LIFE SCIENCES · GRADE 12 · PAPER 1
Plant Hormones
Responding to the Environment: Plants | CAPS Weighting: 13 marks | Term 3
WHAT THIS TOPIC COVERS (CAPS SCOPE)
• General functions of auxins, gibberellins and abscisic acid.
• The role of auxins in geotropism and phototropism.
• The control of weeds using plant hormones.
• Plant defence mechanisms: chemicals and thorns.
1. PLANT HORMONES: THE BIG PICTURE
Plants do not have a nervous system, so they coordinate growth and respond to their environment using chemical
messengers called plant hormones (phytohormones). These hormones are produced in small quantities in one
part of the plant and transported to another part, where they change the rate of cell division, cell elongation or cell
differentiation.
KEY TERM · Plant hormone (phytohormone)
Definition: A chemical substance produced in small amounts in one part of a plant that is transported to another
part, where it regulates growth, development or a response to the environment.
Exam use: Examiners expect you to name the correct hormone AND link it to the correct effect. “A hormone made
the plant grow” earns no marks — name the hormone and describe the effect precisely.
Three hormones — three different jobs
Hormone General function
Auxins Control cell elongation. Responsible for geotropism and phototropism. Used in weed control
(synthetic auxins) and to promote rooting in cuttings.
Gibberellins Promote stem elongation (internode growth), seed germination and fruit growth. Used
commercially to increase fruit size and promote uniform germination.
Abscisic acid (ABA) The main “stress hormone”. Inhibits growth, promotes seed and bud dormancy, and causes
stomata to close under water stress.
Exam tip: Examiners often set a match-the-column or terminology question using all three hormones together in
Section A. Learn the one distinguishing keyword for each: auxin → tropisms / elongation; gibberellin → stem
elongation / germination; abscisic acid → dormancy / stomatal closure / stress.
2. AUXINS: GEOTROPISM AND PHOTOTROPISM
KEY TERM · Tropism
Definition: A directional growth response of a plant to a stimulus coming from one direction. Growth towards the
stimulus is a positive tropism; growth away from it is a negative tropism.
Exam use: “Geotropism” and “phototropism” must always be qualified as positive or negative when describing root
or shoot behaviour — this is a common lost mark.
The core mechanism (learn this once, apply it to both tropisms)
Distinction Juice | CAPS Grade 10–12 Study Notes Page 1
, DISTINCTION JUICE Life Sciences · Grade 12 · Plant Hormones
• Auxin is produced at the tip of the root or shoot.
• It moves towards the side of the root/shoot that is in the shade (phototropism) or the lower side (geotropism), so
auxin becomes unevenly distributed.
• Auxin causes cells to elongate. In shoots, a high auxin concentration increases elongation. In roots, a high
auxin concentration inhibits elongation.
• Because one side elongates faster than the other, the root or shoot bends.
Organ Stimulus Response Auxin distribution Result
Shoot Light Positive Accumulates on Shaded side elongates faster → shoot bends
(one-sided) phototropism shaded side towards light
Root Light Negative Accumulates on Shaded side inhibited → root bends away
(one-sided) phototropism shaded side from light
Shoot Gravity Negative Accumulates on Lower side elongates faster → shoot bends
(horizontal) geotropism lower side upward
Root Gravity Positive Accumulates on Lower side inhibited → root bends downward
(horizontal) geotropism lower side
Writing “auxin makes the root grow towards gravity” without explaining that auxin inhibits growth
COMMON
on the lower side of the root. In roots, more auxin means less growth — the opposite of what
MISTAKE
happens in shoots. Mixing this up is the single biggest mark-loser on this topic.
EXPLAIN-type question — answer formula
When a question asks you to explain how auxin causes a tropic response, use this sequence: Stimulus → unequal
auxin distribution → effect on cell elongation → direction of bending. Each arrow is normally worth its own mark.
EXAMPLE QUESTION
“A potted plant is placed on a windowsill and receives light from one side only. Explain how this plant will respond
over the next few days.” (4)
Answer strategy
1. Identify the response: positive phototropism of the shoot.
2. State the mechanism: auxin accumulates on the shaded side of the shoot.
3. Link to cells: this causes increased cell elongation on the shaded side.
4. State the outcome: the shoot bends/grows towards the light source.
Why this earns full marks: each numbered point matches one likely marking point in the memo. Never skip straight from
“stimulus” to “result” — the mechanism step (auxin distribution → elongation) is where most of the marks sit.
