Topic 8.2
AEROBIC RESPIRATION
Topic Outline
 Energy Conversions
AUDIO
Reducing agent
 Glycolysis
X
 Link Reaction
 Krebs Cycle
O
H
e–
 Electron Transport Chain
 Oxidative Phosphorylation
 Mitochondria (Structure & Function)
Energy is converted to a usable form in cell respiration
Y
Oxidizing agent
Energy Conversions
Chemical energy stored within organic molecules can be released via the process
of cellular respiration (anaerobic or aerobic) for use within the cell
This energy can be converted into an immediately available source (ATP)
• ATP is produced directly via substrate level phosphorylation
The energy can also be converted into a transitional source (hydrogen carriers)
• Hydrogen carriers produce larger amounts of ATP via oxidative phosphorylation
• This involves aerobic respiration (requires oxygen and occurs in mitochondria)
Adenosine Triphosphate (ATP)
ATP is a high energy molecule that serves two key functions:
• It acts as an immediate source of energy when hydrolysed to ADP (+ Pi)
• It can transfer the phosphate, making molecules less stable and more reactive
Adenine
Ribose
High-energy
bond
P
P
P
Adenosine Triphosphate
Adenine
Ribose
Inorganic
phosphate
P
P
+
P
Adenosine Diphosphate (+ Pi)
Phosphorylation of molecules makes them less stable
+
Free
energy
Redox Reactions
Energy can be transferred between organic molecules by means of redox reactions
• Reduction is a gain in hydrogen and electrons or a loss of oxygen
• Oxidation is a loss in hydrogen and electrons or a gain of oxygen
Oxidation
Reduction
Electrons
Loss
Gain
Hydrogen
Loss
Gain
Oxygen
Gain
Loss
OIL RIG:
Oxidation
Is
Loss (of e–)
Reduction
Is
Gain (of e–)
Hydrogen Carriers
Hydrogen carriers (or electron carriers) transfer high energy electrons (+ protons)
between molecules via redox reactions – essentially functioning like chemical taxis
Reduction
H
H + + e–
H+
Organic
molecule
NAD+
Organic
molecule
Oxidation
Cell respiration involves the oxidation and reduction of electron carriers
NADH + H+
(gains energy)
Aerobic Respiration
Aerobic respiration occurs within mitochondria and requires oxygen to proceed
• It releases energy stored within hydrogen carriers for a greater ATP yield
Aerobic respiration involves four key stages:
• Glycolysis – Carbohydrates are partially broken down anaerobically
• Link Reaction – Products are transferred to the mitochondria
• Krebs Cycle – Products are completely broken down (⬆︎ hydrogen carriers)
• Electron Transport Chain – ATP produced from hydrogen carriers
Glycolysis
Glycolysis involves the partial breakdown of glucose under anaerobic conditions
• Via many steps, glucose (6C) is broken down into two pyruvate molecules (3C)
The key events of glycolysis are:
• ATP expenditure – Glucose is phosphorylated by two ATP
• Lysis – The phosphorylated molecule is split into two
• Oxidation – The two triose phosphates are oxidised
• Hydrogen carriers formed – Two NADH molecules released
• ATP production – Four ATP molecules formed (net gain = 2)
In glycolysis, glucose is converted into pyruvate in the cytoplasm
HINT: Aloha!
Glycolysis Overview
NAD+
2H
2 × ATP
2 × ATP
NADH + H+
3C
Pyruvate
3C
Pyruvate
Glucose
(6C Compound)
NAD+
2H
NADH + H+
(3C Compound)
2 × ATP
Net Reaction: Glucose → 2 × Pyruvate + NADH (×2) + ATP (×2)
Glycolysis gives a small net gain of ATP without the use of oxygen
Link Reaction
The link reaction functions to connect the anaerobic processes that occur in the
cytoplasm (i.e. glycolysis) with the aerobic processes that occur in mitochondria
The key events of the link reaction are:
• Pyruvate is transported from the cytoplasm to the mitochondrial matrix
• Pyruvate is oxidised to produce a reduced hydrogen carrier (NADH)
• Pyruvate is decarboxylated to form an acetyl compound (CO2 is produced)
• The acetyl compound is attached to coenzyme A (to form acetyl CoA)
Pyruvate is decarboxylated and oxidised, and converted into acetyl coenzyme A in the link reaction
Link Reaction Overview
CYTOSOL
MITOCHONDRIAL MATRIX
Pyruvate
Acetyl CoA
(3C Compound)
(2C Compound)
NAD+
CO2
2H
CoA
NADH + H+
Net Reaction: Pyruvate → Acetyl CoA + NADH + CO2
Note: Glycolysis forms two pyruvate molecules so all resulting products are doubled
Krebs Cycle
The Krebs cycle (also known as the citric acid cycle) involves a series of oxidation
and decarboxylation reactions that occur within the mitochondrial matrix
The key events of the Krebs cycle are:
• Acetyl CoA combines with a 4C compound to make a 6C compound
• The 6C compound is broken back down into the original 4C compound
• This involves the formation of ATP (1 per cycle) and carbon dioxide (2 per cycle)
• It also produces a large amount of hydrogen carriers (3 NADH + 1 FADH2 per cycle)
