7.3 Passive and active transport mechanisms

Passive and active transport mechanisms are crucial for cellular function. Passive transport moves molecules down concentration gradients without energy, while active transport uses energy to move molecules against gradients. These processes maintain cell homeostasis and enable essential functions.

Diffusion, osmosis, and facilitated diffusion are key passive transport methods. Active transport includes primary types using ATP directly and secondary types leveraging existing gradients. Understanding these mechanisms is vital for grasping how cells regulate their internal environment and interact with surroundings.

Passive vs Active Transport

Energy Requirements and Concentration Gradients

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• Passive transport moves molecules across a membrane down their concentration gradient without using energy, while active transport requires energy to move molecules against their concentration gradient
• The direction of molecule movement in passive transport is determined by the concentration gradient (high to low concentration), whereas active transport can move molecules in the opposite direction of the concentration gradient (low to high concentration)

Types of Passive and Active Transport

• Passive transport includes simple diffusion and facilitated diffusion, which rely on the kinetic energy of molecules and the permeability of the membrane
  • Simple diffusion occurs through the lipid bilayer without the help of membrane proteins
  • Facilitated diffusion involves carrier proteins or channel proteins that facilitate the movement of specific molecules
• Active transport involves the use of carrier proteins or pumps that undergo conformational changes to move molecules across the membrane, utilizing energy in the form of ATP
  • Primary active transport directly uses ATP hydrolysis to move molecules against their concentration gradient (sodium-potassium pump)
  • Secondary active transport relies on the electrochemical gradient generated by primary active transport to move other molecules (sodium-glucose cotransporter)

Diffusion and Osmosis

Principles of Diffusion

• Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration, driven by the random motion of molecules and their kinetic energy
• The rate of diffusion is influenced by factors such as the concentration gradient (larger gradient leads to faster diffusion), temperature (higher temperature increases kinetic energy and diffusion rate), and the size and charge of the molecules (smaller, uncharged molecules diffuse faster)
• Diffusion occurs until the concentrations on both sides of the membrane are equal, reaching a dynamic equilibrium

Osmosis and Tonicity

• Osmosis is a special case of diffusion that involves the movement of water molecules across a semipermeable membrane from a region of high water potential (low solute concentration) to a region of low water potential (high solute concentration)
• The osmotic pressure is the pressure required to stop the net movement of water molecules across a semipermeable membrane in a solution
• Tonicity describes the relative concentration of solutes in a solution compared to another solution, which determines the direction of water movement
  • Isotonic solutions have equal solute concentrations, resulting in no net water movement (normal saline)
  • Hypotonic solutions have lower solute concentrations, causing water to move into the cell and leading to cell swelling or lysis (distilled water)
  • Hypertonic solutions have higher solute concentrations, causing water to move out of the cell and leading to cell shrinkage (concentrated salt solution)

Facilitated Diffusion with Carrier Proteins

Carrier Proteins and Channel Proteins

• Facilitated diffusion is a type of passive transport that involves the use of carrier proteins or channel proteins to facilitate the movement of specific molecules across the membrane
• Carrier proteins have binding sites that are specific to certain molecules, such as glucose or amino acids, and undergo conformational changes to transport the molecules across the membrane
  • Example: GLUT (glucose transporter) proteins facilitate the movement of glucose into cells
• Channel proteins form hydrophilic pores that allow the passage of specific ions or small molecules, such as potassium or chloride ions, down their concentration gradient
  • Example: Potassium channels allow the selective movement of potassium ions across the membrane

Characteristics of Facilitated Diffusion

• The rate of facilitated diffusion is limited by the number of available carrier proteins or channels and can be saturated when all the binding sites are occupied
• Facilitated diffusion is specific to certain molecules, depending on the type of carrier protein or channel involved
• Like simple diffusion, facilitated diffusion does not require energy input and moves molecules down their concentration gradient

Primary vs Secondary Active Transport

Primary Active Transport

• Primary active transport directly uses energy from ATP hydrolysis to move molecules against their concentration gradient
• The sodium-potassium pump (Na+/K+ ATPase) is an example of primary active transport that maintains the concentration gradients of sodium and potassium ions across the cell membrane by using ATP
  • The pump moves three sodium ions out of the cell and two potassium ions into the cell for each ATP molecule hydrolyzed, creating an electrochemical gradient
• Other examples of primary active transport include the calcium pump (Ca2+ ATPase) and the proton pump (H+ ATPase)

Secondary Active Transport

• Secondary active transport utilizes the electrochemical gradient created by primary active transport to move other molecules against their concentration gradient without directly using ATP
• The sodium-glucose cotransporter (SGLT) is an example of secondary active transport that uses the sodium gradient to transport glucose into the cell
  • Sodium ions move down their concentration gradient into the cell, providing the energy to transport glucose against its concentration gradient
• The sodium-calcium exchanger (NCX) is another example of secondary active transport that uses the sodium gradient to remove calcium ions from the cell
  • Three sodium ions move into the cell down their concentration gradient, while one calcium ion is removed from the cell against its concentration gradient
• The coupling of primary and secondary active transport allows for the efficient movement of various molecules across the membrane and maintains cellular homeostasis