Chapter 39 Notes Plant Responses to Internal and External Signals

Concept 39.1

Messages are sent by hormones through a signal-transduction pathway - a hormone binds to a specific receptor on the cell wall - the binding stimulates the production of a secondary messenger - the secondary messenger trigger's the cell’s response to the original signal

Concept 39.1

Concept 39.2 Hormones: chemical signals that coordinate the parts of an organism - compound that is produced by one part of the body and is transported to other parts to bind to receptors in order to trigger responses

Concept 39.2 Tropism: any growth response that results in curvatures of whole plant organs toward or away from stimuli Phototropism: the growth of a shoot toward light - directs growing seedlings so photosynthesis can occur

Concept the tip of the seeding is enclosed in a sheath called the coleoptile; if the tip is removed, the coleoptile would not curve - a signal is transmitted downward from the tip of the elongating region of the coleoptile

Concept 39.2

Plant hormones help coordinate growth, development, and responses to environmental stimuli - response to a hormone depends on its relative concentration compared with other hormones - hormonal balance controls growth and development

Concept 39.2 Auxin - any chemical substance that promotes the elongation of coleoptiles - moves from the shoot tip down to the base of the plant - if concentrations are too high, auxin will inhibit growth

Concept auxin stimulates proton pumps which in turn increases voltage across the membrane and lowers the pH in the cell wall - the acidification activates enzymes called expansins which help allow the cell to grow

Concept 39.2

Cytokinins - stimulate cytokinesis, or cell division - produced in actively growing tissues; are needed to be present along with auxin for cells to divide

Concept 39.2 Cytokinins and auxin control apical dominance, the ability of the terminal bud to suppress the development of axillary buds - if the terminal bud is removed, the inhibition of axillary buds is removed and the plant becomes bushier

Concept 39.2

Gibberellins - stimulate growth in the stems and leaves, but not in the roots - stimulates cell elongation and division - the effects of gibberellins are seen when they are given to a plant that is dwarfed

Concept 39.2

Abscisic Acid - slows down growth - seed dormancy: high levels of ABA inhibit germination; only when the ABA is inactivated will the seed germinate - drought stress: when a plant begins to wilt, ABA causes stomata to close

Concept 39.2 Ethylene - produced by plants in response to environmental stresses - also occurs in fruit ripening and programmed cell death - triple response that allows plants to circumvent an obstacle

Concept 39.2