Unité 2 : Cellules, Développement, Biodiversité et Conservation
Pearson Edexcel · International A-Level · Biology Sujet 2 10:10 Narration en anglais · Sous-titres anglais + 中文 incrustés
Chapitres
Transcript
You began as one fertilised egg cell.
你从一个受精卵开始。
This unit explains how that cell divided, how the copies were shuffled and halved into gametes, how identical genes became different tissues, and how the variety that produces is counted, classified and defended.
本单元讲解这个细胞如何分裂、基因副本如何被打乱并减半形成配子、相同的基因如何变成不同的组织，以及由此产生的多样性如何被计数、分类和保护。
The paper is ninety minutes, eighty marks, all compulsory, and it rewards precise definitions, careful microscope arithmetic and clean calculations - the diversity index and Hardy-Weinberg in particular.
试卷90分钟、80分，全部必答，考察精确的定义、仔细的显微镜计算和整洁的运算——尤其是多样性指数和哈迪–温伯格。
A eukaryotic cell is a set of specialised compartments.
真核细胞是一组分区的结构。
The nucleus stores DNA and its nucleolus builds ribosomes. Rough endoplasmic reticulum folds the proteins its ribosomes make; the Golgi apparatus modifies, packages and ships them; mitochondria supply ATP; chloroplasts carry out photosynthesis.
细胞核储存DNA，核仁合成核糖体；粗面内质网折叠其核糖体合成的蛋白质；高尔基体修饰、包装并运输它们；线粒体供能；叶绿体进行光合作用。
Cells organise into tissues, tissues into organs, organs into systems.
细胞构成组织、组织构成器官、器官构成系统。
A prokaryotic cell has none of these membrane-bound organelles: a cell wall, often a capsule, plasmids beside one circular DNA molecule, 70S ribosomes, sometimes a flagellum.
原核细胞没有这些膜性细胞器：细胞壁、常有荚膜、质粒与一条环状DNA、70S核糖体，有时有鞭毛。
Magnification is how much bigger the image is than the real thing: image size over actual size, with units matched before dividing.
放大倍数是像与物的尺寸之比：像长除以实长，先统一单位再相除。
Resolution is different - the smallest separation at which two objects still look like two.
分辨率则不同——仍能看出是两个物体的最小间距。
A light microscope resolves about two hundred nanometres; an electron beam, with a far shorter wavelength, resolves to a few nanometres.
光学显微镜约能分辨200纳米；波长短得多的电子束可分辨到几纳米。
To see the two membranes of a nuclear envelope as separate, you need that resolution, not just more magnification of a blurry image.
要把核膜的两层膜分开看清，需要的是分辨率，而不只是把模糊的图像再放大。
In meiosis the DNA replicates once but the cell divides twice.
减数分裂中DNA复制一次，细胞却分裂两次。
Meiosis one separates the homologous pairs; meiosis two separates the chromatids, exactly as mitosis does.
减数第一次分裂分开同源染色体；第二次分裂像有丝分裂一样分开姐妹染色单体。
The result is four haploid cells, each genetically different.
结果是四个单倍体细胞，各不相同。
Two processes create that difference: independent assortment - which member of each pair ends in which gamete is random - and crossing over, where homologous chromatids exchange sections in prophase one.
两个过程造就差异：独立分配——每对同源染色体中哪一条进入哪个配子是随机的；以及交叉互换——同源染色单体在前期一交换片段。
A cell in G1 carries two arbitrary units of DNA. Replication in S phase doubles it to four.
G1期的细胞含两个单位的DNA，S期复制后翻倍为四。
Meiosis one separates the homologous chromosomes, so each cell drops back to two units - but each chromosome is still two chromatids.
减数第一次分裂分开同源染色体，每个细胞降回两个单位——但每条染色体仍含两条染色单体。
Meiosis two separates the chromatids: one unit per gamete.
减数第二次分裂分开单体：每个配子一个单位。
Fertilisation restores two.
