Food Chains, Food Webs & Nutrient Cycles

The Plumbing of Life: How Energy and Nutrients Move

Every organism eats something (except decomposers, which absorb organic matter). Every organism is eaten (or decomposes). Energy enters ecosystems as sunlight; nutrients cycle eternally. This chapter explores the two fundamental flows: energy flow (one-way: sunlight → heat) and nutrient cycling (circular: atmosphere → organisms → soil → atmosphere). Exams love testing specific cycles (carbon, nitrogen, phosphorus) and asking students to identify which organisms play which roles. Master this chapter, and you'll answer most ecology questions correctly.

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The Carbon Cycle: Life's Most Important Element

Carbon is the backbone of all organic molecules. It cycles constantly between the atmosphere, living organisms, oceans, and soil.

Major Carbon Reservoirs (Where Carbon is Stored)

1. Atmosphere: ~400 ppm CO2 (carbon dioxide).
2. Living organisms: ~3% of atmospheric CO2; mostly in plants (photosynthesis) and animals (respiration).
3. Soil: Organic carbon in dead plants, decomposing matter, soil organisms.
4. Oceans: Dissolved CO2 in water; bicarbonate ions; limestone and carbonate rocks on ocean floor.
5. Fossil fuels: Coal, oil, natural gas (ancient photosynthesis, buried millions of years).

The Carbon Cycle: Step-by-Step

1. Photosynthesis (Carbon Fixation)

• Plants absorb CO2 from atmosphere.
• Using sunlight, they convert CO2 to glucose (C6H12O6).
• Reaction: 6CO2 + 6H2O + sunlight → C6H12O6 + 6O2.
• Result: Carbon enters the biotic (living) cycle.

2. Consumption by Animals

• Herbivores eat plants (get carbon in plant tissue).
• Carnivores eat herbivores (carbon passes up food chain).
• Result: Carbon circulates through food webs.

3. Respiration (Carbon Release)

• All living organisms respire: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
• Plants respire (day and night); animals respire continuously.
• Result: CO2 is released back to atmosphere.

4. Decomposition

• Dead organisms and waste (feces, leaves) accumulate.
• Decomposers (bacteria, fungi) break down organic matter.
• Reaction: Organic carbon → CO2 (aerobic decomposition) or CH4 (anaerobic, in wetlands).
• Result: Carbon returns to atmosphere and soil.

5. Fossilization (Long-term Storage)

• Under anaerobic conditions (ocean sediments, swamps), organic matter doesn't fully decompose.
• Over millions of years, pressure and heat convert it to coal, oil, natural gas.
• Result: Carbon is locked away for millions of years.

6. Combustion (Fossil Fuel Burning)

• Humans extract and burn fossil fuels.
• Reaction: Fossil fuel + O2 → CO2 + H2O + energy.
• Result: Stored carbon is rapidly released to atmosphere (major cause of climate change).

The Carbon Cycle Diagram (Simplified)

ATMOSPHERE (CO2)
     ↓ (photosynthesis)
PLANTS → ANIMALS → DECOMPOSERS
     ↓ (respiration)
ATMOSPHERE (CO2)

SLOW CYCLE:
PLANTS/ANIMALS → SOIL (buried) → FOSSIL FUELS → (combustion) → ATMOSPHERE (CO2)
TIMELINE: Millions of years

Key Numbers for Exams:

• Atmospheric CO2: ~400–420 ppm (2024); rising 2–3 ppm/year.
• Photosynthesis: Removes ~50 billion tons CO2/year from atmosphere.
• Human emissions: ~37 billion tons CO2/year (mainly fossil fuels, deforestation).
• Net effect: Atmosphere gains ~10–15 billion tons CO2/year (imbalance).

[Memory Hook] "Carbon cycle = photosynthesis (fix) → respiration (release) → decomposition (recycle) → fossils (bury) → combustion (release)."

