Cell Transport Mechanisms: Diffusion & Osmosis

Study Notes

Diffusion, facilitated diffusion, active transport, the Na/K pump, and osmosis are key mechanisms for transporting substances across cell membranes in biology.

Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration.
This movement is driven by the concentration gradient.
It's a passive process, meaning it doesn't require energy input from the cell.
Diffusion continues until the concentration is uniform throughout (equilibrium).
Small, nonpolar molecules, such as oxygen and carbon dioxide, can readily diffuse across the cell membrane.
The rate of diffusion is affected by factors like temperature, concentration gradient, and the size/polarity of the diffusing molecules.
In biological systems, diffusion is crucial for processes like gas exchange in the lungs and nutrient uptake in the intestines.

Facilitated diffusion is the movement of molecules across the cell membrane via transport proteins.
This process is still passive, relying on the concentration gradient.
It's used for molecules that cannot directly diffuse through the lipid bilayer, typically larger or polar molecules like glucose and amino acids.
Channel proteins form pores or channels in the membrane, allowing specific molecules to pass through.
Carrier proteins bind to the molecule, change shape, and release the molecule on the other side of the membrane.
Facilitated diffusion is specific to the molecule being transported, based on the protein's structure.
This process is saturable, meaning the rate of transport plateaus when all transport proteins are in use.

Active transport is the movement of molecules across the cell membrane against their concentration gradient.
This requires the cell to expend energy, typically in the form of ATP.
It's essential for maintaining concentration gradients that are different from equilibrium.
Primary active transport uses ATP directly to move molecules.
Secondary active transport uses the electrochemical gradient created by primary active transport to move other molecules.
Active transport involves specific transport proteins that bind to the molecule and use energy to change shape and move it across the membrane.

The sodium-potassium pump is a primary active transport protein found in the plasma membrane of animal cells.
It uses ATP to move sodium ions (Na+) out of the cell and potassium ions (K+) into the cell.
For each ATP molecule hydrolyzed, the pump moves 3 Na+ ions out and 2 K+ ions in.
This creates an electrochemical gradient, with a higher concentration of Na+ outside the cell and a higher concentration of K+ inside.
The Na+/K+ pump is crucial for maintaining cell volume, nerve impulse transmission, and muscle contraction.
It also helps in secondary active transport by creating an electrochemical gradient.

Osmosis is the movement of water across a semipermeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration).
Water moves to equalize solute concentrations on both sides of the membrane.
This is a passive process, driven by the difference in water potential.
Osmosis is critical for maintaining cell turgor pressure in plants and regulating cell volume in animals.
In animal cells, osmosis can cause cells to swell and burst (hypotonic solution) or shrink and shrivel (hypertonic solution).
Plant cells have a cell wall that prevents them from bursting in a hypotonic solution, instead becoming turgid.
Isotonic solutions have equal solute concentrations, so there is no net movement of water.