A level Biology: Post your doubts here!

[Lujain M.]

w08 q4
you measure the closest distance from the inner alveolar wall to the membrane of the red blood cell, which is roughly 6-8mm depending on where you measure
magnification is x2500 so 6mm or 8mm divide by 2500 = 2.4 or 3.2 micrometers
so answer is B

w10 qp11 q16
its obvious that answer is A with some physics knowledge but with the facts the question has provided
firstly it says sucrose solution with highest water potential ie. closer to 0 as water potential is generally <0.
so water will move into cell via osmosis
this means you can immediately eliminate B where the centre cells are flaccid

over all potential of plant cells in As level biology is determined by the water potential and pressure potential
the thin cell wall cells have less pressure potential so they are likely to take in more water and expand hence the only correct answer is A where the strip is flipped inside out.

Thank you a lot! Do you mind helping me with q34 nov.12 v.2?

 

[6Astarstudent]

Thank you a lot! Do you mind helping me with q34 nov.12 v.2?

An adult human has approximately 5 dm3 of blood, which circulates about once a minute.
100 cm3 of arterial blood carries 19 cm3 of oxygen. 100 cm3 of venous blood carries 12.5 cm3 of
oxygen.
What is the approximate volume in cm3 of oxygen taken up per minute in the lungs?

arterial blood indicates the oxygenated blood, and the venous blood is the deoxygenated blood
difference in volume of oxygen carried is 19 - 12.5 = 6.5 cm^3 of oxygen. <----- this oxygen is gained from the lungs
so 6.5 cm^3 of oxygen is gained every 100 cm^3 of blood solution.
adult human has a total of 5dm^3 of blood solution = 5000 cm^3 blood
so 6.5 x (5000/100) = 325

 

[Lujain M.]

An adult human has approximately 5 dm3 of blood, which circulates about once a minute.
100 cm3 of arterial blood carries 19 cm3 of oxygen. 100 cm3 of venous blood carries 12.5 cm3 of
oxygen.
What is the approximate volume in cm3 of oxygen taken up per minute in the lungs?

arterial blood indicates the oxygenated blood, and the venous blood is the deoxygenated blood
difference in volume of oxygen carried is 19 - 12.5 = 6.5 cm^3 of oxygen. <----- this oxygen is gained from the lungs
so 6.5 cm^3 of oxygen is gained every 100 cm^3 of blood solution.
adult human has a total of 5dm^3 of blood solution = 5000 cm^3 blood
so 6.5 x (5000/100) = 325

Thank you so much! Really appreciate your help!

 

[6Astarstudent]

Thank you so much! Really appreciate your help!

np if you have anymore questions don't hesitate to ask, I'm helping my sister prepare her paper 1 as well

 

[Jinosupreme]

Which of the following are included in the apoplast pathway?
1 living components
2 plant vacuoles
3 cell walls
4 xylem vessels
A 3 only
B 1 and 2 only
C 3 and 4only
D 1,2and 4only

Answer is A, why xylem vessels is not included in apoplast?

 

[6Astarstudent]

Which of the following are included in the apoplast pathway?
1 living components
2 plant vacuoles
3 cell walls
4 xylem vessels
A 3 only
B 1 and 2 only
C 3 and 4only
D 1,2and 4only

Answer is A, why xylem vessels is not included in apoplast?

apoplast pathway is the movement of water between cell walls from root to xylem.
and water moves in the xylem via mass flow of water mainly due to transpiration.

 

[Lujain M.]

Nov. 12 v.2 q12 please! How do I know which bonds exist in which?

 

[6Astarstudent]

Nov. 12 v.2 q12 please! How do I know which bonds exist in which?

answer is C

here's a short summary

primary structure = peptide bonds only, but between molecules NOT ADJACENT STRANDS (they often try to trick students with the adjacent strand Q)
secondary = hydrogen bonds only (some syllabus includes hydrostatic forces)
tertiary = hydrogen bond, covalent (disulphide links), ionic, hydrostatic forces

 

[Lujain M.]

answer is C

here's a short summary

primary structure = peptide bonds only, but between molecules NOT ADJACENT STRANDS (they often try to trick students with the adjacent strand Q)
secondary = hydrogen bonds only (some syllabus includes hydrostatic forces)
tertiary = hydrogen bond, covalent (disulphide links), ionic, hydrostatic forces

Thanks a bunch

 

[Jinosupreme]

Which statement about the light microscope is correct?
A As the smallest distance to see two points as distinct separate points decreases, the
resolution also decreases.
B If the resolution is 220 nm, then a bacterium 0.2 μm in diameter will not be visible.
C If the wavelength of light is 600 nm, then two membranes 300 nm apart will be visible as two distinct membranes.
D Using visible light of a longer wavelength, such as red light, will improve the resolution.

