1. Introduction

During the light reactions of photosynthesis, a chloroplast takes light energy and converts it into electrical energy, which is then used to create chemical energy in the form of NADPH and ATP. These energy transformations occur along the thylakoid membrane (shown below).

A chloroplast, thylakoids, and the thylakoid membrane

The goal of what follows is for you to be able to explain these transformations. Start by familiarizing yourself with the diagram:

  • m: an entire chloroplast
  • n: a stack of three thylakoids
  • o:  the thylakoid membrane
  • 1:  the stroma, which is the fluid between the thylakoids and the inner chloroplast membrane
  • 2: the thylakoid space, the enclosed, fluid-filled compartment inside the thylakoid
  • A and D: light energy, which drives the entire process
  • l,  b, e, f (and some other unlabeled segments): an electrical current, driven by light energy
  • g and h: chemical energy in the form of NADPH (at g) and ATP (at h).

. Let’s go.

2. Chlorophyll, photoexcitation, and photosystems

Chlorophyll is embedded in the thylakoid membrane. If you look at the diagram above, you can find chlorophyll represented by the green circles in the protein complexes surrounding the orange arrows at “l” and “e.” These protein complexes are labeled as “photosystem 2” and “photosystem 1.”

As we saw in the previous tutorial, chlorophyll has a central porphyrin ring (everything within the red circle at “2”), with a magnesium ion (Mg++, number “3”) positioned in the center.

Photoexcitation

This molecular structure allows for photoexcitation. When chlorophyll (shown at “3” in the diagram on the left) absorbs light energy (“1”), its electrons (“4”) can get boosted up to a higher energy level. Once there, they fall back down (“5”), emitting heat (“6”) and light (“7”).

This circular flow of electrons only happens if chlorophyll is artificially extracted from a chloroplast, dissolved in alcohol, and exposed to ultraviolet light. Click here to watch a cool video (not one of mine) showing photoexcitation in a chlorophyll solution.

A photosystem

In a chloroplast, photoexcitation leads to the formation of an electrical current. Here’s how that works.

Chlorophyll molecules are organized into photosystems, which are embedded in the membranes of the thylakoids. A single photosystem is shown on the right. Look at the diagram at the top of this page to see photosystems in the context of the thylakoid membrane.

Within a photosystem, light (“1”) strikes any one of the many chlorophyll molecules (any of the green circles, such as those at “3”). These chlorophylls are collectively referred to as an antenna complex. They work in a way that’s analogous to an old-fashioned television antenna or a more modern satellite dish receiver: they collect energy and then pass it to another part of a system.

The chlorophylls in an antenna complex act as a kind of net. If any one of the chlorophylls captures a photon’s energy, that energy can be passed from one chlorophyll to the next. This energy transfer is not by an electrical current: it’s by a process called resonance energy transfer (which you can read about on Wikipedia).

At some point, the energy arrives at a reaction center (“4”). In the reaction center, a chlorophyll molecule (at “5”) is positioned across from a protein complex called a primary electron acceptor (represented by the red rectangle at “2”). When electrons from the chlorophyll at “5” are boosted to a higher energy level (as shown by the yellow arrow between “5” and “2”), the primary electron acceptor snatches them up. This chemically reduces the primary electron acceptor (because it has gained electrons) and oxidizes the chlorophyll (because it has lost electrons). This loss of electrons, caused by photoexcitation, is the driving force behind the electrical current that’s the basis for transforming light energy into chemical energy during the light reactions.

3. Video: Noncyclic Electron Flow

In the diagram below, you can see how two photosystems (designated by the letters “l” and “e”) are organized in a thylakoid membrane. As electrons flow through this system, they flow through Photosystem 2 and then through Photosystem 1. That pathway is called noncyclic electron flow. 

Why does Photosystem 2 come first? These photosystems were numbered by the order of their discovery in the 1950s and 1960s. You just have to memorize it: during noncyclic electron flow, photosystem 2 comes before photosystem 1. 

To understand what happens during the light reactions, start by watching this video.

4. Reading: Noncyclic Electron Flow

Now study this diagram and text.


The text below is set up so that you can scroll while continuing to look at the image.

