Membrane Structure and Function

Membrane Structure

• Biological membranes primarily form bilayers, studded with membrane-bound and transmembrane proteins.
• Cell membranes exhibit fluidity, enabling lateral movement of lipids and proteins. This is described by the Fluid Mosaic Model.
• Lipid bilayer thickness is approximately 5 nm.
• Transverse diffusion ("flip-flop") is very slow (t½ in days), while flippase-catalyzed transverse diffusion is fast (t½ in seconds). Lateral diffusion is very fast (1 μm/s).

Overview of Membrane Functions

• Compartmentalization: Membranes create enclosed intracellular compartments.
• Scaffold for biochemical activities: Membranes organize enzymes for efficient interaction.
• Selectively permeable barrier: Membranes regulate substance exchange between compartments.

Membrane & Storage Lipids

• Storage lipids: primarily energy storage (e.g., triglycerides)
• Membrane lipids: are polar. 
• Phospholipids are the major lipids found in biological membranes.
  • Glycerophospholipids: have a glycerol backbone, two fatty acid chains, and a phosphate head group. The head group is substituted with other molecules like choline, serine, or inositol.
• Glycolipids: have a carbohydrate head group attached
  • Sphingolipids: are similar to glycerophospholipids but have a sphingosine backbone instead of glycerol.
  • Archaebacterial ether lipids: have different backbone structures.

Phospholipid Structure

• Glycerophospholipids (general structure):
  • Glycerol backbone
  • Two fatty acid chains (saturated or unsaturated)
  • Phosphate head group with a polar substituent (e.g., choline, serine)

Fatty Acids

• Saturated fatty acids: have no double bonds between carbon atoms; example: palmitic acid.
• Unsaturated fatty acids: have one or more double bonds between carbon atoms; example: oleic acid.
• Saturation affects membrane fluidity; unsaturated fatty acids increase fluidity. 

Lipid Ordered and Disordered States

• Lipid-ordered state (gel): lipids are tightly packed.
• Lipid-disordered state (fluid): lipids are more fluid and less ordered.
• Temperature influences the transition between these states.

Membrane Asymmetry

• Phospholipid composition differs between the inner and outer leaflets of membranes.
• Different phospholipids are preferentially located in either the inner or outer layer. 

Membrane Proteins

• Membrane proteins are categorized as integral or peripheral.
• Integral proteins are embedded within the lipid bilayer.
• Peripheral proteins are associated with the membrane surface.
• Transmembrane domains: Integral proteins have transmembrane domains which can be composed of α helices or β barrels.

Glycophorin

• Glycophorin is an integral membrane protein.
• Its structure is shown with an amino terminus on the outside and a carboxyl terminus on the inside.

Types of Transporter Proteins

• Uniporter: Transports one solute across a membrane.
• Symporter: Transports two or more solutes in the same direction across a membrane.
• Antiport: Transports two or more solutes in opposite directions across a membrane.

Glucose Transporters

• GLUT1 - ubiquitous, basal glucose uptake
• GLUT2 - liver, pancreas; glucose uptake; insulin regulation
• GLUT3 - basal glucose uptake
• GLUT4 - muscle and fat; insulin induced glucose uptake
• Other transporters (GLUTs 5-12) exist with specialized roles and distributions

Microdomains (Lipid Rafts) in Plasma Membrane

• Lipid rafts are specialized regions in the plasma membrane that are enriched in sphingolipids and cholesterol.
• They play a role in localized signal transduction modules.

Active and Passive Transport

• Passive transport: Movement of molecules across a membrane without energy input.
• Active transport: Movement of molecules across a membrane with energy input.

Membrane Potentials

• Membrane potential is the electrical potential difference across a membrane.
• Electrochemical gradient determines movement of charged particles
• Ion concentration gradients are maintained by active transport

ATP Synthase

• ATP Synthase is a multisubunit protein involved in the synthesis and use of ATP
• ATP synthase uses energy from a proton gradient to catalyze ATP synthesis
• 3 protons for 1 ATP
• It's located in mitochondria or chloroplasts.

Explore the fundamental aspects of biological membranes, including their structure, fluidity, and key functions. This quiz covers the Fluid Mosaic Model, lipid bilayers, and the roles of membranes in cellular compartmentalization and selectively permeable barriers.