Photosynthesis

Objectives:

By the time you complete this unit, you should be able to explain the core concepts on this page and in the corresponding sections of Chapter 10 (Photosynthesis) in Campbell:

  • How the light reactions capture light energy and convert it to chemical energy.
  • How the Calvin cycle makes use of the outputs of the light reactions.
  • Similarities and differences between photosynthesis and cellular respiration.

For more detailed objectives, see the review section at the bottom of this page.

The photosynthesis coverage in Campbell is excellent; I strongly recommend reading it. On this page, I'll give a brief account of some of the key photosynthesis ideas for Bio 6B, intended to supplement the chapter and give you a clear idea of what to expect for the quiz and midterm.

Before you start, I recommend that you watch this video: Photosynthesis | HHMI BioInteractive Video. It's an excellent video that goes from simple overview to detailed mechanisms. This might be all you need to know, in 11 minutes!

Photosynthesis lecture video

I made a lecture video focusing on the light reactions, emphasizing the interaction between light and matter.

Photosynthesis occurs in chloroplasts

Like cellular respiration, eukaryotic photosynthesis occurs in specialized membrane-bound organelles. The chloroplasts that perform photosynthesis are similar to mitochondria in many ways, especially because they are surrounded by two membranes and contain a very large surface area of folded internal membranes. In both cases, the internal membranes are involved in generating a proton gradient that drives ATP synthesis. In addition, both the mitochondrion and the chloroplast are derived from bacterial cells through endosymbiosis and both contain their own bacteria-like DNA.

Chloroplast structure.

Image credit: Callahan/Wikimedia (micrograph); Molnar & Gair/Wikimedia (diagram).

In mitochondria, the proton gradient is generated across the deep folds (cristae) of the inner membrane. In chloroplasts, the proton gradient is generated across the membranes of the thylakoids, which are separate, but derived from the inner membrane. Thylakoids are flat and grouped into stacks (called grana). This structure gives a high ratio of surface are to volume: plenty of room for the photosynthetic complexes in the membrane, but a low internal volume (the lumen) to fill with protons.

The light reactions power the Calvin cycle

Overall, photosynthesis captures light energy and uses it to produce organic molecules. The light reactions capture light energy and use it to produce chemical energy, while the Calvin cycle uses this chemical energy to convert oxidized inorganic carbon (CO2) into reduced organic carbon (G3P, which can be converted to sugar).

Light reactions and Calvin cycle.

The light reactions involve some mechanisms that are very similar to what you’ve seen in respiration. I’ll describe that in some detail. The Calvin cycle involves some interesting and complex biochemistry, but I’ll only describe it in basic terms.

The light reactions produce ATP and NADPH

There are two important outputs of the light reactions that get used in the Calvin cycle: ATP and NADPH, which carries reducing power in the form of high-energy electrons.

Photosystems absorb light energy

Photosynthesis begins when photosynthetic pigment molecules in photosystems absorb photons of light energy. Photosynthetic pigments include chlorophyll and a few accessory pigments, such as carotenes. When these molecules absorb photons, electrons are excited, or pushed to higher-energy orbitals. Thanks to the complex structure of photosystems, the high energy of the electrons can be shared across multiple pigment molecules by resonance transfer. The end result is that multiple photons can be absorbed, and their energy is passed to the reaction center chlorophylls at the heart of the photosystem. This boosts those electrons to such a high energy level that they are passed on to the electron transport chain, and the reaction center chlorophylls are oxidized.

Photosystems are organized into photosynthetic supercomplexes. This allows many light-absorbing pigment molecules to cooperate in a gigantic antenna complex, sharing energy across many electrons in one system. No individual chlorophyll molecule absorbs enough light energy to power photosynthesis, but the supercomplex accomplishes something that can't be done by its individual molecules. This is one of many examples in biology in which higher-level structures play an essential role.

PS II splits water

A huge energy input from the pigments in the photosystem is required to oxidize the reaction center chlorophylls of PS II. These oxidized chlorophylls in turn become powerful oxidizers, ready to replace their lost electrons. The replacement electrons come from a surprising source: H2O.

In cellular respiration, H2O is the stable, low-energy final destination of the electrons from the electron transport chain. It’s stable because the electrons are primarily held by the oxygen atom, which is highly electronegative. In other words, it would take a very powerful oxidizer to oxidize water. As it turns out, that’s exactly what PS II provides, in the form of those oxidized chlorophylls. 

When the reaction center of PS II oxidizes H2O, two electrons are passed to the reaction center chlorophylls. These will eventually be excited and passed to the ETC. In addition, two protons (H+) are released into the thylakoid space, contributing to the proton gradient. Finally, each oxidized H2O releases one oxygen, contributing to the formation of the O2 we breathe. Without photosynthesis, there would be no eukaryotes. (If you think this through, it also tells you that oxygen-producing photosynthesis must first have been performed by bacteria, before the evolution of prokaryotes. In fact, some of those oxygen-producing photosynthetic bacteria later took up residence inside eukaryotic cells and became chloroplasts.)

