Types of Cellular Transport to Know for General Biology I

Cellular transport is essential for maintaining life, allowing substances to move in and out of cells. This process can be passive, using no energy, or active, requiring energy. Understanding these mechanisms is key to grasping how cells function and maintain homeostasis.

1. Passive transport

   • Movement of substances across a cell membrane without the use of energy (ATP).
   • Relies on concentration gradients; substances move from areas of higher concentration to lower concentration.
   • Includes processes like simple diffusion, facilitated diffusion, and osmosis.

2. Simple diffusion

   • Direct movement of small, nonpolar molecules (e.g., O2, CO2) through the lipid bilayer.
   • Does not require transport proteins or energy.
   • Continues until equilibrium is reached, where concentrations are equal on both sides of the membrane.

3. Facilitated diffusion

   • Movement of larger or polar molecules (e.g., glucose, ions) through specific transport proteins in the membrane.
   • Does not require energy; relies on concentration gradients.
   • Involves channel proteins or carrier proteins to assist in the transport process.

4. Osmosis

   • The diffusion of water molecules across a selectively permeable membrane.
   • Water moves from areas of lower solute concentration to areas of higher solute concentration.
   • Important for maintaining cell turgor and overall homeostasis.

5. Active transport

   • Movement of substances against their concentration gradient, requiring energy (ATP).
   • Essential for maintaining concentration differences across membranes.
   • Involves specific transport proteins known as pumps.

6. Primary active transport

   • Direct use of ATP to transport molecules against their concentration gradient.
   • Example: Sodium-potassium pump (Na+/K+ pump) that maintains Na+ and K+ gradients across the cell membrane.
   • Critical for functions such as nerve impulse transmission and muscle contraction.

7. Secondary active transport

   • Indirect use of ATP; relies on the energy created by primary active transport.
   • Involves the movement of one substance down its gradient to drive the transport of another substance against its gradient.
   • Can be symport (both substances move in the same direction) or antiport (substances move in opposite directions).

8. Endocytosis

   • Process by which cells engulf substances from the external environment.
   • Involves the invagination of the cell membrane to form a vesicle.
   • Allows for the uptake of large molecules, particles, or even other cells.

9. Phagocytosis

   • A type of endocytosis where the cell engulfs large particles or microorganisms.
   • Often referred to as "cell eating."
   • Important for immune responses and the removal of debris.

10. Pinocytosis

    • A type of endocytosis where the cell takes in small droplets of extracellular fluid.
    • Often referred to as "cell drinking."
    • Allows cells to sample their environment and absorb nutrients.

11. Receptor-mediated endocytosis

    • A selective form of endocytosis that involves the binding of specific molecules to receptors on the cell surface.
    • Triggers the invagination of the membrane and formation of a vesicle.
    • Allows for the uptake of specific substances, such as hormones and nutrients.

12. Exocytosis

    • The process by which cells expel materials in vesicles that fuse with the plasma membrane.
    • Important for the secretion of hormones, neurotransmitters, and waste products.
    • Plays a key role in maintaining the balance of substances within the cell.