One small molecule pays for everything.

Why cells need ATP.

Spend ATP on active transport, then on movement, then on an anabolic reaction, and watch the ATP and ADP + Pi counters. Then choose "respire: remake ATP" and see what happens to them.

• Structure: ATP is a phosphorylated nucleotide: adenine, ribose and three phosphate groups (Fig. 1).
• Small amounts: hydrolysing one ATP releases about 30.5 kJ mol⁻¹, a convenient size for most reactions, so little energy is wasted as heat.
• Fast: hydrolysis is a single step catalysed by one enzyme (ATPase), so energy is available instantly.
• Easy to move, kept in: ATP is small and water-soluble, so it moves easily around the cell, but it cannot cross the cell surface membrane.
• Quickly remade: ATP is not stored; a cell recycles its ATP many times a minute, re-phosphorylating ADP during respiration.
• Universal: every known organism uses ATP, which is why it is called the universal energy currency.

ATP is synthesised in two ways: by substrate-linked phosphorylation, the direct transfer of a phosphate group from a substrate molecule to ADP (in glycolysis and the Krebs cycle), and by chemiosmosis, using a proton gradient across the membranes of mitochondria (respiration) and chloroplasts (photosynthesis).

Fuels and their RQ.

Pick carbohydrate, lipid, protein and anaerobic in turn. Count the oxygen molecules taken in and the carbon dioxide molecules produced, read the RQ, and compare the energy bars.

• Why lipids release most energy: most of the energy comes from oxidative phosphorylation, which uses hydrogen atoms carried by reduced NAD and FAD. Fatty acids have far more hydrogen atoms per gram than carbohydrates, so more reduced NAD and FAD, more protons pumped and more ATP. They also need more oxygen.
• Proteins give slightly more than carbohydrates, because they contain a little more hydrogen per gram; the amino groups are removed and excreted as urea, not respired.
• RQ definition: RQ = molecules of CO₂ produced ÷ molecules of O₂ taken in. Because equal numbers of gas molecules occupy equal volumes, volumes can be used instead.
• From an equation: glucose C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O gives 6 ÷ 6 = 1.0. Oleic acid C₁₈H₃₄O₂ + 25.5O₂ → 18CO₂ + 17H₂O gives 18 ÷ 25.5 = 0.71.
• Above 1: an RQ greater than 1 means some anaerobic respiration: yeast making ethanol releases CO₂ without taking in any O₂.

Measuring RQ with a respirometer.

Choose germinating peas, press Run for 10 minutes and record how far the liquid moves in both tubes. Repeat with sunflower seeds and blowfly larvae, then change the temperature.

• Tube A (potassium hydroxide): KOH absorbs all the CO₂ given out, so the drop of coloured liquid moves towards the organisms only because O₂ is taken in. Distance moved = oxygen uptake.
• Tube B (water instead): CO₂ is not absorbed, so the liquid moves by (O₂ taken in − CO₂ given out). CO₂ produced = distance A − distance B.
• RQ = (A − B) ÷ A. In the sim at 25 °C, tube A moves 18.0 mm for all three; tube B moves 0 mm for peas (RQ 1.0), 5.4 mm for sunflower seeds (RQ 0.70) and 1.8 mm for blowfly larvae (RQ 0.90).
• Volume: volume of gas = πr²h, where r is the radius of the capillary bore and h the distance moved. With r = 0.5 mm, 18.0 mm is about 14.1 mm³ of O₂.
• Good practice: stand the tubes in a water bath at constant temperature, let them equilibrate before timing, use the same mass of organisms in each tube, and set up a control with glass beads (or boiled seeds) of the same volume to detect changes in temperature or pressure.