📚 Year 12 SQA Biology: Core Knowledge Review | 核心知识点梳理
The SQA Higher Biology course deepens your understanding of life at molecular, cellular and whole-organism levels. It is divided into three units: DNA and the Genome, Metabolism and Survival, and Sustainability and Interdependence. This article reviews the core knowledge points for Year 12 students, providing bilingual explanations to strengthen both conceptual clarity and exam confidence.
SQA高等生物学课程从分子、细胞和整体生物水平加深你对生命的理解。课程分为三大单元：DNA与基因组、新陈代谢与生存、可持续性与相互依存。本文针对Year 12学生梳理核心知识点，以双语解释强化概念理解与应试信心。
1. DNA Structure and Function | DNA 的结构与功能
DNA (deoxyribonucleic acid) is a double helix composed of two antiparallel strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C) or guanine (G). The strands run in opposite directions, designated as 5′ to 3′ and 3′ to 5′.
DNA（脱氧核糖核酸）是一种双螺旋结构，由两条反平行的核苷酸链组成。每个核苷酸由一个脱氧核糖、一个磷酸基团和四种含氮碱基之一组成：腺嘌呤(A)、胸腺嘧啶(T)、胞嘧啶(C)或鸟嘌呤(G)。两条链方向相反，分别标记为5’到3’和3’到5’。
The backbone of each strand is formed by sugar–phosphate bonds. The two strands are held together by hydrogen bonds between complementary base pairs: A pairs with T (two hydrogen bonds), and C pairs with G (three hydrogen bonds). This base pairing ensures accurate replication and transcription.
每条链的主链由糖-磷酸键构成。两条链通过互补碱基对之间的氢键连接：A与T配对（两个氢键），C与G配对（三个氢键）。这种碱基配对保证了精确的复制和转录。
In eukaryotic cells, DNA is tightly coiled around histone proteins to form nucleosomes, which further condense into chromatin and chromosomes. Prokaryotic cells contain a single circular chromosome and may also possess small circular plasmids. The compact packaging protects DNA and regulates gene expression.
在真核细胞中，DNA紧密缠绕在组蛋白上形成核小体，进而凝缩为染色质和染色体。原核细胞含有一条环形染色体，也可能含有小型环形质粒。致密的包装保护DNA并调控基因表达。
2. DNA Replication | DNA 复制
DNA replication is a semi-conservative process that occurs before cell division. The enzyme DNA helicase unwinds the double helix and breaks hydrogen bonds between base pairs, creating a replication fork. Single-stranded binding proteins stabilise the separated strands.
DNA复制是一个半保留过程，发生在细胞分裂之前。DNA解旋酶解开双螺旋并断裂碱基对间的氢键，形成复制叉。单链结合蛋白稳定分开的链。
DNA polymerase III adds free nucleotides to the 3′ end of each growing strand, requiring a primer synthesised by primase. The leading strand is synthesised continuously in the 5’→3′ direction. The lagging strand is synthesised discontinuously in short Okazaki fragments, which are later joined by DNA ligase.
DNA聚合酶Ⅲ将游离核苷酸加到每条生长链的3’端，需要引物酶合成的引物。前导链沿5’→3’方向连续合成。后随链不连续地合成短小的冈崎片段，之后由DNA连接酶连接。
This process ensures that each new DNA molecule contains one original parental strand and one newly synthesised strand. Proofreading by DNA polymerase corrects most errors, maintaining genetic stability.
该过程确保每个新DNA分子含有一条原有的亲代链和一条新合成的链。DNA聚合酶的校对功能纠正大多数错误，维持遗传稳定性。
3. Gene Expression and Protein Synthesis | 基因表达与蛋白质合成
Gene expression involves two main stages: transcription and translation. During transcription, RNA polymerase binds to a promoter region and unwinds the DNA. It synthesises a single-stranded mRNA molecule complementary to the template strand, substituting uracil (U) for thymine (T).
