Aerobic Respiration

The Link Reaction

Pyruvate can be used in aerobic respiration by converting it to acetyl coenzyme A. This process is called the link reaction and it takes place in the matrix of the mitochondria.

Decarboxylation

Pyruvate is first actively transported from the cytoplasm across the mitochondrial membrane and into the matrix of the mitochondria.
In the mitochondrial matrix, pyruvate is decarboxylated and dehydrogenated to acetate, which is a two-carbon molecule.
- CO₂ is produced as a by-product.
- NAD is reduced to NADH.

Coenzyme A

Acetate then combines with coenzyme A (CoA) to produce acetyl coenzyme A.
Acetyl coenzyme A is used in the Krebs cycle (the next stage of aerobic respiration).
- The link reaction links glycolysis to the Krebs cycle.

Net gain

The net gain from the link reaction is:
- 1 CO₂ molecule.
- 1 NADH molecule.

The Krebs Cycle

The Krebs cycle takes place in the matrix of the mitochondria. The products of the cycle are two coenzymes (NADH and FADH₂), ATP and CO₂.

The Krebs Cycle

Acetyl coenzyme A (coA)

Acetyl coenzyme A acts as a carrier for the two-carbon acetyl group. It reacts with oxaloacetate (a four-carbon molecule) to produce citrate (a six-carbon molecule).
CoA is now available to be recycled and reused in the link reaction.
The production of citrate allows the Krebs cycle to begin.

Illustrative background for 6C &rightarrow; 5C

Illustrative background for 6C &rightarrow; 5C ?? "content

6C → 5C

Citrate is converted to a five-carbon molecule (5C) by decarboxylation and dehydrogenation.
- CO₂ is produced as a by-product.
- NAD is reduced to NADH.

Illustrative background for 5C &rightarrow; 4C

Illustrative background for 5C &rightarrow; 4C ?? "content

5C → 4C

The five-carbon molecule is decarboxylated and dehydrogenated again to a four-carbon compound.
- CO₂ is produced.
- NAD is reduced to NADH.
- ATP is also produced by substrate-level phosphorylation.

Regeneration of oxaloacetate

This 4C molecule is then dehydrogenated again to produce another molecule of NADH. FAD is also reduced to FADH₂.
No decarboxylation takes place at this stage.
These intermediate reactions regenerate oxaloacetate. This allows the cycle to continue again.

Net gain

The net gain of the Krebs cycle is:
- 2 CO₂ molecules.
- 3 NADH molecules.
- 1 ATP molecule.
- 1 FADH₂ molecules.
For each molecule of glucose, there are two cycles (this is because two molecules of pyruvate are produced in glycolysis).

Other respiratory substrates

Fatty acids and amino acids can also be used as respiratory substrates in aerobic respiration.
The substrates are converted to molecules that can easily enter the Krebs cycle.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage in aerobic respiration.

Inner mitochondrial membrane

Oxidative phosphorylation takes place at the inner mitochondrial membrane.
There are several features of the membrane that allows production of ATP on a large scale:
- Three electron carrier proteins (electron transport chain, ETC).
- ATP synthase enzyme.
The space between the inner and outer mitochondrial membranes is called the intermembrane space.

Electron transport chain

NADH and FADH₂ (from the Krebs cycle) are oxidised by the first electron carrier protein in the inner mitochondrial membrane.
This initiates oxidative phosphorylation because NADH and FADH₂ release two protons and two electrons each.
The electrons are then transferred along the ETC.

Proton gradient

As the electrons move down the ETC, they lose energy.
This energy pumps the protons from NADH and FADH₂ into the intermembrane space.
This creates a proton gradient (also known as an electrochemical gradient).

Chemiosmosis

The protons diffuse down the concentration gradient through the ATP synthase enzyme.
As protons flow through the ATP synthase, energy is released.
This energy converts ADP and inorganic phosphate to ATP.
This process is called chemiosmosis.

