6.15B Explain how plant hormones control and coordinate plant growth and…

The idea

Plant hormones are chemical messengers that coordinate growth and development across the whole plant, not just at the point where they are made. Auxins are a group of these hormones produced mainly in the shoot and root tips. They spread through the plant and affect how cells elongate, so their uneven distribution bends a growing shoot or root. In a phototropism, light shining from one side causes auxin to gather on the shaded side of the shoot; cells there elongate more, so the shoot bends towards the light. In a gravitropism, gravity causes auxin to collect on the lower side; in shoots this extra auxin speeds elongation and the shoot bends upwards, while in roots the same build-up slows elongation on the lower side and the root bends downwards. This statement is examined at both Foundation and Higher tier.

What earns the marks

• States that auxins are plant hormones produced mainly in shoot and root tips and transported to where they act.
• Explains that auxin causes cells to elongate, so an uneven distribution of auxin bends the growing organ.
• Describes phototropism as a response to light in which auxin accumulates on the shaded side of a shoot, causing greater elongation there and bending towards the light.
• Describes gravitropism as a response to gravity in which auxin accumulates on the lower side, speeding shoot elongation so the shoot bends upwards and slowing root elongation so the root bends downwards.
• Identifies that plant hormones coordinate growth and development across the whole plant rather than acting only where they are produced.

Common mistakes and how to put them right

• Claims that auxin collects on the lit side of a shoot.

  Put it right by placing the higher auxin concentration on the shaded side, which produces the bend towards light.

• Assumes auxin always speeds up cell elongation.

  Put it right by noting that in roots the higher auxin on the lower side slows elongation, giving downward bending.

• Treats phototropism and gravitropism as the same stimulus.

  Put it right by naming light for phototropism and gravity for gravitropism.

• Says hormones act only at the tip where they are made.

  Put it right by explaining that auxin is transported away from the tips to affect cells elsewhere.

Exam tips

• Read the question carefully to check whether it asks about a shoot or a root, because the same auxin build-up produces opposite bending directions.
• Link each answer to the mechanism: name the stimulus, state where auxin accumulates, then state the effect on cell elongation and the resulting bend.
• Use precise terms such as 'shaded side', 'lower side' and 'elongation' rather than vague phrases like 'grows towards the light'.
• Remember this content is assessed at both Foundation and Higher tier, so practise explaining the mechanism as well as recalling the definitions.