RESPIRATION

I. Virtually all energy on earth comes from sunlight 

A. Plants use energy from the sun to make the bonds which hold organic molecules together

B. When these bonds are broken during respiration the energy is ultimately transferred to ATP, which is then moved about cells and organisms to power their needs

II. Respiration =  the means by which the energy of the sun, captured during photosynthesis, is transferred to ATP and made available for the energy requirements of the cell

A. Recall that energy rich carbohydrate molecules are generally stored as sucrose or starch in plants or glycogen in animals

B. Respiration itself actually begins with glucose molecules

C. Therefore these carbohydrate molecules must be first broken down so the glucose is available for respiration

D. Glucose molecules can then be used as a source of energy under aerobic (with oxygen) or anaerobic (without oxygen) conditions

E. The overall reaction for the complete aerobic oxidation of glucose is:

C₆H₁₂0₆ + 6O₂  -----> 6CO₂ + 6H₂O + energy (ATP)

Note that respiration is the complement of photosynthesis

F. Respiration has four distinct stages:

1. Glycolysis

2. Krebs cycle

3. Electron transport chain

4. Oxidative phosphorylation

H. Through this series of chemical reactions the stored energy in glucose molecules is extracted and transferred to ATP

III. Glycolysis

A. Literally translated glycolysis means splitting of glucose

B. Glycolysis is an anaerobic process that takes place in the ground substance of the cytoplasm (cytosol)

C. The 6-carbon glucose is broken down into two 3-carbon molecules called pyruvate

D. It occurs in a series of 10 steps, each of which is catalyzed by a specific enzyme

E. Note that it requires an input of energy at steps 1 and 3 and that energy is gained at steps 6 (NADH), 7 (ATP) and 10 (ATP)

F. The net yield from each glucose molecule is 2 NADH, 2ATP and 2 molecules of pyruvate

G. The potential energy in a glucose molecule is 686 kcal

1. Two pyruvate molecules have a total energy content of about 546 kcal

2. Therefore most of the potential energy from the glucose molecule remains in the pyruvate bonds

H. Under aerobic conditions the pyruvate formed from glycolysis goes through three additional stages in the mitochondria where it is completely oxidized into CO₂ yielding much more energy

II. Krebs cycle

A. During the Krebs cycle some of the energy stored in the 2 pyruvate molecules (obtained during glycolysis)  is transferred to ATP but most ends up in energy carrying molecules called NADH and FADH₂

B. Recall the structure of mitochondria

1. There are two membranes, the inner, called cristae, is folded and convoluted to increase its surface area

C. The Krebs cycle takes place within the interior of mitochondria, which consists of a dense solution of enzymes, coenzymes, water, phosphates and other molecules involved in respiration

D. The two pyruvates produced via glycolysis enter the mitochondria and one carbon is stripped from each, resulting in the carbon dioxide you exhale

E. The remaining portion of the pyruvates are combined with an enzyme helper called coenzyme A (CoA) to form acetyl-CoA

F. Acetyl-CoA combines with oxaloacetate and this compound goes through a series of chemical reactions to form a small amount of ATP (2 molecules) plus other energy carrying molecules called NADH and FADH₂

G. Recall that cells need energy in the form of ATP

H. Therefore the NADH and FADH₂ from the Krebs cycle will enter the electron transport chain so their energy can be transferred to ATP

III. Electron transport chain

A. Electron transport chain = the process by which the high energy electrons carried by the NADH and FADH₂ (from the Krebs cycle and glycolysis) pass along a series of electron carrier molecules "downhill" to oxygen, ultimately forming water molecules

1. Cytochrome is one of these electron carriers

B. Recall that mitochondria have two membranes and the inner one, the cristae, is highly folded and convoluted

C. The electron transport chain takes place on the surfaces of the cristae

D. As the electrons pass down the electron transport chain a proton gradient is generated across the membrane

IV. Oxidative phosphorylation

A. During oxidative phosphorylation the proton gradient generated by the electron transport chain drives the formation of ATP from ADP

B. Due to the high concentration of protons on one side of the membrane the protons are pumped to the inner mitochondrial matrix through ATP synthase complexes

1. The energy from this process is used to convert ADP to ATP

C. The net result is that each NADH from the Krebs cycle yields 3 ATP and each FADH₂ yields 2 ATP

1. 1 NADH = 3 ATP

2. 1 FADH2 = 2 ATP

D. The electrons are ultimately transferred to oxygen which combines with hydrogen, producing the water you exhale

V. Energy yield from aerobic respiration

A. The net yield from one glucose molecule is 36 ATP which contains 263 kcal

B. Glucose has a potential of 686 kcal

C. Therefore aerobic respiration yields 38% of the potential energy from a glucose molecule

D. Despite the second law of thermodynamics this is a relatively high yield and the process is much more efficient than something like an automobile engine

E. The Krebs cycle is involved in both catabolic and anabolic processes

1. Complex carbohydrates, fats and are broken down in the digestive system and their components enter the respiration process at various points

VI. Anaerobic respiration

A. In most organisms pyruvate usually follows the aerobic pathway and it is completely oxidized to carbon dioxide and water

B. However in the absence of oxygen certain kinds of cells and organisms can still survive on the small amounts of energy produced by glycolysis alone

C. To do this they must constantly recycle the NAD+ needed at in glycolysis

D. Without this recycling (reoxidation), glycolysis would soon stop because the cell would run out of NAD+ as an electron acceptor

E. Anaerobic respiration (fermentation) = process by which cells recycle (reoxidize) the NAD+ needed for glycolysis in the absence of oxygen

F. In animals, many bacteria and fungi this anaerobic process results in the formation of the waste product lactate and the process is called lactate fermentation

1. If you do anaerobic exercise you work faster than you can take in oxygen. You switch to anaerobic respiration, lactate builds up in muscles, lowers the blood pH (causes burning) and reduces the capacity of the muscle fibers to contract

G. In yeasts and certain bacteria pyruvate is broken down to carbon dioxide and ethanol (grain alcohol) under anaerobic conditions

1. This is called alcoholic fermentation

2. Since alcoholic fermentation only involves glycolysis it produces very little ATP

3. Most of the energy (93%) , from the original glucose ends up in the ethanol. That is why alcohol has a lot of calories

H. Anaerobic respiration yields only 2 ATP's from glycolysis because there is no Krebs cycle nor electron transport chain. Therefore, it is a very inefficient system

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