Tropic responses in plants — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 40Tropisms

Stimuli

• Stimuli plants respond to changes in the environment such as light, water and gravity.

Speed

• Speed their responses are usually much slower than those of animals.

Tropism

Tropism a growth response towards a stimulus (positive) or away from it (negative).

Section 2 of 40Types of tropism

Stimulus	Response	Definition	Positive	Negative
Gravity	Gravitropism	Growth towards or away from gravity	Roots grow towards gravity	Shoots grow away from gravity
Light	Phototropism	Growth towards or away from the direction of light	Shoots grow towards light	Roots grow away from light

Section 3 of 40Shoots and roots

Shoots

• Shoots grow upwards, away from gravity and towards light, so the leaves can absorb sunlight.

Roots

• Roots grow downwards, towards gravity and away from light, to anchor the plant and absorb water and minerals.

Section 4 of 40Investigating phototropism

1

Set up three identical plants, A, B and C.

2

Plant A grows with light from one side only.

3

Plant B is placed on a clinostat, which slowly rotates it.

4

Plant C is grown in the dark.

Result A

• Result A the seedlings grow towards the light.

Result B

• Result B all sides get equal light, so they grow straight up.

Result C

• Result C the plant becomes tall and slender with yellowing leaves.

Section 5 of 40Investigating gravitropism

1

Put damp cotton wool in two petri dishes.

2

Place three bean seedlings in each dish, with the radicle facing down, horizontal and facing up.

3

Attach one dish to a support on its side and the other to a clinostat.

4

Place both in a light-proof box for two days and observe growth.

Fixed dish

• Fixed dish all roots grow down and all shoots grow up, whichever way they started.

Clinostat dish

• Clinostat dish roots and shoots grow straight outwards, with no gravitropic response.

Light-proof box

• Light-proof box it cancels the effect of light on growth.

Section 6 of 40Auxin

Auxin

Auxin a growth hormone made in the tips of growing shoots and roots that controls the direction of growth.

Spread

• Spread it diffuses from the tip to other parts.

Cell elongation

• Cell elongation in shoots auxin stimulates the cells behind the tip to elongate, and more auxin means faster growth.

Meristem

• Meristem only the region behind the tip contributes to growth by cell division and elongation.

Section 7 of 40Phototropism in shoots

1

Light shines mainly on one side of the shoot.

2

Auxin from the tip concentrates on the shaded side.

3

The cells on the shaded side elongate faster than those on the lit side.

4

Unequal growth bends the shoot towards the light.

• Even light
  • when light shines all around, auxin is spread evenly and the shoot grows straight.

Section 8 of 40Experiment on auxin and light

Seedling	Treatment	Result	Conclusion
A	Tips removed	No growth in length	Auxin is made in the tip and is needed for cell elongation
B	No light reaches the tips	Grow evenly	Light affects the auxin distribution
C	More light on one side of the tips	Shaded side grows faster	Uneven auxin makes the shoot grow towards the light

Section 9 of 40Gravitropism in shoots and roots

Shoots

• Shoots auxin collects on the lower side, which makes that side grow faster so the shoot grows upwards.

Roots

• Roots auxin collects on the lower side but higher auxin slows cell elongation there, so the upper side grows faster and the root bends downwards.

Section 10 of 40More from the specification

More from the specification

Mammalian nervous system

• Mammalian nervous system the brain and spinal cord (CNS) receive impulses from receptors via sensory neurones and send impulses to effectors through motor neurones

Reflex arc

• Reflex arc receptor, sensory neurone, relay neurone, motor neurone, effector;
  • fast and automatic, such as the pupil reflex narrowing in bright light

Neurones and synapses

• Neurones and synapses electrical impulses cross synapses by neurotransmitters, so signals are fast, unlike plant tropisms

Homeostasis

• Homeostasis the nervous and hormonal systems keep blood glucose and temperature at set levels by negative feedback;
  • muscles are the effectors for shivering

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Pupil reflex

• The automatic change in pupil size in response to light.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Glucose

• Sugar that is the fuel for respiration.

Muscle

• Tissue that contracts to cause movement.

Temperature

• Average energy of the particles in an object.

Neurone

• A nerve cell that carries electrical signals.

Insulin

• The hormone that lowers blood glucose.

