Confusing thermochemistry, help!

[TXhopeful]

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Concepts I grasp:
1. The "rule of thumb" is that breaking a bond requires energy while forming a bond releases energy.
2. Endothermic reactions are positive enthalpy changes (heat into system), and exothermic reactions are negative enthalpy changes (heat out of system).
3. Building a molecule from smaller ones is anabolic (like steroids), while breaking them down is catabolic.

What throws me off is explaining the processes of cellular respiration: breaking glucose into water and CO2, releasing energy in the process.
Catabolic processes are exothermic, because they release energy into the surroundings, but breaking a bond (like glucose into water and CO2) is supposed to be putting energy IN to break it, right?
Vice versa with photosynthesis being an anabolic/endothermic process, when you're using water and CO2 to FORM glucose (form is supposed to be release).

I feel like I'm in a roundabout and keep missing my exit.

 

[drr0zz0r]

*deleted*

 

[aldol16]

You're missing two things. First, the overall enthalpy change of a reaction is the sum of the bond breaking and forming. So if you make more bonds than you break, the overall enthalpy change can be negative. Second, whether a reaction is exergonic or not depends on entropy as well. Catabolic processes inevitably make multiple molecules from one. That means entropy is always increasing in that system and that there can sometimes be a huge entropic drive to a system, propelling it forward.

 

[eteshoe]

Concepts I grasp:
1. The "rule of thumb" is that breaking a bond requires energy while forming a bond releases energy.
2. Endothermic reactions are positive enthalpy changes (heat into system), and exothermic reactions are negative enthalpy changes (heat out of system).
3. Building a molecule from smaller ones is anabolic (like steroids), while breaking them down is catabolic.

What throws me off is explaining the processes of cellular respiration: breaking glucose into water and CO2, releasing energy in the process.
Catabolic processes are exothermic, because they release energy into the surroundings, but breaking a bond (like glucose into water and CO2) is supposed to be putting energy IN to break it, right?
Vice versa with photosynthesis being an anabolic/endothermic process, when you're using water and CO2 to FORM glucose (form is supposed to be release).

I feel like I'm in a roundabout and keep missing my exit.

For glycolysis (breaking down glucose into pyruvate that can be then sent to the TCA cycle) the process occurs since it takes in 2 units of ATP/per glucose molecule and produces 4 ATPs, 2 NADH (both high energy molecules/e- donors). This is a net gain of 2 ATP and 2 NADH per molecule of glucose. Just think about the net effects as the driving forces. Usually nature couples endothermic processes with exothermic ones so that life can keep going.

 

[aldol16]

Usually nature couples endothermic processes with exothermic ones so that life can keep going.

You mean exergonic and endergonic.

 

[eteshoe]

You mean exergonic and endergonic.

Yup. Been up too long

 

[TXhopeful]

@aldol16 @eteshoe
Thank you both for your explanations! I understand now