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K. Muma
Bio 6
Cell Metabolism
Study Objectives:
1. Define oxidation and reduction.
2. Describe the mechanisms of ATP synthesis: substrate level phosphorylation vs.
oxidative phosphorylation
3. Write the overall general equation for cellular respiration.
4. Describe the role of dehydrogenases and coenzymes NAD and FAD in cellular
respiration.
5. Distinguish between anaerobic (lactic acid fermentation) and aerobic respiration in
terms of when they occur and the total number of ATP produced per glucose
molecule.
6. Describe the basic stages of aerobic respiration: glycolysis, transitional stage, Krebs
cycle and Electron Transport System. For each stage include:
What goes into each stage and what is produced in each stage?
Where does each stage occur in the cell?
Is it anaerobic or aerobic?
7. Define lipolysis and explain how glycerol and fatty acids are used for cellular
respiration.
8. Define beta-oxidation, ketogenesis, ketosis, and ketoacidosis
9. Describe protein metabolism. Explain the role of proteases, deamination, and organic
acids in utilizing proteins for cellular respiration.
10. Define glycogenolysis and gluconeogenesis.
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Cell Metabolism Outline
I.
The big picture:
a. The sun provides the energy that powers all life
b. Animals depend on plants to convert solar energy to chemical energy
c. This chemical energy is in the form of organic molecules which animals
then eat to convert into ATP through cellular respiration
II.
Cellular Respiration
a. Overview
i. Cell respiration is the main way that chemical energy is harvested
from organic molecules and converted to ATP
ii. It involves a series of catabolic reactions
iii. This is an aerobic process —it requires oxygen
iv. OVERALL EQUATION
b. Oxidation-Reduction Reactions (review these types of reactions on page
103 of your textbook).
i. Reactions transferring electrons from one molecule to another
ii. Molecules that lose electrons are said to be oxidized
iii. Molecules that gain electrons are said to be reduced
iv. Movement of electrons is usually associated with movement of
hydrogen atoms
v. Look at the overall equation above: which molecule is oxidized
and which is reduced during cellular respiration?
c. Enzymes Involved
i. Dehydrogenases - enzymes that catalyze redox reactions by
removing hydrogen
ii. Most require coenzymes that are able to accept and carry the
electrons (electron carriers)
1. Nicotinamide adenine dinucleotide (NAD+) – derived
from niacin
2. Flavin adenine dinucleotide (FAD) – derived from
riboflavin
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d. Three overall stages of cellular respiration
i. Glycolysis
ii. Krebs Cycle (a.k.a. Citric Acid Cycle)
iii. Electron Transport Chain
e. Mechanisms of ATP Synthesis
i. Substrate-level phosphorylation
1. Enzymes transfer a phosphate group from a substrate to
ADP
2. Occurs during glycolysis and Krebs cycle
ii. Oxidative phosphorylation
1. The phosphorylation of ADP is powered by a series of
redox reactions that transfer electrons from organic
molecules to oxygen
2. Produces the majority of the ATP molecules
3. In electron transport system
III.
Stage 1: GLYCOLYSIS
a. Overview
i. Occurs in the cytoplasm
ii. Does not require oxygen (anaerobic)
iii. Six carbon glucose molecule is broken down into 2 three carbon
molecules of pyruvic acid
iv. Produces 2 net ATP and 2 NADH (electron carrier)
*The image above is a simplified version of glycolysis showing what goes in and what
is produced. Each little gray ball represents a carbon. Also see figure 2-11 in your
textbook.
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b. The Fate of Pyruvic Acid (see figure 2-16)
i. Depends on the availability of oxygen
ii. In aerobic conditions pyruvic acid is converted to acetyl-CoA and
enters the Krebs cycle
iii. In anaerobic conditions NADH + H+ reduces pyruvic acid to form
lactic acid
c. Lactic Acid Fermentation
i. Produces 2 ATP per glucose (less efficient)
ii. When oxygen becomes available again lactic acid is oxidized back
to pyruvic acid and enters the Krebs cycle
IV.
Stage 1½ : TRANSITION
a. Pyruvic acid enters the mitochondrial matrix through facilitated diffusion
b. There it is converted to Acetyl-Coenzyme A to enter Krebs cycle
c. 1 CO2 and 1 NADH is produced in this stage per pyruvate
V.
