This unit follows energy from sunlight into sugar, and from sugar into ATP, the molecule cells spend to do work. By the end, you should be able to trace each stage of photosynthesis and cellular respiration, say where it happens and what it makes, and explain how light, temperature and gas levels change the rates.
Read the overview and big idea first, then work through each topic note and try its checkpoint question before reading the answer.
Every living cell needs a steady supply of ATP, and this unit explains where it comes from. You'll see how plants capture light energy and store it in sugar, and how nearly all organisms, plants included, release that energy again.
Honors Biology Unit 5 covers cellular energy. It starts with ATP, how its hydrolysis releases energy, and how cells couple that energy to reactions that wouldn't happen on their own. It then covers photosynthesis (the light reactions in the thylakoids and the Calvin cycle in the stroma), the factors that speed up or slow down photosynthesis and respiration, and the three stages of cellular respiration (glycolysis, the Krebs cycle and the electron transport chain), plus fermentation when oxygen runs out.
Breaking glucose down releases energy, and building proteins or pumping ions uses energy. ATP carries energy between the two. Under standard biochemical conditions, its hydrolysis releases about 7.3 kcal/mol, enough to drive many uphill reactions when the two are coupled. The energy released varies with cellular conditions.
The two overall equations are mirror images. Photosynthesis takes in and water and gives off , while respiration takes in and gives off and water. Plants do both, since their own cells need ATP too.
Enzymes work fastest near an optimal temperature and pH. Extreme heat or pH can change their shape and reduce their activity. Photosynthesis also depends on light and , and aerobic respiration depends on oxygen. A limiting factor is a condition or resource that restricts the rate.
In the thylakoid membrane, light-energized electrons power the H+ pumps. In the inner mitochondrial membrane, electrons from NADH and do the same job. In both, H+ ions rush back through ATP synthase, and that flow attaches phosphate to ADP. If you understand chemiosmosis once, you understand it in both organelles.
ATP (adenosine triphosphate) is the molecule cells spend to pay for work.
Photosynthesis turns light energy into chemical energy in two linked stages inside the chloroplast.
Neither pathway runs at a fixed speed.
Cellular respiration breaks glucose down to make ATP.
Open this guide for a closer review of the topic.
ATP (adenosine triphosphate) is the molecule cells spend to pay for work. It has an adenine base, a ribose sugar and three phosphate groups. The phosphate groups are negatively charged and repel each other. Hydrolysis produces ADP and inorganic phosphate, which together have lower free energy than ATP and water.
Cells release that energy by hydrolysis, using water to split ATP into ADP and inorganic phosphate (). Under standard conditions this releases about 7.3 kcal/mol ( kcal/mol). Respiration and photosynthesis rebuild ATP by adding a phosphate back onto ADP, so the same molecules get recycled thousands of times a day.
A common mix-up is picturing energy stored inside a "high-energy bond" like a tiny battery. Breaking any bond takes energy. The payoff comes because the products (ADP and ) are much more stable than ATP.
ATP drives endergonic reactions () through energy coupling. Often ATP hands its phosphate directly to another molecule, which makes that molecule reactive enough to finish the reaction.
Worked example. Under standard biochemical conditions, making glutamine from glutamate and ammonia has a free energy change of kcal/mol, so it is unfavorable on its own. Couple it to ATP hydrolysis and add the free energy changes. kcal/mol. The negative total means the coupled reaction is thermodynamically favorable under these conditions.
A reaction has a standard free energy change of kcal/mol. Can coupling it to one ATP hydrolysis make it favorable under standard biochemical conditions? Yes. kcal/mol, so the coupled reaction is thermodynamically favorable under those conditions.
Photosynthesis turns light energy into chemical energy in two linked stages inside the chloroplast. The overall equation is .
The light reactions happen in the thylakoid membranes. Light excites electrons in photosystem II first. It was discovered second, which is why the numbers look backward. PSII replaces its lost electrons by splitting water, and that releases . The electrons move down an electron transport chain that pumps into the thylakoid space, and flowing back out through ATP synthase makes ATP. Photosystem I re-energizes the electrons and passes them to , forming NADPH.
The Calvin cycle runs in the stroma. The enzyme RuBisCO attaches to the 5-carbon sugar RuBP, and the unstable 6-carbon product splits into two 3-PGA. ATP and NADPH then reduce 3-PGA to the 3-carbon sugar G3P, and most G3P is recycled to rebuild RuBP.
Worked example. How much ATP and NADPH does one glucose cost? Each turn of the cycle fixes one and uses 3 ATP and 2 NADPH. Three turns make one net G3P, costing 9 ATP and 6 NADPH. Glucose has 6 carbons, so it takes 2 G3P, which means 6 turns. That is ATP and NADPH per glucose.
A plant is given water made with a heavy form of oxygen (O-18). Where does the heavy oxygen show up? It shows up in the the plant releases, because the light reactions split water to get electrons.
| Feature | Light reactions | Calvin cycle |
|---|---|---|
| Location | Thylakoid membranes | Stroma |
| Inputs | Light, $H_2O$, ADP, $NADP^+$ | $CO_2$, ATP, NADPH |
| Outputs | ATP, NADPH, $O_2$ | G3P (used to make glucose), ADP, $NADP^+$ |
| Uses light directly? | Yes | No, but it stops once ATP and NADPH run out |
Neither pathway runs at a fixed speed. The rate depends on enzyme activity and the supply of raw materials. A limiting factor is a condition or resource that restricts the rate.
