Lecture 18

Photosynthesis

100

What is photosynthesis? Where does it occur? 

What is the full equation for photosynthesis?

Photosynthesis is the process of converting the energy of sunlight into chemical energy(organic molecules). Photosynthesis occurs in the Chloroplast.

6CO₂ + 6H₂O + light--> C₆H₁₂O₆ + 6 O₂

200

What are the two components of photosynthesis? Define what they do.

5pt. bonus Is photosynthesis an anabolic or catabolic process?

The first is Light Reactions: (Photo) Use the energy from sunlight to generate high energy molecules ATP and NADPH. O₂ is created as a byproduct.

2nd is the Calvin Cycle: (synthesis) Uses the high energy molecules ATP and NADPH to reduce or "fix" 6 CO₂ into 1 C₆H₁₂O₆. 

Anabolic process.

300

List and define the structure of chloroplast, and where the 2 components of photosynthesis are located.

1st structure: Thylakoid membranes: extensive membrane system to provide membrane are for photosynthetic light reactions.

2nd structure: Stroma: aqueous environment surrounding thylakoids where the Calvin cycle takes place. 

400

Where and what are photosystems? What happens in photosystems?

Photosystems are located in the thylakoid membrane, where 200-300 chlorophyll(light-abosrbing) molecules and acessory pigments group together forming complexes that are photosystems. 

In photosystems, resonant energy transfer between chlorophylls allowing light energy to eventually reach the reaction center.

500

What happens in Photosytems I and II? What does ETC do?

In photosystem II(the first one): uses light energy to strip elections from water, exciting them; moving to higher free energy.

In photosystem I: uses light energy to re-excite electrons. High free energy electrons are donated to NADP⁺ to form NADPH.

ETC connects Photosystems II and I together, and uses high free energy electrons to create a H⁺ gradient.

Photosynthesis overview cont.d

100

What are the 3 fates for excited electrons in Photosynthetic Pigments(in order)?

Ground State-electron, Excited-state electron, and re-excited state electron.

200

How do chloroplasts use ETC and Chemiosmosis?

By: using energy from e- to pump H⁺ across the membrane, the diffusion of H⁺ down concentration gradient powers ATP synthase(Chemiosmosis), and has oxidative phosphorylation.

300

Describe what happens during Cyclic electron flow.

During cyclic electron flow, electrons are lost when excited in PSI and go back to the first ETC to make additional ATP instead of reducing NADP⁺.

Cylic e- flow uses photosystem I alone, and generates ATP only not NADPH or O₂.

*Short answer: undergoes cyclic electron flow when the plant needs more ATP instead of reducing NADP⁺.

400

Define Z-Scheme.

Z-Scheme is non-cylic electron flow during the light reactions linking PSII and PSI, generating O_2,ATP, and NADPH. Z-scheme focuses on the free energy of relevant electrons. 

500

Label the input/outputs of the structure above

Calvin Cycle

100

What happens in the Calvin Cycle, where does it take place, and what powers it?

In the Calvin cycle, CO₂ is "fixed"(added to carbohydrates) or reduce; and makes sugar and other products. The Calvin cycle has 3 phases: CO₂ fixation, Reduction of G3P, and Regeneration phase. The Calvin Cycle takes place in the chloroplast Stroma; and is powered by light reactions(first component of photosynthesis). 

200

Describe what happens in phase 1 of the Calvin Cycle: fixation of CO₂. What goes in and what comes out?

15 pt. bonus: What is photorespiration and why is it negative for plants?

During phase 1, CO₂ is added to the 5-carbon compound called Ribose-1,5-biphosphate(RuBP) and is catalyyzed by an enzyme called Rubisco. When starting phase 1, CO₂ and RuBP go in and 3-Phosphoglycerate(3-PGA) comes out.

Photorespiration is when Rubisco binds to oxygen instead of CO₂ and produces fewer sugars per unit time & requires ATP and releases CO₂ when converting a product of oxygenation to 3 PGA.

300

Describe what happens in phase 2 of the Calvin Cycle: Reduction of 3-phosphoglycerate(G3P). What goes in and what comes out?

During this phase the molecules generated by Rubisco are reduced using the energy from ATP and NADPH. During the start of phase 2: 3-PGA, NADH, and ATP go in while G3P, NADP⁺, and ADP P_i come out.

400

Describe what happens in phase 3 of the Calvin Cycle: Regeneration phase. What goes in and what comes out?

During this phase: Rubisco substrate regenerates requiring ATP and using the remaining 5 G3P. This phase makes a high free energy Rubisco substrate to make CO₂ addition exergonic. 

What goes in are: 3ATP and what comes out is 3ADP and 3RuBP

500

Is the calvin cycle an exergonic or endergonic process overall?

Endergonic, it requires requires energy input in the form of ATP and NADPH to convert carbon dioxide into glucose.