Question

In Chapter 8, we saw that ATP is the currency of the cell, powering almost all the work carried out in a typical cell. In almost all eukaryotes and aerobic prokaryotes, the majority of this ATP is synthesized from the flow of stored hydrogen ions (protons) in the form of a proton gradient. This week’s forum will explore the universality of proton gradients and provide insight into the role they may have played in the origin of life itself and why, despite their incredible biochemical virtuosity, prokaryotes have failed to develop the complex morphology (multicellularity) observed for eukaryotic cells . Read the article Why Are Cells Powered by Proton Gradients, and answer the questions that follow. ----> LINK ; https://www.nature.com/scitable/topicpage/why-are-cells-powered-by-proton-gradients-14373960

1. (a) Explain how “natural” proton gradients could have arisen at ancient hydrothermal vents. Be sure to include in your response the source of the protons. (b) Do those same “natural” gradients exist today at hydrothermal vents – why or why not?

2. From analysis of Figure 4, explain the meaning of the Figure Legend, “Why chemistry is not enough”.

3. What explanation does the paper provide as to why prokaryotic cells have shown little tendency to develop the morphological complexity observed for eukaryotic cells?

4. Speculating is a fun part of science. Many of the hypotheses put forth in this paper are highly speculative. For example, little data exists to support the hydrothermal vent origin of proton gradients. Based on our class discussions, either propose another possible origin for proton gradients, or propose a question on a point or aspect of the origin that you find unclear or confusing.

Answer 1

1. (a). Russell has long maintained that natural proton gradients played a central role in powering the origin of life. Four billion years ago, alkaline fluids bubbled into mildly acidic oceans. Acidity is a measure of proton concentration, which was about four orders of magnitude higher than in vent fluids. The difference between the acidity of the oceans and the vent fluids gave rise to proton gradient across the vent membranes. This proton gradient force has similar electro chemical potential as modern cells. In this way natural potential gradients could have arisen at hydrothermal vents.

(b). The same natural gradients are not exist today at hydrothermal vents. The reason behind this is that CO₂ levels were about a thousand times higher than they are today, and CO₂ forms carbonic acid in solution, rendering the oceans mildly acidic.

2.   From analysis of figure-4, ‘’why chemistry is not enough” means, in an ancient pathway of photosynthesis (Acetyl-CoA), hydrogen gas is reacted via a few steps with CO₂. Energy is released by this pathway and captured as ATP. But, there is a problem, pointed out by William Martin in collaboration with Russell; all cells that use the Acetyl-CoA pathway today depend on proton gradients. None of them can grow by fermentation i.e., by chemistry of glycolysis. This is because, CO₂ is a stable molecule and does not react easily even with a hydrogen. If it starts to react it’s not easily stopped. Cells need equivalent of spark to get CO₂ to react and for cells that spark is ATP. The problem here is when CO₂ reacts with H₂ only small amount of ATP is released i.e., 1 ATP. If this produced ATP is utilized for the spark then there is no net gain, there is no growth finally, there is no life. Hence, this is the reason why chemistry has not enough life.

3. The reason for prokaryotic cells have shown little tendency to develop the morphological complexity observed for eukaryotic cells is the control of proton gradients. All eukaryotic organisms have mitochondria. It is capable of undergoing oxidative phosphorylation. But prokaryotes lack the mitochondria. No mitochondrial genome, no oxidative phosphorylation. Lacking mitochondria cannot grow large and complex, because they cannot control respiration over a wide area of energetic membranes.

4. After detailed study about the given data, unclear or confusing aspect is why complex life derives from a single common ancestor, which arose just once in the 4 billion years of life on earth.