Biological Membranes

Membranes serve as barriers, defining cell boundaries and compartmentalizing the internal cell structure.

Membranes are complex lipid-based structures that form pliable sheets, composed of a variety of lipids and proteins.
Membrane lipids can be glycosylated.
All cells have a cell membrane separating the internal environment from the external surroundings.
Eukaryotic cells possess numerous internal membranes that create cellular compartments.

Lipids aggregate into distinct structures in aqueous solutions depending on lipid type and concentration.
- Micelles: Form from amphipathic molecules with larger heads than tails (e.g., fatty acids, sodium dodecyl sulfate). These can contain from a few dozen to thousands of lipid molecules. Aggregation occurs above a specific concentration threshold.
- Vesicles (Liposomes): Small bilayers that encapsulate an aqueous cavity, which can hold dissolved substances (e.g., drugs). These vesicles can fuse with cell membranes.
- Membrane Bilayer: Comprises two leaflets of lipid monolayers—hydrophilic head groups face water while hydrophobic tails are buried inside.

Membrane proteins can be integral or peripheral. Integral proteins often span the membrane while peripheral proteins associate loosely with the membrane via ionic interactions.
Functions of membrane proteins:
- Receptors: Detect signals such as light, hormones, neurotransmitters, and pheromones.
- Channels, Gates, Pumps: Facilitate the transport of nutrients and ions across membranes.
- Enzymes: Play roles in lipid biosynthesis and ATP synthesis.

Membranes are selectively permeable; small nonpolar molecules can diffuse passively, while polar molecules require specialized transport proteins.
Types of Transport:
- Simple Diffusion: Nonpolar molecules move down their concentration gradient.
- Facilitated Diffusion: Uses transport proteins to move substances down their electrochemical gradient.
- Active Transport: Moves substances against their gradient, typically costing energy (ATP).
  - Primary Active Transport: Directly driven by ATP.
  - Secondary Active Transport: Driven by the electrochemical gradient created by primary active transport.

Membrane thickness: 30 – 100 Å (3 – 10 nm).
Lipid bilayers form spontaneously and are stabilized by noncovalent forces, notably the hydrophobic effect.
Asymmetric design: Different lipid compositions and carbohydrate moieties are oriented towards the extracellular space. Membranes are also electrically polarized (approximately -60 mV inside).
The fluid mosaic model illustrates the dynamic nature of membranes, where lipid and protein components can move freely, influencing membrane function.

Variance in lipid composition is observed across organisms, tissues, and organelles.
- Cholesterol is common in plasma membranes but absent in prokaryotes.
- Different organelles possess distinct lipid compositions.
- Membranes in archaea feature unique glycerol chirality and linkages compared to bacteria.

Lipid Rafts: Clusters of glycosphingolipids and associated proteins, providing functional complexity to membranes.
Caveolin: A protein that induces membrane curvature required for vesicle formation.
Membrane fusion can occur spontaneously or be mediated by proteins (e.g., during neurotransmitter release).

Understanding membrane structure and function is critical, as membranes play essential roles in transport, communication, and cellular organization.
Active transport mechanisms rely heavily on ATP, underlining the energy demands of cellular function.

Membranes are complex lipid-based structures that form pliable sheets, composed of a variety of lipids and proteins.
Membrane lipids can be glycosylated.
All cells have a cell membrane separating the internal environment from the external surroundings.
Eukaryotic cells possess numerous internal membranes that create cellular compartments.
Common anchor lipids include:
- Glycosylphosphatidylinositol (GPI): A glycolipid that anchors proteins to the membrane.
- Fatty acyl chains: Commonly palmitic (16:0) or myristic acid (14:0) attached to proteins.
- Prenyl groups: such as farnesyl (15 carbons) or geranylgeranyl (20 carbons), used to tether proteins to membranes.

Membrane proteins can be integral or peripheral. Integral proteins often span the membrane while peripheral proteins associate loosely with the membrane via ionic interactions.
Functions of membrane proteins:
- Receptors: Detect signals such as light, hormones, neurotransmitters, and pheromones.
- Channels, Gates, Pumps: Facilitate the transport of nutrients and ions across membranes.
- Enzymes: Play roles in lipid biosynthesis and ATP synthesis.
Seven categories of attachments to the membrane include:
1. Glycosylphosphatidylinositol (GPI) anchors
2. Fatty acylation
3. Prenylation
4. Myristoylation
5. Palmitoylation
6. Non-covalent interactions (with other membrane components)
7. Disulfide bonds (in some cases, linking proteins to membrane structures)