Ch.11

Separation of Environment: Membranes separate the internal environment of the cell from the external environment.
Control of Passage: Regulate the passage of materials into and out of the cell via transport proteins and channels that selectively allow specific ions, nutrients, and waste products to cross, ensuring that nutrients enter and waste products are expelled.
Information Reception: Cells can receive information through specific receptor proteins and sensors embedded in the membrane that bind to signaling molecules (ligands) and relay information to the cell interior, initiating cellular responses.
Structural Integrity: Membranes help maintain the shape of the cell and can assist in motility, providing specific functions unique to different cell types.

General Structure: The plasma membrane is primarily composed of a lipid bilayer, which consists of phospholipids, cholesterol, and proteins.
Key Lipids: Phosphatidylcholine is noted as the most common lipid in cell membranes. Besides phosphatidylcholine, other common phospholipids include phosphatidylethanolamine, phosphatidylserine, and sphingomyelin, each contributing to the membrane's overall charge and recognition properties.

Membrane proteins contribute significantly to the mass of the membrane (approximately 50% of total membrane mass).

Contains both external membranes and internal membranes such as the endoplasmic reticulum, which interact with various cellular components like the nucleus, lysosomes, peroxisomes, and mitochondria.

Phospholipids have unique properties regarding their interaction with water (phospholipids are amphipathic).
Amphipathic Molecules: These structures possess both a hydrophilic (water-attracting) head containing a phosphate group and a hydrophobic (water-repelling) tail consisting of two hydrocarbon chains. This dual nature is crucial for their spontaneous self-assembly into a bilayer in aqueous environments.

Five parts that constitute a molecule of phosphatidylcholine:
1. Hydrophobic Tails: Two hydrocarbon tails.
2. Hydrophilic Head: A phosphate group attached to a choline section.
Example Lipids:
- Phosphatidylserine: A common phospholipid.
- Cholesterol: A sterol contributing to membrane structure.
- Galactocerebroside: A glycolipid derived from animal fats.

The amphipathic structure causes molecules in water to arrange themselves into a bilayer, forming a stable barrier where the hydrophobic tails are shielded from water by the hydrophilic heads. This arrangement minimizes unfavorable interactions between water and nonpolar tails.
Self-Sealing: The bilayer can spontaneously rearrange itself to seal tears due to the energetic favorability of hydrophobic tails being sequestered from water, rather than being exposed at an edge.

Fluidity: Refers to the ease of movement of lipid molecules within the plane of the bilayer, including lateral diffusion, rotation, and flexion of hydrocarbon chains. Flip-flop (transverse diffusion) to the opposite leaflet is very rare without the aid of specific enzymes due to the high energy cost of moving a hydrophilic head through the hydrophobic core.
Influencing Composition:
- Longer tails (>18 carbon atoms) lead to less fluidity due to increased van der Waals interactions and tighter packing.
- Shorter tails (<18 carbon atoms) increase fluidity due to less interaction between molecules.
- Unsaturated fats with double bonds create 'kinks' in their tails, preventing tight packing and allowing greater fluidity, making them more difficult to freeze.

Saturated Fats: Solid at room temperature due to tightly packed hydrocarbon tails, which result in lower fluidity.
Unsaturated Fats: Generally liquid at room temperature due to the presence of double bonds that create kinks in the tails, preventing tight packing and enhancing fluidity.

Cholesterol constitutes about 20% of the cell membrane and plays a crucial role in modulating membrane properties:
- Increases rigidity: By integrating within the bilayer, cholesterol stiffens the membrane.
- Decreases fluidity: Restricts movement of phospholipids.
- Decreases permeability: Reduces the likelihood of passage for certain molecules across the membrane.

Membranes are synthesized in the endoplasmic reticulum (ER):
- Random assembly of phospholipids occurs through the action of an enzyme called scramblase, which randomly equilibrates phospholipids between the two leaflets of the ER membrane, promoting symmetrical growth.
- The Golgi apparatus further modifies and refines this structure. Specific ATP-dependent enzymes called flippases selectively transfer specific phospholipids from the non-cytosolic leaflet to the cytosolic leaflet, establishing and maintaining the asymmetric distribution of phospholipids characteristic of the plasma membrane and other organelles.

Membrane proteins are vital for carrying out various functions within the cell membrane:
- Transporters and Channels: Allow selective transport of substances across the membrane.
- Anchors: Secure other molecules to the membrane.
- Receptors: Bind external signals and cause internal cellular changes.
- Enzymes: Catalyze reactions converting molecules from one form to another.

Transmembrane Proteins: Integral proteins that span the entire membrane, often as alpha-helices or beta-barrels, interacting extensively with both the hydrophobic core and aqueous environments on either side.
Monolayer-Associated Proteins: Integral proteins anchored to only one side of the bilayer, usually contacting the cytosolic leaflet via an amphipathic alpha-helix or other hydrophobic interactions.
Lipid-Linked Proteins: Proteins located entirely outside the bilayer, on either the cytosolic or non-cytosolic face, bound covalently to one or more lipid molecules that are inserted into the bilayer.
Protein-Attached Proteins: Peripheral proteins that associate with the membrane indirectly by noncovalent interactions with other membrane proteins, rather than directly with the lipid bilayer. These can be easily removed without disrupting the membrane.