OCR A-Level Biology June 2023 Mark Scheme 1: Experimental Design | OCR A-Level生物2023年6月评分方案1：实验设计

📚 Experimental Design in OCR A-Level Biology June 2023 Mark Scheme 1 | OCR A-Level生物2023年6月评分方案1中的实验设计

The OCR A-Level Biology June 2023 Paper 1 included a high-mark experimental design question that tested candidates’ ability to plan a rigorous enzyme kinetics investigation using a colorimeter. Successful answers had to demonstrate precise control of variables, appropriate use of a blank, the importance of repeats, and a logical sequence of practical steps. Understanding the mark scheme for this question reveals exactly what examiners reward and the common misconceptions that cost students grades.

OCR A-Level生物2023年6月的试卷1中，有一道高分值的实验设计题，考察考生使用比色计设计严谨酶动力学实验的能力。高分的答案必须展现对变量的精确控制、正确使用空白对照、重视重复性，以及合理的实操步骤。理解这道题的评分方案，可以清晰揭示考官所看重的得分点，以及那些让学生丢分的常见理解误区。

1. Question Context and Overview | 题目背景概览

The question asked students to design an investigation into the effect of a non-competitive inhibitor on the rate of an enzyme-controlled reaction, measuring the decrease in absorbance as a coloured substrate is broken down. Candidates had to name specific equipment, detail how the colorimeter would be calibrated with a blank, state the range of inhibitor concentrations, and explain how the rate would be calculated from absorbance readings taken at fixed time intervals.

该题要求学生设计一个实验，研究非竞争性抑制剂对酶促反应速率的影响，通过测量有色底物分解时吸光度的降低来反映反应速率。考生需要列出具体仪器、详细说明如何使用空白对照校准比色计、设定抑制剂的浓度范围，并解释如何在固定时间间隔记录吸光度来计算反应速率。

2. Specifying Independent, Dependent and Control Variables | 明确自变量、因变量与控制变量

Independent variable: concentration of non-competitive inhibitor (e.g. 0, 0.2, 0.4, 0.6, 0.8, 1.0 mmol dm⁻³). The mark scheme insisted on a minimum of five different concentrations including a zero-inhibitor control. Dependent variable: rate of reaction, measured as change in absorbance per unit time (ΔA min⁻¹). Key control variables: temperature (use of a water bath at 30 °C), pH (buffer solution), enzyme concentration, substrate concentration, and volume of all solutions.

自变量：非竞争性抑制剂的浓度（例如 0、0.2、0.4、0.6、0.8、1.0 mmol dm⁻³）。评分方案要求至少包含五种不同浓度，且必须有一个抑制剂浓度为零的对照组。因变量：反应速率，用单位时间内吸光度的变化表示（ΔA min⁻¹）。关键控制变量：温度（使用 30 °C 水浴），pH（缓冲液），酶浓度，底物浓度，以及所有溶液的体积。

3. Justifying the Use of a Colorimeter and Blank | 比色计与空白对照的使用理由

A colorimeter measures the absorbance of a specific wavelength of light passing through a sample. The mark scheme rewarded candidates who explained that a blank (cuvette containing all reagents except the enzyme or the substrate, replaced with distilled water or buffer) must be used to set zero absorbance before each reading. This compensates for any absorbance by the inhibitor or buffer itself and ensures that the measured absorbance change is solely due to substrate breakdown. An appropriate wavelength (e.g. 470 nm for a blue substrate turning colourless) must be selected using a filter or monochromator.

比色计可以测量特定波长的光穿过样品时的吸光度。评分方案鼓励考生解释为何每次读数前必须使用空白对照（比色皿中含有除酶或底物外的所有试剂，用蒸馏水或缓冲液替代）将吸光度归零。这能消除抑制剂或缓冲液本身可能带来的吸光度干扰，确保测得的吸光度变化仅由底物分解引起。此外，还需选用合适的波长（如测量蓝色底物变为无色，可选用 470 nm 的滤光片或单色器）。

4. Step-by-Step Methodology According to the Mark Scheme | 评分方案认可的实操步骤

Examiners expected a logical, numbered sequence. A top-level answer would include: (1) Label test tubes for each inhibitor concentration and prepare a water bath at 30 °C. (2) Add 2 cm³ of buffer, 1 cm³ of enzyme solution, and 1 cm³ of inhibitor solution to each tube and equilibrate for 5 minutes. (3) Add 1 cm³ of substrate solution, quickly mix, transfer to a cuvette, and insert into the colorimeter. (4) Start the stopwatch immediately and record absorbance every 15 seconds for 3 minutes. (5) Repeat the procedure three times for each concentration. (6) Plot a graph of absorbance against time and use the initial linear portion to calculate the initial rate of reaction.