  1. “A” shows a photon striking the antenna complex of Photosystem 2. The photon excites electrons in chlorophyll, and the energy bounces around until it reaches p680 in Photosystem 2’s reaction center. p680 is named for the wavelength of light that best stimulates that photosystem. Light at 680 nanometers is in the red part of the visible light spectrum.
  2. Electrons jump from p680 to a primary electron acceptor, which passes them to an electron transport chain (indicated by the orange arrow at “b”). The electron transport chain (ETC) is a series of enzymes embedded in the thylakoid membrane, each more electronegative than the previous one. You can think of electronegativity as the “capacity to grab electrons.” Because of this increasing electronegativity, electrons flow from the primary electron acceptor to the other proteins that make up the ETC, such as  PQ (which stands for “plastoquinone”), cytochromes, and PC (which stands for “plastocyanin”).
  3. During each electron handoff, the electrons release a bit of the energy initially absorbed from the photon that hit Photosystem 2’s antenna complex. That released energy is used to perform some work, which is shown at “c.” The work is pumping protons from the stroma to the thylakoid space. As we’ll see (and as you already know from studying ATP synthesis in the oxidative phosphorylation stage of cellular respiration), this is the basis of ATP synthesis. Note that this is an active transport process: protons are getting pumped from where they’re in low concentration (in the stroma) to where they’re in high concentration (in the thylakoid space).
  4. Loss of electrons is oxidation, and p680 is now in an oxidized state. To replace these electrons, an oxygen-evolving complex (also known as a water-splitting complex) that is built into Photosystem 2 grabs electrons from a water molecule. The electrons flow to p680. Without these electrons, the water dissociates into protons and oxygen, as shown in “k.” At “j” you can see these protons accumulating in the thylakoid space (region “2”), joining the protons that were pumped there by a proton pump at “c.”
  5. Now find “e.” Look down a bit, and locate the electron just to the right of PC. This electron was initially boosted by Photosystem 2. It was passed along the electron transport chain, and its energy was used for proton pumping. Depleted of energy, it arrives at Photosystem 1.
  6. Photosystem 1 is also being struck by photons (letter “d”), which excite the electrons in its antenna complex, which bounce around until arriving at Photosystem I’s reaction center, p700. At p700, energized electrons are grabbed by the primary electron acceptor of Photosystem 1, which sends them to a second electron transport chain (shown at “f”).
  7. The electron transport chain that follows Photosystem 1 (again, “f”) sends its electrons toward the mobile electron carrier NADP+. An enzyme called NADP+ reductase uses these electrons to reduce NADP+ to NADPH (shown at “g”).
  8. In step “6,” p700 was oxidized. To replace its lost electrons, it accepts the electron that has been flowing in its direction from Photosystem II, which ultimately can be traced back to a water molecule.
  9. Steps “c” and “k” have been causing protons to accumulate in the thylakoid space. The result is a two-fold gradient. First, it’s a diffusion gradient, with a thousand times more protons inside the thylakoid space than in the stroma (the fluid outside of the thylakoids). Just based on simple diffusion, these protons will “want” to diffuse from where they’re more concentrated to where they’re less concentrated. Second, it’s an electrochemical gradient. These protons are all positively charged. Like charges repel, and these protons “want” to get away from one another.
  10. This proton concentration gradient is a source of potential energy, and the chloroplast makes use of it at letter “i” (on the right side of the diagram). The channel at “i” goes through ATP synthase, the enzyme that uses the kinetic energy of diffusing protons to make ADP and Pi into ATP (which you can see happening at “h”). This movement through ATP synthase is facilitated diffusion: the flow of protons from the thylakoid space to the stroma is energetically downhill, which is why it can be coupled to the uphill process of creating ATP from ADP and Pi.

5. Photoexcitation, Photosystems, and Non-cyclic Electron Flow: Checking Understanding

In mitochondria, protons flow from the intermembrane space to the matrix through ATP synthase

ATP synthesis during noncyclic electron flow parallels what happens in cellular respiration. In cellular respiration, an electron transport chain powered by energy from food pumps protons into the mitochondrial intermembrane space. These trapped protons flow back to the mitochondrial matrix through an ATP synthase channel, creating ATP. Because oxygen is required for the mitochondrial electron transport chain, this process is called oxidative phosphorylation.

In chloroplasts, protons flow from the thylakoid space to the stroma through ATP synthase.

In the light reactions, the protons are trapped, instead, in the thylakoid space (2), and flow back to the stroma (1) through an ATP synthase channel. Because light powers the electron transport chain that generates this proton gradient during photosynthesis, the process is called photophosphorylation.

The fact that ATP synthase is used in life’s two great energy-related processes — cellular respiration and photosynthesis —  is no coincidence. As we’ll learn in Unit 7, the ATP synthase in mitochondria and chloroplasts is an evolutionary homology: it’s a feature derived from a common ancestor, possibly the universal ancestor of all living things. If you’re interested in learning more about ATP synthase and its evolution, start with this article about ATP synthase on Wikipedia (the link will open in a new tab).

To consolidate what you’ve learned above, take the following quiz. Note that this quiz includes some big diagrams. You might have to adjust the display on your screen (Ctrl – on Windows, Command – on Mac) for them to fit on your screen). 

[qwiz style=”width: 800px;” random = “false” qrecord_id=”sciencemusicvideosMeister1961-Photoexcitation, Photosystems and Noncyclic Flow (2.0)”]

[h]Photoexcitation, Photosystems, and Non-Cyclic Electron Flow

[i]

[q labels = “top” dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28b172673e49f” question_number=”1″]

 

[l]ATP synthesis

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]electron flow

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]proton accumulation

[fx] No. Please try again.

[f*] Great!

[l]proton pump

[fx] No. Please try again.

[f*] Excellent!

[l]reduction

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]stroma

[fx] No. Please try again.

[f*] Excellent!

[l]thylakoid membrane

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]water splitting

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[q labels = “top” dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28a891f9e909f” question_number=”2″]Label the structures in this photosystem

 

[l]antenna complex

[fx] No. Please try again.

[f*] Correct!

[l]oxidized chlorophyll

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]primary electron acceptor

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]reaction center

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[q labels = “top” dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|289da8022c49f” question_number=”3″]

 

[l]electron gaining energy

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]chlorophyll

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]incoming photon

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]electron losing energy

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]energy released as light and heat

[fx] No. Please try again.

[f*] Excellent!

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28971ba0bb09f” question_number=”4″]The diagram below depicts a [hangman]

[c]cGhvdG9zeXN0ZW0=

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28908f3f49c9f” question_number=”5″]The molecules indicated at number 3 are [hangman]

[c]Y2hsb3JvcGh5bGw=

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|288a281e96c9f” question_number=”6″]The process depicted below is called [hangman]

[c]cGhvdG9leGNpdGF0aW9u

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28839bbd2589f” question_number=”7″]The region depicted at “1” is the [hangman]

[c]c3Ryb21h

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|287d349c7289f” question_number=”8″]The particle that’s getting pumped at  “c” is a [hangman]

[c]cHJvdG9u

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|2876a83b0149f” question_number=”9″]Chemically speaking, what’s happening to water at “k” is [hangman]

[c]b3hpZGF0aW9u

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28701bd99009f” question_number=”10″]Chemically speaking, what’s happening to NADP+ at “g” is [hangman]

[c]cmVkdWN0aW9u

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|28698f781ec9f” question_number=”11″]The proton pumping at “c” is powered by a flow of [hangman]

[c]ZWxlY3Ryb25z

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|286248d2f649f” question_number=”12″]While the process at “c” is active transport, what’s happening at “i” is facilitated [hangman]

[c]ZGlmZnVzaW9u

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Photoexcitation, Photosystems, and Non-cyclic Electron Flow|2855c5130cc9f” question_number=”13″]The electrons used to reduce NADPat “g” ultimately come from [hangman]

[c]d2F0ZXI=[Qq]

[x][restart]

[/qwiz]

6. The Z scheme

In the previous section, we looked at how a flow of electrons, powered by light, is used to synthesize ATP and NADPH. The Z scheme is a graphical way of depicting how chloroplasts (and other photosynthesizers) carry out this work, boosting the energy level of electrons, then harvesting energy as the electrons flow along an electron transport chain. Understanding the Z scheme will deepen your understanding of the material we just covered (and because of the learning you just did, understanding the Z scheme should be easy).