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The Nitrogen Cycle: The Protein Cycle

Nitrogen is essential for proteins and nucleic acids (DNA). Despite Earth's atmosphere being 78% nitrogen (N2), most organisms cannot use it directly. The nitrogen cycle converts atmospheric N2 into usable forms.

Key Nitrogen Compounds

• N2 (nitrogen gas): 78% of atmosphere; unusable by most organisms.
• NO3 (nitrate): Usable form for plants; found in soil.
• NH4+ (ammonium): Usable form for plants; found in soil, urine.
• NO2 (nitrogen dioxide): Atmospheric pollutant (from combustion).

The Nitrogen Cycle: Step-by-Step

1. Nitrogen Fixation (N2 → NH4+)

• Converts unusable atmospheric N2 into usable ammonia (NH4+).
• Who does it? Nitrogen-fixing bacteria:
  • Free-living: Azotobacter, Cyanobacteria (in soil, water).
  • Symbiotic: Rhizobium bacteria (live in legume root nodules: peas, beans, clover).
• Process: Bacteria use enzyme nitrogenase to break N2 triple bond; energy-expensive (requires ATP).
• Amount: ~200 million tons N fixed globally/year.

2. Assimilation (Uptake by Organisms)

• Plants absorb NO3- and NH4+ from soil via roots.
• Plants use nitrogen to synthesize amino acids, proteins.
• Animals eat plants (get nitrogen in protein).
• Result: Nitrogen circulates through food chains.

3. Ammonification (Decay)

• Dead organisms and animal waste contain nitrogen (proteins, nucleic acids).
• Decomposers (bacteria, fungi) break down proteins.
• Reaction: Proteins → Amino acids → Ammonia (NH3) / Ammonium (NH4+).
• Result: Ammonia returns to soil.

4. Nitrification (Oxidation)

• Nitrifying bacteria oxidize ammonia to nitrite, then nitrate.
• Step 1: Ammonia (NH3) → Nitrite (NO2-) [by Nitrosomonas bacteria].
• Step 2: Nitrite → Nitrate (NO3-) [by Nitrobacter bacteria].
• Result: Highly oxidized, stable form (NO3-) available to plants.
• Location: Happens in soil; requires oxygen (aerobic).

5. Denitrification (N-loss)

• Denitrifying bacteria convert NO3- back to N2 gas.
• Location: Anaerobic environments (swamps, wetlands, deep soil).
• Function: Removes nitrogen from soil; returns it to atmosphere.
• Result: Nitrogen leaves the biotic cycle; returns to atmosphere.

6. Industrial Fixation (Human-made)

• Haber-Bosch process: Convert N2 + H2 → NH3 (ammonia) at high temperature/pressure.
• Used to manufacture synthetic fertilizers.
• Amount: ~100 million tons artificial N/year (roughly half of all fixed nitrogen).
• Problem: Excess fertilizer runoff pollutes waterways (eutrophication).

The Nitrogen Cycle Diagram

ATMOSPHERE (N2)
     ↓ (N-fixation by bacteria)
SOIL (NH4+, NO3-)
     ↓ (assimilation)
PLANTS → ANIMALS
     ↓ (death, waste)
SOIL (proteins, waste)
     ↓ (ammonification)
SOIL (NH4+)
     ↓ (nitrification)
SOIL (NO3-)
     ↓ (denitrification by anaerobic bacteria)
ATMOSPHERE (N2)

Key Players:

• Nitrogen-fixing bacteria: Convert N2 to NH4+.
• Nitrifying bacteria: Convert NH4+ to NO3-.
• Denitrifying bacteria: Convert NO3- back to N2.

[Memory Hook] "N-fixation (break N2) → Assimilation (plants use) → Ammonification (decay) → Nitrification (oxidize) → Denitrification (release to air)."

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The Phosphorus Cycle: The Slower Cycle

Phosphorus is essential for energy molecules (ATP), nucleic acids (DNA/RNA), and bones/teeth. Unlike carbon and nitrogen, phosphorus doesn't exist as a stable gas; it cycles more slowly through soil and water.