考官期望看到逻辑清晰、编号有序的步骤。高分答案应包括：（1）为每个抑制剂浓度标记试管，准备 30 °C 水浴。（2）向每支试管中加入 2 cm³ 缓冲液、1 cm³ 酶溶液和 1 cm³ 抑制剂溶液，水浴平衡 5 分钟。（3）加入 1 cm³ 底物溶液，迅速混匀，转移至比色皿并放入比色计。（4）立即启动秒表，每 15 秒记录一次吸光度，持续 3 分钟。（5）每个抑制剂浓度至少重复实验三次。（6）绘制吸光度对时间的曲线，取初始线性部分计算反应的初始速率。

5. Ensuring Accuracy and Reliability | 保证准确性与可靠性

Accuracy was improved by using a water bath rather than relying on room temperature, and by rinsing cuvettes with the reaction mixture before filling. Reliability was addressed by performing repeats (at least three) and calculating a mean initial rate. If an anomalous result was obtained, the mark scheme indicated that the student should identify it, exclude it from the mean, and repeat that specific run if time permits. The use of a buffer minimised changes in pH, which could denature the enzyme or alter the inhibitor’s ionisation state.

提高准确性的措施包括使用水浴而非依赖室温，以及在装液前用反应混合液润洗比色皿。可靠性则通过至少三次重复并计算平均初始速率来保证。若出现异常值，评分方案指出，考生应识别并剔除该值，不纳入平均值的计算，同时在时间允许的情况下重做该组实验。使用缓冲液可最大限度减少 pH 变化，避免酶变性或抑制剂电离状态发生改变。

6. Dealing with the Non‑competitive Inhibitor Mechanism | 非竞争性抑制剂作用机制的处理

Unlike competitive inhibitors, a non-competitive inhibitor binds to an allosteric site, altering the enzyme’s shape so the substrate cannot bind efficiently. The mark scheme rewarded candidates who linked this to the prediction that increasing inhibitor concentration would reduce the maximum rate (Vₘₐₓ) without changing the affinity (Kₘ) significantly, as the substrate can still bind to uninhibited enzymes. The initial rate should decrease progressively with inhibitor concentration, but the shape of the progress curve may not show a simple linear relationship at later time points due to substrate depletion.

与竞争性抑制剂不同，非竞争性抑制剂结合在别构部位，改变酶的形状，导致底物无法有效结合。评分方案赞赏那些能将此机制与预测联系起来的考生：增加抑制剂浓度会降低最大反应速率（Vₘₐₓ），但不显著改变酶与底物的亲和力（Kₘ），因为底物仍能与那些未被抑制的酶分子结合。初始速率会随抑制剂浓度增加而逐步下降，但由于底物消耗，反应进行到后期时刻的进展曲线可能不再呈现简单的线性关系。

7. Risk Assessment and Ethical Use of Reagents | 风险评估与试剂伦理使用

The June 2023 mark scheme included safety considerations as part of a well-planned design. Candidates were expected to mention wearing eye protection, washing hands after handling enzyme powders or inhibitor solutions, and wiping up spills immediately. If the inhibitor was described as a heavy metal ion (e.g. Cu²⁺ or Hg²⁺), disposal must follow local chemical waste guidelines. Although enzymes and substrates used in school laboratories are generally low‑hazard, a clear risk assessment (low risk with suitable control measures) earned marks.

2023年6月的评分方案将安全考量纳入了设计完善的实验计划之中。考生需提及佩戴护目镜、接触酶粉末或抑制剂溶液后洗手、立即清理溢出物。如果抑制剂是重金属离子（例如 Cu²⁺ 或 Hg²⁺），则必须按照当地化学废料规定处理。尽管学校实验室使用的酶和底物通常危害较低，一份明确的风险评估（在适当控制措施下仍属低风险）依然能够赢得分数。

8. Drawing and Interpreting Results Tables and Graphs | 结果图表的设计与解读

Examiners expected a results table with clear headings, units in parentheses (e.g. Absorbance at 470 nm / arbitrary units), and space for mean values. The graph should plot absorbance (y‑axis) against time (x‑axis) for each inhibitor concentration. Students were advised to draw a line of best fit and use tangents at time zero to determine the initial rate, rather than averaging rates over the entire time course. This is critical because the rate slows as substrate is used up, and only the initial rate accurately reflects enzyme activity under the set conditions.