Why the Z scheme? I have always thought that the N scheme would have made more sense (because the rise in electron energy, followed by the downward sloping loss of energy, followed by another rise looks to me like an “N”). But it’s been called the “Z scheme” since the 1950s. Let’s see how it works.

6a. Z-Scheme Reading

As with the diagram above, we’ll create a linear sequence, even though many parts of the process occur simultaneously. Notice that the Z scheme diagram has a Y-axis, which is electron energy.

  1. A photon arrives at Photosystem II. The energy from this photon bounces around the photosystem until it reaches the reaction center at P680.
  2. An electron in a chlorophyll molecule gets an energy boost that sends it to PS II’s primary electron acceptor. Note that in this diagram, the higher position of the electron at PS II’s primary acceptor means that it has more energy than the original electrons in the unenergized chlorophyll in PS II.
  3. As the PS II’s primary electron acceptor accepts chlorophyll’s electron (oxidizing it), an oxygen-evolving complex in PS II rips electrons away from a water molecule and passes them to p680. The water dissociates into protons and oxygen.
  4. PS II’s primary electron acceptor passes this energized electron to an electron transport chain. As the electron moves from one embedded electron carrier in the chain to the next, it releases energy.
  5. This energy release shown in step 4 is coupled with cellular work: in this case the work of creating ATP from ADP and Pi. (Note: we know how this happens: proton pumping, proton accumulation, and diffusion through ATP synthase. But because the Z scheme is about following the electrons, none of that is in the diagram).
  6. The electron from PS II arrives at PS I in a low energy condition. But, as it’s been traveling, photons have been shining on PS1, and energizing its electrons. The energy bounces to P700, and its electrons are boosted in energy and then snatched away by PS I’s primary electron acceptor. These lost electrons are replaced by the electrons arriving from PS II (which can ultimately be traced to water). Notice that the electron boost from PS I boosts these electrons to their highest energy level.
  7. PS I’s primary electron acceptor passes its energized electrons to PS 1’s electron transport chain. This ETC, however, doesn’t pump protons. Instead, it channels PS I’s high energy electrons to the enzyme NADP+ reductase.
  8. NADP+ reductase uses the incoming electron’s energy to convert oxidized, low-energy NADP+ into high energy, reduced NADPH.

6b. Z Scheme video

6c. The Z-scheme: Checking Understanding

Note again that this quiz includes some big diagrams. You might have to adjust the display on your screen (Ctrl – on Windows, Command – on Mac) for them to fit on your screen). 

[qwiz style=”width: 800px;” random = “true” qrecord_id=”sciencemusicvideosMeister1961-Z-Scheme (2.0)”]

[h]The Z Scheme

[i]

[q labels = “top” dataset_id=”SMV_PSN_The Z scheme|28077b033a49f” question_number=”1″]

 

[l]electron transport chain

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]light

[fx] No. Please try again.

[f*] Good!

[l]Photosystem I

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]Photosystem II

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]splitting water

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]primary electron acceptor

[fx] No. Please try again.

[f*] Excellent!

[l]NADP+ reductase

[fx] No. Please try again.

[f*] Great!

[l]NADPH

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q dataset_id=”SMV_PSN_The Z scheme|280113e28749f” question_number=”2″]In the diagram below, PS I is at

[textentry single_char=”true”]

[c]bw ==[Qq]

[f]WWVzLiBMZXR0ZXIg4oCcb+KAnSBpcyBQUyBJLg==[Qq]

[c]Kg==[Qq]

[f]Tm8uIFRoZSBwaG90b3N5c3RlbXMgaW5jbHVkZSB0aGUgY2hsb3JvcGh5bGwsIHRoZSBwcmltYXJ5IGFjY2VwdG9ycywgYW5kIHNvIG9uLiBJbiBhZGRpdGlvbiwgcmVtZW1iZXIgdGhhdCBpbiB0ZXJtcyBvZiBlbGVjdHJvbiBmbG93LCDCoFBTIDEgY29tZXMgYWZ0ZXI=IFBTIElJLg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_The Z scheme|27faacc1d449f” question_number=”3″]In the diagram below, the primary electron acceptor for PS II is at

[textentry single_char=”true”]

[c]ZQ ==[Qq]

[f]WWVzLiBMZXR0ZXIg4oCcZeKAnSBpcyB0aGUgcHJpbWFyeSBlbGVjdHJvbiBhY2NlcHRvciBmb3IgUFMgSUku[Qq]

[c]Kg==[Qq]

[f]Tm8uIExvb2sgZm9yIHRoZSBpbml0aWFsICYjODIyMDtkZXN0aW5hdGlvbiYjODIyMTsgb2YgYW4gZWxlY3Ryb24gdGhhdCYjODIxNztzIGJlZW4gYm9vc3RlZCBieSBsaWdodCBlbmVyZ3kgaW4gUFMgSUkgKHdoaWNoLCBpbiB0ZXJtcyBvZiBlbGVjdHJvbiBmbG93LCBjb21lcyBiZWZvcmUgUFMgSSku

Cg==

[Qq]

