Cellular Respiration:
Or, How one good meal provides energy for the work of 75 trillion cells
February 16-18, 2004


Readings: Ch 7 109-122* (*The text goes into WAAY more detail than you need to know, so use the text as a supplement only - study the figures, use the CD, but DON'T feel like you need to understand all the text!)
Warm Up 5
is due Weds Feb 18 at 9:30, and Good For 5 is due Fri Feb 20 at 11:30. See top of notes page for info on Good For Amnesty!


Every living thing is a sort of imperialist, seeking to transform as much as possible of its environment into itself... -- Bertrand Russell

I. Cellular Respiration: breaking down sugar in the presence of oxygen (aerobic).

Photosynthesis (you recall...) is the process by which CO2 and H2O are used to make sugars and starches.

During Cellular Respiration, sugar is broken down to CO2 and H2O, and in the process, ATP is made that can then be used for cellular work.

The overall reaction for cellular respiration: (does this reaction look familiar? Overall, it is the reverse reaction of photosynthesis, but chemically, the steps involved are very different.)

C6H12O6 + 6O2 -------------------> 6CO2 + 6H2O + ~38 ATP

 


Whereas only photosynthetic cells can make sugar using photosynthesis, ALL cells need to be able to break down sugars they take in from their environment and turn it into energy to be used in cellular work....


II. Cellular respiration can be broken down into 4 stages:

Essentially, sugar (C6H12O6) is burned, or oxidized, down to CO2 and H2O, releasing energy (ATP) in the process. Why do cells need ATP? ALL cellular work -all the activities of life - requires energy, either from ATP or from related molecules. A lot of oxygen is required for this process! The sugar AND the oxygen are delivered to your cells via your bloodstream.

This process occurs partially in the cytoplasm, and partially in the mitochondria. The mitochondria is another organelle in eukaryotic cells. like the chloroplast, the mitochondria has two lipid bilayers around it, and its own genome (indicating that it may be the result of endosymbiosis long ago). In some ways similar to the chloroplast, the mitochondria also has two main sites for the reactions: The matrix, a liquidy part of the mitochondrion, and the christae, the folded membranes in the mitochondrion.

1: Glycolysis ("splitting of sugar"): This step happens in the cytoplasm.

One Glucose (C6H12O6) is broken down to 2 molecules of pyruvic acid. Results in the production of 2 ATPs for every glucose. (page 113 of your book goes into painful detail about this process, but all YOU need to know is that glucose is split into to 2 molecules of pyruvate!)

2: Transition Reaction: Pyruvic Acid is shuttled into the mitochondria, where it is converyed to a molecule called Acetyl CoA for further breakdown.

3: The Krebs Cycle, or Citric Acid Cycle: Occurs in the mitochondrial matrix, the liquid-y part of the mitochondria.

In the presence of Oxygen gas (O2), all the hydrogens (H2) are stripped off the Acetyl CoA, two by two, to extract the electrons for making ATP, until there are no hydrogens left - and all that is left of the sugar is CO2 - a waste product - and H2O (exhale). The Krebs cycle results in the production of only ~4 ATPs, but produces a lot of NADH, which will go on to the next step... Hans Krebs won the Nobel Prize in 1953 for his discovery of the Citric Acid Cycle.

4: The Electron Transport Chain and Chemiosmosis ("the big ATP payoff"). Occurs in the christae of the mirochondria, the folded membranes inside the chloroplast.

Electrons from Hydrogen are carried by NADH and passed down an electron transport chain to result in the production of ATP. Results in the production of ~32 ATPs for every glucose. Peter Mitchell won the Nobel Prize in 1978 for his work on energy production in mitochondria, called the Chemiosmotic Theory.

Reality Check:
Question: So - all of these steps to do...what?
Answer: BREAK DOWN GLUCOSE TO MAKE ATP ~38 ATPs for every glucose!!!
What's so great about ATP?
Every time you move a muscle, think, breathe, replicate your DNA, every time your heart beats - you use ATP to do this work!


III. Alternative Energy in the Body

What if the body runs out of sugar for glycolysis? Can the body still make ATP?

Answer: Yes, this is what the Lo-carb Atkins and South Beach diets are based on...draining the carbohydrate levels and reserves of your body, so that to make daily energy (ATP), you body has to turn to an alternate source of fuel...fats and proteins! See figure 7.12 in your book - a great summary!

Fats as fuel: Triacylglycerol reserves (located in unpleasantly strategic locations on your body ...) tend to be stored in adipose cells. Lipases are released into the bloodstream and break down fats in the bloodstream (from the fats and oils we eat) or travel to adipose cells.

Proteins as fuel: Excess proteins in our diets cannot be stored like glycogen or fats can, and must be broken down by the body.


IV. Anaerobic Respiration (Fermentation):

If enough oxygen is present is cells, each glucose molecule will produce ~38 ATPs - lots of energy, for lots of cellular work. However, what happens if you (or another organism) run short of oxygen? (Like if you are a sprinter, or being chased by a LION, or in an aerobics class but WAY above your target heart rate?) How does your body get energy to do its cellular work when oxygen is limited?

In suboptimal O2 concentrations, a partial breakdown of sugar occurs that results in just a bit of energy, enough to keep a cell alive or working for a while, but not as efficiently. This process is called fermentation, or anaerobic respiration. Usually the process goes only as far as glycolysis (2 ATPs), and does not enter the mitochondria for further breakdown. However, these 2 ATPs give your body enough energy to cross the finish line...and hopefully avoid being eaten by that lion...

In our cells, anaerobic respiration results in the production of lactic acid, the molecule that builds up when you 'feel the burn' during or after strenuous exercise. The lactic acid, plus the hydrogens (2H) released, cause the muscle to become more acidic, causing pain and burning in the muscle. Ow.

Anaerobic Respiration we force on other living creatures to meet our own desires...:

In yeast cells, fermentation results in the production of 2 ATPs and a waste product, ethyl alcohol. Humans have figured out that if they put yeast cells under sub-optimal oxygen concentrations with grape or malt sugar, lots of tasty fermented alcoholic bevereges like beer, wine, and champagne can be made (and consumed responsibly...!) [Image]

Question: Why do beer and wine have an alcohol concentration of ~8%? Why doesn't the alcohol concentration get any higher than this? Talk about it for a few minutes....


V. A comparison between Photosynthesis and Respiration:

Handout in class.

VI. A final note:

We tend to associate the term respiration with breathing and the lungs, but today we also talked about cellular respiration. What's the connection? In short, the function of the lungs is to pull in the oxygen our mitochondria need to burn sugar and remove the CO2 our mitochondria generate after breaking that sugar down!! We can't burn sugar well without oxygen, and we can't get oxygen to all our cells without our lungs, our blood cells (which carry oxygen) and our heart!


Objectives:

1. Be able to write the overall equation for Cellular respiration.

2. Be able to list

3. Be able to describe what happens if the body runs out of carbohydrate reserves to use as fuel

4. Be able to describe what anaerobic respiration is and what types of products result in