Metabolic Diversity: Choosing an Electron Donor and Acceptor
Microbes are classified by where they get carbon, where they get energy and what they pass electrons to at the end. The energy available depends on the gap between donor and acceptor, which is why oxygen respirers outgrow the alternatives when oxygen is present.
What a learner can do afterwards
- Classifies an organism from its carbon source, energy source and terminal acceptor.
- Ranks acceptors by the energy yield they allow and explains the ranking.
- Predicts which metabolism dominates as a sediment loses oxygen with depth.
1 · Read
Imagine sorting microbes the way you sort lunchboxes. Ask where carbon comes from: air gas or ready made organics. Ask where energy comes from: light or chemicals. Then ask what catches electrons at the end. Those three answers fix the lifestyle completely.
Now rank the catchers the way athletes rank medals. Oxygen accepts most willingly, then nitrate, then sulfate and carbon dioxide last. The wider the gap between giver and catcher, the more energy each fuel molecule banks. That sliding scale is why oxygen users outgrow the rest wherever oxygen reaches.
Picture the mud under a pond sorted by depth for you. Near the top the oxygen users grow fastest on the full payoff. Below them come the nitrate users, and deep down in the oxygen free dark sit the sulfate breathers and methane makers. Read the oxygen profile and you predict the stack.
Two small extras complete your map. Without an outside catcher, cells dump electrons back onto their own waste just to keep going, and most fuel energy stays locked inside: tiny yield. With a catcher, electron flow pumps a gradient whose return through the synthase mints the energy coins.
You read the carbon, energy, and final catcher to predict who wins each layer.
More to read
8.3 Cellular Respiration
Cellular respiration is organised around one question: where do the electrons go at the end. Donors such as glucose give up electrons, carriers shuttle them through a membrane chain, and a terminal acceptor collects them. Oxygen accepts more willingly than nitrate, which beats sulfate and carbon dioxide in turn. The bigger the gap between donor and acceptor, the more energy the chain banks as ATP. That ranking is why oxygen respirers outgrow fermenters and anaerobes wherever oxygen reaches.
8.4 Fermentation
Fermentation is respiration without an external acceptor. Cells dump electrons back onto a product they made themselves, such as lactate or ethanol, just to recycle carriers and keep glycolysis running. The payoff is tiny next to full respiration because most of the fuel energy stays locked in the waste product. Microbes accept that bargain only when no usable external acceptor is around. Speed and simplicity trade against yield.
7.4 Oxidative Phosphorylation
Oxidative phosphorylation converts the donor acceptor gap into ATP through a proton gradient. Electron flow pumps protons across the membrane, and their return through ATP synthase mints the energy currency. Oxygen sits at the bottom of this cascade because it pulls electrons hardest, letting the chain pump the most protons. Swap in a weaker acceptor and the gradient shrinks, so fewer ATP per fuel molecule. The chemistry is one sliding scale from oxygen down to carbon dioxide.
7.5 Metabolism without Oxygen
Sediments sort metabolisms by depth because oxygen runs out downward. Near the surface, aerobes dominate and grow fastest on the full oxygen payoff. Below them come nitrate users, then sulfate reducers and methanogens in the deep oxygen free layers. Each zone is ruled by whichever resident can bank the most energy from the acceptors still available. Read the oxygen profile and you can predict the community stacked beneath it.
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2 · Watch
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Where it sits
Learn first
Nothing; this is a starting point.
Optional connections
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.