[q dataset_id=”SMV_PSN_The Z scheme|27f420606309f” question_number=”4″]In the diagram below, the electron transport chain that provides the energy for making ATP is

[textentry single_char=”true”]

[c]Zg ==[Qq]

[f]QXdlc29tZS4gTGV0dGVyIOKAnGbigJ0gaXMgdGhlIGVsZWN0cm9uIHRyYW5zcG9ydCBjaGFpbiB0aGF0IHByb3ZpZGVzIHRoZSBlbmVyZ3kgdG8gY29udmVydCBBRFAgYW5kIFAgaW50byBBVFAu[Qq]

[c]Kg==[Qq]

[f]Tm8uIFRyeSB0byBmaW5kIHRoZSBlbGVjdHJvbiB0cmFuc3BvcnQgY2hhaW4gdGhhdCBjb25uZWN0cyBQaG90b3N5c3RlbSBJSSB3aXRoIFBob3Rvc3lzdGVtIEku

Cg==

[Qq]

[q dataset_id=”SMV_PSN_The Z scheme|27ecfefbf8c9f” question_number=”5″]In the diagram below, NADP+ reductase is

[textentry single_char=”true”]

[c]bA ==[Qq]

[f]QXdlc29tZS4gTGV0dGVyIOKAnGzigJ0gaXMgTkFEUA==Kw==IHJlZHVjdGFzZS4=[Qq]

[c]Kg==[Qq]

[f]Tm8uIEhlcmUmIzgyMTc7cyBhIGhpbnQuIFlvdSYjODIxNztsbCBmaW5kIE5BRFA=Kw==IHJlZHVjdGFzZSBhdCB0aGUgZW5kIG9mIHRoZSBlbGVjdHJvbiB0cmFuc3BvcnQgY2hhaW4gYXNzb2NpYXRlZCB3aXRoIFBob3Rvc3lzdGVtIEku

Cg==
Cg==[Qq]

[x][restart]

[/qwiz]

7. Two More Music Videos about the Light Reactions

If you want to review through music and animation, here are two more videos. Once you’re ready, scroll down below these videos to the cumulative quiz.

7a. How Noncyclic Electron Flow Creates ATP

7b. Making NADPH and O2; Cyclic Electron Flow

8. Cumulative Quiz: The Light Reactions

We’ve covered a lot of material above. Take the following quiz so you can consolidate what you’ve learned.

[qwiz style=”width: 800px !important;” random = “true” qrecord_id=”sciencemusicvideosMeister1961-Light Reactions Quiz (2.0)”]

[h]Cumulative Quiz: The Light Reactions

[i]

 

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27b771ea7cc9f” question_number=”1″] Which number or letter indicates the thylakoid membrane?

[textentry single_char=”true”]

[c]IG 8=[Qq]

[f]IFllcy4g4oCcb+KAnSBpcyB0aGUgdGh5bGFrb2lkIG1lbWJyYW5lLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBNZW1icmFuZXMgYXJlIG1hZGUgb2YgcGhvc3Bob2xpcGlkcyBhbmQgcHJvdGVpbnMuIFRoZSBwaG9zcGhvbGlwaWRzIGFyZSBvcmdhbml6ZWQgaW50byBhIGJpbGF5ZXIgKHR3byBsYXllcnMpLiBXaGF0IGxvb2tzIGxpa2UgaXQgY291bGQgYmUgYXJyYW5nZWQgaW4gdHdvIGxheWVycz8=

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27b1300a8809f” question_number=”2″] Which number or letter shows the oxidation of chlorophyll in Photosystem II?

[textentry single_char=”true”]

[c]IG w=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGzigJ0gc2hvd3MgdGhlIG94aWRhdGlvbiBvZiBjaGxvcm9waHlsbCBpbiBQaG90b3N5c3RlbSAyLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBGaW5kIFBob3Rvc3lzdGVtIDIuIFRoZW4gZmluZCBhbiBhcnJvdyBtb3ZpbmcgZnJvbSBjaGxvcm9waHlsbCBpbiB0aGUgcmVhY3Rpb24gY2VudGVyIHRvIGEgcHJpbWFyeSBlbGVjdHJvbiBhY2NlcHRvcg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27ab832d8c49f” question_number=”3″] Which number or letter indicates the electron transport chain that powers proton pumping from the stroma to the thylakoid space?

[textentry single_char=”true”]

[c]IG I=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGLigJ0gc2hvd3MgdGhlIGVsZWN0cm9uIHRyYW5zcG9ydCBjaGFpbiB0aGF0IHBvd2VycyB0aGUgcHVtcGluZyBvZiBwcm90b25zIGZyb20gdGhlIHN0cm9tYSB0byB0aGUgdGh5bGFrb2lkIHNwYWNlLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciBhbiBhcnJvdyB0aGF0IHNob3dzIGEgcHJvdG9uIChIKw==KSBtb3ZpbmcgZnJvbSB0aGUgc3Ryb21hICgxKSB0byB0aGUgdGh5bGFrb2lkIHNwYWNlICgyKS4gTm93IGZpbmQgdGhlIGFycm93IHRoYXQgaW5kaWNhdGVzIHRoZSBlbGVjdHJvbiBmbG93IHRoYXQgcG93ZXJzIHRoYXQgcHVtcC4=

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27a58bcf1409f” question_number=”4″] Which number or letter indicates the pumping of protons from the stroma to the thylakoid space?

[textentry single_char=”true”]

[c]IG M=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGPigJ0gc2hvd3MgdGhlIHB1bXBpbmcgb2YgcHJvdG9ucyBmcm9tIHRoZSBzdHJvbWEgKDEpIHRvIHRoZSB0aHlsYWtvaWQgc3BhY2UgKDIp[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciBhbiBhcnJvdyB0aGF0IHNob3dzIGEgcHJvdG9uIChIKw==KSBtb3ZpbmcgZnJvbSB0aGUgc3Ryb21hICgxKSB0byB0aGUgdGh5bGFrb2lkIHNwYWNlICgyKS4=

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|279fb9b15a09f” question_number=”5″] Which number or letter indicates the enzyme-catalyzed creation of the reduced, mobile electron carrier that brings reducing power to the Calvin cycle.

