Jugami Biology

Biology · Lesson 4

Photosynthesis and Respiration

How a leaf turns light into sugar, and how every cell burns it for power.

Nearly every living thing runs on energy that started as sunlight. Plants, algae and some bacteria capture it in photosynthesis and store it in sugar. Then almost all organisms, plants included, release that energy through cellular respiration to make ATP, the cell's energy currency.

Both processes happen in organelles you met in The Cell: chloroplasts and mitochondria.

Ink sketch of a sunlit leaf: arrows show carbon dioxide going in from the air, water coming up the stem (H₂O in), and oxygen going out.O₂ outCO₂ inH₂O infig. 1: a leaf at work

1. The big picture

Photosynthesis uses light energy to build glucose from carbon dioxide and water, releasing oxygen:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Aerobic respiration runs the overall reaction in reverse, releasing the stored energy and capturing part of it as ATP:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (about 30–32 ATP)

Matter cycles: the carbon, hydrogen and oxygen atoms go round and round between the two processes. Energy does not cycle. It flows in as light, and every step loses some as heat, which is why life needs a constant supply of sunlight.

These summary equations hide dozens of steps. Each process is a chain of enzyme reactions, split between different parts of the cell.

2. Energy-flow diagram

Click a part of the diagram, or use the buttons, to read its field notes.

Energy flow between a chloroplast and a mitochondrionLight enters a chloroplast. The chloroplast sends glucose to glycolysis in the cytoplasm and oxygen to the mitochondrion. Pyruvate from glycolysis enters the mitochondrion, which releases carbon dioxide and water that return to the chloroplast, and makes ATP for the cell.cytoplasmlightthylakoids (grana)stromachloroplastmatrixmitochondrionglycolysis(in the cytoplasm)glucosepyruvateO₂CO₂ + H₂O returnATP
Thylakoids
Light-dependent reactions: light, water in; O2, ATP and NADPH out.
Stroma
Calvin cycle: CO2, ATP and NADPH in; G3P out, used to build glucose.
Glycolysis (cytoplasm)
Glucose split into 2 pyruvate; net 2 ATP and 2 NADH.
Matrix
Pyruvate oxidation and the Krebs cycle: CO2 released, NADH and FADH2 loaded.
Inner membrane
Electron transport and chemiosmosis: O2 becomes water; most ATP made here.

3. Light-dependent reactions

These happen in the thylakoid membranes, where chlorophyll and other pigments are grouped into two light-harvesting complexes called photosystems. They are numbered in the order they were discovered, not the order they work.

  1. Photosystem II absorbs light, and the energy boosts electrons out of a special chlorophyll pair. To replace them, an enzyme splits water: 2H2O → 4H+ + 4e− + O2. All the oxygen photosynthesis releases comes from water, not from carbon dioxide.
  2. The electrons pass down an electron transport chain. Their energy pumps H+ ions into the space inside the thylakoid, building up a concentration gradient.
  3. Photosystem I absorbs more light and re-energizes the electrons, which are used to reduce NADP+ to NADPH.
  4. H+ ions flow back out to the stroma through ATP synthase, a rotating enzyme that makes ATP from ADP and phosphate.

ATP and NADPH carry the captured energy to the Calvin cycle. Oxygen is a by-product and leaves the leaf.

4. The Calvin cycle

In the stroma, the Calvin cycle uses that ATP and NADPH to build sugar from CO2. It does not use light directly, though it stops in the dark once the ATP and NADPH run out.

  1. Carbon fixation. The enzyme RuBisCO attaches CO2 to a five-carbon sugar, RuBP. The six-carbon result immediately splits into two three-carbon molecules (3-PGA). RuBisCO is probably the most abundant protein on Earth.
  2. Reduction. ATP and NADPH convert 3-PGA into G3P (glyceraldehyde 3-phosphate), a three-carbon sugar.
  3. Regeneration. Most G3P is rearranged, using more ATP, to rebuild RuBP so the cycle can continue.

