HKDSE Biology: Plant Hormones and Tropisms Tutor Prep Guide | Auxin Transport, Four Tropisms, Four Common Errors

2026-11-10

Tutor lesson-prep guides · Secondary Biology

Students can draw a shoot bending towards light, yet many believe the plant moved towards the lamp or that light pulled the hormone sideways. A tropism is one sentence: cells on one side elongate faster while every cell keeps its position. This lesson uses that single test to explain all four tropisms, then compresses five hormone groups into one table.

Objectives — Students can: (1) explain shoot phototropism and root geotropism with the phrase “cells on one side elongate faster” and state which side decides the direction of bending; (2) describe auxin in three steps – made at the tip, transported laterally, accumulated on the shaded side – and explain why the same concentration promotes shoots but inhibits roots; (3) match each of five plant hormone groups to one physiological role and one experiment or agricultural example.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Cell wall and elongation	Which structure must loosen before a plant cell can elongate?	The cell wall loosens; water uptake and turgor then drive elongation
Diffusion and transport	How does auxin move from the tip to tissue below?	Short distance by diffusion and polar transport
Control in experiments	What is the minimum control to show the tip senses light?	A covered tip against an uncovered tip

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Opening judgement	Ask whether cells move when a shoot bends to light	Write an answer now, compare at the end
6-18 min	One side elongates faster	Show a stem with sides of 2 mm and 6 mm	Work out the ratio and state the bending direction
18-32 min	Auxin in three steps	Made at the tip, moved laterally, accumulated on the shaded side	Retell phototropism using the three steps
32-44 min	Concentration and organs	The same concentration promotes shoots but inhibits roots	Decide the direction of root geotropism
44-56 min	Five hormones and experiment design	Fill the hormone table, then design one controlled test	Write the control and the measured quantity
56-60 min	Exit check	Collect two questions and mark at once	Complete independently

03 Teaching script (read it out)

Scene 1: A tropism is not movement, it is unequal elongation

The picture in most students heads is a plant chasing light, so they answer that the plant moves towards the source. Remove that picture in the first sentence: a plant does not move, cell positions are fixed from germination, and only the length of each cell changes. Use two side lengths: the lit side of a stem reaches 2 mm while the shaded side reaches 6 mm, so the longer side pushes the stem into a curve away from the light. The ratio is 3 times, and students should compute it themselves rather than be told. Then derive all four tropisms from one question repeated twice: which side elongates faster, and why is the hormone concentration different there? Phototropism, geotropism, hydrotropism and thigmotropism all follow. Stress that roots and shoots bend in opposite directions in geotropism even though the hormone is the same, because the same concentration has opposite effects on the two organs; that is the hook for the next scene.

Students do: draw a curved stem, label lit side and shaded side, mark the longer side and state the bending direction in one sentence.

Scene 2: Auxin in three steps: tip, lateral transport, shaded-side accumulation

Step one: the tip both senses light and makes auxin. Cover the tip and the shoot stops bending; cover the region below and it still bends. That single comparison is the backbone of every experiment question. Step two: light causes lateral transport of auxin within the tip, moving it from the lit side to the shaded side, so the shaded side ends up with a higher concentration. Light does not destroy or pull the hormone away. Step three: cells on the shaded side take up water, their walls loosen and they elongate, so the shoot curves towards the light. Add one line only: auxin loosens the cell wall; the molecular detail is not required. Then teach the concentration effect, the most common source of lost marks. The same concentration promotes shoot elongation but inhibits root elongation, so in a horizontal root the lower side has more auxin and therefore elongates less, which bends the root downwards. Finish with apical dominance as the applied case: the apical bud suppresses lateral buds, and removing it lets them grow. Give numbers – lateral buds from 12 mm to 30 mm, a difference of 18 mm – so the idea becomes measurable.

Students do: turn the three steps into a flow chart and mark which step the tip-covering experiment proves.