[textentry single_char=”true”]

[c]IG c=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGfigJ0gc2hvd3MgdGhlIGVuenltZSBOQURQKw==IHJlZHVjdGFzZSBjb252ZXJ0aW5nIE5BRFA=Kw==IHRvIE5BRFBILg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciBhIG1vbGVjdWxlIHRoYXQmIzgyMTc7cyBhY2NlcHRpbmcgYW4gZWxlY3Ryb24gKGFuZCBhIHByb3RvbiksIGNvbnZlcnRpbmcgaXQgaW50byBvbmUgb2YgdGhlIHR3byBrZXkgb3V0cHV0cyBvZiB0aGUgbGlnaHQgcmVhY3Rpb25zLg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|2799e793a009f” question_number=”6″] Which number or letter indicates the accumulation of protons that powers the synthesis of ATP?

[textentry single_char=”true”]

[c]IG o=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGrigJ0gc2hvd3MgcHJvdG9uIGJ1aWxkdXAgaW5zaWRlIHRoZSB0aHlsYWtvaWQgc3BhY2UgZnJvbSB0aGUgc3BsaXR0aW5nIG9mIHdhdGVyICgmIzgyMjA7ayYjODIyMTspIGFuZCBwcm90b24gcHVtcGluZyBieSBwaG90b3N5c3RlbSAyJiM4MjE3O3MgZWxlY3Ryb24gdHJhbnNwb3J0IGNoYWluICgmIzgyMjA7YyYjODIyMTsp[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciBhbiBhcnJvdyBvciByZWdpb24gaW4gdGhlIGRpYWdyYW0gdGhhdCBzaG93cyB0aGUgYWNjdW11bGF0aW9uIG9mIHByb3RvbnMgZnJvbSB0aGUgc3BsaXR0aW5nIG9mIHdhdGVyLCBhbmQgZnJvbSB0aGUgcHVtcGluZyBvZiBwcm90b25zICh3aGljaCBpcyBzaG93biBhdCAmIzgyMjA7YyYjODIyMTspLg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|2793caf46989f” question_number=”7″] Which number or letter indicates the reaction that’s the source of all of the oxygen in our atmosphere?

[textentry single_char=”true”]

[c]IG s=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGvigJ0gc2hvd3MgdGhlIHNwbGl0dGluZyBvZiB3YXRlciAoJiM4MjIwO2smIzgyMjE7KSwgd2hpY2ggcmVzdWx0cyBpbiBwcm90b25zLCBlbGVjdHJvbnMgKHdoaWNoIGdldCB1c2VkIGluIHBob3Rvc3ludGhlc2lzKSwgYW5kIG94eWdlbiAod2hpY2ggYnViYmxlcyBvdXQgb2YgdGhlIGNobG9yb3BsYXN0IGFuZCBpbnRvIG91ciBhdG1vc3BoZXJlKQ==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciBhbiBhcnJvdyBvciByZWdpb24gaW4gdGhlIGRpYWdyYW0gdGhhdCBzaG93cyB0aGUgcHJvZHVjdGlvbiBvZiBveHlnZW4gKE8=Mg==KQ==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|278df8d6af89f” question_number=”8″] Which number or letter indicates the region of the chloroplast that would have a very low pH?

[textentry single_char=”true”]

[c]ID I=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnDLigJ0gaW5kaWNhdGVzIHRoZSB0aHlsYWtvaWQgc3BhY2UuIEJlY2F1c2UgcHJvdG9ucyBhcmUgYmVpbmcgcHVtcGVkIGludG8gdGhpcyByZWdpb24sIGl0IHdpbGwgaGF2ZSBhIGxvdyBwSCAoYmVjYXVzZSBsb3cgcEggbWVhbnMgaGlnaCBwcm90b24gY29uY2VudHJhdGlvbiku[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciB0aGUgcmVnaW9uIHdoZXJlIHByb3RvbnMgYXJlIGFjY3VtdWxhdGluZy7CoEJlY2F1c2UgcHJvdG9ucyBhcmUgYmVpbmcgcHVtcGVkIGludG8gdGhpcyByZWdpb24sIGl0IHdpbGwgaGF2ZSBhIGxvdyBwSCAoYmVjYXVzZSBsb3cgcEggbWVhbnMgaGlnaCBwcm90b24gY29uY2VudHJhdGlvbiku

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27884bf9b3c9f” question_number=”9″] Which number or letter indicates the primary electron acceptor of Photosystem I?

[textentry single_char=”true”]

[c]IG k=[Qq]

[f]IFllcy4gVGhlIGxldHRlciDigJxp4oCdIGluZGljYXRlcyB0aGUgcHJpbWFyeSBlbGVjdHJvbiBhY2NlcHRvciBvZiBQaG90b3N5c3RlbSAxLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBGaWd1cmUgb3V0IHdoaWNoIHBob3Rvc3lzdGVtIGlzIFBob3Rvc3lzdGVtIDEuIFRoZW4gZmluZCB0aGUgcG9pbnQgaW4gdGhhdCBzeXN0ZW0gd2hlcmUgZWxlY3Ryb25zIGhhdmUgdGhlaXIgaGlnaGVzdCBlbmVyZ3kgbGV2ZWwu

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27829f1cb809f” question_number=”10″] Which letter indicates the source of all of the electrons involved in the light reactions of photosynthesis?

