Photosynthesis and Cellular Respiration

Almost all life on Earth runs on two connected processes. Photosynthesis captures light energy and stores it as chemical energy in glucose (C₆H₁₂O₆). Cellular respiration breaks that glucose back down to release usable energy as ATP. Plants do both: they make their own glucose by photosynthesis, then break it down by respiration just like animals do. Animals can only do the second half, so they eat plants or other organisms to get glucose in the first place. Think of glucose as a rechargeable battery. Photosynthesis charges the battery using light. Respiration drains the battery to power the cell. The two reactions are almost exact opposites of each other, and one process's products are the other process's reactants.

Photosynthesis happens in the chloroplast, an organelle found in plant cells and algae but not in animal cells. (Some bacteria, such as cyanobacteria, photosynthesize too, but they do it without chloroplasts, since bacteria have no membrane-bound organelles.) Inside the chloroplast, green pigments called chlorophyll absorb light energy and use it to build glucose out of carbon dioxide and water. The overall equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ In words, six molecules of carbon dioxide plus six molecules of water, powered by light, produce one glucose molecule and six molecules of oxygen gas. That oxygen is released as a waste product, and it's the same oxygen animals breathe in. Photosynthesis happens in two connected stages inside the chloroplast. The light-dependent reactions, in the thylakoid membranes, absorb light, split water (releasing the O₂), and produce ATP and NADPH. The Calvin cycle, in the stroma, uses that ATP and NADPH to build glucose from CO₂. The Calvin cycle does not need light directly, but it depends on the ATP and NADPH the light-dependent reactions just made.

Example: If a plant produces 3 molecules of glucose (C₆H₁₂O₆) through photosynthesis, how many molecules of O₂ are released? The equation shows a 1:6 ratio of glucose to O₂ (one glucose for every six O₂). So 3 molecules of glucose come with 3 × 6 = 18 molecules of O₂.

Cellular respiration is essentially photosynthesis run in reverse. It breaks glucose apart in the presence of oxygen and releases the stored energy as ATP. Every living cell does some form of respiration, in plants and animals alike. The overall equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP One glucose molecule plus six molecules of oxygen produce six molecules of carbon dioxide, six molecules of water, and usable energy in the form of ATP. Respiration happens in stages across two locations. Glycolysis splits glucose into two smaller molecules called pyruvate, in the cytoplasm, outside any organelle, and nets 2 ATP directly. If oxygen is present, the pyruvate moves into the mitochondria, where the Krebs cycle and the electron transport chain finish breaking it down, using the oxygen and releasing far more ATP, plus the CO₂ and H₂O you exhale and produce.

ATP (adenosine triphosphate) is the molecule cells use to store and spend energy moment to moment. It's made of an adenosine molecule attached to three phosphate groups. The bond holding on the third, outermost phosphate group is high in energy. When a cell needs energy for some task, an enzyme breaks that bond, releasing the energy and leaving behind ADP (adenosine diphosphate) plus a free phosphate group. ATP → ADP + phosphate + energy released Cells use that released energy to power almost everything they do: contracting a muscle fiber, pumping ions across a membrane, building proteins, and transmitting a nerve signal. ADP isn't waste, it gets rebuilt back into ATP by cellular respiration, so the cell recycles the same small pool of molecules constantly rather than making a fresh batch from scratch each time.

Everything described above for the Krebs cycle and electron transport chain is aerobic respiration, respiration that requires oxygen. It happens in the mitochondria and produces a large amount of ATP, roughly 30 to 36 ATP molecules total per glucose molecule (older textbooks list numbers up to 38, the exact count depends on how the energy accounting is done). When oxygen is not available, a cell can still run glycolysis in the cytoplasm without it, since glycolysis doesn't use oxygen directly. But glycolysis alone only nets 2 ATP per glucose, and it grinds to a halt if the cell can't get rid of a byproduct that glycolysis leaves behind. That's the job of fermentation, an anaerobic (no oxygen) process that recycles that byproduct so glycolysis can keep running.

Fermentation doesn't make more ATP by itself, its job is to regenerate a molecule glycolysis needs so glycolysis can keep producing its 2 ATP even without oxygen. There are two types you'll see most often. Lactic acid fermentation happens in your own muscle cells during intense exercise, when they're using oxygen faster than your bloodstream can deliver it, and in some bacteria (the ones used to make yogurt and cheese). Pyruvate from glycolysis is converted into lactic acid (lactate). Example: During a sprint, muscle cells run low on oxygen. Glycolysis keeps producing ATP, and the pyruvate it makes is converted to lactic acid instead of entering the mitochondria. The buildup of lactic acid contributes to the muscle burn felt during hard exercise. Alcoholic fermentation happens in yeast and some bacteria. Pyruvate is converted into ethanol and carbon dioxide. This is the reaction bakers rely on for bread to rise (the CO₂ forms bubbles in the dough) and brewers rely on to make alcoholic beverages.

Now that both processes have been covered, it helps to see them side by side. Notice how the products of one are the reactants of the other, which is what makes them a cycle rather than two unrelated reactions.

1. In which organelle does photosynthesis occur, and in which organelle does most of aerobic cellular respiration occur? 2. Write the overall equation for cellular respiration, including the reactants and products. 3. A muscle cell is exercising hard and running low on oxygen. Which process lets its glycolysis keep making ATP, and how much ATP does that process add on top of glycolysis? 4. What is the role of ATP in a cell, and what molecule is left behind after ATP releases its energy? 5. Using elodea-light-distance.csv, calculate the bubble rate (bubbles per minute) at each distance by dividing bubbles_counted by time_minutes. What is the rate at 100 cm and at 40 cm? 6. As the lamp moves closer to the plant (distance decreases), what happens to the rate of oxygen bubble production? Using the photosynthesis equation on this sheet, explain why light is one of the things that changes that rate. 7. Between 20 cm and 10 cm, the rate stops increasing. The Elodea is fully submerged in water, so water is never in short supply. Of the other two reactants in the photosynthesis equation (CO₂ and light energy), which one is most likely limiting the rate at those two closest distances, and why is light no longer the answer? 8. Calculate the percent increase in bubble rate from 100 cm to 60 cm, using percent change = (later rate − earlier rate) ÷ earlier rate × 100.