1. Photosynthesis Part I: Overview & The Light-Dependent Reactions

2. Photosynthesis: The BIG Picture
Photosynthesis is the process by which PHOTOAUTOTROPHS convert the energy in SUNLIGHT into the energy stored in ORGANIC COMPOUNDS.
Don’t forget the 1st Law of Thermodynamics, which says…
The 1st Law of Thermodynamics states that “energy cannot be created or destroyed, but it can be transformed from one form to another.” Emphasize that photoautotrophs do not make energy! They, however, can transform light energy into chemical energy through the process of photosynthesis.
An autotroph is an organism that is a “self-feeder.” Autotrophs can sustain themselves without taking in energy from other organisms; typically, this is done through the process of photosynthesis or chemosynthesis. Plants, algae, and some bacteria are autotrophs.
Organic compounds are molecules that contain carbon and hydrogen, and include carbohydrates (the primary product of photosynthesis), lipids, proteins, and nucleic acids.
What is an autotroph?
What is an organic compound?
light

3. Photosynthesis & Ecology
The energy captured through photosynthesis forms the basis of the ecological pyramid.
The biomass that is generated by producers supports nearly all the living organisms on the planet.
Autotrophs capture energy and “fix” it into organic compounds; heterotrophs consume compounds produced by other organisms to obtain energy.
-All organisms require an input of energy to survive and can be classified into 2 broad categories, depending on how they obtain this energy: autotrophs (producers) are able to transform energy from the environment (sunlight, chemicals) and produce organic compounds, while heterotrophs (consumers) must rely on organic compounds that were produced by other organisms. Autotrophs include plants, algae, and some bacteria; animals, fungi, protozoa, and some bacteria are heterotrophic.
Carbon fixation is defined as the reduction (gain of electrons) of inorganic carbon (carbon dioxide) to organic compounds by living organisms.

4. Leaves: The Photosynthetic Organs of Plants
Leaves perform most of the photosynthesis in plants.
cuticle
upper epidermis
palisade mesophyll
spongy mesophyll
- The cuticle is a waxy layer that helps to prevent the leaf from losing too much water through transpiration (don’t forget – lipids, like waxes, are hydrophobic!). This is a great time to emphasize the evolutionary adaptations that some plants possess to thrive in dry environments; a thicker cuticle helps to block the escape of water by osmosis, thus helping the plant maintain homeostasis despite the dry external conditions. The upper and lower epidermis provides a barrier from the external environment (much as our skin does). The palisade and spongy mesophyll are in red because those cells (within a leaf – don’t forget to emphasize scale here!) have many chloroplasts and perform most of the photosynthesis within a plant. The “spaces” between the cells in the spongy mesophyll layer are for gases (CO2 for use during the Calvin Cycle and O2 prior to its release from the leaf. Each stoma (plural, stomata) is surrounded by a pair of guard cells; stomata are pores that allow gases to enter and exit a leaf. The xylem and phloem make up the vascular tissue of a plant, and transport water from roots to shoots in vascular plants (xylem) and transport sugars from sugar sources to sugar sinks (phloem). The xylem provides the water to be used during the light reactions of photosynthesis, and the phloem transports the carbohydrates from the leaves to other parts of the plant after the Calvin Cycle.
lower epidermis
Xylem & phloem
stoma
Bundle-sheath cells

5. Leaves: The Photosynthetic Organs of Plants
Leaves have a LOT of surface area to facilitate absorption of sunlight.
- Although lots of surface area facilitates the absorption of more sunlight (and thus photosynthesis can occur at a faster rate), having lots of surface area also allows more transpiration to take place through the stoma on the underside of leaves in dry environments; this can lead to desiccation. Many species of plants that exist in these environmental conditions have evolutionary adaptations that limit transpiration.
Are there any drawbacks to leaves having a large amount of surface area?

