1. PHOTOSYNTHESIS SBI4U
Diana Puia

2. CURRICULUM EXPECTATIONS
C3.2 explain the chemical changes and energy conversions associated with the process of photosynthesis (e.g., carbon dioxide and water react with sunlight to produce oxygen and glucose) C3.3 use the laws of thermodynamics to explain energy transfer in the cell during the processes of cellular respiration and photosynthesis. C3.4 describe, compare, and illustrate (e.g., using flow charts) the matter and energy transformations that occur during the processes of cellular respiration (aerobic and anaerobic) and photosynthesis, including the roles of oxygen and organelles such as mitochondria and chloroplasts.

3. AGENDA Which organisms perform photosynthesis?
What is photosynthesis? (Overview)
Where does it happen?
Light as a source of energy
Light-dependent reactions
Photosystem I
Photosystem II
Calvin cycle

4. Which Organisms perform Photosynthesis?
PHOTOAUTOTROPHS
Plants
(a) Sunflowers
Algae
(b)Spirogyra
Protists
(c)Euglena gracilis
Cyanobacteria
(d) Anabaena
MISCONCEPTION #1: Cyanobacteria are sometimes called blue-green algae. The name “cyanobacteria” in fact comes from the Greek translation of the bacteria’s colour. It is incorrect to call them algae because cyanobacteria are prokaryotic, and algae are in fact eukaryotic.
NOTE: blue-green algae bacteria in oceans produce the most O2.

5. What is Photosynthesis?
The process which transforms light energy into chemical energy. OVERALL REACTION:
MISCONCEPTION #2: Students sometimes ignore the word “overall” and assume this is a one step reaction. As teachers, we have to stress the fact that not all reactants are used at once, and not all products are given off at once. The overall equation gives no indication of the complex process in which the overall change is accomplished.
light energy
6CO2 + 6H2O C6H12O6 + 6O2

6. What is Photosynthesis?
LIGHT-DEPENDENT REACTIONS
(the photo part)
Requires H2O, chlorophyll, and light energy (from any light).
Produces O2, ATP, and NADPH
CALVIN CYCLE
(the synthesis part)
Requires ATP, NADPH, and CO2.
Produces glucose (sugars), ADP+Pi, and NADP+
You can see how that not all reactants are used at once, and not all products are given off at once.
6CO2 + 6H2O C6H12O6 + 6O2
light energy

7. Photosynthesis Overview
Diagram showing an overview of photosynthesis. All processes take place inside the chloroplasts: light reactions in the thylakoid membrane, and Calvin cycle in the stroma.

8. Why is photosynthesis important?
Directly or indirectly, photosynthesis nourishes almost the entire living world:
Makes energy-rich organic molecules (glucose) from energy-poor inorganic molecules (CO2 and H2O)
It is the start of all food chains & webs.
It also makes oxygen.

9. Where does photosynthesis happen?
Photosynthesis happens in:
Palisade layer
Spongy layer
In the leaves, photosynthesis happens in mesophyll cells found in the palisade layer and the spongy layer. Upper epidermal cells are colourless and transparent in order to maximize the amount of light reaching mesophyll cells in these layers. The stomata allow the exchange of oxygen, CO2, and water vapour with the atmosphere. Guard cells have chloroplasts and can also perform photosynthesis.

10. Chloroplasts ENDOSYMBIOTIC THEORY:
An ancestor of cyanobacteria was engulfed by an ancestor of today’s eukaryotic cells.
Symbiotic relationship – eukaryote offered protection, cyanobacteria offered food.
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria.
Because of this, chloroplasts continue to contain their own DNA to this day, and whereas DNA in a plant cell nucleus is linear, chloroplast DNA is often circular like that of bacteria!

11. Structure of a chloroplast

12. MISCONCEPTION #3: thylakoids are similar to stacked cookies
MISCONCEPTION #3: thylakoids are similar to stacked cookies. In fact, the lumen (empty space) of all the stacked thylakoids (grana) and the unstacked thylakoids (lamellae) is connected!

13. Light as a Source of Energy
60% absorbed by the atmosphere
40% reaches plants on Earth.
Only 5% of that is used in photosynthesis.

14. What is light? A form of electromagnetic (EM) radiation.
Travels in wave packs as photons (also known as quanta).
Photon wavelength is inversely proportional to energy (the shorter the wavelength, the “bluer” the light, the higher the energy).

15. T.W. Engelmann’s Experiment (1882)
Engelmann video
T.W. Engelmann’s Experiment (1882)
Used Spirogyra – has a long spiral chloroplast throughout its length.
Added aerobic bacteria to the slide.
Placed a triangular prism between the light source and the stage.
Found that bacteria accumulated where oxygen was produced the most (in areas exposed to red and blue-violet light).

