Response to Stimuli Flashcards
what is a stimulus?
a detectable change in the internal or external environment of an organism that leads the o a response in the organism
what does responding to a change require?
detection
involves a stimulus being detected by a receptor cell
what do the different types of receptors do?
•produce electrical activity in nerve cells in réponse to stimuli
•secrète substances in response to stimuli
where do the nerve impulses sent by receptor cells travel to?
a coordinator
either - brain or spinal cord
what produces the response?
an effector
what is each receptor and effector linked to?
central coordinator
what is the sequence of events for either chemical control or nerve cells be summarised as?
stimulus
receptor
coordinator
effector
response
what is the growth response towards a stimulus referred to as?
a tropism
they can be positive or negative causing the plant to grow towards or way from the stimulus
what tropisms do plants respond to?
phototropism
gravitropism
hydrotropism
what is phototropism?
a growth response to light
what is gravitropism?
growth response to gravity
what is hydrotropism?
growth response to water
what does the growth response of plants rely on?
chemical substances that are released in response to a stimulus
these specific growth factors act in a similar way to the hormones that are found in animals
what is an example of a plant growth factor?
indoleacetic acid
IAA
which belongs to a group of substances called auxins
controlling plant elongation
where is IAA synthesised?
at the tips of roots and shoots
what is the sequence of events of the response of the shoots of flowering plants to unilateral light ?
- cells in the tip of the shoot produce IAA
which is then transported down the shoot - the IAA is initially transported evenly throughout all regions as it begins to move down the shoot
- light causes the movement of IAA from the light side to the shaded side of the shoot
- a greater concentration of IAA builds up on the shaded side of the shoot than on the light side
- as IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more
- the shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light
what happens when IAA moves into the elongating region?
it binds to protein receptors on the cell membranes
it lowers the pH by releasing hydrogen bonds
the lowered pH breaks some of the bonds found between the microfibrils in cellulose cell walls
this causes the cell wall to loosen and allows the cells to be more easily stretched when the turgid of the cells increases ( by an increase of water being stored in the vacuole)
controlling growth by elongation?
•IAA molecules bind to a receptor protein on the cell surface membrane
• IAA stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell wall (across the cell surface membrane)
• this acidifies the cell wall (lowers the pH of the cell wall)
• this activated proteins known as expansins, which loosen the bonds between cellulose microfibrils
•at the same time, potassium ion channels are stimulated to open
•this leads to an increase in potassium ion concentration in the cytoplasm, which decreases the water potential of the cytoplasm
•this causes the cell to absorb water by osmosis (water enters the cell through aquaporins) which is then stored in the vacuole
•this increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
THE CELL ELONGATES
what does phototropism affects on the plant?
shoots
top of a stem
what does the concentration of IAA determine?
the rate of cell elongation within the region of elongation
what happens if the concentration of IAA is not uniform on either side of a root or shoot?
uneven growth can occur
what is it called when the shoot grows towards the light?
positive phototropism
what is the response of a horizontally- growing root to gravity?
• cells in the tip of a root produce IAA, which is then transported along the root
• the IAA is initially transported to all sides of the root
•gravity influences the movement of IAA from the upper side to the lower side of the root
• a greater concentration of IAA builds up on the lower side of the root than on the upper side
•as IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side
• the relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards toward the force of gravity
what part of the plant does gravitropism affect?
roots