Answer is B, why C is wrong?

 

[6Astarstudent]

Which statement about the light microscope is correct?
A As the smallest distance to see two points as distinct separate points decreases, the
resolution also decreases.
B If the resolution is 220 nm, then a bacterium 0.2 μm in diameter will not be visible.
C If the wavelength of light is 600 nm, then two membranes 300 nm apart will be visible as two distinct membranes.
D Using visible light of a longer wavelength, such as red light, will improve the resolution.

Answer is B, why C is wrong?

theoretically its correct but practically its wrong.
light microscopes have something like a maximum of x1500 magnification
even when you have enough resolution, 300nm x 1500 = 0.45mm on the best light microscopes in the world.
so you cannot distinguish them

 

[Jinosupreme]

Anyone can help with this

http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_w12_qp_12.pdf
Q21, 23, 24, 31, 33 and 38?

21B
23A
24A
31B
33D
38C

Will really appreciate your help!

 

[6Astarstudent]

Anyone can help with this

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_w12_qp_12.pdf
Q21, 23, 24, 31, 33 and 38?

21B
23A
24A
31B
33D
38C

Will really appreciate your help!

#21
definition of gene is a sequence of DNA on a section of chromosome that has some function ie. making polypeptide chains via mRNA production
B is a description of DNA or a segment of chromosome not gene
#23
A is correct because they showed that both CUG and UUA code for the same amino acid leu

#24
1. is correct because movement of water via symplast pathway is a passive process down water potential gradient
2. is wrong because water moves up the xylem through the elongated tubes via mass flow of water (mainly due to transpiration), not symplast/apoplast
3. is the description of symplast not apoplast, so it is wrong
hence answer is A, only 1 is correct
#31
blood in the vein moves up the feet via contraction of muscles when a person moves. so if you stand really still with no muscle movement the blood will flow up really slowly, so there is an increase in pressure, so answer is B.
#33
D, only 1,3,4 because 2 (deposits of fatty material in arteries and veins) which is artheroma is not related to the heart in anyway, there are no arteries and veins in the heart so it will not cause heart failure. Heart failure can be caused by deposits of fatty material in coronary arteries however.
#38
X is removed
D feeds only on X so D will eventually disappear, or mostly die out
the unlabelled box will feed on more of the primary producer because the competitor X is gone, this leads to more unlabelled box organisms
more unlabelled box organisms means more food source for both B, C so they increase
However, A only feed on D and B and we know that D disappears so A can only feed on B so number of B will decrease
hence only C increases

 

[Jinosupreme]

#21
definition of gene is a sequence of DNA on a section of chromosome that has some function ie. making polypeptide chains via mRNA production
B is a description of DNA or a segment of chromosome not gene
#23
A is correct because they showed that both CUG and UUA code for the same amino acid leu

#24
1. is correct because movement of water via symplast pathway is a passive process down water potential gradient
2. is wrong because water moves up the xylem through the elongated tubes via mass flow of water (mainly due to transpiration), not symplast/apoplast
3. is the description of symplast not apoplast, so it is wrong
hence answer is A, only 1 is correct
#31
blood in the vein moves up the feet via contraction of muscles when a person moves. so if you stand really still with no muscle movement the blood will flow up really slowly, so there is an increase in pressure, so answer is B.
#33
D, only 1,3,4 because 2 (deposits of fatty material in arteries and veins) which is artheroma is not related to the heart in anyway, there are no arteries and veins in the heart so it will not cause heart failure. Heart failure can be caused by deposits of fatty material in coronary arteries however.
#38
X is removed
D feeds only on X so D will eventually disappear, or mostly die out
the unlabelled box will feed on more of the primary producer because the competitor X is gone, this leads to more unlabelled box organisms
more unlabelled box organisms means more food source for both B, C so they increase
However, A only feed on D and B and we know that D disappears so A can only feed on B so number of B will decrease
hence only C increases

Wonderful explanation, thank you so much!

 

[Jinosupreme]

Guys, I'm confused with what you can see using light microscope,
In one year, MJ 2012 paper 11, Q1 http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_s12_qp_11.pdf
They say we can see mitochondria and starch grain through X400 light microscope, and cannot see centriole and chromosomes

BUT in year ON 2010, paper 11 Q1
http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_w10_qp_11.pdf
They say we CAN ONLY see Chlorplast under X400 light microscope BUT NOT Lysosomes, ER, and Mitochondria.

