13. Which of the following proteins acts as an energy transducer?
(1) G-protein.
(2) Bacteriorhodopsin.
(3) Hemoglobin.
(4) Heat shock protein.

Energy transducers are proteins that convert one form of energy into another, enabling biological systems to perform work essential for life. Among various proteins, some specialize in transforming energy from environmental stimuli into usable cellular forms, such as electrical or chemical gradients.

What Is an Energy Transducer?

An energy transducer protein converts energy from one form to another. For example, in sensory cells, energy transducers transform environmental stimuli (like light or chemical signals) into changes in electrical potential or chemical gradients across membranes.

In biological systems, energy transduction is fundamental for processes like photosynthesis, respiration, and sensory perception.

Examining the Protein Options

1. G-protein:

   • Functions as a molecular switch in signal transduction pathways.

   • Transmits signals from activated receptors to downstream effectors by binding and hydrolyzing GTP.

   • Does not directly convert energy forms; instead, it relays signals.

   • Classified as a transducer in signaling but not an energy transducer.

2. Bacteriorhodopsin:

   • A membrane protein found in certain archaea (e.g., Halobacterium).

   • Acts as a light-driven proton pump, absorbing photons and transporting protons across the membrane.

   • Converts light energy into a proton electrochemical gradient, which can be used to generate ATP.

   • A classic example of a biological energy transducer.

3. Hemoglobin:

   • Oxygen transport protein in red blood cells.

   • Binds and releases oxygen but does not transduce energy between forms.

   • Functions in gas transport, not energy transduction.

4. Heat shock protein:

   • Molecular chaperones assisting in protein folding and protection under stress.

   • Do not convert energy forms; their role is in protein maintenance.

Why Bacteriorhodopsin Is the Energy Transducer

• Bacteriorhodopsin absorbs light and uses that energy to pump protons across the membrane.

• This proton pumping creates a proton motive force, a form of stored energy used by the cell to synthesize ATP.

• It directly converts light energy into electrochemical energy, fulfilling the definition of an energy transducer.

Summary Table

Conclusion

Among the given options, bacteriorhodopsin uniquely acts as an energy transducer by converting light energy into a proton gradient across the membrane. This function is central to energy conversion in certain microorganisms and exemplifies biological energy transduction.

Final Answer:
(2) Bacteriorhodopsin