6. Chloroplast Structure: A Review
In eukaryotes, photosynthesis takes place inside chloroplasts inside cells (inside leaves).
Chloroplasts have 3 membranes:
Granum
Outer membrane
Inner membrane
Thylakoid membrane, folded to form thylakoids
Thylakoids are
arranged in stacks
called grana.
-Again, students can get caught up on the scale of where photosynthesis occurs. Emphasize to them that chloroplasts are cellular organelles, found inside cells, which make up tissues, which make up organs (such as leaves). Alternatively, show them the picture on slide 4 and describe that we’re now looking at ONE chloroplast inside one of the cells in the mesophyll.
-Emphasize to students the structure of membranes (phospholipid bilayer with embedded proteins) and the “benefits” of having membranes (able to separate different/incompatible processes into different areas, and the ability to build a concentration gradient by having two distinct and separate areas**).
Chlorophyll and other pigments involved in photosynthesis are embedded in the thylakoid membrane.

7. Light & Pigments
Visible light is made up of different colors of light with different wavelengths.
Light has a dual nature.
Light exhibits properties of both waves and particles (photons).
Emphasize to students that the three primary colors of light ARE NOT THE SAME AS the three primary colors of pigments they already know and love. The three primary colors of light are red, green and blue. Most students have seen the “RGB” designation on a computer or projector screen. Choose a student with a “red” (or some other color) shirt and ask them what color it is. They’ll look at you like you’ve lost it, but on occasion one will respond correctly and tell you that their shirt is really every other color BUT red and that those “other” colors are being absorbed. We PERCEIVE the shirt as red since that this the color of light that is NOT being absorbed, but rather is being reflected back to our eyes, stimulating the cones in our eyes, which sends an electrical signal to the brain that we learned long, long ago to call “red”.

8. Light & Pigments Pigments are molecules that absorb light energy.
Different pigments absorb light of different wavelengths.
Major photosynthetic pigments:
Chlorophyll A
Chlorophyll B
Carotenoids
Xanthophyll
Carotenes
- Sometimes it helps students to think of pigments as “light-collecting buckets.” Emphasize that the energy of light is dependent upon its frequency and not on its wavelength.

9. Light & Pigments What is the advantage to having multiple pigments?
Which colors of light are absorbed well? Not well?
Red, blue, and violet light are absorbed well, while green and yellow light is not well-absorbed.
The advantage of having multiple pigments is having the ability to absorb wavelengths of light that would otherwise be “lost” – either reflected or transmitted. Each pigment has a different absorption spectrum, and absorbs some wavelengths of light that other pigments do not. For example, chlorophyll A absorbs wavelengths of light between approximately 670nm and 690nm that no other pigment absorbs; if chlorophyll A was not present, all light in that range could not be absorbed and thus could not be used to drive photosynthesis.

10. Light & Pigments
What information does this ABSORPTION SPECTRUM tell you?
What information does this ACTION SPECTRUM tell you?
An absorption spectrum (A) shows the wavelengths of light that are best absorbed by each type of pigment. An action spectrum (B) shows how wavelength influences the RATE of photosynthesis. Emphasize the obvious to students: examine the axis labels on the graphs!
The absorption spectrum shows that the pigments absorb red and blue/violet light best, thus reflect the other colors of light.
The action spectrum shows, however, that some photosynthesis occurs at nearly every wavelength of light (including yellow and green), but occurs at the highest rate in blue/violet and red light. Thus, plants appear GREEN!

11. (c) Engelmann’s experiment
Figure 8.9c
Aerobic bacteria
Filament
of alga
400
500
600
700
Figure 8.9c Inquiry: which wavelengths of light are most effective in driving photosynthesis? (part 3: Engelmann’s experiment)
(c) Engelmann’s experiment 11

12. Photosynthesis: An Overview
The net overall equation for photosynthesis is:
Photosynthesis occurs in 2 “stages”:
The Light Reactions (or Light-Dependent Reactions)
The Calvin Cycle (or Calvin-Benson Cycle or Dark Reactions or Light-Independent Reactions)
6 CO2 + 6 H2O C6H12O6 + 6 O2
light
Photosynthesis is an endergonic reaction because it requires an input of energy to occur; that energy comes in the form of light energy and is dependent solely on the frequency of the incident light.
Though the Calvin Cycle is sometimes referred to as the Dark Reactions, emphasize to students that this is actually a very poor name since these reactions do NOT ever occur in the dark. The Calvin Cycle requires ATP and NADPH from the light reactions. As soon as the light is gone, so is the supply of ATP and NADPH so the Calvin Cycle stops, too. The AP exam is most likely to refer to the Calvin Cycle as “Light-Independent Reactions” but even that is a misleading term. Emphasize to students that even though these reactions do not use light directly, they do stop as soon as the light (think energetic photons) is gone.
Is photosynthesis an ENDERGONIC or EXERGONIC reaction?

