5.5.2(plant hormones)

Name the 5 hormones

• Cytokinin

• Abscisic acid

• Auxin - Indole-3-acetic acid

• Gibberellins

• Ethene

Name the roles of cytokinin

• Promote cell division

• Delay leaf senescence

• Overcome apical dominance

• Promote cell expansion

Name the roles Abscisic acid

• Inhibits seed germination and growth

• Causes stomatal closure due to low water availability

Name the roles Auxin

• Promotes cell elongation

• Inhibit growth of side-shoots

• Inhibit leaf abscission(leaf fall)

Name the roles Gibberellins

• Promote seed germination

• promotes growth of stems

Name the roles Ethene

• Promotes fruit ripening

• Promotes leaf abscission

Describe how plant hormones cause a response

• Plant hormones bind to receptors on the plasma membrane when they reach their target cells

Describe and explain what stimulates leaf loss in decidous plants occur

• In darker months, A response is triggered due to shortening of daylength

• Less photosynthesis due to less sunlight leads to light having leaves and also water can freeze in the leaf cells, damaging them

• Ethene stimulates the leaves to die and fall

• Auxin inhibits leaf loss

Describe and explain how leaf loss occurs

• The abscission layer is a layer of cells that develop at the bottom of the leaf stalk

• Ethene stimulates abscission cells to expand, breaking cell walls and cause the leaf to fall

• Auxins are produced by young leaves and inhibit leaf loss

  • As leaves get older, less auxin is produced, leading to leaf loss

Describe and explain how seed germination occurs

• As the embryo of the seed begins growing, it requires more energy

• The embryo releases gibberellins(GA), which go to the aleurone layer, which surrounds the endosperm

• Gibberellic acid stimulates the production of amylase by the aleurone layer

• Amylose causes the breakdown of starch into maltose in the endosperm

• Maltose is then broken down into glucose and used to provide energy for growing the embryo

• It is stimulated by gibberellins and is inhibited by abscisic acid

Describe and explain how stomatal closure occurs

• Plants respond to the stimulus of low water availability

• ABA binds to receptors on guard cell plasma membrane

• The receptors activate a cascade of events that result in ion channels opening

• This causes ions to move out of the guard cell which increases the water potential

• Water then moves out of the guard cells down a water potential gradient by osmosis

• Guard cells become flaccid and the stomata close

Describe what is meant by apical dominance

• Apical dominance is where the main, central stem of the plant grows more strongly than the other side stems

• This prevents side shoots from growing

Why is apical dominance useful

• It is useful as it allows sunlight to reach leaves at the bottom of the trees

  • It is important to devote energy to growing upwards so that it can get more light to undergo more photosynthesis

Describe how apical dominance occurs

• Auxins stimulate the growth of the apical bud

• Auxins inhibit the growth of the side shoots from the lateral bud

Describe and explain how stem elongations occurs

• Gibberellins stimulate the stems of the plants to grow by stem elongation