Water carbohydrates and lipids

Water as a Solvent in Transport

Water acts as a vital biological solvent because its dipole nature allows it to dissolve charged or polar substances and transport them around organisms.

The unequal sharing of electrons between oxygen and hydrogen makes water a polar molecule. Oxygen pulls the shared electrons more strongly, becoming slightly negative (δ⁻). Each hydrogen becomes slightly positive (δ⁺). This permanent separation of charge across the molecule is called a dipole.

When ionic or polar solutes enter water, the dipoles attract them. The slightly positive hydrogens face negative ions. The slightly negative oxygens face positive ions. Water molecules surround each solute particle and pull them apart, dissolving them.

This solvent property is essential for transport in living organisms. Glucose, amino acids, mineral ions, hormones, and urea all dissolve in blood plasma. The circulatory system can therefore carry them around the body. In plants, mineral ions and sucrose dissolve in water for transport through xylem and phloem. Inside cells, the watery cytoplasm provides the medium for metabolic reactions to occur.

Hydrogen bonds also form between adjacent water molecules. These weak attractions occur between the δ⁺ hydrogen of one molecule and the δ⁻ oxygen of another. The resulting cohesion supports the continuous water column drawn up xylem during transpiration.

In contrast, non-polar substances like lipids are hydrophobic and do not dissolve in water. They are transported by proteins, for example as lipoproteins in blood.

Classifying Carbohydrates and Energy Roles

Carbohydrates are organic molecules built from carbon, hydrogen, and oxygen, classified by size into three groups: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides are single sugar units, typically with the general formula (CH₂O)ₙ. The three required examples are glucose, fructose, and galactose. They are small, polar, and soluble, which makes them easy to transport in solution. Glucose is the main respiratory substrate: it is broken down to release energy in cells.

Disaccharides form when two monosaccharides join through condensation. The three required examples are maltose (glucose + glucose), sucrose (glucose + fructose), and lactose (glucose + galactose). They remain soluble and act as transport sugars. For example, sucrose is the main transport sugar in plant phloem.

Polysaccharides are long chains or branched structures made of many monosaccharide units. They are insoluble, so they do not affect water potential or osmotic balance inside cells. This insolubility, combined with their compact shape, makes them ideal energy storage molecules. The three required examples are glycogen, amylose, and amylopectin.

The link between structure and function is consistent. The helix in amylose makes it compact for storage. Branching gives many ends for enzymes to act on simultaneously, allowing fast glucose release. Glycogen is the most branched because animals have higher metabolic rates than plants and need glucose mobilised quickly. Insolubility means these large molecules do not lower the water potential of storage cells, preventing unwanted osmotic water uptake.

Testing for Reducing Sugars and Starch

Two important biochemical tests allow semi-quantitative estimation of carbohydrate concentrations: Benedict’s reagent detects reducing sugars, while iodine solution detects starch.

Benedict’s reagent contains copper(II) sulfate. When heated with a reducing sugar, the sugar donates electrons that reduce Cu²⁺ to Cu⁺. This produces an insoluble brick-red precipitate of copper(I) oxide. As the concentration of reducing sugar increases, more precipitate forms and the colour shifts from blue → green → yellow → orange → brick-red.

Reducing sugars include all monosaccharides (glucose, fructose, galactose) and most disaccharides (maltose, lactose). Sucrose is non-reducing and produces no colour change directly with Benedict’s. To test for sucrose, the sample must first be hydrolysed (e.g. with dilute hydrochloric acid) to release reducing monosaccharides.

Iodine solution (potassium iodide with iodine) detects starch. When iodine enters the helical coils of amylose, the starch–iodine complex absorbs light strongly, producing a blue-black colour. The deeper the colour, the more starch present.

Both tests can be used semi-quantitatively by comparing samples to a series of solutions of known concentration (colour standards).

MisconceptionMany students assume sucrose gives a positive Benedict’s test simply because it is a sugar. Sucrose is non-reducing and gives no colour change unless first hydrolysed. Maltose and lactose, however, are reducing and do react.

Exam TipAlways check whether the named sugar is reducing before predicting a Benedict’s result.

Glycosidic Bonds, Condensation and Hydrolysis

Monosaccharides join through condensation reactions to form disaccharides and polysaccharides, creating glycosidic bonds that are later broken by hydrolysis when sugars are needed.

In a condensation reaction, two monosaccharides come together. A hydroxyl (–OH) group on one sugar reacts with a hydroxyl group on the next. A molecule of water (H₂O) is released, and a new covalent glycosidic bond links the two sugars. For example, an α-1,4 glycosidic bond joins carbon 1 of one glucose to carbon 4 of the next.

Examples of disaccharide formation:

• glucose + glucose → maltose + H₂O
• glucose + fructose → sucrose + H₂O
• glucose + galactose → lactose + H₂O

Polysaccharides such as glycogen, amylose, and amylopectin form by repeated condensation reactions. Each step releases one water molecule and forms one glycosidic bond.

In a hydrolysis reaction, a water molecule is added across a glycosidic bond, breaking it. The –H from water joins one sugar, and the –OH joins the other, releasing the original monosaccharides. Hydrolysis is catalysed by specific enzymes. Amylase hydrolyses starch into maltose, and maltase hydrolyses maltose into glucose. Hydrolysis is essential during digestion and during the breakdown of stored polysaccharides for respiration.

Examiner Insight“Glycosidic bond” earns the mark; “chemical bond” or “sugar bond” does not. For full credit, name the specific bond (glycosidic) and the type of reaction (condensation or hydrolysis), and state that water is released or added.

Exam TipAlways pair the bond name with the reaction type and the role of water.

Triglycerides and Lipid Saturation

A triglyceride is a major lipid formed when one glycerol combines with three fatty acids through condensation reactions, producing three ester bonds.

Each fatty acid joins glycerol when its carboxyl group (–COOH) reacts with a hydroxyl group (–OH) on the glycerol. One water molecule is released per bond formed, so each triglyceride synthesis releases three water molecules in total. The bond formed is called an ester bond, characterised by the C(=O)–O linkage between the fatty acid and glycerol.

Triglycerides are excellent energy stores. They contain many C–H bonds, releasing approximately twice the energy per gram as carbohydrate when respired. They are insoluble, so they do not affect water potential. Stored compactly in adipose tissue, they also provide thermal insulation, cushioning of organs, and (when respired) metabolic water.

Triglycerides can be saturated or unsaturated, depending on the bonds in their fatty acid chains:

A fatty acid with one C=C double bond is monounsaturated, while one with multiple C=C bonds is polyunsaturated.