Metabolic Processes - Photosynthesis

Question 1

What is the relationship between the primary chlorophyll a, accessory pigments, the antenna complex, the reaction centre, and the photosystems?

Answer 1

• Accessory pigments capture light energy and pass it to chlorophyll a (primary pigment)
• Chlorophyll a pass an electron to the primary electron acceptor in what is known as the reaction centre
• Antenna complex: a cluster of pigments embedded in the thylakoid membrane
• The antenna complexes are imbedded in the thylakoid membrane in protein complexes known as photo systems

Question 2

What are the possible outcomes when a pigment molecule absorbs a photon of light?

Answer 2

• Returns to its ground state, releasing heat energy
• Excite an electron in a nearby pigment molecule
• Be transferred to a primary electron acceptor (a molecule that can accept electrons, becoming reduced during photosynthesis)

Question 3

What are the absorption and action spectra and what is the relationship between them?

Answer 3

• Absorption spectrum: the range of wavelengths a given pigment can absorb
• Action spectrum: the overall rate of photosynthesis at each wavelength of light
• Relationship if high absorption %, high rate of photosynthesis for a given pigment

Question 4

How does photosystem I differ from photosystem II?

Answer 4

• Photosystem I: contain chlorophyll a molecules that specialize in absorption of light with a wavelength of 700nm (P700 molecules) (on the right)
• Photosystem II: contain chlorophyll a molecules that specialize in absorption of light with a wavelength of 680nm (P680) (on the left)

Question 5

Non-cyclic photophosphorylation

Answer 5

• Light energy is harvested by PSII, chlorophyll a becomes excited so one of its double bonds breaks and the pair of electrons “jumps” into a higher energy level
• Excited pair of electrons is given to plastoquinone (PQ) which transfers it down the ETC. As electrons are passed down, their energy is used to pump H+ from stroma to thylakoid limen (creates a gradient of H+)
• Electrons continue down the ETC to PSI and are passed to the reaction centre. Another pair of electrons is excited due to light energy and leaves its bond
• Electrons are captured by ferredoxin (Fd) and then passed to NADP reductase (an enzyme that combines the electrons, NADP, and an H+ to produce NADPH)

Question 6

Why is it important for water to be split during the light-dependent reactions?

Answer 6

Since PSII loses electrons during photophosphorylation (electrons become excited, “jump” an energy level, and double bonds are broken), they need to be replaced. If too many electrons are lost by the reaction center chlorophyll a, it could be damaged. To replace the lost electrons, PSII splits water, producing H+ (contributes to H+ gradient), electrons, and oxygen (released as waste)

Question 7

Cyclic photophosphorylation

Answer 7

• PSI operates independently of PSII
• Instead of reducing NADP+ reductase, the electrons passed from PSI to ferredoxin are shuttle back to plastoquinone (PQ). This allows for PQ to be continually reduced and oxidized, moving protons into the thylakoid lumen without PSII
• Results in formation of ATP through chemiosmosis but does not result in formation of NADPH and does not involve water molecule splitting
• Important since light-independent reactions require more ATP than NADPH (also allows cell to produce additional ATP that are used in various cellular processes)

Question 8

What is carbon fixation?

Answer 8

The process of taking inorganic, gas-phase CO2 and converting it into organic carbohydrate molecules
- Chemical bonds in CO2 must be broken and new bonds must be formed in the carbohydrate molecules
- Breaking CO2 bonds requires ATP because CO2 is very stable so breaking its bonds requires a lot of energy
- Also, CO2 is a fully oxidized molecule, so to change it into a carbohydrate, it must be reduced by adding electrons and H+ (provided by NADPH)

Question 9

What happens during the carbon fixation stage of the light-independent reactions?

Answer 9

• CO2 (inorganic) reacts with ribulose-1,5-bisphosphate (RuBP) to produce two 3-phosphoglycerate
• Catalyzed by the enzyme Rubisco (most important enzyme for living things, allows for inorganic carbon to be converted to an organic form, responsible for fixing almost every carbon molecule found in living things, most abundant protein on Earth)

Question 10

What happens during the reduction stage of the light-independent reactions?

Answer 10

• ATP is used to phosphorylate each molecule of 3-phosphoglycerate
• This molecule is reduced by high-energy electrons from NADPH to produce glyceraldehyde-3-phosphate (G3P)

Question 11

What happens during the regeneration stage of the light-independent reactions?

Answer 11

• Some of the G3P molecules are combined + rearranged to regenerate the RuBP needed to restart the cycle (requires 3 molecules of ATP)

Question 12

Why are three cycles of of the light-independent reactions required to produce one G3P?

Answer 12

• For each complete cycle, one CO2 molecule is converted into one reduced carbon (essentially one CH2O unit of carbohydrate)
• One G3P has three of such units
• 9 ATP and 6 NADPH are required (doubled for a single glucose - produced from 2 G3P)