Osmosis: Water Movement Across Cell Membranes
Definition
Osmosis
Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
Water movement is crucial for cellular function, allowing cells to maintain their internal environment and balance solute concentrations.
Water Moves Because Of The Concentration Gradient
- Solutes (e.g., salts, sugars) bind with water molecules, reducing the number of free water molecules in the solution.
- A solution with higher solute concentration has fewer free water molecules.
- Water moves from where there are more free water molecules (lower solute concentration) to where there are fewer free water molecules (higher solute concentration).
- This creates a concentration gradient that drives osmosis.
- Think of water as someone who hates crowds.
- When stuck amongst a group of loud and annoying people, it'd naturally leave to somewhere quieter and less cramped.
Osmosis Crosses The Phospholipid Bilayer Through Aquaporins
- Water molecules are small and polar (hydrophilic), but they can pass directly through the phospholipid bilayer because of their small size.
- Some cells use aquaporins, specialized protein channels that enhance water permeability and allow faster water movement.
- Aquaporins allow water molecules to pass through in single file at their narrowest point, while preventing charged particles like protons (H⁺) from crossing.
- Osmosis is a passive process.
- It requires no direct energy expenditure and is driven entirely by the concentration gradient of free water molecules.
At their narrowest point, aquaporins allow water molecules to pass through in a single file, ensuring efficient transport while preventing charged particles like protons (H⁺) from passing.
Self review- What is the role of solute concentration in osmosis?
- How do aquaporins enhance water movement across membranes?
- Why is the phospholipid bilayer selectively permeable, and how does this affect osmosis?