ATP synthase is powered by a proton gradient

The light reactions produce ATP, which is used as an energy source in the Calvin cycle. The mechanism for producing this ATP is the same as in cellular respiration: a proton gradient drives the rotor on ATP synthase, causing the enzyme to phosphorylate ADP. Just like in respiration, the proton gradient is generated across an internal membrane of the organelle. In photosynthesis, there are two sources of protons: some are transported into the thylakoid space by the electron transport chain, and some are produced in the thylakoid when PS II splits water.

The light reactions power the Calvin cycle: details

We can put the light reactions and the Calvin cycle together in one diagram, like this:

Light reactions and Calvin cycle.

Detailed steps:

  1. Photon energy is captured by Photosystem II (PS II). The absorbed energy excites electrons from chlorophylls within the photosystem. Once the electrons get sufficiently excited, the reaction center chlorophylls within PS II get oxidized.
  2. When PS II loses electrons, it gets replacement electrons by oxidizing H2O. This supplies more electrons for the electron transport chain, as well as releasing protons into the thylakoid space, adding to the proton gradient.
  3. Excited electrons from PS II are passed to the cytochrome complex by Pq. The cytochrome complex passes the electrons through a series of exergonic redox steps while capturing energy to pump protons into the thylakoid space. (You may recall that cytochromes also play important roles in cellular respiration. In addition to the mobile electron carrier cytochrome C, there are cytochrome proteins in the proton-pumping complexes.)
  4. The protons released by PS II and those pumped by the cytochrome complex build up a strong electrochemical proton gradient. The proton gradient powers ATP synthase, which produces ATP. (The ATP produced in this step will be used in the Calvin cycle; photosynthetic cells still need to perform cellular respiration to meet their other ATP needs.)
  5. Electrons from the cytochrome complex are passed to PS I at low energy. PS I captures more photon energy and excites these electrons to a high energy state.
  6. High-energy electrons from PS I are used to make NADPH by the enzyme NADP+ reductase. The overall process of electrons being captured from H2O, passed through the electron transport chain, and ultimately passed to NADPH, is called linear (or noncyclic) electron flow.
  7. Alternatively, the excited electrons from PS I can be passed back to the cytochrome complex, providing redox energy to power more proton pumping. This process, called cyclic electron flow, adds to the proton gradient and powers more ATP production. Cyclic electron flow occurs when the Calvin cycle slows due to a lack of ATP, because linear electron flow alone doesn't produce enough ATP relative to the amount of NADPH. Slowing the Calvin cycle results in a decreased NADP+ concentration, so electrons are passed to the cytochrome complex instead.
  8. Carbon fixation, in which a CO2 molecule is added to an existing organic molecule, is an exergonic process. In an earlier step, CO2 acceptor molecules (RuBP) with extremely high free energy were generated. In a reaction involving multiple molecules at once, high-energy bonds from RuBP are broken, while lower-energy bonds are created when the CO2 is added to become a carboxyl group (COO-). This reaction is catalyzed by the enzyme Rubisco (Ribulose Bisphosphate Carboxylase). Inorganic carbon (CO2) becomes organic carbon in this step.
  9. In the reduction phase of the Calvin cycle, NADPH reduces the carboxyl groups and ATP energy drives the formation of new, higher-energy bonds, ultimately producing G3P. This product can be used to produce sugars or used in the next phase of the Calvin cycle.
  10. In the regeneration phase of the Calvin cycle, some G3P molecules are reorganized to become high-energy RuBP molecules that can act as CO2 acceptors. ATP is required for this process.

Summary

The light reactions produce ATP and excited electrons, carried by NADPH. The Calvin cycle uses these products to turn CO2 into reduced organic molecules.

Photosynthesis compared to respiration

It's tempting to view photosynthesis and cellular respiration as opposite processes, because they have opposite inputs and outputs. However, the mechanisms are much more similar than they are different. Characteristics common to both processes include:

  • Double membrane-bound organelles that generate an electrochemical proton gradient.
  • The proton gradient is generated by an electron transport chain that couples redox reactions to proton transport.
  • The electron transport chain uses cytochrome proteins.
  • ATP synthase produces ATP, powered by proton gradient.
  • Electrons are carried by dinucleotides (NADPH or NADH).
  • There is a redox cycle for organic molecules (oxidizing in the citric acid cycle, reducing in the Calvin cycle).

All these similarities don't exist by chance. Both the mitochondrion and the chloroplast are derived from bacterial cells, and those cells shared many components by common ancestry. This is a common theme in biology: new pathways can evolve by reconfiguring pre-existing genes and proteins to produce different results.

