Exam 4- chap 11 Membrane

Membrane Structure and Architecture

Composition and Organization of Membranes

Biological membranes are essential for compartmentalization in cells, composed primarily of a lipid bilayer with embedded proteins. The amphipathic nature of phospholipids drives the formation of bilayers, which are stabilized by hydrophobic interactions.

• Phospholipids have hydrophilic heads and hydrophobic tails, causing them to self-assemble into bilayers in aqueous environments.

• Bilayer formation is energetically favorable due to the exclusion of water from hydrophobic tails.

• Micelles vs. Bilayers: Bilayers are more stable for phospholipids due to their two hydrophobic tails, while micelles are favored by single-tailed lipids.

Example: The plasma membrane of eukaryotic cells is a classic example of a lipid bilayer with embedded proteins.

Membrane Functions

Major Roles of Membranes

Membranes perform a variety of functions essential for cellular life.

• Compartmentalization of cellular processes

• Regulation of transport (selective permeability)

• Signal transduction and communication

• Energy transduction (e.g., mitochondria, chloroplasts)

• Cell recognition and adhesion

Membrane Proteins: Types and Functions

Classification of Membrane Proteins

Membrane proteins are classified based on their association with the lipid bilayer:

Type	Location	Function
Integral	Span or are embedded in the bilayer	Transport, signaling, structural support
Peripheral	Associated with membrane surface	Enzyme activity, signaling, cytoskeletal attachment
Amphitropic	Reversibly associate with membrane	Regulation, signaling

• Integral proteins often contain transmembrane α-helices or β-barrels.

• Peripheral proteins interact via non-covalent interactions and can be removed by salt or pH changes.

• Amphitropic proteins associate with membranes in response to signals or modifications.

Example: Receptors, channels, and enzymes are common integral membrane proteins.

Membrane Fluidity

Factors Affecting Fluidity

Membrane fluidity is determined by lipid composition and temperature.

• Unsaturated fatty acids increase fluidity due to kinks in their tails.

• Saturated fatty acids decrease fluidity by allowing tight packing.

• Cholesterol modulates fluidity by disrupting packing at low temperatures and restraining movement at high temperatures.

Example: Bacteria adjust fatty acid composition in response to temperature changes to maintain membrane fluidity.

Membrane Domains and Rafts

Lipid Rafts and Protein Targeting

Membranes are not uniform; specialized domains called lipid rafts are enriched in cholesterol and sphingolipids, serving as platforms for signaling and trafficking.

• Rafts organize specific proteins for efficient signaling.

• Targeting proteins to rafts can increase the efficiency of cellular processes.

Example: GPI-anchored proteins are often found in lipid rafts.

Membrane Fusion

Role in Cellular Processes

Membrane fusion is critical for processes such as vesicle trafficking, neurotransmitter release, and cell division.

• Fusion involves proteins that mediate the merging of lipid bilayers.

• SNARE proteins are key mediators of vesicle fusion in neurons.

Example: Neurotransmitter release at synapses requires membrane fusion.

Membrane Transport Mechanisms

Passive vs. Active Transport

Transport across membranes can be passive (no energy required) or active (energy required).

• Passive transport includes simple diffusion and facilitated diffusion.

• Active transport requires energy, often from ATP hydrolysis.

Equation for Free Energy Change in Transport:

$\Delta G = RT \ln \left( \frac{C_2}{C_1} \right) + ZF\Delta \Psi$

• $C_1$ and $C_2$ are concentrations on either side of the membrane.

• $Z$ is the charge, $F$ is Faraday's constant, $\Delta \Psi$ is the membrane potential.

Types of Transporters

• Channels: Allow rapid movement of ions or water (e.g., K+ channels).

• Carriers: Bind and transport specific molecules (e.g., glucose transporter).

• Pumps: Use energy to move substances against gradients (e.g., Na+/K+ ATPase).

Facilitated Diffusion

Facilitated diffusion uses transport proteins to move substances down their concentration gradients without energy input.

• Binding energy lowers activation energy for transport.

• Specificity arises from precise interactions between transporter and substrate.

Example: Glucose transport into cells via GLUT transporters.

Active Transport

Active transport moves substances against their gradients, requiring energy.

• Primary active transport: Direct use of ATP (e.g., Na+/K+ ATPase).

• Secondary active transport: Uses gradients established by primary transporters (e.g., symporters and antiporters).

ABC Transporters use ATP to export hydrophobic drugs and metabolites.

Ion Channels and Gated Transport

Voltage-Gated and Ligand-Gated Channels

Ion channels can be regulated by voltage changes or ligand binding.

• Voltage-gated channels open in response to changes in membrane potential.

• Ligand-gated channels open when specific molecules bind to the channel.

Example: Potassium channels open during action potentials in neurons.

Neurotransmitters and Channel Activation

Neurotransmitters can trigger ligand-gated ion channels, leading to rapid cellular responses.

Neurotransmitter	Receptor Type	Effect
Acetylcholine	Nicotinic receptor	Opens Na+ channels
GABA	GABAA receptor	Opens Cl- channels
Glutamate	NMDA/AMPA receptor	Opens Ca2+/Na+ channels
Additional info:	Other neurotransmitters (e.g., glycine, serotonin) also act via ligand-gated channels.

Summary Table: Major Transporters in Epithelial Cells

Transporter	Location	Function
Na+/K+ ATPase	Basolateral membrane	Pumps Na+ out, K+ in
Glucose transporter (SGLT)	Apical membrane	Co-transports Na+ and glucose into cell
ABC transporter	Various	Exports hydrophobic drugs/metabolites
Additional info:	Other transporters include Cl- channels and antiporters for pH regulation.

Key Equations and Concepts

• Free energy change for transport: $\Delta G = RT \ln \left( \frac{C_2}{C_1} \right) + ZF\Delta \Psi$

• Binding energy lowers activation energy for facilitated diffusion.

• ATP hydrolysis provides energy for active transport.

Additional info:

• Membrane trafficking, signaling, and disease mechanisms are closely linked to membrane structure and transport.

• Disorders such as cystic fibrosis result from defects in membrane transport proteins.