Water (H₂O) is one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is far more "electron-greedy" (electronegative) than hydrogen, so the shared electrons in each covalent bond spend more time near the oxygen. That creates a slightly negative region on the oxygen (δ⁻) and slightly positive regions on each hydrogen (δ⁺).

Think of it like a tug-of-war where oxygen is much stronger than hydrogen — it doesn't rip the rope away completely (that would be an ionic bond), it just pulls the middle point closer to its own side. That uneven "middle point" is called a dipole, and a molecule with a dipole like this is called a polar molecule.

Why this matters — properties that come from hydrogen bonding

• Cohesion — water molecules stick to each other via hydrogen bonds. This is why water flows as a continuous stream and why it can be pulled up a plant stem in one unbroken column.
• Adhesion — water molecules stick to other polar surfaces (like the walls of a xylem vessel). Cohesion + adhesion together explain capillary action.
• Excellent solvent — because water is polar, its δ⁺ ends surround negative ions (like Cl⁻) and its δ⁻ ends surround positive ions (like Na⁺). This pulls ionic and polar substances apart so they dissolve — which is essential because dissolved substances can move freely and react with each other inside cells.

Carbohydrates all contain carbon, hydrogen and oxygen, and they're built in a "Lego-brick" hierarchy: small units (monomers) snap together into bigger and bigger structures.

Glucose is a hexose (6-carbon) sugar that forms a six-membered ring. Carbons 1–5 form the ring, and carbon 6 sticks up above it. There are two nearly identical forms of glucose, and the only difference is which way round the H and OH groups point on carbon 1:

• Alpha (α) glucose — the H is above the ring at carbon 1, the OH is below.
• Beta (β) glucose — the H is below the ring at carbon 1, the OH is above.

This tiny difference matters enormously — it's the reason starch and glycogen (built from α-glucose) form compact, coiled/branched storage molecules, while cellulose (built from β-glucose, not required in this topic but useful context) forms straight, strong structural fibres.

Two monosaccharides join together via a condensation reaction to form a disaccharide, creating a glycosidic bond and releasing one water molecule. The "name" of the bond (1,4 or 1,2, etc.) tells you which numbered carbons on each sugar are joined.

Because disaccharides carry lots of exposed –OH (hydroxyl) groups, they form hydrogen bonds readily with water — which is why they're easily soluble and taste sweet, and why the body can quickly break them down for a fast source of energy.

Polysaccharides are the storage form of carbohydrate — big, insoluble, and packed with energy. Being insoluble is actually a feature, not a limitation: a soluble molecule would lower the water potential of the cell and cause water to rush in by osmosis, swelling and possibly bursting the cell. Starch and glycogen dodge this problem entirely.

Starch (plants) — made of two different polysaccharides

• Amylose — an unbranched chain of α-glucose joined by 1,4 glycosidic bonds. It coils into a helix shape, held together by hydrogen bonds within the molecule. The helix packs the chain very compactly, so a lot of glucose can be stored in a small space.
• Amylopectin — a branched molecule with 1,4 and 1,6 glycosidic bonds. The branches create lots of exposed terminal glucose molecules, which enzymes can hydrolyse quickly when the cell needs energy.

Both forms are insoluble and stored as granules inside plastids (membrane-bound organelles specialised for storage/photosynthesis in plant cells).

Glycogen (animals & fungi)

Glycogen works on the same principle as amylopectin — 1,4 and 1,6 glycosidic bonds — but is even more branched. More branches means more terminal glucose ends, which means glucose can be added or removed faster. This matters because animals are highly metabolically active and need to mobilise glucose quickly. It's stored as visible granules in liver and muscle cells.

Benedict's reagent is a blue solution containing copper(II) sulfate. Reducing sugars (which can donate electrons — remember OILRIG: Oxidation Is Loss, Reduction Is Gain, of electrons) reduce the blue Cu²⁺ ions to insoluble brick-red copper(I) oxide, which forms a precipitate.

Method

1. Add an excess of Benedict's reagent to the sample in a test tube.
2. Heat in a water bath that has been brought to the boil, for a few minutes.
3. Observe the colour change.

Testing for non-reducing sugars (e.g. sucrose)

Sucrose won't react with Benedict's reagent directly because it isn't a reducing sugar. To test for it:

1. Add dilute hydrochloric acid to the sample and heat in a boiling water bath — this hydrolyses any glycosidic bonds present, breaking the disaccharide into its monosaccharides.
2. Neutralise with sodium hydrogencarbonate (check with red litmus paper, then add a little extra so conditions are slightly alkaline — Benedict's test needs this to work).
3. Carry out the standard Benedict's test as normal — a colour change now confirms a reducing sugar was present (i.e. that a non-reducing sugar was hydrolysed into reducing monosaccharides).

