• In the space below, write down the equation (chemical reaction) for photosynthesis (look it up in your notes if you can’t remember it!!!):
_______ + _______ + _______________ —> ____________ + _______
• Now, flip the two sides of the chemical reaction for photosynthesis:
______________ + _______ —> _______ + _______ + _______________
• What you’ve written above is the chemical reaction that takes place constantly inside of each cell in your body—it’s called cellular respiration. The energy produced is stored in molecules called adenosine tri-phosphate (ATP), which release lots of chemical bond energy when they lose a P (phosphate ion, PO4 3-) and become adenosine di-phosphate (ADP). This energy in ATP is what our cells and body use to run everything we do—breathe, move, blink, etc.
• Look at the second equation you wrote above. When glucose is broken down by your cells to make energy, what substance is also necessary to make that reaction happen?
• Hopefully you said: oxygen gas! Oxygen is the key to getting most of the energy out of the chemical bonds in glucose. Most living things prefer to burn glucose in the presence of oxygen (a process called aerobic respiration) because oxygen molecules play a pivotal role in getting maximum energy out of the later stages of cellular respiration. Without oxygen, respiration can happen, but it is less efficient.
• Scientists think that there was little to no oxygen present on Earth when life was first evolving; it wasn’t common until plants produced a lot of it as a by-product (leftover) of photosynthesis. So some simpler organisms like bacteria and yeast (a fungus) evolved earlier, and they don’t use oxygen to get energy from glucose, but instead go through the process of fermentation using anaerobic respiration.
Anaerobic Respiration = respiration without oxygen
• Look at the image on the backside of this page. Just like the first step in cellular respiration in your cells, yeast converts glucose (6-carbon) molecules into two 3-carbon molecules called pyruvate. But in the absence of oxygen, unlike you, yeast will convert the pyruvate molecules into ethanol, commonly known as alcohol, which releases a small amount of energy and CO2 gas. This is known as alcoholic fermentation.
• Bacteria can also break down glucose in the absence of oxygen in a similar process called lactic acid fermentation. Pyruvate molecules are converted into a molecule called lactate (related to lactose sugar in milk), which releases another small amount of energy.
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| Glucose (C6H12O6) enters a cell and is first converted to 2 molecules of pyruvate (C3H3O3 -) in the watery cytosol. When O2 is present, the cell’s energy factory, the mitochondria, turns the pyruvate into lots of ATP (energy), releasing only CO2 and H2O . When O2 is not present, lactic acid or alcoholic fermentation processes get a smaller amount of energy by breaking down the pyruvate molecule into lactate or ethanol in the cytosol. |
Follow-up Questions
What’s the opposite chemical process to photosynthesis? What is used and what is produced?
What’s the difference between aerobic and anaerobic respiration?
What kind of organisms undergo anaerobic respiration? Why do they use this less efficient process?
Most yeast and bacteria are able to perform aerobic respiration with the right conditions. When a beer brewer or wine maker uses yeast to make alcohol, what do they have to do to ensure that the yeast produce the desired product?
During the early parts of strenuous workouts, your body often produces lactic acid, which builds up in your muscles to make you feel sore and fatigued. Why do you think your body does this process that’s similar to what bacteria do?
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