Each turn fixes one carbon. Three turns fix three CO2 and yield one spare G3P, using 9 ATP and 6 NADPH. Glucose has six carbons, so it takes two G3P, or six turns. The plant uses G3P to make glucose, sucrose, starch, cellulose and more.

5. Cellular respiration

StageWhereWhat happensPer glucose
GlycolysisCytoplasmGlucose is split into two pyruvate. No oxygen needed.Net 2 ATP, 2 NADH
Pyruvate oxidationMitochondrial matrixEach pyruvate loses one carbon as CO2 and becomes acetyl-CoA.2 NADH, 2 CO2
Krebs (citric acid) cycleMitochondrial matrixAcetyl-CoA is broken down; its carbons leave as CO2. The cycle turns twice per glucose.2 ATP, 6 NADH, 2 FADH2, 4 CO2
Electron transport chain and chemiosmosisInner mitochondrial membraneNADH and FADH2 give up electrons. The chain pumps H+ out of the matrix; O2 takes the electrons and forms water; H+ flows back through ATP synthase.About 26–28 ATP

The total is about 30–32 ATP per glucose. Each NADH is worth roughly 2.5 ATP and each FADH2 about 1.5, and the exact total depends on how NADH from glycolysis gets into the mitochondrion. Older textbooks give 36–38, from rounding to 3 and 2 and ignoring the cost of moving molecules across membranes.

Oxygen's only job is at the very end, as the final electron acceptor. Without it, the chain backs up and the mitochondria stop making most of their ATP.

6. Fermentation

When oxygen runs short, glycolysis can still make 2 ATP per glucose, but only if NAD+ is recycled from NADH. Fermentation does that recycling. It makes no extra ATP.

Lactic acid fermentation

Muscles and yogurt

Pyruvate is turned into lactate. Your muscle cells do this during hard sprints, and bacteria do it when they turn milk into yogurt.

C6H12O6 → 2C3H6O3

Alcoholic fermentation

Yeast, bread and beer

Yeast turns pyruvate into ethanol and CO2. The CO2 makes bread rise; the ethanol bakes off.

C6H12O6 → 2C2H5OH + 2CO2

How the two processes connect

The products of photosynthesis (glucose and O2) are the reactants of respiration, and the products of respiration (CO2 and H2O) are the reactants of photosynthesis. Both use an electron transport chain and ATP synthase driven by an H+ gradient, in membranes inside organelles that were once free-living bacteria (see Evolution).

A common mistake is to think plants photosynthesize while animals respire. Plants do both: their mitochondria respire day and night. In daylight a growing plant usually photosynthesizes faster than it respires, so overall it takes in CO2 and gives out O2.

Explore: what limits the rate?

This is a simplified model, not real data. It uses the limiting-factor idea: the scarcest input sets the pace. Real curves bend gradually, and temperature, water and the type of plant matter too.

Turn on JavaScript to try the sliders. The idea: in dim light, adding CO2 does nothing because light is limiting; in bright light with little CO2, CO2 is limiting; once both are plentiful, the enzymes set a maximum.

7. Equation builder

Pick a reaction and set the coefficients. A balanced equation has the same number of each kind of atom on both sides.

Turn on JavaScript to use the builder. Answers: photosynthesis 6, 6 → 1, 6; respiration 1, 6 → 6, 6; alcoholic fermentation 1 → 2, 2; lactic acid fermentation 1 → 2.