Impulse

• Electrical signal along a neurone

Section 11 of 40More on this topic

Term	Meaning
Vasoconstriction	Narrowing of skin blood vessels to reduce heat loss
Vasodilation	Widening of skin blood vessels to increase heat loss
Phototropism	Growth response to the direction of light
Gravitropism	Growth response to gravity
Clinostat	Apparatus that slowly rotates a plant to cancel one-sided stimuli
Auxin	Plant growth hormone that controls cell elongation

Section 12 of 40Stimuli and tropisms

Stimulus

• Stimulus plants respond to changes in light, water and gravity for survival.

Speed

• Speed plant responses are usually much slower than animal responses.

Positive response

• Positive response growth towards the stimulus.

Negative response

• Negative response growth away from the stimulus.

Stimulus	Response	Shoot	Root
Light	Phototropism	Positive	Negative
Gravity	Gravitropism (geotropism)	Negative	Positive

Section 13 of 40Survival value of tropisms

1Positive phototropism of shoots

• Positive phototropism of shoots leaves absorb more light for photosynthesis.

2Negative gravitropism of shoots

• Negative gravitropism of shoots leaves are lifted into the light.

3Positive gravitropism of roots

• Positive gravitropism of roots the plant is anchored and reaches water and mineral ions.

4Positive hydrotropism of roots

• Positive hydrotropism of roots roots grow towards water.

Section 14 of 40Plants compared with animals

Plants compared with animals

No nervous system

• No nervous system plants respond by growing, not by muscle contraction.

Control

• Control plant hormones such as auxin control the response.
  • (growth regulators)

Growth, not movement

• Growth, not movement a plant does not move, it grows.

Section 15 of 40Other plant responses

Touch

• Touch tendrils of climbing plants coil around supports.

Hydrotropism

• Hydrotropism roots grow towards moist soil.

Cause

• Cause all depend on unequal growth on opposite sides of an organ.

Time

• Time growth takes time, so responses may take hours or days.

Exception

• Exception the Venus fly trap closes quickly through changes in water pressure in cells.

Section 16 of 40Planning an investigation of a tropism

Independent variable

• Independent variable the direction of the stimulus.

Dependent variable

• Dependent variable the angle of growth after a set time.

Control variables

• Control variables species, age, temperature, moisture and the dish.

Repeats

• Repeats about ten seedlings per condition and a mean reduce the effect of individual differences.

Clinostat

• Clinostat it cancels the effect of the stimulus.

Section 17 of 40More from the specification

1Tropic Responses vs Animal Reflexes

• Plant tropisms involve slow directional growth regulated by plant hormones, contrasting with animal reflexes that use electrical impulses along neurones.

2Neural Impulse Propagation

• A sensory neurone carries an electrical impulse to the central nervous system,
• which coordinates an immediate motor impulse to an effector muscle.

3Pupil Light Reflex

• Sensory receptors in the retina detect illumination levels and send nerve impulses to reflexively constrict the pupil in bright conditions.

4Endocrine Regulation Comparison

• While plant tropisms respond slowly to environmental gradients, the human endocrine system rapidly manages blood glucose levels by releasing insulin.

5Retina

• The layer at the back of the eye with light receptors.

6Synapse

• A junction between two neurones.

7Pupil reflex

• The automatic change in pupil size in response to light.

8Ciliary muscle

• Muscle that changes the lens shape

9Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

10Negative feedback

• A control that reverses a change to restore the set point.

11Motor neurone

• A neurone carrying impulses from the CNS to an effector.

12Glucose

• Sugar that is the fuel for respiration.

13Neurone

• A nerve cell that carries electrical signals.

14Insulin

• The hormone that lowers blood glucose.

15Impulse

• Electrical signal along a neurone

16Concentration

• Amount of solute dissolved in a volume of solution.

17Homeostasis

• Maintaining a constant internal environment.

18Glycogen

• The storage carbohydrate in animals.

Section 18 of 40Auxin in the shoot

Auxin

Auxin a plant growth hormone made in the tip of the shoot.

Effect

• Effect it makes cells behind the tip elongate, and more auxin means faster elongation.

Meristem

• Meristem only the region behind the tip contributes to growth by cell division and elongation.

Section 19 of 40Response to one-sided light

1

Light shines from one side.

2

Auxin moves to the shaded side of the shoot.

3

Cells on the shaded side elongate faster than those on the lit side.

4

The shoot bends towards the light.

• Tip
  • removing it stops the response and covering it stops the response, so the tip detects light and makes auxin.

Section 20 of 40Response to gravity

Root

• Root auxin collects on the lower side of a horizontal root.
• A high auxin concentration inhibits elongation, so the lower side grows more slowly.
• The root bends downwards.

