The Role of Water in Living Systems

Water in Living Systems

• Water is the most abundant substance in living systems.

Structure of Water Molecules

• The geometry of a water molecule is bent or V-shaped.
• A water molecule (H2O) has a slightly positive charge on the hydrogen atoms and a slightly negative charge on the oxygen atom.
• The oxygen atom in a water molecule carries a partial negative charge (δ-).

Hydrogen Bonds

• Hydrogen bonds are a type of weak electrostatic attraction between molecules.
• Hydrogen bonds are strongest when the bonded molecules are oriented to maximize electrostatic interaction.
• Hydrogen bonds are responsible for water's high heat capacity and its ability to dissolve substances.

Properties of Water

• Water has a high heat capacity due to the energy required to break hydrogen bonds.
• Water has high cohesive and adhesive properties due to the strong hydrogen bonds between molecules.
• Water's high surface tension and viscosity are also due to hydrogen bonds.

Water as a Solvent

• Water is a universal solvent, able to dissolve a wide variety of substances.
• Substances that dissolve easily in water are called hydrophilic (water-loving) compounds, such as salts, sugars, and amino acids.
• Substances that are insoluble in water are called hydrophobic (water-fearing) compounds, such as oils and fats.
• The ability of water to dissolve substances is determined by its polarity and hydrogen bonding ability.

Regulation of Body Fluids

• Aldosterone and antidiuretic hormone (ADH) regulate body fluid homeostasis.
• Aldosterone helps regulate the amount of water reabsorbed in the kidneys.
• ADH helps regulate the amount of water reabsorbed in the kidneys and the permeability of the collecting ducts.

Fluid Balance and Osmosis

• Interstitial fluid is found in the spaces between cells.
• Osmosis is the movement of water molecules from a region of high concentration to a region of low concentration through a selectively permeable membrane.
• Osmotic pressure is the pressure required to prevent osmosis.
• Molecular mobility is related to the osmotic effect, as it affects the rate of water movement.
• Isotonic solutions have the same osmotic pressure as the cell's internal environment.
• Hypertonic solutions have a higher osmotic pressure than the cell's internal environment, causing water to leave the cell.
• Hypotonic solutions have a lower osmotic pressure than the cell's internal environment, causing water to enter the cell.