The final electron acceptor

After the electrons have reached the end of the ETC and protons have flowed through the ATP synthase enzyme, they combine with O₂ to form water (H₂O).
- Oxygen is called the final electron acceptor for this reason.

1Cell Biology

2Molecular Biology

2.1Water

2.1.1Water & Hydrogen Bonding

2.1.2IB Multiple Choice - Water & Hydrogen Bonding

2.1.3Extended Response - Water

2.2Carbohydrates & Lipids

2.2.1Monosaccharides

2.2.2Disaccharides & Polysaccharides

2.2.3Triglycerides & Phospholipids

2.2.4Cellulose

2.2.5Starch & Glycogen

2.3Proteins

2.3.1Structure & Functions of Proteins

2.4Enzymes

2.4.1Enzymes

2.4.2Factors Affecting Enzyme Activity

2.4.3Enzyme-Controlled Reactions

2.4.4End of Topic Test - Enzymes

2.5Structure of DNA & RNA

2.5.1DNA & RNA

2.6DNA Replication, Transcription & Translation

2.6.1Protein Synthesis

2.6.2Transcription & Translation

2.6.3End of Topic Test - DNA, Genes & Protein Synthesis

2.6.4Exam-Style Question - Protein Synthesis

2.6.5End of Topic Test - Coronavirus Translation

2.6.6IB Multiple Choice - Transcription

2.6.7IB Multiple Choice - Translation

2.7Cell Respiration

2.7.1Overview of Respiration

2.7.2Anaerobic Respiration

2.7.3End of Topic Test - Anaerobic Respiration

2.7.4Respiration Experiments

2.7.5End of Topic Test - Respiration

2.7.6IB Multiple Choice - Respiration

2.8Photosynthesis

2.8.1Overview of Photosynthesis

3Genetics

4Ecology

4.1Species, Communities & Ecosytems

4.1.1Population Dynamics

4.1.2Communities & Measuring Diversity

4.1.3Relationships Between Populations

4.1.4Overview of Ecosystems

4.1.5Nutrient Cycles

4.1.6Fertilisers

4.1.7Eutrophication

4.1.8Chi-Squared Test

4.2Energy Flow

4.2.1Energy

4.2.2Energy Flow

4.2.3Biomass & Food Chains

4.2.4Heterotrophs & Autotrophs

4.3Carbon Cycle

4.3.1Carbon Cycle

4.4Climate Change

4.4.1Carbon Dioxide

4.4.2The Greenhouse Effect

4.4.3Global Warming

4.4.4Maintaining Biodiversity

5Evolution & Biodiversity

5.1Evidence for Evolution

5.1.1Evidence for Evolution - Fossils & DNA

5.1.2Evidence for Evolution - Anatomy & Geography

5.1.3IB Multiple Choice - Evidence for Evolution

5.1.4Extended Response - DNA & Evolution

5.2Natural Selection

5.2.1Introduction

5.2.2Evolutionary Fitness

5.2.3Natural Selection & Variation

5.2.4End of Topic Quiz - Natural Selection

5.2.5IB Multiple Choice - Natural Selection

5.2.6Speciation

5.3Classification of Biodiversity

5.3.1Introduction to Biodiversity

5.3.2Keystone Species

5.3.3Courtship & Ancestry

5.3.4Classification

5.4Cladistics

5.4.1Evolutionary Relationships

5.4.2IB Multiple Choice - Species & Taxonomy

6Human Physiology

6.1Digestion & Absorption

6.1.1Overview of Digestion

6.1.2Digestion in Mammals

6.1.3Absorption

6.1.4End of Topic Test - Substance Exchange & Digestion

6.1.5End of Topic Test - Substance Ex & Digestion

6.2The Blood System

6.2.1The Circulatory System

6.2.2The Heart

6.2.3Blood Vessels

6.2.4Cardiovascular Disease

6.3Defence Against Infectious Disease

6.3.1Immune System

6.3.2The Immune Response

6.3.3Antibodies

