Cellular respiration is the process by which organisms convert the chemical energy from nutrients into ATP. It occurs in three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis breaks down glucose into pyruvate and occurs in the cytoplasm, producing a small amount of ATP. The Krebs cycle further breaks down pyruvate in the mitochondria, producing more ATP and electron carriers. In the electron transport chain, electrons are passed through protein complexes in the mitochondrial membrane, pumping protons and producing the most ATP through chemiosmosis. Oxygen is the final electron acceptor, with carbon dioxide and water as end products.

1. Cellular Respiration
(Electron Transport Chains)

2. Introduction
The energy from the sun trapped by plants is obtained
by other organisms such as animals.
The plants and animals carry out the chemical energy of
food molecules that is released and partially captured in
the form of ATP (Adenosine Triphosphate).

3. Carbohydrates, fats, and proteins can all be used as fuels
in cellular respiration, but glucose is most commonly
used as an example to examine the reactions and
pathways involved.

4. Living cells require
energy to perform
different tasks. The
chloroplast of the plants
can collect the energy
from the sun through
photosynthesis and store
it in the chemical bonds
of carbohydrate
molecules.
What is Cellular Respiration?
Photosynthesis

5. What is Cellular Respiration?
However, other types of organisms such as fungi,
protozoa, and a large portion of bacteria, are unable to
perform photosynthesis. Thus, these organisms rely on
the carbohydrates formed in plants for their metabolic
processes. The process of converting carbohydrates from
food produced by plants into energy is known as cellular
respiration.

6. Cellular Respiration
Therefore, cellular respiration can be defined as a long
complicated process that breaks down the food molecules to
release energy.
RELEASE ENERGY
Eating Food = Eating Glucose
• Food molecules are glucose specifically.
• We also need oxygen to oxidize thoroughly.
• Cell respiration also enables us to breathe out
carbon dioxide and the water that has made off to
the side.
• But the adenosine triphosphate is what we are
concerned about.
C6 H12 O6 + SUNLIGHT
6O2 6CO2 + 6H2O + ATP
Oxygen acts as oxidizing agent because it accepts electrons to
form water, the waste product of cellular respiration.

7. Cellular Respiration Processes
ETC
Krebs
Cycle
Glycolysis
We can divide cellular respiration into
three metabolic processes: glycolysis,
the Krebs cycle, and electron
transport chain. Each of these occurs
in a specific region of the cell:
1. Glycolysis occurs in the
cytoplasm.
2. The Krebs cycle takes place in
the matrix of the mitochondria.
3. Electron Transport Chain is
carried out on the inner
mitochondrial membrane.
In the absence of oxygen, respiration
consists of two metabolic pathways:
glycolysis and fermentation. Both of
these occur in the cytoplasm.

8. Glycolysis
Glycolysis literally means "splitting sugars." Glucose, a six
carbon sugar, is split into two molecules of a three
carbon sugar. In the process, two molecules of ATP, two
molecules of pyruvic acid and two "high energy" electron
carrying molecules of NADH are produced. Glycolysis can
occur with or without oxygen. In the presence of oxygen,
glycolysis is the first stage of cellular respiration. Without
oxygen, glycolysis allows cells to make small amounts of
ATP. This process is called fermentation.

9. Krebs Cycle/ Citric Acid Cycle
The Krebs Cycle begins after the two molecules of the
three carbon sugar produced in glycolysis are converted
to a slightly different compound (acetyl CoA). Through a
series of intermediate steps, several compounds capable
of storing "high energy" electrons are produced along
with two ATP molecules. These compounds, known as
nicotinamide adenine dinucleotide (NAD) and flavin
adenine dinucleotide (FAD), are reduced in the process.
These reduced forms carry the "high energy" electrons to
the next stage. The Citric Acid Cycle occurs only when
oxygen is present but it doesn't use oxygen directly.

10. Electron Transport Chain (ETC)
Electron Transport requires oxygen directly. The electron
transport "chain" is a series of electron carriers in the
membrane of the mitochondria in eukaryotic cells.
Through a series of reactions, the "high energy" electrons
are passed to oxygen. The energy used in the electron
transport change pumps protons and the process of
pumping of protons is known as chemiosmosis. In the
said process, a hydrogen concentration gradient is
formed, and through phosphorylation ATP is ultimately
produced.

11. Metabolic Pathways
 Glycolysis – it is the process that involves the
catabolism of glucose into two molecules of pyruvic
acid. There are several metabolic fates of a pyruvate.
In aerobic metabolism, the pyruvic acid is converted
to acetyl CoA which enters the Krebs Cycle. Pyruvic
acid in anaerobic metabolism is reduced to lactic acid.

12. Anaerobic Respiration
It is used by some microorganisms in which neither
oxygen (aerobic respiration) nor pyruvate derivatives
(fermentation) is the final electron acceptor. Rather, an
inorganic acceptor such as sulfate or nitrate is used.
Pyruvates were converted to lactate and this lactate
remains in cytoplasm which could either be converted
again to lactic acid as a waste product or enter the Cori
Cycle. Below is the net equation for lactic acid
fermentation.
Glucose + 2 ADP + 2 Pi 2 Lactate + 2 ATP

13. Cori Cycle
This involves the utilization of lactate produced from
glucose by anaerobic glycolysis (lactic acid fermentation)
in the muscle cells and red blood cells. The lactate is
moved to the liver, re-oxidized to pyruvate and turned
back to glucose through gluconeogenesis. Then, it is
returned to the muscle or other peripheral tissues.

14. Aerobic Respiration
It requires oxygen in order to generate ATP.
Although carbohydrates, fats, and proteins can all be
processed and consumed as reactants, it is the preferred
method of pyruvate breakdown in glycolysis and requires
that pyruvate enter the mitochondrion in order to be fully
oxidized by the Krebs Cycle. The products of this process
are carbon dioxide and water, but the energy transferred
is used to break strong bonds in ADP as the third
phosphate group is added to form ATP,
NADH and FADH2.

15. Steps in Glycolysis
There are two major stages of glycolysis: preparatory
phase and pay off phase. In the first stage, the glucose is
prepared for its catabolism by its phosphorylation and
then cleaved to form three-carbon sugar. In this stage, 2
ATP molecules are expended. The second phase involves
the production of 4 molecules of ATP.

16. Preparatory Phase
Stage 1: Glucose
Phosphorylation
Stage 2: Isomerization
Stage 3: Second
Phosphorylation
Stage 4: Cleavage to
two triose phosphates
Stage 5: Isomerization

17. Preparatory Phase
Stage 6: Generation of
1,3-biphosphoglycerate
Stage 7: Substrate-level
phosphorylation
Stage 8: Phosphate
transfer
Stage 9: Synthesis of
phosphoenolpyruvate
Stage 10: Substrate-
levek phosphorylation

18. Overall Reaction of Glycolysis
C6 H12 O6 + 2 NAD+ + 2 ADP + 2 P 2 pyruvic acid
+ 2 ATP + 2 NADH + 2 H+
The reaction above is considered as exergonic due to
the production of 2 molecules of ATP.