outline the need for energy in living organisms, as illustrated by active transport, movement and anabolic reactions, such as those occurring in DNA replication and protein synthesis

Energy in Living Organisms – Cambridge AS & A Level Biology (9700)

1. Syllabus Learning Outcomes & Assessment Objectives

• LO1 – Explain why living organisms need energy. (AO1, AO2)
• LO2 – Describe the structure of ATP and how it is synthesised. (AO1, AO2)
• LO3 – Analyse how ATP hydrolysis is coupled to non‑spontaneous processes. (AO2, AO3)
• LO4 – Outline the major catabolic pathways that generate ATP (glycolysis, link reaction, Krebs cycle, oxidative phosphorylation). (AO1, AO2)
• LO5 – Discuss anaerobic respiration and calculate respiratory quotients (RQ). (AO2, AO3)
• LO6 – Explain photophosphorylation and the Calvin‑Benson cycle. (AO1, AO2)
• LO7 – Evaluate the energy‑requiring processes of active transport, mechanical movement and anabolic reactions (DNA replication, protein synthesis). (AO2, AO3, AO4)
• LO8 – Relate practical investigations to the theory (e.g., measuring ATP, RQ, limiting‑factor experiments). (AO3, AO4)

2. Why Living Organisms Need Energy

All cellular activities that maintain order, enable growth and allow a response to the environment are energetically demanding. The syllabus groups these demands into three broad categories:

3. ATP – The Universal Energy Carrier

3.1 Structure and High‑Energy Bonds

• ATP = adenosine (adenine + ribose) + three phosphate groups (α, β, γ).
• The β‑γ phosphoanhydride bond is a “high‑energy” bond; hydrolysis releases ≈ ‑30.5 kJ mol⁻¹ (ΔG°′).
• Energy release is due to increased entropy, resonance stabilisation of the phosphate ions and reduced electrostatic repulsion.

3.2 Synthesis of ATP

1. Substrate‑level phosphorylation – direct transfer of a phosphate from a high‑energy intermediate to ADP.

   • Glycolysis: phosphoglycerate kinase, pyruvate kinase.
   • Krebs cycle: succinyl‑CoA synthetase.

2. Oxidative phosphorylation (mitochondria) – chemiosmotic coupling of electron transport to ATP synthase.

   • Complexes I–IV pump H⁺ across the inner mitochondrial membrane.
   • ATP synthase uses the H⁺‑motive force to convert ADP + Pᵢ → ATP.

3. Photophosphorylation (chloroplasts) – light‑driven electron transport creates a thylakoid H⁺ gradient that drives ATP synthase.

3.3 Coupling ATP Hydrolysis to Non‑spontaneous Reactions

For a process with ΔG > 0, coupling to ATP hydrolysis makes the overall reaction exergonic:

ΔG_overall = ΔG_process + ΔG_{ATP hydrolysis}

Because ΔG_{ATP hydrolysis} ≈ ‑30.5 kJ mol⁻¹, the sum is negative and the combined reaction proceeds.

4. Catabolic Pathways that Generate ATP (Energy Supply)

4.1 Glycolysis (Cytosol)

• One glucose → 2 pyruvate + 2 ATP (substrate‑level) + 2 NADH.
• Net ATP gain: 2 ATP per glucose.

4.2 Link Reaction (Mitochondrial matrix)

• Pyruvate + CoA + NAD⁺ → Acetyl‑CoA + CO₂ + NADH.
• Produces 1 NADH per pyruvate (2 NADH per glucose).

4.3 Krebs (Citric Acid) Cycle (Mitochondrial matrix)

• Each Acetyl‑CoA yields: 3 NADH, 1 FADH₂, 1 GTP (≈ ATP).
• Per glucose (2 Acetyl‑CoA): 6 NADH, 2 FADH₂, 2 GTP.

4.4 Oxidative Phosphorylation (Inner mitochondrial membrane)

• Electrons from NADH → Complex I → CoQ → Complex III → Cyt c → Complex IV (O₂ → H₂O).
• Electrons from FADH₂ enter at Complex II (bypass Complex I).
• ≈ 2.5 ATP per NADH and ≈ 1.5 ATP per FADH₂ (P/O ratios).
• Overall yield: ≈ 30 ATP per glucose in eukaryotes (including glycolytic ATP).

