Wk 5 Lecture 1 Light-Dependent Reactions Flashcards
What is photosynthesis (general)?
The process of using sunlight (PHOTO) to produce (SYNTHESIS) carbohydrate
Photosynthesis reaction
6CO2 + 6 H2O + light energy -> C6H12O6 + 6O2
What does it mean to say “photosynthesis is the reverse process of cellular respiration?” (discuss in terms of redox, electron energy, and potential energy)
-CO2 is reduced (instead of oxidized) and water is oxidized (instead of reduced)
-Electrons gain energy as they move from water to CO2 (turning into glucose), using light energy captured by chlorophyll
-Potential energy increases
Describe “Photo-“ in photosynthesis
-Conversion of light energy into chemical energy (ATP)
-Light-dependent (light) reactions
-produce O2 from H2O
Describe “-Synthesis” in photosynthesis
-CARBON FIXATION: fixing carbon (CO2) into organic molecules
-Light-independent (dark) reaction
-Calvin-Benson cycle => produce sugar
Photosynthesis is two sets of reactions linked by…
Electrons
What are chloroplasts? (textbook info)
An organelle in plant cells that contains chlorophyll; site of photosynthesis
Which cells of a plant contain chloroplast? What does this mean?
Mesophyll cells => majority of photosynthesis occurs INTERNALLY in leaves
What are mesophyll cells? (textbook info)
A cell found near surfaces of plant, under epidermal cells, that are specialized for lightcapturing reactions of photosynthesis
Where does carbon dioxide enter/oxygen leave in leaves?
Pores called STOMATA
How many membranes surround chloroplasts?
2 membranes
What is the third internal membrane of chloroplasts?
Thylakoids
What are thylakoids? What do they contain?
Flattened, vesicle-like structures inside chloroplasts
-Contain large quantities of pigment (most common is CHLOROPHYLL) and ATP SYNTHASE
What is a stack of thylakoids called?
Grana (granum plural)
What is the stroma?
Fluid-filled space between thylakoids and inner membrane (analogous to mitochondrial matrix)
In plants, cells that photosynthesize (mesophyll cells) typically have how many chloroplasts?
40-50 chloroplasts
Captured light energy can be transferred to other molecules to ultimately produce…
Energy intermediates for cellular work
Sunlight = ? energy
Electromagnetic energy
What is electromagnetic energy?
Energy that travels through space as waves (at speed of light [google lol])
Describe the electromagnetic spectrum (range direction, relationship b/t wavelength and energy, which colors associated)
Short gamma rays/wavelengths (high energy) => very long radio waves/wavelengths (low energy)
Wavelength and energy are inversely proportional
-Short wavelength/high energy = purple
-Long wavelength/low energy = red
What do photosynthetic pigments absorb?
Light (only certain wavelengths)
What light do chlorophylls (chlorophyll a and b) absorb?
Red and blue light
What light do chlorophylls (chlorophyll a and b) transmit?
Green light
What light do carotenoids absorb?
Green and blue light
What light do carotenoids transmit?
Yellow, orange, and red light
Chlorophyll structure
Head: Ring structure that absorbs light w/ a magnesium in the middle
Tail: Isoprenoid tail that anchors chlorophyll in thylakoid membrane
Carotenoid structure
Isoprenoid chain in between two ring structures (each ring responsible for absorbing light)
Having different pigments allows plants to…
Absorb light at many different wavelengths
Which photons excite electrons to a higher energy state, red or blue?
Blue (shorter wavelength, higher energy)
Photosynthetic pigments form aggregates in thylakoids called…
Photosystems
What 2 things does a photosystem consist of?
- Antenna complex
- Reaction Center
What is the antenna complex of a photosystem?
-Accessory pigments that absorb energy and pass excited electron via resonance from antenna pigments
-Surrounds reaction center
What is the reaction center of a photosystem?
-Acts as an electron acceptor
-Contains proteins that capture and process excited electrons
-Has 2 embedded chlorophyll a molecules (special pair)
What are the four possible fates of electrons in reaction centers of photosynthetic pigments that are excited by photons?
Energy released from these electrons can
1. Be emitted in the form of light via fluorescence (drop energy state) and/or
2. Be given off as heat (drop energy state), or
3. Excite an electron in a nearby pigment and induce resonance, or
4. Be transferred to an electron acceptor in a redox reaction
When does fluorescence occur for excited electrons?
Typical of isolated pigments and less frequent in chloroplasts
Where does resonance energy transfer occur for excited electrons?
Antenna pigments
Where do redox reactions occur for excited electrons?
Reaction center pigments
What are the two types of reaction centers? Which is the first step of photosynthesis?
