Water in Living Systems

Water in Living Systems

• Water is the most abundant chemical compound in living systems, making up 60-70% of the human body.
• 2/3 of the total body water is intracellular, with the rest being interstitial fluid (25%) and blood plasma (25%).
• Water acts as a solvent for essential substances such as Na+, K+, glucose, ATP, and proteins.
• The body regulates both the volume and pH of water to maintain a constant environment for cells (homeostasis).
• Water helps regulate body temperature and is a reactant and product in many chemical reactions in the living system.
• It plays a crucial role in transporting oxygen and nutrients to cells and carrying away waste products.
• Water is a medium for the movement of molecules into and out of cellular compartments.
• Water is essential for the structure and function of biomolecules.

Buffers

• A buffer is a solution that resists changes in pH when small quantities of acid or base are added.
• Acidic buffers have a pH less than 7, while basic buffers have a pH greater than 7.
• Buffer solutions are made from a weak acid and its salt (e.g., ethanoic acid and sodium ethanoate) or a weak base and its salt (e.g., ammonia solution and ammonium chloride).

How Buffer Solutions Work

• Acidic buffers: When an acid is added to a buffer, the buffer removes most of the new hydrogen ions by combining with the weak acid to form a weak acid.
• Basic buffers: When an alkali is added to a buffer, the OH- ions react with the weak base and its conjugate acid, shifting the equilibrium to the right.

Chemical Equilibrium

• A chemical reaction is in equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.
• The equilibrium constant (K) is a measure of the ratio of products to reactants at equilibrium.
• When K is greater than 10^2, the products are favored, and when K is less than 10^-2, the reactants are favored.

Buffer Systems in the Body

• The major buffer system in blood is the carbonic acid-bicarbonate system.
• This system helps regulate pH by buffering excess H+ or OH- ions.
• When an acid is added, the system shifts to the left, forming more carbonic acid, which can be removed from the blood through exhalation.
• When an alkali is added, the system shifts to the right, forming more bicarbonate, which can be removed through increased excretion by the kidneys.

Regulation of pH in the Body

• The body has several mechanisms to regulate pH, including:
  • Increasing or decreasing breathing rate to expel or retain CO2.
  • Increasing or decreasing excretion of H+ or HCO3- by the kidneys.
  • Increasing or decreasing the concentration of carbonic acid or bicarbonate in the blood.

Bicarbonate Buffer

• The bicarbonate buffer is a conjugate acid-base pair that helps regulate pH in the blood.
• It is regulated by the kidneys and lungs.
• The acid component (H2CO3) is generated from dissolved CO2 and water, while the base component (HCO3-) is regulated by the kidneys.
• The buffer system is highly effective due to its equilibration with a large reserve of gaseous CO2 in the air space of the lungs.

Phosphate Buffer

• The phosphate buffer system is another buffer active mainly within cells.
• It helps regulate pH by buffering excess H+ or OH- ions.
• The kidneys remove excess HPO4-2 and H2PO4- from the body to maintain pH homeostasis.