Molecules of Life
FoundationWater and inorganic ions underpin life, and the four classes of biomolecules — carbohydrates, lipids, proteins and nucleic acids — each have their own building blocks and roles; with the Benedict's, iodine, grease spot and biuret tests you can identify what any food contains.
- Water's high specific heat capacity, good solvent power and high latent heat of vaporisation support stable body temperature, transport and cooling.
- Inorganic ions such as nitrogen, magnesium, calcium and iron are needed in small amounts but are essential parts of proteins, chlorophyll, bones and teeth, and haemoglobin.
- Carbohydrates come as monosaccharides, disaccharides and polysaccharides: glucose fuels cells directly, starch and glycogen store energy in plants and animals, and cellulose builds cell walls.
文體小知識：科學知識說明文——介紹水、無機離子和四大類生物分子的構件與功能，並示範常用食物試驗的原理，是中四生物的第一課。
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1Water is the most abundant substance in the body, making up about sixty to seventy per cent of body mass. It has several life-friendly properties: its high specific heat capacity means the temperature changes little even when much heat is absorbed or released, keeping body temperature stable; it is an excellent solvent, carrying nutrients, wastes and gases dissolved in the blood; and its high latent heat of vaporisation means a little sweat removes a lot of heat.
2The body also needs a range of inorganic ions. Nitrogen is a key element in proteins and nucleic acids; magnesium sits at the centre of the chlorophyll molecule, so leaves turn yellow without it; calcium is the main component of bones and teeth; and iron is the core of haemoglobin in red blood cells, so a lack of iron lowers oxygen transport and causes anaemia.
3Carbohydrates contain carbon, hydrogen and oxygen, and are the main energy source in the diet. Monosaccharides such as glucose can be absorbed and broken down directly; disaccharides such as maltose and sucrose are made of two monosaccharide units; polysaccharides are long chains of monosaccharides — starch stores energy in plants, glycogen in animals, and cellulose forms plant cell walls.
4Lipids are built from glycerol and fatty acids and do not dissolve in water. Gram for gram they store about twice as much energy as carbohydrates, making them efficient reserves; subcutaneous fat also slows heat loss and cushions organs, while phospholipids form the basis of every cell membrane. Proteins are chains of amino acids — twenty kinds are common — used for growth and repair, catalysis, oxygen transport and defence against pathogens.
5Nucleic acids are polymers of nucleotides: DNA and RNA store and pass on genetic information. To identify which biomolecules a food contains, we use food tests: Benedict's test for reducing sugars, the iodine test for starch, the grease spot test for lipids, and the biuret test for proteins — each judged by a specific colour change.
1Summary 重點摘要
Water and inorganic ions underpin life, and the four classes of biomolecules — carbohydrates, lipids, proteins and nucleic acids — each have their own building blocks and roles; with the Benedict's, iodine, grease spot and biuret tests you can identify what any food contains.
2Vocabulary cards Vocabulary
A biomolecule made of carbon, hydrogen and oxygen, covering monosaccharides, disaccharides and polysaccharides; the main energy source of organisms.
A common six-carbon monosaccharide; the usual substrate of cell respiration, absorbed directly by the digestive tract.
The storage polysaccharide of plants, a long chain of glucose units; it turns blue-black with iodine solution.
The storage polysaccharide of animals, kept mainly in the liver and muscles and quickly broken down to top up blood glucose.
The polysaccharide of plant cell walls; human digestive enzymes cannot break it down, so it counts as dietary fibre.
Water-insoluble biomolecules made of glycerol and fatty acids, used for energy storage, insulation and cell membranes.
The building blocks of proteins; twenty kinds are common, chained in different orders to form different proteins.
Biomolecules made of amino acids, responsible for growth and repair, catalysis, transport and defence.
Nucleotide-based biomolecules that store genetic information, including DNA and RNA.
A food test for reducing sugars: heated with Benedict's solution, a positive result gives a brick-red precipitate.
3Step by step Steps
- Step 1: Water — the medium of lifeWater is the most abundant substance in cells, and three of its properties support life directly. First, its high specific heat capacity means the temperature changes little even when much heat is absorbed or released, so body temperature stays stable. Second, water is an excellent solvent: nutrients, wastes and gases dissolve in blood for transport. Third, its high latent heat of vaporisation means a little sweat removes a lot of heat. For example, sweat evaporating after exercise carries heat away from the skin, using exactly this property.
- Step 2: The role of inorganic ionsInorganic ions are needed in small amounts, yet life cannot go on without them. Nitrogen is a key component of amino acids and nucleotides; without it plants cannot build proteins. Magnesium sits at the centre of every chlorophyll molecule, so magnesium-starved leaves turn yellow. Calcium hardens bones and teeth, and growing children need plenty. Iron is the core of haemoglobin; without it the blood carries less oxygen and anaemia follows. For example, dark green vegetables are rich in magnesium, and a balanced diet supplies many ions at once.
