter 7 Lecture

Membrane Structure and Function

• The plasma membrane regulates the movement of substances into and out of the cell.

  • Passive Transport: Small molecules move across the membrane without energy input; may require transport proteins.

  • Active Transport: Requires energy (ATP) and a transport protein to move substances against their concentration gradient.

  • Bulk Transport: Utilizes vesicles for transport; includes exocytosis and endocytosis.

Fluid Mosaic Model of Membranes

Concept 7.1: Membrane Components

• Lipids and Proteins: Main components; carbohydrates are also essential.

• Phospholipids: Primary components of membranes; amphipathic with hydrophobic tails and hydrophilic heads.

• Form a bilayer with hydrophobic regions protected from water.

• Membrane proteins are also amphipathic; hydrophilic regions face the cytosol or ECM, while hydrophobic regions are embedded in the bilayer.

Structure of Membranes

• Fluid Mosaic Model:

  • Membranes are depicted as fluid bilayers with proteins floating within; proteins can group together to perform common functions.

Membrane Fluidity and Composition

The Fluidity of Membranes (1 of 4)

• Membranes held together by weak hydrophobic interactions; allows lateral movement of lipids and proteins.

• Rarely, lipids may flip-flop between layers.

Factors Affecting Membrane Fluidity

• Temperatures affect fluidity; lower temperatures lead to solidification.

• Unsaturated fatty acids: Increase fluidity compared to saturated fatty acids which pack tightly.

• Cholesterol stabilizes membrane fluidity; restricts phospholipid movement at high temperatures and prevents solidification at low.

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral Proteins: Bound to the membrane surface.

• Integral Proteins: Penetrate hydrophobic core, including transmembrane proteins that span the membrane.

  • Hydrophobic regions consist of nonpolar amino acids; may form alpha helices.

Functions of Membrane Proteins

• Key functions include:

  • Transport for molecules and ions.

  • Enzymatic activity for biochemical reactions.

  • Signal transduction: Communication with external signals.

  • Cell-cell recognition and intercellular joining.

  • Attachment to cytoskeleton and ECM.

The Role of Carbohydrates in Membranes

• Cell-Cell Recognition: Cells bind to surface molecules (often carbohydrates).

  • Glycolipids: Carbohydrates attached to lipids.

  • Glycoproteins: Carbohydrates attached to proteins; act as markers for identification.

Selective Permeability of Membranes

Concept 7.2: Selective Permeability

• Plasma membranes control material exchange; exhibit selective permeability by allowing certain substances to cross more easily.

  • Nonpolar molecules (e.g., hydrocarbons) pass rapidly through lipid bilayers.

  • Polar molecules (e.g., sugars, water, ions) pass slowly due to the hydrophobic core.

Transport Proteins

• Facilitated Diffusion: Hydrophilic substances cross membranes quickly through transport proteins.

  • Channel Proteins: Create hydrophilic passageways for specific molecules (e.g., aquaporins for water).

  • Carrier Proteins: Bind and change shape to shuttle molecules across the membrane.

Active Transport Mechanisms

Concept 7.3: Active Transport

• Requires energy input (from ATP) to move substances against their gradients.

• Examples:

  • Sodium-Potassium Pump: Exchanges Na+ and K+ ions, maintaining concentrations essential for cellular function.

  • Electrogenic Pumps: Generate voltage across membranes, storing energy for cellular work.

Cotransport Mechanism

Concept 7.4: Cotransport

• Active transport of one solute can drive the transport of another substance against its gradient.

• Used in plant cells for sucrose transport and in animal cells for glucose reabsorption in intestines.

Bulk Transport: Exocytosis and Endocytosis

Concept 7.5: Vesicular Transport

• Exocytosis: Transport vesicles fuse with the membrane to release contents outside (e.g., insulin secretion).

• Endocytosis: Macromolecules enter cells via vesicles; types include:

  • Phagocytosis (cellular eating).

  • Pinocytosis (cellular drinking).

  • Receptor-mediated endocytosis: Specific uptake triggered by solute binding to receptors.

The Significance of Membrane Function

• Membrane properties are crucial for maintaining homeostasis and facilitating communication in cellular processes.

• Understanding these mechanisms aids in comprehending cellular health and disease states.