Shoot

• Shoot auxin collects on the lower side of a horizontal shoot.
• It stimulates elongation, so the shoot bends upwards.

Section 21 of 40Experiments on shoot tips

Experiment	Observation	Conclusion
Tip removed from shoot	No growth or bending	The tip makes the substance that controls growth
Tip covered with foil	Shoot grows straight	The tip senses the light
Mica sheet inserted on the shaded side	No bending	Auxin moves down the shaded side
Agar block with auxin on a cut stump	Growth resumes and bends towards the side of the block	Auxin diffuses and stimulates elongation

Section 22 of 40Reasoning in the experiments

Tip removed

• Tip removed the shoot cannot make auxin.

Tip covered

• Tip covered the shoot cannot sense the light.

Even light

• Even light auxin is evenly distributed and the shoot grows straight.

Section 23 of 40Uses of plant hormones

Selective weedkillers

• Selective weedkillers synthetic auxins make broad-leaved weeds grow out of control.

Rooting powder

• Rooting powder it helps cuttings produce roots.

Section 24 of 40More from the specification

More from the specification

Plant Tropic Responses

• Tropisms are directional growth movements where plant organs bend toward or away from environmental stimuli via differential auxin distribution, occurring without nervous or muscular systems.

The Involuntary Pupil Reflex

• In animal nervous systems, the pupil reflex automatically adjusts pupil diameter through antagonistic contractions of circular and radial iris muscles in response to changing light levels.

Effector Muscles in Reflex Arcs

• Motor neurones convey rapid electrical impulses from the central nervous system to effector muscles, generating rapid mechanical contractions that pull tissues away from danger.

Thermoregulation and Blood Glucose Homeostasis

• Complex organisms use integrated nervous and endocrine systems to maintain constant internal core temperature and blood glucose levels despite fluctuating external conditions.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Pupil reflex

• The automatic change in pupil size in response to light.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Glucose

• Sugar that is the fuel for respiration.

Muscle

• Tissue that contracts to cause movement.

Temperature

• Average energy of the particles in an object.

Neurone

• A nerve cell that carries electrical signals.

Insulin

• The hormone that lowers blood glucose.

Impulse

• Electrical signal along a neurone

Section 25 of 40Types of tropic response

Stimulus	Response	Shoot	Root
Light	Phototropism	Positive	Negative
Gravity	Gravitropism (geotropism)	Negative	Positive

Section 26 of 40Investigating phototropism

Set-up

• Set-up seedlings are placed under different lighting.

Seedling A

• Seedling A grows towards the light source.

Seedling B

• Seedling B a clinostat gives all sides equal light, so the shoot grows straight.

Clinostat

Clinostat apparatus that slowly rotates a plant.

Seedling	Treatment	Expected result
A	Tips of stems removed	No bending
B	No light reaches the tips	Grows straight up
C	More light reaches one side of the tips	Bends towards the light

Section 27 of 40Investigating gravitropism

1

Add damp cotton wool to two petri dishes.

2

Place three bean seedlings in each with the radicle horizontal, upwards or downwards.

3

Attach the second dish to a clinostat.

4

Place both in a light-proof box so the seedlings are in darkness.

5

Leave for two days and observe.

1First dish

• First dish all radicles grow downwards and all plumules grow upwards (negative gravitropism).
  • (positive gravitropism)

2Clinostat

• Clinostat gravity acts equally on all sides, so growth is not directional.

3Darkness

• Darkness it removes light as a variable.

Section 28 of 40How auxin explains the results

Auxin

• Auxin a growth hormone made in the shoot tip that makes cells behind it elongate.

Light from one side

• Light from one side auxin moves to the shaded side, so the shoot bends towards the light.

Horizontal shoot

• Horizontal shoot auxin collects on the lower side and the shoot bends upwards.

Horizontal root

• Horizontal root a high auxin concentration inhibits elongation, so the root bends down.

Section 29 of 40Planning a fair test

Same conditions

• Same conditions identical seedlings of the same species and age, the same temperature and moisture and identical dishes.

Independent variable

• Independent variable the direction of the stimulus.

Dependent variable

• Dependent variable the direction or amount of growth, measured with a protractor or ruler.

Repeats

• Repeats several seedlings per condition and a mean.

Control

• Control the clinostat removes the one-sided effect of the stimulus.

Section 30 of 40More from the specification

Organization of the nervous system

• The mammalian nervous system consists of the central nervous system and peripheral nerves containing sensory, relay,
  • (brain and spinal cord)
• and motor neurones.

