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Cells & Molecules > Large Molecules
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Large Molecules

  • Monomer (-OH) + Monomer (-H) → Polymer + H2O(l)
    • Condensation: monomers (e.g. amino acids) join to form polymers (e.g. proteins)
    • Glycosidic bond forms when two carbohydrate monomers join together
    • Hydrolysis: break down of a polymer; reverse reaction
  • Polymers are also called macromolecules (e.g. starch, proteins, triglyceride)

Carbohydrates

  • Organic molecules in which C, H and O bind together in the ratio Cx(H2O)y
  • Serve as an energy source important for the brain and cellular respiration
  • Plants produce carbohydrates by using energy from sunlight
    • 6CO2 + 6H2O + energy (from sunlight) → C6H12O6(carbohydrate) + 6O2
  • Animals eat plant materials to obtain the produced carbohydrates
  • They can then be used in animal metabolism to release energy
    • C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Monosaccharides

Triose (3 carbons)

Product of respiration and photosynthesis

Pentose (5 carbons)
- Ribose
- Deoxyribose

Found in RNA and DNA
nucleic acids

Hexose (6 carbons)
- Glucose
- Fructose
- Galactose

Source of energy in respiration
Main energy source in brain
Found in sweet-tasting fruits

Disaccharides (two sugar residues)

Sucrose (glucose + fructose)

Transport carbohydrates in plants

Maltose (glucose + glucose)

Formed from digestion of starch

Lactose (glucose + galactose)

Carbohydrates found in milk

Polysaccharides (many sugar residues)

Starch (alpha-glucose)

Main storage of carbohydrates 
- in plants

Glycogen (alpha-glucose)

- in humans and animals

Cellulose (beta-glucose)

Important component of the plant cell wall

Starch

  • Consists of amylopectin and amylose (both are made of α-glucose)
    • Amylopectin is branched via 1,6-glycosidic bonds
    • Amylose forms a stiff helical structure via 1,4-glycosidic bonds
    • Both are compact molecules → starch can be stored in small space
  • The ends are easily broken down to glucose for respiration
  • Does not affect water potential as it is insoluble
  • Readily hydrolysed by the enzyme amylase found in the gut and saliva
  • Major carbohydrate used in plants
    • Found as granules (chloroplast)
    • Each granule contains amylopectin combined by a larger amount of amylose
  • Commonly used sources are corn (maize), wheat, potato, rice

Glycogen

  • Branched, storage, polymer of glucose linked via glycosidic bonds
  • Found in skeletal muscle and in the liver
  • Chains are linked by alpha-1,4-linkage, branches are linked by alpha-1,6-linkages
  • Glycogen is broken down to glucose by glycogenolysis (glycogen phosphorylase)
  • Major site of daily glucose consumption (75%) is the brain via aerobic pathways
  • Most of the remainder is utilized by erythrocytes, skeletal muscle, and heart muscle
  • Glucose is obtained from diets or from amino acids and lactate via gluconeogenesis
  • Storage of glycogen in liver are considered to be main buffer of blood glucose levels

Cellulose

  • Polysaccharide consisting of long beta-glucose chains
  • Linked together by hydrogen bonds to form microfibrils
  • Structural function is a important component of plant cell walls
  • Its tensile strength helps plant cells in osmosis //cell does not burst in dilute solutions

Proteins

Structure

  • Proteins are polymers of amino acids
  • Proteins are made up by different combinations of 20 amino acids
    • They have a general structure:
    • The difference between different amino acids is found in the R-group
    • When two amino acids join together, they release -H and -OH groups highlighted in red below
    • Peptide bond is formed between alpha-carbon and nitrogen
    • Condensation reaction
  • Primary structure of a protein
    • Sequence of amino acids
    • Joined together by covalent peptide bonds
  • Secondary structure
    • Hydrogen bonds between amino acids
    • Made of a combination of alpha-helices and beta-pleated sheets
    • Proportion of α-helix and β-sheet depends on sequence (primary structure)
  • Tertiary structure
    • Complex globular shape
    • Folding and twisting of polypeptides (H-bond)
    • Polypeptides contain many peptide bonds
  • Quaternary structure
    • Several polypeptide chains //several tertiary structures combined
    • Haemoglobin has 4 polypeptide chains
    • Collagen has 3 polypeptide chains, twisted around each other
    • Globular proteins are soluble and has folded chains
    • Fibrous proteins are insoluble and long, thin, twisted chains
  • Same amino acid sequence → same shape always

Bonds Found in Proteins

  • Hydrogen bonds
    • Between R-groups are easily broken, but are numerous
    • The more bonds, the stronger the structure
  • Disulphide bonds
    • Between sulphur-containing amino acid cystine
    • Strong bonds found in skin and hair
  • Denaturation
    • Destruction of tertiary structure, can be done by heat
    • Protein structure is lost and cannot reform → dysfunctional

Absorption and Function

  • Absorption of proteins in the digestive tract
    • Proteins are taken in as food
    • They are broken down in the digestive tract into their individual amino acids
    • Amino acids are recombined in the body to form different proteins
    • Good food sources include beans, milk, cheese, fish, meat
  • Several substances are composed of proteins with distinct functions
    • Keratin, collagen are main components in hair, muscles, tendons, skin
    • Enzyme amylase digests starch
    • Haemoglobin transports O2 in the blood stream
    • Insulin regulates glucose storage

Lipids

  • Easily dissolved in organic solvents but not in water
  • Triglycerides (fats and oils)
    • Serves as an energy reserve in plant and animal cells
    • Consists of 3 fatty acids linked by ester bonds to glycerol
    • Excess energy available from food/photosynthesis is stored as triglycerides
    • Can be broken down later to yield energy when needed
    • Fats and oils contain twice as many energy stored per unit of weight as carbohydrates
    • Triglycerides (TG) are also called triacylglycerides (TAG)
  • Saturated fatty acids
    • -COOH group without double bonds in the carbohydrate chain
    • May cause blockage of arteries which can lead to strokes and heart attacks
    • High melting point / solid at room temperature (fats) / typical animal fats
  • Unsaturated fatty acids
    • -COOH group with double bonds in the carbohydrate chain
    • Low melting point / liquid at room temperature (oils)
    • Found in plants
  • Phospholipids
    • Formed by replacing one fatty acids in a triglyceride with a phosphate group
    • Phosphate is polar / hydrophilic / does mix with H2O
    • Fatty acid tails remain non-polar / hydrophobic / insoluble, does not mix with H2O
    • Form a ball called a micelle when placed in a polar solution (e.g. water)