Plant Tropisms And Hormones Flashcards
Animals responding to changes in environmental conditions
Are able to move to where conditions are more favourable and nutrients are available.
A stable internal environment is maintained in the face of change external conditions.
Nervous and hormonal systems promote internal stability.
Plants responding to changes in environmental conditions
Must stay where the seed germinates.
Must get their requirements from their immediate environment.
Plant growth and reproduction is aligned with the seasons.
Plants have no nervous system, hormone carry the messages.
What is a hormone?
A chemical produced by cells in one part of an organism and transported through the organism to where it acts.
Hormones….
Act as intercellular messengers.
In animals will only affect a specific target group cells.
Hormonal effects are slower than nervous responses.
Hormones in plants are responsible for….
Phototropism
Geotropism
Apical dominance- inhibition of lateral (side) branches
Growth of the entire plant- germination, fruits, flowers
Ripening of fruit- conversion of starches to sugars
Abscission- shedding of leaves and flowers
Plants respond to the environment in the following ways…
Sensing light
Sensing gravity
Sensing touch
Sensing light
Phototropism, where a shoot grows towards light, is a response to blue light. The pigment carotene is likely to be included.
Sensing gravity
Geotropism, shoots are negatively geotropic: they grow away from the pull of gravity and therefore upwards, out of the soil. In contrast, roots are positively geotropic: they grow towards the pull of gravity.
Sensing touch
Thigmotropism refers to growth responses that occur as a result of contact with a surface. The cooling and bending of tendrils around objects, as in pea plants and vines, is the result of thigmotropism.
Define: phototropism
Change in the direction of plant growth in response to light.
Define: geotropism
The growth of roots in response to the pull of gravity.
What are auxins?
Auxins are plant hormones that promote growth of new shoots, trigger positive phototropism and negative geotropism of shoots and are responsible for apical dominance.
What is apical dominance?
Where the main central stem of a plant is dominant over the other stems.
Gibberellins
Promote cell elongation. Growth of the entire plant. Promote cell division, flowering in some plants, fruit enlargement and seed germination.
Cytokinins
Are plant hormones that stimulate cell division and differentiation.
Abscisic acid
Is a growth-inhibiting hormone that controls a plants response to stress and dormancy.
Ethylene
Increases cellular respiration and the processes with fruit ripening.
What is homeostasis?
Is the Maintenance of a constant stable internal environment, despite the fluctuations in the external environment, within narrow limits.
That is the tissue fluid around the cells needs to remain in a relatively constant state.
Eg: body temperature in humans is maintained between the temperatures of 36.5-37.5.
What are the advantages of homeostasis?
It has survival value because it means an animal can adapt to a changing environment. (Deal with temperature differences)
The body will attempt to maintain a norm. Eg: body temperature at 37 degrees.
What are the disadvantages of homeostasis?
It can only work within tolerable limits.
Extreme conditions can disable the negative feedback mechanism.
In these instances, death can result, unless medical treatment is executed to bring about the natural occurrence of these feedback mechanisms.
What is a negative feedback system?
A control system where the response produced due to a particular stimulus reduces the size of the original disturbance; which leads to homeostasis.
What does a negative feedback system involve?
The nervous or hormonal systems, or both acting together, to promote stability of the internal environment.
The negative feedback process is as follows:
The level of glucose in the bloodstream drops (factor decreases).
The person requires glucose in the cells to meet the demand for ATP (corrective response).
The body detects this with a particular receptor designed for this function.
These receptors release hormones (glucagon), chemical messages that initiate the start of the feedback mechanism.
The hormones travel to their target tissue (liver) and initiate a corrective response.
Why is the negative feedback loop essential for survival?
It maintains homeostasis within the body.
Constant conditions are required for optimum enzyme function.
