VCE Biology Question Thread

[TheBigC]

I agree with the answers above but do you have to talk about accuracy or precision for this? I feel like this is referring more to validity than accuracy. I did a similar prac and talked about accuracy when I compared my experimental data to the actual 'optimal temperature' of the enzyme (which was found online). You can talk about precision by looking at scatter when you draw a line of best fit on your graph.

If questions request that you discuss accuracy and precision, then yes, though in your case validity refers to whether or not the experiment conducted actually tests the hypothesis being predicted. Altering temperature by differing intervals does not affect the validity of the experiment unless the intervals are unable to sufficiently provide comparable data from which an optimal temperature can be reasonably observed (i.e. it is hypothesised that the optimal temp. is 37 degrees celsius. If you only test temperatures of 100 degrees celsius and 0 degrees celsius, the experiment will be invalidated as you did not directly or closely test the hypothesis).

[vox nihili]

Well the radioactive carbon dioxide would be fixed/reduced into organic carbon in the light independent reactions and then forms G3P => glucose. From there, the glucose (6C) with radioactive carbon is used in respiration to form pyruvates (3C) in glycolysis? So shouldn't they also be radioactive?

Correct. This means there must be an alternative step occurring. I believe that the radioactive glucose-phosphate is being converted into sucrose, in converse to it pursuing the glycolysis pathway. However, I am still attempting to gauge why oxaloacetate immediately presents with high C-14 levels. (Vox, what is your answer?)

Alternative step is not a bad idea, although I think it's probably not what's happening here.

I'll ask another question: you assume correctly that the cell will be undergoing respiration whilst it undergoes photosynthesis. It starts respiring at the very beginning of the experiment—where does the glucose come from?

[darkz]

Alternative step is not a bad idea, although I think it's probably not what's happening here.

I'll ask another question: you assume correctly that the cell will be undergoing respiration whilst it undergoes photosynthesis. It starts respiring at the very beginning of the experiment—where does the glucose come from?

So the leaves are breaking down starch granules into glucose first for respiration and the radioactive carbon is being stored as starch hence the rise in % starch while the % of pyruvate remains at 0%?

[DBA-144]

Except for SACs and homework tasks does anyone think we will be required to draw a graph of some sort?

Thanks.

[PhoenixxFire]

I didn’t see a draw a graph question on any of the practice exams I did, so I doubt but you definitely need to know how to read one.

[TheBigC]

Except for SACs and homework tasks does anyone think we will be required to draw a graph of some sort?

Thanks.

Definitely assessable. You should be very confident in constructing graphs relating to various biological concepts. For example, it is not uncommon to be asked to draw a graph relating to levels of antibodies during the primary and secondary immune responses.

[vox nihili]

So the leaves are breaking down starch granules into glucose first for respiration and the radioactive carbon is being stored as starch hence the rise in % starch while the % of pyruvate remains at 0%?

Not a bad idea, you’re on the right track!!


The plant cell already has glucose stores. These are what it uses to fuel respiration. There’s actually heaps of pyruvate being produced, but none of it has he radioactive carbon incorporated because only the glucose made since the start of the experiment and hence anything derived from it, has radioactive carbon.

The reason you don’t see any pyruvate is basically because there isn’t enough glucose produced in the time the experiment takes to have any significant impact on the glucose stores (it may only make something as low as 1% of the stores by the end of the experiment), so there just isn’t enough time to see that glucose used for respiration.

If we ran the experiment over w few hours, then we would expect to see some oyruvate pop up

[PopcornTime]

Not a bad idea, you’re on the right track!!


The plant cell already has glucose stores. These are what it uses to fuel respiration. There’s actually heaps of pyruvate being produced, but none of it has he radioactive carbon incorporated because only the glucose made since the start of the experiment and hence anything derived from it, has radioactive carbon.

The reason you don’t see any pyruvate is basically because there isn’t enough glucose produced in the time the experiment takes to have any significant impact on the glucose stores (it may only make something as low as 1% of the stores by the end of the experiment), so there just isn’t enough time to see that glucose used for respiration.

If we ran the experiment over w few hours, then we would expect to see some oyruvate pop up

But isn't the experiment taking place over a 140 second time frame, meaning that pyruvate should be produced as photosynthesis and cellular respiration occur at extremely fast rates?

Also, how could you explain the decrease in compounds such as PGAL and glucose-phosphate near end of experiment?

[darkz]

But isn't the experiment taking place over a 140 second time frame, meaning that pyruvate should be produced as photosynthesis and cellular respiration occur at extremely fast rates?

Also, how could you explain the decrease in compounds such as PGAL and glucose-phosphate near end of experiment?

What we're saying is that the glucose produced via photosynthesis is being stored as starch. And then starch granules are being broken down and used in respiration, hence they aren't tagged with the radioactive carbon and shows as 0%

The decrease in PGAL and glucose-phosphate are probably attributed to the fact that there was a fixed amount of radioactive carbon dioxide "injected" into the chamber, so hence the reaction initially rapidly increased due to there being copious quantities of the substrate (CO2), but at the end I'd assume that the supply of the substrate just started to dwindle hence the slowing of the rate of photosynthesis

[PopcornTime]

What we're saying is that the glucose produced via photosynthesis is being stored as starch. And then starch granules are being broken down and used in respiration, hence they aren't tagged with the radioactive carbon and shows as 0%

The decrease in PGAL and glucose-phosphate are probably attributed to the fact that there was a fixed amount of radioactive carbon dioxide "injected" into the chamber, so hence the reaction initially rapidly increased due to there being copious quantities of the substrate (CO2), but at the end I'd assume that the supply of the substrate just started to dwindle hence the slowing of the rate of photosynthesis

Wouldn't stored starch have radioactive carbon atoms from the glucose produced in light-independent reactions? So if the starch is broken down into glucose molecules for glycolysis, wouldn't there be radioactive atoms present?

[PhoenixxFire]

No. The starch being used was made before the radioactive CO2 was introduced. The starch that has radioactive CO2 is just being stored for now - that's why you would see it in the pyruvate if the experiment went on for longer, eventually you would run out of the old starch and would have to use the new radioactive starch.

[darkz]

Wouldn't stored starch have radioactive carbon atoms from the glucose produced in light-independent reactions? So if the starch is broken down into glucose molecules for glycolysis, wouldn't there be radioactive atoms present?

Well as Vox said, because the actual quantity of glucose produced is rather small over the 140 seconds in comparison with the total glucose available in the cell - probably only around 1%, thus there just wasn’t enough time to show the plant consuming the freshly stored glucose - and probably just used existing starch granules

[TheBigC]

No. The starch being used was made before the radioactive CO2 was introduced. The starch that has radioactive CO2 is just being stored for now - that's why you would see it in the pyruvate if the experiment went on for longer, eventually you would run out of the old starch and would have to use the new radioactive starch.

Just to clarify. You wouldn't necessarily run out of 'old' starch, it is just that the percentage composition of starch containing radioactive carbon would increase (over time) and the consequential probability that radioactive starch will be converted into glucose then pyruvate would increase as cellular respiration proceeds.

I am being very pedantic here. I am sure you were aware of this Pheonix. I just do not want others to be mislead.

[0bMcG]

Just wondering in regards to the Kreb Cycle if we should say that the pyruvate molecule or acetyl coenzyme A is used as an input?

Thanks heaps!

[darkz]

Just wondering in regards to the Kreb Cycle if we should say that the pyruvate molecule or acetyl coenzyme A is used as an input?

Thanks heaps!

Well the correct answer would be Acetyl CoA because the pyruvate is converted into Acetyl CoA prior to entering into the krebs cycle