Membrane transport

• Simple diffusion:

  • Passive process that doesn't require energy (no ATP utilized)
  • Allows molecules to move from areas of high concentration to areas of low concentration through the cell membrane
  • Molecules that can move by simple diffusion: oxygen, CO2, steroid hormones, lipid-soluble drugs
• Cell membrane structure:

  • Phospholipid bilayer with polar phospholipid heads and nonpolar fatty acid tails
  • Polar heads repel charged substances from crossing the membrane
  • Nonpolar fatty acid tails allow nonpolar molecules like oxygen, CO2, steroid hormones, and lipid-soluble drugs to pass through
• Factors affecting rate of diffusion (specifically for oxygen and CO2):

  • Surface area: larger surface area increases diffusion rate
  • Concentration gradient: higher gradient leads to more molecules moving in the corresponding direction
  • Thickness of the cell membrane: thicker membrane decreases diffusion rate
  • Weight of the molecule: heavier weight decreases diffusion rate
• Facilitated diffusion:

  • Passive process that requires transport proteins (channels or carriers) to shuttle molecules across the cell membrane
  • Types of facilitated diffusion channels: leaky, voltage-gated, ligand-gated, mechanically gated
  • Osmosis is a type of facilitated diffusion for water movement from areas of high concentration to low concentration or low solute concentration to high solute concentration
  • Aquaporins are channels that allow water to move across the cell membrane
• Leaky channels are important for allowing charged or polar molecules to move across the cell membrane
• Potassium leaky channels are crucial, especially in neurons
• Potassium leaky channels control the resting membrane potential in neurons
• Voltage-gated channels are significant for action potentials in neurons
• Voltage-gated channels open when a specific threshold voltage is reached
• Sodium and calcium are examples of ions that flow through voltage-gated channels
• Ligand-gated ion channels are essential for inducing action potentials, especially at the neuromuscular junction
• Acetylcholine binding opens ligand-gated ion channels, allowing ions like sodium to flow in
• Ligand-gated ion channels are crucial for muscle contraction
• Mechanically gated channels are stimulated by mechanical stress, such as pressure on pain receptors
• Sodium ions flow through mechanically gated channels, inducing action potentials
• Mechanically gated channels are important for activating pain receptors and sending pain signals to the central nervous system
• Carrier-mediated facilitated diffusion involves proteins like glut transporters to move molecules like glucose across the cell membrane
• Glut4 transporters are crucial for moving glucose into adipose and muscle tissues
• Insulin regulates the activity of glut transporters, increasing glucose uptake into cells
• Primary active transport directly uses ATP to move molecules against their concentration gradient
• Sodium-potassium ATPases are vital for moving sodium and potassium ions against their gradients
• Insulin and thyroid hormones can regulate the activity of sodium-potassium ATPases
• Digoxin inhibits sodium-potassium ATPases, affecting heart contractility
• Calcium pumps are another example of primary active transport, moving calcium ions against their concentration gradient
• Calcium ATPases are important for regulating calcium levels in cells
• Calcium pumps play a crucial role in muscle contraction and other cellular processes
• Calcium ATPases are important for muscle relaxation
• During muscle relaxation, calcium ions need to be pushed into the sarcoplasmic reticulum to prevent muscle contraction
• Sarcoplasmic reticulum acts as a calcium storage center
• ATP is required to pump calcium from the muscle cytoplasm into the sarcoplasmic reticulum
• Increased sympathetic nervous system activity can stimulate calcium ATPases to push more calcium into the sarcoplasmic reticulum, leading to increased muscle contractility
• Proton pumps in the stomach are essential for producing hydrochloric acid
• Proton pumps push protons against their concentration gradient from the parietal cell into the stomach lumen
• ATP is needed for this process
• Proton pump inhibitors can control the activity of proton pumps, reducing the production of hydrochloric acid and treating conditions like GERD and peptic ulcer disease
• Secondary active transport involves molecules moving with or against their concentration gradients
• Symport is when molecules move in the same direction, while antiport is when they move in opposite directions
• Sodium is often involved in secondary active transport, moving down its concentration gradient
• Sodium glucose symporter allows glucose to enter cells by piggybacking on sodium
• SGLT2 inhibitors can inhibit this transporter, leading to increased glucose excretion in urine and reduced blood glucose levels in diabetes