WJEC生物 生物分子 碳水 脂质 蛋白质
生物分子的内容在 WJEC 与 Eduqas 两条路线上都排在课程最前面,因为后面遇到的酶、膜运输、呼吸与光合作用都要靠分子结构来解释。WJEC 的 AS Unit 1 名字是 Basic Biochemistry and Cell Organisation,卷面 1 小时 30 分钟、满分 80 分,占整个 A Level 的 20%,占 AS 的一半;这门课 2015 年 9 月起施教,2017 年夏天第一次发放 A Level 成绩(WJEC 官方 spec,单元代码 2400U10-1)。Eduqas 是同一家机构给英格兰学校的版本,把内容拆成三份各 2 小时、各 100 分、各占 33⅓% 的 Component,官方把生化的五个基础主题称作 Core Concepts,三份卷子都可能抽到,其中第一项就是化学元素如何组成生物分子。官方样卷上八道题的分值是 8、13、7、12、8、9、14、9,合计 80 分,题面从给结构式写名称到给数据画图都有。很多学生以为这一章只需要背名词和背检验颜色,实际上几乎每一问都要求把结构或性质直接连到功能上。
The biochemistry core opens both the WJEC and the Eduqas A Level Biology specifications, and every later topic leans on it, since enzymes, membrane transport, respiration and photosynthesis all assume you can read a molecular structure. WJEC AS Unit 1 is titled Basic Biochemistry and Cell Organisation; it lasts 1 hour 30 minutes, carries 80 marks and counts for 20 per cent of the A Level and half of the AS. Eduqas splits the same material into three two-hour components of 100 marks each, and its Core Concepts can surface in any of the three papers, so the food tests repay careful revision.
水的极性、氢键与热学性质
水占大多数细胞质量的一半以上,它的特殊之处来自极性:氧原子带部分负电,两个氢原子带部分正电,于是相邻水分子之间在四个方向都能形成氢键。比热容 4.2 J g-1 K-1 意味着加热同样质量的水比加热等量乙醇需要更多能量,所以水生环境的温度变化缓慢,酶不会因为温度骤升而变性。汽化潜热高,100 摄氏度时约 2260 J g-1,出汗与叶片蒸腾因此能带走大量热量而只损失少量水;表面张力让水在细导管里能连续上升,这一条直接连到植物蒸腾流的解释。作为溶剂,水的极性让离子与糖类溶解其中,运输与代谢才有介质;作为代谢物,水参与水解反应,也参与光合作用的光解,并在呼吸作用中被生成。很多学生以为水的所有性质都归功于氢键,遇到比热容与汽化潜热两问时只写氢键而不写氢键数量多、断开需要吸收能量,分数往往只拿到一半。
Water makes up the bulk of a cell by mass, and its behaviour follows from polarity, since oxygen carries a partial negative charge while the two hydrogens carry partial positive charges, allowing hydrogen bonds in every direction. Its specific heat capacity of 4.2 J g-1 K-1 keeps aquatic habitats and cell contents thermally stable, while a latent heat of vaporisation of about 2260 J g-1 lets sweating and transpiration remove large amounts of heat. As a solvent it dissolves ions and sugars, and as a metabolite it takes part in hydrolysis and photolysis; answers that mention hydrogen bonds without the energy needed to break them score half marks.