基因表达包括两个主要阶段：转录和翻译。转录时，RNA聚合酶结合到启动子区域并解开DNA。它合成一条与模板链互补的单链mRNA，其中尿嘧啶(U)代替胸腺嘧啶(T)。
The primary mRNA transcript in eukaryotes undergoes post-transcriptional modification: a 5′ cap is added, introns are removed by splicing, and a poly-A tail is added to the 3′ end. The mature mRNA then exits the nucleus and attaches to a ribosome.
真核生物的初级mRNA转录本经过转录后修饰：添加5’帽结构，通过剪接去除内含子，并在3’端添加poly-A尾。成熟的mRNA随后离开细胞核并与核糖体结合。
During translation, the ribosome reads the mRNA codons. Each tRNA molecule carries a specific amino acid and has an anticodon complementary to the codon. Amino acids are joined by peptide bonds, forming a polypeptide chain. The process continues until a stop codon is reached.
翻译过程中，核糖体阅读mRNA上的密码子。每个tRNA携带一个特定的氨基酸，并有一个与密码子互补的反密码子。氨基酸通过肽键连接，形成多肽链。过程持续到终止密码子为止。
Gene expression is regulated by transcription factors and environmental signals. In differentiated cells, only specific sets of genes are expressed, enabling cell specialisation. Epigenetic modifications, such as DNA methylation and histone modification, also influence gene activity.
基因表达受转录因子和环境信号调控。在分化细胞中，只有特定的基因组表达，实现细胞特化。表观遗传修饰（如DNA甲基化和组蛋白修饰）也影响基因活性。
4. Mutations and Evolution | 突变与进化
A mutation is a random change in the nucleotide sequence of DNA. Point mutations involve a single base change: substitution, insertion or deletion. Substitutions may be silent, missense or nonsense, while insertions and deletions cause frameshift mutations, altering the entire downstream amino acid sequence.
突变是DNA核苷酸序列的随机变化。点突变涉及单个碱基的改变：替换、插入或缺失。替换可能是沉默、错义或无义，而插入和缺失导致移码突变，改变下游全部氨基酸序列。
Mutations can be neutral, harmful or beneficial. They are the ultimate source of genetic variation, upon which natural selection acts. Evolution occurs when allele frequencies in a population change over generations, driven by selection, genetic drift and gene flow.
突变可以是中性、有害或有利的。它们是遗传变异的最终来源，自然选择在此基础上起作用。当种群的等位基因频率在世代间改变时，进化就发生了，驱动因素包括自然选择、遗传漂变和基因流。
SQA candidates must distinguish between absolute and relative fitness, and understand how selection pressures (directional, stabilising, disruptive) shape populations. Speciation occurs when populations become reproductively isolated, leading to the formation of new species.
SQA考生需区分绝对适合度和相对适合度，并理解选择压力（定向、稳定、分裂）如何塑造种群。当种群发生生殖隔离时形成新物种，即物种形成。
5. Metabolic Pathways and Enzymes | 代谢途径与酶
Metabolic pathways are integrated sequences of enzyme-controlled reactions. Anabolic pathways build complex molecules and require energy; catabolic pathways break down molecules and release energy. Enzymes lower activation energy and are not consumed in the reaction.
代谢途径是酶控反应的整合序列。合成代谢途径构建复杂分子并消耗能量；分解代谢途径分解分子并释放能量。酶降低活化能且在反应中不被消耗。
The induced-fit model describes how the enzyme’s active site changes shape to bind the substrate, forming an enzyme-substrate complex. Enzyme activity is influenced by temperature, pH, substrate concentration and inhibitors. Competitive inhibitors resemble the substrate and compete for the active site; non-competitive inhibitors bind elsewhere and change the active site’s shape.
诱导契合模型描述了酶的活性部位如何改变形状以结合底物，形成酶-底物复合物。酶活性受温度、pH、底物浓度和抑制剂影响。竞争性抑制剂类似底物并竞争活性部位；非竞争性抑制剂在别处结合并改变活性部位形状。
Feedback inhibition allows the end product of a pathway to inhibit an early enzyme, preventing overproduction. This is a key mechanism for metabolic control.
