Anatomy& Physiology
Fluid Mosaic Model
The plasma membrane is described by the fluid mosaic model: a phospholipid bilayer with proteins, cholesterol, and carbohydrates embedded or attached. The lipids move laterally, giving the membrane fluidity, while proteins and other molecules form a mosaic within the bilayer.
Membrane Composition
Membrane lipids are the majority of the membrane, with about being phospholipids. Other components include cholesterol and glycolipids (glycolipids have sugars attached). Carbohydrates are often attached to lipids or proteins and participate in signaling and cell recognition.
Phospholipid Bilayer Orientation
Phospholipids arrange as a bilayer with polar (hydrophilic) heads facing the aqueous exterior and interior, and nonpolar (hydrophobic) tails facing inward. This forms a "butter sandwich" arrangement; the interior and exterior fluids are water-based.
Membrane Fluidity and Cholesterol
Cholesterol sits among the phospholipids and stabilizes the membrane. It modulates fluidity, preventing too much movement when conditions are warm and helping maintain proper fluidity when conditions are cold. In different species or environments, the cholesterol content helps membranes adapt to temperature changes.
Membrane Proteins: Integral vs Peripheral
Integral proteins span the membrane and can be seen from both sides; peripheral proteins are attached to one side of the membrane. Functions include transport, receptors for signaling, and enzymatic activity at the membrane. Most proteins are positioned to interact on or near the intracellular side.
Carbohydrates and Glycolipids
Glycolipids and glycoproteins carry carbohydrate chains that extend outward from the membrane. These sugars act as signals or identifiers (antennas) for cell communication and recognition.
Steroid vs Protein Hormones and Permeability
Steroid hormones are lipids (cholesterol derivatives) and can diffuse through the membrane to interact with intracellular receptors. Protein hormones are polar and cannot cross the membrane easily; they require membrane receptors to convey their signal.
Receptors and Cell Recognition
Membrane components enable cell recognition, crucial for immune function (macrophages, T and B cells) and fertilization (sperm-egg recognition). Receptors, transport proteins, and enzymes at the membrane coordinate signaling and transport across the membrane.