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Chapter 6: How Cells Harvest Energy: Cellular Respiration

Learning Objectives: describe the general function of cellular respiration, list the major events of glycolysis, transition, and Kreb’s cycle, distinguish between oxidative phosphorylation and substrate-level phosphorylation, discuss the structure and function of the electron transport chain, calculate the yield of ATP molecules per glucose molecule for aerobic respiration and fermentation, discuss the concept of a metabolic pool and how the breakdown of carbohydrate, proteins, and fats contributes to the pool.

I. Intro/Review: Organisms can get E by ________________ of food molecules. Food E is in the form of ____________________. A pair of high E electrons can be stored in a covalent bond. (Remember — 1covalent bond = 1 pair electrons). _______________________ is the process by which high E electrons are removed from food molecules & their energy transferred into molecules of ATP. This involves a series of ________________________ reactions. When high E electrons are removed from food molecules, this is oxidation. The electrons drop to lower E levels & their energy is used to make ATP (involves a series of redox reactions). NAD+ (nicotinamide adenine dinucleotide) is a coenzyme of oxidation-reduction, it is oxidized when it gives up electrons and reduced when it accepts electrons. Each NAD+ molecule used over and over again. FAD (flavin adenine dinucleotide) is also a coenzyme of oxidation-reduction that is sometimes used instead of NAD+. It accepts two electrons and two hydrogen ions (H+) to become FADH2.

a. When the electrons are depleted of E, they are accepted by a __________________ (reduction). This acceptor may be 1 of 3 things . . .

i. _____________________ – oxygen (the most common form of cellular respiration)

ii. _______________________ inorganic molecule other than oxygen (sulfate, nitrate)

iii. _______________________ – organic molecule

b. Aerobic Respiration

Summary reaction:

C6H12O6 + 6O2 6CO2 + 6H2O (+36 – 38ATP)

(large amt. of E) (small units of E)

i. Aerobic respiration is a lengthy metabolic pathway in which ATP is made by 2 processes . . .

a. ___________________ – direct transfer of a phosphate group from a substrate to ADP, forming ATP

ADP + P (from substrate) ATP

b. ________________________ – E from the drop of high E electrons (through a series of redox reactions) is used to phosphorylate ADP. The process by which oxidation occurs is called _________________.


d. Glycolysis

i. Begins with one ______________ & ends with two __________________

ii. Occurs in cytoplasm

iii. Requires no O2

iv. 10 steps, 10 enzymes

v. Is the 1st step in all 3 forms of cellular respiration

vi. Glycolysis means “sugar-splitting.”

vii. Part I: Glucose (6C) is broken down into 2 PGAL’s (Phosphoglyceraldehyde – 3 Carbon molecules); uses 2 ATP as Eact phosphorylation @ 2 steps, makes molecules more reactive.

viii. Part II: 2 PGAL’s (3C) are converted to 2 pyruvates. The net ATP production of glycolysis is ______ ATP’s. @ 1 step, each of 2 3C molecules is oxidized, losing a pair of high E electrons along with 2H.+ These are accepted by the coenzyme NAD+, which is reduced to NADH(H+). NADH carries the electrons to the ETC.

2NAD+ 2NADH + 2H+

Step 3 is a point of allosteric control. Excess ATP binds to & inactivates the enzyme — no more glycolysis until the excess is used up.

Summary of the yield:

4 ATPmade- 2ATPused = ____ ATPnet

_____ NADH produced (to ETC)

_____ pyruvate remain (still containing lots of high E e’s)

e. Prep Steps

i. Links glycolysis to the Kreb’s Cycle

ii. Occurs in the matrix

iii. Each of the 2 pyruvates enter the mitochondrion & travel to the matrix.

iv. Enzyme removes the carboxyl group (____________________) which is lost as CO2

v. A pair of high E e’s along with 2H+ is removed by oxidation (X 2 pyruvates) NAD+

NADH + H+ ( ETC)

vi. Result is 2C acetyl group (X2 pyruvates). It is picked up by coenzyme A (forming ____________& transported to the Kreb’s Cycle.

f. Kreb’s Cycle aka Citric Acid Cycle

i. Called a cycle since it regenerates its starting compound

ii. Occurs in the matrix

iii. 8 steps, 8 enzymes

iv. Begins when a 2C acetyl group is transferred from acetyl CoA to 4C oxaloacetate to form 6C citric acid *CoA returns to prep steps for another acetyl group 6C citric acid continues through the cycle. 2 CO2s are lost/turn of cycle. 1 ATP is made/turn by substrate level phosphorylation. During cycle, 3 pairs of high E electrons are transferred to NAD+. 3 NADH carry them to the ETC. 1 pr. of slightly lower E electrons are accepted by FADH. FADH2 carries them to the ETC.

