Tuesday, April 5, 2011

A knitball of stress

Since I can't exactly post on my Tumbrughhhhh about my educational experiences, I shall post them here.

Yesterday I learned more about cell biology than I have the rest of the term (mid-semester, but term sounds much better and plus I'm still kind of accustomed to using it from high school). Anyway, I spent roughly about 3-4 hours watching this amazing lecturer from some random non-profit organisation explaining photosynthesis and cellular respiration in just enough detail and I guess he's really inspired me to try to actually do well in these exams.

If I do really well in these exams, that 7.0 is definitely attainable.

Damned eraser almost ripped my page. We have a definite love-hate relationship.

I'm yet to do a summary for everything; I have to really go through my entire lecture booklet + Jess' notes and I'm planning to redo the practice test. Actually, I don't think I did too badly considering that I hadn't even touched cell biol when I did it the first time. Sort of failed on the true/false questions and almost died because of that but hey, nevermind.

Let's try to consolidate my understanding of photosynthesis. I wasn't bothered taking a detailed photo of the second page just because it's pretty self-explanatory. So, let's get started.


The overall reaction for this process is as follows:

6CO2 + 12H2O -> C6H12O6 + 6H2O + 6CO2 + Energy

So there are two main phases: the light DEPENDENT reaction (which is obviously dependent on light) and the light INDEPENDENT/""""dark"""" reaction/Calvin cycle (happens simultaneously to the light reaction but obviously doesn't require it). Firstly, let's deal with the light dependent reaction.

Yeah I know, really cute drawing right?

Step 1: Light DEPENDENT reaction

There are proteins, or "complexes" embedded within this phospholipid bilayer and it occurs in the thylakoid membrane.

Summary of what happens:

  • Water is broken up into H+ and molecular oxygen (O2) and the electrons given off are the source to drive this """""electron transport chain""""" and yes I know it's technically not THE electron transport chain but that's the best way to put it.
  • ATP synthase is driven by this process and electrons combine with the final electron acceptor NADP+ and is reduced to NADPH. ADP combines with P to form ATP

A more detailed summary:

  • Photons of light penetrate the cell and hit the first complex PSII, exciting electrons and therefore putting them at a high energy state.
  • The electrons move to the next complex and lose energy through movement until it reaches PSI where it is hit by another photon and gets excited again.
  • The energy lost through movement from higher energy states to lower energy states pumps H+ ions from the stroma into the lumen of the thylakoid and establishes a concentration/electric/pH gradient/potential.
  • Eventually, the [H+] inside the lumen reaches a stage where all the hydrogen ions want to be pumped back into the cell and this is where ATP synthase comes into play. The electrons from the chain reach the ATP synthase where they are used to reduce NADP+ to NADPH and the energy from the electrons' drops in states is used to drive this process.
  • Now, obviously it's called ATP synthase for a reason. While all this NADPH is being formed, this whole thing is driving the ADP and Ps together and they form ATP.
  • So, electrons are being used in this chain, yes? One question you may raise: once the electrons are used up, how do you get new ones? Does the process just stop? And ok I'm tired so they're bad questions but basically, the electrons are being constantly replaced through the "electrolysis" of water. Again, a loose term, but essentially, water molecules hit PSII and it's strong enough to pull electrons and it's broken up into H+ ions and more importantly, molecular oxygen or O2
The basic reactions for this step are:

2H2O -> 4H+ + O2 + 2e-
NADP+ + H+ + e- -> NADPH
ADP + P -> ATP

Step 2: Light INDEPENDENT/"Dark" reaction/Calvin cycle

Basically, this is where the CO2 from the overall reaction is converted into usable forms for the cell like glucose or sucrose. Carbon is "fixed" - from a gaseous form to a usable form for energy. Since this is a lot simpler than the light dependent reaction, I don't think I really need two types of summaries. It occurs in the stroma.

What happens:

  • 6 CO2 molecules combine with six 5 carbon molecules called RuBP (which stands for ribulose biphosphate or ribulose-1,5-biphosphate but that's not very important at all) to form 12 molecules of G3P (glyceraldehyde-3-phosphate) which is a 3 carbon molecule. Aside: just concentrate on the number of carbons. We started with 6 carbons from CO2 and combined it with 6 RuBPs so 6 + 5 x 6 = 36 and we form 12 G3Ps so that's 12 x 3 = 36 carbons.
Summary equation: 6CO2 + 6RuBP -> 12G3P
  • Since G3P is at a higher energy state than CO2 or RuBP, you need energy to make the reaction work. This energy comes from the ATPs and NADPHs which were produced in the light dependent reaction. Specifically, 12 ATPs and 12 NADPHs are used in the above reaction and are converted into 12ADPs, 12Ps and 12 NADP+
  • From the 12G3Ps, 10G3Ps are used to remake 6 molecules of RuBP and that's why it's called a cycle. In the process of recombination, 6 ATPs are used to complete this reaction because it requires energy. Aside: 10G3Ps give 3 x 10 = 30 carbons and 6 RuBPs give 5 x 6 = 30 carbons.
Summary equation: 10G3P + 6ATP -> 6ADP + 6P + 6RuBP
  • Now, we started with 12G3Ps and 10 were used to regenerate RuBP, so what happened to the other 2? Well, we know that G3P has 3 carbons and since there are 2 of them, they combine to form a 6 carbon molecule; what haven't we covered? The sugar. So two of them join together to form either a glucose molecule or sucrose - something that provides food to the cell - C6H12O6.
The basic reactions for this step are:

6CO2 + 6RuBP + 12ATP + 12NADPH -> 12G3P + 12ADP + 12P + 12NADP+
10G3P + 6ATP -> 6RuBP + 6ADP + 6P
2G3P -> C6H12O6

Still too confusing? Just know that CO2 is converted into C6H12O6 and these reactions require energy to happen because of the products at a higher energy state. Also know that it's a cycle.

And there you have it: photosynthesis in a nutshell.

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