四种无机离子 Mg2+ Fe2+ Ca2+ PO43-
官方 spec 在生物分子这一块只点了四种无机离子,每一项都要求把一个具体功能写清楚。Mg2+ 是叶绿素的中心离子,缺镁时老叶先变黄,因为叶绿素分子无法组装完成。Fe2+ 是血红蛋白里血红素的中心离子,每个血红素靠它结合一个氧分子,铁被氧化成三价时结合氧的能力下降。Ca2+ 的功能不止一项:它构成骨骼与牙齿里的磷酸钙,参与肌肉收缩时肌动蛋白与肌球蛋白的滑动,还参与神经末梢的胞吐与血液凝固。PO43- 出现在三条线上:核酸骨架里的磷酸二酯键、ATP 的三个磷酸基团、磷脂的极性头部。这四种离子都是小分子,卷面常把它们放进表格题,要求从左栏的离子指到右栏的功能,而右栏往往写着不止一个得分点。常见的错误是把 Mg2+ 与 Fe2+ 的功能互换,或者把 Ca2+ 只写成骨骼成分,漏掉肌肉收缩与胞吐这两分。
The specification names only four inorganic ions here, and each one needs a concrete function; Mg2+ sits at the centre of chlorophyll, and Fe2+ sits at the centre of the haem group in haemoglobin, where each iron binds one oxygen molecule. Ca2+ does several jobs at once, forming calcium phosphate in bone and teeth, triggering the actin and myosin interaction in muscle contraction, and supporting exocytosis at nerve endings and blood clotting. Phosphate appears in the sugar-phosphate backbone of nucleic acids, in ATP and in the polar head of a phospholipid, which is why table questions ask for the function rather than the name.
单糖、双糖与 α β 葡萄糖的差别
单糖按碳原子数分成丙糖、戊糖与己糖,戊糖里的核糖与脱氧核糖构成核酸的五碳糖骨架,己糖里的葡萄糖、果糖与半乳糖分子式都是 C6H12O6,区别只在原子排列。α 与 β 葡萄糖的差别只在第 1 位碳上羟基的方向,但这一点决定了它们能聚合成什么:α 葡萄糖形成淀粉与糖原,β 葡萄糖形成纤维素。双糖由两个单糖缩合而成,麦芽糖是葡萄糖加葡萄糖、乳糖是葡萄糖加半乳糖、蔗糖是葡萄糖加果糖,三种双糖的分子式都是 C12H22O11。还原性来自游离的醛基或酮基,麦芽糖与乳糖都是还原糖,蔗糖两个单糖的还原端都用在成键上,因此属于非还原糖。Benedict 检验题常把蔗糖单独放一格,要求先加稀盐酸煮沸再中和,之后才出现砖红色沉淀。学生最常把非还原糖直接写成阴性反应,漏掉水解这一步,整道小题只拿到一半分。
Monosaccharides are classified by carbon number into trioses, pentoses and hexoses, and ribose and deoxyribose build the sugar-phosphate backbone of nucleic acids while glucose, fructose and galactose share the formula C6H12O6. Alpha and beta glucose differ in one hydroxyl group on carbon 1, yet that detail decides the polymer, since alpha glucose gives starch and glycogen while beta glucose gives cellulose. A reducing sugar carries a free aldehyde or ketone group, so maltose and lactose react with Benedict's reagent while sucrose, whose two reducing ends are locked into the glycosidic bond, needs boiling with acid and neutralising before a brick-red precipitate appears.
缩合与水解:糖苷键、酯键与肽键
三类生物分子用三种键连接,而三种键都由同一对反应生成:缩合去掉一分子水,水解加回一分子水。两个单糖用糖苷键连接,甘油与脂肪酸用酯键连接,两个氨基酸用肽键连接。一条由 n 个单糖组成的直链在缩合过程中失去 n-1 个水分子,这个数目是固定得分点,卷面常给出一条含 5 个残基的链,要求写出失去 4 个水分子。消化侧的水解刚好相反,淀粉与糖原被水解酶切成单糖,蛋白质被切成氨基酸,脂肪被切成甘油与脂肪酸,水解酶做的是加回水并断开对应的键。官方 spec 写出一句容易被忽略的要求:要能用给定的结构式表示缩合与水解如何成键断键,但只需要看懂,不需要默画结构式。这一步最容易丢分的地方是把键名与分子名混着写,例如把肽键写成氢键,或者把甘油的酯键写成糖苷键。
Three classes of biological molecule are joined by three bonds, and all of them form and break through the same reactions; condensation removes one water molecule and hydrolysis puts it back. Sugars link by glycosidic bonds, glycerol and fatty acids link by ester bonds, and amino acids link by peptide bonds, so a straight-chain polymer of n monosaccharides releases n-1 water molecules as it forms. Digestion reverses this, since amylase and maltase cut starch into monosaccharides and protease cuts proteins into amino acids; the specification asks learners to use given structural formulae but only to recognise them, and confusing peptide with hydrogen is a common way to lose the mark.
