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Lipids (DP IB Biology: SL)

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Long Term Storage

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
    • Certain photosynthetic pigments eg. carotenoids
    • Glycolipids, typically as cell-surface recognition molecules/receptors

Examiner Tip

Ensure that you are familiar with the structure of a triglyceride and that you can recognise whether the fatty acids are saturated or unsaturated.

Lipids: Health Claims

  • 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

Body Mass Index
    • 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

Triglycerides Basics - Formation of a triglyceride (1)_2, downloadable AS Level & A Level Biology revision notesTriglycerides Basics - Formation of a triglyceride (2), downloadable AS Level & A Level Biology revision notes

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

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Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.