Cell Membrane: Fluid Mosaic Model

• The cell membrane forms a barrier containing cell contents and is semipermeable, allowing water to pass but not larger molecules.

Fluid Mosaic Model

• Proposed by Jonathan Singer and Garth Nicolson in 1972.
• Phospholipids form a bilayer with hydrophobic ends facing the middle and hydrophilic ends towards watery contents.
• Proteins are interspersed throughout the phospholipid bilayer.
• Carbohydrates are present on the cell membrane surface for cell recognition, important in immune response and rejection of foreign cells.
• Glycolipids form when some carbohydrates bond to lipids.
• Glycoproteins form when most carbohydrates bond to proteins.
• Phospholipids and some proteins can move, making their location 'fluid'.
• The model explains semipermeability and molecule passage.

Movement of Phospholipids

• Phospholipids move laterally within the membrane rapidly (approximately 2 um per second).
• Adjacent phospholipids swap positions 10^7 times per second.
• Unsaturated hydrocarbon tails have 'kinks' that reduce tight packing and increase membrane fluidity.
• Organisms regulate unsaturated fatty acids to compensate for temperature changes as lipids solidify at low temperatures.
• Membranes stay fluid and don't crystallize at low temperatures with high unsaturated hydrocarbon amounts.

Membrane Proteins

• Large protein molecules are embedded in the phospholipid bilayer.
• Integral membrane proteins (IMPs) are permanently attached with some extending from one side to the other as transmembrane proteins.
• Some IMPs only extend partway across the lipid layer.
• Peripheral proteins are attached to the surface and part of the integral protein.
• Different proteins have different functions and compositions.
• Transport proteins assist the movement of ions, small molecules, or macromolecules.
• Membrane receptor proteins communicate with the environment by receiving signals and triggering specific internal responses.
• Membrane enzymes modify molecules near the cell surface.

Signal Transduction

• Occurs when an extracellular molecule activates an IMP receptor; the receptor sends a message into the cell to signal gene activation or alter chemical/cell metabolism.

Diffusion Across the Cell Membrane

• Lipid-soluble molecules (oxygen, carbon dioxide) dissolve in the phospholipid bilayer and diffuse across.
• Water diffuses across the cell membrane through temporary openings made by fluid lipids.
• Aquaporins are integral membrane proteins that form water channels for fast water movement.

Facilitated Diffusion Across Cell Membrane

• Compounds combine with carrier molecules to cross the membrane.
• Carrier molecules move substances (large, water-soluble molecules, amino acids, simple sugars) across the membrane.
• Active transport occurs when energy is used to move a chemical against a concentration gradient.