WJEC生物 淀粉糖原 纤维素 磷脂 蛋白质
WJEC 的 AS Unit 1 叫 Basic Biochemistry and Cell Organisation，考试时间 1 小时 30 分钟，满分 80 分，占总资格 20%，AS 阶段两个单元各占 AS 的一半。这个单元的第一块内容几乎全是结构决定功能的比较题：淀粉跟糖原凭什么能储存，纤维素跟几丁质凭什么能做支架，纤维状蛋白跟球状蛋白凭什么分工不同。很多学生把名词都背下来了，一遇到比较题还是写不出差别，原因不在记忆量，而在没有把结构和性质连起来。
In the WJEC Biology specification, AS Unit 1 is titled Basic Biochemistry and Cell Organisation and is assessed by one paper of 1 hour 30 minutes worth 80 marks. Almost every question in Topic 1 can be reduced to a single demand: explain what a molecule's structure allows it to do. Candidates who memorise names without linking shape, property and function tend to lose the longer structured marks.
四种无机离子与水的极性
WJEC 考纲 1.1(a) 只点了四种无机离子：Mg2+ 参与叶绿素分子，Fe2+ 构成血红蛋白，Ca2+ 构成骨骼与牙齿并参与肌肉收缩和神经冲动传递，磷酸根出现在 ATP、磷脂、核酸与骨骼中。这些离子以离子形式存在，不需要消化就能被吸收，所以缺少时症状往往出现得很快。水的考点更集中：水分子是极性分子，一个水分子的氧端带部分负电荷，氢端带部分正电荷，相邻水分子之间形成氢键。氢键数量多，水的比热容就大，细胞内温度不容易骤变；汽化潜热也大，出汗蒸发带走的热量多。水还是极性溶质的好溶剂，离子与糖类溶在其中才能被运输。水本身参与缩合与水解，属于代谢物。表面张力让水在植物木质部导管里连成不断的水柱。
The WJEC specification 1.1(a) names only four inorganic ions for this topic: magnesium in chlorophyll, iron in haemoglobin, calcium in bone, teeth and neural signalling, and phosphate in ATP, phospholipids and nucleic acids. Water earns its marks through hydrogen bonding: polarity keeps its specific heat capacity and latent heat of vaporisation unusually high, which buffers cells against temperature swings. It also acts as the solvent for ions and sugars and as a metabolite in condensation and hydrolysis.
单糖、双糖与糖苷键的方向
单糖按碳原子数分类：三碳糖出现在呼吸作用的中间步骤，五碳糖是核糖与脱氧核糖，六碳糖是葡萄糖、果糖与半乳糖。双糖由两分子单糖缩合而成，麦芽糖是葡萄糖加葡萄糖，蔗糖是葡萄糖加果糖，乳糖是葡萄糖加半乳糖。缩合反应在两个单糖之间形成一个糖苷键，同时脱去一分子水；水解反应方向相反，需要羧酸酶参与，例如麦芽糖酶、蔗糖酶与乳糖酶，实验室里也可以用稀酸加加热代替。常见的错误是只写脱去一分子水，没有写出生成的键叫糖苷键，比较题里这一分常常就在这里丢掉。三碳糖与五碳糖在题目里出现频率不高，但分类题一旦问到碳原子数就要写对。
Monosaccharides are classified by carbon number: triose sugars sit in the middle of respiration, pentoses include ribose and deoxyribose, and hexoses include glucose, fructose and galactose. Disaccharides form when two monosaccharides condense with the loss of one water molecule; maltose is glucose plus glucose, sucrose is glucose plus fructose, lactose is glucose plus galactose. Hydrolysis reverses that step and needs a carbohydrase such as maltase, sucrase or lactase, although dilute acid with heat will do the same job in a test tube.
