Chapter 21: Metabolic diversity
Lessons from eccentrics
Sometimes understanding a process is aided by looking at variations on the process. We are studying how organisms satisfy their energetic and material needs allowing growth to take place. The primary material we are considering in the previous chapters is carbon, but it generally brings with it hydrogen and oxygen (in a carbohydrate molecule). We will get to other materials (elements, e.g. nitrogen) in a subsequent chapter, but carbon, hydrogen and oxygen make up 95% of most organisms. This chapter first reviews the common patterns of matter and energy acquisition found in most organisms. Then we will then consider several groups of organisms whose energy and carbon processing is eccentric in various ways.
TOPICS
- Review: ’normal’ patterns involving matter and energy
- Acquiring energy — cellular respiration
- Acquiring food — heterotrophs and photosynthetic autotrophs
- Acquiring carbon — heterotrophs and photosynthetic autotrophs
- ‘ Eccentric’ organisms
- Obtaining energy when there is no oxygen
- fermentors
- using alternative electron acceptors
- Chemoautotrophs, making food (carbohydrates) without light
- Photoheterotrophs (=photoenergetic heterotrophs) , separating matter and energy
- Obtaining energy when there is no oxygen
‘The norm’—the most common processes
1. Acquiring metabolic energy
Nearly all organisms utilize the reactions discussed in the chapter on cellular respiration to supply their energetic needs: cellular respiration provides energy in the form of ATP as long as a supply of carbohydrates and oxygen is available. Oxygen is readily available in all terrestrial habitats due to the fact that the atmosphere is 20% oxygen and local depletions are rare because the atmosphere is continually mixed by winds. In contrast, in some aquatic situations (including soils saturated with water), oxygen may be in short supply. Present in such situations are a number of organisms, both prokaryotic (bacteria or archaea) and eukaryotic, that can obtain ATP energy in the absence of oxygen, in a manner that utilizes only the glycolysis part of cellular respiration. Alternatively, there also are organisms, solely prokaryotes, that have mechanisms to make ATP that don’t rely at all on the pathways of cellular respiration.
2. Making your own food: energy flow in photosynthetic autotrophs
If you consider it closely, the energetics of photosynthetic organisms seems unnecessarily involved: the light reactions produce ATP and NADPH in order to synthesize carbohydrates from carbon dioxide. Then the carbohydrates are broken down in cellular respiration to produce NADH (for our purposes the same as NADPH) and ATP. Why bother making carbohydrates?; why not just use the light reactions to obtain ‘energy currency,’ ATP and NADH? To a limited extent, photosynthetic organisms DO satisfy some of their energetic needs directly off of the light reactions, i.e., not all of the ATP and NADPH produced is used to