储存多糖与结构多糖的对比
淀粉、糖原、纤维素与几丁质都是多糖,差别在单体、键型与分支程度,而这些结构差异正好对应功能差异。直链淀粉由 α 葡萄糖通过 1,4 糖苷键连成螺旋,支链淀粉每隔二十多个残基出现一个 1,6 分支,整个分子不溶于水,因此不会把细胞的水势拉低;糖原的分支比支链淀粉更密,通常每八到十二个残基就有一次分支,动物因此能在几分钟内调出葡萄糖。纤维素由 β 葡萄糖通过 1,4 键连成直链,相邻残基翻转 180 度,链与链之间形成大量氢键并聚成微纤维,抗张强度很高,植物细胞壁靠它抗住膨压。几丁质的单体也是 β 构型,部分羟基被含氮的乙酰基取代,昆虫外骨骼与真菌细胞壁因此既坚硬又有韧性。判断题如果问的是储存而不是结构,答出分支密度与不溶性两点,比只写由葡萄糖组成多拿两分。
Starch, glycogen, cellulose and chitin are all polysaccharides, and their differences in monomer, linkage and branching map onto differences in function. Amylose is a helix of alpha glucose joined by 1,4 glycosidic bonds, amylopectin adds 1,6 branches every twenty or so residues, and neither dissolves readily, which keeps water potential stable; glycogen branches every eight to twelve residues so that animals can release glucose within minutes. Cellulose chains of beta glucose alternate by 180 degrees and pack into microfibrils held by many hydrogen bonds, giving the tensile strength that resists turgor, while chitin uses the same beta monomer with some hydroxyl groups replaced by nitrogen-containing groups.
甘油三酯与磷脂的结构差别
脂质的共同点是不溶于水而溶于乙醇一类有机溶剂,这一条由分子结构决定:脂肪酸链与甘油骨架上几乎没有极性基团。甘油三酯由一分子的甘油与三分子脂肪酸通过酯键连接,每连一条酯键放出一分子水,所以完全水解要加回三分子水。磷脂把其中一个脂肪酸换成磷酸基团,分子因此一头亲水、一头疏水,细胞膜的双层结构正是靠这个两性特征自发形成,磷酸头部朝水、脂肪酸尾巴相对。作为储能分子,脂质的氢碳比更高,每克氧化释放约 38 kJ,而糖类约 17 kJ,同时脂质不溶于水,储存时不会影响细胞的水势与渗透平衡,这一点在种子与动物脂肪组织里都很关键。很多学生以为凡是脂质都由甘油构成,遇到固醇类时仍按甘油三酯作答,这道题的分就丢了。
Lipids dissolve in organic solvents such as ethanol but not in water, because their fatty acid chains and glycerol backbone carry almost no polar groups. A triglyceride forms when glycerol joins three fatty acids by ester bonds, releasing one water molecule per bond, so complete hydrolysis adds three back; a phospholipid swaps one fatty acid for a phosphate group, giving a hydrophilic head and hydrophobic tails that self-assemble into a bilayer. Gram for gram, lipid oxidation releases roughly 38 kJ against about 17 kJ for carbohydrate, and because lipid is insoluble it can be stored without disturbing water potential.
