Recording-2025-02-12T15:00:36.784Z

Key phospholipids of interest:
- Phosphatidylcholine: A major component of the cell membrane.
- Phosphatidylserine: Similar to phosphatidylcholine; important for cell signaling and apoptosis.

Both phosphatidylcholine and phosphatidylserine contain:
- Fatty acids
- Glycerol
- Phosphate
Difference: Phosphatidylcholine has choline, while phosphatidylserine has serine.

Cell membranes demonstrate asymmetry with specific lipids concentrated on the inner and outer layers:
- Extra cytosolic phase: Refers to the environment outside the cell.
- Cytosolic phase: Pertains to the inside of a vesicle.
Membrane asymmetry is established and maintained with specific proteins:
- Flipases: Move specific phospholipids (ex. phosphatidylserine) from the outer to the inner leaflet.
- Flopases: Move phospholipids from the inner to the outer leaflet.
- Scramblases: Randomly flip lipids across the membrane, disrupting established asymmetry.

Once established, membrane asymmetry is stable due to:
- Difficulty in lipid flip-flopping (crossing from one face to the other).
- Lipids freely diffuse laterally but seldom flip-flop due to energetic barriers.
Energy is required to establish asymmetry, notably involving enzyme actions (flipases/flopases).

Smooth ER: Lacks asymmetry; uses scramblases to equalize lipid distribution during synthesis.
Golgi Apparatus: Serves as a sorting center:
- Utilizes flipases and flopases to establish and maintain lipid asymmetry for vesicles coming from the ER.

Apoptosis: A regulated form of cell death crucial for tissue remodeling and clearance of damaged cells.
- Involves the activation of caspases, proteolytic enzymes that degrade key cellular components.
- Triggers scramblases to flip phosphatidylserine to the outer leaflet, signaling for phagocytosis by immune cells.

Caspases: Proteases activated to initiate apoptosis, leading to:
- Dimerization of procaspases (inactive form).
- Cascades of biochemical events leading to cellular disassembly (nuclear fragmentation, membrane blebbing).

Scramblases are normally inactive in healthy cells to preserve membrane asymmetry.
During apoptosis, scramblases are activated by caspases, allowing flipping of phosphatidylserine, marking cell for removal by phagocytes.
Flopases may be inactivated by caspases to prevent lipid asymmetry from being restored during cell death.

Types of Membrane Proteins:
- Integral Proteins: Span the membrane; involved in transport and signaling.
- Peripheral Proteins: Loosely attached via weak interactions; contribute to cell signaling and cytoskeletal attachment.
Functions include:
- Transport: Across the membrane (selective permeability).
- Anchoring: To cytoskeleton and extracellular matrix.
- Receptor Activity: Binding to ligands for signal transduction.

Alpha Helices: Common structure for membrane-spanning regions; hydrophobic amino acids oriented towards lipid bilayer.
Beta Barrels: Less common; form pores in membranes, allowing passage of specific molecules.

Fluorescence Recovery After Photobleaching (FRAP):
- A technique to study membrane fluidity by tracking recovery of fluorescence in a bleached area.
- Outcomes: Mobile proteins recover fluorescence, while immobilized proteins do not.

Apoptosis serves vital roles in:
- Development: Removes unnecessary cells (e.g., webbing in amphibians).
- Immune response: Eliminates infected or damaged cells to maintain organism health.
Apoptosis is tightly regulated to prevent inappropriate cell death, which can lead to diseases such as cancer.