Indulge me...photosynthesis

[galleon]

I have a marine botany exam coming up and one of the synthesis questions is "Describe photosynthesis," thus, while practicing/studying for the exam, I thought "why not write the practice essay here?"

Specialized organelles known as plastids occur in plant and algal cells. Individual plastids have different specializations, one of which is to autotrophically synthesize energy in a storable form. These plastids are known as chloroplasts. Inside the chloroplasts there are stacks of hollow structures called thylakoids residing in the chloroplast matrix or stroma. These stacks are known as grana. The thylakoids have a fluid mosaic membrane, embedded with reaction centers for light harvesting, as well as proteins for selective permeability and electron oxidation and reduction.

Initially, light energy strikes Photosystem II in the thylakoid membrane of the chloroplasts, exciting a single electron per P680 (peak light absorbance at 680nm) chlorophyll molecule, along with the electrons of the "crutch" molecule, light harvesting complex. The excited electron(s) exits photosystem two, oxidizing the P680 molecule and reducing the electron acceptor molecule Plastoquinone. The P680 in turn, from the "pull" of the Plastoquinone reduction, oxidizes an electron from water, creating 1/2)O(sub)2 and two H(super)+ protons which remain in the thylakoid space.

The e(super)- received by Plastoquinone is then oxidized by Cytochromes complex, another electron acceptor molecule, and then again from Cytochromes complex by Plastocyanin. As the electron is passed down this electron transport chains, the acceptor molecules sequentially absorb energy held by the electron. This energy is used to pump H(super)+ protons across the membrane bilayer through the acceptor molecules, setting up a proton gradient and lowering the pH of the thylakoid space. Thus, H(super)+ protons flow from the stroma into the thylakoid space via the Cytochromes complex molecule.

The energy from the protons from water and the proton gradient are utilized for chemiosmosis by coupling factor molecules as they passively exit the thylakoid space to the stroma through the molecules. Chemiosmosis the use of the energy for the combining of inorganic P with ADP+ via the ATP synthase enzyme to form ATP. The ATP produced resides in the stroma.

The electron originally excited from Photosystem II is then moved to Photosystem I via the oxidation of Plastocyanin. The P700 (peak absorbance at 700nm) molecule in Photosystem I is reduced by the electron, where it is re-excited by the light energy harvested by the P700 molecule. In its excited state, the the electron moves to an FeS acceptor molecule by the oxidation of P700. FeS is oxidized of the electron by Ferredoxin, where the electron then moves to the final electron acceptor molecule, NADP+, where the NADP reductase enzyme couples a proton with the NADP+ molecule, creating NADPH, which is transferred to the stroma.

The above processes that create ATP and NADPH describes noncyclic photophosphorylation. Cyclic photophosphorylation returns the electron to the Photosystem II reaction center prior to the creation of NADPH, whereby only ATP is created. The mechanism regulating phosphorylation is not yet understood.

Subsequent to the manufacture of NADPH and ATP by the light-dependent reactions described above, Light independent reactions occur in the stroma of the chloroplast. This begins with carboxylation, where the enzyme RuBP carboxylase oxygenase attaches 6 CO2 molecules harvested from the stomata of the leaf or the permeability of the bilayer membrane to 6RuBP to create 12 molecules of PGA (phosphoglyceric acid). In the next phase, reduction, the 12PGA is combined with 12ATP and 12NADPH to create 12ADP+, 12NADP+ and 12 PGAL (phosphoglyceraldehyde). Finally, regeneration extracts 1 carbohydrate molecule (C(sub)6H(sub)12O(sub)6 and 6 molecules RuBP from the PGAL and CO2

To avoid photorespiration (where the RuBP carboxylase oxygenase enzyme binds to an oxygen substrate due to its higher relative concentration than CO2, caused by closure of stomata to prevent water loss and subsequent depletion of CO2 by light dependent reactions), some plants specifically have evolved a "C4" pathway, where the harvest and fixation of CO2 are separated either spatially or temporally. Spatial separation involves the integration of two specialized cells. Mesophyll cells acculmulate CO2 by joining it to phosphenal pyruvate (PEP) via the carboxylase enzyme to create malic acid. The malic acid is transferred to bundle sheath cells via vascular processes for decarboxylation, where malic acid is broken down to yield free CO2 and PEP (which is returned to the mesophyll cells). Fixation to carbohydrates can then occur.