Temperature and pH affect enzyme activity. Warming increases molecular motion, so reaction rates often rise up to an optimum. Optima vary, although many human enzymes work well near 37°C. Higher temperatures reduce activity and can denature an enzyme by changing its shape. A pH far from the optimum can change charges in the enzyme and may also alter its shape.
Light intensity controls the light reactions. As light increases, photosynthesis usually speeds up until it reaches a light saturation point, where another factor, such as supply, limits the rate. More light does not increase the rate at that point. Very intense light can damage the photosynthetic machinery and reduce the rate.
Gas levels matter for both pathways. Without , the aerobic electron transport chain stalls because oxygen is its final electron acceptor. Some cells use fermentation to keep glycolysis running, yielding a net 2 ATP per glucose. Low slows carbon fixation. In hot, dry weather, plants may close their stomata to save water, reducing entry. RuBisCO then reacts more often with instead of , leading to a wasteful process called photorespiration.
A classic lab counts bubbles from a pond plant like Elodea at different lamp distances. More bubbles per minute can suggest faster photosynthesis, but bubble sizes vary, so measuring oxygen volume gives a better estimate.
You move a lamp closer to a plant, but its photosynthesis rate stops rising. What happened? The plant reached its light saturation point, so another factor, such as or temperature, is limiting the rate.
Cellular respiration breaks glucose down to make ATP. The overall equation is , the reverse of photosynthesis.
Glycolysis happens in the cytoplasm and doesn't need oxygen. It spends 2 ATP and makes 4 while splitting glucose into two pyruvate, for a net 2 ATP and 2 NADH. With oxygen present, each pyruvate enters the mitochondrion, loses one carbon as and becomes acetyl-CoA, making 1 NADH each.
The Krebs cycle runs in the mitochondrial matrix and turns twice per glucose. The two turns release 4 and make 6 NADH, 2 and 2 ATP.
The electron transport chain sits in the inner mitochondrial membrane. NADH and drop off electrons, the chain pumps into the intermembrane space, and flows back through ATP synthase to make most of the ATP. Oxygen is the final electron acceptor and becomes water.
Without oxygen, some cells use fermentation to turn NADH back into so glycolysis can keep running. Muscle cells make lactate, while yeast can make ethanol and .
Worked example. Estimate ATP per glucose. Count the carriers first. 2 NADH (glycolysis) + 2 (pyruvate step) + 6 (Krebs) = 10 NADH, plus 2 . At about 2.5 ATP per NADH and 1.5 per , . Add 2 ATP from glycolysis and 2 from the Krebs cycle for about 32. That estimate assumes a high-yield shuttle transfers electrons from cytoplasmic NADH into the mitochondrion. A different shuttle lowers the estimate to about 30. Older textbooks say 36-38, but current estimates are about 30-32.
Why does the Krebs cycle stop when oxygen runs out, even though it never uses directly? Without , the ETC stalls, so NADH cannot be turned back into by the chain. The Krebs cycle eventually lacks the it needs to accept electrons.
| Stage | Location | Needs $O_2$? | Products per glucose |
|---|---|---|---|
| Glycolysis | Cytoplasm | No | 2 pyruvate, 2 ATP (net), 2 NADH |
| Pyruvate oxidation | Mitochondrial matrix | Indirectly | 2 acetyl-CoA, 2 NADH, 2 $CO_2$ |
| Krebs cycle | Mitochondrial matrix | Indirectly | 6 NADH, 2 $FADH_2$, 2 ATP, 4 $CO_2$ |
| Oxidative phosphorylation | Inner mitochondrial membrane | Yes | About 26-28 ATP and water |
| Term | Definition |
|---|---|
| ATP | Adenosine triphosphate, the molecule cells hydrolyze to release energy for work like active transport and muscle contraction. |
| Chemiosmosis | ATP production driven by $H^+$ ions flowing down their gradient across a membrane through ATP synthase. |
| ATP synthase | A membrane enzyme that uses the flow of $H^+$ ions to attach phosphate to ADP, found in both thylakoids and mitochondria. |
| NADPH | The electron carrier made in the light reactions that delivers high-energy electrons to the Calvin cycle. |
| NADH | The electron carrier made in glycolysis and the Krebs cycle that delivers electrons to the electron transport chain. |
| Chloroplast | The plant and algae organelle where photosynthesis happens, with thylakoids for the light reactions and stroma for the Calvin cycle. |
| Mitochondrion | The organelle where the Krebs cycle and electron transport chain take place in eukaryotic cells. |
| Final electron acceptor | A molecule that accepts electrons at the end of an electron transport chain. In aerobic respiration, oxygen accepts electrons and $H^+$ to form water. |
| Stomata | Pores on a leaf that let $CO_2$ in and water vapor and $O_2$ out, and that close to save water in dry conditions. |
Because plants make their own sugar, it's easy to assume they skip respiration. They don't. Plant cells have mitochondria and run respiration all the time to turn that sugar into ATP. During the day photosynthesis usually outpaces respiration, so plants release more than they use.
The old name "dark reactions" causes this one. The Calvin cycle doesn't use light directly, but it runs on ATP and NADPH from the light reactions, so in most plants it runs during the day and slows quickly after dark.
Fermentation itself makes no ATP. All 2 ATP come from glycolysis. Fermentation's only job is to turn NADH back into so glycolysis has what it needs to keep going.
The equation makes it look like oxygen could come from either reactant. Experiments with labeled oxygen show it comes from water, split by photosystem II. The oxygen atoms in end up in the sugar and in new water molecules.