This includes the perfect and complete summary notes from Topics 1 to 3 for the Biology B AS/A level exam using the Pearson Edexcel exam board. These notes are written following the Pearson Edexcel Specification, including all the necessary requirements according to the specification. They also in...
Topic 1: Biological Molecules
1.1 Carbohydrates
- General formula: Cx(H2O)n
- Made up of elements; Carbon, Hydrogen, Oxygen
- Made up of sugar units called saccharides
1. Monosaccharides; Monomers of one sugar unit
- General formula: (CH2O)n - (n= from 3 to 7)
Triose sugars - Three carbon atoms (n=3)
(C3H6O3) - Mitochondria- broken down from glucose into triose sugars during respiration
Pentose sugars - Five carbon atoms (n=5)
(C5H10O5) - Ribose and deoxyribose
- Making genetic material- nucleic acids (DNA and RNA)
Hexose sugars - Six carbon atoms (n=6)
(C6H12O6) - Highly soluble in water
- Isomers; same chemical formula but different structure
1. Glucose; main substrate for respiration (cardiac tissues)
- Carbon atom always bonded to oxygen atom in a stable closed chain
- Isomers; a-glucose and B-glucose
- BUT in B-glucose; H and OH swap in C1
2. Galactose; production of glycolipids and glycoproteins
- Not as soluble
3. Fructose; sweet and very soluble
- Source of energy in respiration
Condensation Reactions Hydrolysis reactions
- Monosaccharides join to form disaccharides and - Disaccharides and polysaccharides split
polysaccharides - Water molecule is added to break glycosidic bond
- Formation of glycosidic bond - covalent bond - Used to digest polysaccharides in the gut (starch,
- One molecule of
water +
+H2O
+ formed/released
glycogen, cellulose
+H2O per bond
2. Disaccharides; Two monosaccharides
- Form 1,4-glycosidic bond (C-O-C)
Disaccharide Source Monosaccharide
Sucrose Stored in plants (sugar cane) a-glucose + fructose
, 3. Polysaccharides; Chains of 11+ monosaccharide units
- Not sugars; do not contain the sweet taste
- Starch and Glycogen;
1. Form compact molecules; large numbers can be stored
2. Glycosidic bonds easily broken; rapid release of glucose for cellular respiration
3. Insoluble; do not cause osmotic water movements/change water potential of cell
4. Flexible
Starch - Plant storage of glucose
(plant) - Long chain of a-glucose units
- Mixture of two polysaccharides;
1. Amylose
- Helix shape
- Longer unbranched polymer of a-glucose molecules
- Joined by 1,4-glycosidic bonds
- Releases glucose slowly over a long period of time
2. Amylopectin
- Helix shape with branches
- Shorter branched polymer of a-glucose molecules
- Joined by 1,4 and branches with 1,6-glycosidic bonds
- Releases glucose rapidly- easily hydrolysed and digested
Glycogen - Animal storage of glucose
(animals - Branched chain of a-glucose units
and fungi) - Joined by 1,4 and branches with 1,6-glycosidic bonds
- Large surface area as it is very branched- contains many 1,6-glycosidic bonds
- Hydrolysed quickly as it forms many bonds- rapid release of energy
Cellulose - Plant structure- component of cell wall; gives strength and structural support
(plant) - Long, straight, unbranched chains of inverted B-glucose units
- Joined by inverted 1,4-glycosidic bonds
- Alternating unit inversion causes cross-linking; hydrogen bonds form between the partially positive
hydrogen atoms of the hydroxyl (-OH) group and the partially negatively oxygen atoms
- Hydrogen bonds hold the chain in strong, rigid threads; microfibrils- they increase tensile strength to
withstand osmotic pressure
- Insoluble
- Permeable
- Hard to digest
Test for reducing sugars; all monosaccharides and some disaccharides (maltose
and lactose)
- Benedict's solution; chemical test
- Contains copper(II) ions which are reduced to copper(I) ions when heated
- Change in colour; increasing concentration
(blue - green - yellow - orange - brick red)
Test for non reducing sugars (do not react with Benedict's solution); sucrose
- Heat sucrose with a few drops of hydrochloric acid to hydrolyse the
glycosidic bonds and allow it to cool
- Neutralise solution with sodium hydrogen carbonate- produces the
monosaccharide unit the sugar
- It will now give a positive Benedict's test
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