Biological molecules

Living things are built from four main kinds of large molecule 分子: carbohydrates 碳水化合物, lipids 脂质, proteins 蛋白质 and nucleic acids 核酸. You can use simple chemical tests to find out which kinds are present in a sample.

Benedict's test: blue turns green, then orange, then brick-red as more reducing sugar is present

Semi-quantitative Benedict's test

The normal Benedict's test only tells you "yes or no". A semi-quantitative 半定量 test gives a rough amount. First you standardise 标准化 the test: you run it on solutions of known concentration 浓度 and record the result for each. Then you can estimate an unknown by either:

• the time to the first colour change (more sugar changes colour faster), or
• comparing the final colour to your set of colour standards.

Testing for non-reducing sugars

Some sugars, such as sucrose, are non-reducing sugar 非还原糖: they give a negative Benedict's test. To detect them:

1. Do a normal Benedict's test first. It stays blue (no reducing sugar).
2. Take a fresh sample and add dilute hydrochloric acid 盐酸, then heat. This acid hydrolysis breaks the sugar into smaller reducing sugars.
3. Cool, then neutralise 中和 the acid with sodium hydrogencarbonate.
4. Now do the Benedict's test again. A brick-red colour shows a non-reducing sugar was present.

Testing for a non-reducing sugar: hydrolyse it with acid first, then the second Benedict's test turns red

Worked example. A solution gives a negative Benedict's test. It is then boiled with dilute hydrochloric acid, neutralised with sodium hydrogencarbonate, and re-tested with Benedict's - now it turns brick-red. What was present, and why is each step needed? The first negative result rules out a reducing sugar. Boiling with acid hydrolyses the glycosidic bond, splitting a non-reducing sugar such as sucrose into its reducing monosaccharides. The neutralising step is essential because Benedict's only works in alkaline conditions - skip it and the test fails even when sugar is present. The positive re-test therefore shows a non-reducing sugar was there all along. Quote the first, negative test as part of the answer: without it, the final red colour cannot tell a non-reducing sugar from a reducing one.

Monomers, polymers and macromolecules

• a monomer 单体 is a small molecule that is a single unit.
• a polymer 聚合物 is a long molecule made of many monomers joined together.
• a macromolecule 大分子 is any very large molecule.

Sugars come in three sizes:

• a monosaccharide 单糖 is a single sugar unit, such as glucose 葡萄糖 and fructose.
• a disaccharide 二糖 is two units joined, such as maltose and sucrose.
• a polysaccharide 多糖 is many units joined into a polymer.

Glucose, fructose 果糖 and maltose 麦芽糖 are reducing sugars. Sucrose 蔗糖 is a non-reducing sugar.

Two ring forms of glucose

Glucose has six carbon atoms and forms a ring. There are two ring forms. They differ only at carbon 1:

• in α-glucose, the –OH group on carbon 1 points down, below the ring.
• in β-glucose, the –OH group on carbon 1 points up, above the ring.

This small difference decides which polysaccharide the glucose can build.

The two ring forms differ only at carbon 1: the –OH points down in α, up in β

Joining and breaking sugars

Monomers are joined by strong covalent bonds 共价键. When two sugars join, a glycosidic bond 糖苷键 forms between them. This happens by condensation 缩合: a molecule of water is removed each time a bond forms.

The reverse is hydrolysis 水解: a water molecule is added to break a glycosidic bond. This is why the non-reducing sugar test needs acid and heat — they hydrolyse sucrose into glucose and fructose.

Condensation removes water to join monomers; hydrolysis adds water to split them

Storage polysaccharides: starch and glycogen

Starch is the energy 能量 store in plants. It is made of two polymers of α-glucose:

• amylose 直链淀粉 — a long, unbranched chain that coils into a spiral.
• amylopectin 支链淀粉 — a chain with many side branches.

Glycogen 糖原 is the energy store in animals. It is like amylopectin but has even more branches, so it can be broken down quickly when energy is needed.

These stores suit their job well: they are compact, they are insoluble 不溶 (so they do not leave the cell), and they do not change the water potential 水势 of the cell (so they do not pull water in by osmosis 渗透). The many branches give many ends, so glucose can be added or removed fast.

Real starch grains inside a potato, stained by iodine. Each grain is a dense, insoluble package of amylose and amylopectin — note the scale: the largest are only about 0.1 mm across

Cellulose

Cellulose 纤维素 is made of β-glucose. Because of the β form, every other glucose is flipped over, so the chains are long and straight. Many straight chains lie side by side and are held together by hydrogen bonds 氢键 into strong bundles called microfibrils 微纤丝. These give the plant cell wall 细胞壁 its strength and stop the cell bursting.

The shape fits the job: amylose coils and amylopectin/glycogen branch (for compact stores), while straight cellulose chains pack into strong fibres

Triglycerides

A triglyceride 甘油三酯 is the main fat or oil. It is non-polar 非极性 and hydrophobic 疏水 (it does not mix with water). It is made from one glycerol 甘油 molecule joined to three fatty acids 脂肪酸 by ester bonds 酯键. Each ester bond forms by condensation, so three water molecules are removed.

