📚 Combined-019: Investigating Osmosis and Water Potential | 综合科学019：渗透作用与水势探究
This article provides a comprehensive revision guide for Edexcel A-Level Combined Science Core Practical 019, which investigates how varying solute concentrations affect the mass of potato tissue. You will learn to determine water potential by applying the principles of osmosis, collecting precise data, and analysing results with a critical eye. Mastering this practical is essential for understanding water transport in plants and for developing key investigative skills assessed in the A-Level qualification.
本文为Edexcel A-Level综合科学核心实验019提供完整的复习指南。该实验通过改变蔗糖溶液的浓度来观察马铃薯组织的质量变化，从而测定组织的水势。你将学习如何运用渗透原理、收集精确数据并用批判性思维分析结果。掌握这项实验对理解植物水分运输以及培养A-Level评价体系中的关键探究技能至关重要。
1. Core Practical Overview | 核心实验概览
Core Practical 019 is a controlled investigation into the relationship between sucrose concentration and the mass of potato cylinders. By immersing potato samples in solutions of known solute potential, you can determine the external concentration at which there is no net movement of water, thus directly identifying the tissue's water potential. This practical reinforces the concept that water moves from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
核心实验019是一项受控探究，研究蔗糖浓度与马铃薯圆柱体质量之间的关系。将马铃薯样品浸入已知溶质势的溶液中，就能找到没有净水移动的外部浓度，从而直接确定组织的水势。该实验强化了这样一个概念：水分会沿着水势梯度从水势较高的区域通过部分透性膜向水势较低的区域移动。
2. Water Potential and Osmosis Theory | 水势与渗透作用理论
scals (MPa), and is the sum of the solute potential (Ψₛ) and the pressure potential (Ψₚ). For dilute aqueous solutions at atmospheric pressure, Ψₚ ≈ 0, so Ψ is approximately equal to Ψₛ, which is always negative or zero because dissolved solutes reduce the freedom of water molecules. Osmosis is the passive movement of water through a partially permeable membrane down a water potential gradient until equilibrium is reached.
溶液或组织的水势(Ψ)是指水分从该系统移出的倾向，测量单位为兆帕(MPa)，它是溶质势(Ψₛ)与压力势(Ψₚ)的总和。对于大气压下的稀水溶液，Ψₚ ≈ 0，因此Ψ近似等于Ψₛ；由于溶解的溶质降低了水分子的自由能，溶质势始终为负值或零。渗透作用是指水分子通过部分透性膜沿水势梯度被动移动，直至达到平衡。
The formal relationship is expressed as:
其关系可用公式表示为：
Ψ = Ψₛ + Ψₚ
In living plant cells, the cell wall exerts a pressure potential (turgor pressure) that contributes to the overall Ψ. In the potato practical, we assume the tissue is plasmolysed or incipient plasmolysed when the external Ψₛ matches the internal Ψ, a condition that enables us to estimate the water potential of the tissue.
在活植物细胞中，细胞壁会产生压力势(膨压)，对总Ψ有所贡献。在本次马铃薯实验中，当外部Ψₛ与组织内部Ψ相等时，我们假定组织处于质壁分离或初始质壁分离状态，这一条件能让我们估算组织的水势。
3. Aim and Hypotheses | 实验目的与假设
The aim is to determine the water potential of potato tuber cells by measuring the percentage change in mass of potato cylinders placed in a range of sucrose solutions. The null hypothesis states that there is no significant difference in the mass change across the sucrose concentrations, while the alternative hypothesis predicts that the direction and magnitude of mass change will depend on the sucrose concentration, with a distinct point of zero net change indicating the tissue water potential.
