1. Cellular respiration
Biological Energy

3. The equation for Cell Respiration:
Chemical Pathways
Cells do not “burn” glucose but instead release energy from the molecules gradually.
Body Temperature?
The process begins with a pathway called glycolysis.
When oxygen is present, there are two further pathways that release LOTS of energy.
When oxygen is not present, glycolysis is followed by different pathways.
Cellular respiration is the process that releases energy by breaking down food molecules in the presence of oxygen.
1) Glycolysis
2) Kreb’s cycle
3) electron transport chain (ETC)
The equation for Cell Respiration:
6O2 + C6H12O6  6CO2 + 6H2O + Energy
Look familiar? What is the relationship between the equation for photosynthesis and the above equation?

5. Chemical Pathways GLYCOLYSIS
One molecule of glucose is broken in half to produce two molecules of pyruvic acid (a 3-carbon compound)
Occurs in the cytoplasm or cytosol of a cell
2 ATP molecules are used to get this pathway started
At the end of glycolysis, 4 ATP molecules are produced so a net gain of 2 ATP for the cell
One glycolysis reaction removes 4 high-energy electrons and passes them to a carrier called nicotinamide adenine diculeotide, NAD+.
NAD+ accepts electrons and becomes NADH and transfers energetic electrons to other molecules from glucose.
Process is so fast that thousands of ATP can be produced in a few milliseconds.
Does not require oxygen.

8. FYI
Phosphorylation of sugars is often the first stage of their catabolism. It allows cells to accumulate sugars because the phosphate group prevents the molecules from diffusing back across their transporter. Phosphorylation of glucose is a key reaction in sugar metabolism because many sugars are first converted to glucose before they are metabolized further.

10. Chemical Pathways Pyruvic acid + NADH  alcohol + CO2 + NAD+
FERMENTATION
Follows glycolysis when oxygen is not present.
Produces ATP.
Cells convert NADH to NAD+ by passing high-energy electrons back to pyruvic acid.
Is anaerobic.
ALCOHOLIC FERMENTATION
Yeasts and some other types of microorganisms form ethyl alcohol and CO2 as wastes.
Pyruvic acid + NADH  alcohol + CO2 + NAD+
LACTIC ACID FERMENTATION
Pyruvic acid is converted to lactic acid
Regenerates NAD+ so glycolysis can continue
Pyruvic acid + NADH  lactic acid + NAD+
BOTH OF THESE PROCESSES BEGIN WITH GLUCOSE

12. The Kreb’s Cycle and Electron Transport
These pathways require oxygen – aerobic.
O2 acts as an electron acceptor.
At the end of glycolysis, 90% of energy in glucose is still unused!
Where is it? – locked in high-energy electrons of pyruvic acid.
Kreb’s & ET extract this unused energy through a series of several steps involving enzymes.
KREBS CYCLE
The second stage of aerobic cell respiration
AKA citric acid cycle & occurs in mitochondrial matrix
Pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions
1) pyruvic acid from glycolysis enters mitochondrion
2) one carbon dioxide comes off of pyruvic acid and forms a 2-carbon compound acetyl- CoA (acetyl coenzyme A)
3) acetyl joins a 4-carbon molecule to produce a 6-carbon compound called citric acid
4) citric acid breaks down into a 4-carbon compound and releases 2 molecules of CO2 (the source of CO2 on your breath!)
5) the cycle begins again with the same 4-carbon compound
6) ADP  ATP ; NAD+  NADH ; FAD  FADH2

15. The Kreb’s Cycle and Electron Transport
The glycolysis pathway requires 10 different enzymes to take the 6- carbon glucose molecule to 2 3-carbon pyruvate molecules.
The Kreb’s Cycle (or citric acid cycle) requires 9 different enzymes to recycle 4-carbon molecule and extract high energy electrons.
So now we understand the importance of proteins in living systems.
PRODUCTS OF KREB’S CYCLE:
4 NADH molecules  to ETC
1 FADH2 molecule  to ETC
1 ATP molecule  to ETC
3 CO2 molecules  to lungs for exhalation
REACTANTS OF KREB’S CYCLE:
Pruvic acid
NAD+
Coenzyme A
ADP
FAD

16. The Kreb’s Cycle and Electron Transport
ELECTRON TRANSPORT CHAIN (ETC)
Occurs in mitochondrial inner membrane or cristae (inner membrane folds)
NADH & FADH2 from Kreb’s carry high-energy electrons to the ETC.
These electrons are used to convert ADP to ATP with ATP synthase.
1) ETC made of carrier proteins located in the inner membrane of mitochondrion.
2) At the end of the ETC, an enzyme combines electrons from the carriers with hydrogen ions and oxygen to form water.
3) Oxygen gets rid of low-energy electrons and hydrogen ions.
4) When 2 high-energy electrons transport down the ETC, that energy is used to transport H+ across the membrane.
5) H+ builds up inside the cell along the inner membrane giving a positive charge on the inside and a negative charge on the outside.
6) When H+ escapes through protein channel and ATP synthase, the protons (H+) rotate the enzyme and from this turbine movement, ADP + Pi  ATP
From Kreb’s and ETC, about 36 ATP molecules can be produced from 1 glucose molecule.
This represents ~38% of energy stored in a glucose molecule.
What happens to the other 62%?
Body Temperature?

19. The Kreb’s Cycle and Electron Transport
ENERGY AND EXERCISE
Quick Energy
During fight or flight or if you need a quick burst of energy during an athletic activity, muscle cells contain small amounts of ATP ready for a few seconds of intense activity.
After those few seconds, muscle cells begin producing ATP through lactic acid fermentation – ever been sore after intense work-outs?
This is because you have built up an “oxygen debt” that can be repaid by heavy breathing.
Long-Term Energy
For physical activity lasting longer than about 90 seconds, cellular respiration is the only way to generate a continuing supply of ATP.
Cell. Resp. generates more ATP than fermentation. This is why cross-country athletes are always told to “pace yourself”.
Glycogen is a carbohydrate and an energy storage polymer of glucose which is stored in muscle and liver. Glycogen stores can last ~ 15 – 20 minutes.
After that, the body breaks down other forms of energy such as lipids – hence, we lose weight when we limit lipid intake and exercise regularly.
Photosynthesis and Cellular Respiration are opposite, or reverse, chemical processes.