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15.2.214-15 · Homeostasis and Control

The Role of Auxin in Elongation Growth — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 29 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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15.2.2 The Role of Auxin in Elongation Growth — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how auxin causes cell elongation by acidifying the cell wall.Communication — plant hormones, or growth regulators, are responsible for most communication in plants.…

  • Growth stages
  • Proton pump
  • Gibberellin
  • Germination
  • Auxins — a class of hormone that regulates many aspects of growth, including elongation
  • IAA — indole 3-acetic acid is the principal auxin and is usually called auxin

Part 1 of 5Plant growth regulators

Communication
  • Communication plant hormones, or growth regulators, are responsible for most communication in plants.
Auxins
  • Auxins a class of hormone that regulates many aspects of growth, including elongation.
IAA
  • IAA indole 3-acetic acid is the principal auxin and is usually called auxin.

Part 2 of 5Where auxin is made and what it does

1Synthesis
  • Synthesis in the growing tips of roots and shoots, in the meristems.
2Growth stages
  • Growth stages cell division by mitosis, elongation by absorbing water, and differentiation.
3Control
  • Control auxin controls growth by elongation.

Part 3 of 5Acidification of the cell wall

1
Auxin binds to a receptor protein on the cell surface membrane.
2
ATPase proton pumps move H+ from the cytoplasm into the cell wall.
3
The cell wall becomes more acidic (lower pH).
4
Expansin proteins are activated and loosen the bonds between cellulose microfibrils.

Part 4 of 5Water uptake and elongation

1
K+ channels are stimulated to open.
2
K+ concentration in the cytoplasm increases, lowering its water potential.
3
Water enters the cell by osmosis through aquaporins.
4
The internal pressure rises.
5
The wall stretches because the expansins have loosened it.
6
The cell elongates.

Part 5 of 5More from the specification

Tactile stimulation of sensory hairs
  • Tactile stimulation of sensory hairs mechanical deflection of trigger sensory hairs inside Venus flytrap lobes generates receptor potentials triggering rapid closure.
Endosperm
  • Triploid food tissue in the seed that feeds the embryo
Midrib
  • Central rib of the leaf between the two lobes.
Sensory hair
  • Touch-detecting hair on the trap lobe.
Venus flytrap
  • Carnivorous plant with snap-closing leaf lobes.
Aleurone layer
  • Protein-rich layer around the endosperm.
Hinge cell
  • Cell at the base of the trap that expands to close it.
Action potential
  • Brief reversal of the membrane potential along an axon.
Gibberellin
  • Hormone stimulating germination and stem elongation.
Germination
  • The start of growth of a seed.
Embryo
  • A little ball of cells that forms from the first cell and develops into a baby.
Amylase
  • Enzyme that breaks starch into maltose.

Key terms

TermMeaning
AuxinPlant hormone controlling elongation growth.
IAAIndole 3-acetic acid, the main auxin.
ExpansinProtein loosening bonds between cellulose microfibrils.
Proton pumpATPase moving H+ across a membrane.
MeristemRegion of dividing cells at a growing tip.
ElongationIncrease in cell length by absorbing water.

Avoid these mistakes

1
Saying auxin makes cells divide only — it makes cells elongate.
2
Saying H+ are pumped out of the wall — they are pumped into it.
3
Saying expansins are activated at high pH — they are activated at low pH.
4
Forgetting water uptake — elongation needs water by osmosis.

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