IGCSE Biology / Chemical coordination in plants
Roots and shoots responding to gravity
Edexcel International GCSE Biology (4BI1), chapter 12: Chemical coordination in plants, specification point 2.83, 2.84. Free lesson.
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Gravitropism is the way roots and shoots grow in response to gravity. Auxin gathers on their lower sides, but the organs respond differently because their cells have different sensitivities to auxin.
What gravitropism means
Gravitropism, also called geotropism, is a plant’s growth response to gravity. Gravity acts downwards on a seedling whatever way up it is lying. This helps its root grow into the soil and its shoot grow upwards.
How auxin makes roots and shoots bend
When a seedling lies on its side, auxin gathers on the lower side of both the root and the shoot. Auxin affects cell elongation, so the difference in growth between the two sides makes each organ curve. In a shoot, auxin causes cells on the lower side to elongate more. That side grows more, so the shoot curves upwards, away from gravity. In a root, auxin inhibits cell elongation on the lower side. That side grows less, so the root curves downwards, towards gravity. Growing towards gravity is positive gravitropism. Growing away from gravity is negative gravitropism. Roots…
Why auxin has opposite effects
The hormone is the same in both organs. The difference is how sensitive the cells are to it. Auxin causes elongation only within a certain range of concentration. The amount on the lower side is within the range that makes shoot cells elongate, but root cells are more sensitive to auxin. In roots, that same amount is already high enough to inhibit elongation. Shoot cells are relatively less sensitive to auxin, so the lower-side concentration makes them elongate. Root cells are more sensitive, so the same concentration inhibits their elongation. The two organs therefore curve in…
Applying the response to an upside-down seed
Even if a seed is planted upside down, gravity still acts downwards and auxin gathers on the lower side of each organ. Each responds in its usual way, so the shoot and root correct their directions as they grow. The lower side of the shoot elongates more, so the shoot curves upwards. Elongation on the lower side of the root is inhibited, so the root curves downwards. Both organs end up growing in the directions the seedling needs.