Reflex actions and rapid responses

• A reflex action bypasses conscious brain processing, transmitting electrical impulses along a three-neurone reflex arc to effector muscles.
  • (such as the knee-jerk or pupil reflex)

Synaptic transmission between neurones

• Neurotransmitter molecules diffuse across synaptic clefts from presynaptic vesicles to bind receptors on postsynaptic neurone membranes, ensuring unidirectional nerve impulse transmission.

Comparison of nervous and endocrine systems

• The nervous system uses electrical impulses along neurones for rapid, short-term responses,
• whereas the endocrine system uses blood-borne hormones to regulate long-term processes such as glucose levels.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Pupil reflex

• The automatic change in pupil size in response to light.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Glucose

• Sugar that is the fuel for respiration.

Muscle

• Tissue that contracts to cause movement.

Neurone

• A nerve cell that carries electrical signals.

Insulin

• The hormone that lowers blood glucose.

Impulse

• Electrical signal along a neurone

Section 31 of 40Tropisms

Tropism

Tropism a directional growth response of a plant to a stimulus.

Positive

• Positive growth towards the stimulus.

Negative

• Negative growth away from it.

Organ	Light	Gravity
Shoot	Positive phototropism	Negative gravitropism
Root	Negative phototropism	Positive gravitropism

• Survival value
  • shoots reach light for photosynthesis and roots grow down to find water and anchor the plant.

Section 32 of 40Role of auxin in the shoot

Auxin

• Auxin made in the tips of shoots and roots, it moves through the plant and controls cell elongation.

Light from one side

• Light from one side auxin moves to the shaded side.

Shaded cells

• Shaded cells they elongate more, so the shoot bends towards the light.

Section 33 of 40Role of auxin with gravity

Horizontal shoot

• Horizontal shoot auxin accumulates on the lower side and promotes growth there, so the shoot bends upward.

Horizontal root

• Horizontal root high auxin inhibits elongation, so the lower side grows less and the root bends downward.

Section 34 of 40Evidence from experiments

1Tip removed

• Tip removed no bending towards light, because the auxin source is lost.

2Tip covered with foil

• Tip covered with foil no bending, because the tip cannot detect light.

3Agar block between tip and shoot

• Agar block between tip and shoot bending still occurs, showing a chemical diffuses.

4Mica barrier on the shaded side

• Mica barrier on the shaded side auxin cannot reach the cells, so no bending occurs.

Section 35 of 40Investigating gravitropism

1

Place cress or bean seedlings on damp cotton wool in petri dishes.

2

Orient one dish upright and another horizontally and keep both in the dark.

3

After two to three days observe the direction of shoots and roots.

4

Record that roots grow down and shoots grow up, whichever way the seed is turned.

Darkness

• Darkness it removes light as a variable.

Clinostat

• Clinostat straight growth shows the effect of gravity has been balanced.

Section 36 of 40Investigating phototropism

Control

• Control use light from all sides or a clinostat so the seedling gets no one-sided stimulus.

Measurement

• Measurement measure the angle of curvature with a protractor from photographs and calculate a mean.

Replicates and variables

• Replicates and variables use enough seedlings and control water and temperature.

Section 37 of 40Uses and other plant responses

Synthetic auxins

• Synthetic auxins used as rooting powder, as selective weedkillers and to make seedless fruit.

Hydrotropism

• Hydrotropism roots grow towards water.

Thigmotropism

• Thigmotropism climbing plants coil around supports after touch.

Slow growth responses

• Slow growth responses plant responses are produced by growth, not muscle contraction.

Section 38 of 40More from the specification

Plant Tropisms vs Animal Nervous Responses

• Plants lack an animal nervous system and respond to environmental stimuli through directional growth tropisms regulated by hormones,
• whereas animals use interconnected neurones to generate rapid behavioural reflexes.

Involuntary Reflex Responses

• An involuntary reflex arc involves rapid impulse transmission along a sensory neurone, relay neurone,
  • (such as the pupil reflex to bright light or withdrawal from pain)
• and motor neurone to protect the body from harm.

Regulation of Blood Glucose Levels by Insulin

• Pancreatic islets monitor blood glucose concentrations;
• when levels rise after a carbohydrate meal, beta cells release insulin to stimulate glucose uptake by liver and muscle cells, lowering blood glucose back to normal set points.

