VCE Biology Question Thread

[IThinkIFailed]

Water IS still outputted by photosynthesis, its just that when you are writing the net equation, you cancel out the water to show that there is a net loss/consumption of water in the process.
You can write the equation in both forms and get full marks.

Thanks for the help, does that mean I should or shouldn’t write water as an output of photosynthesis in an exam?

[PhoenixxFire]

Thanks for the help, does that mean I should or shouldn’t write water as an output of photosynthesis in an exam?

You can do either - VCAA will accept either answer as correct assuming your equation is balanced (ie if you say that water is an ouput then you need to say that 12 H2O is the input, if you say that there's no net ouput of water then you need to say that 6 H2O is the input).

Personally I'd use the full equation because I think it makes it a bit easier to think about when you're considering the stages of photosynthesis separately, but either is fine.

[IThinkIFailed]

In the light dependent reaction, chlorophyll traps light energy which excites an electron so it is “shed” and gets carried by electron carriers. My question is, if the light energy is being absorbed by the electron, how can water be split by light energy as textbooks say so? Wouldn’t the light energy already have been used up?

[zuijinde]

You do not need to know this, but it is my understanding that light energy is 'captured' by chlorophyll though exciting electrons with the chlorophyll molecule (gan be other pigments too). The energy possessed by these electrons is then used to split water into H+ ions, oxygen and 2 free electrons (Or a free electron pair) which then carried through an ETC.

Again, this is just my understanding and is probably not the most accurate, However, you will not need to know this and It is suffice to say that chlorophyll molecules capture light energy which is used this to split water...

Don't think this is quite correct.
The water molecules are photolysed by light itself, not by the electrons. What the photoexcited electrons (electrons that possess energy) do is that when they move from photosystem 2 to photosystem 1 via electron carriers, they give off energy for the H+ ions to actively transport from the stroma to lumen, thus H+ ions can move through the cytochrome complex then onto the ATP synthase.

[Erutepa]

Don't think this is quite correct.
The water molecules are photolysed by light itself, not by the electrons. What the photoexcited electrons (electrons that possess energy) do is that when they move from photosystem 2 to photosystem 1 via electron carriers, they give off energy for the H+ ions to actively transport from the stroma to lumen, thus H+ ions can move through the cytochrome complex then onto the ATP synthase.

I still think that this is not quite right. I have done some reading and it still seems that the energy used to split water (at PSII) comes from light energy absorbed by chlorophyll molecules, not sunlight interacting directly with the water itself. Electrons released from the splitting of water enter the electron transport chain facilitated electron carriers where at PSI, light energy captured by chlorophyll is used to continue to power this ETC to (as you said) produce NADPH and ATP.
here is a little link that i used: https://courses.lumenlearning.com/boundless-biology/chapter/the-light-dependent-reactions-of-photosynthesis/

EDIT: tried to edit original post, but accidentally deleted it, oh well...

[zuijinde]

Hm, I guess I learned something new today!
However, I still believe light directly photolysing the water is viable (which is why when you shine light towards water you get bubbles)

Electrons released from the splitting of water enter the electron transport chain facilitated electron carriers where at PSI, light energy captured by chlorophyll is used to continue to power this ETC to (as you said) produce NADPH and ATP.

To my understanding, the electrons released from the splitting of water mainly replaces the lost electrons from PS2 that have already gone to the electron carriers, rather than the electron carriers. You might need to follow up on this as I'm not too sure myself now 

Cheers!

[IThinkIFailed]

Hm, I guess I learned something new today!
However, I still believe light directly photolysing the water is viable (which is why when you shine light towards water you get bubbles)
To my understanding, the electrons released from the splitting of water mainly replaces the lost electrons from PS1 that have already gone to the electron carriers, rather than the electron carriers. You might need to follow up on this as I'm not too sure myself now 

Cheers!

