Intro to Cellular Respiration: The Production of ATP

Cel­lu­lar res­pi­ra­tion is the process by which organic com­pounds (prefer­ably glu­cose) are bro­ken apart, releas­ing energy that is used to pro­duce ATP mol­e­cules.  Cells need to have ATP because it’s the gaso­line that pow­ers all liv­ing things.  ATP is a high energy nucleotide which acts as an instant source of energy within the cell.

And by laun­dro­mat, I mean your body!

Cel­lu­lar res­pi­ra­tion is like a change machine: you’re turn­ing sug­ars into ATP so it will be a usable form of energy.  If you go to a coin oper­ated laun­dro­mat, they all seem to run on quar­ters for some rea­son.  So you might say, “I don’t have any quar­ters but I want to wash my clothes and I have a $10 bill.”  You put your $10 bill into the change machine and you get 40 quar­ters and now you could use the coin oper­ated wash­ers and dry­ers.  All the chem­i­cal reac­tions in liv­ing things run off of these quar­ters (ATP).  They don’t run off $10 bills (sugars/fats/proteins).

One glu­cose will give you as much as 38 ATP, sim­i­lar to the way a $10 bill will give you 40 quar­ters.  A fat mol­e­cule is more like a $100 bill because it has that much more energy.

Since ATP is found in all liv­ing things it’s some­times called the energy cur­rency of cells, which goes well with this laun­dro­mat analogy.

Here is the over­all sim­pli­fied reac­tion for aer­o­bic respiration:

C₆H₁₂O₆ + 6O₂ —– enzymes & coen­zymes ——> 6CO₂ + 6H₂O + Release of Energy (≤38 ATP) + Heat

In order to make ATP, you need food (sugar) and oxy­gen.  If you don’t have food, you can’t make ATP and you’re going to die.  Even if I brought in all the food in the world and then I dia­bol­i­cally suck all the oxy­gen out of this room, you’re still going to die.  You need oxy­gen to unlock the energy that’s in the food.  Cel­lu­lar res­pi­ra­tion also explains why we are breath­ing oxy­gen and why we exhale car­bon dioxide.

In essence, the energy that was in cova­lent bonds of the glu­cose mol­e­cule is being released.  In actu­al­ity, this process requires sev­eral steps because the sugar is bro­ken down by baby steps, lit­tle by lit­tle, and is cat­alyzed many enzymes and coenzymes.

The effi­ciency of cell respiration

Noth­ing is per­fectly effi­cient in this world.  Even your car engine is only about 25% effi­cient at best.  Only about 25% of the burned gaso­line goes toward mov­ing your car while the other 75% is given off as heat which is why your engine and exhaust sys­tems are very, very hot.

Some nifty num­bers here:  There’s 686kcal (686,000 calo­ries) in a mole of glu­cose.   The actual usable energy obtain­able from the 38 ATP mol­e­cules that may be pro­duced from this glu­cose is 288,800 calo­ries (38 x 7,600 calo­ries per ATP).  There­fore, the com­plete oxi­da­tion of glu­cose is only about 40% effi­cient (288÷686).   The other 60% goes off as heat.  It’s impos­si­ble to con­vert one form of energy into another with­out cre­at­ing heat.  This release of heat is pre­dicted by the law of ther­mo­dy­nam­ics.  In other words, approx­i­mately 40% of the energy that’s cre­ated is used to phos­pho­ry­late ADP into ATP.

Fur­ther­more, this reac­tion explains why the tem­per­a­ture of your body is almost 100°F.  If you start to exer­cise, cel­lu­lar res­pi­ra­tion starts to speed up inside your mus­cle cells to pro­duce more ATP, so your body starts break­ing down sug­ars at a faster rate, you breathe oxy­gen at a faster rate and exhale car­bon diox­ide at a faster rate and give off a lot more heat at the same time.

Oxy­gen acts as a “hydro­gen acceptor.”

What liv­ing things do is they take oxy­gen and trans­fer the hydro­gens that come off of sugar mol­e­cules and stick them onto oxy­gen.  When you attach a cou­ple hydro­gens onto an oxy­gen, you get water.  For this rea­son, it is said that oxy­gen is a hydro­gen acceptor.

We breathe in oxy­gen to remove the hydro­gens off the sug­ars and fats oth­er­wise the extra [H⁺] will cause the acid­ity to increase and pro­teins will unravel (dena­ture) and die.  Remem­ber, an acidic solu­tion is one that has more hydro­gen ions (H⁺) than hydrox­ide ions (OH^-).

Remem­ber the mnemonic OIL RIG:

Oxi­da­tion: The glu­cose is being oxi­dized into car­bon diox­ide. OIL: Oxida­tion Is a Loss (of H⁺ and e^-)

Reduc­tion: Oxy­gen is reduced into water. RIG: Reduc­tion Is a Gain (of H⁺ and e^-)

One of the coen­zymes that helps cel­lu­lar res­pi­ra­tion is NAD+, which con­tains the B Vit­a­min, Niacin.

Now that we’ve described the big pic­ture and we under­stand that sug­ars are the prin­ci­ple form of food used to cre­ate ATP, now we could under­stand why hav­ing a healthy blood sugar level is so impor­tant.  So let’s go off on a slight tan­gent and explain how the sugar lev­els in our blood remain con­stant before div­ing fully into cel­lu­lar respiration.

Next post in the series: How are glu­cose lev­els reg­u­lated in the blood stream?

Cel­lu­lar respiration

• Ana­bolic and Cata­bolic Reactions
• Intro to Cel­lu­lar Res­pi­ra­tion: The Pro­duc­tion of ATP
• How Glu­cose Lev­els are Reg­u­lated in the Blood Stream
• Cell Res­pi­ra­tion Part 1: Anaer­o­bic Res­pi­ra­tion (Gly­col­y­sis and Fermentation)
• Cell Res­pi­ra­tion Part 2: Aer­o­bic Res­pi­ra­tion (Tran­si­tion Reac­tion & Kreb’s Cit­ric Acid Cycle)
• Cell Res­pi­ra­tion Part 3: Aer­o­bic Res­pi­ra­tion (Elec­tron Trans­port System)
• The Catab­o­lism of Fats and Pro­teins for Energy
• The Catab­o­lism of Nucleic Acids
• Oxy­gen Debt

Related posts:

1. Cell Res­pi­ra­tion Part 1: Anaer­o­bic Res­pi­ra­tion (Gly­col­y­sis and Fermentation)
2. Cell Res­pi­ra­tion Part 3: Aer­o­bic Res­pi­ra­tion (Elec­tron Trans­port System)
3. Cell Res­pi­ra­tion Part 2: Aer­o­bic Res­pi­ra­tion (Tran­si­tion Reac­tion & Kreb’s Cit­ric Acid Cycle)
4. Oxy­gen Debt
5. How Glu­cose Lev­els are Reg­u­lated in the Blood Stream