Cells, Development, Biodiversity and Conservation

You began as one fertilised egg cell. This unit explains how that cell divided, how the copies were shuffled and halved to make gametes, how identical genes became different tissues, and how the variety that produces is counted, classified and — increasingly — defended. It is the unit of microscopes, calculations and definitions done precisely.

WBI12 Cells, Development, Biodiversity and Conservation is the second IAS unit: 1 hour 30 minutes, 80 marks, all compulsory. It assesses Topic 3 (Cell Structure, Reproduction and Development) and Topic 4 (Plant Structure and Function, Biodiversity and Conservation). Expect labelling diagrams, microscope and magnification calculations, the index of diversity and Hardy–Weinberg questions, and long answers on conservation evaluation.

A eukaryotic 真核 cell is a set of specialised compartments:

Cells organise into tissues 组织 (similar cells), organs 器官 (cooperating tissues) and organ systems 器官系统. A prokaryotic 原核 cell has none of the membrane-bound organelles: a cell wall, a slime capsule 荚膜, one circular DNA molecule plus plasmids 质粒, 70S ribosomes, and sometimes a flagellum 鞭毛.

Magnification 放大倍数 is how much bigger the image is; resolution 分辨率 is the smallest separation still seen as two. Light microscopes resolve about 200 nm; electron beams, far shorter in wavelength, resolve to a few nanometres — but need thin stained sections (TEM) or gold-coated surfaces (SEM). $M = \text{image size} / \text{actual size}$, keeping units consistent.

Worked check. A mitochondrion measures 14 mm on a photograph labelled ×20 000. Actual length $= 14\,000\ \mu\text{m} / 20\,000 = 0.7\ \mu$m. A nucleus that must be seen with its double membrane needs an electron microscope: the two membranes lie closer than a light microscope's resolution.

Meiosis 减数分裂 is two successive divisions after one DNA replication, producing four genetically different haploid 单倍体 cells from one diploid 二倍体 cell. Two processes create the variation:

• Independent assortment 独立分配 — each homologous pair lines up and separates at random in meiosis I; each gamete gets one of each pair, from either parent at random ($2^n$ combinations).
• Crossing over 交叉互换 — in prophase I, homologous chromatids swap sections, mixing alleles within chromosome pairs.

Mammalian gametes are specialised for delivery and provisioning: the sperm 精子 carries an acrosome 顶体 (digestive enzymes), a midpiece of mitochondria and a flagellum; the egg 卵子 contributes nearly all cytoplasm and a zona pellucida 透明带 that locks out extra sperm after the cortical reaction 皮质反应. In flowering plants the pollen tube delivers two male nuclei: one fertilises the egg, the other the polar nuclei — double fertilisation 双受精.

Mitosis 有丝分裂 produces two genetically identical diploid cells: growth, repair, asexual reproduction. Interphase ($G_1$, S — DNA replication, $G_2$) fills most of the cycle; mitosis itself is prophase (chromosomes condense, spindle forms), metaphase (chromosomes line at the equator), anaphase (spindle fibres pull chromatids to poles), telophase (nuclear membranes reform), then cytokinesis 胞质分裂.

The mitotic index 有丝分裂指数 $=$ cells in mitosis $\div$ total cells counted. High values in a tissue biopsy mean rapid division — a cancer signature.

Worked check. A root-tip squash shows 36 of 450 cells in mitosis: index $= 36/450 = 0.08 = 8\%$. If the cycle takes 20 hours, mitosis occupies $0.08 \times 20 = 1.6$ hours.

A totipotent 全能性 stem cell can form a whole organism (the first divisions of the zygote); a pluripotent 多能性 cell (inner blastocyst 胚泡) can form any tissue but not the whole organism. Differentiation happens by differential gene expression 差异基因表达: every cell keeps the whole genome, but each type switches on only its own subset. One gene can yield several proteins by post-transcriptional modification 转录后修饰 — RNA splicing choices of the pre-mRNA. The final phenotype also reflects the environment (height needs both genes and nutrition) and, for many traits, multiple genes acting together (polygenic 多基因 inheritance giving continuous variation).

Plant cells add a cellulose cell wall 细胞壁 joined to neighbours by plasmodesmata 胞间连丝. Cellulose chains bundle into microfibrils 微纤维 whose criss-cross laying gives tensile strength; secondary thickening with lignin strengthens and waterproofs.

Water and ions matter beyond drinking: nitrate 硝酸盐 builds amino acids, calcium 钙离子 cross-links pectin in walls and signals inside cells, magnesium 镁离子 sits at the heart of chlorophyll — a shortage yellows the leaves.

