C1.2 Cell respiration

ATP—adenosine triphosphate—is a small soluble nucleotide that distributes energy between energy-releasing reactions and energy-requiring cell work.

ATP can move within the cell, release a manageable amount of energy rapidly by hydrolysis and transfer its terminal phosphate to coupled reactions. It is continuously regenerated rather than stored in large quantities.

Its adenine, ribose and three-phosphate structure supports reversible cycling with ADP. Respiration supplies energy to form ATP; ATP hydrolysis then drives transport, synthesis or movement near the point of use.

A membrane pump couples ATP hydrolysis to a conformational change that moves ions against an electrochemical gradient, while ADP and phosphate are returned to ATP-producing pathways.

ATP is an energy-transfer currency, not the original source of energy and not long-term energy storage. Energy comes from reactions such as respiration and is partly transferred through ATP.

Hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and inorganic phosphate releases energy that can be coupled to cell work.

Energy from respiration is required to phosphorylate ADP + phosphate back to ATP. Rapid ATP ↔ ADP cycling links energy-yielding reactions to energy-requiring processes without requiring a large ATP store.

ATP supplies active transport across membranes, anabolic synthesis of macromolecules, movement of whole cells and movement of components such as chromosomes or motor proteins.

ATP hydrolysis powers a membrane pump; respiration then provides energy for ADP + Pi → ATP, allowing the same carrier system to support another round of transport.

ATP hydrolysis releases sufficient energy for many cell tasks, but a numerical kilojoule value is not required. ATP is recycled rather than used once.

Cell respiration is an enzyme-controlled system that transfers energy released from carbon compounds into ATP; glucose and fatty acids are major substrates, although other organic compounds can also be used.

Word equations: glucose + oxygen → carbon dioxide + water; glucose → lactate. In both cases energy released is transferred to ATP, but mitochondrial aerobic stages produce far more.

Cell respiration is chemical energy transfer inside cells; gas exchange is movement of oxygen and carbon dioxide across a surface. Mitochondria are required for aerobic but not human anaerobic respiration.

Cell-respiration rate can be measured as substrate use or product formation per unit time, with results normalized when organisms differ in mass or number.

In a respirometer, soda lime absorbs carbon dioxide, so a pressure or gas-volume decrease reflects oxygen uptake. Keep temperature constant in a water bath and use a control without respiring material to correct for non-biological pressure changes.

Use the calibrated capillary displacement to find oxygen-volume change, subtract the control change, divide by elapsed time, then divide by sample mass if samples are being compared.

A respirometer measures oxygen uptake only when carbon dioxide is appropriately absorbed and the system is sealed. Temperature, pressure, mass, activity and acclimation time must be controlled.

NAD is a hydrogen and electron carrier: it is reduced when it accepts hydrogen during dehydrogenation and later oxidized when it donates electrons.

Oxidation is electron loss. Removing hydrogen together with its electron oxidizes a respiratory substrate, while NAD gains that reducing power; oxidation and reduction therefore occur together in a redox reaction.

Carrier cycle: substrate loses hydrogen/electrons → NAD is reduced → reduced NAD transfers a pair of electrons to the mitochondrial electron transport chain → NAD is regenerated for further dehydrogenation.

Reduced NAD made during glycolysis, the link reaction or the Krebs cycle is reoxidized when its electrons enter the chain, allowing those pathways to keep accepting hydrogen.

NAD is not the terminal electron acceptor. In aerobic respiration oxygen ultimately accepts the electrons, while NAD cycles between oxidized and reduced forms.

Glycolysis is a linear cytoplasmic pathway in which one glucose is converted stepwise into two pyruvate, with a net yield of ATP and reduced NAD.

Phosphorylation uses ATP to activate the six-carbon molecule; lysis splits it into two three-carbon compounds; oxidation transfers hydrogen to NAD; ATP formation then occurs by substrate-level phosphorylation.

Each step is catalysed by a different enzyme. Track the required net outputs rather than intermediate names: two pyruvate, two ATP net and reduced NAD per glucose.

The ATP investment precedes later ATP-producing steps, so gross production is larger than the net gain. NAD must be regenerated for the oxidation step to continue.

Glycolysis does not require oxygen or mitochondria and does not completely oxidize glucose. Do not count invested ATP as part of the net yield.

When oxygen is unavailable, human and yeast cells regenerate NAD using pyruvate-derived reactions so glycolysis can continue with a net yield of two ATP per glucose.

In humans, pyruvate accepts hydrogen/electrons from reduced NAD and becomes lactate, regenerating NAD. In yeast, pyruvate loses carbon dioxide and is reduced to ethanol, also regenerating NAD.