13. Photosynthesis: An Overview
Follow the energy in photosynthesis,
light
Light
Reactions
Calvin
Cycle
Organic compounds (carbs)
ATP
NADPH
light
thylakoids
stroma
- It cannot be overemphasized that energy is not MADE, but is instead transformed through the processes of photosynthesis and cellular respiration. Each of the “boxes” (light/ATP & NADPH/organic compounds) contains energy, but in a different form. Photosynthesis is a process that converts energy from an “un-usable form” (light) into a “usable form” (organic compounds), and requires an intermediate step (ATP/NADPH).
Granum

14. Calvin Cycle Light Reactions [CH2O] (sugar)
Figure 8.5
H2O
CO2
Light
NADP+
ADP +
P i
Calvin
Cycle
Light
Reactions
ATP
Figure 8.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle
NADPH
Chloroplast
[CH2O]
(sugar) O2 14

15. In Review
Click on the hyperlink below and choose “Animation” to begin the review.
/bioflix/bioflix.htm?c7ephotosynthesis
- Online animation of the entire process of photosynthesis, including both the light reactions & the Calvin Cycle. Obviously, you’ll need to be connected to the internet to be able to show this in class. Show only the light reactions as an introduction.

16. Phase 1: The Light Reactions
The light reactions of photosynthesis involve the use of photosystems.
A photosystem is a cluster of pigment molecules bound to proteins, along with a primary electron acceptor.
2 photosystems are involved:
Photosystem II (P680)
Absorbs light best at a wavelength of 680nm
Photosystem I (P700)
Absorbs light best at a wavelength of 700nm
A photosystem works essentially like an antenna to capture light.
The photosystems were named in order of their discovery; in non-cyclic photophosphorylation, photosystem II actually “comes” before photosystem I.

17. (INTERIOR OF THYLAKOID) Protein subunits THYLAKOID SPACE
Figure 8.12
Photosystem
STROMA
Photon
Light-
harvesting
complexes
Reaction-
center
complex
Primary
electron
acceptor e−
Chlorophyll
STROMA
Thylakoid membrane
Thylakoid membrane
Transfer
of energy
Figure 8.12 The structure and function of a photosystem
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Protein
subunits
THYLAKOID
SPACE
(a) How a photosystem harvests light
(b) Structure of a photosystem 17

18. Phase 1: The Light Reactions
Photosystem II [a group of pigment molecules] absorbs the energy in a photon [a particle of light], exciting an electron to a higher energy level.
Thus, PSII is now 1 electron SHORT of what it needs.
This electron is replaced by photolysis – the splitting of water using light.
O2 is released as a byproduct.
Non-Cyclic Electron Flow
- Photolysis – the splitting of water using light – replaces the electron that was “excited out of” photosystem II, and releases oxygen as a byproduct. The H+ ions from water are used to build the concentration gradient during chemiosmosis.

19. Phase 1: The Light Reactions
Non-Cyclic Electron Flow

20. Phase 1: The Light Reactions
The excited electron travels down the electron transport chain, made of increasingly electronegative cytochromes, “losing energy” as it goes. This energy is used to build a concentration gradient of protons [chemiosmosis].
At the same time, Photosystem I [another group of pigment molecules] also absorbs light energy, exciting one of ITS electrons to a higher energy level.
Non-Cyclic Electron Flow
A good time to emphasize to students the concept of electronegativity – an atom’s ability to attract electrons to itself. Also, since energy cannot be created or destroyed (according to the 1st law of thermodynamics), emphasize that the energy the electron “loses” as it goes down the electron transport chain is harnessed to build a concentration gradient as part of coupled reactions.
The coupling of exergonic reactions to endergonic reactions is a theme seen over and over again in AP biology and is certain to be on the exam. Emphasize this concept whenever discussing examples of it, as here and also in cell respiration.

