Photosynthesis

Question 1

What is the general equation for photosynthesis?

Answer 1

light energy + 6CO₂ + 6H₂O = C₆H₁₂O₆ + 6O₂

Question 2

What is a general description of what happens during photosynthesis?

Answer 2

• An anabolic process in which light energy from the sun is converted into chemical energy which is stored in sugars and other fuels
• Ligh reactions: light energy transformed into chemical energy
• Dark reactions (Calvin Cycle): carbon fixed into sugars using energy

Question 3

What happens in the light reactions?

Answer 3

• Light reactions occur in the thylakoid membrane
• Light energy is converted to chemical energy
• Photosystem II:
• 2 photons are absorbed and excite electrons in the different types of chlorophyll
• Energy is delivered to the reaction centre
• Two electrons fall from chlorophyll a to the electron acceptor
• The electrons enter the ETC
• Water is split: H₂O = 1/2O₂ + 2H⁺ + 2e^- to replace the lost electrons
• The H₂O and products remain in the thylakoid space
• Photosystem I:
• 2 photons are absorbed and the same process occurs as in photosystem II
• 2 electrons enter the ETC and are replaced by the two from photosystem II
• ETC:
• The electrons moving along the ETC create a proton gradient
• H⁺ ions move from the stroma to the thylakoid space
• Reduction:
• The two electrons from photosystem I and two H⁺ ions from the stroma are used to reduce NAP⁺ + 2H⁺ + 2e^- = NADPH + H⁺
• Photophosphorylation:
• ADP + P_I = ATP and 2H⁺ are pumped into the stroma

Question 4

What happens in the dark reactions/Calvin Cycle?

Answer 4

• The dark reactions occur in the stroma
• Fixation:
• 3RuBP (5C molecule) + 3CO₂ + RUBISCO = 3 unstable 6C molecules = 6 3C molecules
• Reduction:
• 6 3C molecules + 6ATP + 6NADPH = 1G3P yield + 5G3P for regeneration + 6ADP + 6P_I + 6NAP⁺ + 6H⁺
• Regeneration:
• 5G3P + 3ATP = 3RuBP + 3ADP + 3P_I

Question 5

What happens in C4 photosynthesis?

Answer 5

• CO₂ enters the mesophyll cell
• PEP carboxylase (3C molecule) fixes the CO₂ to form a C₄ acid
• The C₄ acid compound is pumped through the membrane to the bundle sheath cell using ATP and split
• The CO₂ is used for the Calvin Cycle while pyruvate is returned to the mesophyll cell
• The Calvin Cycle produces sugar as in C₃ photosynthesis

Question 6

What happens in CAM photosynthesis?

Answer 6

• The same process occurs as in C₄ photosynthesis except in the mesophyll cell only and in stages
• The fixing of CO₂ to C₄ acid occurs during the night when the stomates are open adn the acid is stored in a vacuole
• During the day CO₂ is released into the Calvin Cycle while th stomates are closed

Question 7

What are the problems with photosynthesis?

Answer 7

• CO₂ enters and H₂O and O₂ leave through the stomates
• On hot days plants close stoamtes to reduce water loss
• CO₂ levels decrease and O₂ levels increase in the leaf
• Sugar production decreases
• Photorespiration increases

Question 8

What is photorespiration?

Answer 8

• RUBISCO fixes O₂ instead of CO₂
• Occurs in the light and uses O₂ and energy and releases CO₂ without forming sugars
• Can drain up to 50% of carbon fixed by the Calvin Cycle
• Evolutionary remenant from a time when there was more O₂ in the atmosphere?
• Protection against intense light?