Summary of Prep/Kreb’s (Matrix Events)

4 NADHs produced

1 FADH2 produced

1 ATP made

3 CO2 lost as waste

[X 2 pyruvates]

g. Electron Transport Chain

i. ATP made by oxidative phosphorylation via chain of e carriers embedded in the inner membrane of mitochondrion. (Each mitochondrion has many ETCs) — occurs across _________ _____________ in prokaryotes

1. Pairs of high E electrons are delivered by NADH & FADH2 (regenerating NAD+ and FAD).

2. Pairs of electrons travel down the chain of carriers, dropping E as they go. When they reach the end of the chain the electrons are depleted of E. They are accepted by oxygen along with 2 H+ producing ___________. The E made available as the electrons travel down the chain is used to power chemiosmosis. It is used to actively transport (pump) pairs of H+ from lower concentration in the matrix to higher concentration in the intermembrane space. This creates a concentration gradient of H+ across the membrane along with an electrical gradient (electrochemical gradient). The pairs of H+ are allowed to return to the matrix via carrier proteins associated with the enzyme ATP synthetase. For each pair of H+ that travels through, the enzyme is empowered to phosphorylate 1 ADP 1ATP. The electrons delivered by NADH has enough potential E to pump 3 pairs of H+ – so that 3 ATP are made for each NADH. The electrons delivered by FADH2 have only enough potential E to pump 2 pairs of electrons, so 2 ATP are made.

Summary of ATP Yield

Glycolysis: 2 net ATP by substrate level phosphorylation + 2 NADH( 6 ATP); total of 8 ATP

Prep Steps (X2): 2 NADH 6 ATP

Kreb’s (X2): 2 ATP by substrate level phosphorylation + 6 NADH (18 ATP) + 2 FADH2 ( 4 ATP); total of 24 ATP

8 + 6 + 24 = 38 ATP (in prokaryotes)

In eukaryotes, there is a 2 ATP “shuttle fee” top transport the NADH (from glycolysis) from the cytoplasm into the matrix.

h. Energy yield from glucose metabolism

· Net yield per glucose

· From glycolysis – ____ ATP

· From citric acid cycle – _____ ATP

· From electron transport chain – ______ or ______ ATP

· Energy content

· Reactant (glucose) 686 kcal

· Energy yield (36 ATP) 263 kcal

· Efficiency is ____%

· Rest of energy from glucose is lost as ___________

II. ATP Production in the Absence of Oxygen

a. Anaerobic respiration

i. Occurs in a few ___________

ii. Similar to aerobic respiration except that the final electron acceptor is an inorganic molecule other than oxygen (sulfate, nitrate).

iii. Produces about the same amount of ATP as aerobic respiration, but the rate is a bit slower. (Oxygen “pulls” electrons more efficiently.)

b. Fermentation

i. In some organisms that ordinarily respire aerobically, fermentation provides a backup pathway that can make some ATP in the temporary absence of oxygen. In the absence of oxygen the ETC shuts down, so that no ATP can be made by oxidative phosphorylation. There is a limited amt. of NAD+ in a cell. When the ETC backs up due to no oxygen, NADH cannot drop off its electrons, so NAD+ is not regenerated. Without NAD+ even glycolysis cannot continue, since there is a step involving NAD+ before ATP production occurs. The purpose of fermentation is to regenerate NAD+ so that glycolysis can continue & some _________ can be made. All of the ATP made via fermentation is from glycolysis – only 2 ATPs /glucose molecule. Fermentation pathways provide an organic molecule to accept electrons from NADH. There are 2 fermentation pathways:

1. Lactic acid fermentation occurs in some bacteria & in skeletal ________________. NADH dumps electrons on pyruvate, yielding lactic acid. Creates “muscle burn.” Eventually recycled in the liver.

2. Alcohol fermentation: occurs in ___________. Pyruvate is converted to 2C acetaldehyde. Produces CO2 needed for bread to rise. NADH dumps electrons onto acetaldehyde yielding 2C ethanol. Important in brewing industry. Too high an ethanol concentration kills yeast.

III. Metabolic Pool Concept – Glucose is not the only food molecule. Cells also get energy from foods other than sugars. Proteins are first broken down into their individual _________ __________. Deamination reactions convert the amino acids into molecules that can take part in the Krebs cycle. Fats are first broken down into ___________ _________. A process called -oxidation then converts the fatty acid tails into acetyl groups that can be combined with coenzyme A to form acetyl-CoA, which feeds into the Krebs cycle.


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