2.3 Water

Water is the lifeblood of biology. Its unique properties, stemming from its polarity, make it essential for life. From acting as a universal solvent to regulating temperature, water's versatility is unmatched in the biological world.

Understanding water's role is key to grasping how life works. It's involved in nearly every biological process, from transporting nutrients to facilitating chemical reactions. Water's importance can't be overstated - it's truly the foundation of life as we know it.

Water's Polarity and Biological Significance

Polarity of Water Molecules

Top images from around the web for Polarity of Water Molecules

• 

  Water – Biology 2e Part I, 2nd edition View original

  Is this image relevant?

• 

  Chemical Bonds · Anatomy and Physiology View original

  Is this image relevant?

• 

  Intermolecular Forces | Chemistry: Atoms First View original

  Is this image relevant?

• 

  Water – Biology 2e Part I, 2nd edition View original

  Is this image relevant?

• 

  Chemical Bonds · Anatomy and Physiology View original

  Is this image relevant?

1 of 3

Top images from around the web for Polarity of Water Molecules

• 

  Water – Biology 2e Part I, 2nd edition View original

  Is this image relevant?

• 

  Chemical Bonds · Anatomy and Physiology View original

  Is this image relevant?

• 

  Intermolecular Forces | Chemistry: Atoms First View original

  Is this image relevant?

• 

  Water – Biology 2e Part I, 2nd edition View original

  Is this image relevant?

• 

  Chemical Bonds · Anatomy and Physiology View original

  Is this image relevant?

1 of 3

• Water molecules are polar due to the unequal sharing of electrons between the oxygen and hydrogen atoms
  • This results in a slightly negative charge near the oxygen and a slightly positive charge near the hydrogens
  • The polar nature of water allows it to form hydrogen bonds with other water molecules and with polar substances (proteins, carbohydrates, nucleic acids)

Significance of Water's Polarity in Biological Systems

• Hydrogen bonding between water molecules contributes to the unique properties of water
  • These properties include cohesion, adhesion, and high specific heat
  • Cohesion allows water to maintain its liquid state and resist separation
  • Adhesion enables water to "stick" to surfaces and move upward against gravity in narrow spaces (xylem vessels in plants)
• The polarity of water enables it to act as an excellent solvent for polar and ionic compounds
  • This facilitates the transport of nutrients and waste products in living organisms
  • Dissolved substances can be easily transported within and between cells

Water's Unique Properties

Cohesion and Adhesion

• Cohesion refers to the attraction between water molecules due to hydrogen bonding
  • This allows water to maintain its liquid state and resist separation
  • Cohesion is responsible for the formation of water droplets and the ability of water to be transported in plants
• Adhesion is the attraction between water molecules and other polar substances
  • This enables water to "stick" to surfaces and move upward against the force of gravity in narrow spaces (xylem vessels in plants)
  • Adhesion is crucial for the movement of water from roots to leaves in plants through capillary action

High Specific Heat and Heat of Vaporization

• Water has a high specific heat capacity, meaning it requires a large amount of energy to change its temperature
  • This property helps regulate temperature in living organisms and their environments
  • It allows water to absorb and release heat slowly, maintaining stable temperatures in cells and bodies of water
• The high heat of vaporization of water allows it to absorb a significant amount of energy during evaporation
  • This helps cool surfaces and regulate body temperature in organisms
  • Evaporative cooling through sweating or panting is an important thermoregulatory mechanism in many animals

Water as a Solvent

Water as the "Universal Solvent"

• Water is known as the "universal solvent" due to its ability to dissolve a wide range of polar and ionic substances
  • Examples of substances dissolved in water include salts, sugars, and amino acids
  • The dissolution of substances in water allows for their transport within and between cells
• Aqueous environments within cells and extracellular spaces provide a medium for chemical reactions
  • Dissolved substances can interact and participate in biochemical processes
  • The aqueous environment facilitates the interaction of biomolecules (enzymes, substrates)

Water's Role in Biochemical Reactions

• Water participates in many biochemical reactions, such as hydrolysis and condensation reactions
  • Hydrolysis reactions involve the breakdown of larger molecules by the addition of water (digestion of proteins, carbohydrates, and lipids)
  • Condensation reactions involve the formation of larger molecules by the removal of water (synthesis of proteins, carbohydrates, and nucleic acids)
• The presence of water is crucial for the synthesis and breakdown of biological macromolecules
  • Examples include the formation and breakdown of peptide bonds in proteins and the synthesis and hydrolysis of ATP

Water's Role in Homeostasis

Osmotic Balance and Water Movement

• Water helps maintain osmotic balance between cells and their environment through the movement of water across semi-permeable membranes via osmosis
  • Osmosis is the diffusion of water from a region of high water potential to a region of low water potential across a semi-permeable membrane
  • Water moves into or out of cells to maintain equal solute concentrations on both sides of the cell membrane
• The movement of water across membranes is crucial for maintaining cell shape and function
  • Cells can swell or shrink depending on the osmotic conditions of their environment (hypotonic, isotonic, or hypertonic solutions)

Temperature Regulation and Water

• The evaporative cooling effect of water helps regulate body temperature in many organisms
  • Mammals and birds regulate body temperature through sweating and panting
  • As water evaporates from the skin or respiratory surfaces, it absorbs heat and cools the body
• Water's high specific heat capacity also contributes to temperature regulation
  • Large bodies of water (oceans, lakes) can absorb and release heat slowly, moderating temperature fluctuations in nearby ecosystems

Transport and Structural Support

• Water is essential for the transport of nutrients, oxygen, and waste products throughout the body via the circulatory system
  • Blood plasma, which is mostly water, serves as a medium for transporting dissolved substances and cells
  • Water helps maintain blood volume and pressure, ensuring proper circulation
• In plants, water provides structural support through the process of turgor pressure
  • Turgor pressure is maintained by the balance of water uptake and transpiration
  • When plant cells are fully hydrated, they exert pressure against the cell wall, providing rigidity and support to the plant

Homeostatic Mechanisms Involving Water

• Homeostatic mechanisms help control water balance within organisms
  • Thirst is a sensation that drives animals to drink water when the body's water levels are low
  • Hormones such as antidiuretic hormone (ADH) regulate water reabsorption in the kidneys, controlling urine concentration and water retention
• The kidneys play a crucial role in maintaining water balance by adjusting the amount of water excreted in urine
  • In response to dehydration, ADH promotes water reabsorption, producing concentrated urine
  • When the body is well-hydrated, ADH levels decrease, allowing for the production of dilute urine and the elimination of excess water