L9 : Life in Redox Gradient Flashcards
(15 cards)
What is a redox gradient?
Refers to spatial variation in oxidation reduction potential
Why are redox gradients significant for life?
Organisms exploit gradient to extract energy
By transferring electrons from reduced compounds to oxidised compounds
Why is aerobic respiration highly favourable?
Uses oxygen at TEA
- Strong oxidising agent
Yields large negative delta G
- High energy yield
What is sulfate reduction?
Anaerobic process
Organisms use SO4 2- as TEA
Produce H2S
Energetically less favourable than aerobic respiration
What is methanotrophy?
Methane oxidation
Often coupled with oxygen or nitrate as EA
What type of environment supports reactions like sulfate reduction or methane oxidation?
Reducing environments
eg. hydrothermal vents or anoxic sediments
Where reduced compounds meet oxidised water
Why are hydrothermal vents thermodynamically interesting?
eg. Mid Atlanticridge
Emit reduced compounds like sulfide and methane into oxidised sea water
Creates steep redox gradients and abundant free energy
Can almost any redox pair support microbial metabolism?
Yes
Microorganisms have evolved to use diverse redox couples
Examples:
- arsenite / arsenate
- phosphite / phosphate
What is the general reaction for oxygenic photosynthesis
H2O + CO2
Sunlight
O2 + organic matter
Why is oxygenic photosynthesis energetically expensive?>
Oxidising water requires large energy input as strong electron donor required
Why use water as electron donor despite cost?
Abundant and replenishable
Unlike more energy efficient donors (eg. H2S) that are scarce
What is chemosynthesis?
Fixation of CO2 into organic matter using energy from chemical reactions (rather than sunlight)
What does a steep downhill slope on redox tower represent?
Highly favourable reaction
What does high Q and low Q indicate?
High Q = lots of product
- Reaction to completion
Low Q = lots of reactant
- Starting reaction with no product has Q=0
What is the reactant quotient?
Q = [B]/[A]
Describes how far a reaction is from equilibrium
