Question

In the experiment of Racker and Stoeckenius, bacteriorhodopsin was oriented in such a way that it pumped \(\mathrm{H}^{+}\) into a vesicle. Each vesicle actually contained many molecules of bacteriorhodopsin. How would the results of the experiment have been affected if \(50 \%\) of the bacteriorhodopsin molecules pumped \(\mathrm{H}^{+}\) into the vesicle and \(50 \%\) pumped \(\mathrm{H}^{+}\) out of the vesicles? a. The same amount of ATP would be made in the presence of light, and no ATP would be made in the dark. b. More ATP would be made in the presence of light, and no ATP would be made in the dark. c. No ATP would be made in the presence of light, and no ATP would be made in the dark. d. No ATP would be made in the presence of light, but some ATP would be made in the dark. some ATP would be made in the presence of light, and some ATP would be made in the dark.

Short answer

c. No ATP would be made in the presence of light, and no ATP would be made in the dark.

Step-by-step solution

Understand the Function of Bacteriorhodopsin

Bacteriorhodopsin is a protein used by Archaea, most notably by Halobacteria, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. This creates a proton gradient that the cell uses to drive ATP synthesis.

Understand the Effect of the Bacteriorhodopsin Orientation

When the bacteriorhodopsin pumps \(\mathrm{H}^{+}\) into the vesicle, it means the protons are being concentrated in a specific area. If half of the bacteriorhodopsin is reversed, half would pump \(\mathrm{H}^{+}\) out of the vesicle. This means that the overall effect is neutralized.

Predict the Effect on ATP Production

ATP (Adenosine Triphosphate) production is driven by a proton gradient across the membrane. Because the hypothetical scenario neutralizes this gradient (half pumps in and half pumps out), there would be no driving force to manufacture ATP both in presence or absence of light.

Key concepts

Bacteriorhodopsin

Bacteriorhodopsin is a fascinating protein found in certain microorganisms like Halobacteria. It plays a crucial role by functioning as a light-driven proton pump. This means that bacteriorhodopsin harnesses sunlight to pump hydrogen ions (H⁺) across a membrane. By doing so, it actively contributes to the creation of a proton gradient. This protein is embedded in the cellular membrane and helps maintain the necessary conditions for various biological processes. One of its main roles is to assist in the production of ATP, the energy currency of the cell.
Bacteriorhodopsin is remarkable because it converts light energy directly into chemical work by moving protons against a concentration gradient. This action doesn't involve the typical electron transport chain used by other organisms for energy conversion, showcasing nature's diversity in solving energy needs.

Proton Gradient

The proton gradient is vital for energy production in cells. It is formed when protons (H⁺ ions) accumulate on one side of a membrane, creating both a concentration and an electrochemical gradient. This gradient acts as a form of stored energy the cell can tap into when needed. In the context of bacteriorhodopsin, this protein uses light to transport protons across the membrane, establishing this gradient.
The importance of the proton gradient cannot be overstated. It's a driving force behind ATP synthesis, as the protons tend to flow back across the membrane through special protein complexes. This movement down their gradient is coupled with the synthesis of ATP from ADP and inorganic phosphate. The process is known as chemiosmosis, and it underscores how integral gradients are to cellular energy conversion.

ATP Synthesis

ATP synthesis is the process by which cells generate adenosine triphosphate (ATP), a molecule that stores and transfers energy within cells. This process is powered by the proton gradient created by proteins like bacteriorhodopsin. The enzyme complex responsible for ATP production is ATP synthase. It is embedded in the same membrane where the proton gradient has been established.
As protons flow through ATP synthase back into the lower concentration side, the enzyme harnesses this movement to catalyze the formation of ATP. Each molecule of ATP formed is a direct result of the energy captured from the gradient. This process highlights the elegant efficiency of cellular energy systems: capturing, storing, and utilizing energy in a neat cycle that supports life.

Membrane Protein Function

Membrane proteins are vital for a variety of biological functions, and they achieve these roles through their ability to be embedded within cellular membranes. These proteins can act as channels, carriers, or pumps, and they are essential for tasks ranging from nutrient transport to signaling.
In the case of bacteriorhodopsin, its function as a proton pump showcases the ability of membrane proteins to convert energy types and support cellular metabolism. By creating a proton gradient, membrane proteins enable processes like ATP synthesis, as discussed earlier. They are versatile in their functions, often responding to environmental cues such as light or chemical signals. This ability allows cells to adapt to changing conditions while maintaining internal stability, illustrating the critical role of membrane proteins in cellular life.