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active loading (AQA A level biology)

what do we have to know abt active loading for mass transport in a plant? i couldn't find any questions specifically abt describing the process and different sources online were telling me different things.

normally i like to have processes written like a MS answer but since i haven't seen any for specifically active loading this is what i have:

- companion cells use ATP to actively transport H+ into surrounding cells (creating H+ conc gradient)
- cotransport of sucrose with H+ down H+ conc gradient (facilitated diffusion) into companion cell thru cotransport protein
- conc. gradient of sucrose in companion cells and sieve tube cells so sucrose diffused into sieve cells thru plasmodesmata
- reduces wp in sieve cells

am i missing anything? also for my second point im confused how smth can be both cotransport and facilitated diffusion, ik the H+ is going down a conc. gradient but the sucrose is going against its conc gradient, should i just write cotransport or would i have to say it's cotransport and facilitated diffusion? pls help ty!!!
(edited 1 year ago)
Original post by zwłoki998
what do we have to know abt active loading for mass transport in a plant? i couldn't find any questions specifically abt describing the process and different sources online were telling me different things.

normally i like to have processes written like a MS answer but since i haven't seen any for specifically active loading this is what i have:

- companion cells use ATP to actively transport H+ into surrounding cells (creating H+ conc gradient)
- cotransport of sucrose with H+ down H+ conc gradient (facilitated diffusion) into companion cell thru cotransport protein
- conc. gradient of sucrose in companion cells and sieve tube cells so sucrose diffused into sieve cells thru plasmodesmata
- reduces wp in sieve cells

am i missing anything? also for my second point im confused how smth can be both cotransport and facilitated diffusion, ik the H+ is going down a conc. gradient but the sucrose is going against its conc gradient, should i just write cotransport or would i have to say it's cotransport and facilitated diffusion? pls help ty!!!

There was a part of last year's 15 marker about describing this. I think I remember getting full marks with just active transport of H+, and diffusion of sucrose via cotransport, although I may not have mentioned that either. I think they mostly check your knowledge of the term companion cell, and then the osmosis aspect moving water in from xylem, moving sucrose from source to sink. The movement of sucrose into phloem was only worth a few marks out of a 6 marker, so they didn't want it described in detail. I hope this is reassuring, but honestly I wouldn't stress about the intricacies. What you're explaining makes sense. :smile:
Reply 2
Original post by DanielK1456
There was a part of last year's 15 marker about describing this. I think I remember getting full marks with just active transport of H+, and diffusion of sucrose via cotransport, although I may not have mentioned that either. I think they mostly check your knowledge of the term companion cell, and then the osmosis aspect moving water in from xylem, moving sucrose from source to sink. The movement of sucrose into phloem was only worth a few marks out of a 6 marker, so they didn't want it described in detail. I hope this is reassuring, but honestly I wouldn't stress about the intricacies. What you're explaining makes sense. :smile:


ohhh ok i see!! tyyy :smile:

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