Revise Carbohydrates and lipids for Cambridge 9700 (2.2)

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a general formula of Cₓ(H₂O)y. They are primarily known for their role as energy sources and for structural purposes.

Monosaccharides

These are the simplest carbohydrates, often called 'simple sugars'. They are the monomers (single units) from which larger carbohydrates are built. Key examples include:

• Glucose: A hexose sugar (C₆H₁₂O₆), crucial for respiration. You must know the ring forms of α-glucose (hydroxyl group on carbon 1 points down) and β-glucose (hydroxyl group on carbon 1 points up). This small difference is vital for their polymer formation.
• Ribose: A pentose sugar (C₅H₁₀O₅), a component of RNA, ATP, and NAD.

Disaccharides

Formed when two monosaccharides join together via a condensation reaction, releasing a molecule of water and forming a glycosidic bond. Examples include:

• Maltose: α-glucose + α-glucose
• Sucrose: α-glucose + fructose (transported in plants)
• Lactose: α-glucose + galactose (milk sugar) Disaccharides can be broken down into their monosaccharide units via hydrolysis, which is the chemical addition of water to break a bond.

Polysaccharides

These are large polymers formed from many monosaccharide units joined by glycosidic bonds. Their large size makes them insoluble, which is ideal for storage as they do not affect the water potential of cells. Their structure directly relates to their function:

• Starch: The primary energy storage in plants, found as grains in chloroplasts and storage organs. It's a mixture of two α-glucose polymers:
  • Amylose: An unbranched chain of α-glucose units linked by α-1,4 glycosidic bonds. Its helical structure makes it compact for storage.
  • Amylopectin: A branched chain of α-glucose units linked by α-1,4 glycosidic bonds in the chains and α-1,6 glycosidic bonds at branch points. The branching provides many terminal ends for enzymes to hydrolyse, allowing for quicker glucose release when energy is needed.
• Glycogen: The main energy storage in animals, stored in the liver and muscles. It is structurally similar to amylopectin but is more highly branched. This extensive branching allows for very rapid mobilisation of glucose to meet the high metabolic demands of animals.
• Cellulose: A structural component of plant cell walls. It's an unbranched polymer of β-glucose units, linked by β-1,4 glycosidic bonds. The orientation of β-glucose monomers (with every other monomer inverted by 180°) allows for long, straight, unbranched chains. Many of these parallel chains form extensive hydrogen bonds with each other, creating strong microfibrils. These microfibrils provide high tensile strength, preventing the plant cell from bursting under turgor pressure.

Condensation Reaction (forming a disaccharide): Monosaccharide + Monosaccharide → Disaccharide + H₂O

Hydrolysis Reaction (breaking down a disaccharide): Disaccharide + H₂O → Monosaccharide + Monosaccharide

Lipids are a diverse group of organic molecules that are insoluble in water but soluble in organic solvents like ethanol. They contain carbon, hydrogen, and oxygen, but with a much lower proportion of oxygen compared to carbohydrates. This high proportion of C-H bonds makes them a rich source of energy.

Triglycerides

These are the most common type of lipid, often referred to as 'fats' (solid at room temperature) or 'oils' (liquid at room temperature). They are formed by one molecule of glycerol chemically bonded to three molecules of fatty acids via condensation reactions. Each bond formed is an ester bond, and three water molecules are released in total.

Fatty Acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end. They can be:

• Saturated: Contain no carbon-carbon double bonds (C=C) in their hydrocarbon chain. The chains are straight, allowing them to pack tightly together. This results in strong intermolecular forces, making triglycerides containing them solid at room temperature (e.g., butter, lard).
• Unsaturated: Contain one or more C=C double bonds. The double bonds introduce 'kinks' or bends in the chain, preventing close packing. This weakens intermolecular forces, making triglycerides containing them liquid at room temperature (e.g., olive oil).

Functions of Triglycerides:

• Energy Storage: They contain more energy per gram than carbohydrates, making them an efficient long-term energy store.
• Insulation: Adipose tissue under the skin provides thermal insulation.
• Protection: Fat deposits cushion vital organs.
• Buoyancy: Being less dense than water, fat helps aquatic animals to float.
• Metabolic Water: Oxidation of lipids produces a significant amount of water, crucial for desert animals.

Phospholipids

These are modified triglycerides and are crucial components of cell membranes. A phospholipid molecule consists of a glycerol molecule bonded to two fatty acids and one phosphate group. The phosphate group is negatively charged and polar, making this part of the molecule hydrophilic ('water-loving'). The two fatty acid 'tails' are non-polar and hydrophobic ('water-repelling'). This dual nature is called amphipathic. In aqueous environments, this property causes phospholipids to spontaneously arrange into a bilayer, with the hydrophilic heads facing the water on either side and the hydrophobic tails shielded in the centre. This bilayer forms the basic structure of all cell membranes.

A series of standard laboratory tests can be used to identify the presence of these biological molecules.

1. Test for Reducing Sugars (e.g., glucose, fructose, maltose)

• Reagent: Benedict's solution (blue).
• Procedure: Add an equal volume of Benedict's solution to the sample in a test tube and heat in a water bath at 80-95°C for 5 minutes.
• Positive Result: The blue solution changes colour. The final colour indicates the concentration of the sugar: Green → Yellow → Orange → Brick-red (highest concentration).

2. Test for Non-Reducing Sugars (e.g., sucrose)

• Procedure: First, perform the test for reducing sugars. If it is negative (remains blue), take a fresh sample. Add a few drops of dilute hydrochloric acid and heat in a water bath to hydrolyse the sugar into its monosaccharides. Then, neutralise the acid with sodium hydrogencarbonate solution. Finally, perform the Benedict's test again.
• Positive Result: A positive colour change (Green/Yellow/Orange/Brick-red) now indicates the original presence of a non-reducing sugar.

3. Test for Starch

• Reagent: Iodine solution (yellow-brown).
• Procedure: Add a few drops of iodine solution directly to the sample.
• Positive Result: The solution turns from yellow-brown to a blue-black colour.

4. Emulsion Test for Lipids

• Procedure: Add about 2 cm³ of ethanol to the sample in a test tube and shake vigorously to dissolve any lipids. Pour this ethanol-sample mixture into a test tube containing water.
• Positive Result: A cloudy white emulsion forms, indicating the presence of lipids.