The shape of the lesson
By the end of the lesson, learners can
- Root hairs and the pathway of water: Each root hair is a long, thin outgrowth of a single cell in the outer layer of the root.
- Tracing water with a stain: To find out which tissue carries water up a plant, stand a cut celery stalk (or a white flower) in water coloured with a dye such as food colouring or eosin.
- Root hair cells: absorb water (osmosis) and mineral ions (active transport).
- Large surface area increases uptake of water and mineral ions.
- Pathway: root hair cells → root cortex cells → xylem → mesophyll cells.
- Stain test: the dye moves with the water, so it marks the xylem.
One picture every idea returns to
Root hair cells absorb water (by osmosis) and mineral ions from the soil. Their long, thin extensions give a large surface area, which increases the uptake of water and mineral ions. Water then follows the pathway root hair cells → root cortex cells → xylem → mesophyll cells. Standing a plant in a coloured dye shows that water travels up the above-ground parts in the xylem.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, From soil to leaf, cell by cell.. Every drop of water a plant uses enters through its roots. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Root hairs and the pathway of water | Each root hair is a long, thin outgrowth of a single cell in the outer layer of the root. Project tab 2 of the lab (Water uptake through root hairs) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Tracing water with a stain | To find out which tissue carries water up a plant, stand a cut celery stalk (or a white flower) in water coloured with a dye such as food colouring or eosin. Project tab 3 of the lab (Celery stalk in a coloured dye) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Tracing water with a stain, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A root hair is part of one cell, not a separate cell and not a whole root. | And the hairs increase the surface area; they do not make the water move faster through the plant. |
| The dye is carried up with the water, so it shows the xylem, not the phloem. | Say the dye "moves with the water in the xylem", not that the dye is "absorbed by the leaves". |
Water uptake: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
State two functions of root hair cells and explain how their shape helps.
Outline the pathway taken by a water molecule from the soil to a mesophyll cell in a leaf.
Plan an investigation, using a suitable stain, to show the pathway of water through the above-ground parts of a plant.
A student measured how far up a celery stalk the dye had travelled.
| Time / min | 0 | 20 | 40 | 60 |
|---|---|---|---|---|
| Distance / mm | 0 | 30 | 62 | 90 |
(a) Calculate the mean rate of movement of the dye over 60 minutes. (b) Suggest one change that would make the dye rise faster, and explain why.
Gardeners move seedlings with a ball of soil around the roots. Explain why.
Explain why a transverse slice of a celery stalk that has stood in red dye shows red dots only near the outside, while the rest of the slice is uncoloured.
Answer key and marking notes
- They absorb water from the soil (by osmosis).
- They absorb mineral ions such as nitrate and magnesium (by active transport).
- The long, thin hair gives a large surface area, which increases the rate of uptake of water and mineral ions.
- It enters a root hair cell.
- It passes through the root cortex cells into the xylem in the centre of the root.
- It travels up the stem and into the leaf in the xylem, then passes into the mesophyll cells.
- Stand a freshly cut celery stalk with leaves (or a white flower) in a beaker of water containing a dye such as food colouring or eosin.
- Leave it in the light for a few hours (warm conditions increase transpiration, so the dye rises faster).
- Look for coloured strands in the stalk and colour in the leaf veins (or petals).
- Cut thin transverse slices with a sharp blade and view with a hand lens: the coloured dots show where the xylem is.
- (a) 90 ÷ 60 = 1.5 mm per minute.
- (b) Put the plant in a warmer place, or in moving air (a fan).
- This increases the rate of transpiration, so water (carrying the dye) is pulled up the xylem faster.
- Pulling a seedling out of the soil tears off many delicate root hairs.
- This reduces the surface area for water uptake, so the seedling cannot take in enough water and wilts; the soil ball protects the root hairs.
- The dye moves only with the water, which travels in the xylem.
- The xylem is in vascular bundles arranged in a ring near the outside of the stalk, so only those bundles are stained.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Where on a young root are root hair cells found?