The resulting monosaccharides expose an aldehyde or ketone functional group that can donate electrons to copper(II) sulfate, allowing the red precipitate to form.

Add a few drops of orange/brown iodine solution directly to the sample.

This test is especially useful in enzyme experiments — e.g. to show that amylase has fully digested starch (the blue-black colour disappears over time).

A colour change from "green" to "brick-red" is useful, but human eyes are bad at judging exact shades. To turn these tests into quantitative data (an actual concentration value), we use a colorimeter.

Step 1 — Serial dilution

Create a set of standard solutions of known concentration by serially diluting a stock solution (e.g. 0, 2, 4, 6, 8, 10 mmol dm⁻³ glucose, or "doubling dilutions" where concentration halves each time).

Step 2 — React identically

Treat every standard solution (and the unknown sample) in exactly the same way: same volume of Benedict's reagent or iodine, same water bath temperature, same time.

Step 3 — Colorimeter reading

A colorimeter shines a specific wavelength of light through the sample in a cuvette and measures how much light is absorbed. The filter colour chosen should contrast with the sample's colour — e.g. Benedict's turns orange/red, so a blue filter is used, because blue light is strongly absorbed by orange/red solutions.

Step 4 — Calibration curve

Plot absorbance (y-axis) against known concentration (x-axis) for your standards. This produces a calibration curve. Measure the absorbance of your unknown sample, then read across and down from the curve to estimate its concentration.

These two reaction types appear everywhere in biology — carbohydrates, lipids, and (later) proteins and nucleic acids all use the same basic pattern.

In carbohydrate chemistry specifically, the bond formed/broken is called a glycosidic bond, and it forms between two hydroxyl (–OH) groups on adjacent monosaccharides. The name (1,4-glycosidic, 1,6-glycosidic, 1,2-glycosidic) tells you which numbered carbons on each ring are linked.

Lipids are macromolecules made of carbon, hydrogen and oxygen — but with a much lower proportion of oxygen than carbohydrates, which is exactly why they're non-polar and hydrophobic (insoluble in water). Triglycerides are the type of lipid that makes up fats and oils.

A triglyceride is built from two types of monomer:

• Glycerol — a small alcohol molecule (C₃H₈O₃) with three hydroxyl (–OH) groups, one on each carbon.
• Fatty acids (×3) — each has a long hydrocarbon "tail" (the R group, typically 4–24 carbons) with a methyl group (CH₃) at one end and a carboxyl group (–COOH) at the other. Shorthand formula: RCOOH.

To break a triglyceride back down (during digestion, for example), the reverse — hydrolysis — occurs: water is added to break each ester bond, releasing the glycerol and fatty acids again.

Functions of lipids

Fatty acids vary in two ways: the length of the hydrocarbon (R group) chain, and whether the chain is saturated or unsaturated.

Most vegetable oils are unsaturated (exceptions: coconut and palm oil, which are saturated despite being plant-derived — a classic exam trap!).

Key Terms

Formulas & Quick Facts

• Fatty acid shorthand: RCOOH
• Glycerol formula: C₃H₈O₃
• 1 triglyceride = 1 glycerol + 3 fatty acids − 3 H₂O
• Benedict's positive result: blue → green → yellow → orange → brick-red
• Iodine positive result: orange/brown → blue-black
• OILRIG: Oxidation Is Loss, Reduction Is Gain (of electrons)

• I can explain why water is a polar molecule and how hydrogen bonds form between water molecules
• I can describe cohesion, adhesion, and water's role as a solvent
• I can name and distinguish monosaccharides, disaccharides, and polysaccharides
• I can describe the structural difference between alpha and beta glucose
• I can name the monosaccharides that combine to form maltose, sucrose, and lactose, and their glycosidic bond types
• I can describe the structure of amylose, amylopectin, and glycogen, and link structure to function
• I can carry out and interpret Benedict's test for reducing and non-reducing sugars
• I can carry out and interpret the iodine test for starch
• I can explain how to make sugar/starch tests semi-quantitative using serial dilutions and a colorimeter
• I can define condensation and hydrolysis reactions and explain the role of water in each
• I can describe the formation of a triglyceride via esterification (condensation)
• I can distinguish saturated, monounsaturated, and polyunsaturated fatty acids, and cis vs trans fats