Energy Flow & Nutrient Cycles: every key term you need (+ practice quiz)

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Photosynthesis

Process converting CO₂ + H₂O + light → glucose + O₂; occurs in chloroplasts.

Respiration

Process converting glucose + O₂ → CO₂ + H₂O + energy (ATP); occurs in mitochondria.

ATP

Adenosine triphosphate; energy currency of cells; energy released when losing phosphate group.

Photosystem

Light-harvesting complex in chloroplast; absorbs photons, transfers energy to electron transport chain.

Calvin Cycle

Light-independent reactions fixing CO₂ into sugars; requires ATP and NADPH from light reactions.

Stomata

Pores in leaf leaves opening for gas exchange; closed at night to conserve water.

C3 Photosynthesis

Standard pathway; first stable product has 3 carbons; efficient in cool, moist climates.

C4 Photosynthesis

Modified pathway; first stable product has 4 carbons; more efficient in hot, dry climates.

CAM Photosynthesis

Crassulacean acid metabolism; opens stomata at night to conserve water; drought-adapted.

Enzyme

Protein catalyst speeding reactions; lowers activation energy; not consumed in reaction.

Photosynthetic Efficiency

Percentage of light energy converted to chemical energy; typically 1-2% in field conditions.

Cellular Respiration

Metabolic process releasing energy from organic molecules; aerobic requires O₂, anaerobic does not.

Glycolysis

Process splitting glucose into pyruvate; occurs in cytoplasm; produces small amount of ATP.

Krebs Cycle

Aerobic respiration cycle producing NADH and FADH₂; occurs in mitochondrial matrix.

Electron Transport Chain

Series of proteins transferring electrons, pumping protons, producing large amount of ATP.

Chemiosmosis

ATP synthesis driven by proton gradient across mitochondrial membrane.

Fermentation

Anaerobic respiration producing ATP without oxygen; produces lactate or ethanol and CO₂.

Nitrogen Fixation

Conversion of N₂ → ammonia by bacteria; essential for protein synthesis.

Nitrogenase

Enzyme catalyzing nitrogen fixation; requires energy (ATP) to break N≡N bond.

Root Nodules

Structures on legume roots housing nitrogen-fixing bacteria; mutualistic relationship.

10% rule

Only about 10% of energy at one trophic level is stored in the next; the rest is lost as heat, waste, and unconsumed biomass.

Gross vs. net primary productivity

GPP is total energy captured by photosynthesis; NPP = GPP − plant respiration, the energy actually available to consumers.

Trophic efficiency

Fraction of energy transferred between levels, usually 5-20%; explains why food chains rarely exceed 4-5 links.

Biomass pyramid

Standing biomass per trophic level; can be inverted in aquatic systems where fast-turnover phytoplankton support more zooplankton mass.

Nitrogen fixation

Conversion of atmospheric N2 to ammonia (NH3/NH4+) by bacteria such as Rhizobium in legume nodules or by lightning and Haber-Bosch.

Nitrification

Bacterial oxidation of ammonium to nitrite (NO2−) then nitrate (NO3−), the form plants take up most readily.

Denitrification

Anaerobic bacteria convert nitrate back to N2 (and N2O), returning nitrogen to the atmosphere; common in waterlogged soils and wetlands.

Assimilation and ammonification

Assimilation: plants build N into proteins; ammonification: decomposers release NH4+ from dead organic matter.

Phosphorus cycle

Has no significant gaseous phase; phosphorus weathers from rock, cycles through organisms, and settles in sediments, making it a common limiting nutrient.

Limiting nutrient

Nutrient in shortest supply relative to need; phosphorus often limits freshwater lakes, nitrogen limits oceans, so adding it triggers algal blooms.

Carbon sinks and sources

Sinks (oceans, forests, soils) absorb more carbon than they release; sources (fossil fuel burning, deforestation) do the reverse.

Residence time

Average time an atom stays in a reservoir; carbon stays about 5 years in the atmosphere but millions of years in limestone.

Hydrologic cycle drivers

Solar energy powers evaporation and transpiration (evapotranspiration); gravity drives precipitation, runoff, and infiltration.

Aquifer recharge

Water percolating through soil to refill groundwater; slowed by pavement, compaction, and overdrafting.

Chemosynthesis

Bacteria at hydrothermal vents oxidize H2S to make sugars, supporting food webs with no sunlight.

Sulfur cycle

Sulfur moves from rocks and volcanoes to atmosphere (SO2) and oceans; human combustion adds SO2 that forms acid rain.