Learning Outline

Getting Energy

BIO 095

Introduction

This review covers the mechanisms by which the energy trapped in nutrient molecules (such as glucose) is transferred to ATP.

ATP molecule. ATP is a modified adenine nucleotide. That is, it's an adenine (nitrogen base), ribose (sugar), and phosphate group with an EXTRA 2 phosphates. These extra two phosphates require "extra" energy to push them onto the nucleotide, then "pop off" fairly easily—giving the energy back. Thus, ATP is used to "trap" energy released with nutrients break down, then transfer it quickly to cell processes that need the energy. ATP is thus a kind of "rechargeable cell battery."

 

ATP cycle. ATP is "adenosine triphosphate" and with 3 P (phosphate) groups is the "charged up" energy-containing form. When a P breaks off, ADP (adenosine diphosphate) is formed. ADP, with only 2 phosphates, is the "discharged" version of the molecule. ADP and P can be "recharged" by using energy from nutrient molecules.

The key here is focusing on "what's really happening" without getting bogged down in the details of the chemistry.

• What's really happening is really simple

  • The cell transfers energy from fuel molecules eventually to ATP
    
    

Basic definitions

Metabolism—body chemistry

• Catabolism—chemistry that breaks big molecules into small ones
  
  
• Anabolism—chemistry that builds small molecules into big ones

Metabolic pathway—series of chemical reactions, one leading to the next, and so on

Respiration—literally "re-breathing" and refers to bringing in oxygen (O2) and releasing carbon dioxide (CO2)

• Cellular respiration—chemical process in cells that uses oxygen and gives off carbon dioxide, really referring to energy transfer
  
  
• Aerobic respiration—respiration pathway that requires oxygen
  
  
• Anaerobic pathway—pathway that does not require oxygen
  
  
  • Actually, this step is technically "fermentation" not "respiration" but the term "anaerobic respiration" is still often used

Coenzyme—coenzymes "help" enzymes
In this story, it is helpful to think of coenzymes as "escorts" that move molecular fragments from one chemical pathway to another; examples: NAD and FAD

Substrate—any molecule acted upon by one or more enzymes

Summary of chemical changes

 C₆H₁₂O₆  +  O₂  ———>  H₂O  +  CO₂  +  energy (in ATP)  

[this equation is not balanced]

Cellular Respiration Pathways
Notice that the cell may start with glucose, but other nutrients (amino acids from proteins, fatty acids & glycerol from lipids) can also enter the pathway and contribute energy for recharging ATP

Step 1: Glycolysis

Breaks glucose (C₆) into two pyruvic acids (2 C₃) and releases energy

Enough energy is released for 2 ATP molecules

Anaerobic

Occurs in cytosol outside of mitochondria

 

Step 2: Transition reaction

If pyruvic acid is to continue, it enters the mitochondrion and one carbon is removed --forming Acetyl (C₂)

Coenzyme A (CoA) temporarily binds to acetyl and escorts it into the citric acid cycle

This begins the aerobic process (although O₂ will not actually be used until later, the molecule will enter this pathway until and unless O₂ is there at the end of the line)

Step 3: Citric Acid Cycle

Also known as Krebs Cycle (for Hans Krebs) or TCA (tricarboxylic acid) Cycle

Acetyl rides this "ferris wheel" where it is broken apart, releasing its energy

The Cs and Os simply fall away, forming the waste CO₂ 

Most of the energy released in the form of energized electrons from H (the H⁺ proton also tags along for the trip)

• The high-energy electrons (and H⁺) are picked up by coenzymes NAD and FAD and escorted to the Electron Transport System
  
  

Step 4: Electron Transport System (ETS)

Also called Electron Transport Chain (ETC)

High-energy electrons (and H⁺) are dropped off at molecules in the cristae

The electrons are shuttled from molecule to molecule losing their energy as they go (passed like a hot potato, eventually "cooling off")

