A level Biology: Post your doubts here!

[mehria]

okay but stillf precence of sucrose makes solute potential high in phloem ?
AM I right ?

yes

 

[joanna omar]

PP1 doubts In

june 10 (v1) ---->Q22,Q28,Q33

sumeru mehria

 

[Karim Nabil]

but i think most of sucrose is converted to starch in phloem sieve tube cells...that is then unloaded to the sink/storage organs

sucrose is actively loaded to the phloem sieve tubes, this lowers water potential so water enters to the tubes by osmosis creating a high hydrostatic pressure, therefore sucrose moves in mass flow from sieve tubes near to source to tubes near to sink, then it is diffused to the sink as sucrose,as solute potential in sieve tubes now is less negative, water move out from sink cells to the sieve tubes by osmosis maintaining pressure gradient between source and sink,hence this allows sucrose to unloaded from source to sink, at the sink, an enzyme called invertase hydrolyses sucrose to glucose and fructose , this now can be used for respiration or stored as starch,as a result sucrose concentration decrease in sink. thus maintaining concentration gradient for sugar uptake

 

[Karim Nabil]

Ahh...you are really confusing me........a highly concentrated solution has lowest solute potential, which means that it has more solute molecules and less water molecules.....that's why concentrated solutions are higly negative...

look sumeru, first of all, the more negative the value, the more solute, therefore water potential is lesser and so on
example: solution A has water potential of -250KPa, solution B has water potential of -400KPa
now analyse your data as following:

• -250=less negative,then less solute then more water
• -400=more negative,then more solute, then more water

 

[sumeru]

look sumeru, first of all, the more negative the value, the more solute, therefore water potential is lesser and so on
example: solution A has water potential of -250KPa, solution B has water potential of -400KPa
now analyse your data as following:

• -250=less negative,then less solute then more water
• -400=more negative,then more solute, then more water

Well, thank you very much for that information, but according to our Cambridge's course book , the lower the solute potential.(solute's potential not concentration)
the ,lower the water potential....and this seem's quite confusing.........

 

[sumeru]

look sumeru, first of all, the more negative the value, the more solute, therefore water potential is lesser and so on
example: solution A has water potential of -250KPa, solution B has water potential of -400KPa
now analyse your data as following:

• -250=less negative,then less solute then more water
• -400=more negative,then more solute, then more water

And according to our book, lower solute potential means high concentration of solutes....so the sign gets more negative and water potential decreases.........so, yes this is now pretty confusing.......

 

[Snowysangel]

yes

What now I'm confused. Sucrose increases the concentration of the solute, so the solute potential should become more negative and hence lower, shouldn't it?

 

[asma tareen]

Andconcentraing to our book, lower solute potential means high concentration of solutes....so the sign gets more negative and water potential decreases.........so, yes this is now pretty confusing.......

I think you mean low water potential means high concentration of solute :/

 

[Karim Nabil]

And according to our book, lower solute potential means high concentration of solutes....so the sign gets more negative and water potential decreases.........so, yes this is now pretty confusing.......

ok i will ask my teacher, she will be more specific than us, and then post it here with details

 

[mehria]

What now I'm confused. Sucrose increases the concentration of the solute, so the solute potential should become more negative and hence lower, shouldn't it?

read ths.. hope it helps n clear all of ur confusion.... we knw that sucrose will mve frm a region f higher conc to a region of low conc..that's the reaason y it moves from companion cells to sieve tube cells through plasmodesmata...

 

[♣♠ Magnanimous ♣♠]

read ths.. hope it helps n clear all of ur confusion.... we knw that sucrose will mve frm a region f higher conc to a region of low conc..that's the reaason y it moves from companion cells to sieve tube cells through plasmodesmata...

this site is awesome.. thanks for sharing ..

 

[mehria]

this site is awesome.. thanks for sharing ..

u're welcum...i've already uploaded the whole AS Biology syllabus...

 

[joanna omar]

read ths.. hope it helps n clear all of ur confusion.... we knw that sucrose will mve frm a region f higher conc to a region of low conc..that's the reaason y it moves from companion cells to sieve tube cells through plasmodesmata...

Okay this is clear to all of us but the confusion is on the water potentail and how sucrose is affecting :/

 

[sumeru]

Okay this is clear to all of us but the confusion is on the water potentail and how sucrose is affecting :/

ok, at which part are you actully confused?

 

[mehria]

Okay this is clear to all of us but the confusion is on the water potentail and how sucrose is affecting :/

if there is more sucrose then solution will be hyperactive n hyperactive means less water molecules n less water potential...
we define solute potential as the tendency of solute to lower the water potential....

 

[joanna omar]

if there is more sucrose then solution will be hyperactive n hyperactive means less water molecules n less water potential...
we define solute potential as the tendency of solute to lower the water potential....

Yeah thats true thank you so much

 

[joanna omar]

ok, at which part are you actully confused?

PP1 doubts In
june 10 (v1) ---->Q22,Q28,Q33

 

[mehria]

Yeah thats true thank you so much

my pleasure

 

[mehria]

PP1 doubts In
june 10 (v1) ---->Q22,Q28,Q33

for Q22=> in semi conservative method the bacteria is first grown in a culture medium containing N15.. ther r left there till all of the bacteria contains N15 then it is transfered to N14 but here not all of them r left to grow for longer tym... in ths question some of them r kept for 50min, some for 100min n so on.... when these r taken out n put inside a confugratng machine then it shows that some bacteria contain N15 as they will b at the bottom n some will have N14( at the top)
so the correct answer is C

Q33=> X is representng inpiratory reserve volume n Y is representing expiratory reserve volume... Vital capacity is the maximum air breathed IN n OUT.... so for moderate exercise neither X will increase nor Y... as the volume of air breathed in n out will be lesser than vital capacity of lungs

 

[joanna omar]

my pleasure

thank you loads
Nov 2010 V1 ---> Q38 and 39