Major Phosphorus Reservoirs

1. Rock: Phosphate-bearing minerals (apatite).
2. Soil: Released by weathering of rock.
3. Living organisms: Incorporated into organic molecules.
4. Ocean sediments: Accumulate over millions of years.

The Phosphorus Cycle: Step-by-Step

1. Weathering (Rock Dissolution)

• Rain (slightly acidic with CO2) dissolves phosphate-bearing rocks.
• Reaction: Rock (phosphate minerals) + H2O + CO2 → Soluble phosphate (PO4 3-).
• Result: Phosphate enters soil.
• Timeline: Slow; takes years to decades.

2. Assimilation (Plant Uptake)

• Plants absorb phosphate from soil via roots.
• Plants use phosphorus for nucleic acids, ATP, phospholipids.
• Animals eat plants (get phosphorus in organic molecules).

3. Death & Decomposition

• Dead organisms and waste return to soil.
• Decomposers break down organic matter; phosphate is released.
• Phosphate returns to soil, ready for plant uptake.

4. Leaching (Transport to Oceans)

• Soil phosphate is water-soluble; rain washes it into rivers and streams.
• Result: Phosphate reaches oceans.

5. Ocean Sedimentation

• Phosphate settles to ocean floor as sediment.
• Over millions of years, accumulates in sedimentary rocks.
• Timeline: Very slow; millions of years.

6. Uplift (Geological Return)

• Tectonic activity lifts ocean sediments.
• Erosion of uplifted rock releases phosphate back to land.
• Timeline: Millions of years (very slow).

The Phosphorus Cycle Diagram

ROCK (phosphate minerals)
     ↓ (weathering)
SOIL (soluble phosphate)
     ↓ (plant assimilation)
PLANTS → ANIMALS
     ↓ (death, waste)
SOIL (organic phosphate)
     ↓ (decomposition)
SOIL (soluble phosphate) / WATER (leaching)
     ↓
OCEAN (dissolved)
     ↓ (sedimentation)
OCEAN SEDIMENTS (rock)
     ↓ (geological uplift, millions of years)
ROCK

Key Difference from C & N Cycles: Phosphorus has NO atmospheric phase (no P2 gas). It cycles only through soil and water. This makes it slower; weathering and uplift take millions of years.

[Memory Hook] "Phosphorus = rock → soil → organisms → water → ocean → rock (very slow, no gas phase)."

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The Sulfur Cycle: The Often-Forgotten Cycle

Sulfur is essential for amino acids and proteins. Like phosphorus, sulfur cycles through soil, water, and atmosphere, but also exists as sulfur dioxide (SO2) from combustion and volcanic activity.

The Sulfur Cycle: Simplified

1. Weathering & Assimilation:

• Sulfate rocks weather; plants absorb sulfate (SO42-).
• Plants use sulfur for amino acids (methionine, cysteine).

2. Decomposition:

• Dead organisms release sulfur-containing compounds.
• Bacteria decompose these, releasing hydrogen sulfide (H2S, "rotten egg" smell).

3. Atmospheric Phase:

• Volcanic eruptions release SO2 (sulfur dioxide).
• Combustion of fossil fuels releases SO2 (major air pollutant).
• Bacterial decomposition releases H2S (minor source).

4. Oxidation & Precipitation:

• Atmospheric SO2 oxidizes to sulfuric acid (H2SO4); forms acid rain.
• Acid rain deposits sulfate back to soil.

5. Return to Soil:

• Sulfate accumulates in soil and water.
• Cycle repeats.

[Memory Hook] "Sulfur = rock → organisms → atmosphere (SO2, H2S) → acid rain → soil."

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Bioaccumulation vs. Biomagnification: A Critical Distinction

These two concepts are frequently confused in exams.

Bioaccumulation

Definition: Accumulation of a substance (usually toxic) in an organism over time.

Process:

1. Organism exposed to toxin (pesticide, heavy metal) from environment.
2. Organism can't break down toxin efficiently.
3. Toxin accumulates in body tissues over time.

