BIOC 4520: Metabolic Process Lec 8

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Questions and Answers

What role did methanogenic bacteria play in early Earth’s atmosphere?

They produced oxygen through photosynthesis

They generated methane from CO2 and hydrogen (correct)

They produced oxygen radicals as a byproduct

They thrived in oxygen-rich environments

Which of the following organisms evolved oxygenic photosynthesis?

Methanogenic bacteria

Purple sulfur bacteria

cyanobacteria (correct)

Purple nonsulfur bacteria

Why is photorespiration considered a wasteful process in plants?

It results in the loss of ATP

It decreases plant productivity by approximately 30% (correct)

It accelerates the production of oxygen radicals

It does not recycle carbon

What is the primary consequence of oxygen competing with carbon dioxide for the active site of Rubisco?

Decreased photosynthetic efficiency Signup and view all the answers

What is the role of photorespiration in plants?

To recover carbon and prevent loss of C Signup and view all the answers

What effect do mutations in the photorespiratory pathway have on plants?

They are deleterious to plant function Signup and view all the answers

Which of the following statements is true about folic acid?

It derives its name from a Latin word meaning 'leaf' Signup and view all the answers

What is the effect of high carbon dioxide levels on photorespiration?

Reduces the occurrence of photorespiration Signup and view all the answers

What is the main function of ribulose bisphosphate carboxylase/oxygenase (RUBISCO) in photosynthetic organisms?

To facilitate the competition between CO2 and O2 Signup and view all the answers

What is not a consequence of the glycolate pathway in photorespiration?

Increased ATP production Signup and view all the answers

What role does glycine decarboxylase (GDC) play in plant cells during photorespiration?

It facilitates the decarboxylation of glycine, releasing CO2 Signup and view all the answers

During the conversion of P-glycolate to PGA + CO2, which of the following occurs?

Complex ATP-consuming reactions take place Signup and view all the answers

Which subunits are found in the glycine decarboxylase complex?

P, L, T, H Signup and view all the answers

What are the primary gases present in early Earth's atmosphere?

Hydrogen, methane, CO2, and ammonia Signup and view all the answers

How did methanogenic bacteria produce methane?

From hydrogen and CO2 or acetate Signup and view all the answers

What is the significance of hydrogen sulfide (H2S) in purple sulphur bacteria?

It replaces water in the photosynthesis process Signup and view all the answers

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

They serve as terminal electron acceptors Signup and view all the answers

Why was the initial absorption of O2 by sediments important for early life forms?

It prevented the toxic accumulation of O2 in the atmosphere Signup and view all the answers

What was a likely consequence of the presence of oxygen in Earth's atmosphere during the evolution of early life forms?

Inhibition of organic compound formation Signup and view all the answers

What type of photosynthesis do cyanobacteria perform?

Oxygenic photosynthesis Signup and view all the answers

Which organisms are classified as autotrophs in the early Earth context?

Purple sulphur bacteria Signup and view all the answers

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

Chain reactions leading to oxidative damage Signup and view all the answers

Which enzyme is responsible for the detoxification of hydrogen peroxide in plant cells?

Ascorbate peroxidase Signup and view all the answers

What occurs when high light intensity causes over-reduction of photosystems?

Formation of reactive oxygen species Signup and view all the answers

Which process describes the transition of an excited molecule to a triplet state via a 'forbidden' change in electron spin?

Intersystem crossing Signup and view all the answers

What happens to an excited singlet state (ESS) molecule after it absorbs a photon?

It can return to ground state in different ways. Signup and view all the answers

Which option describes one way an absorbed photon can be dissipated by an excited molecule?

Internal conversion Signup and view all the answers

What is NOT a function of the excited triplet state (T1) after intersystem crossing?

Formation of reactive oxygen species Signup and view all the answers

How long do reactive oxygen species (ROS) typically last in biological systems?