- Photosystem I
- Photosystem II (first step)
What do the two photosystem work together to produce?
An enhancement effect (compliment each other)
What does photosystem II trigger?
Chemiosmosis
Explain chemiosmosis of photosystem II
-Energy reaches reaction center => the special pair chlorophyll (p680) is oxidized (by donation of a high energy electron to electron acceptor PHEOPHYTIN)
-Electron is passed to an ETC instead of being pulled back to the special pair chlorophylls (via electron shuttle - PLASTIQUINONE [PQ]) => produces proton gradient => drives production of ATP via ATP synthase
What is pheophytin? (textbook info) Is it reduced or oxidized?
A pigment that is structurally identical to chlorophyll (but lacks magnesium atom in its head) but functionally different; pheophytin does not become excited by photons or resonance => accepts excited electrons from reaction center pair chlorophylls => redox rxn between these two pigments is KEY to transforming light into chemical energy
special pair pigments are oxidized; pheophytin is reduced
How does plastiquinone play a role in producing an electrochemical gradient/proton motive force? (textbook info) (3 steps)
- PQ picks up 2 electrons from photosystem II (pheophytin) and 2 protons from chloroplast stroma (PQ is reduced)
- Carries electrons through membrane to lumen side of thylakoid => delivers to molecules w/ higher redox potential in cytochrome complex
- PQ drops off protons in thylakoid lumen after being oxidized by cytochrome complex
High concentration of protons in thylakoid lumen => proton electrochemical gradient that drives H+ from thylakoid lumen into stroma
What is the exergonic flow of protons down the electrochemical gradient coupled to? How? (textbook info)
The endergonic synthesis of ATP from ADP and an inorganic phosphate
Stream of protons flows through ATP synthase, causing conformational changes in the enzyme that drive production of ATP
What is the this process (synthesis of ATP in chloroplasts initiated by energy from light) called? What is it similar to in mitochondria? What is the key difference?
Photophosphorylation; similar to oxidative phosphorylation
Mitochondria: ATP exported and fuels many different cellular processes
Chloroplasts: ATP remains w/in organelle and is used fro production of carbohydrate
Direction of PSII, chemiosmosis, and photophosphorylation
PSII => chemiosmosis => photophosphorylation
Electrons in the ETC participate in… gradually stepped down in…
Redox reactions; in potential energy
Redox reactions from ETC result in…
Protons being pumped from one side of the membrane to inside the thylakoid
What is oxygenic photosynthesis? (lecture)
The exclusive process by which PSII oxidizes water (only protein complex that can do it in this way) to get electrons
Splits water to…
1. Replace its lost electrons
2. Produces oxygen
Formula for oxygenic photosynthesis; what do the hydrogen ions contribute to
2H2O -> 4 H+ + 4e- + O2
The hydrogen ions contribute to the electrochemical gradient
How many photons needed to produce one O2 molecule from water? Why? (textbook)
4 photons; oxygenic photosynthesis formula proportions (for every photon used 1 electron is excited and leaves PSII => 1 electron replaced for every 1 photon => 4 photons for 4 electrons as per formula)
What does photosystem I produce? How? (7 steps)
NADPH
- Pigments absorb 2 photons and pass energy to reaction center
- 2 electrons are excited in reaction center special pair chlorophyll molecules (p700)
- Reaction center pigments are oxidized
- Excited electrons are passed through series of carriers inside photosystem I
- Then passed to a molecule called FERREDOXIN (electron donor)
- Then to enzyme called NADP+ reductase
- NADP+ reductase transfers the two electrons and a proton to reduce NADP+ and form NADPH (high reducing power)
How do photosystems I and II interact?
“Z-scheme”/noncyclic phosphorylation
What is the physical link between ETC following photosystem II to photosystem I?
Plastocyanin, a small diffusible protein
What does plastocyanin do specificially?
- It is reduced by electrons that reach end of cytochrome complex
- It can shuttle over 1000 electrons/second b/t cytochrome complex and PSI
- Replaces electrons carried away from special pair chlorophylls (p700) in PSI reaction center
For each O2 molecule produced by PSII (via oxygenic photosynthesis), how many electrons are transferred along Z-scheme and how many NADPH molecules are produced?
4 electrons, 2 NADPH
What is the enhancement effect?
Explained by Z-scheme
-Photosynthesis is more efficient when both 680 and 700 nm wavelengths are available b/c both photosystems can run at maximum rates
What is cyclic electron flow?
PSI can occasionally transfer electrons to PSII ETC via ferredoxin to PQ instead of reducing NADP+ => increases ATP production
Why do cyclic and noncyclic electron flow coexist?
To product additional ATP and meet the energy demand for manufacturing sugars from CO2
