Plasma Membrane Proteins

Plasma Membrane Proteins

• Ion channels: allow specific ions to move through water-filled pores; most plasma membranes include specific channels for several common ions
• Carrier proteins: carry specific substances across membrane by changing shape; examples include amino acid carriers
• Enzymes: catalyze reactions inside or outside cell; examples include lactase that splits lactose in the small intestine
• Linker proteins: anchor filaments inside and outside the plasma membrane, providing structural stability and shape for the cell
• Receptor proteins: recognize specific ligands and alter cell function; examples include antidiuretic hormone receptors in the kidneys
• Cell-identity markers (glycoproteins): distinguish cells from others; examples include major histocompatibility (MHC) proteins

Mechanisms of Membrane Transport

Passive Transport

• Diffusion: movement of solutes or water from high to low concentration
• Facilitated diffusion: requires specific channel or carrier molecule, no energy used

Active Transport

• Requires energy (ATP)

Simple Diffusion

• Movement of substance across membrane from high to low concentration

Facilitated Diffusion

• Requires membrane proteins; examples include channel-mediated and carrier-mediated facilitated diffusion

Carrier-Mediated Facilitated Diffusion

• Carrier proteins bind to molecules and change shape to move them across the membrane

Secondary Active Transport Mechanisms

• Use energy stored in Na+ or H+ concentration gradients to drive transport of other solutes against their concentration gradients
• Examples include antiporters and symporters

Antiporters

• Carry two substances across the membrane in opposite directions

Symporters

• Carry two substances across the membrane in the same direction

Ion Channels

• Allow passive movement of specific ions down their electrochemical gradient
• Regulated or "gated" holes through the membrane
• Examples include K+, Na+, Cl-, Ca2+ ion channels
• Mechanisms of gating include voltage, ligands, temperature, pH, and mechanical stress

Primary Active Transport Mechanisms (Pumps)

• Sodium-Potassium pump (Na-K-ATPase)
• Requires energy from ATP hydrolysis
• Crucial for maintaining cell volume and ionic gradients responsible for setting resting membrane potential and generating action potentials

Transporters (Carriers)

• Allow passive movement of solutes across membrane down their concentration gradient
• Examples include GLUT1 glucose transporter
• Exhibit saturation kinetics

Tonicity

• Measure of a solution's ability to change cell volume by altering water content
• Osmosis occurs between fluid compartments separated by semi-permeable membranes

Isotonic Solution

• Solution with same concentration of solutes as the cell

Hypotonic Solution

• Solution with lower concentration of solutes than the cell

Hypertonic Solution

• Solution with higher concentration of solutes than the cell

Passive Membrane Transport - Diffusion

• Random spread of particles from high to low concentration
• Depends on amount of substance, concentration gradient, temperature, surface area, and diffusion distance

Osmosis

• Passive net movement of water through a selectively permeable membrane from high to low water concentration
• Occurs when a membrane is permeable to water but not solutes
• Water can pass through plasma membrane by simple diffusion or through aquaporins (integral membrane proteins)

Function of the Plasma Membrane

• Flexible yet sturdy semi-permeable regulator
• Covers and protects the cell
• Controls what goes in and comes out
• Links to other cells
• Displays "flags" to tell other cells "who" it is