CHAPTER 3.2 SECONDARY PRODUCTION

Learning Outcomes

• By the end of Chapter 3 students should be able to:
  • Explain primary production
  • Understand the ecological importance of photosynthesis.
  • Describe the biochemical mechanism of photosynthesis (light reactions ➔ Calvin cycle).
  • Describe secondary production
  • Trace energy flow through trophic levels.
  • Relate secondary production to aerobic respiration at the cellular scale.
  • Outline the main biogeochemical cycles
  • Water, carbon, nitrogen, phosphorus (not detailed in the slides but listed as course goals).
  • Analyse biotic linkages
  • Biotic ↔ biotic (predation, competition, symbiosis).
  • Abiotic ↔ abiotic (physical & chemical interactions such as weathering, evaporation).
  • Biotic ↔ abiotic (organisms modifying pH, light penetration, nutrient pools).

Secondary Production

• Definition
  • Generation of new biomass by heterotrophs (consumers).
  • Represents the quantity of tissue synthesized from assimilated food.
  • Sometimes restricted to herbivores (with “tertiary production” for carnivores) but more commonly includes all heterotrophic biomass generation.
• Responsible organisms
  • Animals, protists, fungi, many bacteria.
• Conceptual linkage
  • Mirrors primary production but at consumer levels; depends on the transfer of organic matter between trophic levels.

Consumer Categories & Detritus

• Primary consumer – herbivore that eats producers.
• Secondary consumer – carnivore that eats herbivores.
• Tertiary consumer – carnivore that eats other carnivores.
• Detritivores / decomposers – organisms (bacteria, fungi, some invertebrates) that derive energy from detritus.
• Detritus – non-living organic material (dead bodies, feces, fallen leaves, wood) that can re-enter food webs via decomposers or be eaten directly by higher-level consumers.

Energy Flow Fundamentals

• Ultimate source for almost all ecosystems = sunlight.
• Producers capture light via photosynthesis:
  • $6 CO<em>2 + 6 H</em>2O \xrightarrow{light} C<em>6H</em>{12}O<em>6 + 6 O</em>2$
• Energy transfers through:
  • Food chains (linear)
  • Food webs (interconnected chains, more realistic)
  • Food/energy pyramids (graphical energetics)
• First Law of Thermodynamics
  • Energy can be transferred or transformed but cannot be created or destroyed.
• Second Law of Thermodynamics
  • Every energy transformation increases the entropy of the universe; usable energy is partly lost as heat.
  • Example: chemical → kinetic energy in a cheetah; heat + metabolic by-products released.

Biotic & Abiotic Components in Energy Diagrams

• Abiotic nutrient pool (CO₂, O₂, H₂O, minerals) ⇄ Producers ⇄ Consumers ⇄ Decomposers.
• Photosynthesis fixes carbon; aerobic respiration returns CO₂.
• Heat is dissipated at every transformation.

Trophic Relationships & Nutrient Transfers

• Arrows illustrate:
  • Consumption (producer → consumer)
  • Litterfall & deposition (biotic → non-living pool)
  • Decomposition & translocation (detritus → nutrients → producers)
• Two parallel chains often coexist:
  1. Grazing food chain – starts with living producers.
  2. Detrital food chain – starts with detritus; decomposers and detritivores dominate.

Ecological Pyramids

• Pyramid of Energy
  • Shows total incoming energy at each trophic level.
  • Energy expressed as dry-mass equivalents (kJ m⁻² yr⁻¹ or kcal units).
  • Energy always decreases upward due to heat loss; therefore pyramid is never inverted.
• 10 % Rule
  • Only ~$10\%$ of energy at one level is incorporated into biomass at the next.
  • Causes diminishing biomass & numbers in higher trophic levels.
• Major energy losses between levels
  • Uneaten material / capture inefficiency.
  • Indigestible fractions (egestion).
  • Metabolic heat after digestion & assimilation.
• Pyramid of Biomass
  • Depicts total dry mass (g m⁻²) of living tissue per trophic level at sampling time.
  • Calculated: $\text{Biomass}=\bar{W}\times N$ (average weight × number of organisms).
• Pyramid of Numbers
  • Simply counts individuals present per level.
  • Often upright but can invert when one large producer supports many small herbivores (e.g., tree ecosystem).