1 x 10^-6 seconds Signup and view all the answers

Study Notes

Early Earth’s Atmosphere

Early Earth's atmosphere was anaerobic, rich in hydrogen, methane, CO2, and ammonia
The presence of these gases was likely essential for the evolution of life
Haldane proposed that O2 was absent on early Earth, otherwise organic compounds would have been oxidized to CO2 and H2O

Methanogenic Bacteria

The earliest life forms were methanogenic bacteria (methane-producing Archaea)
These bacteria are strictly anaerobic and are killed by O2
They produce methane from hydrogen and CO2, or acetate
- CO2 + 4 H2 ATP generation CH4 + 2 H2O
- CH3COOH ATP generation CH4 + CO2
Acetate and CO2 sustain anaerobic respiration by acting as terminal electron acceptors, allowing the production of ATP (simple “glycolysis”)
Methanogens thrive in habitats resembling early Earth: rich in CO2 and H2, such as hot volcanic, sulphurous, anaerobic springs

Evolution of Photosynthesis

Some prokaryotes became autotrophic, generating their energy
Purple sulfur bacteria are photosynthetic bacteria that reduce carbon to carbohydrates
They do not release oxygen, and use hydrogen sulfide (H2S) instead of water
Cyanobacteria evolved oxygenic photosynthesis, releasing oxygen
- 2H2O O2 + 4H+ + 4e-
Oxygen initially was absorbed by sediments and Fe2+ was oxidized to Fe3+
This prevented an immediate increase in atmospheric O2, which would have been lethal to methanogens

The Problem of Oxygen

Oxygen is a reactive molecule that can extract electrons from other molecules, causing oxidative damage
This damage is caused by reactive oxygen species (ROS)
ROS are short-lived but can create a chain reaction
Plants have developed detoxification mechanisms to combat ROS, such as ascorbate peroxidase (APX) - H2O2 + Asc H2O + MDHA
High light intensity can overload photosystems and lead to the production of ROS
- Excess energy electrons cannot be passed to NADP+
- This leads to electron leakage to molecular oxygen

Photosynthesis and Respiration

Photosynthesis is the process where plants convert light energy into chemical energy
Respiration is the process where plants break down glucose to generate energy
In photosynthetic organisms, there is a significant level of respiration in the light

Photorespiration: The Role of Rubisco

Rubisco is a bifunctional enzyme that can use both O2 and CO2
It competes with CO2 in the active site, allowing the enzyme to bind to O2
O2 binding can reduce productivity by around 30% in plants like wheat, rice, and potato
This process is called photorespiration
While photorespiration reduces productivity, it is essential for plant survival
Mutations in the photorespiratory pathway are harmful
High CO2 levels can minimize the problem

### The Glycolate Pathway

The glycolate pathway is a complex, ATP-consuming process that recovers carbon fragments from photorespiration.
This pathway involves three organelles: chloroplasts, mitochondria, and peroxisomes.
It loses carbon as CO2 through the mitochondrial decarboxylation of glycine.
The process converts two moles of P-glycolate to PGA + CO2.

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

Glycine decarboxylase (GDC) is an essential enzyme in the photorespiratory pathway.
It accounts for a significant portion of mitochondrial protein in plant leaves.
It is a tetrameric protein with four subunits: P, L, T, and H.
GDC catalyzes the conversion of glycine to CO2, NADH, and 5,10-methylenetetrahydrofolate.
Serine hydroxymethyl transferase (SHMT) is another essential enzyme in the pathway.
It catalyzes the transfer of a one-carbon unit to glycine.
The one-carbon unit is carried on tetrahydrofolate (THF).

### Folate

Folate is a vital nutrient for plants, humans, and other organisms.
It plays a critical role in cell division and growth.
A deficiency in folate can lead to anemia.
Folic acid is a synthetic form of folate.
It was first isolated from spinach leaves.

Photorespiration: Overall Importance

While photorespiration reduces productivity, it offers several benefits:
- It recovers carbon from photorespiration
- It provides a turnover of reducing power and ATP.
- It helps minimize photoinhibition by removing excess energy.
C4 photosynthesis is a more efficient mechanism for fixing carbon dioxide in plants and alleviates some challenges associated with photorespiration.

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