Distinction Juice | CAPS Grade 10–12 Study Notes Page 2
Plant Hormones
Responding to the Environment: Plants | CAPS Weighting: 13 marks | Term 3
WHAT THIS TOPIC COVERS (CAPS SCOPE)
• General functions of auxins, gibberellins and abscisic acid.
• The role of auxins in geotropism and phototropism.
• The control of weeds using plant hormones.
• Plant defence mechanisms: chemicals and thorns.
1. PLANT HORMONES: THE BIG PICTURE
Plants do not have a nervous system, so they coordinate growth and respond to their environment using chemical
messengers called plant hormones (phytohormones). These hormones are produced in small quantities in one
part of the plant and transported to another part, where they change the rate of cell division, cell elongation or cell
differentiation.
KEY TERM · Plant hormone (phytohormone)
Definition: A chemical substance produced in small amounts in one part of a plant that is transported to another
part, where it regulates growth, development or a response to the environment.
Exam use: Examiners expect you to name the correct hormone AND link it to the correct effect. “A hormone made
the plant grow” earns no marks — name the hormone and describe the effect precisely.
Three hormones — three different jobs
Hormone General function
Auxins Control cell elongation. Responsible for geotropism and phototropism. Used in weed control
(synthetic auxins) and to promote rooting in cuttings.
Gibberellins Promote stem elongation (internode growth), seed germination and fruit growth. Used
commercially to increase fruit size and promote uniform germination.
Abscisic acid (ABA) The main “stress hormone”. Inhibits growth, promotes seed and bud dormancy, and causes
stomata to close under water stress.
Exam tip: Examiners often set a match-the-column or terminology question using all three hormones together in
Section A. Learn the one distinguishing keyword for each: auxin → tropisms / elongation; gibberellin → stem
elongation / germination; abscisic acid → dormancy / stomatal closure / stress.
2. AUXINS: GEOTROPISM AND PHOTOTROPISM
KEY TERM · Tropism
Definition: A directional growth response of a plant to a stimulus coming from one direction. Growth towards the
stimulus is a positive tropism; growth away from it is a negative tropism.
Exam use: “Geotropism” and “phototropism” must always be qualified as positive or negative when describing root
or shoot behaviour — this is a common lost mark.
The core mechanism (learn this once, apply it to both tropisms)
Distinction Juice | CAPS Grade 10–12 Study Notes Page 1
, DISTINCTION JUICE Life Sciences · Grade 12 · Plant Hormones
• Auxin is produced at the tip of the root or shoot.
• It moves towards the side of the root/shoot that is in the shade (phototropism) or the lower side (geotropism), so
auxin becomes unevenly distributed.
• Auxin causes cells to elongate. In shoots, a high auxin concentration increases elongation. In roots, a high
auxin concentration inhibits elongation.
• Because one side elongates faster than the other, the root or shoot bends.
Organ Stimulus Response Auxin distribution Result
Shoot Light Positive Accumulates on Shaded side elongates faster → shoot bends
(one-sided) phototropism shaded side towards light
Root Light Negative Accumulates on Shaded side inhibited → root bends away
(one-sided) phototropism shaded side from light
Shoot Gravity Negative Accumulates on Lower side elongates faster → shoot bends
(horizontal) geotropism lower side upward
Root Gravity Positive Accumulates on Lower side inhibited → root bends downward
(horizontal) geotropism lower side
Writing “auxin makes the root grow towards gravity” without explaining that auxin inhibits growth
COMMON
on the lower side of the root. In roots, more auxin means less growth — the opposite of what
MISTAKE
happens in shoots. Mixing this up is the single biggest mark-loser on this topic.
EXPLAIN-type question — answer formula
When a question asks you to explain how auxin causes a tropic response, use this sequence: Stimulus → unequal
auxin distribution → effect on cell elongation → direction of bending. Each arrow is normally worth its own mark.
EXAMPLE QUESTION
“A potted plant is placed on a windowsill and receives light from one side only. Explain how this plant will respond
over the next few days.” (4)
Answer strategy
1. Identify the response: positive phototropism of the shoot.
2. State the mechanism: auxin accumulates on the shaded side of the shoot.
3. Link to cells: this causes increased cell elongation on the shaded side.
4. State the outcome: the shoot bends/grows towards the light source.
Why this earns full marks: each numbered point matches one likely marking point in the memo. Never skip straight from
“stimulus” to “result” — the mechanism step (auxin distribution → elongation) is where most of the marks sit.
Distinction Juice | CAPS Grade 10–12 Study Notes Page 2
