Lecture 15 - Membranes and Transport

Lecture 15: Membranes and Transport

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.

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.

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.

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 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 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.

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.

Integral membrane proteins possess hydrophobic transmembrane segments (~20 amino acids).
Charged amino acids localized in aqueous domains; Tyr and Trp cluster at interfaces.

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

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.

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

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.

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).

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.

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

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