Ch 3 · Transpiration · O Level Biology 5090
Topic 3 of 4 in Transport in Plants — O Level Biology 5090 — 48 lessons.
Introduction
Most of the water a plant absorbs is lost from its leaves as vapour. This loss, transpiration, is the engine that pulls water and mineral ions up the xylem. You need the two-step description of how water leaves a leaf, the effect of environmental factors on the rate and transpiration pull, cohesion and wilting.
Key ideas — What transpiration is
Transpiration is the loss of water vapour from leaves. It happens in two stages, and both must appear in any description:
🧑🏫 Teacher's explanation
Think of a leaf like a wet sponge sitting in a breezy, dry room. The water doesn't just jump out of the sponge; first, it has to turn into steam (evaporation) inside the tiny holes of the sponge, and then that steam has to drift out into the room (diffusion).
📖 Definitions
🔍 Worked example — Easy
Identify the two distinct processes that occur during transpiration in the order they happen.
🔍 Worked example — Standard
A student observes that a plant in a humid environment transpires slower than a plant in a dry environment. Explain why using the concept of concentration gradients.
🔍 Worked example — Exam-style
Calculate the percentage increase in transpiration rate when the temperature increases from 15^° C to 25^° C, assuming the rate increases from 0.5 mm/min to 0.8 mm/min.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Factors that affect the rate of transpiration
🧑🏫 Teacher's explanation
Imagine you are standing in a crowded room where everyone is trying to exit through a single door. If the room is very hot, people move much faster and more energetically toward that door. This is like temperature increasing the kinetic energy of water molecules, making them evaporate and diffuse more rapidly.
📖 Definitions
🔍 Worked example — Easy
A student uses a potometer to measure the volume of water taken up by a plant. At 20^° C, the volume is 5 ml in 10 minutes. At 30^° C, the volume is 8 ml in 10 minutes. Calculate the percentage increase in the rate of transpiration.
🔍 Worked example — Standard
Explain why increasing the wind speed around a plant increases the rate of transpiration.
🔍 Worked example — Exam-style
A plant is placed in a highly humid environment. Predict and explain the effect on the rate of transpiration.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Measuring transpiration with a potometer
A potometer measures the rate at which a cut shoot takes up water, which is very close to the rate of transpiration.
🧑🏫 Teacher's explanation
Imagine you are drinking through a straw. The speed at which the liquid rises in the straw depends on how hard you are sucking. In a plant, 'sucking' is actually the evaporation of water from the leaves. As water turns into vapour and leaves through the stomata, it creates a 'pull' that drags the water column upwards.
📖 Definitions
🔍 Worked example — Easy
A leafy shoot is placed in a potometer. An air bubble moves 4.5 cm in 5 minutes. Calculate the rate of water uptake in cm/min.
🔍 Worked example — Standard
A student measures the distance a bubble moves in a potometer under two conditions. At 20^°C, the bubble moves 2.0 cm in 10 minutes. At 30^°C, the bubble moves 5.0 cm in 10 minutes. Calculate the percentage increase in the rate of water uptake.
🔍 Worked example — Exam-style
A student observes that when a fan is placed near the potometer, the bubble moves faster. Explain why this occurs, referring to the concentration gradient.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) The student increased the humidity inside the container. State the effect this would have on the distance moved by the air bubble per minute. [1] (b) Explain why the rate of transpiration changes when humidity is increased. [3] [4 marks]
Key ideas — Transpiration pull, cohesion and wilting
how water climbs the xylem. As water evaporates from mesophyll cells, they draw more water from neighbouring cells and ultimately from the xylem in the leaf veins. This creates a tension at the top of each xylem vessel. Water molecules attract one another by cohesion, so the pull is transmitted down the whole column of water, which rises as a continuous thread. This force is the transpiration pull. An air bubble breaks the column, which is why shoots are cut under water.
🧑🏫 Teacher's explanation
Imagine a long, thin straw submerged in a glass of water. If you suck on the straw, you create a vacuum/low pressure at the top, and the entire column of water rises to meet your mouth. In a plant, the 'suction' is created by evaporation at the leaf surfaces.
📖 Definitions
🔍 Worked example — Easy
A student uses a potometer to measure the rate of transpiration. In 5 minutes, an air bubble moves 2.5 cm along the capillary tube. Calculate the rate of transpiration in cm/min.
🔍 Worked example — Standard
Explain why a plant stem might wilt during a period of intense sunlight and high temperature.
🔍 Worked example — Exam-style
A plant is moved from a dark room to a bright, windy outdoor area. Predict the effect on the transpiration rate and explain the mechanism of water movement that will be increased.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain how this environment affects the rate of transpiration. [2] (b) Describe how the loss of water from the leaves leads to the wilting of the plant stem. [3] [5 marks]
Worked example
Question. A student used a potometer in still air and with a fan. The bubble moved 12 mm in 5 minutes in still air and 31 mm in 5 minutes with the fan. (a) Calculate the rate of uptake with the fan in mm per minute. [1] (b) Explain why the rate was higher with the fan. [3] (c) Explain how loss of water from the leaves causes water to move up the xylem. [3]
🔍 Worked example — Easy
A plant's water uptake was measured using a potometer. The air bubble moved 25 mm in 10 minutes. Calculate the rate of transpiration in mm min^-1.
🔍 Worked example — Standard
A student observes that a plant wilts when the temperature increases from 20circC to 35circC and the humidity decreases. Explain the effect of these environmental changes on the rate of transpiration.
🔍 Worked example — Exam-style
Explain the mechanism of 'transpiration pull' and how it relates to the structure of the xylem. [3 marks]
⚠️ Common errors
Common mistakes
Practice
Answers