NADPH vs. NADH

The structures of NADPH and NADH are very similar; both consist of two adenine nucleotides joined together. They do similar jobs: receiving electrons from one molecule and passing them on to another. Why is it that NADPH is used in photosynthesis, while NADH is used in cellular respiration? Why couldn't one of these electron carriers be used for both processes? The most important difference between NADPH and NADH isn't in the molecules themselves, but in the way they are managed within the cell:

  • NAD+ mostly exists in its oxidized form. Cellular respiration relies on rapid oxidation of food molecule, so the [NAD+]/[NADH] ratio is kept very high. There is usually a high concentration of NAD+, ready to act as an oxidant in glycolysis, pyruvate oxidation, or the citric acid cycle. This speeds up respiration. As soon as NAD+ gets reduced to NADH, it gets oxidized again by the high electron demand of the electron transport chain.
  • NADPH mostly exists in its reduced form. On the other hand, a high concentration of NADPH is needed to drive the Calvin cycle forward. The light reactions continually reduce NADP+, so it is always ready to act as a reducing agent.

These two dinucleotide electron carriers function in many other processes in cells, with NAD+ usually ready to act as an oxidizer and NADPH ready to act as a reducing agent.

Plants have mitochondria, too

The light reactions generate ATP, so many students end up thinking that photosynthetic cells don't need to do cellular respiration to generate ATP. This is mistaken, because the ATP generated in the light reactions gets used in the Calvin cycle. Cells still need ATP for other processes, so they must perform respiration. Mesophyll cells in the leaves of plants have plenty of chloroplasts, but they also have mitochondria.

Review

Terms & concepts

  • Carbon fixation
  • Chloroplast
  • Cytochrome complex
  • Electron transport chain in photosynthesis
  • Excited electrons
  • NADP+/NADPH
  • Photosynthetic pigments
  • Photosystems I and II (Note that, for historical reasons, the one that comes first is called PS II.)
  • Proton gradient in photosynthesis
  • Reaction center chlorophylls
  • Redox processes in photosynthesis
  • Resonance transfer
  • Stroma
  • Thylakoid

Review questions

  1. In the overall process of photosynthesis, CO2 (fully oxidized carbon) becomes reduced to form sugar molecules. Getting reduced means that electrons are added. Where do the electrons come from?
  2. What products of the light reactions get used in the Calvin cycle?
  3. Where does PS II get the electrons that will get excited by light energy? Where does it pass these electrons after they're excited?
  4. Where does PS I get the electrons that will get excited by light energy? Where does it pass these electrons after they're excited?
  5. In one step of photosynthesis, water gets split. Is this process endergonic or exergonic? What makes it happen?
  6. Is the whole process of the Calvin cycle, including all inputs and outputs, endergonic or exergonic? Explain.
  7. In the Calvin cycle, what gets oxidized and what gets reduced? (You don't need to memorize the names of the various intermediates; think in terms of inputs and outputs.)
  8. Describe the pathway that electrons take through the electron transport chain and the rest of photosynthesis, from beginning to end.
  9. "Rubisco" is short for ribulose bisphosphate carboxylase. What does carboxylase mean?
  10. Is the carbon fixation step of the Calvin cycle endergonic or exergonic? Explain.
  11. Why is ATP needed in the regeneration of RuBP, the CO2 acceptor, in the Calvn cycle?
  12. Compare and contrast the functions of the electron transport chains in photosynthesis and respiration.
  13. Does photosynthesis overall produce a net gain of ATP?
  14. Where does the O2 come from in photosynthesis?
  15. What is cyclic vs. linear (or noncyclic) electron flow in photosynthesis? Why are both pathways necessary?
  16. For the proton gradient in photosynthesis, where do the protons come from?
  17. Why do the photosynthetic cells of plants need mitochondria?
  18. There's a photo in Campbell showing a flask of dissoled chlorophyll emitting bright red fluorescence after being stimulated by short-wavelength light. Why doesn't this happen during photosynthesis?
  19. What colors of light work best for providing energy for  photosynthesis? Why?

References & further reading

Photosynthesis in general

Photosynthesis | HHMI BioInteractive Video. An excellent video that goes from simple overview to detailed mechanisms. This might be all you need to know, in 11 minutes!

Photosynthesis from OpenStax. A chapter from a free online textbook; this book is designed to be the equivalent of Campbell Biology.

Photosynthesis from Khan Academy. A complete unit, covering more or less the same set of concepts as Campbell.

Photosynthesis on PDB-101

The great protein site PDB-101 has some interesting resources related to the proteins involved in photosynthesis:

  • Chloroplast. A detailed image of the molecular organization of photosynthesis.
  • Cytochrome bc1. The proton-pumping complex at the heart of the electron transport chain in photosynthesis.
  • Photosynthetic Supercomplexes. As the name implies, the functional organization of photosynthesis involves complexes of complexes, which facilitates the processes of gathering light energy and exciting electrons.
  • Photosystem I.
  • Photosystem II.
  • Rubisco.
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