Why Do Some Chemical Reactions Take Time?

[jobyts]

Why some of the chemical reactions are slow? Is it directly proportional to the size and shape of the molecules - since the complex shape makes 2 molecules less probable coming in contact with the appropriate alignment for the chemical reaction to happen?

I'm trying to find an answer within physics or chemistry domain for the question - why plants grow slow.
If we provide all the necessary ingredients, what is preventing them the chemical reaction to complete and produce the results within, say, few seconds?

 

[pzona]

Well, you're basically asking for a fundamental course in chemical kinetics. Yes, the molecule's shape and size do play a part, but other important factors include physical state, temperature, concentration, catalysts, etc. Google "chemical kinetics" or look in any chemistry textbook for more information, and then if you have more specific questions, feel free to ask them.

 

[alxm]

Why some of the chemical reactions are slow? Is it directly proportional to the size and shape of the molecules - since the complex shape makes 2 molecules less probable coming in contact with the appropriate alignment for the chemical reaction to happen?

Well, this is a factor. When the geometry of two molecules inhibits a reaction, chemists say it's 'sterically hindered'.

But it's not usually the main factor in determining the kinetics. Basically, it's usually the case that either they can get close enough, or they can't. If they do get close enough, what determines if a reaction occurs is whether or not the colliding molecules have sufficient kinetic energy to react. See, molecules attract at very long range, and repel each other at short range. If they can react, it's only once they get very close that a bond is formed and the repulsion is overcome. So there's an energy barrier to the reaction. You can think of it as rolling a ball over a hill - it needs a certain amount of speed to get over, even if it's ultimately going to end up downhill of where it started.

Technically, this is expressed by the Eyring and Arrhenius equations, which connect the kinetic energy (due to temperature) and the 'height' of the energy barrier with the reaction rate.

However, this doesn't do much toward explaining why plants and animals grow at the speeds they do. See, most of the important reactions in living things are catalyzed (by enzymes), and don't occur by just random bumps. This means a much more complicated kinetics, and a different temperature dependency.

The question of why plants and animals grow at the speeds they do doesn't really have a simple answer. There are very many limiting factors and mechanisms that govern their growth, and temperature is only one small factor. The main limiting factors are always light, water and nutrients. Think about how different plant species in the same environment may grow at vastly different rates.

As so often in biology, the basic answer would really be that plants grow at a certain rate because they're genetically programmed to do so. Given their environment, ecological niche etc, they've basically evolved to grow as fast or slow as is optimal for them. (or at least, as optimal as they've been able to evolve during the time they've been occupying that niche)