Energy and Carbon Source
How Photoheterotrophs Generate Energy
Photoheterotroph vs Photoautotroph
They are microorganisms that cannot make their food like any autotrophs. Some common examples are purple non-sulfur bacteria and green non-sulfur bacteria. Humans also exhibit this type of nutrition.
1. Using Bacteriorhodopsin
It is a chlorophyll-based mechanism similar to photosynthesis. The light activates the molecules of the pigment bacteriorhodopsin resulting in the movement of electrons through the electron transport chain. This flow of electrons through the proteins pumps hydrogen ions across the membrane. The energy stored in the proton gradient is used for ATP synthesis. The electron flow in photoheterotrophs is cyclic because the electrons released from the reaction center flow through the transport chain and return to the reaction center. Purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria are examples of such photoheterotrophs.
2. Using Purple-Rhodopsin-Based Proton Pumps
These pumps are integral membrane proteins. Such pump consists of a single protein bound to retinal, a Vitamin A derivative. The pump may have accessory pigments like the carotenoids associated with the protein. On absorbing light, the retinal molecule isomerizes, changing the molecule’s shape and pumping a proton across the membrane. The proton gradient generates ATP, transports solutes across the membrane, or drives the flagellar motion. One flavobacterium is insufficient to reduce carbon dioxide using sunlight. Instead, it uses energy from its rhodopsin system to convert it through the process of anaplerotic fixation, as shown below: CO2 + pyruvate + ATP (from photons) → Malate + ADP +Pi, where malate is obtained by breaking down other carbohydrates: Carbohydrate + O2 → Malate + CO2 + Energy This method of carbon fixation is used by Halobacteria, Flavobacteria, and Vibrios when reduced carbon compounds are scarce. However, energy is plentiful in the form of sunlight.
Similarities
Use sunlight as the source of energy (ATP)Use electron transport chain to generate ATP
However, there are some critical differences between the two, based on which we place them under two groups.
