urinary system membrane transport system

• What are the four primary means by which water and other small molecules can cross into or out of cells?

  • Diffusion
  • Facilitated Diffusion
  • Primary Active Transport
  • Secondary Active Transport
• What is diffusion?

  Diffusion is powered by the random movement of molecules in a solution, resulting in net movement from high concentration to low concentration.
• Why does diffusion not saturate?

  Because the random movement of molecules is not limited by available carriers or pumps.
• How does the permeability of a membrane affect net flux?

  Net flux is proportional to the permeability of the membrane.
• Which substances can cross cell membranes by diffusion?

  Substances that can dissolve in the membrane, typically hydrophobic or lipid-soluble substances.
• What is osmosis?

  Osmosis is the diffusion of water down its concentration gradient.
• What are the key features of facilitated diffusion?

  • Utilizes carrier proteins or pores to move substances
  • Movement is passive from high to low concentration
  • Occurs only across cell membranes
  • Subject to saturation due to limited carriers or pores
  • Allows competition between related substances for the same carrier
• What is $T_m$ in facilitated diffusion?


  Tm is the transport maximum, the maximum rate of transport when the carrier protein is fully saturated.
• What are the characteristics of primary active transport?

  • Employs protein pumps to move substances
  • Moves molecules against their concentration gradient
  • Requires cellular energy, usually ATP
  • Subject to saturation due to limited protein pumps
  • Example: $Na-K ATPase$ which pumps 3 Na ions out for 2 K ions in (cost is 1 ATP)
• What is the role of $Na-K ATPase?$


  The Na-K ATPase pumps 3 $Na^+$ ions out of the cell and 2 $K^+$ ions into the cell, using 1 ATP.
• What is the function of primary active transport in cells?

  To move ions or small molecules against their concentration gradient, essential for establishing concentration gradients.
• What are the features of secondary active transport?

  • Utilizes proteins similar to facilitated diffusion
  • Couples the movement of several different molecules
  • Can saturate due to limited protein availability
  • Includes co-transport and counter-transport
  • Relies on the strong Na gradient established by normal active transport mechanisms such as $Na-K ATPase$
• What are the two types of secondary active transport?

  Co-transport and counter-transport.
• How is secondary active transport powered?

  By coupling with the strong Na gradient established by primary active transport mechanisms like $Na-K ATPase.$
• What is co-transport?

  Co-transport moves two or more molecules in the same direction across the membrane.
• What is counter-transport?

  Counter-transport moves molecules in opposite directions across the membrane.