Classification arranges life by shared characteristics; the three-domain system 三域系统 (Bacteria, Archaea, Eukarya) rests on molecular evidence — ribosomal RNA sequences — that revealed Archaea are closer to us than to bacteria. Within a habitat, diversity has two measures:

• Species richness 物种丰富度 — how many species.
• Index of diversity 多样性指数 — $D = \dfrac{N(N-1)}{\sum n(n-1)}$, where $N$ is all individuals and $n$ each species' count. Higher $D$, more diverse.

Worked check. Habitat A: 3 species with counts 40, 35, 25 ($N = 100$). $D = 100 \times 99 / (40 \times 39 + 35 \times 34 + 25 \times 24) = 9900 / (1560 + 1190 + 600) = 9900/3350 = 2.96$. Habitat B: 3 species, counts 90, 6, 4: $D = 9900/(8010 + 30 + 12) = 1.23$. Same richness, very different balance — $D$ sees what richness cannot.

Within one species, the heterozygosity index 杂合度指数 $H =$ (number of heterozygotes) $\div$ (number of individuals) measures genetic variety. A species found nowhere else is endemic 特有种 — and its island home makes it both precious and fragile.

A niche 生态位 is a species' way of life: its role, its requirements, its tolerances. Adaptations come in three registers — anatomical (thick fur), physiological (enzyme variants), behavioural (migration timing).

The Hardy–Weinberg equation 哈迪–温伯格方程 tracks allele frequencies in a large, randomly breeding population with no selection, migration or mutation:

$q^2$ = frequency of the homozygous recessive genotype. If 4 % of a population shows the recessive phenotype, $q^2 = 0.04$, so $q = 0.2$, $p = 0.8$, carriers $2pq = 0.32$.

Worked check. Of 500 foxes, 45 are homozygous recessive: $q^2 = 0.09$, $q = 0.3$, $p = 0.7$. Expected carriers $= 2pq \times 500 = 0.42 \times 500 = 210$.

Conservation questions want balance. Zoos and seed banks 种子库 keep species against extinction and run captive breeding, but cost space, lose natural behaviour and serve few individuals; habitat protection and corridors 走廊 preserve ecology at scale but need land, money and political will. Credit the specific argument, not the sentiment.

For the microbiology here (and Unit 4's): grow bacteria on nutrient agar or in broth, using aseptic technique 无菌技术 — sterilise loops by flaming, flame bottle necks, work near an updraught, seal plates, incubate below body temperature so human pathogens cannot grow. Count colonies on spread or pour plates (each colony from one cell or a clump: colony-forming units), or count cells with a haemocytometer, or follow turbidity 浊度 in broth.

Worked check. In the exponential phase, the growth-rate constant $k = (\log_{10} N_t - \log_{10} N_0)/(0.301 \times t)$. From $1 \times 10^4$ to $4 \times 10^7$ cells in 4 hours: $\log$ difference $= 3.602$, so $k = 3.602/(0.301 \times 4) \approx 3.0$ divisions per hour — about a 20-minute generation time.

1. Calculate the actual width of a nucleus that appears 24 mm wide at ×4 000. Give your answer in µm.
2. Name the two processes in meiosis that create genetic variation, and state when each happens.
3. A cell has 2 a.u. of DNA in G1. Sketch how DNA content changes through S phase, meiosis I and meiosis II.
4. Explain why all cells of a blastocyst can become any tissue, but a mature muscle cell cannot.
5. Explain two ways xylem is adapted to its function, one structural and one chemical.
6. Two habitats each contain 5 plant species. Explain why the index of diversity can still differ between them.
7. In a population of 1 000, 16 people show a recessive condition. Calculate q, p and the expected number of carriers.
8. A species of snail is endemic to one island. Explain why endemic species face higher extinction risk.
9. State three conditions a population must meet for the Hardy–Weinberg equation to hold.
10. Give one benefit and one limitation of seed banks compared with protecting habitat.

Answers: 1 $24\,000/4\,000 = 6\ \mu$m; 2 crossing over in prophase I, independent assortment in metaphase/anaphase I (and II); 3 rises to 4 in S, halves to 2 after meiosis I, halves to 1 after meiosis II, restored to 2 at fertilisation; 4 blastocyst cells are pluripotent — all genes still switchable; a muscle cell has switched off other pathways permanently in ordinary conditions; 5 hollow, lignified dead cells form continuous water columns with no cytoplasm to resist flow; lignin waterproofs and strengthens walls against collapse; 6 D weighs the evenness of abundance, not just the species count; 7 $q^2 = 0.016$, $q \approx 0.126$, $p \approx 0.874$, carriers $2pq \approx 0.22$, about 220 people; 8 a single local event (storm, disease, introduced predator, habitat loss) removes the whole species at once; 9 large population, random mating, no selection/mutation/migration; 10 seed banks store genetic diversity cheaply and safely against habitat loss (benefit) but species exist out of their ecology — no interactions, no evolution in place, and recolonisation is uncertain (limitation).