16. The Absorption Spectrum of Chlorophyll A and B
Photosynthesis happens more rapidly in the blue and red ends of the spectrum. Green light is not absorbed; it is reflected, and that’s why plants look green.

17. Photosynthetic Pigments
Chlorophyll a (blue-green)
Chlorophyll b (yellow-green)
Carotenoids (yellow-orange)
Xanthophylls (yellow)
Anthocyanins (red, violet, blue)
Anthocyanins are found in vacuoles; the others are found in chloroplasts.

18. CHLOROPHYLL COMPOSED OF 2 PARTS: Porphyrin ring Hydrocarbon chain
TWO COMMON TYPES:
Chlorophyll a – methyl (CH3)
Chlorophyll b – aldehyde (CHO)
Porphyrin ring is where the loose, excitable electrons are found. Hydrocarbon chain is hydrophobic and it anchors the chlorophyll molecule in the lipid bilayer of the thylakoid membrane.

19. Coloured leaves? Right: mosaic forest (same season)
Bottom: summer vs. autumn leaves
MISCONCEPTION #4: leaves that are not green are dying. In the picture on the right, we see a mosaic forest made up of different coloured trees within the same season (ex. Maple). Their accessory pigments such as carotene are arranged towards the top layer of the leaves, as an evolutionary advantage to attract pollinators. In the bottom picture, we are shown leaves in the summer and then in the fall. Leaves contain chlorophyll as well as other accessory pigments. In the summer, the green colour reflected by chlorophyll overwhelms the colour reflected by the other pigments, and we perceive trees as green. In the fall, leaves break down cholorphyll and conserve it for the spring.

20. Capturing light energy: photosystems
PHOTOSYSTEMS CONSIST OF:
Antenna complex – captures light first
Reaction centre – chlorophyll a molecule

21. Capturing light energy: photosystems
Main photosynthetic pigment:
Chlorophyll a (two types: p680 and p700)
Accessory pigments:
Chlorophyll b – broader spectrum used for photosynthesis.
Carotenoids – absorb excessive light that would damage chlorophyll.
A photosystem is just a set of chlorophyll and accessory pigment molecules close to each other. They absorb light and transfer the energy from molecule to molecule and finally to the reaction centre (chlorophyll a). The electron is then stripped away by a molecule called the “primary electron acceptor”. The primary electron acceptor is reduced and the reaction centre chlorophyll a molecule is oxidized (remember LEO says GER).

22. Photoexcitation
What happens when a chlorophyll molecule interacts with light energy (photons)?
Before light strikes the molecule, electrons are at ground state.
Photon of light hits.
Electron excited to higher
energy state.
Electron falls back down to
ground state and gives off a
photon of energy (flourescence) and
some heat.

23. Chlorophyll Fluorescence
Fluorescence DEMO
Chlorophyll Fluorescence
Isolated chlorophyll molecules fluoresce when separated from the photosynthetic membrane in which they are normally embedded.
If illuminated in bright white light, an isolated solution of chlorophyll will fluoresce, giving off red light and heat.

24. Photosystems Photosystem I (PSI) Photosystem II (PSII)
The two photosystems work together to start the process of photosynthesis
Photosystem I (PSI)
Photosystem II (PSII)
Contains p700 chlorophyll a. Absorption peaks at 700nm red light. Found in the thylakoid membrane.
Contains p680 chlorophyll a. Absorption peaks at 680nm red light. Found in the thylakoid membrane.
The 2 photosystems differ in the type of Chlorophyll a present. The numbers refer to the wavelength (in nm) of red light they absorb.
The p700 and p680 chlorophyll a molecules are identical, but they differ in absorption wavelengths because of the effects of the proteins they are associated with in the reaction centre.

