Lecture 20: Light Reactions.

Question 1

Describe the role of the light reactions in photosynthesis

Answer 1

The light reactions convert solar energy to the chemical

energy of ATP and NADPH

Question 2

• Describe the basic properties of light and how wavelength affects energy

Answer 2

• Light is a form of electromagnetic energy: also called
  electromagnetic radiation
• Wavelength characterizes the type of
  electromagnetic energy

Question 3

• Define the electromagnetic spectrum and know the range of light that drives
photosynthesis

Answer 3

• The electromagnetic
  spectrum is the entire range
  of electromagnetic energy, or radiation
• Plants absorb visible light between 380-750 nm that drives photosynthesis.

Question 4

• Define photosynthetic pigments and describe their role in

absorbing/transmitting light

Answer 4

• Pigments: substances that absorb visible light
• Wavelengths that are not
  absorbed are reflected or
  transmitted

Question 5

Pigments and their function?

Answer 5

• Chlorophyll a is the main photosynthetic pigment
• Chlorophyll b: accessory
  pigment that broadens the
  spectrum of light used for
  photosynthesis
• Carotenoids: accessory
  pigments that absorb
  excessive light which would
  damage chlorophyll

Question 6

• Describe how Chlorophyll is excited by light

Answer 6

• When a pigment absorbs light: it goes from a stable ground state to an unstable excited state
• When excited electrons fall back to the ground state, heat and photons
  (fluorescence) are given off
• When Chlorophyll absorbs light, it transfers energy to another chlorophyll molecule and so on until the reaction center is reached.

Question 7

• Describe the structure and function of a photosystem

Answer 7

• A photosystem consists of a reaction-center complex
  surrounded by light-harvesting complexes
• The light-harvesting complexes transfer energy of photons to the reaction center complex
• A primary electron acceptor in the reaction center complex accepts excited electrons and is reduced

Transfer of e-:
- The solar-powered transfer of an
electron from a chlorophyll a
molecule to the primary electron
acceptor is the first step of the
light reactions

Question 8

• Know the two types of Photosystems involved in photosynthesis, the
wavelength of light they optimally absorb, and the name of their respective
reaction center chlorophyll a molecules

Answer 8

Photosystem II (PS II) functions first and is best at absorbing a
wavelength of 680 nm
→ The reaction-center chlorophyll a of PS II is called P680

Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
→ The reaction-center chlorophyll a of PS I is called P700

Question 9

• Describe the flow of electrons within and between Photosystems II and I;
including any enzymes that are represented, where and how ATP and
NADPH production is occurring, and where water is being split

Answer 9

• light strikes pigments on light harvesting complex on Photosystem II. Excited e- transferred to p680 and then to primary e- acceptor.
• H2O is split and e- transferred from H atoms to P680. O2 released as by-product.
• e- from the primary electron acceptor of PSII travel down the ETC to PSI. As e- pass through the cytochrome complex, protons are pumped into the lumen, producing ATP.
• same process occurs at PSI in which excited e- reaches P700 and then primary electron acceptor.
• e- then continue to travel down ETC from the primary e- acceptor of PSI through Fd. NADP reductase catalyzes transfer of e- from Fd to NADP, which is then reduced to NADPH (used in the calvin cycle).