08 May 2021

4.1. [cont] Biological molecules: DNA & Water

Biological molecules 
#4 is DNA
#5 is water

4. DNA

DNA is the molecule that carries genetic material in most organisms.

  • two strands coiled together to form a double helix 
  • each strand contains chemicals called bases 
  • cross-links between the strands are formed by pairs of bases 
  • the bases always pair up in the same way: 
    • A with T
    • C with G 
DNA double helix

5. Water

Water is a solvent in organisms.

  • Mosts cells contain 75% water.
  • Many substances move around a cell when they are dissolved in water.
  • Many reactions take place in water.
  • Digestion
    • food can be digested ONLY in liquid form - enzymes need to be in water to catalyze reactions
  • Excretion
    • excretory products, like urea, are dissolved in water to dilute them --> allows for removal from body
  • Transport systems 
    • Bloodstream = plasma + cells. Plasma is 92% water and transports CO2, urea, digested food, hormones
    • Plants:
      • in xylem, water transports mineral ions
      • in phloem, water transports sugars and amino acids for use or storage

Try this!
[source: workbook]

For past papers, click here.

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