21. Phase 1: The Light Reactions
The electron lost from Photosystem I is replaced by the electron that was excited and subsequently lost from Photosystem II.
The excited electron from Photosystem I travels down another electron transport chain, “losing energy” as it goes, and ultimately REDUCES NADP+ to NADPH [an electron carrier].
Non-Cyclic Electron Flow
Emphasize that the “loss in energy” is exactly what drives other reactions.

22. 2 P680 1 Light Pigment molecules Photosystem II (PS II) Primary
Figure
Primary
acceptor 2 e−
P680 1
Light
Figure How linear electron flow during the light reactions generates ATP and NADPH (steps 1-2)
Pigment
molecules
Photosystem II
(PS II) 22

23. 2 2 H+ + 3 / P680 1 Light Pigment molecules Photosystem II (PS II) 1 2
Figure
Primary
acceptor 2
2 H+
H2O e− + / 2 1 3 O2 e− e−
P680 1
Light
Figure How linear electron flow during the light reactions generates ATP and NADPH (step 3)
Pigment
molecules
Photosystem II
(PS II) 23

24. 4 Electron transport chain 2 2 H+ + 3 / P680 1 5 Light Pigment
Figure 4
Electron
transport
chain
Primary
acceptor Pq 2
2 H+
H2O e−
Cytochrome
complex + / 2 1 3 O2 Pc e−
P680 1 e− 5
Light
ATP
Figure How linear electron flow during the light reactions generates ATP and NADPH (steps 4-5)
Pigment
molecules
Photosystem II
(PS II) 24

25. Photosystem I (PS I) Photosystem II (PS II)
Figure 4
Electron
transport
chain
Primary
acceptor
Primary
acceptor Pq e− 2
2 H+
H2O e−
Cytochrome
complex + / 2 1 3 O2 Pc e−
P700 e−
P680 1 5
Light
Light 6
ATP
Figure How linear electron flow during the light reactions generates ATP and NADPH (step 6)
Pigment
molecules
Photosystem I
(PS I)
Photosystem II
(PS II) 25

26. Photosystem I (PS I) Photosystem II (PS II)
Figure 4
Electron
transport
chain 7
Electron
transport
chain
Primary
acceptor
Primary
acceptor Fd Pq e− 8 2
NADP+ e−
2 H+
H2O e− e−
Cytochrome
complex H+ + +
NADP+
reductase / 2 1 3 O2
NADPH Pc e−
P700
P680 1 e− 5
Light
Light 6
ATP
Figure How linear electron flow during the light reactions generates ATP and NADPH (steps 7-8)
Pigment
molecules
Photosystem I
(PS I)
Photosystem II
(PS II) 26

27. Phase 1: The Light Reactions
The energy LOST from the electrons as they travelled down the electron transport chain is used in the process of chemiosmosis to make ATP.
What is ATP?!?
Ask students to define “osmosis”. Their answer should say something similar to: “Osmosis is the net movement of solvent molecules through a semipermeable membrane into a region of higher solute concentration, in order to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves, without input of energy, across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. It is important to note that although osmosis does not require input of energy (a passive process), it does use kinetic energy and can be made to do work.
Help students to better understand that “chemiosmosis” is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. 

28. ATP What are the 3 parts of an ATP molecule? ADENINE
3 PHOSPHATE GROUPS
Why is ATP so unstable?
- Adenosine triphosphate or ATP is the energy currency of the cells and is used to power cellular activity. ATP is made up of adenine (a nitrogenous base – more specifically, a purine), ribose (a pentose sugar), and 3 phosphate groups. ATP is a very unstable molecule because of the negative charges of the 3 neighboring phosphate groups; those negative charges repel, so it is very easy to “pop off” one (or even two) of the phosphate groups to do cellular work. A phosphate group from ATP (which then becomes ADP) is added to a molecule (often a protein) by a kinase, an enzyme responsible for phosphorylation; phosphorylation typically activates the molecule that has gained a phosphate group.
How is ATP used to do cellular work?
RIBOSE

