WATER MOLECULE (AP BIO)

Why is water so important to living organisms?

• Living organisms contain more water than any other compound, and most environments where organisms live are dominated by water.

• Water has special properties that are essential for life.

Why is water a polar molecule?

• Oxygen has a higher electronegativity than hydrogen (it pulls the shared electrons closer to itself)

• This gives oxygen a partial negative charge (δ−) and hydrogen a partial positive charge (δ+).

What type of bond holds the hydrogen and oxygen atoms together in a water molecule?

• Polar covalent bonds.

• The electrons are shared between oxygen and hydrogen (NOT shared equally)

What is a hydrogen bond?

• A hydrogen bond is the attraction between the positive hydrogen H (δ+) of one water molecule and the negative oxygen O (δ−) of another water molecule.

• The hydrogen bond is between DIFFERENT water molecules, not between the H and O inside the same water molecule

What are cohesion and adhesion?

• Cohesion is water sticking to other water molecules.

• Adhesion is water sticking to other polar substances

How do cohesion and adhesion help plants?

• They help water move upward through the xylem by capillary action.

• Cohesion keeps water molecules together, while adhesion helps them stick to the xylem walls.

What is high specific heat, and why does water have it?

• High specific heat means water requires a lot of energy to change temperature.

• Hydrogen bonds require energy to break, so water can absorb lots of heat without its temperature changing quickly.

How does water’s high specific heat help organisms?

• It helps stabilize temperatures.

• For example: water in sweat absorbs heat from the body, and when the sweat evaporates, it removes heat and cools the body.

How does water moderate climate?

• Water has a high heat capacity, so large bodies of water can absorb and release large amounts of energy.

• This helps keep nearby climates from having extreme temperature changes.

Why does ice float on liquid water?

• Hydrogen bonds hold water molecules farther apart when water freezes, giving ice more space between molecules and therefore a lower density than liquid water.

Why is it important that ice floats?

• Floating ice forms a layer on the surface of lakes and ponds that protects the water underneath from extreme cold.

• This helps organisms survive winter. If ice sank, more of the lake could freeze and fewer organisms would survive.

Why is water an excellent solvent?

• Water is polar, so its partially positive and negative ends can attract and surround ions and other polar molecules, allowing them to dissolve.

How does water dissolve NaCl (salt)?

• The δ− oxygen side of water is attracted to Na⁺, while the δ+ hydrogen side is attracted to Cl⁻.

• Water molecules surround the ions and help separate them

What is pH, and what does the pH formula represent?

• pH measures the concentration of H⁺ ions in a solution.

• The formula is pH = −log[H⁺].

What do the different pH values mean?

• pH < 7 → acidic

• pH = 7 → neutral

• pH > 7 → basic

How are H⁺ concentration and pH related

• More H⁺ → lower pH → more acidic

• Less H⁺ → higher pH → more basic

• Because pH is logarithmic, a change of 1 pH unit = a 10× change in H⁺ concentration.

What are acids, bases, and water dissociation?

• Water can dissociate (separate) into H⁺ and OH⁻.

  
  

  • Acids increase H⁺ concentration.

  • Bases increase OH⁻ concentration.

  • Pure water produces equal amounts of H⁺ and OH⁻, making it neutral.

  
  

What are buffers, and why are they important?

• Buffers help keep pH relatively stable.

• Living systems are sensitive to pH changes, so buffers can act as acids or bases to respond to changes in H⁺ or OH⁻.

How does the carbonic acid–bicarbonate buffer help control pH?

• It helps fix pH when there is too much H⁺ or OH⁻.

• 1. Too much H⁺ → pH is too LOW

  ➡ The reaction goes LEFT

  ➡ This removes some of the extra H⁺

  ➡ pH goes back toward normal

  
  

  2. Too much OH⁻ → pH is too HIGH

  ➡ The reaction goes RIGHT

  ➡ This makes more H⁺

  ➡ The H⁺ cancels out some OH⁻

  ➡ pH goes back toward normal

Why are buffers and pH important to proteins?

• Buffers keep pH stable.

• If pH changes too much, proteins can change shape and stop working properly.

• Enzymes work best at specific pH levels.