Plant Hormones: Auxins, Phototropism, Gravitropism & Gibberellins
Plants coordinate and control their growth responses to directional environmental stimuli such as light (phototropism) and gravity (gravitropism or geotropism) using plant hormones called auxins. In particular, unequal distributions of auxin have opposite effects in shoots versus roots: high auxin concentrations stimulate cell elongation in shoots, but inhibit cell elongation in roots. Examiners regularly test the mechanism of shoot and root bending, the seedling growth required practical, and the commercial uses of auxins, gibberellins, and ethene in agriculture.
- Phototropism in Shoots
- A plant growth response to light direction. Auxin is produced in the shoot tip; when exposed to unilateral (one-sided) light, auxin moves to the shaded side. High auxin stimulates cell elongation on the shaded side, causing the shoot to bend towards the light (positive phototropism) to maximise light absorption for photosynthesis.
- Gravitropism (Geotropism) in Roots & Shoots
- Growth response to gravity. When a root or shoot is placed horizontally, gravity causes auxin to accumulate on the lower side. In roots, high auxin inhibits cell elongation on the lower side, so the upper side elongates faster and the root bends downwards (positive gravitropism). In shoots, high auxin stimulates elongation on the lower side, bending the shoot upwards (negative gravitropism).
- Commercial Uses of Auxins
- 1. Selective weedkillers: disrupt normal growth patterns in broad-leaved weeds, killing them while leaving narrow-leaved grass and cereal crops untouched. 2. Rooting powders: dipping plant cuttings in auxin stimulates rapid root development. 3. Tissue culture: promoting growth of cloned plant cells in nutrient agar.
Xylem vs Phloem Structure, Root/Stem Cross-Sections & Potometer
Switch between the three diagrams examiners test on GCSE Biology papers. Use Hide Labels to practice naming parts from memory.
How to score 3/3 marks on Xylem vs Phloem questions:
- 1. Direction & Substance: Xylem carries water and mineral ions upwards only (roots to leaves). Phloem carries dissolved sugars (sucrose) both up and down (translocation).
- 2. Living vs Dead Cells: Xylem vessels are dead hollow tubes with no end walls. Phloem consists of living cells with perforated sieve plates.
- 3. Energy Requirement: Transpiration pull in xylem is passive (no ATP needed). Translocation in phloem is active and relies on mitochondria in companion cells to supply ATP.
Key Concepts & Exam Definitions (6)
- Phototropism in Shoots
- A plant growth response to light direction. Auxin is produced in the shoot tip; when exposed to unilateral (one-sided) light, auxin moves to the shaded side. High auxin stimulates cell elongation on the shaded side, causing the shoot to bend towards the light (positive phototropism) to maximise light absorption for photosynthesis.
- Gravitropism (Geotropism) in Roots & Shoots
- Growth response to gravity. When a root or shoot is placed horizontally, gravity causes auxin to accumulate on the lower side. In roots, high auxin inhibits cell elongation on the lower side, so the upper side elongates faster and the root bends downwards (positive gravitropism). In shoots, high auxin stimulates elongation on the lower side, bending the shoot upwards (negative gravitropism).
- Commercial Uses of Auxins
- 1. Selective weedkillers: disrupt normal growth patterns in broad-leaved weeds, killing them while leaving narrow-leaved grass and cereal crops untouched. 2. Rooting powders: dipping plant cuttings in auxin stimulates rapid root development. 3. Tissue culture: promoting growth of cloned plant cells in nutrient agar.
- Gibberellins (Higher Tier)
- Plant hormones important in initiating seed germination. Used commercially in agriculture to: 1. End seed dormancy so seeds germinate at any time of year. 2. Promote flowering regardless of weather/day length and grow bigger flowers. 3. Increase fruit size (e.g., seedless grapes).
- Ethene in Fruit Ripening (Higher Tier)
- A gaseous hormone that controls cell division and stimulates enzymes that cause fruit to ripen. In the food industry, fruit (like bananas and tomatoes) is harvested while unripe and firm (so it is not bruised during shipping), then sprayed with ethene gas in warm warehouses just before hitting supermarket shelves.
- Germinating seedlings required practical
- Place cress/mustard seeds on damp cotton wool in Petri dishes under three light conditions: full overhead light, partial/one-sided light (box with a slit), and complete darkness. Control variables: temperature, water volume, seed species, and number of seeds. Measure seedling height daily and observe curvature.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Why do seedlings grown in complete darkness grow taller than seedlings grown in full light, yet have yellow leaves?