6.3.4Primary & Secondary Response

6.3.5Vaccines

6.3.6HIV

6.3.7End of Topic Test - Immune System

6.3.8Exam-Style Question - Immune System

6.3.9End of Topic Test - Immune System

6.4Gas Exchange

6.4.1Lung Structure & Alveoli

6.4.2Ventilation

6.4.3Lung Disease

6.4.4Lung Disease Data

6.5Neurons & Synapses

6.5.1Neuron Structure

6.5.2Transmission of an Impulse

6.5.3Speed of Transmission

6.5.4End of Topic Test - Neurones & Action Potentials

6.5.5Synapses

6.5.6Types of Synapse

6.5.7Medical Application

6.5.8End of Topic Test - Synapses

6.5.9End of Topic Test - Nervous Comm&Coord

6.6Hormones, HomeoStasis & Reproduction

6.6.1Overview of Homeostasis

6.6.2Blood Glucose Concentration

6.6.3Controlling Blood Glucose Concentration

6.6.4End of Topic Test - Blood Glucose

6.6.5Type I & Type II Diabetes

6.6.6Human Reproductive Systems

6.6.7Hormonal Control of the Menstrual Cycle

7AHL: Nucleic Acids

7.1DNA Structure & Replication

7.1.1DNA

7.1.2RNA

7.1.3DNA Replication

7.1.4DNA Replication - Summary

7.1.5IB Multiple Choice - DNA Replication

7.2Transcription & Gene Expression

7.2.1Introduction to Transcription & Protein Synthesis

7.2.2Types of RNA

7.2.3Transcription

7.2.4RNA Processing in Eukaryotes

7.2.5IB Multiple Choice - Transcription

7.3Translation

7.3.1Translation

7.3.2Retroviruses

7.3.3Scientific Practices: Coronavirus

7.3.4IB Multiple Choice - Translation

8AHL: Metabolism, Cell Respiration & Photosynthesis

8.1Metabolism

8.1.1Molecules to Metabolism

8.2Cell Respiration

8.2.1Glycolysis

8.2.2Aerobic Respiration

8.3Photosynthesis

8.3.1Light-Dependent Reaction

8.3.2Light-Independent Reaction

8.3.3End of Topic Test - Photosynthesis Reactions

8.3.4Limiting Factors

8.3.5Photosynthesis Experiments

8.3.6End of Topic Test - Photosynthesis

8.3.7End of Topic Test - Photosynthesis

9AHL: Plant Biology

9.1Transport in the Xylem of Plants

9.1.1Structure

9.1.2Cohesion-Tension Theory

9.1.3Investigating Transpiration Rate

9.2Transport in the Phloem of Plants

9.2.1Structure

9.2.2Translocation

9.3Growth in Plants

9.3.1Plant Tropisms

10AHL: Genetics & Evolution

10.1Meiosis

10.1.1Stages of Meiosis

10.2Inheritance

10.2.1Linked Genes

10.2.2Sex-Linked Genes

10.2.3Non-Nuclear Inheritance

10.2.4Chi-Squared Test

10.2.5End of Topic Quiz - Inheritance

10.2.6IB Multiple Choice - Non-Mendelian Genetics

10.2.7Introduction to Non-Mendelian Inheritance

10.2.8Extended Response - Inheritance

10.2.9Grade 4-5 (Scientific Practices) - Inheritance

10.3Gene Pools & Speciation

10.3.1Populations

10.3.2Mutations, Genetic Drift, & Gene Flow

10.3.3Speciation

10.3.4Rate of Speciation

10.3.5Allopatric & Sympatric Speciation

11AHL: Animal Physiology

11.1Antibody Production & Vaccination

11.1.1Immune System

11.1.2The Immune Response

11.1.3Antibodies

11.1.4Primary & Secondary Response

11.1.5Vaccines

11.2Movement

11.2.1Skeletal Muscle

11.2.2Sliding Filament Theory

11.2.3Contraction

11.3The Kidney & Osmoregulation

11.3.1Kidneys & Osmoregulation

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