受精后恢复为二。
Exam questions ask you to draw or complete exactly this graph, and to label which division is which.
考题常让你画出或补全这张图，并标明两次分裂各自的位置。
Each gamete is specialised for its job.
每种配子都为职责特化。
The sperm carries an acrosome of digestive enzymes and a midpiece of mitochondria driving its flagellum.
精子携带含消化酶的顶体和布满线粒体的中段驱动鞭毛。
The egg supplies nearly all the cytoplasm and is wrapped in the zona pellucida; when one sperm fuses with the membrane, the cortical reaction hardens the zona so no second sperm enters.
卵子提供几乎全部细胞质，外覆透明带；当一条精子与膜融合，皮质反应使透明带变硬，阻止第二条进入。
In flowering plants the pollen tube delivers two male nuclei: one fertilises the egg to form the embryo, the other joins the polar nuclei to form the endosperm - double fertilisation.
开花植物中，花粉管送来两个雄核：一个与卵细胞融合成胚胎，另一个与极核结合成胚乳——双受精。
Interphase fills most of the cell cycle - growth in G1, DNA replication in S, preparation in G2.
间期占据细胞周期的大部分——G1生长、S期DNA复制、G2准备。
Mitosis itself has four stages. In prophase chromosomes condense and the spindle forms; in metaphase they line up on the equator attached at their centromeres; in anaphase spindle fibres shorten and pull the chromatids to opposite poles; in telophase nuclear membranes reform and the chromosomes uncoil.
有丝分裂本身分四期：前期染色体凝缩、纺锤体出现；中期染色体排列在赤道板；后期纺锤丝牵引染色单体移向两极；末期核膜重建、染色体解旋。
The mitotic index - cells in mitosis over total cells - tells you how fast a tissue divides; a biopsy with a high index is a cancer signature.
有丝分裂指数——分裂期细胞除以总细胞数——反映组织分裂速度；活检样本指数高是癌症的信号。
The first divisions of a zygote give totipotent cells - able to form a whole organism plus placenta.
受精卵最初的分裂产生全能细胞——能发育成完整生物体加胎盘。
The inner mass of the blastocyst is pluripotent: any body tissue, but not the whole organism.
胚泡内细胞团是多能的：能形成任何体组织，但不能单独成体。
Differentiation never throws genes away; it switches them on and off through transcription factors.
分化从不丢弃基因，而是通过转录因子开关基因。
Even one gene can serve several tissues: the pre-mRNA is spliced differently, keeping different exon combinations.
甚至一个基因可服务多种组织：前体mRNA经不同剪接保留不同的外显子组合。
And the final phenotype is always genotype times environment, with polygenic traits giving the bell curves of continuous variation.
而最终的表型永远是基因型乘以环境，多基因性状给出连续变异的钟形曲线。
Plant cells add a cellulose wall, cross-linked into microfibrils that resist stretching in every direction, joined to neighbours by plasmodesmata.
植物细胞加上纤维素壁，交联成微纤维束，在各方向抗拉伸，胞间连丝连接相邻细胞。
Xylem vessels are dead hollow cells with lignified walls: water travels up, and the lignin both waterproofs and supports.
木质部导管是死亡的空心细胞，壁木质化：水向上运输，木质素既防水又支撑。
Phloem is living - sieve tube elements with companion cells - carrying sugars both directions.
韧皮部是活的——筛管分子伴伴胞——双向运输糖类。
In a root cross-section the xylem forms a central star with phloem between its arms; in the stem the vascular bundles form a ring with phloem outside the xylem.
根的横切面上木质部构成中央星形，韧皮部位于臂间；茎中维管束排成一环，韧皮部在外。
Nitrate builds amino acids, calcium cross-links wall pectin, magnesium sits inside chlorophyll.
硝酸盐合成氨基酸，钙交联壁中的果胶，镁位于叶绿素中心。
Classification now rests on molecular evidence: ribosomal RNA sequencing split the old prokaryotes into Bacteria and Archaea, with everything else in Eukarya.
