Lipids are excellent storage compounds

• Lipid macromolecules, like carbohydrates, contain?carbon, hydrogen?and?oxygen?atoms
• However, unlike carbohydrates, lipids contain?a low proportion of oxygen
• More of the oxygen required for their respiration has to come from?the air
• This allows lipids to be?energy-dense, maximising the energy content per gram versus carbohydrates
  • They contain?2× more energy per gram?than most carbohydrates
  • Less body mass?is required to store a given amount of energy

   

  Lipids are?insoluble?so do not affect osmosis, so do not risk upsetting the water balance of the organism

• When lipids are respired,?a lot of water is produced?compared to the respiration of carbohydrates
  • This is called?metabolic water?and can be used as a?dietary water source?when drinking water is unavailable
    • A?camel's hump?is not a water sac, it is a lipid-rich storage organ that yields metabolic water for the camel in its dry desert habitat
    • A?bird's egg?also makes use of lipid-rich yolk to provide energy and metabolic water to the growing chick

     

  All these features make lipids ideal for long term energy storage

Forms of lipid storage

• In?animals, lipids are stored in various areas
  • Subcutaneous?fats are stored below the skin
  • Visceral?fats are stored around the major internal organs

  There are genetic?and?gender differences?between how individuals store fat

• Fat is stored in?adipose cells, which are specialised to contain large globules of fat
• Adipose cells shrink?when the fat is respired to generate metabolic energy
• In many plants,?seeds?have evolved to store fats to provide energy for a growing seedling plant
• Olives,?sunflowers,?nuts,?coconuts?and?oilseed rape?are good examples of crops whose oils are harvested for edible oil production by humans

Other roles of lipids

• As well as energy storage molecules, lipids have a?number of other roles
  • Physical?protection of soft organs?eg. visceral fat around the heart
  • Thermal insulation?from subcutaneous fat eg. whale blubber
  • Subcutaneous fat as a?buoyancy aid?eg. in seals (fat is less dense than water so assists flotation)
  • Waterproofing secretions?eg. birds' preening glands or waxy cuticles on leaf surfaces
  • Electrical insulation?eg. the myelin sheath around certain nerve axons

  Exam Tip

• Lipids have been?associated with poor health?for a long time, even though they perform vital functions in tissues and organs
• High-fat diets tend to supply?more chemical energy?than an individual needs
  • Consuming excess fat can cause an individual to become?overweight?or?obese?due to the?storage of fat?in adipose tissue

  Body Mass Index?(BMI) is a rough and ready measure of a person's mass in relation to their height

• The calculation of BMI is as follows

• • A BMI below 18.5 is considered?underweight
  • A BMI 18.5-24.9 is considered?normal
  • A BMI of 25.0-29.9 is considered?overweight
  • A BMI of 30.0-39.9 is considered?obese
  • A BMI of 40.0 or more is considered?morbidly obese

  BMI is a crude measurement?as it works against individuals who are heavily muscular but who are also extremely lean

• Overweight and obese people have a higher risk of developing?type II diabetes?and?high blood pressure?and?coronary heart disease
• Because many risk factors combine in the prevalence of these conditions,?lipids are by no means the only cause

NOS: Evaluating claims; health claims made about lipids in diets need to be assessed

• Popular literature, TV and social media make?claims about various foods?and their health benefits
• A food product labelled, 'Low Sugar' may in fact contain a lot of trans-fats but?hides that information, or doesn't label it at all!
• Many health claims are based on?pseudoscience, or backed up with only very small trials or small samples sizes
• Only?scientifically controlled studies?are able to prove?causal links?between food choices and health risks
• Techniques such as?randomised clinical trials?provide data to?inform government policy?and consumers about their food choices
• There remain?complex challenges?for consumers, food producers and governments to ensure a food supply that puts people?at least risk of disease?whilst ensuring that enough food is produced

Formation of Lipids

Triglycerides are formed by?esterification

• An?ester bond?forms when the hydroxyl (-OH) group of the glycerol bonds with the carboxyl group (-COOH) of the fatty acid
  • The formation of an ester bond is a?condensation reaction
    • For each ester bond formed a water molecule is released

    Three fatty acids?join to?one glycerol?molecule to form a?triglyceride

  • Therefore for one triglyceride to form,?three water molecules are released

Formation of a triglyceride from a glycerol molecule and three fatty acid molecules by the process of esterification