Cellular Respiration

Chapter Objectives
After the end of this chapter the students will be able to:
Describe the structure of ATP and its role in cellular metabolism
Describe how ATP is produced in a cell.
Locate where the different processes of cellular respiration occur in the cell.
Describe in detail each stage of aerobic respiration.
Explain the processes of alcoholic fermentation and lactate production.
Explain the role of electron donors and acceptors.

Introduction
3
 Photosynthetic organisms can produce glucose from the products of cellular respiration.
 Then either use the glucose to make ATP or store it in other energy-rich molecules like starch.
 When these organisms are eaten by animals, their glucose molecules are obtained.
 To withdraw chemical energy, first the food (storable energy) breaks down in to monomers
and then turn in to ATP(usable energy).
 ATP = Adenosine Triphosphate.
 Cells only run on ATP, like a car only runs on gasoline.
1/1/2023 By: Asmamaw Menelih

Introduction
4

ATP: Energy for Cells
The full name for ATP is Adenosine Tri-Phosphate
All nucleotides contain:
a nitrogenous base (this one contains adenine)
a pentose sugar
a phosphate group
The ATP molecule is based on this nucleotide.
ATP is sometimes described as a phosphorylated nucleotide.
A nucleotide containing the nitrogenous base
adenine

ATP is essentially the adenine nucleotide with two extra phosphate groups
Adding the extra phosphates requires energy, particularly when the third phosphate is added.
As a result, energy is stored in the ATP molecule
 when the bonds that hold this third phosphate in place are broken, the energy is released again.
The structure of the ATP molecule

ATP is formed by phosphorylation with Adenine nucleotide
phosphorylated nucleotide the adding of one or more phosphate groups to a molecule
The inter-conversion of ADP and ATP

Chemical Structure of ATP
3 Phosphates Ribose Sugar
Adenine Base

Function of ATP
Sunlight energy cannot be used directly by plants and any other organisms to ‘drive’ the synthesis of
proteins or any other molecules.
The same applies to the energy held in a glucose molecule.
These two energy sources must be used to produce ATP, which is used to transfer the energy to the relevant
cellular process.
Coupled reactions to transfer energy in cells

Function of ATP
ATP is adapted to immediate source of energy for cells. because it:
Releases energy in relatively small amounts that are closely matched to the amounts of energy required in
many biological processes occurring inside cells
Releases energy in a single-step hydrolysis reaction, so the energy can be released quickly
Is able to move around the cell easily, but cannot escape from the cell

The following processes are examples of activity of cells that require energy from ATP:
The synthesis of macromolecules – such as proteins
Active transport across a plasma membrane
Muscle contraction
Conduction of nerve impulses
The initial reactions of respiration (the later reactions release energy from glucose to
form more ATP)
Function of ATP

Function of ATP
 Generally, cells use ATP for:
Chemical work – ATP supplies energy to synthesize macromolecules, and therefore the organism
Transport work – ATP supplies energy needed to pump substances across the plasma membrane
Mechanical work – ATP supplies energy for cellular movements

Almost all the ATP produced in cells is formed in the same way.
It obviously involves ADP and Pi joining to form ATP and this requires an input
of energy.
The formation of ATP involves an enzyme called ATP synthase
The ATP synthase is in one of the enzymes found membranes of mitochondrion
& chloroplast.
How is ATP produced in a cell?
The structure of ATP synthase
 ATP synthase is like a water wheel.
 When the rotor is spin by hydrogen ions passing through it, the energy of the spinning
is used to activate sites in the catalytic knob that convert ADP and Pi to ATP.
 In both photosynthesis and aerobic respiration, many of the reactions generate the
hydrogen ions that will pass through the ATP synthase to produce ATP

Two types of metabolic reactions involved during energy production
Anabolism
 larger molecules are made
 requires energy
 Eg. Photosynthesis
 Catabolism
 larger molecules are broken down
 releases energy
 Eg. Cellular Respiration
How is ATP produced in a cell?

There are two main pathways by which respiration can produce ATP:
The aerobic pathway (aerobic respiration) – this requires the presence of oxygen, and
The anaerobic pathway (anaerobic respiration and fermentation) – this can take place in the
absence of oxygen
How is ATP produced in respiration?

Cellular Respiration
17
• Before cells can use the energy in complex
carbohydrates (a major source of energy for
organisms), the large polymers are broken down
into simple sugars such as glucose .
• Then, glucose is split even further to release
energy.

