12.1 Energy
- Syllabus
- 9700–2028–2029
- Topic
- 12.1
- Level
- A2
Learning objectives
12.1.1Energy needs in living organisms• Energy needs in living organisms: active transport, movement, anabolic reactions (DNA replication, protein synthesis)12.1.2ATP features as universal energy currency• ATP features as universal energy currency12.1.3ATP synthesis• ATP synthesis:- Substrate-linked reactions- Chemiosmosis in mitochondria and chloroplast membranes12.1.4Energy values of respiratory substrates• Relative energy values: carbohydrates, lipids, proteins as respiratory substrates12.1.5Respiratory quotient (RQ)• Respiratory quotient (RQ): ratio of CO2 produced to O2 taken in12.1.6RQ calculations• Calculate RQ values from respiration equations12.1.7Respirometer investigations• Investigate using respirometers: determine RQ of germinating seeds or invertebrates
Cellular work needs energy that ATP can couple to immediate tasks
Living cells need a continuous energy supply because maintaining life requires work. Energy is transferred to particular processes; it is not a material that a cell simply stores and spends unchanged.
- Transport: Active transport moves substances against a concentration gradient, and bulk transport such as exocytosis also requires coordinated cellular work.
- Synthesis and growth: Anabolic reactions join smaller units to build larger molecules such as DNA or proteins; cell growth therefore requires energy input.
- Movement and contraction: Motor proteins, muscle contraction and the spindle-driven movement of chromosomes all require energy to change position or shape.
- Division and control: Cell division reorganises and separates cell contents, while homeostatic control continually adjusts cellular conditions; these coordinated changes also require energy.
- Immediate coupling: ATP acts as a short-term, immediately usable carrier that transfers energy from energy-releasing reactions to a nearby energy-requiring task. It is the coupling intermediary, not the original respiratory substrate and not a long-term store.
Energy-releasing reactions → ATP as a transferable immediate carrier → a specific cellular task. The same principle can support different work types, but each task still requires its own molecular machinery and control.
Do not say that ATP is energy itself or that respiration directly “makes energy”. This card explains why work needs energy and the boundary of ATP’s role; ATP structure, hydrolysis and synthesis are developed in the next objectives, while respiratory-substrate values are later in Topic 12.1.
ATP is a small rechargeable energy carrier, not the cell’s long-term fuel store
ATP (adenosine triphosphate) is a phosphorylated nucleotide made from adenine, ribose and three phosphate groups. It is a small, soluble carrier of chemical potential energy that links energy-releasing reactions to cellular work.
- Structure: Adenine + ribose form the adenosine part; three phosphate groups make ATP a triphosphate nucleotide.
- Hydrolysis: ATP + water → ADP + inorganic phosphate (Pi). Removing the terminal phosphate releases energy that can be coupled to a cellular process; the amount is useful and controllable rather than an uncontrolled bulk release.
- Immediate use: ATP hydrolysis can occur close to the process requiring energy, so ATP can transfer energy within a cell rather than the cell moving a large store of fuel to every task.
- Rechargeable cycle: Energy-releasing reactions phosphorylate ADP with Pi to reform ATP. Hydrolysis and resynthesis therefore make a continually recycled ATP/ADP carrier cycle.
- Boundary: ATP is a short-term energy carrier and coupling molecule, not the original respiratory substrate and not a long-term energy store.
Energy released elsewhere → ADP + Pi are converted back to ATP → ATP is hydrolysed where work is required → ADP and Pi can be re-used. This coupling lets cells match energy transfer to particular tasks.
Do not call ATP “energy” itself, and do not treat the phosphate groups as a complete explanation without naming hydrolysis and coupling. The detailed routes that resynthesise ATP, including substrate-linked phosphorylation and chemiosmosis, belong to the next objective; respiratory-substrate energy values are later.
ATP is synthesised by direct phosphate transfer or chemiosmosis
ATP is synthesised when ADP is phosphorylated: ADP + Pi → ATP. Cells supply the phosphate and energy by two named routes.
| Route | How ATP is synthesised | Where it occurs |
|---|---|---|
| Substrate-linked phosphorylation | An enzyme transfers phosphate directly from a phosphorylated respiratory intermediate to ADP | Reactions such as glycolysis and the Krebs cycle |
| Chemiosmosis | Electron transfer builds a proton gradient across a membrane; protons flow through ATP synthase, which phosphorylates ADP | Inner mitochondrial membranes and chloroplast thylakoid membranes |
Substrate-linked phosphorylation transfers phosphate directly from a substrate; chemiosmosis depends on a membrane proton gradient and ATP synthase. Both form ATP from ADP, but they are not the same mechanism.
