The Rate of Photosynthesis | 光合作用速率

📚 The Rate of Photosynthesis | 光合作用速率

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. In IGCSE Science, understanding the rate of photosynthesis and the factors that affect it is essential for explaining plant growth and crop yield.

光合作用是绿色植物、藻类和一些细菌将光能转化为储存在葡萄糖中的化学能的过程。在IGCSE科学中，理解光合作用的速率及其影响因素，对于解释植物生长和农作物产量至关重要。

1. The Basic Equation of Photosynthesis | 光合作用的基本方程式

The overall equation for photosynthesis shows carbon dioxide and water reacting in the presence of light and chlorophyll to produce glucose and oxygen. The equation is often written with word symbols, but the chemical equation is also required.

光合作用的总体方程式表明二氧化碳和水在光与叶绿素的存在下反应生成葡萄糖和氧气。该方程式常用文字表述，但化学方程式也需掌握。

Carbon dioxide + Water → Glucose + Oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note that light energy is not a substance, so it is written above or below the arrow, not as a reactant. Chlorophyll acts as a catalyst in this process.

注意光能并非物质，因此写在箭头上下方，不作为反应物。叶绿素在此过程中起催化作用。

2. Measuring the Rate of Photosynthesis | 测量光合作用速率

The rate of photosynthesis can be measured by the volume of oxygen produced per unit time, the rate of carbon dioxide uptake, or the increase in biomass (dry mass) of the plant.

光合作用速率可通过单位时间内产生氧气的体积、二氧化碳的吸收速率，或植物生物量（干重）的增加来测量。

Oxygen production – using an aquatic plant such as Elodea, counting bubbles or collecting gas in a syringe.

氧气产生 – 使用水生植物如伊乐藻，数气泡或在注射器中收集气体。
Carbon dioxide uptake – using a pH indicator like hydrogencarbonate solution; as CO₂ is removed, the solution becomes more alkaline (pink).

二氧化碳吸收 – 使用碳酸氢盐溶液等pH指示剂；随着CO₂被吸收，溶液变得更碱性（粉红色）。
Biomass increase – measuring the change in dry mass of a plant over time, which requires oven-drying.

生物量增加 – 在一定时间内测量植物干重的变化，需要烘干。

3. The Pondweed (Elodea) Experiment | 伊乐藻实验

A classic IGCSE practical involves placing a piece of Elodea under a lamp and counting the number of oxygen bubbles released per minute. The distance of the lamp is changed to vary light intensity.

一个经典的IGCSE实验是将伊乐藻放在灯下，并计算每分钟释放的氧气气泡数。通过改变灯泡的距离来改变光强度。

To make results more reliable, a ruler is used to set precise distances, and the experiment is repeated at each distance to calculate a mean. The test tube may also be kept in a water bath to maintain constant temperature.

为使结果更可靠，使用刻度尺设定精确距离，并在每个距离重复实验以计算平均值。试管也可置于水浴中以保持恒温。

Light intensity ∝ 1 / (distance)²

This inverse square relationship means that doubling the distance reduces light intensity to one quarter. It is a key calculation in this topic.

这个平方反比关系意味着距离加倍，光强度降至原来的四分之一。这是本主题的关键计算。

4. Effect of Light Intensity | 光强度的影响

As light intensity increases, the rate of photosynthesis increases proportionally at first, because more photons provide more energy to drive the light-dependent reactions.

随着光强度增加，光合作用速率起初成比例增加，因为更多光子提供更多能量来驱动光依赖反应。

However, when light intensity is no longer limiting, the rate reaches a plateau. Other factors such as carbon dioxide concentration or temperature then become the limiting factors.

然而，当光强度不再成为限制因素时，速率达到平台期。此时其他因素如二氧化碳浓度或温度变为限制因素。

In a graph of rate against light intensity, the curve rises steeply then levels off. The initial linear part can be used to determine the effect of light alone.

在速率对光强度的图表中，曲线先急剧上升然后趋于平缓。初始线性部分可用于判断光单独的影响。

5. Effect of Carbon Dioxide Concentration | 二氧化碳浓度的影响

Carbon dioxide is a raw material of photosynthesis. Increasing CO₂ concentration usually increases the rate, until the plant’s enzymes or other factors limit further increase.

二氧化碳是光合作用的原料。增加CO₂浓度通常会提高速率，直到植物的酶或其他因素限制进一步的增加。

In greenhouse farming, growers often burn paraffin heaters to raise both CO₂ level and temperature, which can significantly improve crop yield.

在温室种植中，种植者常燃烧石蜡加热器以提高CO₂浓度和温度，这能显著提高作物产量。

On a graph, the shape is similar to light intensity: a steep rise followed by a plateau. The plateau occurs because light intensity or temperature is now the limiting factor.

在图表上，曲线形状与光强度类似：先陡升后平台。平台出现是因为此时光强度或温度成为限制因素。

6. Effect of Temperature | 温度的影响

Temperature affects the rate of photosynthesis through enzyme activity. The enzymes involved in photosynthesis have an optimal temperature, usually around 25–35 °C for temperate plants.