Temporal separation of fixation and harvest occurs by way of CAM (Crassulacean Acid Metabolism). The stomata open at night to gain CO2 without significant water loss. The CO2 is combined with PEP to form malic acid, which is stored in the cell vacuole until daytime, when the CO2 can be regenerated into carbohydrates, and is therefore decarboxylated back into free CO2 from malic acid.

[mapster]

Specialized organelles known as plastids occur in plant cells.

You lost me...

[Geoff]

OK, Tom now has some competition.

G~

[Casey]

wow what a brain

[fordtrucksrme]

I see your lips moving but I hear seagulls.

[Jimbo]

"exciting a single electron per P680 (peak light absorbance at 680nm) chlorophyll molecule"
Cool so whats the different wavelengths that chloropyll molecules come in. Are they mixed in different types of algea to cover a large range?

[tdwyatt]

Originally posted by galleon
...Temporal separation of fixation and harvest occurs by way of CAM (Crassulacean Acid Metabolism). The stomata open at night to gain CO2 without significant water loss. The CO2 is combined with PEP to form malic acid, which is stored in the cell vacuole until daytime, when the CO2 can be regenerated into carbohydrates, and is therefore decarboxylated back into free CO2 from malic acid.

Heh!!! Nice job!

As Dr. Plaxco would have said, "...now show me the stoichemetric conservation of net charbohydrate production utilizing nocternal malic acid synthesis vs. photon-driven carbon sourcing..." He was always such a dog for that type of exam, buth then again, his questions were never so straightforward...

[galleon]

Jim: it all depends on the algae. There are several pigments of varying wavelength peak efficiencies, most of them being chlorophylls: LHCII, xanthophyll, P960, P890, P870, P840, P700, P680, and the list goes on. The main antennae chlorophylls are those peaking at 680 and 700 nm for PSII and PSI, respectively. The reason "peak" is used is because excitation by inefficient wavelengths results in the de-excitation of the chlorophyll molecule electrons, sometimes emitting light at a longer wavelength (flourescence of diatoms comes to mind). The Stokes shift (wavelength absorption vs. wavelength emission/heat) is actually very small for the majority of wavelengths absorbed and re-emitted during redox of chlorophyll/plastoquinone, hence relative high energy and "brightness" of fluorescence.

Tom: that depends on the plant, the humidity, and the photoperiod . Gotta love immersion protist autotrophism... so much simpler, none of that photorespiration stuff to worry about underwater. Just need the good old motion of the ocean to keep the gases exchanged. Which, BTW, should clue hobbyist in as to why algae grows better in random flow then laminar flow. More chaotic flow results in more surface area exposed to readily gas diffusing energy which leads to more diffusion via viscous entrainment, which means more photosynthesis, which means...

[Piero]

thanks for sharing. Lost me a few times...but still great stuff!

[galleon]

Figured I'd give this a bump after acing the exam, thanks for the indulgence

[Mrs Smith]

Congratulations!

It is simply amazing that with the diversity of life on this planet, so many of them use similar biochemical methods of energy transport, storage and utilization.

I'll spare you a similar discussion of the nuances of Nephrotic syndrome vs. Nephritic syndrome, as corals don't, for the most part, have kidneys. Now fish...

Congrats on the exam!
Danielle

[Spanky]

Originally posted by galleon
Figured I'd give this a bump after acing the exam, thanks for the indulgence

[Stang69]

Ouch!!!! Man I hate thinking that hard


Jeff

[dark horge]

Originally posted by galleon
Figured I'd give this a bump after acing the exam, thanks for the indulgence

That's it?? A bump???
Whatever happened to a free round of drinks for everyone?

[tdwyatt]

Originally posted by dark horge
That's it?? A bump???
Whatever happened to a free round of drinks for everyone?

or at least those of us that indulged you by reponding...