Photosynthesis

5. Name the three membranes and the three aqueous compartments found in a chloroplast.

Chloroplasts are found in the cells of mesophyll, the interior tissue of the cell. The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast). Thylakoids may be stacked in columns called grana. Chloroplasts contain stroma, a dense interior fluid.

Light reaction summary:

generate ATP and increase the potential energy of electrons by moving them from water to NADPH. -Split H2O, release O2, reduce NADP+ to NADPH, Generate ATP from ADP by phosphorylation

Cyclic Electron Flow:

uses only PSI and produces ATP only. No oxygen is released and it generates surplus ATP, which satisfies the higher demand in the Calvin cycle.

Write the summary equation for the process of photosynthesis.

6CO2 + 12 H2O + Light Energy ------> C6H12O6 + 6O2 + 6H2O

Carbon Fixation:

: cycle incorporates each CO2 molecule, one at a time, by attaching it to a five carbon sugar, RuBP. The enzyme that catalyzes this first step is RuBP carboxylase or rubisco. The product of this reaction is a 6 carbon intermediate so unstable that is splits in half forming two molecules of 3-phosphoglycerate.

16. What are the three parts of a photosystem, and how do they function together to harvest light? Identify the first redox reaction of photosynthesis.

A photosystem is composed of a reaction-center complex surrounded by light harvesting complexes that funnel the energy of photons to the reaction center complex. When a special pair of reaction center chlorophyll a molecules absorb energy, one of it's electrons is boosted to a higher energy level and transferred to the primary electrons acceptor. -The first redox reaction of photosynthesis: the solar powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the light reactions. As soon as the chlorophyll electron is excited to a higher energy level, the primary electron captures it.

8. What two products are made by the light reactions for use in the Calvin Cycle?

ATP and NADPH

14. Why are several different pigments used in photosystems? What is the main pigment used?

Chlorophyll a and chlorophyll b and carotenoids. The number and variety of pigment molecules enable a photosystem to harvest light over a larger surface area and a larger portion of the spectrum than could any single pigment molecule alone.

4. Explain how each reactant is imported into a leaf, and how each product is exported from the leaf.

CO2 enters and O2 exits the lead through pores called stomata.

7. In which compartment are the enzymes of the Calvin Cycle found?

Calvin cycle occurs in the stroma.

20. Explain what carbon fixation is. Why is it important for both animal and plant life?

Incorporating each CO2 molecule by attaching it to a 5 carbon sugar, RuBP.

How is it similar and different from NAD+?

Light absorbed by chlorophyll drives a transfer of the electrons and protons from water to the acceptor, NADP+, where they are stored temporarily. The electron acceptor NADP+ is the first cousin to NAD+ and differs solely by the presence of an extra phosphate group in the NADP+ molecule. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons along with a proton.

13. Where in photosynthesis—the light reactions or the Calvin Cycle—is the coenzyme reduced? Where is it oxidized?

Light energy is initially converted to chemical energy in the form of NADPH (reducing power) that can be passed along to an electron acceptor, reducing it and ATP, the energy currency of cells.

6. In which membrane are the enzymes of the light reactions embedded?

Light reactions occur in the thylakoids.

17. Describe the steps of the light reactions.Summarize the path energy takes from entering the light reactions as a photon to moving onward as energized electrons.

Linear Electron Flow: the primary pathway, involved both photosystems and produces ATP and NADPH using light energy. A photon hits a pigment and its energy is passed among pigment molecules until it excited P680. An excited electron from P680 is transferred to the primary electron acceptor. Water is split by enzymes and the electrons are transferred from the hydrogen atoms to P680+ and reducing it to P680. O2 is released as a by-product of this reaction. Each electron falls down an electron transport chain from the primary electron acceptor of PS II to PS I. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane. Diffusion of protons across the membrane drives ATP synthesis. In PS I, transferred light energy excites P700 which loses an electron to an electron acceptor. P700+ accepts an electron passed down from the PS II via the electron transport chain. Each electron falls down an electron transport chain from the primary electron acceptor of PS I to the protein Fd. The electrons are then transferred to NADP+ and reduce it to NADPH. The electrons of NADPH are available for the reactions of the Calvin cycle. The process also removes an H+ from the stroma.

11. Which coenzyme carries electrons in photosynthesis?

NADP+

10. What byproduct is created when electrons are stripped away? With what energy are the electrons excited?

O2 is created when electrons are stripped away. Electrons are excited by ATP.

18. Explain how ATP is made by photophosphorylation, (FROM what reactants, and WITH what energy source) and where in chloroplasts it occurs. Where is this ATP USED?

The light reactions generate ATP using chemiosmosis to power the addition of a phosphate group to ADP=photophosphorylation.

15. According to the absorption spectrum of chlorophyll, which wavelengths/colors of visible light does it absorb? Which wavelengths are reflected and transmitted? What early experiment led to this discovery (involving a prism and bacteria)?

The chlorophyll molecules of chloroplasts absorb violet blue and red light and reflect or transmit green light. The experiment that led to this discovery was Engelmann's experiment in which he illuminated filamentous alga with light that had been passed through a prism, exposing different segments to different wavelengths. He used aerobic bacteria that concentrate near oxygen sources to determine which segments of the alga were releasing the most O2 and photosynthesizing the most. There were in the part with violet blue or red light.

Calvin cycle

The cycle begins with 15 carbon's worth of carbohydrate in the form of 3 molecules of the 5 carbon sugar RuBP. Now there are 18 carbons worth of carbohydrate in the form of 6 molecules of G3P. one molecule exits the cycle to be used by plant cell and the other five molecules are recycled to regenerate the three molecules of RuBP.

What are the three phases of the Calvin Cycle, and what does each accomplish?

The three phases are carbon fixation, reduction and regeneration of the CO2 acceptor (RuBP)

9. What is the original source of electrons for the manufacture of sugar?

Water is split, providing a source of electrons and protons. The light reactions generate ATP and increase the potential energy of electrons by moving them from water to NADPH. The Calvin cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. The Calvin Cycle forms sugar from CO2

Reduction

each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, becoming 1,3-biphosphoglycerate. Next, a pair of electrons donated from NADPH reduces 1,3-biphosphoglycerate which loses a phosphate group, becoming G3P. every three molecules of CO2 that enter the cycle, there are 6 molecules of G3P formed. Only one molecule of this 3 carbon sugar can be counted as a net gain of carbohydrate.