How Cells Harvest Energy

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CHAPTER 7
LECTURE
SLIDES

Respiration
• Organisms can be classified based on
how they obtain energy:
• Autotrophs
– Able to produce their own organic molecules
through photosynthesis

• Heterotrophs
– Live on organic compounds produced by
other organisms

• All organisms use cellular respiration to
extract energy from organic molecules

Cellular respiration
• Digestion – enzymes breaking down large
macromolecules into smaller ones.
• Cellular respiration is a series of reactions
• Oxidations – loss of electrons
• Dehydrogenations – lost electrons are
accompanied by protons
– A hydrogen atom is lost (1 electron, 1 proton)

Redox
• During redox reactions, electrons carry
energy from one molecule to another
• Nicotinamide adenosine dinucleotide
(NAD+)
– Is an electron carrier
– NAD+ accepts 2 electrons and 1 proton to
become NADH
– Reaction is reversible

NAD+ to NADH
• How may electrons does NAD+ need to be
neutral?
– 1 electron

• How many electrons and protons in a
hydrogen atom?
– 1 proton
– 1 electron

• How many protons and electrons are needed
to make NAD+ to NADH?

NAD+ + 2H → NADH + H+

6

Cellular Respiration
• In overall cellular energy harvest
– Dozens or redox reactions take place
– Number of electron acceptors including NAD+

• In the end, high-energy electrons from
initial chemical bonds have lost much of
their energy
• Transferred to a final electron acceptor

Type of Metabolisms
• Generally speaking, the type of terminal
(final) electron acceptor determines the
type or metabolism.
• Aerobic respiration
– Final electron receptor is oxygen (O2)

• Anaerobic respiration
– Final electron acceptor is an inorganic
molecule (not O2)

• Fermentation
– Final electron acceptor is an organic molecule

Aerobic respiration
C6H12O6 + 6O2

6CO2 + 6H2O

∆G = -686kcal/mol of glucose
∆G can be even higher than this in a cell
• This large amount of energy must be
released in small steps rather than all at
once.

NADH
NAD+
+

ATP
2e –

Controlled
release of
energy for
synthesis
of ATP

H+

2e –
H+

H2O

1

2

O2

Electron carriers
• Many types of carriers used
– Soluble, membrane-bound, move within
membrane

• All carriers can be easily oxidized and
reduced
• Some carry just electrons, some electrons
and protons
• NAD+ acquires 2 electrons and a proton to
become NADH

Oxidation
Dehydrogenase

NAD+

+ 2H
2 H+ + 2 e–

Reduction

NADH + H+
(carries
2 electrons)

Metabolism Harvests Energy in Stages
• Combustible reactions and reactions in
biological systems are essentially the same
thing.
• Instead of one reaction where immense heat
is generated (like burning a log), biological
systems transfer electrons to intermediate
electron carriers thus coupling reactions.
• The electron transport chain is a series of
redox reactions produces potential energy in
the form of an electrochemical gradient.

ATP
• Cells use ATP to drive endergonic
reactions
– ΔG = -7.3 kcal/mol

• 2 mechanisms for synthesis
1. Substrate-level phosphorylation
• Transfer phosphate group directly to ADP
• During glycolysis

2. Oxidative phosphorylation
• ATP synthase uses energy from a proton gradient

Substrate-level phosphorylation – phosphate
groups are added to substrates in
enzyme catalyzed reactions.
Enzyme

Enzyme
P

ADP
+

P
Substrate

P
Product

ATP

Oxidation of Glucose
The complete oxidation of glucose proceeds
in stages:
1. Glycolysis
2. Pyruvate oxidation
3. Krebs cycle
4. Electron transport chain & chemiosmosis

Cellular respiration occurs in three main
stages
• Stage 1: Glycolysis
– Glycolysis begins respiration by breaking
glucose (6 carbons) into 2 molecules of
pyruvate (3 carbons)
– This stage occurs in the cytoplasm
 Stage 2: The citric acid cycle
– The citric acid cycle breaks down pyruvate into carbon
dioxide and supplies the...
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