ADVERTISEMENT
ADVERTISEMENT

ZIMSEC O Level Biology Notes: Chemical building blocks of Life: Bio-molecules

ZIMSEC O Level Biology Notes:Properties and Biological significance of water, Hydrogen bonding, Biology Notes: Water and its properties: Density and heat fusion

ADVERTISEMENT

ZIMSEC O Level Biology Notes: Chemical building blocks of Life: Bio-molecules

Water

Structure of a water molecule

  • Water is essential for life.
  • Approximately 75% earth’s surface is water.
  • Life evolved in water.
  • An average adult body is 50 to 65 percent water
  • This rises to about 80% in a child
  • A water molecule is made up of 2 hydrogen atoms and one oxygen atom.
  • It has a bent shape and an angle of 104.5°.
  • Creates a slightly negative oxygen and a slightly positive hydrogen allowing formation of hydrogen bonds.

Hydrogen bonding

  • A type of polar/intermolecular interaction.
  • Critical for:
  • Maintaining protein structure, that is primary, secondary and tertiary.
  • Takes part in enzymatic reactions.
  • Movement of water in plant stems.
  • Ionization of water: H2O -> H+ + OH
  • Generally it breaks down to form a hydrogen ion (H+), and hydroxide ion (OH-).
  • At 25°C, 1 liter of water contains 10-7 moles of H+ ions: 10-7 moles/liter.

pH

ADVERTISEMENT
  • Is a convenient way of indicating H (Hydrogen ion) concentration
  • It is used to measure alkalinity or acidity of a solution
  • pH = – log [H+]
  • For water, pH = – log[10-7] = 7
  • Since for each H+ in pure water, there is one OH-, pH of 7 indicates that it is neutral.
  • Water also acts as a buffer due to a reservoir for H.
  • This means it maintains relatively constant pH over buffering range.

 

Biology Notes: Water and its properties: Density and heat fusion

ZIMSEC O Level Biology Notes:Properties and Biological significance of water

Property: High heat of fusion

  • This is a measure of the heat energy required to melt a solid (ice).
  • Liquid water must lose a large amount of heat energy to freeze.

Biological role

  • Contents of cells and their environment are therefore less likely to freeze.
  • This is an advantage for bodies of living organisms and organisms that live in water.

Property: Density and freezing

  • The density of water decreases below 4°C and ice tends to form on top and floats.
  • Since ice floats, it forms at the top first and at the bottom last.

Biological role

  • Ice insulates the water below it, thus increasing the chances of survival of organisms in water.
  • It also helps maintain circulation in large bodies of water (water expands and rises at 4°C).
ADVERTISEMENT

Related Posts

Next Post

Leave a Reply