CELL BIOLOGY WORKSHOP
Energy conversion using electron transport chains and proton movement occurs within cells.
- Chloroplasts, in plant cells, are the site of photosynthesis which converts light energy to chemical energy which is then used in cellular respiration.
- Mitochondria, in eukaryotic cells, are the site of cellular respiration which is the process of breaking down chemical energy for use within the cell.
A proton pump is coupled to the light reaction in photosynthesis (Refer to p 90 Fig. 5.15). Energy from the sun is absorbed by chlorophyll pigments in the thylakoid membrane of the thylakoids which occur in stacks (grana) located in the stroma of the chloroplasts (organelles in some plant cells such as mesophyll cells located in the leaves). These cells also typically contain mitochondria. In Figure 5.15 we can see that the electron from the splitting of a water molecule, H2O, once again moves along a complex transport chain and then reduces NADP+ to NADPH. Here NADP+ is the electron acceptor. As the electrons move along, a proton pump is set up to produce a concentration gradient for H+ ions which are accumulating in the thylakoid lumen. The H+ ions will move only through the special ' ATP synthase protein complex' where the energy from the concentration gradient is coupled to drive the reaction ADP => ATP.
What is the Calvin cycle?
This refers to the Calvin-Benson cycle which takes place in the stroma of the chloroplasts, ie. sites of photosynthesis. (Refer to p 93, Fig. 5.17)
What does Rubisco do in the Calvin cycle?
Rubisco catalyses the fixation of CO2. It is, therefore, an enzyme: ribulose bisphosphate carboxylase-oxygenase (Rubisco). It assists in the attachment of a CO2 molecule to a molecule of RuBP (short for ribulose biphosphate) in the Calvin-Benson cycle. The CO2- RuBP (6C) intermediate is unstable and splits rapidly into 2 molecules of PGA (phosophoglyceric acid, 3C). Notice how we can keep track of the number of carbon atoms involved. Plants with this pathway in photosynthesis are called C3 plants because of the 3C PGA compound which is the first stable product of carbon fixation. From PGA, energy (12ATP) and reducing power (12NADPH) are used to cycle 6 times, picking up a total of 6 CO2 molecules, regenerating RuBP on each cycle, but at the end of the 6 cycles, producing a 6C glucose molecule. The glucose can then be condensed (H2O removed as glucose molecules are joined together) to form other sugar molecules transported to sites of respiration, or storage sites ready to fuel other reactions at a later date.
C4 and CAM are slightly different but important pathways of carbon fixation.
(Refer to p 95, Fig. 5.22)
Firstly, a 4C compound is the first stable product of C4 carbon fixation (ie the "dark reaction" of photosynthesis), hence the name. CO2 is taken up and attached to PEP (phosphoenolpyruvate) in a carboxylation reaction assisted by an enzyme called PEP carboxylase to form a 4C compound called oxaloacetate. This is immediately converted to another 4C compound (such as malate, depending on the species). Importantly, this all happens outside the chloroplast in the cytosol of the mesophyll cells. The mesophyll cells lack Rubisco, but do provide ATP and NADPH to make PEP and malate. C4 plants have bundle sheath cells which are located next to the vascular bundle tissue (phloem) which transports sugar to other parts of the plant. So the sugar factory is right next door to the transportation. Malate is moved from the mesophyll cell into the chloroplast inside the bundle sheath cell. Here, malate is broken down (decarboxylated) to CO2 and pyruvate. The CO2 is then used in the Calvin-Benson cycle just like it was in C3 plants, Rubisco joining CO2 and RuBP (see above), and the pyruvate is moved back into the mesophyll cell where it is used to make PEP to complete the cycle.
Why? Well, what wasn't mentioned before was that Rubisco is actually in demand by both CO2 and O2 (oxygen), hence its name. O2 attaches to Rubisco in photo-respiration. If there is more O2 compared to CO2 then more photo-respiration and less caboxylation (carbon fixation) will occur. The C4 mechanism actually concentrates CO2 in the bundle sheath cells where all the Rubisco is and it effectively reduces the amount of Rubisco being used for photo-respiration and therefore increases the relative amount of carboxylation, ie carbon fixation or if you like, sugar production. The concentration of CO2 at the site of fixation reduces the need for the stomata (leaf pores) to be wide open so they tend to partially close and hence reduce the loss of water due to transpiration (evaporation of water from the surface of the mesophyll cells through the stomata and into the open atmosphere). C4 plants tend to perform relatively well in conditions of low humidity and high temperature.