25. Non-cyclic (linear) electron flow and chemiosmosis
Electron transport
PSI: a photon of 700nm hits, and transduction happens. One of its electrons is pulled away by the electron acceptor, and it’s then sent to a “shuttle molecule” (this is a missing step in this diagram). The electron is then sent to Ferredoxin which is the first electron receptor. FD pulls the electron harder from the shuttle molecule. Fd containing the electron and NADP+ are both substrates of an enzyme called NADP reductase. Once these are both coming in, the electron can be transferred from Fd to NADP+. But NADP+ is not an electron carrier, so it can only take the electron if the third active site is filled by an H+. 1NADP+ + 2H+ + 2electrons -> 1NADPH (this should be called NADPH2). The result is NADPH. But PSII needs to happen first in order for p700 to get its electrons back to replenish them.
PSII: Similarly, photons hit p680, electrons are lost to the primary electron acceptor, and are then lost to Plastoquinone. This is a shuttle molecule which moves to the active site of the cytochrome b6-f complex. Pq needs a H+ atom to shuttle over and so it moves H+ from the stroma into the lumen. The cytochrome b6-f complex dumps the H+ because it only takes the electron. Plastocyanin is connected to the cytochrome b6-f complex and PSI, so the electron gets transferred back to PSI. This happens twice because NADPH needs 2 electrons.
Splitting H2O: PSII is attached to an enzyme molecule which splits water into protons and electrons (called water-splitting complex). This Z-protein is the only enzyme known that is able to break down water; this process takes a lot of energy. So, water is the source of all the electrons involved in photosynthesis, and this is why we need to water our plants. The oxygen molecules get given off into the atmosphere for us to breathe.
MISCONCEPTION #5: blue arrow! Protons can not get through the phospholipid bilayer. H+ accumulates in the lumen which makes the pH lower (more acidic). This means that electrochemical energy gets created inside.
ATP synthesis: ATP synthase or ATPase is a transmembrane protein which allows H+ to pass through, and the energy gets stored in the bonds of ATP. ATP then goes to the Calvin Cycle.
MISCONCEPTION #6: ATP stores the most energy! In fact, NADPH stores way more energy than ATP because energy is stored in the H+.

26. Non-cyclic electron flow (Z-diagram)
Energy video
Pathways and energy changes are shown in a graph form.

27. Cyclic electron flow A non-sustainable pathway.
If Ferredoxin is close to the cytochrome b6-f complex, it interacts with the b6-f instead of NADP reductase. If the electrons are passed to b6-f they then get transported to Plastocyanin and back to PSI. So, no NADPH is made but we still get ATP. This will lead to the plant dying because it can’t get through the Calvin cycle and it can’t make glucose. No oxygen is released.

28. Melvin Calvin ( )
Determined the details of the cycle in the early 1960’s.
Received a Nobel prize in Chemistry in 1961.

29. Calvin Cycle (dark light-independent reactions)
The Calvin Cycle needs:
ATP (from the light reactions)
H atoms from NADPH
(from the light reactions)
CO2(from the environment)
3 phases:
Carbon fixation
Reduction reactions
Regeneration of RuBP
MISCONCEPTION #7: Calvin cycle can also be called the “dark reactions”. This is in fact incorrect. This process is still driven by light.

30. Calvin Cycle video
MISCONCEPTION #8: One Calvin cycle makes one glucose! In fact, you need to bring in 6 CO2 in order to make one glucose. But you don’t bring in the 6 Carbons at once, the Calvin cycle has to happen twice for one glucose molecule to be formed.
Ribulose-1,5-bisphosphate (RuBP) is a 5-carbon chain with two phosphates attached. This molecule has to be present in the stroma for the Calvin cycle to start, and it comes from the parent cell after division. CO2 comes from the atmosphere through the stomata and it is dissolved in water. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a large, slow enzyme compared to other enzymes, so the cell needs to make lots of them (there are millions in each cholorplast). Rubisco is the most common enzyme in the world. It has 3 active sites: one big one where RuBP attaches, one where CO2 attaches, and one where water attaches for a hydrolysis reaction (which breaks down the 6-carbon chain into 2 PGA). This helps the C from CO2 attach to the RuBP to make a 6-carbon chain. Now this is very unstable and immediately makes 2 PGA’s (1PGA = 3-carbon chain). The energy of ATP is used and ATP is broken into ADP+Pi. The phosphate goes to PGA (low energy) to make 1,3-BPG (high energy). This happens twice and so it uses 2 ATP. Now NADPH gives the H+, the Pi of the 1,3-BPG (acid) comes off and it makes two G3P (aldehyde). You need two NADPH here. To make an aldehyde from an acid, you need H+. G3P holds a lot of energy because it took the H+ from NADPH (NADPH loses energy in this process). Most things in plants can not use G3P in this form, so some G3P makes glucose with the help of an enzyme called G3P-carboxylase, through a condensation reaction. We can not just take the two G3P’s and link them together to make glucose because RuBP needs to be regenerated, or else the cycle does not continue. In reality, we begin with 3 RuBP and 3 CO2; it makes 6PGA and use 6 ATP; it makes 6ADP and 6BPG; it makes 6G3P, 6Pi, 6NADP+ using 6NADPH. One G3P exits the cycle, gets attached to the enzyme that makes glucose; but, you need 2 G3P to make one glucose. The other 5 G3P go through a series of reactions that put together the 3 RuBP but they need the Pi from 3 ATP. This whole thing happens twice to make 1 glucose from 2 G3P.