Comparison of Hormonal and Nervous Communication

• Nervous communication transmits electrical impulses along axons within milliseconds for immediate localized response,
• while hormones travel in the blood system to regulate sustained, widespread metabolic functions.

Plant Tropisms vs Animal Nervous Responses

• Plants lack an animal nervous system and respond to environmental stimuli through directional growth tropisms regulated by hormones,
• whereas animals use interconnected neurones to generate rapid behavioural reflexes.

Involuntary Reflex Responses

• An involuntary reflex arc involves rapid impulse transmission along a sensory neurone, relay neurone,
  • (such as the pupil reflex to bright light or withdrawal from pain)
• and motor neurone to protect the body from harm.

Regulation of Blood Glucose Levels by Insulin

• Pancreatic islets monitor blood glucose concentrations;
• when levels rise after a carbohydrate meal, beta cells release insulin to stimulate glucose uptake by liver and muscle cells, lowering blood glucose back to normal set points.

Comparison of Hormonal and Nervous Communication

• Nervous communication transmits electrical impulses along axons within milliseconds for immediate localized response,
• while hormones travel in the blood system to regulate sustained, widespread metabolic functions.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Pupil reflex

• The automatic change in pupil size in response to light.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Glucose

• Sugar that is the fuel for respiration.

Neurone

• A nerve cell that carries electrical signals.

Insulin

• The hormone that lowers blood glucose.

Impulse

• Electrical signal along a neurone

Hormone

• A chemical messenger carried in the blood.

Concentration

• Amount of solute dissolved in a volume of solution.

Section 39 of 40Key terms

Term	Meaning
Tropism	A growth response of a plant to a directional stimulus.
Phototropism	Growth in response to the direction of light.
Gravitropism	Growth in response to gravity.
Auxin	A plant growth hormone that causes cell elongation.
Clinostat	A device that slowly rotates a plant to cancel the effect of gravity or one-sided light.
Cell elongation	Existing cells getting longer.
Meristem	The growing region behind the tip.
Radicle	The embryonic root of a seedling.

Term	Meaning
Tropism	Directional growth response of a plant to a stimulus
Phototropism	Growth response to light
Gravitropism	Growth response to gravity
Hydrotropism	Growth response to water
Positive response	Growth towards the stimulus
Negative response	Growth away from the stimulus

Term	Meaning
Auxin	Plant hormone controlling cell elongation
Elongation	Cells getting longer
Meristem	Region behind the tip where growth occurs
Agar	Jelly through which a chemical can diffuse
Mica	Impermeable sheet used as a barrier
Synthetic auxin	Man-made auxin used as weedkiller or rooting powder

Term	Meaning
Radicle	Embryonic root
Plumule	Embryonic shoot
Clinostat	Apparatus that slowly rotates plants
Phototropism	Growth response to light
Gravitropism	Growth response to gravity
Auxin	Plant growth hormone

Term	Meaning
Tropism	Directional growth response to a stimulus
Auxin	Plant growth hormone controlling elongation
Hydrotropism	Growth response to water
Thigmotropism	Growth response to touch
Clinostat	Apparatus that rotates plants to cancel one-sided stimuli
Selective weedkiller	Synthetic auxin killing broad-leaved weeds

Section 40 of 40Exam technique

1

Saying auxin makes new cells divide — it makes existing cells elongate.

2

Saying shoots show positive gravitropism — they show negative gravitropism.

3

Saying auxin collects on the lit side — it collects on the shaded side.

4

Saying auxin speeds up growth in roots — high auxin inhibits root elongation.

1

Saying the plant moves towards light — it grows towards light.

2

Saying shoots are positively gravitropic — they are negatively gravitropic.

3

Giving a tropism without naming the stimulus.

4

Saying plants respond as fast as animals — plant responses are slow.

1

Saying auxin makes cells divide — it makes cells elongate.

2

Saying auxin collects on the lit side — it collects on the shaded side.

3

Saying a high auxin concentration stimulates root elongation — it inhibits it.

4

Saying the whole shoot detects light — the tip does.

1

Saying the clinostat adds gravity — it cancels one-sided gravity.

2

Doing the gravitropism experiment in light — it must be in darkness.

3

Saying plumules are the roots — plumules are the shoots, radicles are the roots.

4

Using one seedling per condition — several are needed.

1

Saying auxin moves to the lit side — it moves to the shaded side.

2

Saying roots are positively phototropic — they are usually negatively phototropic.

3

Saying plant responses use muscles — they are produced by growth.

4

Saying a foil-covered tip still senses light — it does not.