Chapter 5: The Dynamic Cell - Active Transport Mechanisms

Active Transport Mechanisms

• Active transport requires energy (ATP).
• Types:
  • Primary transport
  • Secondary transport (Co-transport)
  • Bulk transport
    • Endocytosis
      • Phagocytosis
      • Pinocytosis
      • Receptor Mediated Endocytosis
    • Exocytosis (Secretion)

Electrochemical Gradient

• Electrical gradient: difference in charge across the plasma membrane.
• Inside cells: electrically negative.
• Higher [K^+] and lower [Na^+] than extracellular fluid.
• Na^+ concentration gradient drives it into the cell; electrical gradient also drives it inward.
• K^+ electrical gradient drives it into the cell, but the concentration gradient drives K^+ out.
• Electrochemical gradient: combined concentration gradient and electrical charge.

Pumps & ATP

• Cells expend energy to move molecules against a concentration gradient.
• Requires transport protein and ATP.
• Sensitive to metabolic poisons that interfere with ATP supply.
• Sodium-potassium pump maintains ion gradients for nerve impulse conduction.

Moving Small Molecules

• Primary active transport: Moves ions across a membrane, creates a charge difference, directly dependent on ATP.
• Secondary active transport: Movement of material due to the electrochemical gradient established by primary active transport; does not directly require ATP.

Carrier Proteins

• Uniporter: carries one specific ion or molecule.
• Symporter: carries two different ions or molecules, in the same direction.
• Antiporter: carries two different ions or molecules, in different directions.
• These transporters can also transport small, uncharged organic molecules like glucose.
• These three types of carrier proteins are also in facilitated diffusion, but they do not require ATP to work in that process.

Sodium-Potassium Pump

• Most important active transport protein.
• Pumps Na^+ out of cells and K^+ into cells.
• Uses 1/3 of all energy expended by cells.
• Energy from ATP.

Sodium-Potassium Pump Steps

1. Cytoplasmic Na^+ binds to pump.
2. Na^+ binding stimulates phosphorylation by ATP.
3. Phosphorylation changes protein shape, reducing affinity for Na^+, which is released outside.
4. New shape has high affinity for K^+, which binds extracellularly and triggers phosphate release.
5. Loss of phosphate restores original shape, lowering affinity for K^+.
6. K^+ is released; affinity for Na^+ is high again; the cycle repeats.

Secondary Transport (Co-transport)

• Active transport of one solute indirectly drives transport of other solutes.
• Coupling of "downhill" diffusion of one substance to the "uphill" transport of another.
• Plants use H^+ gradient (generated by ATP-powered proton pumps) to drive active transport of amino acids, sugars, and nutrients.

Bulk Transport

• Used for large molecules and particles that cannot pass through the membrane.
• Involves vesicle formation to take substances in or out of the cell.

Bulk Transport: Endocytosis

• Plasma membrane invaginates, forming a pocket around the target particle.
• The pocket pinches off, resulting in the particle containing itself in a newly created intracellular vesicle formed from the plasma membrane.

Bulk Transport: Phagocytosis

• Cell takes in large particles (e.g., other cells or large particles).
• Neutrophils engulf microorganisms via phagocytosis.
• Plasma membrane's inward-facing surface becomes coated with clathrin.
• Membrane extends, surrounds the particle, and encloses it.
• Clathrin disengages, and the vesicle merges with a lysosome to break down the material.

Bulk Transport: Pinocytosis

• Cell "drinks" molecules, including water, from extracellular fluid.
• Results in smaller vesicles than phagocytosis; vesicles do not need to merge with lysosomes.

Bulk Transport: Receptor-Mediated Endocytosis

• Targeted endocytosis using receptor proteins with specific binding affinity for certain substances.
• Clathrin attaches to the plasma membrane's cytoplasmic side.
• Uptake targets a single type of substance that binds to receptors on the cell membrane's external surface.
• Failure can cause diseases like familial hypercholesterolemia (defective LDL receptors).

Bulk Transport: Exocytosis

• Expels material from the cell into the extracellular fluid.
• Waste material is enveloped in a membrane, which fuses with the plasma membrane.
• Examples: Extracellular matrix protein secretion and neurotransmitter secretion.