Mol Lecture #31

Question 1

Photosynthesis and Light-dependent Reactions

Answer 1

• Photons coming from the sun acting on the pigments

Question 2

Photosystem Overview

Answer 2

• Pigments are organized into protein-containing complexes called photosystems
• PI: NADPH production, lost electrons (that we’ll use) come from photosystem II, chlorophyll in reaction center is called p680.
• PII: ATP production, lost electrons come from water splitting, chlorophyll in the reaction center is P700.

Question 3

Electron Transfer system

Answer 3

→ electron carriers: plastoquinone, ferredoxin, plastocyanin
→ Cytochrome complex- electron carrier and H+ pump
→ Chemiosmosis via ATP synthase

Question 4

Antenna complex and reaction centers

Answer 4

• Lumen of the thylakoid
• Photosystem can be divided into antenna complex and reaction center
  →Antenna complex
  → Reaction center

Question 5

Antenna complex (ring around)

Answer 5

• light excites electrons and energy is transferred between electrons in pigment molecules

Question 6

Reaction center

Answer 6

• electrons are transferred to the primary electron acceptor

Question 7

Photosystem Pathway: Start with PII

Answer 7

• light energy is coming in and exciting electrons in the antenna complex than move onto the electron receiving the energy in the reaction complex. Electrons is donated to the primary acceptor

Question 8

Photosystem Pathway: Water splitting

Answer 8

• how we’re getting the electrons for this process

Question 9

Photosystem Pathway: Cytochrome Complex (key) + PI

Answer 9

• pumping electrons into the thylakoid lumen to create a gradient for ATP synthase → making ATP
• In PI the electron is transferred to NADP+ to create NADPH and H+

Question 10

NADP+/NADPH as an electron shuttle

Answer 10

• In all cells there’s an NAD/NADH pool (CR) and a NADP/NADPH pool (P)
  –> P is the one used in photosynthesis
• NAD+ and NADH do not have a phosphate group

Question 11

Electron Replacement

Answer 11

• Water is split to form H+ and O2, and e’s to replace the ones that go into ETS.
• Water splitting complex at the bottom of PII.

Question 12

Electron Flow (Overview)

Answer 12

• As electrons pass, we’re pumping h+ through the cytochrome complex into the thylakoid lumen, creating this gradient helps to create ATP through the ATP synthase

Question 13

Cyclic Electron flow (works in favor of PII)

Answer 13

• In some cases, PI works independently of PII
• Here, NADP+ is not reduced and the e’s are passed back to the Cytochrome Complex so that only ATP is produced.
• Allows us to continue to use our H+ gradient (making ATP), but we’re no longer making NADPH so we cannot go into Light-independent reactions.

Question 14

Light-independent Reactions

Answer 14

Overview of the Calvin Cycle
- (using ATP and NADPH from dependent)
- Photosynthesis: 6CO2 + ^H2O (light energy) → C6H12O6 + 6O2

Question 15

Calvin Cycle: Process of light-independent reactions

Answer 15

• To make a carbon-containing organic molecule we need 3 turns (3CO2)
• 3CO2+6NADPH+5H2O+9ATP → 1G3P (glyceraldedehyde 3-phosphate)+ 2H+ +6NADP+ + 9ADP + 9Pi
• 1 G3P for every 3 ‘turns’ of the cycle, so need 6 ‘turns’ of the cycle to get 1 glucose (twice)

Question 16

3 phases of the Calvin Cycle

Answer 16

• Carbon Fixation- CO2 is added to a molcule of Ribulose-1,5- bisphosphate CRuBP, 5C, and cleaved to make 2 molecules of 3-phosphoglycerate (3 PGA, 3C)
• Reduction- 3-phosphoglycerate is phosphorylated with P from ATP and is reduced with electrons from NADPH to make glyceraldehyde-3-phosphate
• Regeneration- after 3 turns 1 G3P is released and 5 G3P go to regenerating the 3 RuBP molecules used in the 3 turns.

Question 17

Slight Overview of the Calvin Cycle (Do not need to know this much detail)

Answer 17

• RuBP is split into 2 3GPA → phosphorylate with ATP (–> ADP)
• Takeaway 2 phosphates and gain 2 molecules of G3P
• Bisphosphate kinase adds electrons through the NADPH

Question 18

Important Steps and Molecules: Reaction 1 (Carbon Fixation by Rubisco

Answer 18

• Carboxylation phase catalyzed by rubisco (enzyme)
  → Catalyzes the carbon fixing reaction in which a molecules of CO2 is added to ribulose-1,5, bisphosphate (RuBP) to form 2 molecules of 3-phosphoglycerate (3PGA).
• Rubisco makes up 50% of protein in plant leaves, most abundant protein on earth
• Called C3 pathway because carbon is first fixed in a 3-carbon containing molecule (3PGA)

Question 19

Photorespiration (P.1)

Answer 19

• Rubisco is both a carboxylase and an oxygenase
• Rubisco can also use oxygen to combine RuBP; instead of getting 2 molecules of 3PGA we get 1 3PGA and 1 2PG
  → Bad because it leads to the build-up of toxic products, and we’re not fixing carbon.

Question 20

Photorespiration (P.2)

Answer 20

• Photorespiration- when O2 is used instead of CO2 by Rubisco
  → results in ‘waste’ of RuBP, no fixed carbon, build-up of toxic components
• Rubisco is 100-fold more efficient at using CO2, when plants evolved, there was very low O2 concentrations. Now we have very high levels of O2. Oxygen concentration is 500 fold higher than carbon dioxide. Plants have evolved methods to get around this issue.