Bioenergetics & Ecology: Photosynthesis, Food Webs, Matter Cycles & Symbiosis

A core theme of the GED Science test is the fundamental contrast between how ENERGY and MATTER move through the biosphere: energy flows in a ONE-WAY path—entering as sunlight captured by photosynthetic producers and dissipating as metabolic heat at each trophic level—whereas matter (carbon, nitrogen, oxygen, and water atoms) is recycled continuously in closed biogeochemical loops between organisms and the environment. In this lesson we connect the cellular equations of photosynthesis and cellular respiration (L.b.1) to ecosystem food webs and the 10% energy pyramid (L.b.2), trace the carbon, nitrogen, and water cycles alongside carrying capacity (L.b.3), and classify ecological interactions including predation, competition, mutualism, commensalism, and parasitism (L.c.1).

1. Photosynthesis & Cellular Respiration (L.b.1): • Photosynthesis (in chloroplasts of plants, algae, cyanobacteria): Captures solar energy to build chemical energy: 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ (glucose) + 6O₂. • Aerobic Cellular Respiration (in mitochondria of BOTH plants and animals!): Breaks down glucose in the presence of oxygen to generate usable cellular energy (ATP) and heat: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP + Heat. Remember that plants perform BOTH photosynthesis (in light) and cellular respiration (24 hours a day)! 2. Food Webs, Trophic Levels & the 10% Energy Rule (L.b.2): • Trophic Hierarchy: Level 1 = Producers / Autotrophs (plants, phytoplankton) → Level 2 = Primary Consumers (herbivores) → Level 3 = Secondary Consumers (carnivores/omnivores eating herbivores) → Level 4 = Tertiary Consumers (apex predators). Decomposers (fungi, bacteria) recycle dead organic matter from all levels. • The 10% Rule: Only about 10% of the chemical energy stored at one trophic level is converted into biomass at the next higher trophic level; the other ~90% is used for cellular respiration and lost to the environment as heat. Thus if producers capture 10,000 kcal, primary consumers get ~1,000 kcal, secondary consumers get ~100 kcal, and tertiary consumers get ~10 kcal. 3. Matter Cycles & Carrying Capacity (L.b.3): • Carbon & Oxygen Cycles: Photosynthesis removes CO₂ from the air and releases O₂; respiration, decomposition, and fossil-fuel combustion return CO₂ to the air and consume O₂. • Nitrogen Cycle: Atmospheric N₂ gas (78% of air) cannot be used directly by plants. Nitrogen-fixing bacteria (in soil and legume root nodules) convert N₂ into ammonia/nitrates (NH₄⁺ / NO₃⁻) that plants absorb to build proteins and DNA; denitrifying bacteria return N₂ to the air. • Carrying Capacity (K): The maximum population size an environment's finite resources (food, water, habitat, space) can sustain long-term. Without limiting factors, populations grow exponentially (J-curve); as limiting factors slow growth, the population levels off around K (logistic S-curve). 4. Ecological Relationships (L.c.1): • Predation (+ / −): Predator hunts and kills prey (populations oscillate in out-of-phase cycles). • Competition (− / −): Two species compete for the same limited resource. • Symbiosis (close long-term interaction): - Mutualism (+ / +): Both species benefit (e.g., bees pollinating flowers; nitrogen-fixing bacteria in legume roots). - Commensalism (+ / 0): One species benefits and the other is unaffected (e.g., barnacles riding on a whale). - Parasitism (+ / −): Parasite feeds on host without immediately killing it (e.g., ticks on a deer, tapeworms).