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Updated Mar 23, 2026

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3 pages

Fun WJEC A Level Biology: Knowledge Organizers, Past Papers & More!

Eliza

@3liza

Respiration and Energy Production in Cells: A Comprehensive Guide

Cellular... Show more

1 / 3

$*'universal energy carrier'→ ATP transfers energy for all biochemical reactions in the cells of all living organisms. $ADP + iP \xrightarro$

Stages of Cellular Respiration

This page details the three main stages of cellular respiration: glycolysis, the link reaction, and the Krebs cycle.

Glycolysis Summary:
- Occurs in the cytoplasm
- Converts 6C glucose to 3C pyruvate
- Net gain of two ATP
- Produces reduced NAD
Link Reaction:
- Takes place in the matrix of mitochondria
- Converts 3C pyruvate to acetyl coenzyme A
- Involves decarboxylation (removal of CO₂) and dehydrogenation (removal of hydrogen)
- Reduces NAD
Krebs Cycle:
- Occurs in the mitochondrial matrix
- Produces 1 ATP per cycle
- Generates FADH₂ (equivalent to 2 ATP in ETC) and NADH₂ (equivalent to 3 ATP)
- Happens twice per glucose molecule

Highlight: The Krebs cycle is a key process in aerobic respiration, occurring in the mitochondrial matrix and producing significant amounts of reduced coenzymes for the electron transport chain.

The page also provides information on different types of organisms based on their respiratory requirements:

Aerobes: Most life forms that use oxygen to respire
Facultative anaerobes: Organisms that can respire for a short time without oxygen
Strict anaerobes: Some bacteria species that can only respire in the absence of oxygen

Vocabulary: Decarboxylation is the removal of CO₂, while dehydrogenation is the removal of hydrogen.

$*'universal energy carrier'→ ATP transfers energy for all biochemical reactions in the cells of all living organisms. $ADP + iP \xrightarro$

The Krebs Cycle and ATP Production

This page provides a detailed diagram and explanation of the Krebs cycle, a crucial process in aerobic respiration that occurs in the mitochondrial matrix.

The Krebs cycle begins with a 4C oxaloacetate molecule and involves several steps:

2C acetyl fragment (from acetyl coenzyme A) combines with oxaloacetate to form 6C citrate
Through a series of reactions, the 6C citrate is converted back to 4C oxaloacetate
During this process, CO₂ is released (decarboxylation) and hydrogen is removed (dehydrogenation)

Highlight: The Krebs cycle produces 15 ATP per cycle, with a total of 30 ATP per glucose molecule (as the cycle occurs twice for each glucose molecule).

ATP production in the Krebs cycle:

1 ATP produced directly through substrate-level phosphorylation
4 pairs of hydrogen atoms carried by NAD (each pair generates 3 ATP in the ETC): 4 x 3 = 12 ATP
1 pair of hydrogen atoms carried by FAD (generates 2 ATP in the ETC): 1 x 2 = 2 ATP

Example: For each glucose molecule, the Krebs cycle occurs twice, resulting in 30 ATP (15 x 2) from this stage alone.

The page also summarizes the total ATP production during aerobic respiration:

Glycolysis: 2 ATP
Krebs cycle: 30 ATP
Total: 38 ATP per glucose molecule

Vocabulary: FAD (Flavin Adenine Dinucleotide) and NAD (Nicotinamide Adenine Dinucleotide) are important coenzymes in the electron transport chain.

$*'universal energy carrier'→ ATP transfers energy for all biochemical reactions in the cells of all living organisms. $ADP + iP \xrightarro$

ATP: The Universal Energy Carrier

ATP (Adenosine Triphosphate) plays a crucial role in cellular energy transfer for all living organisms. It consists of adenine, ribose, and three phosphate groups.

Definition: ATP (Adenosine Triphosphate) is the universal energy carrier that transfers energy for all biochemical reactions in cells.

ATP is synthesized through the process of phosphorylation, where ADP combines with inorganic phosphate. This reaction is catalyzed by ATP synthase:

ADP + iP → ATP

The reverse process, where ATP is broken down to release energy, is catalyzed by ATPase:

ATP → ADP + iP

Highlight: ATP hydrolysis releases 30.6 kJ/mol of energy, which is used for various cellular processes.

Advantages of ATP include:

Solubility for easy transport
Only one enzyme needed for hydrolysis
Easy to hydrolyze
Easily transported across membranes

The guide also introduces the concept of metabolism, which encompasses all chemical reactions in cells. These reactions are categorized into:

Catabolic reactions: Breaking down large molecules
Anabolic reactions: Building up large molecules

Vocabulary: Aerobes are organisms that use oxygen to respire, while anaerobes can respire without oxygen.

The electron transport chain (ETC) is a crucial part of aerobic respiration, involving the movement of electrons from higher to lower energy levels, releasing energy in the process. This energy is used to pump protons, creating an electrochemical gradient that drives ATP synthesis.