饱和与不饱和脂肪酸的物理与健康差别
脂肪酸链上的碳碳双键决定了脂肪的物理状态与健康影响。饱和脂肪酸不含双键,碳链是直的,分子之间能紧密排列,分子间作用力强,室温下多为固体,动物脂肪与黄油都属于这一类。不饱和脂肪酸至少含一个顺式双键,双键处出现折角,分子无法紧密堆叠,熔点因此下降,室温下多为液体,植物油与鱼油属于这一类,双键数量越多折角越多,熔点越低。官方 spec 只要求讨论饱和与不饱和脂肪对人类健康的影响,没有要求背具体百分比,答题时写成饱和脂肪摄入过多与心血管疾病风险升高相关、不饱和脂肪在室温下更易代谢即可。题目若要比较两种脂肪的熔点或溶解性,先写双键带来的形状,再写分子间作用力,顺序反了往往只给一半分。常见的错误是把折角说成化学键被破坏,折角只是几何形状,并没有断键。
Whether a fatty acid is saturated or unsaturated depends on the carbon-carbon double bonds in its chain, and that detail controls both melting point and health effects. Saturated chains run straight and pack closely, so the intermolecular forces are strong and the fat is usually solid at room temperature, as in butter and animal fat; an unsaturated chain bends at each cis double bond, packs loosely and melts lower, which is why plant oils stay liquid. The specification asks for a discussion of health implications rather than a memorised percentage, so link high saturated intake to raised cardiovascular risk and note that bending at a double bond is geometry, not bond breaking.
氨基酸与蛋白质的四级结构
氨基酸由同一套骨架加一个可变的 R 基组成:一个氨基、一个羧基、一个氢原子和一个 R 基连在同一个碳上。二十种氨基酸的差别全在 R 基,而 R 基决定了它们在蛋白质里的位置与作用。两个氨基酸缩合时,一个的羧基与另一个的氨基成键并放出一分子水,这条键叫肽键,许多氨基酸连起来就是多肽。一级结构是氨基酸的排列顺序,它决定后面的折叠方式,一个位置出错就可能让整条链失去功能,镰状细胞贫血就是血红蛋白 β 链第六位谷氨酸被缬氨酸替换的结果。二级结构是 α 螺旋与 β 折叠片,靠主链之间的氢键维持;三级结构是整条链在氢键、离子键、二硫键与疏水相互作用下形成的三维形状;四级结构是多条多肽链加辅基拼成的整体,血红蛋白由两条 α 链与两条 β 链加四个血红素构成,能结合四个氧分子。学生最常把二硫键写在二级结构里,这一步在评分方案里是明确的零分。
Every amino acid uses the same backbone, a central carbon carrying an amine group, a carboxyl group, a hydrogen atom and a variable R group, and the twenty standard amino acids differ only in that R group. Condensation between the carboxyl group of one amino acid and the amine group of the next releases water and forms a peptide bond, and the resulting sequence is the primary structure that dictates every later fold; a single substitution causes disease, as in sickle-cell anaemia. Hydrogen bonds along the backbone create alpha helices and beta pleated sheets, ionic, hydrogen, disulfide and hydrophobic interactions shape the tertiary structure, and haemoglobin shows quaternary structure with two alpha chains, two beta chains and four haem groups; disulfide bridges belong to the tertiary level, not the secondary one.
纤维状蛋白与球状蛋白的功能分工
蛋白质按形状分成纤维状与球状两类,形状由一级结构决定,功能又由形状决定。纤维状蛋白的链很长,链与链之间靠氢键与共价交联固定,不溶于水,形成能抗张的结构。胶原蛋白由三条多肽链绞成三螺旋,存在于肌腱、皮肤与血管壁,它的稳定性来自重复排列,而不是酶那样精确的活性位点。球状蛋白把疏水基团藏在内侧、亲水基团露在外侧,因此能溶于水,表面积与体积之比较大,适合承担需要接触底物或被识别的任务,酶、抗体、胰岛素与血红蛋白都属这一类。血红蛋白的四个亚基协同结合氧,第一个氧分子结合之后,其余三个的结合能力上升,这一条解释了氧解离曲线为什么呈 S 形。阅卷时最常见的问题是只答可溶与不可溶,不写形状带来的功能差异,两分的题往往只拿到一分。
Proteins fall into fibrous and globular classes, and the shape that decides the class comes from the primary sequence while the function follows the shape. A fibrous protein runs long chains cross-linked by hydrogen bonds and covalent links, stays insoluble and resists tension, such as collagen, whose three polypeptides twist into a triple helix in tendon, skin and blood vessel walls. A globular protein buries its hydrophobic groups and exposes its hydrophilic ones, so it dissolves and suits roles in enzymes, antibodies and haemoglobin, where oxygen binding is cooperative.