(Refer to p 97, Fig. 5.24)
CAM stands for Crassulacean Acid Metabolism. It is a slight variation on C4 metabolism. The same reactions in C4 happen in CAM plants, however they happen in the same cell but are separated in time. CAM plants open their stomata at night and fix CO2 into a 4C compound such as malate which is temporarily stored in the vacuole of the mesophyll cell. At the break of daylight when the light reaction can provide ATP and NADPH, malate, as in C4 metabolism, is moved into the chloroplast, and decarboxylated into CO2 + pyruvate. CO2 then enters the Calvin-Benson cycle and pyruvate is moved out of the chloroplast into the cytosol of the mesophyll cell and used to regenerate more PEP, ready for the night time uptake of atmospheric CO2.
Why? The temporal separation of the uptake of atmospheric CO2 through open stomata (night) and the use of energy from solar radiation during the day to produce sugar compounds allows these plants to survive extremely hot and dry conditions. Closure of stomata during the day greatly reduces overall water loss by the plant. Temporary storage of the 4C compound (eg. malate for instance) in the vacuole is a way to effectively concentrate CO2 ready for the Calvin-Benson cycle and thus ensures efficient photosynthesis.
Glucose (6C, 6 carbons) is broken down into 2 pyruvate (2 x 3C) molecules ready for further breaking down either with oxygen in the Citric Acid Cycle, OR without oxygen in Fermentation [fermentation to either lactic acid (2 x 3C) or ethanol + CO2 (2 x 2C + 2 x 1C) ]
(Refer to p 80, Fig. 5.4)
Overall, from 1 glucose (6C) molecule to 2 pyruvate (3C) molecules, glycolysis produces:
Remember, the citric acid cycle OR fermentation happens after the glycolysis step.
Citric acid cycle: (Refer to p 82, Fig. 5.6)
In the presence of oxygen pyruvate is broken down into Acetyl CoA which, in Eukaryotes, enters the mitochondria to be used in the citric acid cycle. So the citric acid cycle happens inside mitochondria (the sites of respiration). Respiration is the breaking down of sugars (fuel molecules), by the removal of electrons from C-C and C-H bonds. The electrons are then accepted by coenzymes (NAD and FAD) to produce "reducing power" in the form of NADH and FADH2 which are used in electron transport chains (in mitochondria membrane). The electron transport chains are coupled to proton (H+) pumps producing H+ concentration gradients, and hence a type of potential energy, which is finally coupled to the synthesis of ATP. Overall, from 1 glucose molecule we get 2ATP from glycolysis, and another 34-36ATP from the citric acid cycle (or oxidative respiration, i.e. with oxygen. Don't forget that without oxygen we only end up with 2ATP).
Oxidation-reduction (redox) reactions are extremely important to living systems.
B-oxidation of lipids
Lipids (polycarbons) are hydrolysed into fatty acids, ie water molecules are put back into the condensed lipid molecules. The fatty acids are long carbon chains which enter a process of beta-oxidation in which FAD and NAD+ are reduced to FADH2 and NADH respectively (Refer to p 81, Fig. 5.5), and carbon atoms two at a time are broken down into a 2 carbon compound called Acetyl-CoA which enters the mitochondria to participate in the citric acid cycle
ATP goes to a compound called ADP
Adenosine triphosphate, A~P~P~P can break one of its phosphate bonds to release a useful amount of energy and result in Adenosine diphosphate, A~P~P + P (+ energy).
This reaction can be reversed under the right conditions, such as during the respiration in mitochondria, when there is surplus energy to produce ATP from ADP and P.
(Refer to p 83 Fig 5.7) Respiration in mitochondria typically involves the breakdown of high energy sugars originally produced during photosynthesis in plants. In Figure 5.7, the main concepts to understand are as follows.
NADH and FADH2 come from the citric acid cycle which occurs within the bounds of the inner membrane of the mitochondrion (Figure 5.7). These compounds provide the electrons to be passed to the special protein complexes embedded in the inner membrane. The proteins accept the electron and pass it along an electron transport system and eventually to oxygen, the final electron acceptor, which reacts with H+ ions to produce water.
So where does the energy rich ATP come from? As the electron is passed along the electron transport system, protons (H+ ions) are "pumped" into the water filled space between the inner and outer mitochondrial membranes. Thus, the concentration of H+ increases and a concentration gradient is set up as the H+ ions want to move from the higher concentration to a lower concentration - so they have the potential to move. The only places they can move from high to low concentration is via a special protein channel which deliberately takes advantage of the potential the H+ have to move back into the matrix (centre) of the mitochondrion by coupling the movement to the ADP==>ATP reaction.