Cellular
respiration
Aerobic
respiration
Krebs cycle
Electro
transport chain
Anaerobic
respiration
Glycolysis
1/1/2023 By: Asmamaw Menelih 18

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• Oxidative phosphorylation: oxygen dependent production of ATP
• Matrix: fluid in the mitochondrion in which the reactions of the Krebs cycle take place
• ATP synthase: enzyme involved in the formation of ATP
• Substrate level phosphorylation: when another molecule (substrate) is able to transfer a phosphate group
directly to ADP (ATP synthase enzyme is NOT involved in the formation of ATP)
• Respiratory substrates: organic molecules that can be respired such as glucose, fatty acids, amino acids etc
• phosphorylated nucleotide: the adding of one or more phosphate groups to a nucleotide molecule
• Decarboxylation: removing of carbon from a molecule
• Dehydrogenation: removing of hydrogen from a molecule
Basic Concepts

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1. Glycolysis: "splitting sugars."
• one six-carbon molecule of glucose is broken into two
molecules of the three-carbon compound pyruvate.
• ATP is produced from ADP and Pi, and nicotinamide
adenine dinucleotide (NAD+) is reduced to form
NADH.
Cellular respiration occurs in 4 stages:

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1. Glycolysis:
3. Citric Acid Cycle:
4. Oxidative Phosphorylation:
2. Pyruvate processing

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1. Glycolysis:
• Each acetyl CoA is oxidized to two molecules of CO2.
• More ATP and NADH are produced, and Flavin adenine
dinucleotide (FAD) is reduced to form FADH2.
2. Pyruvate processing (Link Reaction):

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1. Glycolysis:
4. Oxidative Phosphorylation:
• Extracts energy from NADH & FADH2.
• Passes electrons from higher to lower energy states.
• Produces 32 or 34 molecules of ATP
2. Pyruvate processing (Link Reaction):

Stages of Cellular Respiration
25
Glycolysis:
• Glycolysis, takes place in the cytoplasm.
• It does not take place inside the mitochondria
because:
• The glucose molecule cannot diffuse through
the mitochondrial membranes (it is a medium-
sized molecule and is not lipid soluble)
• There are no carrier proteins to transport the
glucose molecule across the membranes.

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Glycolysis:
• Splitting glucose into two 3-carbon
molecules of pyruvate in cytoplasm of cell.
• Does NOT require O2.
• Two ATP are used to activate glucose
• Two electrons and one hydrogen ion are
accepted by NAD+ (nicotinamide adenine
dinucleotide) resulting two NADH.
• Produced four ATP and gain net of two
ATP.

Glycolysis steps

At the end of glycolysis from a single Glucose Molecule:
There is a net gain of two ATP (two molecules are used initially and then
four are produced).
Two molecules of reduced NAD ( NADH)
The molecules of pyruvate pass into the mitochondria through carrier
molecules in the mitochondrial membrane.
Summary of Glycolysis

29
Pyruvate processing (Link Reaction):
• Each pyruvate is processed to release
one molecule of CO2, and the remaining
two carbons are used to form the
compound acetyl CoA.
• The oxidation of pyruvate results in more
NAD+ being reduced to NADH.

 One fate of pyruvate is that it enters to TCA cycle for complete oxidation.
 But there are intermediate processes that convert pyruvate to a acetyl coA.
 The enzyme complex converts pyruvate into Acetyl-CoA by the following chemical changes:
 Decarboxylation of pyruvate (loss of CO2)
 Formation of acetyl group
 Linkage of acetyl group to coenzyme A forming acetyl - CoA.
 Overall Summary of Link reaction
 pyruvate + CoA + NAD⇌ acetyl CoA + CO2 + reduced NAD
Pyruvate processing (Link Reaction)

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Kreb’s Cycle:
• Occurs inside the mitochondrial “matrix”.
• The pyruvate molecules are transformed
into acetyl CoA.
• Begins by the addition of a two-carbon
acetyl group to a four-carbon molecule
(oxaloacetate), forming a six-carbon
molecule (citric acid)
• H atoms added to NAD and FAD to form
NADH and FADH2.
• C and O atoms given off as ‘waste’ product
CO2.
• 2 ATP produced.

