C1.2 Cell respiration

Cell respiration transfers energy from organic compounds into ATP through glycolysis, anaerobic pathways and aerobic mitochondrial stages, supporting usable cellular work and measurable biological activity.

Learning objectives

C1.2.1ATP distributes energy• ATP is a small soluble nucleotide and universal energy currency• It links energy-yielding respiration to energy-requiring cellular processes C1.2.2Life processes using ATP• ATP supplies active transport, anabolic synthesis, and movement• Examples include membrane pumps, macromolecule synthesis, and chromosome movement C1.2.3ATP ↔ ADP interconversions• ATP hydrolysis to ADP and phosphate releases energy for coupled reactions• Respiration phosphorylates ADP to ATP; ATP is recycled rather than stored C1.2.4Cell respiration system• Cell respiration is enzyme-controlled release of energy from organic compounds• Carbon compounds are oxidized stepwise so energy can be transferred to ATP C1.2.5Anaerobic vs. aerobic respiration in humans• Aerobic respiration uses oxygen and mitochondria, producing CO₂, water, and high ATP• Anaerobic respiration in human cytoplasm produces lactate and low ATP C1.2.6Variables affecting rate• Rate depends on metabolic demand, organism size, oxygen, substrate, temperature, and pH• Respirometers measure oxygen uptake while soda lime absorbs CO₂

ATP is a small, soluble nucleotide found in all living cells. It transfers manageable amounts of energy between energy-yielding reactions and energy-requiring processes, and can move between nearby reaction sites in the aqueous cell.

$ATP + H_2O \rightarrow ADP + P_i$

$ADP + P_i \rightarrow ATP$

Cells recycle ATP rapidly rather than storing large reserves of it. Glucose, glycogen and lipids are more stable energy stores; respiration transfers some of their energy into ATP when and where work is needed.

Work powered by ATP	Example	How coupling helps
transport	membrane pump moves ions against a gradient	phosphorylation changes pump conformation
chemical synthesis	amino acids joined into a protein	coupled reactions make an otherwise unfavourable step proceed
mechanical movement	motor proteins move chromosomes or contractile filaments	ATP-driven conformational cycles generate force

ATP hydrolysis is useful because it is coupled directly to another process, often by transferring a phosphate or changing a protein's conformation. Free energy is not released as a detached substance that a cell later collects.

One ATP transfer supplies a small task; continuous ATP regeneration allows many molecular events to combine into cell movement, active transport and growth.

Cell respiration is the enzyme-controlled release of energy from organic compounds to produce ATP. Stepwise oxidation transfers energy in small amounts instead of releasing it suddenly as heat and light.

Glucose is converted to pyruvate, which enters a high-ATP aerobic route with oxygen or low-ATP anaerobic routes producing lactate or ethanol and carbon dioxide.

In humans	Aerobic route	Anaerobic route
oxygen	required for continued electron transport	not required
location	glycolysis in cytoplasm, then mitochondria	cytoplasm only
glucose oxidation	complete	incomplete
products	CO₂ and H₂O	lactate
ATP yield	high	net 2 ATP per glucose

Variable	Why rate may change
ATP demand	active muscle or transport increases ADP supply and pathway flux
organism size	smaller endotherms lose heat faster per unit mass and often need higher mass-specific respiration
oxygen	shortage limits aerobic electron transport and shifts ATP production toward low-yield anaerobic pathways
respiratory substrate	availability and entry route limit carbon and electron supply
temperature	enzyme activity rises to an optimum, then denaturation lowers rate
pH	altered charges and protein structure change respiratory enzyme activity

A measured correlation does not prove a single cause. For example, exercise changes ATP demand, oxygen delivery, temperature and substrate use together; a controlled experiment isolates one variable.

The organism consumes O₂ and releases CO₂. Soda lime absorbs the CO₂, so total gas volume falls by the volume of O₂ taken up; reduced pressure moves the capillary fluid toward the organisms.

A sealed respirometer contains a respiring organism and carbon dioxide absorbent; oxygen uptake lowers pressure and draws capillary fluid toward the chamber.

$\text{oxygen uptake rate}=\frac{\pi r^2 d}{t}$

Use a control tube with an equal volume of inert beads, keep temperature constant in a water bath, allow equilibration before sealing, repeat measurements, and prevent direct contact between organisms and soda lime.

organic substrate → stepwise oxidation in respiration → ATP regeneration → coupled transport, synthesis or movement → ADP + Pᵢ recycled

With adequate oxygen, aerobic respiration completes oxidation and yields much more ATP. Without enough oxygen, human cells can regenerate NAD through lactate formation so glycolysis continues, but only its small ATP yield remains.

Respiration rate can be estimated from O₂ uptake only when CO₂ is absorbed and temperature, pressure, organism amount and measurement time are controlled.

3 marks

Explain the properties of ATP that make it useful for distributing energy within cells.

5 marks

Outline, with examples, the wide range of uses of adenosine triphosphate (ATP) in cells.

1 mark

Which reaction does not cause a net release of energy?

8 marks

Explain the need for energy in cells and how energy is released through cell respiration.

8 marks

Carbon dioxide is released during cell respiration. Explain anaerobic and aerobic respiration.

3 marks

Describe how the apparatus measures the oxygen consumption of the mouse.

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—C1.3 PhotosynthesisRead

C1.2 Cell respiration

Learning objectives

ATP Is a Transfer Currency, Not a Long-Term Store

ATP Couples Energy Release to Three Kinds of Cellular Work

Respiration Releases Energy in Controlled Steps

Respiration Rate Reflects Demand, Supply and Enzyme Conditions

A Respirometer Turns Oxygen Uptake into a Rate

SL Checkpoint: Follow Energy from Fuel to Work

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