# OCR Biology A Exam Thread (Breadth - May 26, 2016 and Depth - June 7, 2016) watch

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1. (Original post by skwonc)
Thanks but what on earth is chi squared? googled and still dont get it..
It is a test to see if there is a difference between the observed frequency and the expected frequency.
for example, it is expected that 25% of the population have blue/green eyes while 75% have brown eyes. then i collect data and find out that 38% of my class have blue/green eyes and the rest brown. i would then use the chi squared formula (which i forgot) but if the value i get is higher than the critical value then i can say that there IS a difference between the observed and expected frequencies.
2. (Original post by Desouza123)
It says it under the maths section
Yes but it doesn't say it anywhere in the modules area, does it?
3. (Original post by 4nonymous)
Yes but it doesn't say it anywhere in the modules area, does it?
Nah, I'm not going to bother learning it
4. (Original post by Desouza123)
Nah, I'm not going to bother learning it
I'vve not been taught it so I'm not gonna either
5. Is the squash slide similar to a wet mount? When preparing a slide for light microscopy..
6. You don't need to know the chi test, stop confusing people by saying you do.. jeez
7. Is transpiration this?

Transpiration is the loss of water from the stomata. Water is able to enter the plant as the plant has a higher solute concentration than the surrounding dirt, meaning the dirt will have a higher water conc compared to plant and the water can move in by osmosis. Once in the plant, it will move through either the apoplast pathway (cell walls and intercellular spaces) or by the symplast pathway (continuous cytoplasm) either way once the pathway's reach the endodermis there is a layer of suberin (casparian strip) which blocks the water from entering the xylem in the apoplast. So water in apoplast moves to symplast. There is a continuous flow of water going UP the xylem due to the strong cohesive forces between water molecules. Once the water evaporates from the mesophyll layer the water will leave the plant through the stomata, lowering the water potential meaning water can continuously move by osmosis into the mesophyll cell. And then I would talk about the stomato being open more in the day-time etc..

Or have I gone into too much detail?
8. Wow. Doing some really mean exam styled questions 'Human genomes contain many more genes than bacterial geonmes and they are much longer' Discuss the way in which this affects the packing of DNA in eukaryotes and prokaryotes. The mark scheme is just gibberish to me

prokaryotic genes arranged into operons; number of genes controlled together; reduces space needed for control elements; single chromosome;
9. (Original post by alevelnerd123)
Is transpiration this?

Transpiration is the loss of water from the stomata. Water is able to enter the plant as the plant has a higher solute concentration than the surrounding dirt, meaning the dirt will have a higher water conc compared to plant and the water can move in by osmosis. Once in the plant, it will move through either the apoplast pathway (cell walls and intercellular spaces) or by the symplast pathway (continuous cytoplasm) either way once the pathway's reach the endodermis there is a layer of suberin (casparian strip) which blocks the water from entering the xylem in the apoplast. So water in apoplast moves to symplast. There is a continuous flow of water going UP the xylem due to the strong cohesive forces between water molecules. Once the water evaporates from the mesophyll layer the water will leave the plant through the stomata, lowering the water potential meaning water can continuously move by osmosis into the mesophyll cell. And then I would talk about the stomato being open more in the day-time etc..

Or have I gone into too much detail?
That's fine, but rather than saying the soil has a higher water potential it's better for the examiner to see that you say that the roots have a lower water potential...
if u get me
10. (Original post by Carrot_Cake_13)
Wow the doing some really mean exam styled questions 'Human genomes contain many more genes than bacterial geonmes and they are much longer' Discuss the way in which this affects the packing of DNA in eukaryotes and prokaryotes. The mark scheme is just gibberish to me

prokaryotic genes arranged into operons; number of genes controlled together; reduces space needed for control elements; single chromosome;
your first 9 words are gibberish
11. (Original post by Littleonezz)
your first 9 words are gibberish
Yeah let me edit 'the' and it would make sense.
12. (Original post by alevelnerd123)
Is transpiration this?

Transpiration is the loss of water from the stomata. Water is able to enter the plant as the plant has a higher solute concentration than the surrounding dirt, meaning the dirt will have a higher water conc compared to plant and the water can move in by osmosis. Once in the plant, it will move through either the apoplast pathway (cell walls and intercellular spaces) or by the symplast pathway (continuous cytoplasm) either way once the pathway's reach the endodermis there is a layer of suberin (casparian strip) which blocks the water from entering the xylem in the apoplast. So water in apoplast moves to symplast. There is a continuous flow of water going UP the xylem due to the strong cohesive forces between water molecules. Once the water evaporates from the mesophyll layer the water will leave the plant through the stomata, lowering the water potential meaning water can continuously move by osmosis into the mesophyll cell. And then I would talk about the stomato being open more in the day-time etc..

Or have I gone into too much detail?
If it asks about transpiration just talk about how water enters leaf from xylem & evaporates from the spongy mesophyll which increases the water vapour concentration inside the leaf compared to outside which results in water vapour diffusing down the concentration gradient out of the stomata. If it asks for transpiration stream, then you talk about the casparian strip etc.
13. I think gas exchange in insects and transcription/translation will come up
14. (Original post by brodingoson)
That's fine, but rather than saying the soil has a higher water potential it's better for the examiner to see that you say that the roots have a lower water potential...
if u get me
Yeah okay!

And to the other person, my bad. I just meant transpiration and transpiration stream all together because it took me ages to understand it all lol..
15. hey guys, i know this is the first time the paper is being done, but does anyone have any rough predictions of grade boundaries for the depth paper?
16. (Original post by andy135)
hey guys, i know this is the first time the paper is being done, but does anyone have any rough predictions of grade boundaries for the depth paper?
67% for an A
17. What topics do you think will come up on this exam tomorrow?
18. (Original post by andy135)
hey guys, i know this is the first time the paper is being done, but does anyone have any rough predictions of grade boundaries for the depth paper?
We haven't done the paper yet so don't even know the level of difficulty of the paper... Impossible to comment..
19. (Original post by Normy18)
What topics do you think will come up on this exam tomorrow?
Literally the whole spec.. I think we might have a 6 marker on one of:
Transpiration
Transcription and translation
DNA replication
Cardiac cycle

On the breadth there wasn't much on
Diffusion
Active transport
Meiosis and mitosis (other than 1 multiple choice question)
Plant defence mechanisms (callose and chemicals)
20. By the way, does anyone have any tips on time management? Should I do the 6 markers first and then the rest of the paper or what?

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