1.1

1. What is a hydrogen bond?

• A hydrogen bond is a weak electrostatic attraction between a hydrogen atom, which is covalently bonded to an electronegative atom (usually oxygen or nitrogen), and another electronegative atom with a lone pair of electrons. It occurs due to the partial positive charge on the hydrogen and the partial negative charge on the electronegative atom.
  
  

2. Where are hydrogen bonds found in water?

• Hydrogen bonds are found between the hydrogen atoms of one water molecule and the oxygen atom of another water molecule. These bonds form because the oxygen atom is more electronegative than the hydrogen atoms, creating partial positive and partial negative charges, which leads to an attraction between the molecules.
  
  

3. Where are hydrogen bonds found in DNA?

• Hydrogen bonds are found between the complementary nitrogenous bases of DNA strands. The nitrogenous bases (adenine, thymine, cytosine, and guanine) form base pairs with each other, and hydrogen bonds help hold the two DNA strands together in a double helix structure.
  
  a. How many hydrogen bonds are found between each complementary base pairing?
  
  

  • Adenine (A) and Thymine (T): 2 hydrogen bonds
    
    

  • Cytosine (C) and Guanine (G): 3 hydrogen bonds
    
    

4. What is the structure of an amino acid?

• An amino acid has a central carbon (C) atom bonded to:
  
  

  • An amino group (NH₂).
    
    

  • A carboxyl group (COOH).
    
    

  • A hydrogen atom.
    
    

  • A variable R group (side chain), which determines the properties of the amino acid.
    
    

• a. What are the three options for the R group?
  
  

  • Nonpolar (hydrophobic): An R group that is hydrophobic and does not interact well with water (e.g., methyl group).
    
    

  • Polar (hydrophilic): An R group that is hydrophilic and interacts with water (e.g., hydroxyl group).
    
    

  • Charged (acidic or basic): An R group that carries a charge, either positive (basic) or negative (acidic), and can form ionic bonds (e.g., carboxyl group or amino group).
    
    

• b. For each R group option, describe the polarity and justify your response.
  
  

  • Nonpolar R group: Hydrophobic, meaning it repels water. Nonpolar side chains do not form hydrogen bonds or ionic interactions, making them water-insoluble.
    
    

  • Polar R group: Hydrophilic, meaning it is attracted to water. These side chains can form hydrogen bonds with water molecules.
    
    

  • Charged R group: Can be either hydrophilic (for charged groups) or hydrophobic. Acidic side chains have a negative charge and basic side chains have a positive charge, allowing for ionic interactions.
    
    

5. What are three properties of water?

• High heat capacity: Water can absorb or release large amounts of heat without significant changes in temperature.
  
  

• Cohesion: Water molecules stick to each other due to hydrogen bonding.
  
  

• Adhesion: Water molecules stick to other substances due to hydrogen bonding.
  
  

a. How did hydrogen bonding allow for each property?

• High heat capacity: Hydrogen bonds absorb heat when they are broken and release heat when they are formed, allowing water to resist temperature changes.
  
  

• Cohesion: The hydrogen bonds between water molecules allow them to stick together, resulting in water’s high surface tension.
  
  

• Adhesion: Hydrogen bonds enable water molecules to adhere to polar surfaces, like the walls of a capillary tube.
  
  

6. Describe why water is considered a polar molecule?

• Water is considered a polar molecule because it has a bent shape, and the oxygen atom is more electronegative than the hydrogen atoms. This creates a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms. The unequal distribution of electrons leads to a dipole, where one side of the molecule is more negatively charged and the other side is more positively charged.
  
  

7. Using the properties of water, describe how water can move up a capillary tube to move from the roots to the leaves in a plant.

• Water moves up a capillary tube due to cohesion and adhesion:
  
  

  • Cohesion: Water molecules stick to each other via hydrogen bonds, which allows them to move together as a column.
    
    

  • Adhesion: Water molecules also stick to the walls of the capillary tube due to hydrogen bonds, helping the water move upward against gravity.
    
    

• This movement is also aided by transpiration, where water vapor leaves the leaves, creating a pull that draws more water up from the roots.
  
  

8. Using the properties of water, describe how a water strider can walk on water.

• The ability of a water strider to walk on water is due to the high surface tension of water, which is a result of hydrogen bonding. The cohesive forces between water molecules allow the surface to resist external force. The water strider's legs are coated with a hydrophobic substance, which prevents it from breaking the surface tension. As a result, the insect can remain on the surface without sinking.