Aerobic Respiration: Linking Organismal & Ecosystem Energetics

• Cellular respiration couples ecosystem-scale energy flow to ATP production inside cells.
• Overall equation (aerobic):
  $C<em>6H</em>{12}O<em>6 + 6\,O</em>2 \rightarrow 6\,CO<em>2 + 6\,H</em>2O + \text{Energy (ATP + heat)}$

Catabolic Pathways

• Aerobic respiration – uses O₂ as final electron acceptor; yields max ATP (~30–32 per glucose).
• Anaerobic respiration – uses alternative acceptors (NO₃⁻, SO₄²⁻, etc.).

Redox Basics

• Oxidation – loss of electrons; Reduction – gain of electrons.
• Electron donor = reducing agent; electron acceptor = oxidizing agent.
• Energy in organic molecules is released when electrons shift to more electronegative atoms (e.g., O₂).

Stages of Cellular Respiration

1. Glycolysis (cytosol)
   • "Splitting sugar" – converts 1 glucose ➔ 2 pyruvate.
   • Two phases:
     • Energy investment: uses 2 ATP.
     • Energy payoff: produces 4 ATP + 2 NADH.
   • Net gain: $2\,ATP$ (substrate-level) + $2\,NADH$.
   • Occurs with or without O₂.
2. Pyruvate Oxidation (mitochondrial matrix)
   • Pyruvate transported via active carrier.
   • Three-step enzymatic complex:
   1. Carboxyl group released as $CO_2$.
   2. Remaining 2-C fragment oxidized; electrons to $NAD^+ \rightarrow NADH$.
   3. Coenzyme-A attaches ➔ Acetyl-CoA (high-energy thioester bond).
3. Citric (Krebs) Cycle
   • 8 enzymatic steps; oxaloacetate regenerated.
   • Per acetyl-CoA: $1\,ATP$ (as GTP), $3\,NADH$, $1\,FADH<em>2$, $2\,CO</em>2$.
4. Oxidative Phosphorylation
   • Electron Transport Chain (ETC)
     • Located on inner mitochondrial membrane.
     • Series of protein complexes + mobile carriers (ubiquinone Q, cytochromes c).
     • Electrons move from high to low free energy ➔ final acceptor $O<em>2$ forms $H</em>2O$.
   • Chemiosmosis
     • ETC pumps $H^+$ into intermembrane space ➔ electrochemical gradient (proton-motive force).
     • $H^+$ flows back via ATP synthase (rotor–stator enzyme) ➔ phosphorylation of $ADP + P_i \rightarrow ATP$.
   • Yields ~$26–28\,ATP$ per glucose.

ATP Accounting (ideal maximum per glucose)

• Glycolysis …… $2\,ATP$
• Citric cycle …… $2\,ATP$
• Oxidative phosphorylation …… $26–28\,ATP$
• Total ≈ $30–32\,ATP$ (≈34 % of glucose energy captured, the rest lost as heat).

Chemiosmosis in Mitochondria vs. Chloroplasts

• Mitochondria
  • Protons pumped into intermembrane space, diffuse back into matrix.
  • Energy source = oxidation of food molecules.
• Chloroplasts
  • Protons pumped into thylakoid space, diffuse back into stroma.
  • Energy source = light-driven electron flow (photosystems II & I).
• BOTH use ATP synthase and proton gradients; differ only in membrane orientation & energy source.
• Light reactions produce ATP + NADPH in stroma, supplying energy & reducing power for the Calvin cycle.

Key Numerical & Conceptual Take-aways

• Only ~$10\%$ of energy passes between successive trophic levels ➔ limits food-chain length (typically <6 levels).
• Energy pyramids are always upright; biomass & number pyramids may invert under special circumstances (e.g., aquatic systems with fast-turnover phytoplankton).
• Cellular respiration efficiency ≈ $34\%$; remainder contributes to ecosystem-level heat loss discussed in thermodynamic context.
• Ecosystem energetics & cellular metabolism are tightly linked: photosynthesis captures light energy ➔ organic molecules ➔ respiration liberates that energy as ATP & heat; biogeochemical cycles close the chemical loops.