考官期望看到设计清晰的结果表格，表头带单位并用括号标注（例如 470 nm 处的吸光度 / 任意单位），并留有填写均值的位置。图表应将不同抑制剂浓度下的吸光度（y 轴）对时间（x 轴）作图。建议学生绘制最佳拟合曲线，并在时间零点做切线以确定初始速率，而不是对整个时间范围内的速率求平均值。这一点极为关键，因为随着底物的消耗速率会减慢，只有初始速率才能准确反映既定条件下的酶活力。

9. Common Errors Highlighted in the Mark Scheme | 评分方案揭示的常见错误

Frequent mistakes included: failing to include a zero‑inhibitor control, confusing the dependent variable (some candidates erroneously measured absorbance directly as the rate), omitting a blank calibration step, not controlling temperature with a water bath, and describing only one repeat. Some candidates also incorrectly stated that the non‑competitive inhibitor would increase absorbance due to its colour, without correcting for it via the blank. The mark scheme penalised vague language such as ‘same amount’ instead of exact volumes.

常见错误包括：遗漏抑制剂浓度为零的对照组、混淆因变量（部分考生错误地直接以吸光度作为速率）、省去空白校准步骤、未使用水浴控温、只描述了一次重复。还有些考生错误地声称非竞争性抑制剂本身的颜色会使吸光度升高，却未通过空白对照来校正。评分方案对“相同量”这样模糊的措辞扣分严厉，必须写明具体体积。

10. How the Mark Scheme Links Practical Skills to Assessment Objectives | 评分方案如何将实验技能关联评估目标

This question primarily assessed AO3 (Analyse, interpret and evaluate) and AO2 (Apply knowledge). Marks were awarded for showing understanding of enzyme kinetics, the rationale behind each practical step, and the ability to justify choices such as the wavelength or buffer selection. The mark scheme rewarded candidates who demonstrated a genuine ‘hands‑on’ familiarity—for example, mentioning the need to zero the colorimeter between readings, or explaining why the cuvette must be handled by the frosted sides to avoid fingerprints on the optical surfaces.

该题主要考察 AO3（分析、解读与评价）和 AO2（知识应用）。只要展示出对酶动力学的理解、每步操作背后的原理，并能为诸如波长或缓冲液的选择提供充分理由，就能得分。评分方案奖赏那些能体现真实“动手”经验的考生——例如提及每两次读数之间需重新调零比色计，或解释为何必须手持比色皿的毛玻璃面以避免光学面留下指纹。

11. Examiner Commentary on Model Answers | 考官对高分答案的点评

The principal examiner’s report noted that the strongest candidates wrote concisely, yet covered every required detail. They used standard biological terminology like ‘initial rate of reaction’, ‘calibration curve’, and ‘allosteric site’ accurately. They structured the answer with bullet points or short paragraphs, making it easy for the examiner to locate the marks. Less successful responses rambled or included irrelevant details, such as describing a different enzyme system that they had memorised, without adapting it to the question.

主考官报告指出，最出色的考生作答简洁却面面俱到。他们准确使用了“反应初始速率”、“校准曲线”、“别构部位”等标准生物学术语。答案以要点或短段落编排，让考官能一目了然地找到得分点。相比之下，失分较多的回答冗长铺陈，或加入无关细节，例如生搬硬套记忆中的另一套酶实验体系，却未能针对此题做出调整。

12. Further Practice and Application to Other Contexts | 延伸练习与其他场景的应用

Teachers are encouraged to use the June 2023 Mark Scheme 1 as a blueprint for training students in experimental design across all six OCR practical activity groups (PAGs). The same principles—explicit variable identification, justified methodological choices, rigorous repeats, and clear data presentation—apply equally to investigations on photosynthesis, respiration, osmosis, and microbial growth. By internalising the mark scheme expectations, learners can approach any practical‑based question with confidence and precision.

教师可以借助2023年6月评分方案1作为蓝图，训练学生应对OCR全部六个实验活动组（PAGs）中的实验设计题。相同的原则——明确识别变量、为方法选择提供理由、严格的重复操作、清晰的数据呈现——同样适用于光合作用、呼吸作用、渗透作用以及微生物生长等探究。当学生将评分方案的期望内化于心，便能在面对任何实践性考题时信心十足、表述精准。

Published by TutorHao | Biology Revision Series | aleveler.com

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