- AIn the outer layer, a short distance behind the root tip
- BIn the centre of the root, around the xylem
- CAt the very tip, forming a cap over the root
- DOnly on the stem, just above the soil surface
The large surface area of root hairs increases the uptake of:
- Awater and mineral ions
- Bsucrose and amino acids
- Coxygen and glucose only
- Dcarbon dioxide for photosynthesis
W = xylem, X = mesophyll cell, Y = root hair cell, Z = root cortex cell. In which order does water pass through these as it moves from the soil to a leaf?
- AY, Z, W, X
- BZ, Y, W, X
- CY, W, Z, X
- DY, Z, X, W
A celery stalk stands in red dye. Thin slices are cut at different heights after 30 minutes and after 3 hours. Which result is expected?
- ARed dots are found higher up the stalk after 3 hours than after 30 minutes
- BThe whole slice is red at every height after 30 minutes
- CRed dots are found only in the leaves after 30 minutes
- DRed appears in the centre of each slice and spreads outwards
Dye in a celery stalk rises 84 mm in 2 hours. What is the mean rate of movement of the dye in mm per minute?
- A0.7 mm per minute
- B42 mm per minute
- C1.4 mm per minute
- D168 mm per minute
By which process does water move from the soil into a root hair cell?
- AOsmosis
- BActive transport
- CTranslocation
- DTranspiration
A 1 cm length of root has a surface area of 0.3 cm2 without its root hairs and 2.4 cm2 including its root hairs. How many times greater is the surface area with root hairs?
- A8 times
- B2.1 times
- C7 times
- D0.125 times
Celery stalks were left in red dye for one hour in different conditions. The table shows how far the dye rose. Which conclusion is best supported by the results?
- AWater and dye are drawn up mainly by water loss from the leaves
- BDye rises at the same rate whatever the conditions
- CTemperature is the only factor that affects how far the dye rises
- DThe stalk itself pumps the dye up, so leaves are not needed
Quiz key
Root hair cells are in the outer layer of the root, in a zone just behind the growing tip, where they grow out into the soil. The centre of the root holds the xylem and phloem. The tip itself is protected by a root cap and has no hairs. Root hairs are part of the root, not the stem.
Root hairs absorb water and mineral ions from the soil, and their large surface area increases the uptake of both. Sucrose and amino acids are made in the plant and carried in phloem. Root hairs do take in some oxygen, but the syllabus point is about water and ions, and glucose is not absorbed from the soil. Carbon dioxide for photosynthesis enters through the leaves.
Water enters a root hair cell (Y), crosses the root cortex cells (Z), enters the xylem (W) and finally reaches the mesophyll cells (X). Z first misses the root hair entry point. Y, W, Z puts the xylem before the cortex, but the xylem is in the centre of the root. Ending with W puts the xylem after the leaf cells.
The dye travels up the xylem with the water, so the longer the stalk stands in dye, the higher the red dots are found. Only the xylem is stained, not the whole slice. The dye has to pass up the stalk before reaching the leaves. In celery the stained xylem is in bundles near the outside, not in the centre.
2 hours = 120 minutes, so rate = 84 ÷ 120 = 0.7 mm per minute. 42 is the rate per hour. 1.4 comes from dividing by 60 instead of 120. 168 comes from multiplying instead of dividing.
Water enters root hair cells by osmosis, across the partially permeable cell membrane. Active transport is used for mineral ions, not water. Translocation is the movement of sucrose and amino acids in phloem. Transpiration is water vapour loss from the leaves.
2.4 ÷ 0.3 = 8, so the root hairs make the surface area 8 times greater. 2.1 is the difference in area, not a ratio. 7 times is the increase over the original (8 − 1), not how many times greater the total is. 0.125 is the ratio the wrong way round.
Removing the leaves (4) cut the rise from 26 mm to 4 mm, and conditions that increase transpiration (warmth, wind) made the dye rise further, so the loss of water from the leaves draws water and dye up. The heights clearly differ. Wind changed the result at the same temperature (1 and 2), so temperature is not the only factor. Without leaves the dye barely rose, so the stalk is not pumping it.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.