alpha 与 beta 葡萄糖的构型差异
1.1(d) 考的是同一个葡萄糖的两种环式构型：差别只在碳 1 上羟基的位置，alpha 构型的羟基在环平面下方，beta 构型的羟基在环平面上方。缩合时 alpha 葡萄糖形成的 1,4 糖苷键让整条链自然盘成螺旋，beta 葡萄糖必须翻转 180 度才能接上，于是链保持笔直。螺旋结构适合密集堆积做储存，笔直链之间能形成大量氢键，几十条并排聚成微纤丝，抗拉强度就出来了。很多学生以为 alpha 和 beta 只是叫法不同，实际上这一步决定了后面四种多糖的全部功能。阅卷时如果只写 alpha 螺旋、beta 直链，不补一句为什么，通常拿不到那条说明分。
The point of 1.1(d) is that glucose exists in two ring forms that differ only in the position of the hydroxyl group on carbon 1: alpha below the ring plane, beta above it. Alpha 1,4 links let the chain coil into a helix, while beta glucose has to be rotated 180 degrees before it can bond, so the chain stays straight. That single difference is what makes storage polysaccharides compact and structural polysaccharides strong.
淀粉与糖原的储存理由
淀粉由直链淀粉与支链淀粉混合组成，直链部分盘成螺旋，支链部分是 alpha 1,4 主链加上 alpha 1,6 分支；糖原的分支比支链淀粉更密，末端数量多，磷酸化酶能同时从许多末端切下葡萄糖，所以动物调动血糖的速度更快。两者都不溶于水，不产生溶于细胞质的颗粒，渗透压几乎不受影响；分子量大又紧凑，占的体积小。水解产物是葡萄糖，可以径直接入呼吸作用。植物把淀粉存放在叶绿体与淀粉粒里，动物把糖原存放在肝细胞与肌细胞里。碘-碘化钾检验淀粉显蓝黑色，这是实验室里最快的判别手段，糖原遇碘只显红褐色，颜色差异本身就是一道分区题。
Starch is a mixture of amylose, which coils into a helix, and amylopectin, which carries alpha 1,4 chains with alpha 1,6 branches; glycogen is more densely branched, so phosphorylase can strip glucose from many ends at once and animals can mobilise fuel quickly. Neither polymer dissolves, so no solute particles appear in the cytoplasm and water potential barely shifts, and both are compact for their mass. Iodine in potassium iodide turns starch blue-black, whereas glycogen gives only a reddish-brown colour.
纤维素与几丁质为何适合做支架
纤维素是 beta 葡萄糖的 1,4 聚合物，直链之间形成大量氢键，几十条链并排成微纤丝，抗拉强度很高。植物细胞壁要能承受膨压，又要让水和溶质通过，纤维素不溶于水、也不主动阻挡小分子，正好满足这两条。几丁质与纤维素骨架相近，只是每个葡萄糖单位上的羟基被乙酰氨基取代，多了氮，硬度与防水性更好，出现在真菌细胞壁与节肢动物外骨骼。与储存多糖对比，结构多糖不需要被轻易水解，键更稳定，分解它的酶也完全不同。考试里常见的问法是把四种多糖按储存与结构分成两组，再各给两条理由，答案要落到分支、溶解度与氢键这三处。
Cellulose is a beta 1,4 polymer of glucose whose straight chains hydrogen bond into microfibrils with high tensile strength, which suits a wall that must resist turgor yet stay permeable to water and small solutes. Chitin shares that backbone but carries acetyl amino groups in place of hydroxyl groups, adding nitrogen and giving a harder, more water-resistant material in fungal walls and insect exoskeletons. Structural polysaccharides are built to resist hydrolysis, so the enzymes that digest them differ from those acting on starch and glycogen.
