Biology Forums - Study Force

suppose there is a toxin that stops the enzyme  Rubicon from functioning.  Without  functioning  RuBisCo, a plant cell cannot fix carbon and the Calving cycle stops. Soon after the Calving cycle stops, the lifht - dependent reactions will also stop, why?

Haven't heard of the enzyme, Rubicon. Assuming you meant Rubisco, the utilization of NADPH, ATP, regeneration of RuBP, sugar production, and utilization of CO2 would not happen since CO2 would not be able to react with RuBP to start the cycle.

We define the function of enzymes as lowering the required activation energy for a reaction to occur. Hence, it is reasonable to state that if RuBisCO didn't exist, the same reaction could take place given ideal conditions (i.e. in solution of correct pH and temperature, with the adequate amounts of substrate). However, as the activation energy required for this reaction to occur is quite high, the reaction would only occur extremely, extremely slowly - or even not at all without an adequate input of environmental energy.

This allows for the answer to your second question. If you look at evolutionary history, you'll see that complex organisms such as animals are fairly recent. Millions of years ago, the most complex life-forms that existed were either unicellular or basic multicellular organisms who relied on photosynthesis to obtain their chemical energy. Assuming they used a similar or identical mode of photosynthesis as seen in modern plants (and some photosynthetic bacteria), without RuBisCO these animals would have become extinct in a very short time - they could not obtain the necessary chemical energy to survive. Given that these organisms are likely to have evolved over time to the more complex organisms we see today, we might not be alive if RuBisCO had never existed.

However, given what we are fairly sure about regarding evolution, there would have likely been some variation in the ancestor species that would have allowed for the adaptation and evolution of other means of photosynthesis/other forms of chemical energy harvesting. Evolution is nature's contingency!