Track the inputs and outputs of matter and energy in photosynthesis, cellular respiration and fermentation, and explain how enzymes and feedback regulate the cellular processes that keep a cell alive

Learning goal: Explain the cellular processes that regulate the characteristics of life: identify the inputs and outputs of matter and energy in photosynthesis, chemosynthesis, cellular respiration and fermentation, describe how cells build macromolecules from carbon and a few other elements, explain how enzymes speed reactions and why temperature and pH change their activity, and describe positive and negative feedback. Design and interpret a fermentation investigation with yeast.

Life is regulated by cellular processes. Living things grow, respond to their surroundings, reproduce, maintain steady internal conditions and use energy. Each of these characteristics is carried out by chemical reactions inside cells. Ohio's course emphasizes inputs and outputs of matter and the transfer and transformation of energy in these processes, not the names of every enzyme or intermediate along the way, and this lesson keeps to that scope.

The elements of life and the macromolecules. Cells acquire carbon from their environment and combine it with hydrogen, oxygen, nitrogen, phosphorus and sulfur to build four families of macromolecules: carbohydrates from simple sugars, lipids from fatty acids and glycerol, proteins from amino acids and nucleic acids from nucleotides. Biosynthesis, the building of these large molecules, joins small units into chains, and overall it requires an input of energy, supplied by ATP; the finished macromolecule holds more chemical potential energy than the small units did. Be careful with the bond language here, because it is a common trap: breaking any chemical bond always requires energy, and forming a bond always releases energy. Whether a whole reaction stores or releases energy depends on the balance between the bonds broken and the bonds formed, which is why building a protein costs energy overall while burning glucose releases it overall (the chemistry course works this balance with bond energies). A protein is a long chain folded from combinations of 20 common amino acids, and what a protein does follows from its amino acid sequence and the shape the chain takes as a result. Enzymes, the proteins that catalyze most of a cell's reactions, are the clearest example of shape determining function.

Photosynthesis, stated as inputs and outputs. In chloroplasts, plants and algae take in carbon dioxide and water and, using the energy of light, produce sugar and release oxygen: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. Matter is conserved: six carbons, twelve hydrogens and eighteen oxygens enter and the same numbers leave, rearranged. Energy is transformed, not created: light energy becomes chemical potential energy in glucose (more precisely, in the arrangement of atoms in glucose and oxygen compared with carbon dioxide and water). Some bacteria and archaea, including those at deep-sea vents where no light reaches, build sugar from carbon dioxide using energy released by inorganic chemical reactions such as the oxidation of hydrogen sulfide; that process is chemosynthesis, and it supports entire vent communities.

Cellular respiration. Cells release the energy stored in glucose by breaking it down with oxygen: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy. The first stage takes place in the cytoplasm; the rest, which yields most of the energy, takes place in mitochondria. The energy is captured in ATP, the molecule that powers active transport, biosynthesis and movement. Textbook estimates of the yield are around 30 ATP per glucose in eukaryotic cells, with the exact figure depending on conditions, and a substantial share of the energy is released as heat, which is why respiration keeps warm-blooded animals warm. Notice that the respiration equation is the photosynthesis equation reversed. Together the two processes cycle carbon and oxygen through ecosystems, while energy flows one way: in as light, out as heat.

Fermentation: releasing energy without oxygen. When oxygen is absent or scarce, many cells run only the first stage of respiration and then pass its products into fermentation. The fermentation reactions themselves make no ATP: they convert an electron carrier (NADH) back into the form the first stage needs (NAD⁺), so that stage, glycolysis, can keep running. The energy yield is small, two ATP per glucose, all from glycolysis, but it lets the cell keep going. Yeast carry out alcoholic fermentation: glucose → 2 ethanol + 2 CO2, the source of the bubbles in bread dough and the alcohol in fermented drinks. Muscle cells, especially when working so hard that they break glucose down faster than their mitochondria can use the products, and the bacteria that turn milk into yogurt and cabbage into sauerkraut, carry out lactic acid fermentation: glucose → 2 lactic acid. Which organelles are involved? Fermentation takes place in the cytoplasm, outside any membrane-bound organelle; the yield is low not because of the location as such but because glucose is only partly broken down: most of its chemical energy is still in the ethanol or lactic acid, and none of it passes through the oxygen-using stages that make most ATP in respiration. No membrane-bound organelle is needed at all: the enzymes that run fermentation are dissolved in the cytoplasm (they were made earlier by ribosomes), which is also why bacteria, which have no mitochondria, ferment in the same way. Inputs: glucose (the enzymes speed the reactions but are not used up); outputs: ethanol and carbon dioxide (yeast) or lactic acid (muscle and yogurt bacteria), plus two ATP per glucose made by glycolysis.