本实验的目的是通过测量置于一系列蔗糖溶液中的马铃薯圆柱体的质量变化百分比，来确定马铃薯块茎细胞的水势。零假设指出，不同蔗糖浓度下的质量变化无显著差异；备择假设则预测，质量变化的方向和幅度取决于蔗糖浓度，并存在一个净质量变化为零的特定点，该点即指示组织的水势。
4. Variables and Controls | 变量与控制
The independent variable is the concentration of sucrose solution (e.g., 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm⁻³). The dependent variable is the percentage change in mass of the potato cylinders. Key controlled variables include: temperature (keep all solutions at room temperature and avoid heat sources), time of immersion (typically 30 minutes), surface area to volume ratio (use cylinders cut with the same cork borer size), and the blotting technique to remove excess surface water before weighing.
自变量是蔗糖溶液的浓度(例如0.0、0.2、0.4、0.6、0.8、1.0 mol dm⁻³)。因变量是马铃薯圆柱体的质量变化百分比。需要控制的关键变量包括：温度(所有溶液保持在室温，远离热源)、浸泡时间(通常为30分钟)、表面积与体积比(使用相同规格的打孔器切取圆柱体)以及称重前去除表面多余水分的吸干技巧。
- Use a water bath or monitor room temperature to maintain thermal consistency.
- 使用水浴或监测室温以保持热一致性。
- Cut potatoes to a uniform length using a scalpel and ruler, ensuring each cylinder is approximately 3 cm long.
- 用解剖刀和尺子将马铃薯切成统一长度，确保每个圆柱体约3 cm长。
- Blot each cylinder gently but firmly with a paper towel before every weighing.
- 每次称重前用纸巾轻轻但果断地吸干每个圆柱体。
5. Equipment and Materials | 仪器与材料
A full equipment list is essential for reproducibility. Typical items include: a large potato, a cork borer (6 mm diameter), a scalpel, a ruler, six beakers or boiling tubes, 1.0 mol dm⁻³ sucrose stock solution, distilled water, a balance measuring to 0.01 g, paper towels, forceps, a stopwatch, and a marker pen for labelling. You may also use a volumetric pipette and pipette filler to measure solution volumes accurately.
完整的仪器清单对实验的可重复性至关重要。典型器材包括：一个大马铃薯、一个直径6 mm的打孔器、解剖刀、尺子、六个烧杯或沸腾管、1.0 mol dm⁻³蔗糖储备液、蒸馏水、精度0.01 g的天平、纸巾、镊子、秒表以及用于标记的记号笔。也可使用移液管和洗耳球精确量取溶液体积。
| Material | Purpose |
| Cork borer | Produce cylinders of equal cross-sectional area |
| 1.0 mol dm⁻³ sucrose | Stock solution for serial dilution |
| Balance (0.01 g) | Precise mass measurement |
| 材料 | 用途 |
| 打孔器 | 制取横截面积相等的圆柱体 |
| 1.0 mol dm⁻³蔗糖 | 用于连续稀释的储备液 |
| 天平(0.01 g) | 精确称量质量 |
6. Method: Step-by-Step Procedure | 方法：逐步步骤
Prepare a serial dilution of sucrose solutions: label six beakers and use distilled water and stock 1.0 mol dm⁻³ sucrose to make 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm⁻³ concentrations. Use a cork borer to remove six cylinders from the potato, trim each to exactly 3 cm in length, and blot immediately. Record the initial mass of each cylinder on the balance; work quickly to minimise evaporation and water uptake from the air.
制备蔗糖溶液的连续稀释：标记六个烧杯，用蒸馏水和1.0 mol dm⁻³蔗糖储备液配制0.0、0.2、0.4、0.6、0.8、1.0 mol dm⁻³各浓度。用打孔器从马铃薯中取出六条圆柱体，每条修整为恰好3 cm长，并立即吸干。在托盘天平上记录每个圆柱体的初始质量；动作要快，以尽量减少蒸发及从空气中吸收水分。
Place one cylinder into each labelled beaker, ensuring it is fully submerged. Start the stopwatch and leave the cylinders for exactly 30 minutes. After the immersion period, remove a cylinder with forceps, blot it using the same technique as before, and reweigh. Repeat for all cylinders in the same order, maintaining consistent blotting pressure and timing.
