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.

1Biological Molecules

1.1Monomers & Polymers

1.1.1Monomers & Polymers

1.1.2Condensation & Hydrolysis Reactions

1.2Carbohydrates

1.2.1Structure of Carbohydrates

1.2.2Types of Polysaccharides

1.2.3End of Topic Test - Monomers, Polymers and Carbs

1.2.4Exam-Style Question - Carbohydrates

1.2.5A-A* (AO3/4) - Carbohydrates

1.3Lipids

1.3.1Triglycerides & Phospholipids

1.3.2Types of Fatty Acids

1.3.3Testing for Lipids

1.3.4Exam-Style Question - Fats

1.3.5A-A* (AO3/4) - Lipids

1.4Proteins

1.4.1The Peptide Chain

1.4.2Investigating Proteins

1.4.3Primary & Secondary Protein Structure

1.4.4Tertiary & Quaternary Protein Structure

1.4.5Enzymes

1.4.6Factors Affecting Enzyme Activity

1.4.7Enzyme-Controlled Reactions

1.4.8End of Topic Test - Lipids & Proteins

1.4.9A-A* (AO3/4) - Enzymes

1.4.10A-A* (AO3/4) - Proteins

1.5Nucleic Acids

1.5.1DNA & RNA

1.5.2Nucleotides

1.5.3Polynucleotides

1.5.4DNA Replication

1.5.5Exam-Style Question - Nucleic Acids

1.5.6A-A* (AO3/4) - Nucleic Acids

1.6ATP

1.6.1Structure of ATP

1.6.2Hydrolysis of ATP

1.6.3Resynthesis of ATP

1.6.4End of Topic Test - Nucleic Acids & ATP

1.7Water

1.7.1Importance of Water

1.7.2Structure of Water

1.7.3Properties of Water

1.7.4A-A* (AO3/4) - Water

1.8Inorganic Ions

1.8.1Inorganic Ions

1.8.2End of Topic Test - Water & Inorganic Ions

2Cells

2.1Cell Structure

2.1.1Introduction to Cells

2.1.2Eukaryotic Cells & Organelles

2.1.3Eukaryotic Cells & Organelles 2

2.1.4Prokaryotes

2.1.5A-A* (AO3/4) - Organelles

2.1.6Methods of Studying Cells

2.1.7Microscopes

2.1.8End of Topic Test - Cell Structure

2.1.9Exam-Style Question - Cells

2.1.10A-A* (AO3/4) - Cells

2.2Mitosis & Cancer

2.2.1Mitosis

2.2.2Stages of Mitosis

2.2.3Investigating Mitosis

2.2.4Cancer

2.2.5A-A* (AO3/4) - The Cell Cycle

2.3Transport Across Cell Membrane

2.3.1Cell Membrane Structure

2.3.2A-A* (AO3/4) - Membrane Structure

2.3.3Diffusion

2.3.4Osmosis

2.3.5Active Transport

2.3.6End of Topic Test - Mitosis, Cancer & Transport

2.3.7Exam-Style Question - Membranes

2.3.8A-A* (AO3/4) - Membranes & Transport

2.3.9A-A*- Mitosis, Cancer & Transport

2.4Cell Recognition & the Immune System

2.4.1Immune System

2.4.2Phagocytosis

2.4.3T Lymphocytes

2.4.4B Lymphocytes

2.4.5Antibodies

2.4.6Primary & Secondary Response

2.4.7Vaccines

2.4.8HIV

2.4.9Ethical Issues

2.4.10End of Topic Test - Immune System

2.4.11Exam-Style Question - Immune System

2.4.12A-A* (AO3/4) - Immune System

3Substance Exchange

3.1Surface Area to Volume Ratio

3.1.1Size & Surface Area

3.1.2A-A* (AO3/4) - Cell Size

3.2Gas Exchange

3.2.1Single-Celled Organisms

3.2.2Multicellular Organisms

3.2.3Control of Water Loss

3.2.4Human Gas Exchange

3.2.5Ventilation

3.2.6Dissection

3.2.7Measuring Gas Exchange

3.2.8Lung Disease

3.2.9Lung Disease Data

3.2.10End of Topic Test - Gas Exchange

3.2.11A-A* (AO3/4) - Gas Exchange

