Ch 2 · Water Uptake · O Level Biology 5090
Topic 2 of 4 in Transport in Plants — O Level Biology 5090 — 38 lessons.
Introduction
A maize plant can move several litres of water from soil to air on a hot day. Every drop follows the same route: into a root hair, across the root, up the xylem and out through the leaf. This lesson traces that pathway, shows how a dye experiment reveals it and, for candidates, explains why root hairs are shaped as they are. Make sure you can define osmosis and active transport first.
Key ideas — The pathway of water through the plant
Learn this sequence exactly as the syllabus states it:
🧑🏫 Teacher's explanation
Imagine a crowded subway station during rush hour. People (water molecules) want to move from a very crowded platform (the soil) to a less crowded corridor (the root cells) because that is where there is more 'space' or lower concentration. This movement happens naturally without anyone pushing them; they just flow toward the empty space.
📖 Definitions
🔍 Worked example — Easy
List the sequence of tissues/cells that water travels through, starting from the soil and ending in the leaf mesophyll.
🔍 Worked example — Standard
Explain why water moves from the soil into the root hair cell.
🔍 Worked example — Exam-style
A student uses a red dye to trace water movement in a plant. The dye is placed in the soil. Describe how the student can use this to prove the pathway of water through the stem.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State which tissue will be stained blue. [1] (b) Explain why the dye is found in this tissue and not in the surrounding parenchyma cells. [2] [3 marks]
Key ideas — Investigating the pathway with a stain
Results. Only the xylem vessels in each vascular bundle are stained: a ring of red dots in transverse section, continuous red lines in longitudinal section. Leaf veins and petals become tinged as the dye reaches the mesophyll.
🧑🏫 Teacher's explanation
Imagine you are drinking a bright red fruit smoothie through a straw. If you look at the straw, you can see the red liquid moving up towards your mouth. In this experiment, the plant's xylem acts exactly like that straw. We use a colored dye to 'trace' the path of the water so we can actually see where it goes.
📖 Definitions
🔍 Worked example — Easy
A student places a white carnation in water containing red dye. After 4 hours, the petals show red streaks. Identify the specific tissue responsible for this color movement and state the process driving the movement.
🔍 Worked example — Standard
A student observes a transverse section of a celery stem. They see red dots arranged in a ring. Explain why the dye is only present in these specific spots and not in the surrounding pith or cortex.
🔍 Worked example — Exam-style
In an experiment, a plant is placed in a warm, breezy room. A control plant is placed in a dark, still room. Compare the expected results regarding the speed of staining in the experimental group versus the control group.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the specific tissue that will be stained by the dye. [1] (b) Describe how the student can use a longitudinal section of the stem to confirm the direction of water movement. [2] [3 marks]
Key ideas — Why root hairs absorb so efficiently
surface area and rate of absorption. Each root hair is a long, narrow extension of a single epidermal cell, and thousands sit behind every root tip. Together they give an enormous surface area in contact with soil water.
🧑🏫 Teacher's explanation
Imagine you are trying to soak up a spill on a kitchen floor using a sponge. If you use a tiny, flat sponge, it will take a long time to absorb the liquid. However, if you use a sponge with many deep, thin crevices and folds, it can hold much more liquid and soak it up much faster. This is exactly what a root hair cell does for a plant.
📖 Definitions
🔍 Worked example — Easy
A root hair cell has a surface area of 200 mum^2. After a mutation, the cell loses its long extension, reducing its surface area to 50 mum^2. Calculate the percentage decrease in surface area.
🔍 Worked example — Standard
Explain why a root hair cell contains a high density of mitochondria.
🔍 Worked example — Exam-style
A student observes that a plant in nutrient-poor soil grows slower than one in nutrient-rich soil. Using your knowledge of root hair cells, explain how the surface area of the root hairs affects the rate of mineral uptake.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A student placed a celery stalk in red dye for four hours, then cut sections at 2 cm, 6 cm and 10 cm above the base. (a) Predict which section shows the least red colour and explain why. [2] (b) State the pathway water takes from the soil to a leaf mesophyll cell. [2] (c) Waterlogged soil contains little oxygen. Explain why plants in waterlogged soil absorb fewer mineral ions. [3]
🔍 Worked example — Easy
A student uses a red dye to track water movement. If the dye is placed in the soil, describe the sequence of tissues the dye must pass through to reach the leaf mesophyll cells.
🔍 Worked example — Standard
A plant is placed in a solution with a higher water potential than the cell sap of the root hair cells. Predict the direction of water movement and state the process.
🔍 Worked example — Exam-style
Explain why a plant with a reduced surface area of root hairs would show a decreased rate of mineral ion uptake. [3]
⚠️ Common errors
Common mistakes
Practice
Answers