Biochemical Cycles: Carbon, Oxygen & Water

CARBON CYCLE

Role of Plants (Producers)

• Photosynthesis removes atmospheric carbon: $6CO<em>2 + 6H</em>2O + \text{light} \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Carbon Storage
– Newly-made glucose is built into cellulose, lignin, starch, oils, etc.
– Entire forest biomes = long-term “carbon sinks.”
• Transfer to Animals
– Herbivores eat leaves, fruits, roots → carbon enters food web.
– Every trophic level after herbivores ultimately traces its carbon to plant biomass.

Role of Animals (Consumers)

• Respiration returns carbon: $C<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + \text{ATP}$
• Decomposition (with fungi & bacteria)
– Breaks down organic matter → releases $CO_2$ to air and soluble carbonates to soil/water.
• Fossil-Fuel Formation
– Burial + heat + pressure over $\text{millions of years}$ → coal, oil, natural gas.
– Represents "geologic-time" carbon storage.

Cycle Summary & Significance

• Plants withdraw $CO<em>2$; animals, plants (at night), decomposers, and combustion return it. • Balance keeps atmospheric $CO</em>2$ near $\approx 0.04\%$ (pre-industrial). Excess from deforestation & burning fuels → climate change.

OXYGEN CYCLE

Role of Plants (Producers)

• Daytime photosynthesis releases $O<em>2$ (see photosynthesis equation above). • Major sources: tropical rainforests, temperate forests, and oceanic phytoplankton. • Maintains $\approx 21\%$ $O</em>2$ in troposphere—critical for aerobic metabolism.

Role of Animals (and Plants at Night)

• Respiration: consumes $O<em>2$, releases $CO</em>2$ (see respiration equation).
• Life-support interdependence: without plant $O<em>2$, aerobic organisms suffocate; without animal/decomposer $CO</em>2$, plants starve.

Cycle Summary

• DAY: Plants → $O<em>2$, take in $CO</em>2$.
• NIGHT: Plants + animals → use $O<em>2$, return $CO</em>2$.
• Self-regulating feedback keeps global $O<em>2$ and $CO</em>2$ within life-compatible ranges.

WATER CYCLE

Role of Plants

• Transpiration
– Root uptake → stomata release → atmospheric water vapour.
– Drives up to $10\%$ of global atmospheric moisture; key for cloud formation & regional rainfall.
• Water Uptake & Storage
– Cell turgor prevents wilting, moderates microclimate, reduces flood peaks, and limits soil erosion.

Role of Animals

• Respiration & Perspiration
– Exhale humid air; sweat evaporates → adds vapour.
• Excretion
– Urine & feces return liquid water + dissolved nutrients to soil and aquatic systems.
• Decomposition
– Body water → soil moisture, vapour.

Cycle Summary

1. Evaporation & Transpiration: liquid → vapour.

2. Condensation & Cloud Formation.

3. Precipitation: rain, snow.

4. Absorption/Infiltration & Run-off: water passes through biota, soil, rivers, aquifers.

5. Biological Storage/Release: plants & animals modulate timing/quantity.

CROSS-CYCLE CONNECTIONS, IMPLICATIONS & EXAMPLES

• Photosynthesis couples carbon & oxygen cycles; transpiration links water to both.
• Example Scenario: Clear-cutting a rainforest
– Immediate $\uparrow$ atmospheric $CO<em>2$ (lost sink), $\downarrow$ $O</em>2$ production, $\downarrow$ regional rainfall (less transpiration) → drought & habitat loss.
• Ethical/Practical Angle: Sustainable forestry & renewable energy mitigate anthropogenic disruption.
• Climate Change Link: $\approx 10,000$ million tonnes (Pg) of fossil-carbon emitted annually—exceeds natural re-absorption, intensifying greenhouse effect.
• Philosophical Note: Cycles illustrate Earth’s closed-system chemistry—matter is conserved, but human behaviour skews distribution and rates.

KEY FORMULAE & NUMERICAL REFERENCES

• Photosynthesis: $6CO<em>2 + 6H</em>2O + \text{light energy} \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Aerobic Respiration: $C<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + \text{ATP + heat}$
• Typical atmospheric concentrations: $CO<em>2 \approx 400$ ppm (pre-industrial $280$ ppm) ; $O</em>2 \approx 209,500$ ppm.
• Global annual fossil-fuel emissions: $\approx 10,000$ Mt $CO_2$-e (shown as "CO2 10000" in resource page).

STUDY REMINDERS

• Link diagrams/flow charts to each bullet set.
• Practice redrawing cycles with arrows noting: inputs, outputs, reservoirs, time scales.
• Be prepared to explain how removing one component (e.g., decomposers) breaks the cycle.
• Connect to previous lessons on trophic levels, energy flow, and ecosystem services.

QUICK QUIZ PROMPTS

• Why is the carbon cycle slower when carbon enters geologic reservoirs?
• How do oceans participate simultaneously in all three cycles?
• What would happen to global oxygen if phytoplankton populations collapsed?