The TCA cycle
Krebs's Cycle:

Summary of Krebs's Cycle
Krebs cycle take place in the fluid matrix of the mitochondrion
Acetyl coenzyme A (2C) reacts with oxaloacetate to form Citrate (6C).
Citrate loses a carbon atom to form a five-carbon compound and CO2 is produced
 The five-carbon compound is then further decarboxylated to form a four-carbon compound and
CO2 is again produced
One molecule of ATP is also produced by substrate level phosphorylation
The four-carbon compound undergoes several molecular transformations to form Oxaloacetate
In several reactions in the cycle, reduced NAD and in one reaction, reduced FAD is produced

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Electron Transport:
 Occurs in the membrane of mitochondria.
 The process by which energy is transferred from
NADH and FADH2 to ATP.
 NADH and FADH2 provide electrons and H+ to the
mitochondria.
 The electrons move via an electron transport chain in
the inner membrane of the mitochondrion.

36
Electron Transport:
 The electrons end up in oxygen.
 Some hydrogen bonds with oxygen to make H2O as a waste
product.
 Generate 32 of the 36 ATP molecules produced from each
glucose molecule!.

37
Electron Transport:
 On the cristae, the following events take place:
 The hydrogen atoms carried by NADH and FADH2 are
released and split into protons (hydrogen ions) and
electrons
 The electrons pass along a series of electron carriers lose
energy as they pass from one carrier to the next
 The molecules that act as electron carriers in the electron
transport chain are:
 reduced NAD dehydrogenase (also a proton pump)
 ubiquinone (also a proton pump), and
 cytochromes (these are proteins that contain iron)

Electron Transport:
Oxidative phosphorylation takes place in the inner mitochondrial membrane.
Reduced NAD and reduced FAD are passed to the electron transport chain.
The hydrogens are removed from the two hydrogen carriers and each is split into its constituent proton
(H+) and electron (e−).
The energetic electron is transferred to the first of a series of electron carriers.

Electron Transport:
 Hydrogen carrier molecules
 Two molecules are important in this transfer process:
Nicotinamide Adenine Dinucleotide (NAD)
Flavine Adenine Dinucleotide (FAD)
 Both are coenzymes and are capable of accepting hydrogen ions.
 NAD + 2H ⇌ reduced NAD
 NAD++2H ⇌ NADH+ + H+

Electron Transport:
 Hydrogen carrier molecules pass protons into the mitochondrial matrix through protein channels in
the inner membrane.
The enzyme ATP synthase is associated with each channel.
As the protons pass through the channel, their electrical potential energy is used to synthesis ATP in the
process called chemiosmosis
The sites of events of respiration in a cell

Summary Cellular Respiration
42
• The actual yield of ATP per glucose (32 ATP) is lower than the theoretical calculation (38 ATP) because the proton motive
force is used to drive other mitochondrial activities, such as the active transport of Pi into the mitochondrial matrix.
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Summary Cellular Respiration
43
• Theoretical Calculation
Cellular Respiration.mp4

Anaerobic Respiration
44
 Pyruvate is a pivotal metabolite in cellular respiration.
 If O2 is not available to the cell, fermentation, an anaerobic process, occurs in the cytoplasm.
 During fermentation, glucose is incompletely metabolized to lactate, or to CO2 and alcohol
(depending on the organism).
 If O2 is available to the cell, pyruvate enters mitochondria by aerobic process.
 Certain cells in our body can produce energy without oxygen-- muscle cells.

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 An anaerobic process that reduces pyruvate to either lactate or alcohol and CO2.
 NADH passes its electrons to pyruvate.
 Alcoholic fermentation: carried out by yeasts, produces carbon dioxide and ethyl
alcohol.
 Used in the production of alcoholic spirits and breads.
 Lactic acid fermentation: carried out by certain bacteria and fungi, produces lactic acid
(lactate).
 Used commercially in the production of cheese, yogurt, and sauerkraut.
 In mammalian red blood cells and in skeletal muscle

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Products of Fermentation
47

 The immediate source energy for cells is ATP
 The full name for ATP is Adenosine Tri-Phosphate
 ATP will release in energy in small amount with a single hydrolysis rxn and can be move around cells easily.
 ATP is important for chemical, transport and mechanical work of cells.
 ATP produced in similar way in all cells. i.e ADP+Pi=ATP
 The enzyme that catalyze ATP production is known as ATP Synthesase
 Cells can Produce ATP through cellular respiration
 The Four stages of cellular respiration are Glycolysis, link rxn, citric acid cycle and Oxidative
phosphorylation
Chapter Summary

Cellular Respiration

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