5.3.4 The Light Reactions: A Summary

Question 1

notes

Answer 1

• In photosystem I an excited electron is used to reduce NADP+ to NADPH + H+. The electron hole created in photosystem I is then filled in with an electron from photosystem II.
  • Cyclic electron flow occurs in photosystem 1 when NADPH builds up. Instead of reducing NADP+, the electron is passed down an electron transport chain until it returns to photosystem 1.
  • In photosystem II an excited electron is passed down an electron transport chain of proteins in order to pump protons into the lumen for ATP synthesis.
  • The photolysis of water provides electrons to photosystem II and produces oxygen and H+ in the lumen.
• Within the cells of the mesophyll are chloroplasts, which have a membrane network within them called the thylakoid. Embedded within the thylakoid are photosystems, which are groups of proteins, pigments, and other molecules. Photosystems are the site of the light reactions.
• In photosystem 2 an excited electron is passed down an electron transport chain. The energy in the excited electron is used to fuel the production of ATP. The photolysis of water provides electrons to photosystem 2 and leads to the production of oxygen (O2) and hydrogen ions (H+) within the lumen of the thylakoid.
• In photosystem 1 an excited electron is passed to ferredoxin and is then used to reduce NADP+ to
  NADPH + H+. The electron hole that is left is filled in with an electron from photosystem 2.

Question 2

Which of these statements concerning the light-dependent reactions of photosynthesis is not true?

Answer 2

• The energy lost from the excited electrons can be used to split carbon dioxide if it is present in large quantities

Question 3

Which of the following sequences correctly represents the flow of electrons in the light-dependent reactions of photosynthesis?

Answer 3

• water, Photosystem II, Photosystem I, NADP

Question 4

During photosynthesis, photophosphorylation and photolysis both occur in the

Answer 4

• chloroplasts

Question 5

Water’s role in photosynthesis is to

Answer 5

• provide electrons to the light-dependent reactions

Question 6

The end products of the light-dependent reactions of photosynthesis include

Answer 6

• ATP, NADPH, and O2