The energy lost by electrons is used to pump the protons (H+) into the intermembrane space, like water behind a dam

As the protons flow back through the dam (down their concentration gradient), this powers the "phosphorylation of "or "adding phosphate to" ATP (oxidative phosphorylation)

The electrons unite with their protons, forming H₂ which is dangerous

• H₂ is immediately "burned" by O₂, forming waste H₂0 
  
  
• A total of 36-38 ATPs are available from aerobic respiration (compare to only 2 for anaerobic alone)
  
  
• Lactic acid
  
  
  • Forms when pyruvic acid does not enter the aerobic pathway
    
    
  • Happens when not enough oxygen or when energy is needed more quickly than aerobic respiration can handle
    
    
  • Later converted back to glucose

Cellular Respiration Summary
This image is just so that you can see the overall process and look for the main events—you are not expected to remember every detailed step!
(click image to see other resolutions & credits)

ATP yield in aerobic cellular respiration
Step	Coenzyme yield (eventually transfers energy to ATP in ETS)	ATP yield	ATP source
Early glycolysis		-2	Two ATP from cytoplasm needed to phosphorylate glucose and fructose 6-phosphate
Late glycolysis		4	From substrate phosphorylation (anaerobic)
	2 NADH	4 (or 6)	Oxidative phosphorylation. (aerobic) Depending on which mechanism transports NADH into the mitochondrion, either 2 or 3 ATP are produced (per NADH).
Transition	2 NADH	6	Oxidative phosphorylation (aerobic)
Citric acid cycle		2	Substrate phosphorylation (anaerobic)
	6 NADH	18	Oxidative phosphorylation (aerobic)
	2 FADH2	4	Oxidative phosphorylation (aerobic)
TOTAL		36 (or 38)	These are theoretical yields of ATP molecules per glucose molecule that enters the pathway, assuming all conenzymes transfer their energy).
Like the EPA mileage estimates for cars/trucks, the "actual mileage may vary" depending on how and where the cells are "driven." "Oxidative phophorylation" implies involvement of the ETS.

 

Summary of Catabolic Pathways

There are two main pathways in which energy-containing nutrients give up their energy to regenerate ATP.

Aerobic pathway

If there is sufficient oxygen available for the last step of oxidative phosphorylation in the ETS, then the nutrient is processed all the way through the system.

• This maximizes the amount of ATP generated per nutrient molecule.

This takes a lot of time

• Look at all the steps involved! OF COURSE it takes time.
  
  
• But it's worth it if you have the time, because you get "good mileage" for the amount of nutrients available

Aerobic respiration is good for steady, long-term use of muscles (endurance activities)

Hikers. Endurance activities such as hiking rely mainly on energy from the aerobic pathway.

Anaerobic pathway

If there is nutrient available, but not enough oxygen . . . and the cell needs energy . . . then some of the nutrient molecules follow the anaerobic pathway.

• Glycolysis produces a net yield of 2 ATPs (which is better than nothing)
  
  
• Substrates cannot enter the citric acid (Krebs) cycle because the oxygen is not available
  
  
• Substrates are instead converted to lactic acid (which still contains a lot of energy)
  
  
  • Have you ever felt the lactic acid burn your muscle tissue when your muscles are using the anaerobic pathway?
    
    
  • This is just a temporary solution . . the lactic acid must still be processed later.
    
    
    • Because oxygen is needed later to process the lactic acid, we sometimes say that the anaerobic pathway incurs an "oxygen debt"
      
      
      • This partly explains the postexercise oxygen consumption (EPOC) experienced when you continue to breathe heavily after you've stopped exercising

Because only glycolysis is used to generate ATP, this is much quicker than using the aerobic pathway

Also used when you need a lot of energy FAST, as in rapid, high-strength movements such as sprinting or lifting a heavy weight.

Lifting weights. Quickly lifting a heavy weight would likely call upon the use of the anaerobic pathway to quickly regenerate ATP.