incredibly difficult bio question

[Black Cat.1]

I hope u guys can help
'Although the mechanism by which geotropism occurs is not fully understood, it is known to be associated with the distribution of amyloplasts. assuming that a hormone is involved , suggest a model that describes a 'substance X' and its distribution in the plant that would explain the positive geotropism of roots'.

this had me scratching my head of ages... :S

[duquesne9995]

Wikipedia might help 
http://en.wikipedia.org/wiki/Amyloplast

In the root cap (a tissue at the tip of the root) there is a special subset of cells, called statocytes. Inside the statocyte cells, some specialized amyloplasts are involved in the perception of gravity by the plant (gravitropism). These specialized amyloplasts—called statoliths—are denser than the cytoplasm and can sediment according to the gravity vector. The statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels. The gravitropic signal then leads to reorientation of auxin efflux carriers and subsequent redistribution of auxin streams in root cap and root as a whole. The changed relations in concentration of auxin leads to differential growth of the root tissues. Taken together, the root is then turning, following the gravity stimuli. They are also found in the endodermic layer of the inflorescence stem. The redistribution of auxin causes the shoot to turn in a direction opposite that of the gravity stimuli

So I think that what it is saying is that amyloplasts (which synthesise and store starch) are denser than cytoplasm so then some of them sediment at the bottom of the plant because of gravity and then this stimulates a signal so that auxin is distributed in that part too, and auxin inhibits rate of growth in root cells so then the upper part of the root grows faster and then the root bends towards gravity, showing positive geotropsim.
The 'substance X' in the question might be the auxins, which are hormones.

Hope that helps.

[Black Cat.1]

yeah i referred to the wiki page myself but it made little sense to me (at least a know now that there are 3 types of leucoplast!), but i didnt quite understand how the actual process occurred. And how would you put this as a stimulus response mechanism diagram? i have no clue

down ward flow of amyloplasts due to gravity factor-> gravitropic receptors in root cells -> production of auxins from meristematic root tissue -> differential elongation due to differing auxin (substance x) concentration...?

i dont think theyll ask this on the exam.,... the heinemann book has so much extra crap in it that noone reAlly needs.... its such a bother. Anyway, thanks for the reply, ive really been confused about this question for a while!

[lexitu]

I think it's a pretty neat question, actually Could be one of the harder exam questions where you need to apply knowledge to extra information.

Do you have access to the heinemann answers? I managed to find their response to the question if you want to have a look at it.

[Black Cat.1]

I think it's a pretty neat question, actually Could be one of the harder exam questions where you need to apply knowledge to extra information.

Do you have access to the heinemann answers? I managed to find their response to the question if you want to have a look at it.

really? that would be great lexitu! highly appreciated! i think, even if they put it on the exam, they would provide enough info to construct an appropriate stimulus-response model if necessary (although this q doesnt directly ask for one).... hahaha thanks for all the help ppl!

[lexitu]

Yes exactly, they would need to provide the background info so you wouldn't be on your own needing to search wikipedia

Heinemann's answer:

Substance X is produced or stored in amyloplasts in roots. It moves away from the direction of gravity, where it promotes the growth of cells. These cells grow longer and the root turns towards gravity.
or
Auxins in the root promote the growth of root cells. Substance X moves in the direction of gravity and inhibits cell growth so that cells grow downwards.

[curry_bro]

Yes exactly, they would need to provide the background info so you wouldn't be on your own needing to search wikipedia

Heinemann's answer:

Substance X is produced or stored in amyloplasts in roots. It moves away from the direction of gravity, where it promotes the growth of cells. These cells grow longer and the root turns towards gravity.
or
Auxins in the root promote the growth of root cells. Substance X moves in the direction of gravity and inhibits cell growth so that cells grow downwards.

hey lexitu, where exactly did u get the answers to the heinemann book? ive been looking everywhere but cant seem to find it

[lexitu]

I can't seem to remember now - they're not with the textbook and CD that comes with it?

[curry_bro]

apparently the 2012 edition had no cd, according to landmarks!!! WHAT! yeah, and theres no answers section in the textbook either... its actually misiing endocytosis from the glossary as well, after having mentioned endocytosis...

[lexitu]

Hmm that's not good! Maybe ask your teacher?

I'd be happy to post some snippets here for any difficult questions