IB DP Biology: HL复习笔记2.2.3 Lipids

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