Example:

• Fish exposed to mercury in water.
• Mercury accumulates in fish's tissues over weeks/months.
• Fish's mercury concentration becomes higher than surrounding water.

Formula: Organism's toxin level > Environmental toxin level.

Biomagnification

Definition: Increase in toxin concentration at EACH successive trophic level.

Process:

1. Toxin accumulates in producers (plants, algae).
2. Herbivores eat many producers, consuming all their accumulated toxin.
3. Carnivores eat many herbivores, consuming all their accumulated toxin.
4. At each step, toxin concentration INCREASES because toxin isn't metabolized (isn't energy; organism can't break it down).

Example:

• Pesticide (DDT) sprayed on crops.
• Concentration in plants: 1 part per million (ppm).
• Herbivores eat many plants; DDT concentration in herbivores: 10 ppm.
• Carnivores eat many herbivores; DDT concentration in carnivores: 100 ppm.
• Top predators eat many carnivores; DDT concentration: 1,000+ ppm (TOXIC).

Key Insight: Why does biomagnification happen?

• Energy transfer is 10% efficient (90% lost).
• Toxins are 100% efficient (not metabolized; can't be lost).
• Result: Toxin concentration increases as organism size decreases (top predators have smallest population but highest toxin).

Comparison Table

Feature	Bioaccumulation	Biomagnification
Definition	Toxin builds up in ONE organism	Toxin increases across trophic levels
What increases?	Toxin in one body	Toxin in successive trophic levels
Cause	Continuous exposure + slow metabolism	Food chain + 10% energy rule + 100% toxin retention
Example	Fish in polluted water accumulates mercury	DDT in plants → herbivores → carnivores

[Exam Trap] A question might ask, "Which process involves toxin concentration increasing at higher trophic levels?" Answer: Biomagnification. Bioaccumulation is a single organism's accumulation.

[Memory Hook] "Bioaccumulation = ONE organism (over time). Biomagnification = UP the food chain (level to level)."

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Biogeochemical Cycles: Master Summary

Cycle	Element/Molecule	Reservoirs	Key Process	Slow/Fast
Carbon	C, CO2	Atmosphere, organisms, soil, fossils, oceans	Photosynthesis ↔ Respiration	Fast (respiration); Very slow (fossilization)
Nitrogen	N2, NH4+, NO3-	Atmosphere, soil, organisms	N-fixation → Nitrification → Denitrification	Medium (bacteria-dependent)
Phosphorus	PO4 3-	Rock, soil, organisms, water	Weathering → Assimilation → Sedimentation	Very slow (no atmospheric phase)
Sulfur	SO2, H2S, SO4 2-	Rock, soil, organisms, atmosphere	Weathering → Decomposition → Oxidation	Medium (volcanic + industrial input)
Water	H2O	Ocean, atmosphere, ice, soil, organisms	Evaporation → Condensation → Precipitation	Fast (24-hour to yearly cycles)

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Exam Traps & Memory Hooks

[Exam Trap] Nitrogen cycle has an atmospheric phase (N2 gas); phosphorus cycle does NOT. This makes the P-cycle much slower.

[Exam Trap] "Nitrogen-fixing bacteria" break N2 triple bond; they're NOT the same as nitrifying bacteria (which oxidize ammonia).

[Exam Trap] Biomagnification requires bioaccumulation to occur first; but bioaccumulation doesn't always lead to biomagnification.

[Exam Trap] Decomposers participate in ALL nutrient cycles. They're not "producers" or "consumers"; they're essential recyclers.

[Memory Hook] "Carbon = atmosphere (fast); Nitrogen = atmosphere (slow, bacteria); Phosphorus = NO atmosphere (very slow); Sulfur = atmosphere (volcanic + pollution)."

