How do auxins bring about positive phototropism in plants?

  1. Phototropins are enzymes found in the plasma membrane of particular cells. They are phosphorylated when hit by blue light.
    2) This phosphorylation triggers the movement of auxin across the plant shoot by active transport as transporter proteins pump the hormone out of cells.
    3) As a result, there is more auxin on the shaded side of the plant, producing a concentration gradient.
    4) Auxin promotes active transport of hydrogen ions into the cell wall by ATPase action. Where there is more auxin, more ions are brought in.
    5) The hydrogen ions lower the pH of the cell wall, reaching the optimum for the enzymes that hydrolyse bonds withincellulose fibres. The ions also disrupt hydrogen bonds between cellulose fibres. This makes the cell wall less rigid.
    6) As the cell absorbs water by osmosis, it swells and elongates. Because of the reduced rigidity of the cell wall, the cell can stretch more than cells with lower auxin concentrations and therefore more restrictive cell walls.
    7) Consequently the cells on the shaded side of the plant grow longer than those facing the light due to the difference in auxin concentration. This causes the plant to bend towards the light.
Answered by Katherine F. Biology tutor

9309 Views

See similar Biology A Level tutors

Related Biology A Level answers

All answers ▸

Explain why multicellular organisms needs to develop a specialised exchange system and transport system, yet unicellular organisms do not


Describe the problems that arise due to cystic fibrosis. Describe why cystic fibrosis occurs, with reference to its deficient gene. Describe the use of liposomes and viruses in treating cystic fibrosis


What happens in the light dependent reaction in photosynthesis?


What are the risks and benefits concerning genetic modification of crops?


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

© MyTutorWeb Ltd 2013–2024

Terms & Conditions|Privacy Policy