29. Phase 1: The Light Reactions
Protons (H+) are pumped from the STROMA into the THYLAKOID SPACE, across the thylakoid membrane.
This builds up a concentration of H+ in the thylakoid space.
This concentration gradient represents POTENTIAL ENERGY.
In chemiosmosis, protons diffuse back to the stroma through ATP synthase.
This is known as the proton motive force.
This causes ATP synthase to spin (like a turbine) and forces ADP and a phosphate group together.
This forms ATP!
What does the word “pumped” imply?
- The word “pumped” implies that energy is required (and it is, since the H+ ions are moving AGAINST their concentration gradient). The energy required to pump protons across the thylakoid membrane against the concentration gradient came from the energy “lost” by the excited electrons as they moved down the electron transport chain after being excited by photons.
Where did the energy come from to do this?

30. Cytochrome complex NADP+ reductase To Calvin Cycle ATP synthase
Figure 8.16
Cytochrome
complex
NADP+
reductase
Photosystem II
Photosystem I
Light
4 H+ 3
Light
NADP+ + H+ Fd Pq
NADPH e− Pc e− 2
H2O 1 / 2 1 O2
THYLAKOID SPACE
(high H+ concentration)
+2 H+
4 H+ To
Calvin
Cycle
Figure 8.16 The light reactions and chemiosmosis: the organization of the thylakoid membrane
Thylakoid
membrane
ATP
synthase
STROMA
(low H+ concentration)
ADP +
ATP P i H+ 30

31. ATP H+ H+ H+ H+ H+ H+ ADP H+ P thylakoid membrane thylakoid space
stroma
ATP synthase e-
NADPH
NADP+ e-
Photosystem I (P700)
Photosystem II (P680)
5000 e-
4999 e-
5000 e-
5000 e-
5000 e-
4999 e-
This “animation” walks through the steps of noncyclic electron flow, as outlined on the previous 3 slides. The “5000 e-” is meant for illustrative purposes only; no matter how many electrons were contained in photosystems II and I, if there was no way to replace those electrons, eventually the number of electrons would be 0. If that were to occur, there would no electrons to be excited by light, and the light reactions would grind to a halt. The electron that was “excited away” from photosystem I is replaced by the electron that was “excited away” from photosystem II; photosystem II’s lost electron is replaced through photolysis – the splitting of water – which releases ½ a molecule of O2 as a byproduct. This is where the oxygen comes from that is produced during photosynthesis, and is why autotrophs need water to perform photosynthesis! The oxygen is released through the stomata. The electron that was excited away from Photosystem I ends up reducing [adding an electron to] NADP+ to form NADPH, an important electron carrier that is needed in the Calvin Cycle.
Make sure to point out to students the coupled reactions that occur; as the electron travels down the electron transport chain, its “lost energy” is used to pump protons from the stroma to the thylakoid space to build a concentration gradient. Then, as those protons diffuse back across the thylakoid membrane through ATP synthase to achieve equilibrium, they cause ATP synthase to spin (like a turbine), which forces ADP and the phosphate group together, forming ATP.
Don’t forget to point out that the membrane is key here! If there was no thylakoid membrane (or if its integrity was disrupted and therefore “leaky”), it would be impossible to build this concentration gradient – not to mention that the cytochromes, photosystems, and ATP synthase would not exist/be functional!
Make sure to make the connection with “osmosis” when discussing “chemiosmosis;” for students who understand the idea of the movement of molecules from an area of high concentration to an area of low concentration (as in osmosis), discuss the idea that this is essentially the same process, just with protons (H+) instead of water molecules. H+ H+ H H H+ O H+ H+ H+ H+ e- H+ O H+
(2 H+ & ½ O2)

32. This is another animation of noncyclic electron flow.
This “animation” walks through the steps of noncyclic electron flow, as outlined on the previous 2 slides.
However in cyclic electron flow, NADPH is not made, water is not split, but ATP is generated through chemiosmosis. 32

33. Phase 1: The Light Reactions
Light-Reactions Animation
Animation web address:

34. Phase 1: The Light Reactions
Quick recap:
In the light reactions, the energy in LIGHT is used to excite electrons to make ATP (through chemiosmosis) and produce NADPH (an electron carrier) to power the Calvin Cycle.
The light reactions occur in/across the thylakoid membrane inside the chloroplasts (inside the cells…)
Light and water are required (reactants).
Oxygen, ATP, and NADPH are produced (products).
- Be sure students can articulate where/how light and water are used in the light reactions, along with where/how oxygen, ATP, and NADPH are produced.

