Biological Properties and Functions of Water and Mineral Salts

Definition and Measurements:
- Water possesses a high specific heat capacity, specifically calculated as $4.2\,kJ\,kg^{-1}\,℃^{-1}$ .
- This numeric value implies that $4.2\,joules$ of energy is required to raise the temperature of $1\,gram$ of water by $1\,℃$ .
Thermal Dynamics:
- Water requires the gain of a significant amount of heat energy to increase the temperature of $1\,g$ of water by $1\,℃$ .
- Conversely, it must lose a large volume of heat energy to decrease the temperature of $1\,g$ of water by $1\,℃$ .
- This characteristic allows water to resist rapid changes in its temperature.
- Water effectively absorbs a large amount of heat energy with only a small rise in its own temperature.
Biological and Environmental Significance:
- Stable Environments: Provides a stable thermal environment in which organisms can live.
- Aquatic Habitats: Oceans, seas, and lakes maintain a thermally stable environment for the survival of aquatic organisms.
- Internal Regulation: Helps maintain body temperature within ranges that allow body cells to function effectively.

Definition and Mechanism:
- Water has a high latent heat of vaporization.
- A significant amount of heat energy is required to break the hydrogen bonds existing between water molecules to facilitate the phase change from the liquid state to vapor (gas).
- Vaporization (汽化): The transformation from a liquid state to a gas state.
Cooling Applications:
- Because a large amount of heat is needed to evaporate water, the process is highly efficient for cooling organisms.
- Sweating (流汗): In animals, a large amount of heat must be absorbed from the body to evaporate water from the skin surface, resulting in a cooling effect on the body.
- Transpiration (蒸腾作用): In plants, substantial heat is absorbed from the leaf surface to evaporate water. This results in a cooling effect on leaves and prevents overheating of plant tissues.

Molecular Arrangement and Phase Comparison:
- Liquid State: Features unstable hydrogen bonds with a non-lattice structure (非晶格结构). The intermolecular space in the liquid state is small.
- Solid State (Ice): Features stable hydrogen bonds arranged in a specific lattice structure (晶格结构). The intermolecular space in the solid state is large.
- Physical Consequences: Due to the lattice structure, ice has a larger volume than liquid water, making ice less dense than water. Consequently, ice floats on water.
Density Anomaly and Ecological Impact:
- Water reaches its maximum density at exactly $4\,℃$ .
- At temperatures of $0\,℃$ and below, ice forms.
- Because it is less dense than the liquid water below it, ice floats on the surface and creates an insulating layer (绝缘层).
- This insulating layer prevents the water in the bottom layers of ponds and lakes from freezing.
- This mechanism ensures that aquatic organisms in ponds and lakes can survive in liquid water during the winter months.

Osmotic Balance in Animals:
- The concentration of dissolved inorganic salts in water is vital for maintaining the osmotic balance between the blood and the interstitial fluid (组织间液).
Turgidity in Plants:
- A high osmotic concentration within plant cells encourages water to enter the cells via osmosis.
- This intake of water causes the cells to become turgid.
- Turgidity is a primary mechanism providing structural support in plants.

Lubrication (润滑):
- Water exhibits low viscosity (低黏稠度).
- Biological lubricants such as mucus (粘液) and synovial fluid (关节骨液) consist primarily of water.
- Mucus: Assists the movement of food substances through the intestinal tract (肠道).
- Synovial Fluid: Lubricates the joints to ease movement and reduce friction.
Providing Moisture:
- Water provides necessary moisture to respiratory surfaces, such as the alveoli (肺泡) in the lungs.
- This moisture allows respiratory gases (such as oxygen and carbon dioxide) to dissolve in the water before diffusion across the respiratory surfaces can occur.

General Properties:
- Minerals in the body exist in ionic form (离子形式).
- Common ions include $Ca^{2+}$ , $Fe^{2+}$ , $K^{+}$ , $Na^{+}$ , $Mg^{2+}$ , $Cl^{-}$ , and $PO_4^{3-}$ .
- Mineral salts constitute approximately $1-1.5\%$ of the content within a cell.
Constituent Functions (组成成分):
- $Mg^{2+}$ : Essential for the synthesis of chlorophyll (叶绿素) in plants.
- $Fe^{2+}$ : Essential for the synthesis of hemoglobin (血红蛋白) for oxygen transport.
- $Ca^{2+}$ and $PO_4^{3-}$ : Essential for building strong and healthy bones and teeth.
Cellular Function Maintenance:
- $Ca^{2+}$ : Facilitates muscle contraction and relaxation.
- $HCO_3^{-}$ : Maintains pH homeostasis (稳态) in the body.
- $Na^{+}$ : Maintains cellular osmotic pressure (渗透压).
- $Cl^{-}$ : Acts as activators (活化剂) for enzymes.

Carbon Dioxide ( $CO_2$ ) Transport Pathways:
- (a) Carbon dioxide carried directly in the Red Blood Cells (RBC).
- (b) $HCO_3^{-}$ dissolved in plasma as carbonic acid.
- (c) $CO_2$ dissolved directly in the plasma.
Chemical Equilibrium for $CO_2$ Transport:
- $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^{-} + H^{+}$