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Biology

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Updated Mar 23, 2026

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3 pages

Fun WJEC A Level Biology: Knowledge Organizers, Past Papers & More!

Eliza

@3liza

Respiration and Energy Production in Cells: A Comprehensive Guide

Cellular respiration is a fundamental process in living organisms, involving the breakdown of glucose to produce ATP, the universal energy carrier. This guide covers the key stages of respiration, including glycolysis,... Show more

$*'universal energy carrier'→ ATP transfers energy for all biochemical reactions in the cells of all living organisms. $ADP + iP \xrightarro$

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Stages of Cellular Respiration

This page details the three main stages of cellular respiration: glycolysis, the link reaction, and the Krebs cycle.

Glycolysis Summary:
- Occurs in the cytoplasm
- Converts 6C glucose to 3C pyruvate
- Net gain of two ATP
- Produces reduced NAD
Link Reaction:
- Takes place in the matrix of mitochondria
- Converts 3C pyruvate to acetyl coenzyme A
- Involves decarboxylation (removal of CO₂) and dehydrogenation (removal of hydrogen)
- Reduces NAD
Krebs Cycle:
- Occurs in the mitochondrial matrix
- Produces 1 ATP per cycle
- Generates FADH₂ (equivalent to 2 ATP in ETC) and NADH₂ (equivalent to 3 ATP)
- Happens twice per glucose molecule

Highlight: The Krebs cycle is a key process in aerobic respiration, occurring in the mitochondrial matrix and producing significant amounts of reduced coenzymes for the electron transport chain.

The page also provides information on different types of organisms based on their respiratory requirements:

Aerobes: Most life forms that use oxygen to respire
Facultative anaerobes: Organisms that can respire for a short time without oxygen
Strict anaerobes: Some bacteria species that can only respire in the absence of oxygen

Vocabulary: Decarboxylation is the removal of CO₂, while dehydrogenation is the removal of hydrogen.

$*'universal energy carrier'→ ATP transfers energy for all biochemical reactions in the cells of all living organisms. $ADP + iP \xrightarro$

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

The Krebs Cycle and ATP Production

This page provides a detailed diagram and explanation of the Krebs cycle, a crucial process in aerobic respiration that occurs in the mitochondrial matrix.

The Krebs cycle begins with a 4C oxaloacetate molecule and involves several steps:

2C acetyl fragment (from acetyl coenzyme A) combines with oxaloacetate to form 6C citrate
Through a series of reactions, the 6C citrate is converted back to 4C oxaloacetate
During this process, CO₂ is released (decarboxylation) and hydrogen is removed (dehydrogenation)

Highlight: The Krebs cycle produces 15 ATP per cycle, with a total of 30 ATP per glucose molecule (as the cycle occurs twice for each glucose molecule).

ATP production in the Krebs cycle:

1 ATP produced directly through substrate-level phosphorylation
4 pairs of hydrogen atoms carried by NAD (each pair generates 3 ATP in the ETC): 4 x 3 = 12 ATP
1 pair of hydrogen atoms carried by FAD (generates 2 ATP in the ETC): 1 x 2 = 2 ATP

Example: For each glucose molecule, the Krebs cycle occurs twice, resulting in 30 ATP (15 x 2) from this stage alone.

The page also summarizes the total ATP production during aerobic respiration:

Glycolysis: 2 ATP
Krebs cycle: 30 ATP
Total: 38 ATP per glucose molecule

Vocabulary: FAD (Flavin Adenine Dinucleotide) and NAD (Nicotinamide Adenine Dinucleotide) are important coenzymes in the electron transport chain.

$*'universal energy carrier'→ ATP transfers energy for all biochemical reactions in the cells of all living organisms. $ADP + iP \xrightarro$

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

ATP: The Universal Energy Carrier

ATP (Adenosine Triphosphate) plays a crucial role in cellular energy transfer for all living organisms. It consists of adenine, ribose, and three phosphate groups.

Definition: ATP (Adenosine Triphosphate) is the universal energy carrier that transfers energy for all biochemical reactions in cells.

ATP is synthesized through the process of phosphorylation, where ADP combines with inorganic phosphate. This reaction is catalyzed by ATP synthase:

ADP + iP → ATP

The reverse process, where ATP is broken down to release energy, is catalyzed by ATPase:

ATP → ADP + iP

Highlight: ATP hydrolysis releases 30.6 kJ/mol of energy, which is used for various cellular processes.

Advantages of ATP include:

Solubility for easy transport
Only one enzyme needed for hydrolysis
Easy to hydrolyze
Easily transported across membranes

The guide also introduces the concept of metabolism, which encompasses all chemical reactions in cells. These reactions are categorized into:

Catabolic reactions: Breaking down large molecules
Anabolic reactions: Building up large molecules

Vocabulary: Aerobes are organisms that use oxygen to respire, while anaerobes can respire without oxygen.