将一条圆柱体放入每个已标记的烧杯中，使其完全浸没。启动秒表，将圆柱体静置恰好30分钟。浸泡结束后，用镊子取出圆柱体，按与之前相同的吸干技术吸干并重新称重。以相同顺序对所有圆柱体重复此操作，保持一致的吸干压力和计时。
7. Data Collection and Recording | 数据收集与记录
Construct a results table before the practical to log the initial mass, final mass, mass change, and percentage mass change for each sucrose concentration. The table should include space for repeated measurements if time allows for replication. Present data to two decimal places, consistent with the balance precision.
实验前先设计一个结果表，记录每种蔗糖浓度下的初始质量、最终质量、质量变化以及质量变化百分比。如果时间允许进行重复，表格应预留重复测量空间。数据精确到小数点后两位，与天平精度一致。
| Sucrose conc. / mol dm⁻³ | Initial mass / g | Final mass / g | Mass change / g | % change |
| 0.0 | 2.45 | 2.62 | +0.17 | +6.94 |
| 0.2 | 2.48 | 2.55 | +0.07 | +2.82 |
| 0.4 | 2.50 | 2.51 | +0.01 | +0.40 |
| 0.6 | 2.47 | 2.32 | −0.15 | −6.07 |
| 0.8 | 2.51 | 2.28 | −0.23 | −9.16 |
| 1.0 | 2.49 | 2.20 | −0.29 | −11.65 |
Record any anomalous results that appear out of sequence and consider repeating that particular concentration if time permits. Note room temperature at the start and end, because temperature fluctuations can affect the rate of osmosis and water potential values.
记录任何有顺序偏差的异常结果，如时间允许应考虑对该浓度进行重复。记录实验开始和结束时的室温，因为温度波动会影响渗透速率和水势数值。
8. Calculating Percentage Change in Mass | 计算质量变化百分比
The percentage change in mass is preferred over absolute mass change because it normalises differences in initial cylinder mass, allowing fair comparison across samples. The formula used is:
采用质量变化百分比而非绝对质量变化更为合理，因为这将初始圆柱体质量的差异标准化，使不同样品之间能够公平比较。所用公式为：
% change = (final mass − initial mass) / initial mass × 100
g yields a mass loss of 0.15 g, giving a percentage change of (−0.15 / 2.47) × 100 = −6.07% l solution), while negative values indicate net water loss (hypertonic external solution).
例如，一个初始质量2.47 g、最终质量2.32 g的圆柱体，质量减少了0.15 g，因此质量变化百分比为(−0.15 / 2.47) × 100 = −6.07%。正值表明净吸水（外部溶液低渗），负值则表明净失水（外部溶液高渗）。
9. Graphical Analysis | 图像分析
Plot a graph of percentage change in mass (y-axis) against sucrose concentration (x-axis). Conventionally, concentration is the independent variable and the percentage change is the dependent variable. Draw a line of best fit, which is typically a smooth curve or a straight line that shows a downward trend. The point where the curve crosses the x-axis (0% mass change) represents the isotonic concentration at which the water potential of the external solution equals that of the potato tissue.
绘制质量变化百分比(y轴)对蔗糖浓度(x轴)的曲线图。习惯上，浓度是自变量，质量变化百分比是因变量。画一条最佳拟合线，通常是一条向下倾斜的光滑曲线或直线。曲线与x轴相交的点(质量变化为0%)代表等渗浓度，在该浓度下外部溶液的水势等于马铃薯组织的水势。
If you prefer a more quantitative fit, calculate the linear regression for the data points and solve for the x-intercept. However, many potato osmosis data sets exhibit slight curvature, so a polynomial trendline or a second-order fit may be more accurate. In your analysis, explain why linear extrapolation can still be used within the steepest range near the intercept.