8. How do we know?

  1. Before 1644 (published 1648) Jan Baptist van Helmont grows a willow in a weighed pot of soil for five years. The tree gains about 164 pounds while the soil loses only about 2 ounces. He concludes the tree is made from water (published in 1648). He missed the air.
  2. 1771 Joseph Priestley finds that a sprig of mint "restores" air in a sealed jar where a candle has burned out, so a mouse can live in it.
  3. 1779 Jan Ingenhousz shows that only the green parts of plants do this, and only in light.
  4. 1782 Jean Senebier shows that plants take in "fixed air" (carbon dioxide) to do it.
  5. 1804 Nicolas-Théodore de Saussure measures the gases and plant mass and shows that water is also used: the plant gains more mass than the carbon dioxide alone accounts for.
  6. 1882 Theodor Engelmann shines a spectrum of light onto a filament of green alga. Oxygen-seeking bacteria crowd around the parts lit by red and blue light, revealing which colors drive photosynthesis.
  7. 1937 Hans Krebs and William Johnson describe the citric acid cycle (Krebs shared the 1953 Nobel Prize). The same year, Robin Hill shows isolated chloroplasts can release oxygen.
  8. 1941 Samuel Ruben, Martin Kamen and colleagues give algae water labeled with heavy oxygen-18. The label appears in the oxygen gas, showing it comes from water.
  9. 1946–1950s Melvin Calvin, Andrew Benson and James Bassham use radioactive carbon-14 to trace the path of carbon through the Calvin cycle. Calvin wins the 1961 Nobel Prize in Chemistry.
  10. 1961 Peter Mitchell proposes chemiosmosis: an H+ gradient across a membrane drives ATP synthesis. Doubted at first, it won him the 1978 Nobel Prize in Chemistry.

Quiz: Energy Lab Practical

Talk about it

  1. A tree's trunk is mostly carbon. Where did that carbon come from, and why did van Helmont get it wrong?
  2. If you breathe out a carbon atom today, trace one path it could take back into your body.
  3. Why do your muscles burn during a sprint but recover when you rest and breathe hard?
  4. Peter Mitchell's idea was rejected by many scientists for years. What kind of evidence would it take to change your mind about a scientific idea?
  5. Greenhouse growers sometimes add CO2 to the air. When would that help, and when would it be wasted?

Sources

  1. Ruben, S., Randall, M., Kamen, M. and Hyde, J. L. (1941). Heavy oxygen (O18) as a tracer in the study of photosynthesis. Journal of the American Chemical Society 63: 877–879. doi:10.1021/ja01848a512
  2. Bassham, J. A., Benson, A. A. and Calvin, M. (1950). The path of carbon in photosynthesis. Journal of Biological Chemistry 185: 781–787. doi:10.1016/S0021-9258(18)56368-7
  3. Krebs, H. A. and Johnson, W. A. (1937). The role of citric acid in intermediate metabolism in animal tissues. Enzymologia 4: 148–156. Reprinted in FEBS Letters 117 (1980): doi:10.1016/0014-5793(80)80564-3
  4. Mitchell, P. (1961). Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism. Nature 191: 144–148. doi:10.1038/191144a0
  5. Hinkle, P. C. (2005). P/O ratios of mitochondrial oxidative phosphorylation. Biochimica et Biophysica Acta – Bioenergetics 1706: 1–11. doi:10.1016/j.bbabio.2004.09.004
  6. Engelmann, T. W. (1882). Ueber Sauerstoffausscheidung von Pflanzenzellen im Mikrospektrum. Pflügers Archiv 27: 485–489. doi:10.1007/BF01802976
  7. Hill, R. (1937). Oxygen evolved by isolated chloroplasts. Nature 139: 881–882. doi:10.1038/139881a0
  8. Gest, H. (2000). Bicentenary homage to Dr Jan Ingen-Housz, MD (1730–1799), pioneer of photosynthesis research. Photosynthesis Research 63: 183–190. doi:10.1023/A:1006460024843
  9. The Nobel Prize in Chemistry 1961 (Melvin Calvin). nobelprize.org
  10. The Nobel Prize in Physiology or Medicine 1953 (Hans Krebs and Fritz Lipmann). nobelprize.org
  11. The Nobel Prize in Chemistry 1978 (Peter Mitchell). nobelprize.org
  12. Clark, M. A., Douglas, M. and Choi, J. (2018). Biology 2e, chapters 7 (Cellular Respiration) and 8 (Photosynthesis). OpenStax. openstax.org/books/biology-2e