Cellular Respiration: Releasing
Chemical Energy
Reactants:
glucose
oxygen
Products:
carbon dioxide
water
(ATP)
Energy is taken out of glucose and this energy transferred to
ATP. The carbon dioxide is what is left after the energy is
removed from the sugar. Oxygen is reduced, and as it gains
electrons (and hydrogens), it forms water.
Cellular respiration is an exergonic metabolic pathway
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External vs. Cellular Respiration
• External respiration is breathing.
• Cellular respiration is the breaking
down of food molecules to release
energy (ATP). During cellular
respiration, glucose is gradually
oxidized and breaks down into
carbon dioxide.
• External respiration is related to
cellular respiration, but they are
not the SAME thing.
• Cellular respiration occurs within
the cytoplasm and mitochondria of
cells.
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ALL living things need ENERGY!
goal of cellular respiration is to make ATP!
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Cellular respiration is biochemical pathway
catalyzed by enzymes that provides the
energy cells need to function.
Cellular Respiration Occurs in Three Stages:
1. Glycolysis
2. Krebs Cycle
3. Electron Transport Chain
(ETC)
Remember: goal of cellular respiration is to make ATP!
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Glycolysis
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Glycolysis- glyco = sugar / lysis = break
The process that begins the breakdown of
sugar. It converts glucose into 2 pyruvic acids
(also called pyruvate).
6 carbons
3 carbons
Glycolysis takes place in the cytoplasm of cells.
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LOOK AT ALL THOSE STEPS!
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Glycolysis Energy Investment
• One glucose is broken into two G3Ps
using ATP for the activation energy
needed to start the reaction.
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Energy Payoff Phase
• Electrons and Phosphates (ENERGY) are
stripped from the G3P molecules producing
ATP as well as electron carriers, NADH.
Additionally, The pyruvates still contain energy.
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Steps of Glycolysis
1. Energy Investment:
– 2 ATP are used to add phosphates to glucose
energizing the molecule causing it to split into a
three carbon sugar (G3P). (Note: some ATP
needed to start the process - Activation Energy)
2. Energy Pay-off
– Electrons (and hydrogens) are removed by an
electron carrier (NAD+) which becomes reduced
(reduction is gain) NAD+ → NADH and the
phosphates later get removed forming ATP
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Summary on Glycolysis
• Takes place in the cytoplasm
• Glucose is split into pyruvates(2)
• produce some ATP (2)
• NAD+ is reduced to form NADH (2)
Remember: goal of cellular respiration is to make ATP!
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When oxygen is present…
• Glycolysis does not require
oxygen, but oxygen is needed
for the next step of to take
place. Aerobic respiration
requires oxygen.
• The pyruvate diffuses into the
matrix of the mitochondria.
Not this Matrix...
This Matrix...
• The matrix is the internal fluid of
the mitochondria.
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ALL
Eukaryote
Cells
have
Mitochondria
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Mitochondria are everywhere!!
animal cells
plant cells
Me too!
Me too!
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So, lets continue the journey.....
• Next, in the matrix, for EACH molecule of pyruvate, a
CO2 is removed (pieces of broken down sugar)
• NADH is formed (that’s two more “energy” carriers)
• A coenzyme attaches forming Acetyl CoA (still has
some energy left in it)
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The Krebs cycle is a biochemical pathway
that further breaks down what is left of sugar.
• It strips energy out of acetyl coA and puts it into ATP
and electron carriers, as it does, carbon dioxide is
released.
• The Krebs cycle was discovered by Hans Kreb
• It is also called the citric acid cycle because citric acid
is the first intermediate formed in the cycle.
• must be done twice (once for each Acetyl CoA)
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Krebs Cycle
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Acetyl CoA enters the Krebs Cycle
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A more complicated version....
Don’t memorize this!
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Steps of the Krebs Cycle
there are actually more than 4!
1. Acetyl Co A joins with an oxaloacetate molecule to form citric
acid.
2. NAD+ removes electrons (hydrogen) from citrate forming
3.
4.
NADH and an intermediate molecule, ketoglutarate. (citric
acid is oxidized and NAD+ is reduced)
NAD+ removes electrons (hydrogen) from ketoglutarate
forming NADH and an intermediate molecule, succinate. This
step also removes phosphates producing ATP.
Electrons (Hydrogen) removed by electron carrier FAD orm
FADH2 and the molecule becomes malate.
5. Malate loses electrons reducing NAD+ into to NADH and
reform oxaloacetate acid, which can be used to join with
another acetyl CoA.
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Krebs Cycle
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The main points of the Krebs cycle
• Throughout the entire process of the Krebs cycle
– Sugar is completely broken down (oxidized)
– As sugar is broken down, carbon dioxide is
released (6)
– Some ATP is produced (2 molecules)
– Electrons (Energy) are transferred to NADH (8)
and FADH2 (2) which are used by the an electron
transport chain and ATP synthase to produce more
ATP.
• Each NADH is worth 3 ATP
• Each FADH2 is worth 2 ATP
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Energy Summary So Far...
• Glycolysis
– 2 ATP
– 2 NADH (worth 6 ATP)
• Krebs Cycle (including pyruvate->Acetyl CoA)
– 2 ATP
– 8 NADH ( worth 24 ATP)
– 2 FADH2 (worth 4 ATP)
Remember: goal of cellular respiration is to make ATP!
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Electron Transport Chain
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Electron Transport Chain
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Electron transport Chain
1. Electrons from Krebs Cycle are carried by NADH and
FADH2 to the mitochondrial membrane and a series of
proteins call the the electron transport chain. (ETC)
2. Electrons move down the chain of membrane proteins,
from high to low energy (redox reactions). Energy is
harnessed and used to pump protons (hydrogen ions) from
the matrix into the inter-membrane space of the
mitochondria. (This is active transport)
3. Oxygen is the “final electron acceptor”, which then bonds
with hydrogen ions forming water.
4. The protons act as a battery flowing through ATP synthase
which produces ATP through the process of
chemiosmosis.
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ATP Production
• 38 ATP are produced during cellular respiration
per glucose molecule.
2 are produced during glycolysis
2 are produced during the Krebs Cycle
34 are produced by the electron transport chain
Remember: goal of cellular respiration is to make ATP!
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Overview of Cellular Respiration
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Overview of Cellular Respiration
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But what if oxygen is not present?
•
•
•
When oxygen is not present anaerobic
respiration follows glycolysis.
Pyruvate is still broken down by glycolysis in
the cytoplasm without the use of oxygen.
This produces 2 ATP per glucose.
Glycolysis is then followed by fermentation.
– There are two types of fermentation
• Lactic acid fermentation
• Alcoholic fermentation
What if you
don’t have
mitochondria?
Remember: goal is still to make ATP!
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Lactic acid fermentation
• In lactic acid fermentation pyruvic acid (pyruvate) is
converted to lactic acid.
• When you exercise, often times lactic acid builds up
due to limited oxygen. (“feel the burn”)
• The point is to regenerate NAD+ to repeat glycolysis.
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Alcoholic fermentation
• In alcoholic fermentation pyruvic acid is converted to
ethyl alcohol. Carbon dioxide is also made.
• This process is used to produce products like breads,
beer and wine.
• The point is to regenerate NAD+ to repeat glycolysis.
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A few last notes on respiration...
• Amino acids (protein) and lipids
can be broken down during
cellular respiration. As they are
broken down by digestion/
hydrolysis, they are modified
into molecules that are part of
the Krebs cycle.
• Cellular respiration also
releases heat energy that can
help organisms maintain
temperature homeostasis.
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