What happens if you cut the tip off a plant shoot or cover the tip with an opaque cap?
Explain how a horizontal root bends downwards due to gravity (4 steps).
Why is positive gravitropism advantageous to a germinating root?
State three commercial uses of gibberellins.
How is ethene used by supermarkets to reduce food waste?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain how auxin causes a plant shoot to bend towards light shining from one side, and explain how this response helps the plant survive. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Auxin is produced in the tip of the shoot and moves to the shaded side (away from the light) [1 mark]. 2. Because there is a higher concentration of auxin on the shaded side, it stimulates cells on the shaded side to elongate faster than cells on the lit side [1 mark]. 3. This unequal growth causes the shoot to bend towards the light source (positive phototropism) [1 mark]. 4. Leaves are positioned to absorb more light energy, increasing the rate of photosynthesis and glucose production for growth [1 mark].
- Q2
A germinating bean seed is pinned horizontally in a dark box. After 48 hours, the shoot has curved upwards and the root has curved downwards. Explain how auxin causes these two opposite directions of growth. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. In both the horizontal shoot and the horizontal root, gravity causes auxin to accumulate on the lower side [1 mark]. 2. Auxin has opposite effects on cell elongation in shoots compared to roots [1 mark]. 3. In the shoot, the high auxin concentration on the lower side stimulates cell elongation, so the lower side grows faster and bends the shoot upwards (negative gravitropism) [1 mark]. 4. In the root, the high auxin concentration on the lower side inhibits cell elongation [1 mark]. 5. Therefore, cells on the upper side of the root elongate faster than those on the lower side, bending the root downwards (positive gravitropism) [1 mark].
- Q3
A farmer sprays a field of wheat with a selective weedkiller containing synthetic auxins to kill dandelion weeds. Explain why the dandelions die while the wheat crop survives. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. Dandelions are broad-leaved plants, whereas wheat is a narrow-leaved cereal crop [1 mark]. 2. Because dandelions have a much larger leaf surface area, they absorb far more of the synthetic auxin than the wheat plants [1 mark]. 3. The excessive auxin causes the dandelion cells to grow at an uncontrollably rapid, unsustainable rate, exhausting their food reserves and killing the weeds [1 mark].
- Q4
Describe how a student could use Petri dishes, cotton wool, water, and cress seeds to investigate the effect of light direction on the growth of newly germinated seedlings. Include two variables that must be controlled. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Set up three Petri dishes lined with damp cotton wool and place 10 cress seeds in each dish, leaving them in a warm place to germinate [1 mark]. 2. Place Dish 1 in a cardboard box with a slit cut in one side (unilateral light), Dish 2 in full overhead light, and Dish 3 in a completely dark box [1 mark]. 3. Control variables: add the same volume of water daily to each dish, keep all dishes at the same temperature, and use seeds of the same species/age [1 mark]. 4. After 5 to 7 days, measure the height of each seedling with a ruler (using string to measure curved stems) and record the direction of shoot bending [1 mark].
Common Exam Questions (4)
How does auxin affect cell growth in shoots compared to roots?
This is the most common exam trap. In shoots, auxin stimulates (speeds up) cell elongation, so the side with more auxin grows faster and bends towards the light or upwards. In roots, auxin inhibits (slows down) cell elongation, so the bottom side with more auxin grows slower and the root bends downwards towards gravity.
Where does auxin accumulate when light shines on a shoot from one side?
Auxin is produced in the tip of the shoot and diffuses backwards. When light shines from one side, auxin accumulates on the shaded side of the shoot. The cells on the shaded side elongate faster than those on the lit side, causing the shoot to bend towards the light (positive phototropism).
Why are selective weedkillers made from synthetic auxins?
Most weeds are broad-leaved plants, whereas crops like wheat and grass are narrow-leaved. Broad-leaved weeds absorb much more of the synthetic auxin spray, causing their cells to grow uncontrollably fast until the weed runs out of nutrients and dies, leaving the narrow-leaved grass or cereal crops unharmed.
What are the commercial roles of gibberellins and ethene?
Gibberellins are plant hormones that initiate seed germination: they are used commercially to end seed dormancy (for malting barley in brewing), promote flowering throughout the year, and grow larger seedless fruit. Ethene is a hydrocarbon gas produced by plants that controls cell division and stimulates the ripening of fruit during storage and transport.
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