Objectives for topic 3.2




MOLECULES OF LIFE

Carbohydrates (elements (CH2O)n)


Organization:


Monosaccharides
Disaccharides
Polysaccharides


Monosaccharides




5C pentoses (eg ribose, deoxyribose)
6C hexoses (eg glucose, fructose, galactose)


Glycoside linkage to form disaccharides

The two sugars are joined by condensation and may be broken by hydrolysis.
Different monosaccharides can be used:


sucrose = glucose + fructose
lactose = glucose + galactose
maltose = glucose + glucose

Polysaccharides:

Common ones based upon glucose


Branched polysaccharides:
Amylose & amylopectin (starches) are synthesised in plants.
Glycogen is synthesised in animals. It is more highly branched than starches = more compact.

Unbranched polysaccharides:
Cellulose in plant cell walls.

FUNCTIONS OF THE CARBOHYDRATESSugars (mono and disaccharides) - small molecules, soluble in water:
  • maintenance of osmotic balance (e.g. salts in blood plasma, plant cell turgidity)
  • transport of energy reserves (e.g. glucose in blood or sucrose in sap)
  • energy substrate (respiration and photosynthesis)
  • energy store (sugar cane)
  • flavouring (fruits).
  • reward (nectar)
  • precursors (building blocks) of polysaccharides, nucleotides and amino acids.
Polysaccharides - large molecules, insoluble in water.


Amino Acids and Proteins : elements C, H, O, N, S


Amino acids
amino group, carboxyl group, hydrogen and a variable side group (residue) each joined to a central carbon atom.


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Amino end and carboxyl end can be ionised NH3+ and COO- to give acidic and basic characteristics. At pH 7 both groups are ionised.
The residues are side chains which give the individual properties to the amino acid (acidic, basic, neutral and nonpolar).


Functions of amino acids:


  • protein synthesis
  • energy reserve
  • hormones (thyroxin)
20 different amino acids used in protein synthesis though others do occur in nature.
Essential amino acids cannot be synthesised by the organism and must form a part of their diet




LIPIDS.

Lipids (elements C, H, O)
More hydrogen than carbohydrates (more reduced). Insoluble in water, soluble in organic solvents (alcohols, acetone, chloroform etc)
Fatty acids: carboxylic acid + long hydrocarbon chain

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STRUCTURAL
biological membranes (phospholipids, cholestrol, steroids, glycolipids), cushioning (fat deposits round the kidneys)
ELECTRICAL INSULATION
myelin sheath round axons
THERMAL INSULATION
subcutaneous fat deposits.
WATER PROOFING
waxes and oils
ENERGY STORE AND SUBSTRATE
very condensed form of energy (37 kJ g-1) used by animals and seeds.
HORMONES
steroids