15. Coordination and Response in Plants

15.1 Coordination and Response in Plants

Open the complete PDF with diagrams and page layout

Page 1

 Cambridge (CIE) O Level

Biology
15.1 Coordination & Response in Plants
Contents
 Tropisms

Page 2

 Tropisms
Gravitropism & Phototropism
Tropisms
Plants Plants can respond to changes in the environment (stimuli) for survival, e.g. light, water,

 gravity
Their responses Their responses are usually much slower than animals
 They grow either towards a stimulus (known as a positive response) or away from a

 stimulus (known as a negative response)
The responses The responses are known as tropisms
 Stimulus Name of Definition Positive Negative

 response response response
 Light Phototropism Growth towards or Growth towards Growth away

 away from the light source from the light

 direction of the light (e.g. by shoots) source (e.g.

 source roots)
 Water Hydrotropism Growth towards or Growth towards Growth away

 away from water water (e.g. by from water (e.g.

 roots) shoots)
 Gravity Gravitropism Growth towards or Growth towards Growth away

 away from the source of from the source

 source of gravity gravity (e.g. by of gravity (e.g.

 roots) shoots)
 It is very important to a plant that its roots and shoots grow in the right directions
Shoots Shoots must grow upwards, away from gravity and towards light, so that leaves are able

 to absorb sunlight
 This means that shoots have a positive phototropic response and a negative

 gravitropic response
Roots Roots need to grow downwards into the soil, away from light and towards gravity, in

 order to anchor the plant and absorb water and minerals from the soil particles.
 This means that roots have a negative phototropic response and a positive

 gravitropic response
Plant Gravitropism Diagram

Page 3

 Placing a plant on its side shows the gravitropic responses (also known as geotropic

 responses)
AuxinsChemical Control of Tropisms
 Plants respond to stimuli by producing a growth hormone called auxin which controls

 the direction of growth of roots or stems
Therefore we say plants Therefore we say plants control their growth chemically
Auxin Auxin is mostly made in the tips of the growing stems and roots and can diffuse to other

 parts of the stems or roots;spreading from a high concentration in the shoot tips down

 the shoot to an area of lower concentration.
 This is an important point.
Only the region behind the tip of a shoot is able to contribute

 to growth by cell division and cell elongation.
This part of a shoot is called the

 meristem.
Auxin stimulates the cells behind the tip to elongate (get larger);the more

 auxin there is, the faster they will elongate and grow
 If light shines all around the tip, auxin is distributed evenly throughout and the cells in the

 meristem grow at the same rate – this is what normally happens with plants growing

 outside
 When light shines on the shoot predominantly from one side though, the auxin produced

 in the tip concentrates on the shaded side, making the cells on that side elongate and

Page 4

 grow faster than the cells on the sunny side
 This unequal growth on either side of the shoot causes the shoot to bend and grow in

 the direction of the light
Auxin and Phototropism Responses
 Positive phototropism in plant shoots
 Auxin plays a role in a plants response to gravity, affecting plant shoots and roots in

 different ways
 When shoots grow away from gravity it is known as negative geotropism
 Gravity modifies the distribution of auxin so that it accumulates on the lower side of

 the shoot
 As seen in the phototropic response, auxin increases the rate of growth in shoots,

 causing the shoot to grow upwards
 When roots grow towards gravity it is known as positive geotropism
 In roots, higher concentrations of auxin results in a lower rate of cell elongation
 The auxin that accumulates at the lower side of the root inhibits cell elongation
 As a result, the lower side grows at a slower rate than the upper side of the root
 This causes the root to bend downwards
 Examiner Tips and Tricks

Page 5

 You should be able to explain the results of an experiment like this into the effect of

 light on the growth of shoots.
Investigating Tropisms
Investigating Phototropisms
Three identical plants Three identical plants are set up as shown below (A, B and C)

Page 6

Page 7

 Investigating the phototropic response
 The seedlings in A grow towards the light source
 In B the effect of the light only coming from one direction has been cancelled out by

 using a clinostat
A clinostat A clinostat is a device that revolves slowly and repeatedly, so the shoots are evenly

 exposed to light
 This means all sides of the seedlings get equal exposure to light so they do not curve

 towards the light source but grow straight up
 In C the seedlings grow straight up looking for light and the plant becomes tall and

 slender with yellowing leaves due to the lack of light
Investigating Gravitropism

Page 8

 Investigating the gravitropic response (set-up)
Add some damp cotton wool to two Petri dishes
Place 3 bean seedlings in the cotton wool in each petri dish
 A – radicle facing downwards
 B – horizontally
 C – radicle (root grows from here) facing upwards
Cover each dish with a lid
Attach one petri dish to a support so that it’s on its side

Page 9

Attach the second petri dish to a clinostat (as shown in the diagrams above).
Place both in a light-proof box (so that the seedlings are in complete darkness), leave

for two days and then observe the growth of the seedlings
 Investigating the gravitropic response (results)
In the first petri dish, all radicles (roots) have grown downwards (positive gravitropic

response) regardless of which way they were initially facing (horizontal, up or down) and

all plumules (shoots) have grown upwards (negative gravitropic response)
In the second petri dish, all radicles and all plumules have all grown neither up nor down

but straight outwards in whichever direction they were placed as the effect of gravity

has been cancelled out by the revolving of the clinostat – they have shown no

gravitropic response at all
The experimentThe experiment needs to be done in a lightproof box to cancel out the effect of light on

the growth of the seedlings

🌳 Student Search Tree & Frequently Asked Exam Questions: coordination and response in plants

Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).

Q1: How to solve coordination and response in plants examination questions step-by-step?

1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.

Q2: What are the common mistakes and experimental sources of error for coordination and response in plants?

Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.

🔍 Related Student Searches & Problem Trees for coordination and response in plants:

Where can I find coordination and response in plants past paper questions by topic with the mark scheme?

On DABOURHUB every coordination and response in plants question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.

What is the most common coordination and response in plants exam question and why do students lose marks on it?

The repeated pattern on Cambridge papers is a short stem that asks you to apply coordination and response in plants and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each coordination and response in plants item here is written as a full-mark answer.

Is there an alternative to Save My Exams or Physics and Maths Tutor for coordination and response in plants?

Yes: the coordination and response in plants notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.

How do I revise coordination and response in plants in two weeks before the exam?

Three passes: read the coordination and response in plants notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.