Photosynthetic Light Reactions

Question 1

What aspects of plants does light affect?

Answer 1

Stomatal movements, timing of leaf loss, germination, and growth

Question 2

How do plants sense light?

Answer 2

With photosynthetic pigments, phytochrome, and blue light receptors

Question 3

What are the properties of light?

Answer 3

• Electromagnetic radiation: shorter wavelength = higher energy
• sun creates electromagnetic radiation ; including visible light, UV, infrared, xrays
• composed of energy photons
• UV damages DNA and proteins - blocked by atmosphere

Question 4

Why is chlorophyll green?

Question 5

Why are carotenoids yellow?

Question 6

Explain carotenoids

Answer 6

• accessory pigments in photosynthesis that absorb wavelengths (unlike chlorophyll a)
• prevent damage to chlorophyll molecules by light

Question 7

Explain chlorophyll A

Answer 7

-essential pigments of photosynthesis
- found in cyanobacteria, algae, and plants

Question 8

What do carotenoids and chlorophyll have in common?

Answer 8

1. Light-absorbing parts contain conjugated double bonds - common among pigments
2. Embedded in the thylakoids (sacs in the chloroplast of a cell) in units called photosystems

Question 9

Define photosystems

Answer 9

• Structural units of protein complexes involved in photosynthesis - contain chlorophyll and carotenoids
• Contain an antenna complex and a reaction centre

Question 10

How is energy transferred within the antenna complex?

Answer 10

Once light hits the antenna complex molecules, energy is transferred from molecule to molecule until photons hit chlorophyll a in the reaction center and electron transfer occurs

Question 11

How is light energy converted into chemical energy?

Answer 11

When an electron absorbs energy, it goes to an excited state, but has to eventually come back down to ground state. To go back to their original state, they must release a photon (packet of energy) that is the exact size of the jump that the electron had to take

Question 12

What could happen when electrons return to their original energy level?

Answer 12

1. Fluorescence
2. Heat
3. Resonance energy transfer that occurs in the antenna complex: from 2 molecules of chlorophyll a to the reaction center

Question 13

Explain what happens to the 2 molecules of chlorophyll a during PS 2?

Answer 13

One electron is ejected from each chlorophyll a and that electron is accepted by the primary electron acceptor. It then moves through the electron transport chain to PS 1

Question 14

How do the 2 chlorophyll a’s in PS 2 get their electrons back after emitting them?

Answer 14

They get them back from water, which also produces oxygen and protons. Oxygen is released and protons accumulate in the thylakoid space

Question 15

How does the chlorophyll a in PS 1 lose electrons?

Answer 15

They lose electrons when their antenna pigments receive light energy. The electrons are accepted by NADP+ to form NADPH

Question 16

How do the 2 chlorophyll a’s in PS 1 get their electrons back after emitting them?

Answer 16

Through the electron transporters, they receive the electrons that came from PS 2.

Question 17

Explain how a proton gradient is established?

Answer 17

A gradient establishes when protons accumulate from water splitting and dissipates when protons flow into the stroma. The energy of the dissipation is used to produce ATP

Question 18

How are light reactions used in the calvin cycle?

Answer 18

1. 5-C acceptor molecule (RuBP) reacts with a molecule of CO2
2. The new 6-C molecules breaks to make two 3-C molecules (PGA)

The cycle must run 6 times (RUBSICO fixes 6 CO2 molecules) to make a 6-C sugar like glucose