Question

Contrast the different processes by which a pigment molecule may lose the energy of an absorbed photon.

• How is the water that is the byproduct of cellular respiration produced?

Which type of DNA repair system can repair errors in double-stranded DNA? Single stranded DNA?

• How does the reduction of pyruvate to lactic acid during fermentation allow glycolysis to continue in the absence of oxygen?

Answer 1

The absorbed photon boosts one of the electrons of pigment's molecule in its ground state to excited state. The excited state is unstable, so excited electrons quickly fall back to the ground state orbital, releasing excess energy in the process. The released energy may be dissipated as heat. Other than this, electron can transfer its energy to other compounds in electron transport chain to generate potential energy for Harvesting it to synthesize energy conserving ATPs and NADH through electron transport chain.

Water is produced by four-electron reduction of oxygen during electron transport chain of mitochondria. NADPH and FADH2 are the reduced cofactors produced in Kreb's cycle. NADPH and FADH2 transfer their electrons through electron transport chain, to generate proton gradient for ATP synthesis ultimately to oxygen, ultimately to oxygen.

Mechanisms for single-strand breaks repair:

Base excision repair (BER)

Nucleotide excision repair (NER)

Mechanisms for double-strand breaks repair:

non-homologous end joining (NHEJ),

microhomology-mediated end joining (MMEJ), and

homologous recombination.

Under anaerobic condition, pyruvate undergoes fermentation to produce lactic acid to meet the energy requirement of cells through substrate level phosphorylation. Since pyruvate is converted to lactic acid, glycolysis continues to produce pyruvate and meets the energy requirements under limited oxygen since glycolysis is independent of oxygen. Also, NADH, produced by glycolysis, is reoxidized during fermentation, supporting glycolysis to continue.