The process of translocation?? can someone please explain it to me :/

Watch
HeartBlink
Badges: 22
Rep:
?
#1
Report Thread starter 3 years ago
#1
I don't get the bit about co-transport of sucrose into sieve tube element (phloem)
So my textbook says, sucrose diffuses (facilitated) from palisade cells to companion cells, down a concentration gradient. I get this
Then H+ is pumps out of companion cells by active transport into the spaces withing its cells (don't fet why since sucrose it already in the companion cels now....)
And then they diffuse back in, along with a sucrose molecules through co-transport proteins.
This baffled me, i spent 50 minutes just trying to figure this out.
First of all, it says along with a sucrose molecules, what is the sucrose molecule doing in the spaces within cell walls/surroundings, i thought it had diffused into companion cells?? :mad2:

It should have been
1. Diffuses into companion cells
2. Diffuses into sieve tube element.

Why are companion cells needed?
Why is active transport needed?
And why is co-transport needed?
I've got my alevel in like a week and i still don't know this :cry2:
The AQA textbook is crap
0
reply
3pointonefour
Badges: 18
Rep:
?
#2
Report 3 years ago
#2
Just go with the phloem, dude
1
reply
HeartBlink
Badges: 22
Rep:
?
#3
Report Thread starter 3 years ago
#3
I looked at the MS for a question, and it says
"at source sucrose is actively transported into phloem/sieve tube element, by companion cells"
And on the side, it says accept "co transport with H ions"

I'm so confused, can someone explain to me
0
reply
HeartBlink
Badges: 22
Rep:
?
#4
Report Thread starter 3 years ago
#4
(Original post by 3pointonefour)
Just go with the phloem, dude
That's not my question
0
reply
3pointonefour
Badges: 18
Rep:
?
#5
Report 3 years ago
#5
(Original post by LunaCat)
That's not my question
Don't worry, I hope you'll implant the knowledge you've gained in your exam!
Just be yourcellf!
1
reply
dhruvap
Badges: 11
Rep:
?
#6
Report 3 years ago
#6
So this is what happens,

1) H+ ions are actively pumped out of the companion cell. So now high conc outide.
2) H+ ions diffuse back into companion cell through a co-transport protein. This brings sucrose with it.
3) Concentration of sucrose molecules is high in the companion cells, and it diffuses into the sieve tube element through the plasmodesmata
4) Water potential in the STE decreases (as you are adding a solute)
5) Water moves in from the neighbouring Xylem, this creates a high hydrostatic pressure
6) At the same time sucrose is actively being pumped into the sink (growing region)
7) So sucrose moves from source to sink along the pressure gradient.

Also water follows the sucrose and some of it goes back into the xylem.
1
reply
Deliciate
Badges: 14
Rep:
?
#7
Report 3 years ago
#7
1. H+ ions are actively transported/loaded out of the companion cell
2. The H+ ions can only diffuse back into the companion cell if they are accompanied by sucrose molecules, this is cotransport
3. The sucrose molecules diffuse out the of the companion cell into the sieve tube through the plasmodesmata & down their concentration gradient.

I do OCR but I think your textbook is incorrect or they phrased it incorrectly. The point of the H+ ions being actively loaded out of the companion cell is so it can diffuse back in (via simple diffusion) but this time it can be accompanied by sucrose molecules. This enables the sucrose molecules to then diffuse into the sieve tube.

To answer your questions...
Companion cells are needed to provide energy for the H+ ions to be actively loaded out of the companion cell in step one. We know companion cells have lots of mitochondria so this provides the ATP for this process.

Active transport is needed for the H+ ions to leave the companion cell so cotransport can occur. If active transport did not happen then the H+ ions would be unable to leave the companion cells via simple diffusion as it needs to go against its concentration gradient. Co-transport is needed so sucrose molecules can diffuse into the companion cells (via cotransport) and then diffuse out of the companion cells in to sieve tube (via simple diffusion) so it acts as a source. Without co-transport sucrose would be unable to be loaded into the sieve tube.
1
reply
NeedHelpHelpNeed
Badges: 8
Rep:
?
#8
Report 3 years ago
#8
1. H is actively transported out of companion cells (into surrounding cells which I don't think we need to know specifics of, but probably the palisade cells you mentioned) using ATP
2. A H concentration gradient is thus created by doing this; there's a high H conc. outside the cell, and low inside the companion cell.
3. This causes H to move down its concentration gradient back into the companion cells, however, it does this via facilitated diffusion through a co-transporter proteins which only 'allows' H back in if it has sucrose molecules with them, so sucrose moves into companion cells too with H .
4. Now that there is a high conc. of sucrose in the companion cells, it can diffuse through the plasmodesmata into the sieve tube elements, again, down a conc. gradient.
I think you may have misunderstood your textbook or it got it wrong, but sucrose doesn't diffuse into companion cells before the H is pumped, but it does afterwards!This is for OCR at least
1
reply
HeartBlink
Badges: 22
Rep:
?
#9
Report Thread starter 3 years ago
#9
(Original post by dhruvap)
So this is what happens,

1) H+ ions are actively pumped out of the companion cell. So now high conc outide.
2) H+ ions diffuse back into companion cell through a co-transport protein. This brings sucrose with it.
3) Concentration of sucrose molecules is high in the companion cells, and it diffuses into the sieve tube element through the plasmodesmata
4) Water potential in the STE decreases (as you are adding a solute)
5) Water moves in from the neighbouring Xylem, this creates a high hydrostatic pressure
6) At the same time sucrose is actively being pumped into the sink (growing region)
7) So sucrose moves from source to sink along the pressure gradient.