One glycerol plus three fatty acid tails; a straight tail is saturated, a kinked one unsaturated

Fatty acids are of two kinds:

• saturated 饱和 — no carbon–carbon double bonds 双键; these fats are usually solid.
• unsaturated 不饱和 — one or more double bonds; these oils are usually liquid.

Triglycerides make a good long-term energy store: they release about twice as much energy per gram as carbohydrates, they are insoluble, and they store little extra mass because they hold no water. Under the skin they also give insulation 隔热 and protect the organs.

Phospholipids

A phospholipid 磷脂 is like a triglyceride, but one fatty acid is replaced by a phosphate 磷酸 group. This gives the molecule two ends with different behaviour:

• a hydrophilic 亲水 ("water-loving") polar 极性 phosphate head.
• two hydrophobic ("water-fearing") fatty acid tails.

This split personality is why phospholipids form the membranes around cells.

The hydrophilic heads face the water; the hydrophobic tails hide inside, forming a bilayer

Amino acids and the peptide bond

Proteins are polymers of amino acids 氨基酸. Every amino acid has the same general structure around a central carbon atom: an amino group 氨基 (–NH₂), a carboxyl group 羧基 (–COOH), a hydrogen atom, and a variable side chain 侧链 (the R group). The R group is different in each amino acid.

Two amino acids join by condensation. The bond formed between the amino group of one and the carboxyl group of the next is a peptide bond 肽键, and a water molecule is removed. Many amino acids joined this way make a polypeptide 多肽. Adding water (hydrolysis) breaks a peptide bond.

Every amino acid has an amino group, a carboxyl group and an R group; two join by a peptide bond

Four levels of protein structure

The four levels: primary → secondary → tertiary → quaternary structure

The folded shape is held together by four kinds of interaction between R groups:

• hydrophobic interactions 疏水作用 (non-polar R groups cluster away from water).
• hydrogen bonding.
• ionic bonds 离子键 (between charged R groups).
• covalent bonding, including strong disulfide bonds 二硫键.

Globular and fibrous proteins

• globular proteins 球状蛋白质 fold into a rounded shape, are usually soluble 可溶, and do jobs in the body (for example enzymes and haemoglobin).
• fibrous proteins 纤维状蛋白质 form long strands, are usually insoluble, and give structure and support (for example collagen).

Haemoglobin — a globular protein

Haemoglobin 血红蛋白 carries oxygen 氧气 in red blood cells. It has a quaternary structure made of four polypeptide chains: two alpha (α-globin) chains and two beta (β-globin) chains. Each chain holds a haem group 血红素. At the centre of each haem group is an iron 铁 atom, and this is where one oxygen molecule binds. Four chains mean one haemoglobin molecule can carry four oxygen molecules.

A real haemoglobin molecule, worked out from X-ray data. Count them: two alpha chains (red), two beta chains (blue), and one green haem group held in each — so four oxygen molecules in total

Collagen — a fibrous protein

Collagen 胶原蛋白 gives strength to skin, tendons 肌腱, bone and blood vessel walls. One collagen molecule is three polypeptide chains wound tightly around each other in a triple strand, held by hydrogen bonds. Many of these molecules lie side by side, slightly staggered, and are cross-linked into thick fibres 纤维. The staggered, cross-linked arrangement makes collagen very strong when pulled.

Water is a small molecule, but its two O–H bonds are polar: the oxygen end is slightly negative and the hydrogen ends are slightly positive. So one water molecule attracts its neighbours, forming weak hydrogen bonds between them. These hydrogen bonds explain water's useful properties:

• solvent action — water is a good solvent 溶剂, so many substances dissolve in it. This lets reactions happen and lets substances be carried around the body.
• high specific heat capacity 比热容 — water needs a lot of energy to warm up, so its temperature stays steady. This protects living things from quick temperature changes.
• latent heat of vaporisation 汽化潜热 — water needs a lot of energy to evaporate 蒸发. So when water evaporates (for example as sweat dries), it carries away a lot of heat and cools the body.

Water is polar (δ− oxygen, δ+ hydrogens), so its molecules attract each other by hydrogen bonds — the reason for all the properties above

• Learn each food test as reagent + positive result + colour change (Benedict's → brick-red; iodine → blue-black; biuret → purple; emulsion → white).
• Benedict's detects reducing sugars; for a non-reducing sugar you must hydrolyse with acid first, then re-test.
• Name the bond precisely: glycosidic (carbohydrates), ester (lipids), peptide (proteins) — all made by condensation (water removed).
• Link structure to function: cellulose (straight chains, H-bonds → strong), glycogen/starch (branched/coiled → compact store).
• For proteins, name the bond at each level: primary (peptide), secondary (hydrogen), tertiary (R-group interactions), quaternary.