Need for Energy in Living Organisms

· Energy is needed for all metabolic activities in living organisms.
· Active transport: energy is required to move ions/molecules against a concentration gradient using carrier proteins.
· Movement: energy is needed for muscle contraction, movement of cilia/flagella, and movement of organelles or vesicles inside cells.
· Anabolic reactions: energy is required to build larger molecules from smaller molecules.
· Key examples of anabolic reactions: DNA replication and protein synthesis.
· Energy released from respiration is transferred to ATP, which directly supplies energy to cell processes.

ATP as the Universal Energy Currency

· ATP = adenosine triphosphate, made of adenine, ribose, and three phosphate groups.
· ATP is a good universal energy currency because it is small, soluble, and can move easily around cells.
· ATP releases energy quickly by hydrolysis of the terminal phosphate bond: ATP → ADP + Pi + energy.
· ATP releases energy in small, manageable amounts, so less energy is wasted as heat.
· ATP can be rapidly regenerated from ADP + Pi during respiration and photosynthesis.
· ATP is used directly to phosphorylate molecules, making them more reactive.
· ATP is not a long-term energy store; carbohydrates and lipids are better for storage.

This diagram shows the structure of ATP, including adenine, ribose, and the three phosphate groups. It helps explain why ATP can release energy when the terminal phosphate group is hydrolysed. Source

ATP Synthesis

· ATP is made by adding inorganic phosphate to ADP: ADP + Pi → ATP.
· ATP is synthesised by substrate-linked reactions and by chemiosmosis.
· Substrate-linked ATP synthesis: phosphate is transferred directly from a phosphorylated substrate to ADP.
· Chemiosmosis: ATP is made when protons (H⁺ ions) diffuse through ATP synthase down a proton gradient.
· Chemiosmosis occurs in the membranes of mitochondria during respiration and chloroplasts during photosynthesis.
· In mitochondria, chemiosmosis occurs across the inner mitochondrial membrane.
· In chloroplasts, chemiosmosis occurs across the thylakoid membrane.

This image shows chemiosmosis in mitochondria, where a proton gradient is used to drive ATP synthesis. It links directly to the syllabus requirement that ATP is synthesised by chemiosmosis in mitochondrial membranes. Source

Respiratory Substrates and Relative Energy Values

· Respiratory substrates are molecules that can be oxidised in respiration to release energy for ATP synthesis.
· Main respiratory substrates: carbohydrates, lipids, and proteins.
· Carbohydrates are commonly used as respiratory substrates and release energy quickly.
· Lipids have the highest energy value because they contain many hydrogen atoms relative to oxygen atoms.
· Lipids are more reduced than carbohydrates, so they release more energy when oxidised.
· Lipids require more oxygen for respiration, so they usually have a lower RQ than carbohydrates.
· Proteins can be respired, but they are usually used only when carbohydrates and lipids are unavailable.
· Before proteins are respired, amino acids must be deaminated to remove the amino group.
· Typical energy value pattern: lipids > proteins > carbohydrates.

Respiratory Quotient (RQ)

· Respiratory quotient (RQ) = ratio of carbon dioxide produced to oxygen taken in during respiration.
· Formula: RQ = CO₂ produced ÷ O₂ consumed.
· RQ can be calculated from a balanced respiration equation.
· For glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
· RQ for glucose = 6 ÷ 6 = 1.0.
· For lipids, RQ is usually about 0.7 because more oxygen is needed relative to carbon dioxide produced.
· For proteins, RQ is usually about 0.8, but exact values depend on the protein or amino acid respired.
· In exam questions, always use the numbers of molecules or volumes of CO₂ and O₂ given.
· If gases are measured under the same conditions, gas volumes are proportional to number of molecules, so volumes can be used in the RQ formula.

Calculating RQ from Equations

· Step 1: Check the respiration equation is balanced.
· Step 2: Identify the number of CO₂ molecules produced.
· Step 3: Identify the number of O₂ molecules used.
· Step 4: Calculate RQ = CO₂ ÷ O₂.
· Example carbohydrate: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, so RQ = 6 ÷ 6 = 1.0.
· Example lipid: C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O, so RQ = 16 ÷ 23 = 0.70.
· A higher RQ suggests more carbohydrate is being respired.
· A lower RQ suggests more lipid is being respired.

Simple Respirometers and RQ Investigations

· A respirometer measures changes in gas volume during respiration.
· Suitable organisms include germinating seeds or small invertebrates such as blowfly larvae.
· Germinating seeds are useful because they are actively respiring and do not photosynthesise if kept in darkness.
· Soda lime / potassium hydroxide absorbs carbon dioxide produced during respiration.
· If CO₂ is absorbed, any decrease in gas volume is due to oxygen uptake.
· A manometer or capillary tube with coloured liquid can show movement caused by changes in gas pressure.
· A control tube with glass beads of equal volume helps show that changes are due to respiration, not environmental factors.
· Apparatus should be left to equilibrate before measurements are taken.
· Keep temperature constant, often using a water bath, because temperature affects respiration rate and gas volume.
· Repeat measurements and calculate a mean to improve reliability.
· Use a known capillary radius and distance moved to calculate gas volume: volume = πr²h.

Determining RQ Using a Respirometer

· To find oxygen uptake, run the respirometer with soda lime/KOH present to absorb CO₂.
· The decrease in gas volume gives the volume of oxygen consumed.
· To help determine CO₂ produced, repeat the experiment without soda lime/KOH.
· Without CO₂ absorption, the gas volume change depends on both oxygen used and carbon dioxide produced.
· Use the measurements to calculate CO₂ produced and O₂ consumed, then calculate RQ = CO₂ ÷ O₂.
· In CIE practical questions, focus on identifying the independent variable, dependent variable, and key control variables.
· Important control variables: temperature, mass/number of organisms, time, volume of apparatus, and type/age of organism.
· Safety: handle soda lime/KOH carefully because it is corrosive.

Common Exam Mistakes

· Do not say ATP “stores large amounts of energy”; ATP is an immediate energy carrier, not a long-term store.
· Do not confuse substrate-linked ATP synthesis with chemiosmosis.
· Do not forget that chemiosmosis requires a membrane, a proton gradient, and ATP synthase.
· Do not calculate RQ using water or ATP numbers; use only CO₂ produced and O₂ consumed.
· Do not ignore balancing in respiration equations.
· Do not say lipids have more energy because they are “bigger” only; the key reason is that they contain more hydrogen atoms and are more reduced.
· Do not forget that soda lime/KOH absorbs CO₂, so gas volume decrease shows O₂ uptake.