Chapter 7 Part B: (Osmosis and Water Balance in Cells)

Osmosis

Definition and Mechanism

Osmosis is a fundamental process in biology describing the movement of water across a selectively permeable membrane. It is a type of passive transport, meaning it does not require cellular energy (ATP).

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Water can move slowly through the phospholipid bilayer but much more rapidly via specialized transmembrane protein channels called aquaporins.

• Aquaporins facilitate the rapid movement of water in and out of cells, which is essential for maintaining cellular water balance.

• Passive transport refers to the movement of substances down their concentration gradient without energy input.

Why Water Needs Channels

• Although water is a small molecule, its movement through the hydrophobic core of the lipid bilayer is limited.

• Aquaporins provide a hydrophilic pathway, allowing water to bypass the lipid bilayer and move more efficiently.

Determinants of Water Movement

Solute Concentration and Free Water

The direction and rate of osmosis are determined by the concentration of solutes in the water.

• Water forms a hydration shell around hydrophilic (polar or charged) molecules such as proteins and ions.

• Free water refers to water molecules not bound in hydration shells and is available for osmosis.

• Ions and other charged molecules cannot easily cross the membrane due to the hydrophobic interior of the lipid bilayer.

Example: Comparing Solutions

• A 1 M solution of sugar has less free water than a 0.1 M solution of sugar because more water molecules are bound in hydration shells in the more concentrated solution.

Effects of Osmosis on Water Balance

Osmosis and Cell-Environment Interactions

Osmosis affects the water balance between the cell and its environment, which is crucial for cell survival.

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration.

• This movement is driven by differences in solute concentrations, affecting the balance between the cell and its surroundings.

Water Balance of Living Cells

Tonicity and Its Effects

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water. It is determined by the relative concentrations of solutes inside and outside the cell.

• Isotonic solution (iso = same): Solute concentration is the same as inside the cell; no net water movement occurs across the plasma membrane.

• Hypotonic solution (hypo = less): Solute concentration is less than inside the cell; the cell gains water.

• Hypertonic solution (hyper = more): Solute concentration is greater than inside the cell; the cell loses water.

Application of Tonicity: Effects on Animal Cells

Solution Type	Effect on Animal Cell	Description
Hypotonic	Lysed	Cell swells and may burst due to water influx.
Isotonic	Normal	No net movement of water; cell maintains normal shape.
Hypertonic	Shriveled	Cell loses water and shrinks.

Key Terms

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Aquaporin: Membrane protein channel that facilitates rapid water movement.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Isotonic: Equal solute concentration inside and outside the cell.

• Hypotonic: Lower solute concentration outside the cell; cell gains water.

• Hypertonic: Higher solute concentration outside the cell; cell loses water.

Example: Red Blood Cells in Different Solutions

• In a hypotonic solution, red blood cells swell and may burst (lysed).

• In an isotonic solution, red blood cells remain normal.

• In a hypertonic solution, red blood cells shrink (shrivel).

Equations

• Osmosis is driven by differences in water potential ($\Psi$), which can be described as: $\Psi = \Psi_s + \Psi_p$ where $\Psi_s$ is the solute potential and $\Psi_p$ is the pressure potential.

Additional info: Water potential is a key concept in plant physiology, describing the tendency of water to move from one area to another.