温度通过酶活性影响光合作用速率。光合作用相关酶有最适温度，温带植物通常为25–35°C左右。

As temperature increases from a low value, the rate increases because molecules move faster and more enzyme-substrate complexes form. Above the optimum, enzymes denature, causing the rate to fall sharply.

当温度从低温上升时，速率增加，因为分子运动加快，形成更多酶-底物复合物。超过最适温度后，酶变性，导致速率急剧下降。

Unlike light and CO₂, temperature can cause the rate to drop after the optimum, so the graph is a rising curve that peaks and then declines.

与光和CO₂不同，温度会使速率在最适点后下降，因此图形是一条上升、达到峰值后下降的曲线。

7. Limiting Factors | 限制因子

A limiting factor is a condition that slows down the rate of a reaction when it is in short supply. For photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因子是一种当供应不足时会降低反应速率的条件。对光合作用而言，主要限制因子是光强度、二氧化碳浓度和温度。

To determine which factor is limiting, scientists can increase one variable while keeping others constant and observe whether the rate changes. If the rate does not change, that factor is not limiting.

要确定哪个因子是限制性的，科学家可在保持其他变量恒定的情况下增加一个变量，并观察速率是否改变。若速率不变，则该因子不是限制性的。

This concept is tested with data interpretation questions, especially when graphs show plateaus.

这一概念常通过数据解读题测试，尤其是图形显示平台时。

8. Experimental Design: Investigating Light Intensity | 实验设计：探究光强度

To investigate the effect of light intensity on the rate of photosynthesis, a student can use the following method:

为了探究光强度对光合作用速率的影响，学生可以使用以下方法：

Place a piece of Elodea in a large test tube filled with water and a small amount of sodium hydrogencarbonate (which supplies CO₂).

将一段伊乐藻放入装满水和少量碳酸氢钠（提供CO₂）的大试管中。
Place the test tube at a fixed distance from a lamp. Use a ruler to measure the distance.

将试管放置在距灯固定距离处，用刻度尺测量距离。
Count the number of oxygen bubbles produced in one minute. Repeat three times and calculate the mean.

计算一分钟内产生的氧气气泡数。重复三次并计算平均值。
Repeat at different distances, for example 10 cm, 20 cm, 30 cm and 40 cm.

在不同距离下重复，例如10 cm、20 cm、30 cm和40 cm。
Convert distance to relative light intensity using 1/d².

使用1/d²将距离转换为相对光强度。

All other variables must be kept constant, including temperature, CO₂ concentration, the size of the Elodea and the angle of the lamp.

所有其他变量必须保持恒定，包括温度、CO₂浓度、伊乐藻的大小和灯的角度。

9. Data Analysis and Graph Skills | 数据分析与绘图技能

When plotting rate (y-axis) against light intensity (x-axis), the graph should show a straight line through the origin at low intensities, then curving to a plateau. The plateau indicates that light is no longer the limiting factor.

绘制速率（y轴）对光强度（x轴）的曲线时，图形应在低强度时呈通过原点的直线，随后弯曲至平台。平台表明光不再是限制因子。

To calculate the mean rate, use the formula:

计算平均速率时，使用公式：

Rate = number of bubbles ÷ time (in minutes)

When a table of raw data is given, check units and convert to relative light intensity before interpreting. Doubling the distance does not halve the light intensity; it quarters it.

当给出原始数据表时，先检查单位，并在解释前转换为相对光强度。距离加倍不会使光强度减半，而是减为四分之一。

10. Common Errors and Exam Tips | 常见错误与考试要点

Error: Confusing the products and reactants. Remember oxygen comes from water, not from carbon dioxide.

错误：混淆产物和反应物。记住氧气来自水，而不是二氧化碳。
Error: Measuring bubbles as “number per minute” without repeating; this gives unreliable results.

错误：测量“每分钟气泡数”但不重复；这会导致结果不可靠。
Error: Ignoring temperature control; a warm lamp can heat the water, increasing the rate independently of light.

错误：忽视温度控制；灯泡发热会加热水，从而独立于光而增加速率。
Tip: Always state “light intensity is proportional to 1/d²” when explaining graph shapes.

要点：解释图形形状时，务必说明“光强度与1/d²成正比”。
Tip: Use the term “limiting factor” when the rate plateaus, and explain which factor is limiting based on the experiment conditions.

要点：当速率达到平台时使用“限制因子”一词，并根据实验条件解释哪个因子是限制性的。

11. Summary | 总结

The rate of photosynthesis is controlled by light intensity, carbon dioxide concentration and temperature. Understanding how to measure this rate and interpret experiments is a core skill for IGCSE Science revision.

光合作用速率受光强度、二氧化碳浓度和温度的控制。理解如何测量该速率并解读实验，是IGCSE科学复习的核心技能。

Remember the classic limiting factor graph shapes, the inverse square law for light, and the need to control variables in practical work. With these tools, you can confidently solve photosynthesis exam questions.

记住经典的限制因子图形形状、光的平方反比定律，以及在实验操作中控制变量的必要性。掌握这些工具，你就能自信地解答光合作用考试题。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

The Rate of Photosynthesis | 光合作用速率