[textentry single_char=”true”]

[c]IG Q=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGTigJ0gaW5kaWNhdGVzIHdhdGVyLCB3aGljaCBpcyB0aGUgc291cmNlIG9mIGFsbCBvZiB0aGUgZWxlY3Ryb25zIGludm9sdmVkIGluIHRoZSBsaWdodCByZWFjdGlvbnMu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBXb3JrIGJhY2t3YXJkIHRvIGZpbmQgdGhlIGZpcnN0IGVsZWN0cm9ucyB0aGF0IGdldCBib29zdGVkIGJ5IGxpZ2h0IGluIFBob3Rvc3lzdGVtIElJLiBXaGVyZSBhcmUgdGhvc2UgZWxlY3Ryb25zIGNvbWluZyBmcm9tPw==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|277cf23fbc49f” question_number=”11″] Which letter indicates the reaction center for Photosystem II?

[textentry single_char=”true”]

[c]IG M=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGPigJ0gaW5kaWNhdGVzIHA2ODAsIHdoaWNoIGlzIHBob3Rvc3lzdGVtIElJJiM4MjE3O3MgcmVhY3Rpb24gY2VudGVyLg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBGaW5kIFBob3Rvc3lzdGVtIElJLiBOb3cgZmluZCB0aGUgbGFzdCBjaGxvcm9waHlsbCBtb2xlY3VsZSB0aGF0JiM4MjE3O3MgaW4gdGhlIGNoYWluIG9mIGVuZXJneSB0cmFuc2ZlcnMgdGhhdCBsZWFkcyB0byBlbGVjdHJvbnMgYmVpbmcgdHJhbnNmZXJyZWQgdG8gYSBwcmltYXJ5IGVsZWN0cm9uIGFjY2VwdG9yLg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27774562c089f” question_number=”12″] Which letter indicates ATP?

[textentry single_char=”true”]

[c]IG g=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGjigJ0gaW5kaWNhdGVzIEFUUA==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBBVFAgc3ludGhlc2lzIGlzIHBvd2VyZWQgYnkgdGhlIGVsZWN0cm9uIHRyYW5zcG9ydCBjaGFpbiB0aGF0IGJyaW5ncyBlbGVjdHJvbnMgZnJvbSBQaG90b3N5c3RlbSBJSSB0byBQaG90b3N5c3RlbSBJLg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|2771e3074149f” question_number=”13″] Collectively, these pigment molecules make up an antenna complex.

[textentry single_char=”true”]

[c]ID M=[Qq]

[f]IFllcy4gVGhlIGNobG9yb3BoeWxsIG1vbGVjdWxlcyBpbmRpY2F0ZWQgYnkgJiM4MjIwOzMmIzgyMjE7IG1ha2UgdXAgdGhlIGFudGVubmEgY29tcGxleC4=[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgcGlnbWVudCBtb2xlY3VsZXMgcmVmZXJyZWQgdG8gaW4gdGhlIHF1ZXN0aW9uIGFyZSBjaGxvcm9waHlsbCBtb2xlY3VsZXMuIFdoYXQgY29sb3IgaXMgY2hsb3JvcGh5bGw/

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|276ca5ec8049f” question_number=”14″] During the light reactions, protons get pumped into this space.

[textentry single_char=”true”]

[c]ID U=[Qq]

[f]IFllcy4gRHVyaW5nIHRoZSBsaWdodCByZWFjdGlvbnMsIHByb3RvbnMgYXJlIHB1bXBlZCBmcm9tIHRoZSBzdHJvbWEgdG8gdGhlIHRoeWxha29pZCBzcGFjZSAoYXQgJiM4MjIwOzUuJiM4MjIxOyk=[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgcHVtcGluZyBvZiBwcm90b25zIGFzc29jaWF0ZWQgd2l0aCBBVFAgc3ludGhlc2lzIGFsd2F5cyBpbnZvbHZlcyBwdW1waW5nIHRoZW0gaW50byBhbiBlbmNsb3NlZCBzcGFjZS4gV2hhdCB0aW55IGVuY2xvc2VkIHNwYWNlcyBhcmUgc2hvd24gb24gdGhpcyBkaWFncmFtPw==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27671e5042c9f” question_number=”15″] The pH of this fluid is significantly higher (less acidic) than the pH in the thylakoid space.

[textentry single_char=”true”]

[c]ID Y=[Qq]

[f]IFllcy4gRHVyaW5nIHRoZSBsaWdodCByZWFjdGlvbnMsIHByb3RvbnMgYXJlIHB1bXBlZCBmcm9tIHRoZSBzdHJvbWEgdG8gdGhlIHRoeWxha29pZCBzcGFjZSAoYXQgJiM4MjIwOzUuJiM4MjIxOykgVGhpcyBsb3dlcnMgdGhlIHBIIGluIHRoZSB0aHlsYWtvaWQgc3BhY2UsIGFuZCByYWlzZXMgdGhlIHBIIGluIHRoZSBzdHJvbWEgKCYjODIyMDs2JiM4MjIxOyku[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgcHVtcGluZyBvZiBwcm90b25zIHRoYXQmIzgyMTc7cyBhc3NvY2lhdGVkIHdpdGggQVRQIHN5bnRoZXNpcyBtb3ZlcyBwcm90b25zIGludG8gdGhlIHRoeWxha29pZCBzcGFjZSAobG93ZXJpbmcgdGhlIHBIIHRoZXJlKSBhbmQgb3V0IG9mIHRoZSBzdHJvbWEsIChyYWlzaW5nIHRoZSBwSCB0aGVyZSku

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|276206764009f” question_number=”16″] A by-product that results from the splitting of water that occurs during the light reactions

[textentry single_char=”true”]

[c]ID M=[Qq]

[f]IFllcy4gVGhlIHNwbGl0dGluZyBvZiB3YXRlciBjcmVhdGVzIG94eWdlbiAoJiM4MjIwOzMmIzgyMjE7KSB3aGljaCBpcyBhIGJ5LXByb2R1Y3Qgb2YgdGhlIGxpZ2h0IHJlYWN0aW9ucy4=[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBCZWNhdXNlIHRoaXMgaXMgYSBieS1wcm9kdWN0LCBsb29rIGZvciBzb21ldGhpbmcgdGhhdCYjODIxNztzIGxlYXZpbmcgdGhlIGxpZ2h0IHJlYWN0aW9ucywgd2hpY2ggaXMgdGhlcmVmb3JlIG5vdCBjb250cmlidXRpbmcgdG8gdGhlIG5leHQgcGhhc2Ugb2YgcGhvdG9zeW50aGVzaXMgKHRoZSBDYWx2aW4gY3ljbGUpLg==

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|275c7eda0289f” question_number=”17″] The source of the electrons in the light-powered electrical current that is part of the light reactions.