反馈抑制使得途径的终产物抑制早期酶，防止过量生成。这是代谢调控的关键机制。
6. Cellular Respiration | 细胞呼吸
Cellular respiration is a series of catabolic pathways that release energy from glucose. The overall equation is:
细胞呼吸是一系列从葡萄糖中释放能量的分解代谢途径。总方程式为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)
Respiration occurs in three main stages. Glycolysis takes place in the cytoplasm: glucose is broken down into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH.
呼吸作用分三个主要阶段。糖酵解发生在细胞质中：葡萄糖分解为两个丙酮酸分子，净生成2 ATP和2 NADH。
If oxygen is present, pyruvate enters the mitochondrial matrix for the citric acid cycle (Krebs cycle). Here, acetyl-CoA is oxidised, releasing CO₂ and generating ATP, NADH and FADH₂. The high-energy electron carriers then pass electrons to the electron transport chain on the inner mitochondrial membrane.
若有氧气存在，丙酮酸进入线粒体基质进行柠檬酸循环（克雷布斯循环）。在此乙酰辅酶A被氧化，释放CO₂并产生ATP、NADH和FADH₂。高能电子载体随后将电子传递到线粒体内膜上的电子传递链。
As electrons move through the chain, hydrogen ions are pumped across the membrane, creating a proton gradient. ATP synthase uses this proton-motive force to synthesise ATP via oxidative phosphorylation. Oxygen acts as the final electron acceptor, forming water.
当电子沿链传递时，氢离子被泵过膜，形成质子梯度。ATP合酶利用这一质子驱动力通过氧化磷酸化合成ATP。氧气作为最终电子受体，生成水。
7. Photosynthesis | 光合作用
Photosynthesis converts light energy into chemical energy in the chloroplasts of plant cells. The overall equation is:
光合作用在植物细胞的叶绿体中将光能转化为化学能。总方程式为：
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The light-dependent reactions occur on the thylakoid membranes. Chlorophyll and accessory pigments in photosystems II and I absorb light, exciting electrons. Photolysis of water produces electrons, protons and oxygen. The electrons pass along an electron transport chain, generating ATP and reducing NADP⁺ to NADPH.
光反应发生在类囊体膜上。光系统Ⅱ和Ⅰ中的叶绿素及辅助色素吸收光能，激发电子。水的光解产生电子、质子和氧气。电子沿电子传递链传递，生成ATP并将NADP⁺还原为NADPH。
The Calvin cycle (light-independent stage) occurs in the stroma. Carbon dioxide is fixed to RuBP by the enzyme RuBisCO. The resulting 3-phosphoglycerate is then reduced using ATP and NADPH from the light reactions to form G3P, some of which is used to regenerate RuBP and some to synthesise glucose.
卡尔文循环（暗反应）发生在基质中。CO₂通过RuBisCO酶固定到RuBP上。生成的3-磷酸甘油酸利用光反应产生的ATP和NADPH还原形成G3P，一部分G3P用于再生RuBP，一部分用于合成葡萄糖。
8. Food Supply and Crop Protection | 食物供应与作物保护
Global food security depends on maximising photosynthetic efficiency and reducing losses. Key factors limiting photosynthesis include light intensity, carbon dioxide concentration, temperature and water availability. Glasshouses and polytunnels allow growers to control these conditions, enhancing yield.
全球粮食安全依赖于最大化光合效率并减少损失。限制光合作用的关键因素包括光照强度、二氧化碳浓度、温度和水供应。温室和塑料大棚使种植者能够控制这些条件，提高产量。
Plants require essential nutrients, particularly nitrogen, phosphorus and potassium (NPK). Fertilisers replenish soil nutrients. Nitrogen is absorbed as nitrate ions (NO₃⁻) and is a key component of amino acids and nucleotides. The nitrogen cycle involves nitrogen fixation, nitrification, assimilation, ammonification and denitrification.