4.5 Anaerobic Respiration (Fermentation)

4.6 Respiratory Quotient (RQ) Calculation

RQ = CO₂ produced / O₂ consumed (molar). Example for glucose:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
RQ = 6 CO₂ / 6 O₂ = 1.0

Typical RQ values:

• Carbohydrate metabolism: ≈ 1.0
• Fat metabolism: ≈ 0.7
• Protein metabolism: ≈ 0.8

5. Photophosphorylation & the Calvin‑Benson Cycle (Energy from Light)

5.1 Light‑dependent Reactions (Thylakoid membranes)

• Photosystem II → H₂O → O₂ + electrons (photolysis).
• Electrons flow through the plastoquinone pool, Cyt b₆f complex (pump H⁺), plastocyanin to Photosystem I.
• Photosystem I reduces NADP⁺ → NADPH (using 2 photons).
• ATP synthase uses the H⁺ gradient → ≈ 3 ATP per NADPH (variable with species).

5.2 Light‑independent Reactions (Calvin‑Benson Cycle, Stroma)

1. Carbon fixation – 3 CO₂ + 3 RuBP → 6 3‑phosphoglycerate (PGA).
2. Reduction – PGA + ATP + NADPH → glyceraldehyde‑3‑phosphate (G3P).
3. Regeneration of RuBP – Uses additional ATP to reform RuBP.

Overall requirement per CO₂ fixed: 3 ATP + 2 NADPH.

5.3 Pigments & Action Spectra (Practical Link)

• Chlorophyll a (peak ≈ 660 nm), chlorophyll b (≈ 645 nm), carotenoids (≈ 470 nm).
• Action spectrum experiments relate wavelength to rate of O₂ evolution or CO₂ uptake – a typical A‑Level practical.

6. Energy‑Requiring Cellular Processes (Energy Demand)

6.1 Active Transport

• Na⁺/K⁺‑ATPase – 1 ATP moves 3 Na⁺ out, 2 K⁺ in; maintains resting potential.
• H⁺‑ATPase (vacuolar) – pumps H⁺ into vacuole; creates gradient for secondary transport.
• Ca²⁺‑ATPase (sarcoplasmic reticulum) – restores low cytosolic Ca²⁺ after contraction.

6.2 Mechanical Movement

• Muscle contraction – each myosin power stroke hydrolyses 1 ATP; cycle: attachment → power stroke → detachment → re‑cocking.
• Bacterial flagellar rotation – MotA/MotB uses the proton‑motive force; ATP required for flagellin synthesis and assembly.
• Cytoplasmic streaming (plants) – actin‑myosin interactions transport organelles; ATP provides the “walking” force.

6.3 Anabolic Reactions

DNA Replication (per replication fork)

• Helicase – ≈ 2 ATP per base pair opened.
• Primase – uses NTPs to lay RNA primers.
• DNA polymerase – incorporates dNTPs; energy from cleavage of the dNTP’s phosphates.
• DNA ligase – 1 ATP per phosphodiester bond formed.

Protein Synthesis (translation)

Energy cost per amino acid incorporated:

• tRNA charging (aminoacyl‑tRNA synthetase) – 2 ATP equivalents (ATP → AMP + PPᵢ).
• Initiation – 1 GTP hydrolysed to position the initiator tRNA.
• Elongation – 2 GTP per peptide bond (EF‑Tu delivers aa‑tRNA; EF‑G mediates translocation).
• Termination – 1 GTP hydrolysed by release factors.

7. Quantitative Summary of ATP (and GTP) Use

8. Linking Energy Supply to Energy Demand

• Catabolic pathways generate ATP (and NADH/NADPH) in proportion to substrate availability and cellular demand.
• All‑osteric regulation (e.g., ATP inhibition of phosphofructokinase) and feedback inhibition ensure that ATP production matches the consumption by active transport, movement and biosynthesis.
• When ATP demand spikes (e.g., intense muscle activity), glycolysis and oxidative phosphorylation accelerate; when demand falls, the ATP/ADP ratio rises, down‑regulating catabolism.

9. Practical Investigations (A‑Level)

10. Suggested Diagram (for revision)