 

[textentry single_char=”true”]

[c]ID I=[Qq]

[f]IFllcy4gTnVtYmVyICYjODIyMDsyJiM4MjIxOyByZXByZXNlbnRzIHdhdGVyLCB0aGUgc291cmNlIG9mIHRoZSBlbGVjdHJvbnMgaW52b2x2ZWQgaW4gdGhlIGxpZ2h0IHJlYWN0aW9ucy4=[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBMb29rIGZvciBhbiBpbnB1dCBvZiBtYXR0ZXIgKG5vdCBlbmVyZ3kpIHRvIHRoZSBsaWdodCByZWFjdGlvbnMgKHRoZXJlJiM4MjE3O3Mgb25seSBvbmUpLg==

Cg==

IA==[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|2756877b8a49f” question_number=”18″] During the light reactions, the energy of electrons powers the pumping of[hangman]from the stroma to the thylakoid space.

[c]IHByb3RvbnM=

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|2750ffdf4cc9f” question_number=”19″] The flow of protons from the thylakoid space to the stroma occurs through [hangman] diffusion.

[c]IGZhY2lsaXRhdGVk

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|274b53025109f” question_number=”20″] In the light reactions, ATP is synthesized as protons diffuse through an ATP [hangman] channel.

[c]IHN5bnRoYXNl

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27463b284e49f” question_number=”21″] The pumping of protons from the stroma into the thylakoid space occurs through [hangman] transport.

[c]IGFjdGl2ZQ==

Cg==

IA==[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|27408e4b5289f” question_number=”22″] The enzyme that converts NADP+ into NADPH is called NADP+ [hangman]

[c]IHJlZHVjdGFzZQ==[Qq]

[x][restart]

[/qwiz]

What’s Next?

  1. In most AP Bio programs, that’s all you need to know about the Light Reactions (but check with your teacher about the supplementary topic below).
  2. Proceed to Topic 3.5, Part 5: The Calvin Cycle (the next tutorial in AP Bio Unit 3)

Supplementary Topic: Cyclic Electron Flow Creates ATP without splitting water or generating NADPH

The term “non-cyclic electron flow” – the Z-scheme’s zig-zag flow of electrons from water to NADP+ – only makes sense if there is a non-linear, cyclical alternative. There is. In cyclic electron flow, a proton gradient is created for ATP synthesis, but there is no splitting of water, no production of oxygen, and no reduction of NADP+ to NADPH.

Before you dig into what’s below, check with your teacher. While cyclic flow is covered in most introductory college biology textbooks, it’s not listed in the College Boards’ learning objectives for photosynthesis.

Cyclic electron flow combines Photosystem I’s light-trapping machinery with Photosystem II’s electron transport chain. Here’s how it works.

Light shining on Photosystem I (number “1” at left), excites electrons, which bounce around the photosystem (represented by “2”) until they reach reaction center p700. P700’s electrons are boosted to a higher energy level (at “3”), and then snatched up by Photosystem I’s primary electron acceptor (at “4”). So far, nothing new. But in cyclic electron flow, instead of electrons flowing to the right, to NADP+ reductase, they flow to the left, to the electron transport chain of Photosystem II (note that right and left only make sense in the context of the diagrams you’ve been looking at). As you can see above, electrons follow what’s called a “shunt pathway” (“5”) that brings them to the electron transport chain of photosystem II (“6”). The flow of energized electrons down this electron transport chain powers proton pumps (not shown in this diagram), which generates the electrochemical gradient that powers ATP synthesis (“7”).

The process is called cyclic because at the end of the electron transport chain the electrons return to photosystem I. This returns electrons to p700, which had become oxidized between steps “2” and “3.” So, no new electrons are required to enter the cycle (hence, no splitting of water, and no production of oxygen).

What causes this shunt pathway to occur? As we’ll see in the next tutorial, as the Calvin cycle transforms carbon dioxide into sugar, it uses three units of ATP for every two units of NADPH. If NADPH were to build up in the stroma, it would act as a competitive inhibitor for NADP+ reductase. In other words, with so much of the product (NADPH) around, it would be hard for the substrate (NADP+) to get access to NADP+ reductase’s active site. With electrons unable to flow to NADP+, they’d be forced to turn the other direction (left, instead of right, in the diagram above), which takes them down the shunt pathway. Because the shunt pathway fills the cytochrome complex in the electron transport chain with electrons, Photosystem II shuts down (because it has nowhere to send its electrons to). When NADPH starts to be consumed by the Calvin cycle, electrons once again start to flow to NADP+, and Photosystem II can start up again.

A final point about cyclic electron flow is this: don’t think of it as a weird exception to “normal” non-cyclic electron flow. There’s an entire category of bacteria known as photoheterotrophs whose photosynthetic repertoire consists solely of cyclic electron flow. In other words, these organisms only use photosynthesis to create ATP (through cyclic flow): they get their carbon by absorbing it from the environment. Additionally, it’s possible that cyclic electron flow was the first type of photosynthesis to evolve, and that the water-splitting, oxygen-producing, carbon dioxide-reducing photosynthesis that we see in chloroplasts is an evolutionary elaboration of this first photosynthetic pathway.

