ATP: the cell’s energy currency
· ATP = adenosine triphosphate, a nucleotide used as the immediate energy currency of the cell.
· ATP is useful because it is small, soluble, mobile, and releases usable amounts of energy in one-step reactions.
· ATP hydrolysis: ATP → ADP + Pi, releasing energy for cell processes.
· ATP synthesis: ADP + Pi → ATP requires an input of energy from respiration.
· ATP powers active transport, anabolic reactions (for example protein synthesis), and movement such as cilia/flagella action, chromosome movement, or muscle contraction.
This diagram shows the ATP–ADP cycle: ATP hydrolysis releases energy for cell work, while energy from respiration is used to reform ATP from ADP and phosphate. It is useful for linking energy release and energy coupling in cells. Source
What cell respiration is
· Cell respiration is the set of reactions that produce ATP using energy released from carbon compounds.
· Main substrates are glucose and fatty acids, but many organic compounds can be respired.
· Do not confuse cell respiration with gas exchange: respiration is a metabolic pathway inside cells; gas exchange is movement of O₂/CO₂ between organism and environment.
· Aerobic respiration requires oxygen and occurs mainly in the mitochondrion.
· Anaerobic respiration in humans does not require oxygen, does not use mitochondria, and occurs in the cytoplasm.
This overview diagram shows how glycolysis, the link reaction/Krebs cycle, and the electron transport chain fit together. It is useful for seeing where ATP, CO₂, NADH, and O₂ are involved across the whole pathway. Source
Aerobic vs anaerobic respiration in humans
· Aerobic respiration uses oxygen, gives a much higher ATP yield, and occurs in the cytoplasm + mitochondria.
· Anaerobic respiration uses glucose only, gives a low ATP yield (2 ATP per glucose), and occurs in the cytoplasm only.
· In humans, anaerobic respiration forms lactate.
· In aerobic respiration, waste products are carbon dioxide and water.
· Simple word equation (aerobic): glucose + oxygen → carbon dioxide + water.
· Simple word equation (anaerobic, human): glucose → lactate.
· Exam tip: fatty acids can be used in aerobic respiration, but not in human anaerobic respiration.
Measuring respiration rate
· Respiration rate can be measured by oxygen uptake, carbon dioxide production, heat production, or substrate use.
· Always identify the independent variable, dependent variable, and appropriate controlled variables.
· Typical practicals may use respirometers, germinating seeds, yeast, or small invertebrates.
· Be able to calculate rate = change ÷ time from raw data.
· Watch units carefully, for example cm³ O₂ min⁻¹ or ppm CO₂ min⁻¹.
HL only: NAD, redox and hydrogen transfer
· NAD is a hydrogen carrier in respiration.
· Oxidation in respiration often means removal of hydrogen (dehydrogenation) from a substrate.
· When a substrate loses hydrogen, it is oxidized; when NAD gains hydrogen, it is reduced to reduced NAD.
· Respiration involves linked redox reactions: one substance is oxidized, another is reduced.
· Reduced NAD later transfers electrons to the electron transport chain.
HL only: glycolysis
· Glycolysis occurs in the cytoplasm.
· Glucose (6C) is converted to 2 pyruvate (3C) by a series of enzyme-catalysed steps.
· Key ideas to know: phosphorylation, lysis, oxidation, and ATP formation.
· Glycolysis produces a net gain of ATP and reduced NAD.
· Glycolysis does not require oxygen, so it occurs in both aerobic and anaerobic respiration.
This image shows the pathway from glucose to pyruvate and highlights where ATP is used and produced and where NAD is reduced. It is most useful for visualizing the sequence of the glycolysis stage. Source
HL only: anaerobic respiration and fermentation
· In human anaerobic respiration, pyruvate is converted to lactate.
· This regenerates NAD, allowing glycolysis to continue.
· The net yield remains 2 ATP per glucose because only glycolysis produces ATP.
· In yeast, anaerobic respiration also regenerates NAD, but the end products are ethanol + carbon dioxide.
· Yeast anaerobic respiration is used in brewing and baking.
· In baking, CO₂ causes dough to rise; in brewing, ethanol is the useful product.
This diagram shows the decision point after glycolysis: with oxygen, pyruvate enters aerobic respiration; without oxygen, it is used to regenerate NAD+ by fermentation. It is excellent for comparing aerobic and anaerobic fates of pyruvate. Source
HL only: link reaction and Krebs cycle
· In aerobic respiration, pyruvate enters the mitochondrion.
· Link reaction: pyruvate is oxidized and decarboxylated to form an acetyl group (2C), releasing CO₂ and producing reduced NAD.
· The acetyl group joins coenzyme A to form acetyl-CoA.
· In the Krebs cycle, the acetyl group combines with oxaloacetate (4C) to form citrate (6C).
· Through the cycle, citrate is converted back to oxaloacetate, so the cycle can continue.
· Per turn of the cycle, acetyl groups are oxidized and decarboxylated, producing CO₂, ATP, and reduced NAD.
· Students only need to name citrate and oxaloacetate as intermediates.
This diagram shows the Krebs (citric acid) cycle, including the entry of the acetyl group, formation of citrate, release of CO₂, and regeneration of oxaloacetate. It helps students track the carbon changes and the cyclical nature of the pathway. Source
HL only: electron transport chain, chemiosmosis and oxygen
· Reduced NAD transfers energy to the electron transport chain (ETC) in the inner mitochondrial membrane.
· It donates electrons to the first carrier and is converted back to NAD.
· As electrons flow along the ETC, energy is released and used to generate a proton gradient across the inner mitochondrial membrane.
· This proton gradient stores potential energy.
· Chemiosmosis occurs when protons flow back through ATP synthase.
· ATP synthase uses that energy to phosphorylate ADP to ATP.
· Oxygen is the terminal electron acceptor. It accepts electrons from the ETC and protons from the matrix to form water.
· Without oxygen, electron flow stops and aerobic ATP production by oxidative phosphorylation stops.
This diagram shows how proton flow through ATP synthase powers the formation of ATP from ADP + phosphate. It directly supports the idea of chemiosmosis in oxidative phosphorylation. Source
HL only: lipids vs carbohydrates as respiratory substrates
· Lipids yield more energy per gram than carbohydrates.
· This is because lipids contain more hydrogen and less oxygen, so they can be more fully oxidized.
· Carbohydrates can enter respiration through glycolysis, so they can be used in anaerobic respiration.
· Fatty acids are broken down into 2C acetyl groups that enter via acetyl-CoA.
· Therefore lipids feed into aerobic pathways, not glycolysis.
Checklist: can you do this?
· State the roles of ATP, ADP, NAD, and oxygen in cell respiration.
· Write the simple word equations for aerobic respiration and anaerobic respiration in humans.
· Compare aerobic and anaerobic respiration by oxygen requirement, location, ATP yield, substrate use, and end products.
· Explain how glycolysis, the Krebs cycle, and chemiosmosis contribute to ATP production.
· Calculate and interpret a rate of respiration from experimental or secondary data.
Common exam traps
· Do not say ATP is stored in large quantities; it is made and used continuously.
· Do not confuse anaerobic respiration in humans (lactate) with yeast fermentation (ethanol + CO₂).
· Do not say oxygen is needed for glycolysis; it is needed as the final electron acceptor in aerobic respiration.
· Do not confuse oxidation with just “adding oxygen” — in respiration it is often loss of hydrogen/electrons.
· Do not forget that mitochondria are needed for aerobic respiration but not anaerobic respiration in humans.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.