Biological Molecules: Question 4

Worked solution

Part (a): Formation of a triglyceride

A triglyceride is formed from one glycerol molecule and three fatty acid molecules. Each fatty acid’s carboxyl group (-COOH) undergoes a condensation reaction with one of the three hydroxyl groups (-OH) on the glycerol molecule, forming an ester bond and releasing a molecule of water. Since this reaction happens three times (once for each fatty acid), three ester bonds are formed and three molecules of water are released in total.

Part (b): Triglycerides as an energy store

Triglycerides are non-polar molecules and so are insoluble in water; storing energy in this form does not disturb the water potential of the cell, unlike storing an equivalent mass of a soluble carbohydrate. Triglycerides also contain a much higher proportion of carbon-hydrogen bonds than carbohydrates, and comparatively little oxygen; when these bonds are broken during respiration, more energy is released per gram than from an equal mass of carbohydrate. This combination of insolubility and high energy density makes triglycerides a compact, efficient, long-term energy store in adipose tissue.

Part (c): Why phospholipids form a bilayer

Each phospholipid molecule has a hydrophilic phosphate head, which is attracted to water, and two hydrophobic fatty acid tails, which are repelled by water. When surrounded by water on both sides, phospholipid molecules automatically arrange themselves so that the hydrophilic heads face outwards, in contact with the water, while the hydrophobic tails point inwards, away from the water, shielding themselves from it. This produces a stable bilayer, with hydrophilic surfaces on the outside and a hydrophobic core, the structural basis of the cell-surface membrane.

Part (d): A property of water from hydrogen bonding

Water molecules are polar, and hydrogen bonds form between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of a neighbouring molecule. Because a large amount of energy is needed to break these many hydrogen bonds, water has a high specific heat capacity. This means the cytoplasm of cells, and the bodies of aquatic organisms such as fish, resist rapid changes in temperature even when the surrounding environment fluctuates, protecting enzyme-controlled reactions from being disrupted by sudden temperature changes. (Other biologically important properties arising from hydrogen bonding, such as high latent heat of vaporisation enabling evaporative cooling, or cohesion enabling water transport in xylem, would also be acceptable here.)

Final answers

• (a) One glycerol + three fatty acids, joined by three ester bonds, releasing three water molecules.
• (b) Non-polar and insoluble (no effect on water potential); high proportion of C-H bonds gives a high-energy, compact store.
• (c) Hydrophilic heads face outward towards water on both sides; hydrophobic tails cluster inward, away from water, forming a bilayer.
• (d) High specific heat capacity (from hydrogen bonding) gives thermal stability, protecting enzyme-controlled reactions in cells/aquatic organisms from rapid temperature change.