35. Phase 1: The Light Reactions
How could we know that?
Cyclic electron flow
Cyclic phosphorylation is a more ancient biochemical pathway.
Most photosynthetic bacteria & all photosynthetic eukaryotes use cyclic phosphorylation.
Cyclic electron flow produces ATP, but does not produce NADPH.
Only photosystem I is used
Electrons are “recycled”
Water is not split
- In general, simpler mechanisms are older (in terms of evolutionary time). Cyclic photophosphorylation is simpler than noncyclic electron flow, and since most photosynthetic bacteria use it, we can be quite confident that it is an “older” mechanism.
This is quite helpful because the Calvin Cycle requires more ATP than it does NADPH, so cyclic electron flow helps “make up” the difference!

36. Phase 1: The Light Reactions

37. ATP H+ H+ H+ H+ H+ H+ ADP H+ P thylakoid membrane thylakoid space
stroma
ATP synthase e-
NADP+
Photosystem I (P700)
Photosystem II (P680)
5000 e-
5000 e-
4999 e-
5000 e-
This “animation” walks through the steps of cyclic electron flow, as outlined on the previous 2 slides.
In cyclic electron flow, NADPH is not made, water is not split, but ATP is generated through chemiosmosis. H+ H+ H+ H+ H+ H+ H+

38. Phase 1:The Light Reactions
Non-Cyclic Electron Flow
Cyclic Electron Flow
2 photosystems (PS II & PS I) are used.
Water is split through photolysis to replace the “lost” electron.
Oxygen is released.
NADPH is produced.
ATP is produced.
Only 1 photosystem (PS I) is used.
Water is not split to replace electrons – the electron is “recycled” back to the photosystem.
Oxygen is not released.
NADPH is not produced.
ATP is produced.
Cyclic vs. Noncyclic Photophosphorylation Animation

39. Phase 1: The Light Reactions
Putting it all together…
The light reactions (light-dependent reactions) transfer the energy in sunlight into chemical energy in the form of ATP and NADPH, which are used to power the Calvin Cycle.
Light and water* are required for the light reactions to occur (reactants).
ATP, NADPH*, and oxygen gas (O2)* are produced through the light reactions (products).
Emphasize to students that chlorophyll and intact thylakoid membranes are also required for the light reactions; they aren’t listed here since they aren’t “consumed” or “produced,” per se.
Try to get students to figure out what the red asterisks mean prior to revealing that text box.
*Denotes items that are not produced
during cyclic phosphorylation

40. Photophosphorylation Animation
Scroll across the bottom of the screen to activate controls and press PLAY. There’s WAY too much emphasis placed on the intermediates in this animation. Students just need to know the basics including the establishment of a proton gradient to fuel the formation of ATP which is used to fuel the “dark” reactions.

41. Tracing Pathway of CO2
Scroll across the bottom of the screen to activate controls and press PLAY.

42. Where, oh where does the O2 come from?
Emphasize not only the source of the oxygen plants release as a “waste” by-product of photosynthesis but also the experimental design presented in this animation.

43. Chemiosmotic Mechanism
Scroll across the bottom of the screen to activate controls and press PLAY.

44. In Review
Click on the hyperlink below and choose “Animation” to begin the review.
/bioflix/bioflix.htm?c7ephotosynthesis
- Online animation of the entire process of photosynthesis, including both the light reactions & the Calvin Cycle. Obviously, you’ll need to be connected to the internet to be able to show this in class. Use the entire slide as a segue to Photosynthesis 02.

45. Created by:
Cheryl Boggs
Richmond, VA