The pathways are the same through glycolysis; they differ in the reaction that regenerates NAD and therefore in final products: human cells form lactate, whereas yeast forms ethanol + carbon dioxide.

Carbon dioxide released by yeast expands bread dough, while ethanol production is used in brewing. Neither fermentation route adds ATP beyond the two net ATP made in glycolysis.

The purpose of reducing pyruvate-derived molecules is NAD regeneration, not a high ATP yield. Human lactate formation does not release carbon dioxide.

In the mitochondrial matrix, the link reaction oxidizes and decarboxylates pyruvate and transfers the remaining two-carbon acetyl group to coenzyme A.

One carbon is released as carbon dioxide, hydrogen/electrons reduce NAD, and the 2C acetyl group joins coenzyme A to form acetyl-CoA for entry into the Krebs cycle.

Per pyruvate: one CO₂, one reduced NAD and one acetyl-CoA; no ATP is formed directly. Per glucose, these outputs double because glycolysis produces two pyruvate.

Carbohydrate-derived pyruvate and fatty-acid breakdown can both supply 2C acetyl groups; coenzyme A carries those groups into the Krebs cycle.

The link reaction is not a cycle and does not directly synthesize ATP. It links glycolysis or lipid breakdown to acetyl-group oxidation.

In the mitochondrial matrix, each 2C acetyl group from acetyl-CoA joins 4C oxaloacetate to form 6C citrate; subsequent reactions regenerate oxaloacetate.

The cycle includes two decarboxylations that release the two incoming carbons as carbon dioxide and four oxidations/dehydrogenations that transfer hydrogen to NAD or FAD.

Each turn yields carbon dioxide, reduced NAD, reduced FAD and one ATP or equivalent while regenerating oxaloacetate. One glucose supplies two acetyl groups, so the cycle turns twice.

Citrate is the named 6C intermediate and oxaloacetate the regenerated 4C acceptor; intermediate names between them are not required for this objective.

The Krebs cycle produces only a small amount of ATP directly. Most captured energy leaves in reduced carriers for later oxidative phosphorylation.

Reduced NAD from glycolysis, the link reaction and the Krebs cycle transfers a pair of electrons to the first carrier of the electron transport chain in the inner mitochondrial membrane.

As electrons pass between carriers, reduced NAD is converted back to NAD and released energy drives proton pumping from the matrix into the intermembrane space. Reduced FAD can also contribute electrons downstream.

The inner membrane's low proton permeability maintains both a concentration difference and electrical potential: an electrochemical proton gradient. Protein-complex names are not required.

Electron donation reoxidizes NAD so earlier respiration stages can continue, while proton pumping stores transferred energy in a gradient ready for ATP synthase.

The electron transport chain builds the gradient; it does not phosphorylate ADP directly. Pumping is matrix → intermembrane space, and chemiosmosis is the return flow.

Chemiosmosis converts the proton gradient into ATP. The inner mitochondrial membrane restricts proton movement except through ATP synthase, so H⁺ diffuses from the intermembrane space back into the matrix through this enzyme. The flow drives phosphorylation of ADP to ATP; this is oxidative phosphorylation. At the end of the electron transport chain, oxygen accepts electrons and H⁺ to form water. Without oxygen, electron flow stops, the gradient collapses and oxidative ATP production cannot continue.

• H⁺ flows down its electrochemical gradient through ATP synthase.
• ATP synthase uses this energy to phosphorylate ADP.
• Oxygen is the terminal electron acceptor and forms water.
• Oxygen removal stops the electron transport chain and oxidative phosphorylation.

Lipids release more energy per gram than carbohydrates because they contain less oxygen and more oxidizable carbon and hydrogen; they also yield more metabolic water.

Carbohydrates are hydrolysed to sugars that enter glycolysis quickly. Fatty acids are broken into 2C acetyl groups that enter aerobic respiration as acetyl-CoA and generate many reduced electron carriers.

Glycolysis and anaerobic respiration occur only when carbohydrate is the substrate. Lipid oxidation depends on mitochondrial aerobic pathways, whereas glycerol from triglycerides can enter carbohydrate metabolism.

During prolonged aerobic activity, repeated fatty-acid breakdown supplies many acetyl-CoA molecules and reduced carriers, giving a high ATP yield; rapid anaerobic ATP demand instead depends on carbohydrate.

Higher energy per gram does not mean lipid is always the fastest or only fuel. Oxygen availability, mobilization, enzyme capacity and activity intensity determine substrate use.