Stage 2: KREBS CYCLE
a. Overview
i. Occurs in the matrix of the mitochondria
ii. Requires oxygen (aerobic)
iii. Completes the breakdown of glucose to CO2 and harvests the
energy as:
1. 2 ATP
2. 6 NADH
3. 2 FADH2
(Numbers based on per one glucose molecule)
b. Events (see figure 2-12):
i. Acetate joins the four carbon compound oxaloacetate to form the 6
carbon compound citrate
ii. 2 decarboxylation events release 2 CO 2
iii. Four oxidation events generate 3 NADH and 1 FADH2
iv. 1 molecule of ATP is formed via substrate-level phosphorylation
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VI.
Stage 3: Electron Transport Chain
a. The hydrogen being delivered to the ETC by the coenzymes are split into
electrons and H+ ions
b. Electrons from NADH and FADH2 are passed down a chain of protein
complexes embedded in the inner membrane of the mitochondria
c. Electrons fall to lower energy levels as they are passed down the chain
(releases energy)
d. Oxygen is the final electron acceptor
e. The negative oxygen binds to 2 H+ to form water
f. Chemiosmosis (see figure 2-13)
i. The energy released by electrons moving down the chain is used to
pump H+ from the matrix to the intermembrane space
ii. This creates a proton gradient (potential energy)
iii. This gradient drives protons back in through a protein called
ATPsynthase
iv. This creates kinetic energy that ATPsynthase harnesses to catalyze
ADP + P  ATP (oxidative-phosphorylation)
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VII.
Metabolic pool concept: any organic molecule can be used in respiration
a. Lipid Metabolism
i. Lipolysis – the hydrolysis of triglycerides into glycerol and fatty
acids
1. Catalyzed by the enzyme lipase
ii. The glycerol
1. is converted into glyceraldehyde phosphate a glycolysis
intermediate
2. then enters into Krebs cycle
3. complete oxidation of glycerol yields 18 ATP molecules
iii. The fatty acid chains
1. Are broken apart into 2 carbon acetic acid fragments (Betaoxidation)
2. Coenzyme A is attached to the acetic acid fragments
forming Acetyl CoA
3. Enters the Krebs cycle
4. Complete oxidation yields ~54 ATP
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iv. Ketogenesis
1. If Acetyl CoA production exceeds the capacity of the Krebs
cycle to process it, the liver will convert it to ketone bodies
which are released into the blood
2. Ketones can be used as an energy source in skeletal and
cardiac muscle
a. Examples of ketones: acetoacetic acid, Bhydroxybutyric acid, acetone
3. Ketosis - an increase in circulating ketone bodies
a. Occurs when lipids are the primary energy source
(starvation and diabetes mellitus)
b. May lead to ketoacidosis – decreased blood pH
c. Depresses nervous system, may become comatose
d. Compensatory response to ketosis: increased
ventilation and large amounts of ketones excreted in
urine
b. Protein Metabolism
i. Proteins are hydrolyzed into individual amino acids by proteases
ii. Amino acids are deaminated in the liver (amine group is removed)
iii. Amine group is removed as ammonia
iv. Combined with CO2 to form urea which is excreted by the kidneys
v. Generates organic acids which can be converted to glucose or enter
Krebs cycle to be oxidized for energy
c. Glucose Synthesis
i. Aerobic metabolism of glucose is the most efficient way for cells
to make ATP
ii. It is the primary source of energy in cells and normally the ONLY
source neurons prefer
iii. There is multiple metabolic pathways for producing glucose to
ensure that there is a continuous supply for the brain
iv. Synthesis of Glucose
1. Glycogenolysis – the breakdown of glycogen to glucose
a. The liver and skeletal muscle contains high
concentrations of glycogen
2. Gluconeogenesis - synthesis of glucose from noncarbohydrates
a. Can start with glycerol, lactic acid or various amino
acids
b. Occurs in the liver and kidneys
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Post-lecture Practice
Cell Respiration Worksheet
1. Overall equation for cellular respiration:
+
What you
eat.
+
What you
inhale.
What you
exhale.
+
What your
cells use
for energy!
2. Fill out the table below to organize the main ideas of cellular metabolism.
(Assume per glucose molecule for number of electron carriers and ATP):
Aerobic or
Anaerobic?
Location in
cell
Starting
Molecule
Ending
Molecules
# of Electron
Carriers
made
Number
of ATP
made
Glycolysis
Transition
stage
Krebs Cycle
(citric acid
cycle)
Electron
Transport
Chain
Fermentation
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