Also water follows the sucrose and some of it goes back into the xylem.
(Original post by Deliciate)
1. H+ ions are actively transported/loaded out of the companion cell
2. The H+ ions can only diffuse back into the companion cell if they are accompanied by sucrose molecules, this is cotransport
3. The sucrose molecules diffuse out the of the companion cell into the sieve tube through the plasmodesmata & down their concentration gradient.

I do OCR but I think your textbook is incorrect or they phrased it incorrectly. The point of the H+ ions being actively loaded out of the companion cell is so it can diffuse back in (via simple diffusion) but this time it can be accompanied by sucrose molecules. This enables the sucrose molecules to then diffuse into the sieve tube.

To answer your questions...
Companion cells are needed to provide energy for the H+ ions to be actively loaded out of the companion cell in step one. We know companion cells have lots of mitochondria so this provides the ATP for this process.

Active transport is needed for the H+ ions to leave the companion cell so cotransport can occur. If active transport did not happen then the H+ ions would be unable to leave the companion cells via simple diffusion as it needs to go against its concentration gradient. Co-transport is needed so sucrose molecules can diffuse into the companion cells (via cotransport) and then diffuse out of the companion cells in to sieve tube (via simple diffusion) so it acts as a source. Without co-transport sucrose would be unable to be loaded into the sieve tube.
(Original post by NeedHelpHelpNeed)
1. H is actively transported out of companion cells (into surrounding cells which I don't think we need to know specifics of, but probably the palisade cells you mentioned) using ATP
2. A H concentration gradient is thus created by doing this; there's a high H conc. outside the cell, and low inside the companion cell.
3. This causes H to move down its concentration gradient back into the companion cells, however, it does this via facilitated diffusion through a co-transporter proteins which only 'allows' H back in if it has sucrose molecules with them, so sucrose moves into companion cells too with H .
4. Now that there is a high conc. of sucrose in the companion cells, it can diffuse through the plasmodesmata into the sieve tube elements, again, down a conc. gradient.
I think you may have misunderstood your textbook or it got it wrong, but sucrose doesn't diffuse into companion cells before the H is pumped, but it does afterwards!This is for OCR at least
Thank you everyone i get it now!

And no, i didn't get it wrong at all, i said what the textbook says word by word and I'm not surprised it is wrong because this happened ALOT in this textbook which is so misleading :/ really the teachers should complain about it to AQA, as it is AQA approved... whatever that means.

I wish I did OCR :/ the textbook for AQA is so contradicting with the mark scheme as well
0
reply
3pointonefour
Badges: 18
Rep:
?
#10
Report 3 years ago
#10
(Original post by LunaCat)
Thank you everyone i get it now!

And no, i didn't get it wrong at all, i said what the textbook says word by word and I'm not surprised it is wrong because this happened ALOT in this textbook which is so misleading :/ really the teachers should complain about it to AQA, as it is AQA approved... whatever that means.

I wish I did OCR :/ the textbook for AQA is so contradicting with the mark scheme as well
Good luck! I hope great things will stem from your upcoming exam!
0
reply
macpatgh-Sheldon
Badges: 20
Rep:
?
#11
Report 3 years ago
#11
Hi young man,

1. Of course I do not know which book you use but I recommend to my AQA students the excellent books by Glenn and Susan Toole (clear, succinct and detailed but concise descriptions; beautiful lucid illustrations; bullet pointed facts that are easy to follow and remember) Looks like you are an AS student - I would strongly recommend that you get at least the A2 (year 2) edition (if you have a week to an exam, also the AS one) - btw, I don't hold any shares in OUP! .
  1. I looked at the MS for a question, and it says
    "at source sucrose is actively transported into phloem/sieve tube element, by companion cells"
    And on the side, it says accept "co transport with H ions"



2. The above from your mark scheme is correct - the sucrose does not diffuse BACK at all because in that second stage it moves (by co-transport with H+ into the SIEVE TUBE ELEMENTS NOT BACK INTO COMPANION CELLS (you are entitled to be confused! as not only is your book wrong, but so are some of the posts above)

3. Be happy that you are doing AQA - it is the board (in biology) in which it is easiest to work out exactly what is in the mark scheme (OCR is not bad, and Edexcel is abominable!)

M
1
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Poll: What factors affect your mental health most right now? Post-lockdown edition

Anxiousness about restrictions easing (21)
6.09%
Uncertainty around my education (37)
10.72%
Uncertainty around my future career prospects (40)
11.59%
Lack of purpose or motivation (47)
13.62%
Lack of support system (eg. teachers, counsellors, delays in care) (17)
4.93%
Impact lockdown had on physical health (16)
4.64%
Social worries (incl. loneliness/making friends) (37)
10.72%
Financial worries (24)
6.96%
Concern about myself or my loves ones getting/having been ill (15)
4.35%
Exposure to negative news/social media (21)
6.09%
Difficulty accessing real life entertainment (10)
2.9%
Lack of confidence in making big life decisions (35)
10.14%
Worry about missed opportunities during the pandemic (25)
7.25%

Watched Threads

View All