Scene 3: Five hormone groups: one table, one role, one example

Do not tell five separate stories. Build a four-column table: hormone, main role, experimental evidence, application or observable phenomenon. Auxin promotes cell elongation and maintains apical dominance; evidence is the oat coleoptile bending experiment; application is rooting of cuttings. Gibberellin promotes stem elongation and seed germination; evidence is a dwarf mutant regaining height after spraying, with alpha-amylase rising in the aleurone layer. Cytokinin promotes cell division and delays leaf ageing; evidence is shoot or root formation in tissue culture depending on its ratio to auxin. Abscisic acid closes stomata when water is short and maintains seed dormancy; evidence is stomatal closure in drought and after external application. Ethene ripens fruit and promotes abscission; evidence is unripe fruit softening faster beside ripe fruit. After the table, run a matching drill: five scenarios, choose one hormone each, and justify why not the other four. That justification step is what explanation questions actually test.

Students do: complete the table and add one observable phenomenon per role.

04 Question types and worked steps

Type	Example	Steps	Answer
Explanation question	Why does a shoot bend towards light?	Unequal elongation plus lateral auxin transport plus higher concentration on the shaded side	All three steps are needed; “it grows towards light” scores nothing
Direction question	Which way does a horizontal root bend?	First decide which side has more auxin, then check whether that organ is promoted or inhibited	State inhibition for roots; do not copy the shoot conclusion
Experiment design	Prove the tip is the light-sensitive region	Independent variable (what is covered), control group, measured quantity (angle or millimetres), controlled variables	No control or no measurable quantity loses the mark
Application question	Why does decapitation stimulate lateral buds? How is fruit ripened?	Hormone source removed or applied, then physiological response, then observable result	Keep each treatment tied to one hormone

Sanity check (teach this): Sanity check: the whole chapter uses one test – one side elongates faster – so students are not memorising four paragraphs; phototropism and geotropism sit side by side because the directions are opposite while the mechanism is identical, and that contrast is the real learning point; the concentration effect is compressed into “shoot and root optima differ by about two orders of magnitude” without asking for values; every experiment answer is checked for a control and a measurable quantity; the five-hormone section keeps identical columns so students compare across rows instead of memorising five blocks.

05 Independent practice (3 questions)

Question	Answer	What to watch
Direction	A horizontal seedling: which way do root and shoot bend, and why?	Check that inhibition in the root is stated
Three-step retell	Write the full mechanism of phototropism in three steps	Check that lateral transport appears
Design	Design a test showing the tip senses light	Check control, variables and measured quantity

06 Common mistakes and what to say

Mistake	Student writes	Say this
Plants move	Answering that the plant moves towards the light	Does any part actually travel? No. Only cell length changes, so say which side elongates faster.
Light pulls auxin away	Saying light destroys auxin on the lit side	Light does not drag the hormone away; the tip transports it laterally to the shaded side, and the two stories predict different experiments.
Copying the shoot into the root	Saying the lower side of a root grows faster because auxin is higher	The same concentration inhibits roots, so the direction is right but the reason is the opposite of the shoot.
Missing controls	Writing only “it stops bending when covered”	How do you know it stopped because of the cover and not because you damaged it? Where is the control and the measurement?

07 Homework (paper, section and question numbers)

Biology chapter index — All chapters for this subject
Full prep guide index — Matching chapters in other subjects
Redo this chapter — Practice section, questions 1, 3 and 5: direction, three-step retell, experiment design

08 Exit check (last 5 minutes)

Question	Answer	Decision
One test	Cells on one side elongate faster; positions never change	Saying the plant moves means returning to scene one
Three steps	Made at the tip, moved laterally, accumulated on the shaded side	Missing lateral transport means repeating scene two
Concentration effect	Shoots promoted, roots inhibited	Treating both organs the same means returning to scene three

09 Tutor FAQ

Should the acid growth hypothesis be taught?

The syllabus needs only that auxin loosens the cell wall so the cell takes up water and elongates; proton pump detail obscures the main line.

Must all four tropisms be memorised?

Phototropism and geotropism must be derivable; hydrotropism and thigmotropism need a definition and one example only.

Is apical dominance examinable?

It is a strong applied example that links directly to removing a hormone source, so it earns three minutes of lesson time.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.