分类如今以分子证据为基础：核糖体RNA测序把原核生物拆分为细菌域和古菌域，其余归真核域。
Within a habitat, species richness simply counts species; the index of diversity also weighs how evenly individuals are spread. N times N minus one over the sum of n times n minus one.
在栖息地内，物种丰富度只数物种数；多样性指数还衡量个体分布的均衡度：N乘N减1，除以各物种n乘n减1之和。
Two habitats with the same three species can score 2.96 and 1.23 - richness identical, balance completely different.
同样三个物种的两块样地可得2.96和1.23——丰富度相同，均衡完全不同。
Within one species, the heterozygosity index measures genetic variety; a species found nowhere else is endemic, and its single home makes it fragile.
在物种内部，杂合度指数衡量遗传多样性；只分布于一地的物种是特有种，唯一的家园使其脆弱。
The Hardy-Weinberg equation tracks allele frequencies in a large, randomly mating population with no selection, mutation or migration.
哈迪–温伯格方程追踪大种群、随机交配、无选择突变迁移条件下的等位基因频率。
p squared, 2pq and q squared are the genotype frequencies.
p平方、2pq与q平方是各基因型频率。
Given the recessive phenotype - q squared - take the square root for q, subtract from one for p, and 2pq gives the carriers.
已知隐性表型即q平方，开方得q，用1减得p，2pq即携带者比例。
Of five hundred foxes, forty-five recessive means q equals zero point three and about two hundred and ten carry the allele.
500只狐狸中45只隐性，则q为0.3，约210只为携带者。
If the next generation drifts from these predictions, one of the conditions has failed - usually selection.
若下一代偏离预测，必是某条件失效——通常是选择。
Conservation questions want balance: zoos and seed banks protect and breed but hold few individuals; habitat protection and corridors preserve whole communities.
保护类题目同样讲平衡：动物园与种子库能保育繁育，但个体稀少；栖息地保护与走廊保全整个群落。
Aseptic technique protects both directions.
无菌技术双向保护。
Flame the inoculating loop to red heat, flame bottle and tube necks, lift lids briefly, seal plates with tape, and incubate at twenty-five degrees so human pathogens cannot multiply.
接种环烧红灭菌，瓶口管口过火，开盖短暂，胶带封皿，25度培养以免人类病原菌繁殖。
Measure growth by direct counts with a haemocytometer, by turbidity in broth, or by viable counts with dilution plating.
测生长可用血细胞计数板直接计数、肉汤浊度或稀释涂布的活菌计数。
The growth curve has four phases - lag while enzymes are made, log with constant doubling, stationary as deaths balance divisions, then death.
生长曲线分四期——延迟期合成酶、对数期恒定倍增、稳定期生死相抵、衰亡期。
The growth-rate constant k is the log difference divided by 0.301 times the time in hours.
增长率常数k等于对数差除以0.301乘时间小时数。
Close with calculation.
以计算收尾。
Crossing over and independent assortment together shuffle four chromatids into four different gametes.
交叉互换与独立分配把四条染色单体组合成四个不同配子。
A nuclear envelope is narrower than two hundred nanometres, so only an electron microscope resolves it.
核膜两层间距小于200纳米，只有电镜能分辨。
For the pond sample, N times N minus one over the sum of n times n minus one gives 2.39; remove the last two species and D falls to 1.48 - diversity collapses even though richness only lost two.
池塘数据中N(N-1)除以Σn(n-1)得2.39；去掉最后两个物种后D降至1.48——丰富度只少两个，多样性却崩塌。
Forty-five recessive foxes in five hundred give about two hundred and ten carriers.
500只中45只隐性，携带者约210只。
And a small captive population drifts from Hardy-Weinberg because chance hits small samples - and mates are few, so mating is not random.
小型圈养种群偏离哈迪–温伯格，是因为小样本受随机摆布，且可交配对象有限、交配不再随机。