Enzymes. A reaction needs a starting push called activation energy. Enzymes are biological catalysts, most of them proteins (a few are RNA molecules, including the part of the ribosome that joins amino acids together), that lower the activation energy for a specific reaction, so it happens far faster at the cell's temperature without the enzyme being used up. The reactant, called the substrate, binds at the enzyme's active site, a pocket whose shape and chemistry match the substrate; in the induced-fit model the enzyme adjusts slightly around the substrate, like a hand closing on a ball, a refinement of the older lock-and-key picture. Because protein enzymes are folded chains, their activity depends on conditions. Most cells work within a narrow range of temperature and pH. At low temperature reactions are slow because molecules move and collide less. As temperature rises, rate rises, until the heat disrupts the protein's folded shape; this change, denaturation, is usually irreversible and the enzyme stops working. A pH outside the enzyme's optimal range alters the charges on the protein, changes its shape and changes how molecules in the cell interact. Substrate concentration matters too: rate rises with substrate until every active site is busy, then levels off.

Feedback keeps conditions steady. Cells and organisms regulate their internal functions as external conditions vary. In negative feedback, a change triggers a response that reduces the change: when blood glucose rises after a meal, insulin is released, cells take up glucose and the level falls back; when it falls, a different hormone raises it. A feedback loop has a sensor that detects the variable, a control center that compares it with a set point and an effector that acts. In positive feedback, a change triggers a response that increases it, driving a process to completion: during blood clotting, the first clotting factors activate more of themselves until the clot forms. Negative feedback maintains homeostasis; positive feedback is rarer and usually ends when some event, such as the clot being complete, stops it.

A worked model in the prose. A student sets up three bottles, each with 100 mL of water, one teaspoon of sugar and half a teaspoon of dried yeast, and stretches a balloon over each neck. Bottle A sits in ice water at about 5 °C, bottle B at room temperature of about 22 °C and bottle C in a warm water bath at about 35 °C. After 30 minutes the balloon circumferences are, hypothetically, 0 cm, 6 cm and 14 cm of inflation. Interpretation: the gas is carbon dioxide from alcoholic fermentation; at 5 °C the yeast enzymes work so slowly that no measurable gas forms; at 35 °C, near the yeast's optimum, gas forms fastest. If a fourth bottle at 55 °C gave 3 cm, the drop would be consistent with heat denaturing the enzymes. The balloon's circumference gain is only a rough indicator of the gas produced (balloons stretch unevenly), not a measure of ethanol or ATP, so it is an indirect, approximate measure of fermentation rate; matched balloons from one packet reduce the error.

Misconceptions. "Plants photosynthesize and animals respire" is half true and half false: plants respire too, all day and all night, and photosynthesis is what they do in addition. "Energy is made in the mitochondria" is wrong; energy is transformed from the bonds of glucose into ATP, and some is lost as heat. "Warmer is always faster for enzymes" is wrong beyond the optimum, where denaturation collapses the rate. "Fermentation happens in the mitochondria" is wrong; it happens in the cytoplasm, and its low yield comes from leaving most of the glucose energy in ethanol or lactic acid.

Investigation you can do. Use the three-bottle yeast setup above, with goggles and adult supervision for the warm bath. Measure balloon circumference with a tape every five minutes for 30 minutes and record it in a table with time down the side and temperature across the top. Keep sugar, yeast, water volume and bottle size the same; change only temperature. Then repeat at room temperature changing one other variable. Food source: compare equal masses of table sugar, flour (mostly starch, so this compares food substrates, not sugars) and a sweetener whose ingredient label you have read, because many packet sweeteners are bulked with fermentable sugars such as dextrose. Acidity: replace one tablespoon of the water with lemon juice so the total volume stays the same, check the pH of both bottles with indicator paper, and note that lemon juice adds a little sugar as well as acid, so this comparison is not perfectly clean. A result in which gas production rises with temperature up to the mid-30s and falls at higher temperatures supports the claim that fermentation is enzyme-driven and temperature-sensitive; a result in which a sweetener with no fermentable ingredient gives little or no gas supports the claim that yeast need a sugar they can break down. What the model cannot show: it cannot detect ethanol or ATP, it cannot distinguish fermentation from any respiration using oxygen trapped in the bottle, and a balloon measures volume roughly, so small differences are not meaningful.

