2.3 - Carbohydrates and lipids

• Glucose:

  • Has the formula C6H12O6
  • It forms a hexagonal ring (hexose)
  • Glucose is the form of sugar that fuels respiration
  • Glucose forms the base unit for many polymers
• Glucose:

  A) CH2OH

  B) O

  
• Alpha glucose Formation: DDUD
• Beta glucose formation: UDUD
• Galactose:

  • Formation: UDUU
  • A hexose sugar
  • Formula C6H12O6 but is less sweet
  • Most commonly found in milk but also found in cereals
• Fructose:

  • A hexose sugar
  • Commonly found in fruits and honey
  • It is the sweetest naturally occurring carbohydrate
• Fructose

  A) CH2OH

  B) CH2OH

  C) Fructose

  D) OH

  E) OH

  F) OH

  G) O

  
• Ribose:

  A) CH2OH

  B) OH

  C) O

  D) OH

  E) OH

  
• Ribose:

  • A pentose sugar, it has a pentagonal ring
  • It forms the backbone of RNA
  • Deoxyribose differs as shown in the diagram with a H
• Monosaccharides are the monomers of polysaccharides
• Maltose:

  • A dimer of glucose
• Lactose:

  • A dimer of glucose and galactose
  • most commonly found in milk
• Sucrose:

  • A dimer of glucose and fructose
  • Known as table sugar
• Polysaccharides:

  • contain more than 2 molecules
  • Often very long and branched
  • Glycosidic bonds can be 1-4 or 1-6
• Cellulose:

  • Cellulose is made by linking together β-glucose molecules.
  • Condensation reactions link carbon atom 1 to carbon atom 4 on the next B-glucose.
  • The glucose subunits in the chain are oriented alternately upwards and downwards.
  • The consequence of this is that the cellulose molecule is a straight chain, rather than curved.
• Cellulose:

  • Cellulose molecules are unbranched chains of B-glucose.
  • Hydrogen bonds link the the molecules together.
  • The linked molecules form bundles called cellulose microfibrils.
  • They have very high tensile strength.
  • The tensile strength of cellulose (the basis of cell walls) prevents plant cells from bursting, even under very high (water) pressure.
• Starch structure:

  • Starch is made by linking together a-glucose molecules
  • Condensation reactions link carbon atom 1 to carbon atom 4 on the next a-glucose
  • All the glucose molecules in starch can be orientated in the same way
  • The consequence of this is that the starch molecule is curved, rather than straight.
  • Size of the molecule is not fixed
• Starch Function:

  • Starch is only made by plant cells.
  • Molecules of both types of starch are hydrophilic but are too large to be soluble in water.
  • Starch does not affect the osmotic balance of cells,
  • It is easy to add or remove extra glucose molecules to starch
  • Therefore starch is useful in cells for glucose, and consequently energy, storage.
  • In seeds and storage organs such as potato cells glucose is held as starch.
  • Starch is made as a temporary store in leaf cells when glucose made faster by photosynthesis than it can be exported to other parts of the plant.
• Amylopectin:

  • The chain is branched, so has a more globular shape.
  • Due it’s branched nature amylopectin can be much larger consisting of 2,000-200,000 units
• Amylose:

  • The chain of α-glucose molecules is un-branched and forms a helix.
  • Typically amylose is made up of 300-3,000 glucose units
• Glycogen:

  • The molecule varies in size, typically it consists of 30,000 units
  • Glycogen is not just a simple chain, it branches many times
  • Condensation reactions link carbon atom 1 - 4 on the next  α-glucose.
  • Branches occur where a condensation reaction instead links carbon atom 1 – 6.
  • As a result the molecule is compact
• Glycogen:

  • Glycogen does not affect the osmotic balance of cells, i.e. cause too much water to enter them
  • It is easy to add or remove extra glucose molecules to starch
  • Therefore glycogen is useful in cells for glucose, and consequently energy, storage.
• Glycogen:

  • Glycogen is made by animals and also some fungi.
  • It is stored in the liver and some muscles in humans.
  • It is used in cells where large stores of dissolved glucose would cause osmotic problems.
• Fatty acid:

  A) CH3

  B) CH2

  C) COOH

  
• Types of fatty acids:

  A) Saturated

  B) Monounsaturated

  C) Polyunsaturated

  
• Cis-isomers:

  • Very common in nature
  • The hydrogen atoms are on the same side of the two carbon atoms
  • The double bond causes a bend in the fatty acid chain
  • Therefore cis-isomers are only loosely packed
  • Triglycerides formed from cis-isomers have low melting points – they usually liquid at room temperature
• Trans-isomers:

  • Rare in nature – usually artificially produced to produce solid fats, e.g. margarine from vegetable oils.
  • The hydrogen atoms are on opposite side of the two carbon atoms
  • The double bond does not causes a bend in the fatty acid chain
  • Trans-isomers can be closely packed
  • Triglycerides formed from trans-isomers have high melting points – they usually solid at room temperature
• Isomers

  A) CIS-ISOMER

  B) TRANS-ISOMER

  
• Triglyceride:

  A) H

  B) H

  C) H

  D) C

  E) OH

  F) GLYCEROL

  G) 3 FATTY ACIDS

  H) Triglyceride

  I) ester bond

  
• Lipid functions:

  • Structure:  Phospholipids are a main component of cell membranes
  • Hormonal signalling:  Steroids are involved in hormonal signalling (e.g. estrogen, progesterone, testosterone)
  • Insulation:  Fats in animals can serve as heat insulators while sphingolipids in the myelin sheath (of neurons) can serve as electrical insulators
  • Protection:  Triglycerides may form a tissue layer around many key internal organs and provide protection against physical injury
  • Storage of energy:  Triglycerides can be used as a long-term energy storage source
• Lipid energy storage:

  • Lipids are normally used for long-term energy storage whereas carbohydrates are used for short-term energy storage
  • The lipids that are used are fats.
  • They are stored in specialized groups of cells called adipose tissue.
  • Adipose tissue is located immediately beneath the skin and also around some organs including the kidneys.
• Reasons for using lipids for long-term energy storage:

  • The amount of energy released in cell respiration per gram of lipids is double that for carbohydrates (and protein)
  • Lipids add 1/6 as much to body mass as carbohydrates: fats are stored as pure droplets whereas when 1g glycogen is stored it is associated with 2g of water.
  • This is especially critical for active animals as energy stores have to be carried.
• Why is glycogen needed?

  • This is because glycogen can be broken down to glucose rapidly
  • Then can be transported easily by the blood to where it is needed
  • Fats in adipose tissue cannot be mobilized as rapidly
  • Glucose can be used either in anaerobic or aerobic cell respiration whereas fats and fatty acids can only be used in aerobic respiration
• Glycogen:

  • The medium-term energy storage molecule in animals. It is stored in the liver and muscles. The energy stored in glycogen is more readily available than the energy stored in fat.
• Glucose:

  • Is in the bloodstream for immediate use and will either be used in respiration to yield ATP or converted to glycogen or fat
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