四个食品检验的判读
淀粉用碘-碘化钾，阳性结果是蓝黑色，这是四个检验里唯一不需要加热的。还原糖用本尼迪特试剂，沸水浴加热后出现砖红色沉淀，葡萄糖、麦芽糖与乳糖都属于还原糖。蔗糖是非还原糖，必须先与稀盐酸煮沸水解，再用碳酸氢钠中和，然后才做本尼迪特检验，跳过中和这一步会得到假阴性。蛋白质用双缩脲试剂，颜色由浅蓝转紫，检出的是肽键而不是某种特定的氨基酸。脂肪用乳化试验，先加乙醇再加等量水，出现白色乳浊液说明样品里含脂类。很多学生以为双缩脲能测氨基酸，其实单个氨基酸没有肽键，结果仍是浅蓝，这一条在选择题里反复出现。
The specified practical work lists four food tests: iodine in potassium iodide for starch, Benedict's test for reducing and non-reducing sugars, the biuret test for protein and the emulsion test for fats and oils. Sucrose must be boiled with dilute hydrochloric acid and then neutralised with sodium hydrogencarbonate before Benedict's reagent is added, because it is not a reducing sugar. The biuret test detects peptide bonds rather than amino acids, so a single amino acid solution stays light blue.
甘油三酯与磷脂的缩合产物
一分子甘油与三分子脂肪酸缩合，形成三个酯键，同时脱去三分子水，产物是甘油三酯。磷脂的结构是一分子甘油上接两分子脂肪酸与一个磷酸基团，磷酸基团再连一个含氮碱基或小分子醇；这样一个分子一头亲水一头疏水，属于两性分子。把磷脂放进水里，疏水尾自动相对，亲水头朝外，形成双分子层，这就是细胞膜的基本骨架。脂肪酸链中含有碳碳双键的是不饱和脂肪酸，双键让链出现弯折，分子之间贴不紧，熔点较低；没有双键的是饱和脂肪酸，链笔直，能紧密堆积。室温下呈液态的植物油不饱和程度高，动物脂肪饱和程度高，这一点常被用来解释两者在烹饪与健康上的差别。
A triglyceride forms when one glycerol condenses with three fatty acids, producing three ester bonds and three water molecules. In a phospholipid the same glycerol carries two fatty acids and one phosphate group, which may be linked to a nitrogen-containing base or a small alcohol, making the molecule amphipathic with a hydrophilic head and hydrophobic tails. In water those tails turn inwards to form a bilayer, which is the framework of every cell membrane.
饱和脂肪与不饱和脂肪的健康含义
大纲把饱和与不饱和脂肪对人体健康的影响单独列成一条，答题要用相关性的表述，不要写成绝对因果。饱和脂肪摄入过高与血液中低密度脂蛋白胆固醇升高相关，而低密度脂蛋白胆固醇偏高与心血管疾病风险增加相关。用不饱和脂肪替代一部分饱和脂肪，低密度脂蛋白胆固醇通常会下降，这一点是这条考点的核心动作。答题时要写清替代与相关两层意思，不要写成吃了脂肪就会得心脏病。很多学生以为所有脂肪都有害，其实磷脂与必需脂肪酸是细胞膜和部分激素合成的原料，问题出在种类与总量上。加工食品里的反式脂肪属于另一类，它不是本题要求的内容，写进答案也不会加分。
The specification asks for the implications of saturated and unsaturated fat on human health, and the safest wording is associational rather than absolute. High intakes of saturated fat are associated with raised low-density lipoprotein cholesterol, which is itself associated with higher cardiovascular risk, while replacing part of that saturated fat with unsaturated fat usually lowers it. Claims that any fat intake causes heart disease overstate what the evidence supports.
氨基酸与肽键的连接方向
每个氨基酸以同一个碳原子为中心，接一个氨基、一个羧基、一个氢原子和一个 R 基团；R 基团不同，20 种标准氨基酸的酸碱性、极性与体积就不同。缩合发生在一个氨基酸的羧基与下一个氨基酸的氨基之间，脱去一分子水，形成肽键。因此多肽是有方向的：一端是游离的氨基，叫 N 端，另一端是游离的羧基，叫 C 端。蛋白质水解时肽键断开，由蛋白酶催化，产物是短肽与氨基酸。R 基团的性质决定这条链后来能不能折叠成特定形状，也决定它待在细胞质里还是嵌在膜的疏水区。常见的错误是把肽键画在两个 R 基团之间，这种图在结构题里一分不给。
Every amino acid has the same central carbon carrying an amino group, a carboxyl group, a hydrogen atom and an R group; the twenty standard amino acids differ only in that R group, which sets acidity, polarity and size. Condensation links the carboxyl group of one amino acid to the amino group of the next, releasing water and forming a peptide bond, so a polypeptide has a free amino end and a free carboxyl end. Proteases hydrolyse those bonds, releasing peptides and free amino acids.