四种食品检验的试剂、条件与结果
Unit 1 的指定实验里最常考的就是四种食品检验,评分方案把试剂、条件与结果分开给分。检验淀粉用碘的碘化钾溶液,阳性结果是蓝黑色,这一步不需要加热。检验还原糖用 Benedict 试剂,需要在热水浴中加热,颜色随还原糖浓度从绿、黄、橙过渡到砖红色,卷面常要求写出颜色顺序而不只是一个砖红。检验非还原糖要先取一份样液加稀盐酸煮沸水解,再用碳酸氢钠中和,然后重做 Benedict 检验,漏掉中和这一步酸性会压住颜色变化。检验蛋白质用双缩脲试剂,阳性结果呈紫色,试剂由氢氧化钠与硫酸铜溶液配成,加入顺序颠倒会先生成蓝色的氢氧化铜沉淀。检验脂质用乳液实验:先用乙醇把脂质溶解,再加入水,出现乳白色浑浊即阳性。错得最多的是把碘液当成 Benedict 的替代品,或者写出砖红色却漏写加热条件,这两种答案都只能拿到结果分的一半。
Four food tests carry most of the practical marks in Unit 1, and each one is marked separately for reagent, condition and result. Iodine in potassium iodide turns blue-black with starch without heating, while Benedict's reagent needs a hot water bath and runs through green, yellow and orange to brick-red as reducing sugar concentration rises, so writing only brick-red understates the answer. A non-reducing sugar is boiled with dilute hydrochloric acid and neutralised with sodium hydrogencarbonate before the Benedict's test is repeated; biuret reagent gives purple with protein, and the emulsion test dissolves lipid in ethanol before water is added.
阅卷口径与四周复习排期
WJEC 的官方 AO 权重表把 Unit 1 拆得很清楚:在整个 A Level 里,这个单元的 AO1 占 7.0%、AO2 占 9.0%、AO3 占 4.0%,换算到 AS 内部是 17.5%、22.5% 与 10.0%,也就是说应用与分析比纯记忆占的比重大。AO2 主要考把结构连到功能、把数据转成结论、把单位算对,所以复习时别只抄定义,要把每一条性质写成一句有因果的答题句。四周可以这样排:第一周水与无机离子,把比热容、汽化潜热与四种离子的功能各写成一页对照;第二周糖类与脂质,重点练糖苷键与酯键的成键断键、储存多糖与结构多糖的对比;第三周蛋白质,把四级结构、纤维状与球状、血红蛋白的协同结合串成一条线;第四周做限时真题,按 1 小时 30 分钟做完 80 分的卷子,再把错题归到知识、数据、表述三类。A2 的 Unit 5 是外部评分的实验考试,实验技能从 Unit 1 就开始积累,不要等到第二年才补。
The official assessment objective table splits Unit 1 clearly, since AO1 carries 7.0 per cent of the whole A Level, AO2 carries 9.0 per cent and AO3 carries 4.0 per cent, which becomes 17.5, 22.5 and 10.0 per cent inside the AS, so application and analysis outweigh recall. Marks for AO2 come from linking a structure to a function, turning data into a conclusion and keeping units correct, which is why a definition copied from a textbook rarely earns full credit. A four-week plan can run water and ions first, sugars and lipids second and proteins third, with timed papers filed under knowledge, data or expression, since the A2 practical examination is externally marked.
牛津AQA国际GCSE生物?OxfordAQA International GCSE Biology (9201): Onl?进口原版¥399.0📱 去京东购买京东精选
需要一对一辅导？Need One-on-One Tutoring?
A-Level / GCSE / IB 生物与全科辅导，按 WJEC 与 Eduqas 考纲逐项过
16621398022 同微信
Visit aleveler.com
或截图后用「扫一扫 → 相册」选择本图