The thing you said about electrons released from spitting of water replacing PS2’s lost electrons is what I’ve been seeing on the internet as well, like crash course and Wikipedia. Is there a “no fail” explanation that VCAA likes?
Would something like this suffice?: Light energy from the sun is used to split water, resulting in electrons which enter the electron transport chain and provide energy to pump out hydrogen ions into the intermembrane space.

[zuijinde]

The thing you said about electrons released from spitting of water replacing PS2’s lost electrons is what I’ve been seeing on the internet as well, like crash course and Wikipedia. Is there a “no fail” explanation that VCAA likes?
Would something like this suffice?: Light energy from the sun is used to split water, resulting in electrons which enter the electron transport chain and provide energy to pump out hydrogen ions into the intermembrane space.

Yes, it should suffice.
Be careful with the last statement, it is the thylakoid lumen (which is the liquid inside a thylakoid disk), rather than the intermembrane space (which is usually in reference to the chloroplast envelope)

[Erutepa]

Hm, I guess I learned something new today!
However, I still believe light directly photolysing the water is viable (which is why when you shine light towards water you get bubbles)
To my understanding, the electrons released from the splitting of water mainly replaces the lost electrons from PS2 that have already gone to the electron carriers, rather than the electron carriers. You might need to follow up on this as I'm not too sure myself now 
Cheers!

I have looked elsewhere and it seems that the information from my original source over-simplifies things. It is the case that the free electrons produced by the splitting of water does replace the  electrons lost from PS2. However, I think for VCE knowledge, saying that the splitting of water provides electrons for the ETC does suffice.

[darkz]

The discussion here is great! But just going to point out for anyone browsing so they don't freak out - for VCE, knowledge of the biochemical pathways of photosynthesis including photosystems is not required.

[f0od]

What is the difference between NADPH or NADH and NADPH2 or NADH2? (particularly for photosynthesis and respiration) Our teacher keeps alternating between the two, so I'm not sure which one I'm supposed to use. Thanks!

[darkz]

What is the difference between NADPH or NADH and NADPH2 or NADH2? (particularly for photosynthesis and respiration) Our teacher keeps alternating between the two, so I'm not sure which one I'm supposed to use. Thanks!

NADPH and NADP+ are used in photosynthesis
NAD+ and NADH are used in respiration

You pretty much just need to know the above differentiation. e.g. you don't need to know the biochemical difference between them

[IThinkIFailed]

Just some quick questions about experimental design.
We just did a practical on leaf discs, which float if photosynthesis occurs. We used concentration of CO2 as our independent variable, to see if it effects the rate of photosynthesis, which is judged by how fast half of the discs float (ET50)
I just want to know, would this be a good hypothesis?
If the concentration of bicarbonate in the solution is increased, then the ET50 for the leaf discs to float will decrease because the rate of photosynthesis has increased.

Thanks for any feedback!

[PhoenixxFire]

Just some quick questions about experimental design.
We just did a practical on leaf discs, which float if photosynthesis occurs. We used concentration of CO2 as our independent variable, to see if it effects the rate of photosynthesis, which is judged by how fast half of the discs float (ET50)
I just want to know, would this be a good hypothesis?
If the concentration of bicarbonate in the solution is increased, then the ET50 for the leaf discs to float will decrease because the rate of photosynthesis has increased.

Thanks for any feedback!

Hey!
Sometimes teachers have different opinions on how a hypothesis should be written, so if you have any examples of hypotheses from your teacher then it'd be a good idea to copy them. Using the if, then format should be fine though. I'd leave out the part about the cause though, so just say "If the concentration of bicarbonate in the solution is increased, then the ET50 for the leaf discs to float will decrease." There's no need to explain anything in your hypothesis.

[ssillyssnakes]

Alright so I have a question.
In my SAC, one of the questions asked us to explain how the reaction between gelatin and bromelain is a condensation reaction or something along those lines. In the heat of the moment, I just tried to vaguely justify why it is and I got full marks, but wouldn't it be a hydrolysis reaction? As the gelatin is being broken down into amino acids?
My teacher says that hydrolysis is just a type of condensation reaction but I can't find anything online that says that? Is that true?