如果你更倾向于量化拟合，可以计算数据点的线性回归并求解x轴截距。然而，许多马铃薯渗透实验的数据集会呈现轻微曲线，因此多项式趋势线或二阶拟合可能更为精确。在分析中，你需要解释为何在靠近截距的最陡范围仍可使用线性外推。
10. Determining Water Potential from the Graph | 从图像确定水势
Once the isotonic concentration (C) is read from the graph, use a standard table or calibration curve that relates sucrose concentration to solute potential at the experimental temperature. For instance, at 20 °C, a 0.3 mol dm⁻³ sucrose solution has a solute potential of approximately −0.83 MPa. The water potential of the potato tissue is therefore equal to this value, assuming no contribution from pressure potential in the experimental setup.
一旦从图上读出等渗浓度(C)，就利用在实验温度下蔗糖浓度与溶质势关系的标准表或校准曲线。例如，在20 °C时，0.3 mol dm⁻³蔗糖溶液的溶质势约为−0.83 MPa。因此，马铃薯组织的水势就等于该数值，前提是实验装置中压力势的贡献为零。
Ψₚₒₜₐₜₒ = Ψₛ (isotonic) ≈ −0.83 MPa
Report your value with appropriate units and compare it to published potato tuber water potentials, which typically range from −0.5 to −1.0 MPa depending on variety and turgor state.
报告数值并附上恰当的单位，并将其与公开发表的马铃薯块茎水势值进行比较，后者视品种和膨压状态通常在−0.5到−1.0 MPa之间。
11. Evaluation and Sources of Error | 评估与误差来源
Identify systematic and random errors that could affect the reliability of your results. Systematic errors include improperly calibrated balance, inaccuracies in making sucrose dilutions, and inconsistent blotting technique. Random errors arise from slight differences in potato cylinder surface area, variation in tissue composition, and timing inaccuracies. A thorough evaluation should also consider the assumption that all water uptake or loss is due solely to osmosis; evaporation and metabolic activity may introduce minor deviations.
识别可能影响结果可靠性的系统误差和随机误差。系统误差包括未校准的天平、制备蔗糖稀释液时的偏差以及不一致的吸干技术。随机误差则源于马铃薯圆柱体表面积的微小差异、组织成分的变异以及计时误差。进行深入评价时还应考虑水分摄入或散失全部由渗透作用引起的假设；蒸发和代谢活动可能引入微小偏差。
- Control the extent of blotting by using a template (e.g., two gentle presses on standard paper towel)
- 使用模板控制吸干程度(如在标准纸巾上轻按两次)
- Take multiple replicate measurements for each concentration to calculate a mean, reducing random errors
- 对每种浓度进行多次重复测量，计算平均值以减少随机误差
- Ensure the potato cylinders are completely submerged and not touching the container walls, which could affect water exposure
- 确保马铃薯圆柱体完全浸没且不接触容器壁，以免影响水分接触
12. Modifications and Further Investigations | 改进与延伸探究
To extend this investigation, consider altering the tissue type (e.g., carrot, apple) to compare water potentials or investigating the effect of temperature on the rate of mass change. You could also apply the same method to study the effect of salt concentration on plasmolysis in onion epidermis, linking it with microscopic observation. Another valuable modification is to use a non-penetrating solute such as mannitol to minimise solute uptake by cells, thus preserving the assumption of a purely osmotic effect.
若要扩展这项探究，可考虑更换组织类型(如胡萝卜、苹果)以比较水势，或探究温度对质量变化速率的影响。你也可以采用相同方法研究盐浓度对洋葱表皮细胞质壁分离的影响，并结合显微镜观察。另一项有价值的改进是使用甘露醇等非穿透性溶质，以尽量减少细胞对溶质的吸收，从而保持纯渗透作用的假设。
vides an excellent opportunity to discuss the concept of water potential units and the van' smotic pressure and i is the ionization constant. Linking practical data to theoretical models deepens understanding and strengthens A-Level synoptic skills.
最后，这项实验为讨论水势单位以及范特霍夫关系(Π = iCRT)提供了绝佳机会，其中Π为渗透压，i为电离常数。将实验数据与理论模型联系起来，能加深理解并强化A-Level的综合分析能力。
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