3.3Digestion & Absorption

3.3.1Overview of Digestion

3.3.2Digestion in Mammals

3.3.3Absorption

3.3.4End of Topic Test - Substance Exchange & Digestion

3.3.5A-A* (AO3/4) - Substance Ex & Digestion

3.4Mass Transport

3.4.1Haemoglobin

3.4.2Oxygen Transport

3.4.3The Circulatory System

3.4.4The Heart

3.4.5Blood Vessels

3.4.6Cardiovascular Disease

3.4.7Heart Dissection

3.4.8Xylem

3.4.9Phloem

3.4.10Investigating Plant Transport

3.4.11End of Topic Test - Mass Transport

3.4.12A-A* (AO3/4) - Mass Transport

4Genetic Information & Variation

4.1DNA, Genes & Chromosomes

4.1.1DNA

4.1.2Genes

4.1.3Non-Coding Genes

4.1.4The Genetic Code

4.1.5A-A* (AO3/4) - DNA

4.2DNA & Protein Synthesis

4.2.1Protein Synthesis

4.2.2Transcription & Translation

4.2.3End of Topic Test - DNA, Genes & Protein Synthesis

4.2.4Exam-Style Question - Protein Synthesis

4.2.5A-A* (AO3/4) - Coronavirus Translation

4.2.6A-A* (AO3/4) - Transcription

4.2.7A-A* (AO3/4) - Translation

4.3Mutations & Meiosis

4.3.1Mutations

4.3.2Meiosis

4.3.3A-A* (AO3/4) - Meiosis

4.3.4Meiosis vs Mitosis

4.3.5End of Topic Test - Mutations, Meiosis

4.3.6A-A* (AO3/4) - DNA,Genes, CellDiv & ProtSynth

4.4Genetic Diversity & Adaptation

4.4.1Genetic Diversity

4.4.2Natural Selection

4.4.3A-A* (AO3/4) - Natural Selection

4.4.4Adaptations

4.4.5Investigating Natural Selection

4.4.6End of Topic Test - Genetic Diversity & Adaptation

4.4.7A-A* (AO3/4) - Genetic Diversity & Adaptation

4.5Species & Taxonomy

4.5.1Courtship Behaviour

4.5.2Phylogeny

4.5.3Classification

4.5.4DNA Technology

4.5.5A-A* (AO3/4) - Species & Taxonomy

4.6Biodiversity Within a Community

4.6.1Biodiversity

4.6.2Index of diversity

4.6.3Agriculture

4.6.4End of Topic Test - Species,Taxonomy& Biodiversity

4.6.5A-A* (AO3/4) - Species,Taxon&Biodiversity

4.7Investigating Diversity

4.7.1Genetic Diversity

4.7.2Quantitative Investigation

5Energy Transfers (A2 only)

5.1Photosynthesis

5.1.1Overview of Photosynthesis

5.1.2Photoionisation of Chlorophyll

5.1.3Production of ATP & Reduced NADP

5.1.4Cyclic Photophosphorylation

5.1.5Light-Independent Reaction

5.1.6A-A* (AO3/4) - Photosynthesis Reactions

5.1.7Limiting Factors

5.1.8Photosynthesis Experiments

5.1.9End of Topic Test - Photosynthesis

5.1.10A-A* (AO3/4) - Photosynthesis

5.2Respiration

5.2.1Overview of Respiration

5.2.2Anaerobic Respiration

5.2.3A-A* (AO3/4) - Anaerobic Respiration

5.2.4The Link Reaction

5.2.5The Krebs Cycle

5.2.6Oxidative Phosphorylation

5.2.7Respiration Experiments

5.2.8End of Topic Test - Respiration

5.2.9A-A* (AO3/4) - Respiration

5.3Energy & Ecosystems

5.3.1Biomass

5.3.2Production & Productivity

5.3.3Agricultural Practices

5.4Nutrient Cycles

5.4.1Nitrogen Cycle

5.4.2Phosphorous Cycle

5.4.3Fertilisers & Eutrophication

5.4.4End of Topic Test - Nutrient Cycles

5.4.5A-A* (AO3/4) - Energy,Ecosystems&NutrientCycles

6Responding to Change (A2 only)

