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Cell Metabolism
BIG PICTURE


The sun provides the
energy that powers all
life
Animals depend on
plants to convert solar
energy to chemical
energy
 This chemical energy
is in the form of
organic molecules
Cellular Respiration




The main way that chemical energy is
harvested from organic molecules and
converted to ATP
Series of catabolic reactions
This is an aerobic process —it requires oxygen
OVERALL EQUATION
Carbon
Glucose
Oxygen
Dioxide
Water
Energy
Oxidation-Reduction Reactions


Reactions transferring electrons from one
molecule to another
 Molecules that lose electrons are said to be
oxidized
 Molecules that gain electrons are said to be
reduced
Movement of electrons is usually associated with
movement of hydrogen atoms
Which molecule is oxidized and
which is reduced during cellular
respiration?
Glucose
Oxygen
Carbon
Dioxide
Water
Energy
Glucose loses electrons (and hydrogen)
Oxidation
Reduction
Oxygen gains electrons (and hydrogen]
Enzymes Involved

Dehydrogenases - enzymes that catalyze
redox reactions by removing hydrogen

Most require coenzymes that are able to
accept and carry the electrons (electron
carriers)


Nicotinamide adenine dinucleotide
(NAD+) – derived from niacin
Flavin adenine dinucleotide (FAD) –
derived from riboflavin
Electron Carriers
Electron Carriers
NAD+
FAD
Stages of cellular respiration
Mechanisms of ATP Synthesis

Substrate-level
phosphorylation


Enzymes transfer a
phosphate group
from a substrate to
ADP
Occurs during
glycolysis and
Krebs cycle
Mechanisms of ATP Synthesis

Oxidative phosphorylation
The phosphorylation of ADP is powered by
a series of redox reactions that transfer
electrons from organic molecules to
oxygen
 Produces the majority of the ATP
molecules
 In electron transport system

Stage 1: GLYCOLYSIS




Occurs in the cytoplasm
Does not require oxygen (anaerobic)
Six carbon glucose molecule is broken down into
2 three carbon molecules of pyruvic acid
Produces 2 net ATP and 2 NADH
(electron carrier)
INVESTMENT STAGE
P
A
Y
O
F
F
S
T
A
G
E
Stage 1: GLYCOLYSIS
The Fate of Pyruvic Acid

Depends on the availability of oxygen
In aerobic conditions pyruvic acid is
converted to acetyl-CoA and enters the
Krebs cycle
 In anaerobic conditions NADH + H+ reduces
pyruvic acid to form lactic acid

Lactic Acid Fermentation


Produces 2 ATP per glucose (less efficient)
When oxygen becomes available again lactic acid
is oxidized back to pyruvic acid and enters the
Krebs cycle.
Stage 1½ : TRANSITION



Pyruvic acid enters the mitochondrial matrix through
facilitated diffusion
There it is converted to Acetyl-Coenzyme A to enter
Krebs cycle
1 CO2 and 1 NADH is produced in this stage per
pyruvate
Stage 2: KREBS CYCLE



Occurs in the matrix of the mitochondria
Requires oxygen (aerobic)
Completes the breakdown of glucose to CO2 and
harvests the energy as:
 2 ATP
Citric
acid
 6 NADH
 2 FADH2
4 CO2
(per 1 glucose)
Two of these cycles
per 1 glucose molecule
Oxaloacetic
acid
Stage 2: KREBS CYCLE


Acetate joins the 4
carbon compound
oxaloacetate to form
the 6 carbon
compound citrate
2 decarboxylation
events release 2 CO 2
Stage 2: KREBS CYCLE



Four oxidation events
generate 3 NADH and 1
FADH2
1 molecule of ATP is
formed via substrate-level
phosphorylation
Note these numbers are
per cycle. For each
glucose molecule you
have two cycles.
Stage 3: Electron Transport Chain


The hydrogen being delivered to the ETC by the
coenzymes are split into electrons and H+ ions
Electrons from NADH and FADH2 are passed
down a chain of protein complexes embedded in
the inner membrane of the mitochondria
Stage 3: Electron Transport Chain



Electrons fall to lower
energy levels as they are
passed down the chain
(releases energy)
Oxygen is the final
electron acceptor
The negative oxygen
binds to 2 H+ to form
water
Stage 3: Electron Transport Chain

Chemiosmosis


The energy released by electrons moving down the
chain is used to pump H+ from the matrix to the
intermembrane space
This creates a proton gradient (potential energy)
Stage 3: Electron Transport Chain


This gradient drives protons
back in through a protein
called ATPsynthase
This creates kinetic energy
that ATPsynthase
harnesses to catalyze
ADP + P  ATP
(oxidative-phosphorylation)
Summary of ETC
Metabolic pool concept: any organic
molecule can be used in respiration
Lipid Metabolism

Lipolysis – the hydrolysis of triglycerides
into glycerol and fatty acids

Catalyzed by the enzyme lipase
Lipid Metabolism

The glycerol



is converted into
glyceraldehyde phosphate a
glycolysis intermediate
Which then enters into Krebs
cycle
Complete oxidation of glycerol
yields 18 ATP molecules
Lipid Metabolism

The fatty acid chains



Are broken apart into 2
carbon acetic acid
fragments (Beta-oxidation)
Coenzyme A is attached to
the acetic acid fragments
forming Acetyl CoA
Enters the Krebs cycle

Complete oxidation yields
~54 ATP
Ketogenesis

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


Ketones can be used as an energy source in
skeletal and cardiac muscle
Examples of ketones: acetoacetic acid, Bhydroxybutyric acid, acetone
Ketosis


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Ketosis - an increase in circulating ketone
bodies
Occurs when lipids are the primary energy
source (starvation and diabetes mellitus)
May lead to ketoacidosis – decreased blood pH


Depresses nervous system, may become
comatose
Compensatory response: Increased ventilation
and large amounts of ketones excreted in urine
Protein Metabolism


Proteins are hydrolyzed
into individual amino
acids by proteases
Amino acids are
deaminated in the liver
(amine group is removed)


Amine group is removed as
ammonia
Combined with CO2 to form
urea which is excreted by
the kidneys
Protein Metabolism: Deamination

Also generates organic acids which can be
converted to glucose or enter Krebs cycle
to be oxidized for energy
Protein Metabolism
Glucose Synthesis
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
Aerobic metabolism of glucose is the most
efficient way for cells to make ATP
It is the primary source of energy in cells
and normally the ONLY source in neurons
There is multiple metabolic pathways for
producing glucose to ensure that there is
a continuous supply for the brain
Synthesis of Glucose

Glycogenolysis – the breakdown of
glycogen to glucose

The liver and skeletal muscle contains high
concentrations of glycogen
Synthesis of Glucose



Gluconeogenesis synthesis of glucose
from non-carbohydrates
Can start with glycerol,
lactic acid or various
amino acids
Occurs in the liver and
kidneys