Molecular Excitation and Photosynthesis Concepts

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Questions and Answers

What mechanism allows an excited molecule to return to its ground state while dissipating energy?

Phosphorescence

Radiative decay

Intersystem crossing

Internal conversion (correct)

What state does an excited molecule transition to during intersystem crossing?

Non-radiative state

Excited singlet state

Excited triplet state (correct)

Ground singlet state

Which of the following best describes the process of fluorescence?

Emission of light at a longer wavelength after energy absorption (correct)

A forbidden transition between two states

An energy transfer to a neighboring molecule

A change in electron spin state

What does 'PIB' refer to in the context of photosynthetic organisms?

Post-illumination burst Signup and view all the answers

When does CO2 exchange occur in a leaf placed in darkness?

During the post-illumination burst Signup and view all the answers

What gas was likely absent in the atmosphere of early Earth, according to Haldane's proposal?

Oxygen Signup and view all the answers

Which type of bacteria are strictly anaerobic and produce methane?

Methanogenic Bacteria Signup and view all the answers

What role do acetate and CO2 play in the metabolism of methanogens?

Terminal electron acceptors for anaerobic respiration Signup and view all the answers

Which of the following is a characteristic of Purple Sulphur Bacteria?

Uses hydrogen sulfide instead of water Signup and view all the answers

What is the primary energy-generating process for methanogens?

Anaerobic respiration without O2 Signup and view all the answers

How did early Earth conditions contribute to the evolution of life forms?

Anaerobic conditions enabled the formation of organic compounds Signup and view all the answers

What significant evolution occurred with cyanobacteria?

They developed the ability for oxygenic photosynthesis. Signup and view all the answers

What was the initial fate of oxygen produced by cyanobacteria in ancient environments?

It was absorbed by sediments. Signup and view all the answers

What was the consequence of the prevention of an increase in atmospheric O2?

Prevention of lethal conditions for anaerobic organisms Signup and view all the answers

What is a consequence of reactive oxygen species (ROS) in biological systems?

They promote oxidative damage in cells Signup and view all the answers

How does high light intensity affect photosystems?

It leads to over-reduction and formation of ROS Signup and view all the answers

What role does ascorbate peroxidase (APX) play in the detoxification process?

It reduces reactive oxygen species into harmless substances Signup and view all the answers

What happens when a photon is absorbed by a ground singlet state molecule?

The electron is raised to a higher energy level Signup and view all the answers

What can happen to high energy electrons in the process of photosynthesis?

They may leak to molecular oxygen instead Signup and view all the answers

Which of the following best describes reactive oxygen species (ROS) in terms of their lifespan?

They are short-lived and last about 1 microsecond Signup and view all the answers

Study Notes

Excited Molecules and Energy Dissipation

Excited molecules can dissipate energy via internal conversion (IC), fluorescence, or intersystem crossing (ISC).
ISC involves a change in electron spin, resulting in an excited triplet state (T1).
Excited singlet states (ESS) and excited triplet states (ETS) can transfer excitation energy to nearby molecules

Photosynthesis and Oxygen

Early Earth's atmosphere was anaerobic and lacked oxygen, which was essential for the evolution of life.
Methanogenic bacteria were the earliest life forms, thriving in an atmosphere rich in CO2 and H2.
Cyanobacteria evolved oxygenic photosynthesis, which significantly impacted the Earth's atmosphere by introducing oxygen.

Oxygen Toxicity and Reactive Oxygen Species (ROS)

Oxygen, while essential for life, is also toxic due to the formation of reactive oxygen species (ROS).
ROS are short-lived but can extract electrons from other molecules, leading to oxidative damage.

Photorespiration and Rubisco

Photorespiration is a process where Rubisco, a bi-functional enzyme, uses oxygen instead of carbon dioxide.
Rubisco's oxygenation activity leads to the formation of glycolate, which is then processed by the glycolate pathway.
The glycolate pathway is a complex and energy-consuming process that aims to recover carbon from photorespiration.

Glycine Decarboxylase (GDC) and Serine Hydroxymethyl Transferase (SHMT)

GDC and SHMT are key enzymes involved in the glycolate pathway.
GDC is particularly abundant in photosynthetic parts of plants, accounting for up to 30% of mitochondrial protein in some leaves.

Folate

Folate is a crucial vitamin involved in various metabolic processes, including single-carbon metabolism.
Folate deficiency can lead to anemia.
Mitchell isolated folic acid from spinach leaves in 1941.
The folate structure consists of a tripartite molecule with various oxidation levels.
Folate is polyglutamylated in vivo and is photolabile.

Competition Between CO2 and O2

Oxygen competes with carbon dioxide for the active site of Rubisco.
This competition highlights the trade-off between photosynthesis and photorespiration.

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Description

This quiz explores the concepts of excited molecules, energy dissipation mechanisms, and the role of photosynthesis in Earth's atmospheric evolution. It covers the transition from anaerobic conditions to the emergence of oxygen through cyanobacteria. Additionally, the quiz addresses the dual nature of oxygen, highlighting its necessity for life and its potential toxicity through reactive oxygen species.