Membrane Structure and Function
Membrane Components
The plasma membrane consists of a phospholipid bilayer.
Phospholipids are amphipathic, containing both hydrophilic and hydrophobic regions.
Hydrophilic phosphate heads face outward, while hydrophobic tails face inward.
The plasma membrane also contains membrane proteins, most of which span the phospholipid bilayer and are amphipathic.
Other components include carbohydrates, cholesterol, and glycoproteins.
Fluid Mosaic Model
The membrane is described by the fluid mosaic model, featuring two fluid layers of phospholipids.
Lipids and proteins can move easily within their respective layers.
Lipid rafts contain patches of proteins that work together.
Membrane Proteins
Transmembrane proteins are embedded within and completely cross the membrane.
They have hydrophobic regions within the membrane and hydrophilic regions on either side.
Peripheral proteins are not embedded in the membrane but are found on the cytoplasmic or extracellular side, anchored by the cytoskeleton or other proteins.
Membrane Protein Function
Transport: Move molecules across the membrane.
Enzymatic Activity: Catalyze reactions.
Intercellular Joining: Hold cells together.
Attachment: Anchor to the cytoskeleton or extracellular matrix.
Signal Transduction: Bind signaling molecules to transmit messages.
Cell-to-Cell Recognition: Allow cells to recognize each other via unique membrane proteins.
Membrane Fluidity
Fluidity is affected by temperature, saturated fatty acid tails, and cholesterol.
Increased temperature increases fluidity.
Increased saturated fatty acid tails decrease fluidity.
Cholesterol acts as a fluidity buffer, preventing the membrane from becoming too fluid or too rigid.
Transport Across Plasma Membrane
The plasma membrane is selectively permeable.
Permeable to small, nonpolar molecules (e.g., and ).
Not permeable to large or hydrophilic molecules, which require transport proteins.
Diffusion
Molecules move down their concentration gradient (from high to low concentration).
Does not require energy.
Equilibrium occurs when movement in both directions is equal.
Osmosis
Water moves across a membrane to reach equilibrium.
Water can cross directly or through aquaporins (transport proteins).
Does not require energy.
Water Balance of Cells
Tonicity refers to the impact of the surrounding solution on a cell’s water loss or gain.
Isotonic: Equal water movement in and out of the cell.
Hypotonic: Surrounding solution has lower solute concentration, causing water to enter the cell.
Hypertonic: Surrounding solution has higher solute concentration, causing water to exit the cell.
Facilitated Diffusion
Large and/or hydrophilic molecules are facilitated by transport proteins.
Molecules move down their concentration gradient.
Two types of transport proteins: channel proteins (open tunnel) and carrier proteins (undergo conformational change).
Active Transport
Molecules are pumped against their concentration gradient (from low to high concentration).
Requires energy (ATP) and carrier proteins.
Example: Sodium-potassium pump maintains a high concentration of sodium outside the cell and a high concentration of potassium inside the cell.
Cotransport
One transport protein moves two molecules in the same direction.
One molecule moves down its concentration gradient, while the other moves against its concentration gradient.
Bulk Transport
Large molecules cross the membrane via vesicles.
Exocytosis: Vesicle fuses with the plasma membrane to release contents outside the cell.
Endocytosis: Plasma membrane folds inward to form a vesicle and bring molecules into the cell.
Types of Endocytosis
Phagocytosis: Intake of food particles.
Pinocytosis: Intake of extracellular fluid.
Receptor-mediated endocytosis: Intake of molecules bound to receptors on the cell surface.