Lecture 15: Membranes and Transport

Chapter Overview

• Biological Membranes

• Membrane Proteins

• Membrane Dynamics

• Membrane Transport

Membrane Lipids

• Lipid structure varies by type and concentration:

  • Micelles: Formed by amphipathic molecules (fatty acids, detergents) with a single leaflet.

  • Bilayers: Comprise two leaflets of lipid monolayers with a hydrophilic head and hydrophobic tail.

  • Vesicles (liposomes): Result from small bilayers forming a central cavity to enclose dissolved molecules.

  • Importance of hydrophobic and polar characteristics in membrane structure.

Membranes

• Complex lipid-based structures defining cell boundaries, vital for:

  • Selective import and export of metabolites and ions.

  • Compartmentalization within eukaryotic cells, essential for separating energy-producing and consuming reactions.

  • Sensing external signals and transmitting information within the cell, critical for processes like nerve signal transmission.

Fluid Mosaic Model of Membranes

• Membranes are flexible structures (3-10 nm thick) forming spontaneously in aqueous solutions, stabilized by the hydrophobic effect.

• Composed of:

  • Integral proteins that penetrate the bilayer.

  • Peripheral proteins and sugars located externally.

Membrane Composition

• Composition varies among organisms, tissues, and organelles:

  • Differences in phospholipids, proteins, sterols, galactolipids (in plants).

  • Asymmetry exists between the two leaflets, influencing function and signaling capabilities.

Membrane Fluidity

• Membrane fluidity varies with temperature and lipid composition:

  • Cold temperatures can cause hardening; unsaturated fatty acids help maintain fluidity at low temperatures.

  • High temperatures necessitate more saturated fatty acids to prevent disintegration.

• Organisms can adjust fatty acid composition to maintain fluidity under varying temperature conditions.

Sterols in Membranes

• Sterols modulate membrane fluidity:

  • At warmer temperatures: decrease fluidity.

  • At cooler temperatures: prevent tight packing and increase fluidity.

  • Cholesterol predominantly found in animal plasma membranes, absent in mitochondria.

Membrane Proteins

• Functions include:

  • Catalyzing reactions, transporting molecules, and relaying signals.

• Types of proteins:

  • Peripheral membrane proteins: Loosely associated with polar head groups.

  • Integral membrane proteins: Tightly bound, often spanning the entire membrane with distinct domains.

Membrane Protein Structures

• Integral membrane proteins possess hydrophobic transmembrane segments (~20 amino acids).

• Charged amino acids localized in aqueous domains; Tyr and Trp cluster at interfaces.

Lipid Anchors

• Some proteins are anchored via reversible covalent linkages to lipids, a process involving prenylation.

Membrane Protein Functional Roles

• Receptors: Detect external signals (e.g., hormones).

• Channels and Pumps: Transport nutrients and ions across membranes.

• Enzymes: Participate in biosynthesis and energy production processes.

Membrane Dynamics

• Membranes are highly dynamic:

  • Lateral diffusion: Quick movement of lipids within the same leaflet.

  • Transverse diffusion: Rare spontaneous flips between leaflets, aided by lipid transporters.

  • Integral proteins can be tethered to the cytoskeleton, maintaining membrane structure.

Lipid Rafts

• Clusters of specific lipids and proteins within membranes that facilitate organized signaling and transport processes.

Membrane Fusion

• Mechanisms of membrane fusion include spontaneous processes and fusion mediated by SNARE proteins, which facilitate communication between vesicles and the plasma membrane.

  • Key in exocytosis and endocytosis events.

Membrane Transport Mechanisms

• Cell membranes allow passive diffusion of small nonpolar molecules but require proteins for polar molecule movement.

• Types of transporters and processes:

  • Uniport: One type of molecule transported.

  • Symport: Two molecules transported in the same direction.

  • Antiport: Two molecules transported in opposite directions.

• Transport mechanisms include:

  • Simple diffusion: Nonpolar movement down a gradient.

  • Facilitated diffusion: Uses proteins to move molecules down an electrochemical gradient.

  • Active transport: Requires energy to move molecules against a gradient (primary and secondary).

Specific Transporters**

• Aquaporins: Facilitate rapid water transport; critical in kidney function.

• Bicarbonate Transporters: Maintain electrochemical balance in erythrocytes.

• Glucose Transporters: Propel glucose import significantly, utilizing symport mechanisms.

ATPases**

• Enzymes phosphorylated by ATP, crucial in maintaining ion gradients and synthesizing ATP via proton gradients.

ABC Transporters**

• These transporters utilize ATP to move substances against gradients, including drugs; CFTR mutations can disrupt chloride transport, leading to diseases such as cystic fibrosis.