一到四级结构各自由什么键维持
一级结构是氨基酸的排列顺序，只靠肽键维持。二级结构是局部的规则折叠，alpha 螺旋与 beta 折叠片，靠主链上羰基氧与氨基氢之间的氢键维持。三级结构是整条链的三维折叠，靠离子键、氢键、二硫键与疏水作用共同维持，其中二硫键是共价键，最不容易被打断。四级结构指两条或更多多肽链聚成有功能的整体，血红蛋白就是四条链加四个血红素基团。加热或极端 pH 会破坏二、三、四级结构，一级结构不变，这个过程叫变性。很多学生以为变性后肽键也断了，其实只是维持形状的那些键被破坏，所以冷却后有时还能部分复性。
Primary structure is the sequence of amino acids held by peptide bonds, and secondary structure is local regular folding into alpha helices and beta pleated sheets held by hydrogen bonds between backbone carbonyl oxygen and amino hydrogen. Tertiary structure is the overall three dimensional fold, maintained by ionic bonds, hydrogen bonds, disulfide bridges and hydrophobic interactions, with disulfide bridges the hardest to break because they are covalent. Quaternary structure describes two or more polypeptide chains assembling into one functional unit, as in haemoglobin with its four chains and four haem groups.
纤维状蛋白与球状蛋白的功能对应
纤维状蛋白的多条链平行排列，长度远大于宽度，三级折叠很少，不溶于水，序列重复性高，适合承受拉力，例如肌腱里的胶原蛋白与毛发角蛋白。球状蛋白折叠紧凑接近球形，疏水 R 基团藏在内部，亲水 R 基团朝外，因此可溶于水，适合在溶液里执行代谢功能，例如酶、血红蛋白、胰岛素与抗体。大纲第 (j) 条问的正是这种结构到功能的对应关系，答题要写成因为结构如何，所以能做什么。把形状、溶解度与功能三条对齐写，通常一次就能拿满。纤维状蛋白被拉断的地方常常在链之间而不是链内，这也是考题里区分两类蛋白的细节。
Fibrous proteins run as long parallel chains with little tertiary folding, are insoluble and highly repetitive, which suits tensile roles such as collagen in tendon and keratin in hair. Globular proteins fold compactly with hydrophobic R groups buried and hydrophilic ones on the surface, so they dissolve and carry out metabolic work as enzymes, haemoglobin, insulin and antibodies. The specification's point 1.1(j) asks exactly for that structure to function link, so answers should state the structural feature, the property it produces and the job that property allows.
结构式只要求认得与伸展题写法
大纲在 Topic 1 末尾写得很清楚：学生要能用给定的结构式说明缩合与水解时哪个键形成或断开，包括肽键、糖苷键与酯键，后面还有一句 recognise and understand but not reproduce。意思是考试会给你结构式，你负责指出位置，不要求从空白纸默写整个分子。复习时把三种连接在给定图上圈出来，比抄十遍结构式有用。每份卷都设一道伸展题，通常要求把结构、性质与功能三层连起来，写的时候按这个顺序推，每层各给一条具体证据。常见的错误是把性质与功能混成一句，例如只写纤维素很结实，没有写氢键与微纤丝怎么带来这份强度。
At the end of Topic 1 the specification states that learners should use given structural formulae to show how bonds are formed and broken by condensation and hydrolysis, including peptide, glycosidic and ester bonds, and then adds that the formulae must be recognised and understood but not reproduced. In practice the paper supplies the drawing and the candidate marks where the bond forms or breaks. Extended response questions reward a chain of reasoning from structure to property to function, with one piece of evidence at each step.
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