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Practice MCQs

1. Which process converts atmospheric CO2 into organic compounds? A) Respiration B) Photosynthesis C) Decomposition D) Combustion

2. The 10% rule applies to: A) Carbon cycle B) Nitrogen cycle C) Energy flow (food chains) D) Phosphorus cycle

3. Which organisms are responsible for nitrogen fixation? A) Plants B) Animals C) Nitrogen-fixing bacteria D) Decomposers

4. Nitrogen gas (N2) is converted to usable forms by: A) Nitrifying bacteria B) Denitrifying bacteria C) Nitrogen-fixing bacteria D) Decomposers

5. The process by which NO3- is converted back to N2 is: A) Nitrification B) Ammonification C) Denitrification D) Nitrogen fixation

6. Phosphorus, unlike carbon and nitrogen, lacks: A) Living organisms B) An atmospheric phase C) Soil reservoir D) Decomposition

7. Which cycle is slowest in returning nutrients to organisms? A) Carbon cycle B) Nitrogen cycle C) Phosphorus cycle D) Water cycle

8. Bioaccumulation refers to: A) Toxin concentration increasing at higher trophic levels B) Toxin building up in a single organism over time C) Toxin metabolism by an organism D) Toxin breakdown by decomposers

9. Biomagnification occurs because: A) Toxins are metabolized efficiently B) Toxins are not metabolized; they accumulate at each trophic level C) Toxins only affect herbivores D) Toxins are broken down by bacteria

10. In a food chain: Plant (DDT = 1 ppm) → Herbivore → Carnivore → Top Predator. If each level consumes 10 organisms from the level below, what's the DDT concentration in the top predator (approximately)? A) 1 ppm B) 10 ppm C) 100 ppm D) 1,000+ ppm

11. Which of the following is involved in the nitrogen cycle but NOT in the carbon cycle? A) Atmosphere B) Organisms C) Nitrogen-fixing bacteria (N2 fixation) D) Respiration

12. The major human activity that disrupts the carbon cycle is: A) Photosynthesis B) Decomposition C) Burning fossil fuels D) Nitrogen fixation

13. Phosphorus-bearing rocks weather to release: A) NO3- B) PO4 3- C) SO2 D) N2

14. Nitrifying bacteria convert: A) N2 to NH4+ B) NH4+ to NO3- C) NO3- to N2 D) Proteins to amino acids

15. Which reservoir stores most of Earth's phosphorus? A) Atmosphere B) Organisms C) Rock and sediments D) Soil only

16. The carbon cycle's atmospheric phase involves: A) N2 gas B) CO2 gas C) SO2 gas D) P gas (no such thing)

17. Decomposers' role in nutrient cycles is to: A) Fix nitrogen from atmosphere B) Break down dead organic matter; release nutrients to soil C) Photosynthesis D) Consume living animals

18. Which process occurs ONLY in anaerobic (oxygen-free) environments? A) Photosynthesis B) Nitrification C) Denitrification D) Respiration

19. Acid rain is caused primarily by: A) Carbon dioxide in atmosphere B) Nitrogen oxides (NO, NO2) C) Sulfur dioxide (SO2) from combustion D) Methane emissions

20. The Haber-Bosch process is used for: A) Nitrogen fixation (artificial fertilizer production) B) Nitrification C) Denitrification D) Carbon fixation

21. If a pesticide (like DDT) is sprayed on crops, which organism will have the HIGHEST concentration? A) Crop plant B) Herbivore eating crop C) Carnivore eating herbivore D) Top predator (hawk eating carnivore)

22. Legume plants (peas, beans) have root nodules containing: A) Nitrifying bacteria B) Rhizobium nitrogen-fixing bacteria C) Denitrifying bacteria D) Decomposers

23. Which cycle directly involves the atmosphere as a major phase? A) Phosphorus cycle B) Carbon cycle C) Nitrogen cycle D) Both B and C

Answer Key: 1-B, 2-C, 3-C, 4-C, 5-C, 6-B, 7-C, 8-B, 9-B, 10-D, 11-C, 12-C, 13-B, 14-B, 15-C, 16-B, 17-B, 18-C, 19-C, 20-A, 21-D, 22-B, 23-D