Cell Transport & Homeostasis: Passive and Active Transport

Cell Transport & Homeostasis

Passive Transport

Passive transport refers to the movement of substances across the cell membrane without the use of energy from the cell. This process relies on the natural motion of molecules and the existence of concentration gradients.

• Diffusion: Movement of molecules from an area of high concentration to an area of low concentration. This process continues until equilibrium is reached on both sides of the membrane. Example: Oxygen and carbon dioxide gas exchange in the lungs.

• Osmosis: A special type of diffusion involving the movement of water molecules across a selectively permeable membrane. Water moves from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). Example: Water uptake by plant roots.

• Facilitated Diffusion: Movement of molecules across the membrane via transport proteins. This process is used for molecules that cannot directly diffuse through the lipid bilayer. Example: Glucose transport into cells.

Transport Proteins:

• Channel Proteins: Allow specific ions and water to pass through the membrane quickly.

• Carrier Proteins: Change shape to move specific molecules across the membrane.

Active Transport

Active transport is the movement of substances across the cell membrane that requires an input of energy, usually in the form of ATP. This process allows cells to move substances against their concentration gradients (from low to high concentration).

• Sodium-Potassium Pump: An example of active transport where sodium ions are pumped out of the cell and potassium ions are pumped in. This maintains essential concentration gradients for nerve impulse transmission.

• Vesicle Transport: Larger molecules and particles are transported by vesicles in processes called endocytosis and exocytosis.

• Endocytosis: The cell membrane engulfs material, forming a vesicle that brings substances into the cell. Example: White blood cells engulfing bacteria.

• Exocytosis: Vesicles containing substances fuse with the cell membrane, releasing their contents outside the cell. Example: Secretion of hormones or neurotransmitters.

Comparison of Passive and Active Transport

Feature	Passive Transport	Active Transport
Energy Requirement	No energy required	Requires energy (ATP)
Direction of Movement	Down concentration gradient	Against concentration gradient
Examples	Diffusion, Osmosis, Facilitated Diffusion	Sodium-Potassium Pump, Endocytosis, Exocytosis

Key Equations:

• General diffusion equation: $J = -D \frac{dC}{dx}$ where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: Maintaining homeostasis in cells depends on the selective movement of substances across the membrane, allowing cells to regulate their internal environment and respond to changes in their surroundings.