Whereas in MJ 2010, paper 11 Q2
http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_s10_qp_11.pdf
Diagram gives a cell with mitochondria, nucleolus and nucleus and etc.
But in X400 light microscope, we can only see nucleus and nucleolus, BUT NOT mitochondria.

And in Q5 of MJ 2012, paper 11, Q5,
http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_s12_qp_11.pdf
Question say microscope has resolution of 200nm, which organelles would not be resolved,
Answer is RIBOSOMES will not be resolved.
Other options are chloroplast, lysosomes and mitochondria.
Does that means, other 3 can be resolved, means can see chloroplast, mito, and lysosomes?

Hence, doesn't it contracdicts with each other?
Can lysosomes and mitochondria be seen?

 

[6Astarstudent]

Guys, I'm confused with what you can see using light microscope,
In one year, MJ 2012 paper 11, Q1 http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_s12_qp_11.pdf
They say we can see mitochondria and starch grain through X400 light microscope, and cannot see centriole and chromosomes

BUT in year ON 2010, paper 11 Q1
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_w10_qp_11.pdf
They say we CAN ONLY see Chlorplast under X400 light microscope BUT NOT Lysosomes, ER, and Mitochondria.

Whereas in MJ 2010, paper 11 Q2
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_s10_qp_11.pdf
Diagram gives a cell with mitochondria, nucleolus and nucleus and etc.
But in X400 light microscope, we can only see nucleus and nucleolus, BUT NOT mitochondria.

And in Q5 of MJ 2012, paper 11, Q5,
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_s12_qp_11.pdf
Question say microscope has resolution of 200nm, which organelles would not be resolved,
Answer is RIBOSOMES will not be resolved.
Other options are chloroplast, lysosomes and mitochondria.
Does that means, other 3 can be resolved, means can see chloroplast, mito, and lysosomes?

Hence, doesn't it contracdicts with each other?
Can lysosomes and mitochondria be seen?

yes there are lots of flaws in the multiple choice paper, you just have to pick the most appropriate answer or rely on your own practical experiences.
In most cells and in general, mitochondria should not be seen, and lysosome is never seen.

I'll do a short summary for you, basicaly from the W10 and S10 questions mitochondria cannot be seen

for the MJ 2012, paper 11, Q5 it says which can be resolved. resolved does not equate to be seen/identified. the resolution is 200nm so you can resolve a mitochondira which is normally 1micrometer. However, the magnification is only 400x so you will see it as something not even half a millimeter through the lenses, as a small dot. You can resolute it (see the picture) but you cannot identify it.
same goes for the MJ 2010, paper 11 Q2, you cannot see mitochondria because its such a small dot.

We're lucky its multiple choice so you can get the answer via other things eg.
starch grains/nucleus/etc are always seen
RER/SER/ribosome/lysosome etc never seen

golgi apparatus and mitochondria etc are the more vague ones.

 

[Jinosupreme]

Anyone can explain

Q38
http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Biology%20(9700)/9700_w04_qp_1.pdf
Answer 38 is D. I understand why add growth, but don't understand why add other life processes and also respiratory heat losses.

Then and also
Q39
http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_w12_qp_13.pdf

Answer is D. Same question, why I need to add lost in energy to calculate my photosynthesis?

 

[Jinosupreme]

Help in question
http://olevel.sourceforge.net/papers/9700/9700_s13_qp_11.pdf
Q27, 32, 33, 34, 39.

Answer is
27B
32D
33A
34A
39A (I dont understand why is nitrogen fixation is aerobic, I thought it has nitrogenase, why need anaerobic condition?)

 

[6Astarstudent]

Anyone can explain

Q38
http://papers.xtremepapers.com/CIE/...and AS Level/Biology (9700)/9700_w04_qp_1.pdf

Answer 38 is D. I understand why add growth, but don't understand why add other life processes and also respiratory heat losses.

Then and also
Q39
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_w12_qp_13.pdf

Answer is D. Same question, why I need to add lost in energy to calculate my photosynthesis?

the only method for a plant to produce energy is photosynthesis, so energy for growth, respiratory heat loss, other life process all came from photosynthesis, hence you add all of them.

same goes for next question, the 27000 is energy plant can provide to herbivore (energy used in growth), the lost 3000 is from other processes. all these energy are made via photosynthesis so you add them.

 

[6Astarstudent]

Help in question
http://olevel.sourceforge.net/papers/9700/9700_s13_qp_11.pdf

Q27, 32, 33, 34, 39.

Answer is
27B
32D
33A
34A
39A (I dont understand why is nitrogen fixation is aerobic, I thought it has nitrogenase, why need anaerobic condition?)

do you want me to explain why each one is the answer or why the other options are wrong?