A-Level Biology - Advanced Analysis & Synthesis

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Chi-squared ($\chi^2$) Goodness of Fit Application

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A statistical test to determine if observed data fits a theoretical ratio (e.g., 3:1 Mendelian inheritance). Formula: $\sum \frac{(O-E)^2}{E}$. **Critical:** Expected values must be >5. If $\chi^2_{calc} > \chi^2_{crit}$ (at df=n-1, p=0.05), reject the null hypothesis (the difference is significant).

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Osmoregulation: Countercurrent Multiplication in the Loop of Henle

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A mechanism to create hyperosmotic medulla. **1.** Descending limb permeable to water (impermeable to salts); water leaves, filtrate concentrates. **2.** Ascending limb actively pumps out Na+/Cl- (impermeable to water); filtrate dilutes. **3.** Interaction sets up a gradient increasing from cortex (300 mOsm) to inner medulla (1200 mOsm).

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Spiral Xylem & Sclerenchyma in Plant Support

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Xylem vessels contain lignin, a hard polymer providing mechanical strength to withstand tension (cohesion-tension). Sclerenchyma fibers (dead at maturity) have thick secondary walls and lignin, providing tensile and flexural strength to resist bending and wind stress. Synoptic link: Turgor pressure in parenchyma contrasts with this rigid support.

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Hardy-Weinberg Principle Conditions

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States allele frequencies remain constant generation to generation **if**: 1. No mutations, 2. Large population (no genetic drift), 3. Random mating, 4. No gene flow (migration), 5. No natural selection. Used as a null model to detect evolutionary forces: $p^2 + 2pq + q^2 = 1$.

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Electron Transport Chain (ETC) & Chemiosmosis Coupling

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Electrons move down redox potentials through carriers (NADH dehydrogenase, cytochrome complexes), releasing energy to pump protons ($H^+$) into the intermembrane space. The electrochemical gradient ($\Delta p$) drives $H^+$ back through ATP synthase (Complex V), phosphorylating ADP to ATP. **Link:** Inhibition (e.g., cyanide) stops proton pumping, collapsing the gradient and halting ATP production.