This week in AP bio, we progressed from the cellular respiration unit to more specifically, the photosynthesis unit. On Monday we resumed one more lecture to finalize the cellular respiration unit, then on Tuesday we started lecturing on photosynthesis. We did a few online packets to help us gain basic knowledge first as well.
On Monday, we started with learning that most cellular respiration requires O2 to produce ATP and without O2, the electron transport chain will cease to operate. In that case, glycolysis couples w fermentation or anaerobic respiration to produce ATP. The formula for cellular respiration is C6H12O6 + 6O2 → 6H2O + 6CO2.
Anaerobic respiration uses an electron transport chain w a final electron acceptor other than O2 for example sulphate. Fermentation uses substrate level phosphorylation instead of an electron transport chain to generate ATP. Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis afterwards. Two common types are alcohol fermentation and lactic acid fermentation. In alcohol fermentation, pyruvate is converted to ethanol in two steps: releasing CO2 from pyruvate, then reducing acetaldehyde to ethanol. An example of alcohol fermentation being used in a real life scenario is yeast being used in brewing, winemaking and baking.
Lactic acid fermentation is a completely different type though. Pyruvate is reduced by NADH, forming lactate as an end product w no release of CO2. An example of lactic acid fermentation in a real world scenario is when some fungi and bacteria is used to make cheese and yogurt. Human muscle cells also use lactic acid fermentation to generate ATP when O2 is scarce.
In comparing fermentation with anaerobic vs aerobic, we must acknowledge a few things. All use glycolysis to oxidize glucose and harvest chemical energy of food and in all 3, NAD+ is the oxidizing agent that accepts electrons during glycolysis. The processes have different final electron acceptors: an organic molecule (pyruvate) in fermentation and O2 in cellular respiration. Cellular respiration produces 32 ATP per glucose mol: fermentation produces 2ATP per glucose mol. Obligate anaerobes carry out only fermentation or anaerobic respiration and cant survive in the presence of O2. Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration. In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes.
FINALLY we can get into photosynthesis. We delved into what actually happens during photosynthesis and what drives the reaction.
I learned that CO2 comes into stoma, oxygen and water come out. Water needs to evaporate to drive the process of taking water up into the leaves – creates negative pressure so it fills void. NADPH is created by photosystem I. The main ingredient is carbon dioxide – binds to RuBP. To combine them, we use an enzyme called RuBisCo. It takes CO2 and sticks it to 5 carbon sugar to create a 6 carbon sugar (becomes unstable) so then the sugar breaks down. C3 plants do great in cool weather but suffer in cold weather – by why? It’s because the CO2 concentration is used up throughout the day while oxygen increases because there’s no way out. Therefore RuBisCo is more likely to bind w oxygen, so it takes components out of the reaction that should function properly – this is also known as photorespiration. It prevents current photosynthesis by taking reactants out of the cycle. We also addressed questions such as “How can we have plants in a hot location?” The answer is to have an open stomata at night so less water will evaporate. It will store CO2 at night to be used for reactions during the day OR you can do the light dependent reactions somewhere else so O2 won’t join to RuBisCo. The xylem is responsible for moving water soluble things from the ground to the plant while the phloem is responsible for taking the products and moving it to where it needs to go.
This week we did more lecturing than I’d like to, but I think I got a good grasp on the basics of photosynthesis. As for the labs, I sped through the virtual ones relatively easily. I’m excited to see what we’ll be doing for the rest of the photosynthesis unit, hopefully do some labs involving plants.
WEBSITES
http://www.phschool.com/science/biology_place/biocoach/cellresp/intro.html
https://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookPS.html
AP Biology 