AP Biology · Lesson 2

Energy coupling and photosynthesis

Know Know it

Every reaction has a change in free energy. An exergonic reaction releases free energy: its products hold less than its reactants. An endergonic reaction needs free energy put in, because its products hold more.

ATP is the cell's energy currency. ATP hydrolysis removes its last phosphate, giving ADP and phosphate and releasing about 30 kilojoules per mole under standard conditions. In energy coupling, the cell links this exergonic step to an endergonic one, often by moving the phosphate onto a reactant or a protein. The total free-energy change of the pair is negative, so the pair can proceed. Cells keep remaking ATP from ADP and phosphate with energy from respiration or, in plants, from light.

Photosynthesis, overall: 6 CO2 + 6 H2O + light energy gives C6H12O6 + 6 O2. It happens in chloroplasts in two linked stages.

Light-dependent reactions (thylakoid membranes): chlorophyll in two photosystems absorbs light, mostly red and blue, and passes energized electrons down an electron transport chain. Water is split to replace those electrons, which releases oxygen gas. The chain pumps H+ into the thylakoid space, and H+ flowing back out through ATP synthase makes ATP. At the end of the chain, the electrons reduce NADP+ to NADPH.

Calvin cycle (stroma): the enzyme rubisco attaches CO2 to a five-carbon sugar, RuBP. ATP and NADPH then turn the products into the three-carbon sugar G3P. Some G3P leaves the cycle to build glucose and other sugars; the rest, with more ATP, rebuilds RuBP. The cycle returns ADP and NADP+ to the light reactions.

Trap The oxygen comes from water

The oxygen gas a plant releases comes from the water split in the light-dependent reactions. The oxygen atoms of CO2 end up in the sugar and water made by the Calvin cycle, not in the oxygen gas.

Worked example Easy

Which of these reactions in a cell is endergonic?

  1. Linking amino acids into a proteinAnswer
  2. Hydrolysis of ATP to ADP and phosphate
  3. Breaking glucose down to carbon dioxide and water
  4. Hydrolysis of starch into glucose

How to solve it

  1. Endergonic means the products hold more free energy than the reactants.
  2. Breaking large molecules down usually releases energy; building them up usually needs it.
  3. Linking amino acids builds a large molecule, so it is endergonic.

Why each choice is right or wrong

  • A. Correct. A protein holds more free energy than the separate amino acids, so each peptide bond needs energy put in, usually from ATP.
  • B. Incorrect. ATP hydrolysis releases free energy, which is why cells use it to drive other reactions.
  • C. Incorrect. Breaking glucose down to carbon dioxide and water releases a large amount of free energy, so it is exergonic.
  • D. Incorrect. Hydrolysis of starch into glucose releases a little free energy, so it is exergonic, not endergonic.

Explain Explain it

Task verbs: explain, predict

To explain a photosynthesis result, trace the link: light, then the light-dependent reactions, then ATP and NADPH, then the Calvin cycle, then sugar. A change at one end shows up at the other. Water and light feed the first stage; CO2 feeds the second.

Worked example Medium

Which molecules made in the light-dependent reactions does the Calvin cycle use to build sugar?

  1. Oxygen and water
  2. Glucose and oxygen
  3. ADP and NADP+
  4. ATP and NADPHAnswer

How to solve it

  1. The light reactions make three things: oxygen, ATP and NADPH.
  2. Oxygen leaves the plant; it is not used to build sugar.
  3. So the Calvin cycle uses ATP and NADPH.

Why each choice is right or wrong

  • A. Incorrect. Oxygen is released as a waste product of splitting water, and the Calvin cycle does not use it.
  • B. Incorrect. Glucose is built from the Calvin cycle's own product, G3P, so it is not an input from the light reactions.
  • C. Incorrect. ADP and NADP+ flow the other way: the Calvin cycle returns them to the light reactions to be recharged.
  • D. Correct. ATP supplies energy and NADPH supplies the electrons that turn fixed CO2 into the sugar G3P.

Predict Your turn

In a fictional cell, joining molecule X to molecule Y has a free-energy change of +20 kilojoules per mole. Predict whether this reaction can go forward on its own in the cell, and say why. Then explain how coupling it to ATP hydrolysis, which has a free-energy change of -30.5 kilojoules per mole, lets it go forward, and give the total free-energy change of the coupled pair.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Predicts that the reaction cannot go forward on its own because its free-energy change is positive (it is endergonic)
  • Explains that the free energy released by ATP hydrolysis is used to drive the reaction
  • Gives the total free-energy change as -10.5 kilojoules per mole and says a negative total lets the pair go forward

Model answer

The reaction cannot go forward on its own, because its free-energy change of +20 kilojoules per mole is positive: it is endergonic. The cell couples it to ATP hydrolysis, which is exergonic, so the free energy released when ATP is hydrolyzed drives the joining of X and Y. Often ATP passes its phosphate to X, which makes X reactive enough to join Y. The total free-energy change is +20 + (-30.5) = -10.5 kilojoules per mole, and because the total is negative, the coupled pair can go forward.

Apply Apply it

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