- Step 3: Monosaccharides and disaccharidesCarbohydrates fall into three classes by size. Monosaccharides, the smallest units, such as glucose and fructose, dissolve easily, cross cell membranes directly and are the first molecules broken down in respiration. Disaccharides pair two monosaccharides: maltose is two glucose units; sucrose is glucose plus fructose. For example, table sugar is split into glucose and fructose by enzymes in the gut before it can be absorbed into the blood.
- Step 4: Polysaccharides — storage and structurePolysaccharides chain together hundreds or thousands of monosaccharides, forming large, poorly soluble molecules ideal for long-term storage or structure. Starch is the storage polysaccharide of plants, abundant in rice and potatoes. Glycogen stores energy in animals, packed in the liver and muscles for quick release into the blood. Cellulose forms plant cell walls and gives mechanical support; humans cannot digest it, but as dietary fibre it keeps the gut moving. For example, athletes load up on rice before a race to fill their muscle glycogen stores.
- Step 5: LipidsLipids are built from one glycerol molecule joined to fatty acids, and include fats, oils and phospholipids. Gram for gram they store about twice as much energy as carbohydrates, making them efficient reserves. Subcutaneous fat conducts heat slowly, reducing heat loss and cushioning organs against knocks. Phospholipids, with one water-loving and one water-hating end, form the skeleton of every cell membrane. For example, penguins and seals survive icy seas because their thick blubber layer slows heat loss.
- Step 6: Proteins and nucleic acidsProteins are chains of amino acids — twenty kinds are common — and different sequences give millions of different proteins: some build and repair tissue, some catalyse reactions (enzymes), some carry oxygen (haemoglobin) and some defend the body (antibodies). Nucleic acids, DNA and RNA, are polymers of nucleotides that store genetic information and direct protein synthesis. For example, building and repairing muscle demands plenty of protein, which is why growing teenagers and athletes need a protein-rich diet.
- Step 7: Four common food testsFood tests are basic S4 skills; match each test to its target and its positive result. Benedict's test: heat with Benedict's solution; a brick-red precipitate means reducing sugars. Iodine test: brown-yellow turning blue-black means starch. Grease spot test: a translucent stain on paper means lipids. Biuret test: turning violet means protein. For example, run all four tests on fresh milk and you will find protein, lipid and reducing sugar (lactose) — one food often contains several classes of biomolecules.
4Diagrams Diagrams
5Worked example Example
A student ran food tests on crushed steamed bun and fresh milk. The bun suspension turned blue-black with iodine solution, and no precipitate formed when it was heated with Benedict's solution. The milk turned violet with biuret solution, left a translucent stain on paper, and gave a brick-red precipitate when heated with Benedict's solution. Identify the biomolecules in each food and state the evidence for each conclusion.
- The bun turned blue-black with iodine solution — a positive iodine test, so the bun contains starch.
- No brick-red precipitate formed when the bun was heated with Benedict's solution — a negative Benedict's test, so the bun contains no reducing sugar (starch itself is not a reducing sugar).
- The milk turned violet with biuret solution and left a translucent stain on paper — positive biuret and grease spot tests, so milk contains protein and lipid.
- The milk gave a brick-red precipitate with Benedict's solution — a positive test, so milk contains reducing sugar (lactose).
- Answer: The steamed bun contains starch (blue-black with iodine) but no reducing sugar. Fresh milk contains protein (violet with biuret), lipid (translucent stain) and reducing sugar (brick-red precipitate with Benedict's solution). In data questions, state the test, the observation and the conclusion to earn full marks.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the building blocks of carbohydrates, lipids and proteins?
Q2Which polysaccharides store energy in animals and in plants?
Q3Iodine solution turns blue-black on mashed potato. What does this show?
Q4Why is the high latent heat of vaporisation of water important for cooling the body?
Q5A food extract turns violet when biuret solution is added. What conclusion can be drawn?
8Recap Recap
- Water's high specific heat capacity, good solvent power and high latent heat of vaporisation support stable body temperature, transport and cooling.
- Inorganic ions such as nitrogen, magnesium, calcium and iron are needed in small amounts but are essential parts of proteins, chlorophyll, bones and teeth, and haemoglobin.
- Carbohydrates come as monosaccharides, disaccharides and polysaccharides: glucose fuels cells directly, starch and glycogen store energy in plants and animals, and cellulose builds cell walls.
- Lipids (glycerol plus fatty acids) store energy efficiently, insulate the body and form cell membranes; proteins (twenty kinds of amino acids) handle growth, repair, catalysis and defence.
- Match each food test to its target: Benedict's for reducing sugars (brick-red precipitate), iodine for starch (blue-black), grease spot for lipids (translucent stain), biuret for proteins (violet).