植物需要必需营养元素，尤其是氮、磷和钾（NPK）。肥料补充土壤养分。氮以硝酸根离子（NO₃⁻）形式被吸收，是氨基酸和核苷酸的关键成分。氮循环包括固氮、硝化、同化、氨化和反硝化作用。
Crop protection involves managing pests, diseases and weeds. Chemical pesticides can be effective but may harm non-target species and lead to resistance. Biological control uses natural predators or parasites; integrated pest management combines methods to minimise environmental impact. Breeding programmes develop resistant varieties.
作物保护涉及管理病虫害和杂草。化学农药有效，但可能危害非目标物种并导致抗药性。生物防治利用天敌或寄生虫；综合害虫管理结合多种方法以最小化环境影响。育种计划培育抗性品种。
9. Population Dynamics and Biodiversity | 种群动态与生物多样性
A population is a group of individuals of the same species living in a particular area. Population size is influenced by births, deaths, immigration and emigration. Growth can follow an exponential (J-shaped) curve when resources are unlimited, but logistic (S-shaped) growth includes a carrying capacity (K).
种群是生活在特定区域的同一物种的个体群体。种群大小受出生、死亡、迁入和迁出影响。当资源无限时，增长可呈指数型（J形）曲线，但逻辑斯蒂（S形）增长包括环境容纳量（K）。
Density-dependent factors, such as competition, predation and disease, regulate population size as density increases. Density-independent factors, like natural disasters, affect populations regardless of size. Survivorship curves (Type I, II, III) describe the pattern of mortality over an organism’s lifespan.
密度制约因子（如竞争、捕食和疾病）在密度增加时调节种群大小。非密度制约因子（如自然灾害）的影响与种群大小无关。存活曲线（Ⅰ、Ⅱ、Ⅲ型）描述生物一生中死亡率的模式。
Biodiversity encompasses genetic, species and ecosystem diversity. It is measured using techniques such as quadrats, transects and mark-recapture. The Simpson’s Diversity Index quantifies species richness and evenness, giving a value between 0 and 1. High biodiversity increases ecosystem stability.
生物多样性包括遗传多样性、物种多样性和生态系统多样性。使用样方、样带和标记重捕等技术进行测量。辛普森多样性指数量化物种丰富度和均匀度，得出0到1之间的值。高生物多样性增强生态系统稳定性。
10. Conservation and Threats to Biodiversity | 保护与生物多样性威胁
Major threats to biodiversity include habitat destruction, overexploitation, invasive species, pollution and climate change. Habitat fragmentation reduces population sizes and isolates gene pools, decreasing genetic diversity. Overfishing, poaching and deforestation directly reduce species numbers.
生物多样性面临的主要威胁包括栖息地破坏、过度开发、入侵物种、污染和气候变化。栖息地破碎化缩小种群规模并隔离基因库，降低遗传多样性。过度捕捞、偷猎和森林砍伐直接减少物种数量。
Conservation strategies aim to preserve genetic variation and ecosystem integrity. In situ conservation protects species within their natural habitats, e.g. nature reserves. Ex situ conservation involves breeding programmes in zoos and seed banks. International agreements, such as CITES, restrict trade in endangered species.
保护策略旨在保存遗传变异和生态系统完整性。就地保护在自然栖息地内保护物种，如自然保护区。迁地保护包括动物园中的繁育计划和种子库。CITES等国际协定限制濒危物种贸易。
Sustainable practices, including sustainable forestry and agriculture, reduce environmental impact. Ecotourism can provide economic incentives for conservation. Maintaining biodiversity is essential for ecosystem services, such as pollination, nutrient cycling and climate regulation.
可持续实践（包括可持续林业和农业）减少环境影响。生态旅游可为保护提供经济激励。维持生物多样性对于授粉、养分循环和气候调节等生态系统服务至关重要。
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