6.1Nervous Communication

6.1.1Survival

6.1.2Plant Responses

6.1.3Animal Responses

6.1.4Reflexes

6.1.5End of Topic Test - Reflexes, Responses & Survival

6.1.6Receptors

6.1.7The Human Retina

6.1.8Control of Heart Rate

6.1.9End of Topic Test - Receptors, Retina & Heart Rate

6.2Nervous Coordination

6.2.1Neurones

6.2.2Action Potentials

6.2.3Speed of Transmission

6.2.4End of Topic Test - Neurones & Action Potentials

6.2.5Synapses

6.2.6Types of Synapse

6.2.7Medical Application

6.2.8End of Topic Test - Synapses

6.2.9A-A* (AO3/4) - Nervous Comm&Coord

6.3Muscle Contraction

6.3.1Skeletal Muscle

6.3.2Sliding Filament Theory

6.3.3Contraction

6.3.4Slow & Fast Twitch Fibres

6.3.5End of Topic Test - Muscles

6.3.6A-A* (AO3/4) - Muscle Contraction

6.4Homeostasis

6.4.1Overview of Homeostasis

6.4.2Blood Glucose Concentration

6.4.3Controlling Blood Glucose Concentration

6.4.4End of Topic Test - Blood Glucose

6.4.5Primary & Secondary Messengers

6.4.6Diabetes Mellitus

6.4.7Measuring Glucose Concentration

6.4.8Osmoregulation

6.4.9Controlling Blood Water Potential

6.4.10ADH

6.4.11End of Topic Test - Diabetes & Osmoregulation

6.4.12A-A* (AO3/4) - Homeostasis

7Genetics & Ecosystems (A2 only)

7.1Genetics

7.1.1Key Terms in Genetics

7.1.2Inheritance

7.1.3Linkage

7.1.4Multiple Alleles & Epistasis

7.1.5Chi-Squared Test

7.1.6End of Topic Test - Genetics

7.1.7A-A* (AO3/4) - Genetics

7.2Populations

7.2.1Populations

7.2.2Hardy-Weinberg Principle

7.3Evolution

7.3.1Variation

7.3.2Natural Selection & Evolution

7.3.3End of Topic Test - Populations & Evolution

7.3.4Types of Selection

7.3.5Types of Selection Summary

7.3.6Overview of Speciation

7.3.7Causes of Speciation

7.3.8Diversity

7.3.9End of Topic Test - Selection & Speciation

7.3.10A-A* (AO3/4) - Populations & Evolution

7.4Populations in Ecosystems

7.4.1Overview of Ecosystems

7.4.2Niche

7.4.3Population Size

7.4.4Investigating Population Size

7.4.5End of Topic Test - Ecosystems & Population Size

7.4.6Succession

7.4.7Conservation

7.4.8End of Topic Test - Succession & Conservation

7.4.9A-A* (AO3/4) - Ecosystems

8The Control of Gene Expression (A2 only)

8.1Mutation

8.1.1Mutations

8.1.2Effects of Mutations

8.1.3Causes of Mutations

8.2Gene Expression

8.2.1Stem Cells

8.2.2Stem Cells in Disease

8.2.3End of Topic Test - Mutation & Gene Epression

8.2.4A-A* (AO3/4) - Mutation & Stem Cells

8.2.5Regulating Transcription

8.2.6Epigenetics

8.2.7Epigenetics & Disease

8.2.8Regulating Translation

8.2.9Experimental Data

8.2.10End of Topic Test - Transcription & Translation

8.2.11Tumours

8.2.12Correlations & Causes

8.2.13Prevention & Treatment

8.2.14End of Topic Test - Cancer

8.2.15A-A* (AO3/4) - Gene Expression & Cancer

8.3Genome Projects

8.3.1Using Genome Projects

8.4Gene Technology

8.4.1Recombinant DNA

8.4.2Producing Fragments

8.4.3Amplification

8.4.4End of Topic Test - Genome Project & Amplification

8.4.5Using Recombinant DNA

8.4.6Medical Diagnosis

8.4.7Genetic Fingerprinting

8.4.8End of Topic Test - Gene Technologies

8.4.9A-A* (AO3/4) - Gene Technology

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