In the Krebs cycle, oxidation of acetyl groups is coupled to reduction of hydrogen carriers, liberating CO 2
Krebs Cycle Overview
Acetyl CoA
Input:
Acetyl CoA
Outputs:
2 × CO2
1 × ATP
NAD+
2H
6C
NADH
+ H+
Krebs
Cycle
FAD
2H
FADH2
CO2
NAD+
2H
5C
4C
NADH
+ H+
3 × NADH
1 × FADH2
NAD+
ATP
CO2
2H
NADH + H+
Electron Transport Chain
Hydrogen carriers produced by prior reactions (glycolysis, link reaction, Krebs cycle)
are transported to the mitochondrial cristae (i.e. the inner mitochondrial membrane)
• This is the location of the electron transport chain (and ATP synthase)
Electron Transport Chain
ATP synthase
Energy released by oxidation reactions is carried to the mitochondrial cristae by reduced NAD and FAD
Establishing a Proton Gradient
Hydrogen carriers transfer high energy electrons to the electron transport chain
• The electrons lose energy as they are shuttled between electron carriers
• This energy is used to pump protons into the intermembrane space (from matrix)
+
NADH
+
+
-
Transfer of electrons between carriers in the electron transport chain is coupled to proton pumping
Chemiosmosis
The build up of protons within the intermembrane space
INTERMEMBRANE
creates an electrochemical gradient (proton motive force)
The protons return to the matrix via a transmembrane
enzyme called ATP synthase (via a process of chemiosmosis)
• This enzyme catalyses ATP synthesis (from ADP + Pi)
This formation of ATP is called oxidative phosphorylation
because it was coupled to oxidation of hydrogen carriers
In chemiosmosis protons diffuse through ATP synthase to generate ATP
MATRIX
The Role of Oxygen
Aerobic respiration requires the presence of oxygen in order to proceed
• Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O (+ 36ATP)
Oxygen is the final electron acceptor in the electron transport chain
• De-energised electrons must be removed for the chain to keep functioning
Oxygen also binds to hydrogen ions in the matrix to maintain a proton gradient
• Chemiosmosis requires higher levels of protons in the intermembrane space
Oxygen combines with the electrons and protons to form water molecules
Oxygen is needed to bind with free protons to maintain hydrogen gradient, resulting in water formation
Electron Transport Chain Overview
Watch the video for a summary of key events in the electron transport chain
Summary of Aerobic Respiration
Glucose
2 ATP
GLYCOLYSIS
Complete combustion of glucose
molecule via aerobic respiration:
2
NADH
Glycolysis: 2 ATP + 2 NADH
Pyruvate × 2
2 CO2
LINK REACTION
Link Reaction: 2 CO2 + 2 NADH
Krebs Cycle: 4 CO2 + 2 ATP
6 NADH + 2 FADH2
2
NADH
Acetyl CoA × 2
2 ATP
4 CO2
KREBS
CYCLE
Electron Transport Chain: 32 ATP
6
2NADH
FADH
2
6 O2
ELECTRON
TRANSPORT CHAIN
32 ATP
6 H2O
Analysis of diagrams of aerobic respiration to deduce where decarboxylation and oxidation occurs
Mitochondrion
The mitochondrion is the organelle responsible for aerobic respiration and has a
highly specialised structure in order to best perform this function:
• Cristae – The inner membrane is highly folded to increase the SA:Vol ratio
(the inner membrane is the site of the electron transport chain)
• Intermembrane Space – There is a very small space between the membranes
(maximises the gradient upon proton accumulation)
• Matrix – Contains appropriate enzymes and suitable pH for the Krebs cycle
• Outer membrane – Contains transport proteins for the shuttling of pyruvate
The structure of the mitochondrion is adapted to the function it performs
Mitochondrion Diagram
Intermembrane space
Small space to quickly
accumulate protons
Inner membrane
Contains ETC and ATP synthase
for oxidative phosphorylation
Cristae
Highly folded to
raise SA:Vol ratio
Matrix
Appropriate enzymes and
suitable pH for Krebs cycle
Outer membrane
Contains transport proteins
Annotation of a diagram of a mitochondrion to indicate the adaptations to its function
Mitochondrion Micrograph
The mitochondrion is a sausage-shaped structure with internal foldings (cristae)
Electron Tomography
A
Electron tomography models internal structures of
B
cells by generating images at different angles with
TEM and then compiling a 3D representation
This can produce images of active mitochondria:
• The intermembrane space is shown to be a
C
consistent width throughout a mitochondrion
• Cristae are shown to be continuous infoldings
of the inner mitochondrial membrane
A
Electron tomography used to produce images of active mitochondria
B
C
Topic Review
Can you do the following?
• Outline redox reactions
• Describe the process of glycolysis
• Outline the key events of the link reaction
• List the outputs of the Krebs cycle
• Explain chemiosmosis
• Draw and describe mitochondrial structures
• Explain how electron tomography is used