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AQA BIOL2~3rd June 2013~AS Biology (Now Closed)

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Reply 3180
Avatar for Chump
Chump
Original post by oak12
take care, anyone with any questions?


Hiya, I'm still not sure on this meiosis thing because originally there are 46 chromosomes in a diploid cell right? So they divide in meiosis 1, so 1 homologous chromosome in each cell but then they divide again with the chromatids seperating, producing 4 cells. However wouldn't that mean there are 11.5 chromosome in each cell, as 46/4 is that? Sorry may just be being stupid but not quite sure, and I don't want to go into the exam like that...
Reply 3181
Avatar for oak12
oak12
9
Original post by Tplox
Thank you so much
Can you also explain
Uptake of water by root hairs
Passage of water into the xylem
Movement of water up the stem in the xylem
Movement of water across the cells of a leaf




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uptake of water by root:

- mineral ions are actively transported into the xylem
- lowers water potential in the xylem
- water follows by osmosis
- thru the root hair cells, cortex cells, endodermis cells and into the xylem
- can move by symplast and apoplast pathway
- generates root pressure (small pressure that pushes on column of water in the xylem)

movement of water up the xylem:

- water vapour lost from the leaves by transpiration
- water moves from the top of the xylem into the leaves by osmosis
- this is called the transpirational pull
- this pulls up the column of water in the xylem, because the water particles stick together, this is cohesion
- water particles also stick to lignin in the wall, to prevent collapsing, this is called adhesion
- this means that as the column is being pulled up, its pulls the walls of the xylem inwards, this is tension

movement of water across the leaf:

- evaporation occurs of moist lining of spongy mesophyll cells
- water vapour builds up in air space
- then moves out thru open stomata by diffusion
- moist lining replaced by water in the spongy mesophyll cells
- this lowers the water potential of the spongy mesophyll cells, so water moves into it from the top of the xylem
- this is the transpirational pull
Reply 3182
Avatar for oak12
oak12
9
Original post by Chump
Hiya, I'm still not sure on this meiosis thing because originally there are 46 chromosomes in a diploid cell right? So they divide in meiosis 1, so 1 homologous chromosome in each cell but then they divide again with the chromatids seperating, producing 4 cells. However wouldn't that mean there are 11.5 chromosome in each cell, as 46/4 is that? Sorry may just be being stupid but not quite sure, and I don't want to go into the exam like that...


ok,

interphase - there are 46 chromosomes (2 chromatids)

end of meiosis 1 - each cell has 23 chromosomes (2 chromatids)

end of meiosis 2 - each cell has 23 chromosomes (1 chromatid)

the confusing thing is that at the end of meiosis there are 23 chromatids in each cell, but these are also called chromosomes, so you say there are 23 chromosomes​
Reply 3183
Avatar for Chump
Chump
Original post by oak12
ok,

interphase - there are 46 chromosomes (2 chromatids)

end of meiosis 1 - each cell has 23 chromosomes (2 chromatids)

end of meiosis 2 - each cell has 23 chromosomes (1 chromatid)

the confusing thing is that at the end of meiosis there are 23 chromatids in each cell, but these are also called chromosomes, so you say there are 23 chromosomes​


Ah, makes more sense now. Thank you Sir :wink:
Reply 3184
Avatar for LH78
LH78
what are the predicted topics to come up?
Reply 3185
Avatar for oak12
oak12
9
Original post by LH78
what are the predicted topics to come up?


maybe,

these haven't really appeared, so a few may:

- aspects of meiosis
- exchange in insects
- exchange in fish
- absorption in roots
- xerophytes
- conjugation
Reply 3186
Avatar for Tplox
Tplox
2
Are homologous chromosomes 2 pairs of chromosomes, or 1 chromosome?


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Reply 3187
Avatar for oak12
oak12
9
Original post by Tplox
Are homologous chromosomes 2 pairs of chromosomes, or 1 chromosome?


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homologous chromosomes are a pair of chromosomes, one maternal and the other paternal, that carry the same genes
anyone got the nelson thornes book for as bio? ok so if you go to the page where is talks about the movement of water up a stem. ignore the first paragraph about movement of water in a leaf and go to the second and third. is the second paragraph talking about water moving up the stem because of symplast and the second talking about the movement of water up the stem because of apoplast? also can someone please explain how you know standard deviations overlap and the effect of this thanks in advance!
Reply 3189
Avatar for oak12
oak12
9
Original post by Sheldon Cooper10
anyone got the nelson thornes book for as bio? ok so if you go to the page where is talks about the movement of water up a stem. ignore the first paragraph about movement of water in a leaf and go to the second and third. is the second paragraph talking about water moving up the stem because of symplast and the second talking about the movement of water up the stem because of apoplast? also can someone please explain how you know standard deviations overlap and the effect of this thanks in advance!


you can either observe when the standard deviations overlap e.g. standard deviation bars on a bar chart (like the fev1 past paper question), or you'll have to calculate it, e.g:

Group A = 15 (+- 3)
Group B = 12 (+- 1)

for Group A the spread is 12-18
for Group B the spread is 11-13

because the standard deviations overlap, it implies that the difference between the means is not significant, and may be due to chance
are you saying they overlap because they both contain the no 12? but if it was 11-12 and 12-18 there wouldn't be an overlap and therefore the difference between the means is greater and therefore more variation? does this mean the bigger the difference in the range of values obtained from the SD that theres more variation?
What's root pressure? And can you run through magnification ; like how to convert certain units
Reply 3192
Avatar for saaaarn
saaaarn
The exam's in 6 hours :afraid:
Im gna fail this!
Original post by saaaarn
The exam's in 6 hours :afraid:


I know!!! STRESS!!!

Still going over beta-glucose... should NOT be doing this at this time. But hey ho, I'm probably gonna pull an all-nighter tonight...
What time you gna sleep
Original post by Micheal123456
Im gna fail this!


That makes two of us!!! :biggrin:
Original post by Micheal123456
What time you gna sleep


Me?

NEVARRRRRR :eek::eek::eek::eek::eek:

No seriously I am staying up as late as I humanely can. If that means staying up till the exam then so be it.

You?
Reply 3198
Avatar for saaaarn
saaaarn
Original post by Micheal123456
What's root pressure? And can you run through magnification ; like how to convert certain units


Root pressure is caused by the active transport of mineral ions from root cells into the xylem, so the water potential of the xylem is lowered (more negative).
Water then moves by osmosis into the xylem.
This creates a higher pressure at the base of the xylem which causes a small pushing force from the roots
As for magnification... I think of it as 'AIM?' A=I/M.
Just measure the distance in millimetres then convert the millimetres into micrometres by x 1000. That's normally all you need to do...
I'm tired so sorry if none of this makes sense haha
Reply 3199
Avatar for saaaarn
saaaarn
Original post by Sabby888
I know!!! STRESS!!!

Still going over beta-glucose... should NOT be doing this at this time. But hey ho, I'm probably gonna pull an all-nighter

I know... Think I'm gonna get some sleep and wake up at 6 or I feel I may fall asleep whilst doing the exam :P have you done all the past papers? Try and get some sleep if you can :smile:

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