Worked matter and energy accounting: A plant leaf takes in 6 molecules of carbon dioxide and 6 of water and makes one glucose and 6 oxygen. Atoms in: C 6, H 12, O 12 + 6 = 18. Atoms out: glucose C6H12O6 has C 6, H 12, O 6; six O2 add 12 oxygens; totals C 6, H 12, O 18. Matter balances. Energy: light energy entered and is now chemical potential energy in the glucose and oxygen, which the plant or an animal that eats it can release later by respiration. In respiration the same atoms are returned as carbon dioxide and water, and the energy leaves as ATP and heat. Matter cycles; energy flows through.

Worked balloon data: Hypothetical inflation at 30 minutes: 5 °C, 0 cm; 22 °C, 6 cm; 35 °C, 14 cm; 55 °C, 3 cm. Rate rises from 5 to 35 °C, consistent with faster molecular motion and enzyme action, then falls sharply at 55 °C, consistent with denaturation. The peak lies somewhere between 22 and 55 °C; the data cannot say exactly where, because only four temperatures were tested. To locate it, a student would add bottles at 30, 40 and 45 °C. The gas is carbon dioxide from the reaction glucose → 2 ethanol + 2 CO2, taking place in the yeast cytoplasm.

Worked enzyme graph reading: A hypothetical graph of the activity of a cellular enzyme (such as catalase) against pH rises from pH 4, peaks near pH 7 and falls to almost nothing by pH 10. Reading it: the optimum is near 7, which suits an enzyme working inside cells whose cytoplasm is near neutral. At pH 4 and pH 10 the changed charges alter the active site's shape, so substrate binds poorly. A second enzyme from the stomach with a peak near pH 2 would be plotted as a separate curve; the two curves together show that "optimal pH" is a property of each enzyme, not a universal number.

Worked refinement of a fermented product: A dairy wants a yogurt for people who cannot digest lactose. The bacteria that ferment milk convert some lactose to lactic acid, which is why yogurt has less lactose than milk, but not none. Design choice: add the enzyme lactase to the milk before or after fermentation so that remaining lactose is split into glucose and galactose, which the customers can absorb. Inputs: milk sugar, bacterial cultures, lactase; outputs: lactic acid, which thickens and sours the milk, plus the simpler sugars. A test batch is checked by measuring residual lactose, and the design is refined by adjusting fermentation time. The same reasoning refines yogurt for other dietary concerns: for a customer limiting sugar, a design team might test whether a longer fermentation with no added sugar lowers the remaining sugar; for bodybuilders, straining concentrates the protein. Each such idea is a hypothesis to be checked by measuring the finished product's composition, and any change to a food for infants or other sensitive groups also needs a validated food-safety process. Each variant changes an input or a condition of the same fermentation and is judged by measuring the output that matters to that customer.

Supported record: Make a six-column table with one row each for photosynthesis, cellular respiration and alcoholic fermentation and the headings Process, Inputs of matter, Outputs of matter, Energy in, Energy out, Location in the cell. Fill each row in your own words, then write one sentence under the chart explaining why fermentation releases so much less energy per glucose than respiration (hint: where is most of the energy at the end?).

Independent performance: Carry out the yeast investigation at three temperatures and then with one additional variable of your choice, and write a report. Success criteria: (a) a data table records balloon circumference every five minutes for every bottle, and the variable changed and the variables held constant are listed; (b) a line graph plots inflation against time with one line per bottle; (c) the explanation identifies the gas, writes the fermentation equation in words, names the part of the cell where it happens and states the inputs and outputs; (d) it explains the temperature pattern using enzyme shape and denaturation, and says where the optimum probably lies and how you would find it; (e) it states two things the balloon model cannot show; (f) it proposes one refinement to the method that would reduce measurement error.