Cyclic Electron Flow Quiz

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Cyclic Electron Flow (2.0)”]

[h]Cyclic electron flow

[q labels = “top”]

 

 

[l]ATP

[fx] No. Please try again.

[f*] Correct!

[l]boosted electron

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]light energy

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]NADP+ Reductase

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]primary electron acceptor

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]PS II (inactive)

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]PS I

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]shunt pathway

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]shut down ETC

[fx] No. Please try again.

[f*] Great!

[l]working ETC

[fx] No. Please try again.

[f*] Excellent!

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|e88da73b1b6f9″ question_number=”23″]The “shunt pathway” that brings electrons from photosystem I to photosystem I’s ETC is at

[textentry single_char=”true”]

[c]IG g=[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGjigJ0gaW5kaWNhdGVzIHRoZSBzaHVudCBwYXRod2F5Lg==[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBGaW5kIFBob3Rvc3lzdGVtIEkgKGF0ICYjODIyMDtlJiM4MjIxOykuIFdoaWNoIGFycm93IHNodW50cyBlbGVjdHJvbnMgZnJvbSB0aGlzIHBhdGh3YXkgb3ZlciB0byB0aGUgZWxlY3Ryb24gdHJhbnNwb3J0IGNoYWluIGZyb20gUGhvdG9zeXN0ZW0gSUk/

Cg==

[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|1ca84927a6dfc4″ question_number=”24″]The diagram shown below is most likely demonstrating

[c]cHJvZHVjdGlvbiBvZiBveHlnZW4=[Qq]

[f]IE5vLiBCZWNhdXNlIFBob3Rvc3lzdGVtIElJIChhdCAmIzgyMjA7ZiYjODIyMTspIGlzIHNodXQgZG93biwgdGhlcmUmIzgyMTc7cyBubyBzcGxpdHRpbmcgb2Ygd2F0ZXIuIEhlcmUmIzgyMTc7cyBhIGhpbnQuIFN0YXJ0aW5nIGZyb20gJiM4MjIwO2osJiM4MjIxOyBub3RlIGhvdyB0aGUgZWxlY3Ryb25zIGFyZSBmbG93aW5nLg==[Qq]

[c]cmVkdWN0aW9uIG9mIE5BRFA=Kw==[Qq]

[f]IE5vLiBOb3RlIGhvdyB0aGUgcGF0aHdheSB0byBOQURQKw==IHJlZHVjdGFzZSAoYXQgJiM4MjIwO2MmIzgyMjE7KSBpcyBncmF5ZWQgb3V0LCBhbmQgaG93IHRoZXJlIGFyZSBubyBlbGVjdHJvbnMgZmxvd2luZyBpbiB0aGF0IGRpcmVjdGlvbi4gSGVyZSYjODIxNztzIGEgaGludC4gU3RhcnRpbmcgZnJvbSAmIzgyMjA7aiwmIzgyMjE7IG5vdGUgaG93IHRoZSBlbGVjdHJvbnMgYXJlIGZsb3dpbmcu[Qq]

[c]Y3ljbGljIGVsZW N0cm9uIGZsb3c=[Qq]

[f]RXhjZWxsZW50LiBUaGlzIGlzIGEgZGVwaWN0aW9uIG9mIGN5Y2xpYyBlbGVjdHJvbiBmbG93Lg==[Qq]

[c]bm9uLWN5Y2xpYyBlbGVjdHJvbiBmbG93[Qq]

[f]IE5vLiBMaW5lYXIgZWxlY3Ryb24gZmxvdyBzdGFydHMgd2l0aCB3YXRlciBhbmQgZW5kcyB3aXRoIE5BRFA=Kw==LiBIZXJlJiM4MjE3O3MgYSBoaW50LiBTdGFydGluZyBmcm9tICYjODIyMDtqLCYjODIyMTsgbm90ZSBob3cgdGhlIGVsZWN0cm9ucyBhcmUgZmxvd2luZy4=[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|1ca7fc521e7bc4″ question_number=”25″]In the process shown below, the energy for ATP production is generated by

[textentry single_char=”true”]

[c]aw ==[Qq]

[f]IFllcy4gTGV0dGVyIOKAnGvigJ0gaW5kaWNhdGVzIHRoZSBlbGVjdHJvbiB0cmFuc3BvcnQgY2hhaW4gZnJvbSBQaG90b3N5c3RlbSBJSS4gVGhpcyBFVEMgdXNlcyBlbGVjdHJvbiBlbmVyZ3kgdG8gcHVtcCBwcm90b25zIGZyb20gdGhlIHN0cm9tYSB0byB0aGUgdGh5bGFrb2lkIHNwYWNlLCBhbmQgZGlmZnVzaW9uIG9mIHByb3RvbnMgZnJvbSB0aGF0IHNwYWNlIHRocm91Z2ggQVRQIHN5bnRoYXNlIGdlbmVyYXRlcyBBVFAu[Qq]

[c]ICo=[Qq]

[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBGaW5kIGFuIGVsZWN0cm9uIHRyYW5zcG9ydCBjaGFpbiB0aGF0IHNlZW1zIHRvIGJlIGFzc29jaWF0ZWQgd2l0aCBBVFAgcHJvZHVjdGlvbiwgd2hpY2ggaXMgaGFwcGVuaW5nIGF0IGxldHRlciAmIzgyMjA7Zy4mIzgyMjE7[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|1ca72d19fc37c4″ question_number=”27″]The process below can be described as [hangman] electron flow.

[c]Y3ljbGlj[Qq]

[q dataset_id=”SMV_PSN_Light Reactions Cumulative|1ca7ad288a33c4″ question_number=”26″]The process below generates ATP but not [hangman].

[c]TkFEUEg=[Qq]

[/qwiz]

Looking for the link to the next topic? It’s above this section about cyclic electron flow.