Lipids release more energy per gram than carbohydrates or proteins
| Respiratory substrate | Approximate energy value | Relative explanation |
|---|---|---|
| Carbohydrate | 17 kJ g⁻¹ | Less reduced than lipid, with fewer C–H bonds per unit mass |
| Protein | 17 kJ g⁻¹ | Similar gross energy value to carbohydrate; amino acids must first be deaminated and proteins have essential non-energy roles |
| Lipid | 37 kJ g⁻¹ | Many C–H bonds and a high hydrogen proportion make lipid more reduced |
Oxidising a more reduced substrate transfers more hydrogen/electrons to NAD and FAD. Reoxidation of more reduced coenzyme supports greater proton pumping, a larger chemiosmotic ATP yield and therefore more energy release per gram. This explains the high lipid value.
Energy value is energy released per unit mass, not RQ and not a statement that cells always respire lipid first. Protein is not reliably an intermediate energy category between carbohydrate and lipid.
Respiratory quotient reveals the balance of carbon dioxide production and oxygen uptake
Respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration: RQ = CO₂ produced ÷ O₂ consumed. It is a dimensionless gas-exchange clue, not a direct measure of energy expenditure.
- Why substrates differ: Carbohydrates, lipids and proteins have different molecular compositions, so their oxidation can produce different CO₂:O₂ relationships.
- Using RQ as evidence: A measured RQ can be compared with the typical pattern expected for a respiratory substrate, helping identify which substrate is contributing most under the measured conditions.
- Mixed substrates: A value between typical single-substrate patterns can indicate that more than one substrate is being respired; it should be treated as evidence about the mixture, not as proof of one pure substrate.
- Aerobic boundary: The usual RQ interpretation assumes oxygen uptake is part of the measured respiration. Anaerobic pathways do not all provide a normal finite RQ in the same way, so the context must be stated.
- Interpretation boundary: RQ is a ratio of gases over a defined interval. It is not the same as an energy value, an ATP yield, or a complete diagnosis of cellular metabolism.
Respiratory substrate and pathway → relative CO₂ output and O₂ uptake → measured RQ → cautious inference about substrate use. The inference is strongest when conditions are controlled and the sample is not simultaneously using several pathways or substrates.
Do not invert the ratio or treat one RQ reading as an absolute label. The next card teaches how to calculate RQ from a balanced equation; this card establishes meaning and interpretation only.
Balanced gas coefficients give respiratory quotient
RQ=molecules (or moles) of O2 taken inmolecules (or moles) of CO2 produced
- Balance the complete aerobic respiration equation. 2. Read the coefficient of CO2 for the numerator. 3. Read the coefficient of O2 for the denominator. 4. Divide and give suitable precision. Do not use substrate, water or atom totals.
Glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O, so RQ = 6/6 = 1.00.
Lauric acid: C12H24O2 + 17O2 → 12CO2 + 12H2O, so RQ = 12/17 = 0.71 (2 d.p.). The lower lipid RQ reflects greater oxygen demand relative to CO2 output.
Coefficients represent molecule or mole ratios; matching gas volumes may be used only under comparable conditions. An unbalanced equation gives an invalid RQ even if the division is performed correctly.
Matched respirometers separate oxygen uptake from carbon dioxide output
Use equal masses and comparable stages of germinating seeds or small invertebrates in sealed experimental tubes, plus an equal-volume inert control. Keep temperature in a water bath, apparatus gas volume, time and starting manometer position constant. Check for leaks and calibrate displacement to gas volume.
Condition A contains a CO2 absorbent such as soda lime or potassium hydroxide, isolated from organisms by gauze/cotton: CO2 is removed, so corrected gas-volume decrease x estimates O2 uptake. Condition B uses a matched sample without absorbent: corrected net decrease y = O2 uptake - CO2 output. Therefore CO2 output = x - y and RQ = (x - y)/x. Repeat with independent matched samples and calculate means.
For a capillary manometer, lower gas pressure draws the marker towards the organism tube. Measure distance over a defined time and convert using capillary cross-sectional area when volume or rate is required. Subtract control drift before calculation.
Prevent direct contact between organisms and corrosive absorbent; handle alkali with eye/skin protection and follow laboratory risk controls. Use appropriate organisms, avoid harmful temperatures or prolonged confinement, and release or dispose of material according to the approved protocol. Raw movement alone is not RQ.