Lecture_3

Molecular Biology of the Cell: Cell Membrane Structure and Function

Author Information

• Dante Neculai

• Zhejiang University Medical School, 2025.03.05

Learning Objectives

• Components and function of the lipid bilayer.

• Association and confinement of membrane components to specific domains.

• Differences between channels and transporters.

• Active membrane transport.

• Understanding chemical synapses.

Content Overview

The Lipid Bilayer

• Forms the plasma membrane and membranes of organelles (endoplasmic reticulum, Golgi apparatus, mitochondrial, nuclear).

• Double layer of phospholipids, about 5 nm thick, relatively impermeable to most water-soluble molecules.

• Approximately 50% of the mass of animal cell membranes is due to lipid molecules (~10^9 lipid molecules).

• Key Lipid Types:

  • Phospholipids: Most abundant.

  • Sphingolipids: Important in cellular signaling.

  • Sterols: Such as cholesterol, which stabilize membranes.

• Membrane proteins span the lipid bilayer, functioning as sensors or receptors to transfer information across the membrane.

The Lipid Bilayer Structure

Views of a Cell Membrane

• Diagrammatic representation shows lipid bilayer (5 nm) with embedded protein molecules.

Lipid Composition

• Amphiphilic nature with hydrophobic tails and hydrophilic heads.

• Phospholipid Components:

  • Includes choline group, phosphate, glycerol, and fatty acid tails.

• Typical phospholipid structure includes:

  • Hydrophilic Head Group: Combines with a charged group (e.g., choline).

  • Hydrophobic Tails: Long hydrocarbon chains that repel water.

Major Phospholipids in Mammalian Plasma Membranes

• Include phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, sphingomyelin.

• Illustrates variations between their structures and roles in the membrane.

Cholesterol's Role

• In cell membranes, cholesterol influences fluidity and stability of the lipid bilayer.

• Cholesterol creates regions of varying fluidity within the bilayer.

Formation of Lipid Bilayers

• Phospholipids spontaneously form bilayers due to their amphiphilic nature, avoiding exposure of hydrophobic tails to water.

• Stability: Achieved through closed structures which minimize hydrophobic tail exposure.

Liposomes and Two-dimensional Fluidity

• Liposomes are vesicles formed by lipid bilayers, showcasing potential in drug delivery.

• Bilayers are considered fluid, with mobility affected by temperature and lipid composition.

Asymmetry of the Lipid Bilayer

• The different lipid compositions of the two monolayers are functionally essential, influencing enzyme binding and signaling.

• Key Points:

  • Cytosolic monolayer contains distinct lipids compared to the external layer.

  • Asymmetry important for recognizing live versus dead cells.

Glycolipids and Their Distribution

• Present on the surface of all eukaryotic membranes; consist of sphingosine with sugar moieties.

• Function in cell recognition and protection against harsh environmental conditions.

Membrane Proteins

Types and Interactions

• Can be integrated into the bilayer in various forms: single alpha helices, β-barrels, and through lipid anchors.

• Glycosylation often occurs, stabilizing structures through disulfide bonds.

Transport Mechanisms Overview

Principles of Membrane Transport

• Lipid bilayers restrict polar molecules; solute concentrations vary between cytosol and extracellular fluid.

• Movement across the membrane can be passive (down a gradient) or active (against a gradient).

Types of Transporters

Channels vs. Transporters

• Channels: Form pores; allow rapid diffusion.

• Transporters: Undergo conformational changes for specific solute movement.

Passive and Active Transport

• Principles defined by the concentration gradient.

• Active transport is energy dependent, mediated through specific pumps.

Input of Energy in Transport

• Transport relies on metabolic energy, often coupling the uphill movement of one solute with the downhill movement of another.

• Examples of Pumps: ATP-driven pumps, coupled transporters (symporters, antiporters).

Chemical Synapses

Mechanism of Transmission

• Action potentials lead to neurotransmitter release at synapses, binding to receptors on the postsynaptic cell.

• Ion channels: Open following binding, affecting excitatory or inhibitory signaling.

Neuronal Functionality

Action Potentials

• Generated through the coordinated activity of voltage-gated ion channels, particularly Na+ channels.

• Influencing excitability and signal propagation through the neuron's structure effectively.

Myelination and Signal Propagation

• Myelin sheaths insulation increases action potential propagation speed by facilitating saltatory conduction.

Experimental Techniques

• Techniques such as patch-clamp recordings measure ion channel activities, determining their roles in cellular signaling.

Reference

• Molecular Biology of the Cell, 6th Edition, Chapters 10 and 11.