Carbon, Nitrogen, and Population Ecology

Overview

• Part 2 of Topic 19: Organisms and Their Environment (IGCSE Biology).
• Covers nutrient cycles (carbon and nitrogen) and population ecology.
• Focus on key processes, organisms involved, and human impacts.

Carbon Cycle

• Carbon forms main part of biological molecules and cycles between atmosphere and organisms.
• Photosynthesis: plants remove CO2 from the atmosphere to form organic carbon.
• Feeding: carbon passes through trophic levels when organisms eat plants or other organisms.
• Respiration: plants, animals, and microorganisms release CO2 back to the atmosphere.
• Decomposition: dead organisms release CO2 via decomposers.
• Fossil fuels: under certain conditions, buried carbon forms fossil fuels over millions of years.
• Combustion: burning coal, oil, and gas releases extra CO2 into the atmosphere.
• Deforestation: reduces plant CO2 uptake and burning trees releases CO2, increasing atmospheric levels.

Nitrogen Cycle

• Nitrogen essential for amino acids, DNA, and ATP.
• Atmospheric N2 is unusable directly by most organisms.
• Nitrogen fixation: conversion of N2 to ammonia (NH3) by bacteria (soil or root nodules) or by lightning.
• Nitrification: nitrifying bacteria convert ammonia → nitrite → nitrate; plants preferentially absorb nitrate ions.
• Assimilation: plants use nitrate to make amino acids and proteins; animals obtain nitrogen by eating plants/animals.
• Decomposition: bacteria and fungi break down dead organic matter into ammonium compounds.
• Waste products: urea, ammonia, and uric acid contain nitrogen from deamination.
• Denitrification: denitrifying bacteria convert nitrates back to N2 gas in low-oxygen soils, completing the cycle.

Populations, Communities, and Ecosystems

• Population: group of organisms of one species in the same area at the same time.
• Community: all populations of different species in an ecosystem.
• Ecosystem: community plus the abiotic environment interacting as a unit.

Factors Affecting Population Growth

• Food supply: abundant food increases growth; scarcity slows growth.
• Competition: organisms compete for limited resources (food, water, space).
• Predation: higher predation tends to reduce population growth rates.
• Disease: spreads quickly in crowded populations and can cause major declines.

Population Growth Curve (Sigmoid)

• Sigmoid (S-shaped) growth occurs under limiting resources.
• Four phases:
  • Lag Phase: slow growth as population adapts to environment.
  • Exponential (Log) Phase: rapid growth; high birth rate, low death rate.
  • Stationary Phase: birth and death rates balance; population size stabilizes due to resource limits.
  • Death Phase: death rate exceeds birth rate; population declines.
• Causes of population decline: inadequate resources and disease spread.

Key Terms and Definitions

• Photosynthesis: process by which plants fix atmospheric CO2 into organic carbon.
• Respiration: metabolic process releasing CO2 from organic molecules.
• Nitrogen Fixation: conversion of atmospheric N2 into ammonia (NH3).
• Nitrification: microbial conversion of ammonia to nitrites and nitrates.
• Denitrification: microbial conversion of nitrates back to N2 gas.
• Decomposition: breakdown of dead organisms into simpler compounds by decomposers.
• Deamination: removal of amino groups from amino acids, producing urea.
• Trophic Level: position of an organism in a food chain or web.

Action Items / Next Steps (Study Recommendations)

• Review processes and organisms involved in carbon and nitrogen cycles.
• Memorize key terms and the sequence of steps in each nutrient cycle.
• Practice drawing labeled diagrams of both cycles and the sigmoid growth curve.
• Apply concepts to real-world impacts: deforestation, fossil fuel combustion, and soil oxygen status.
• Test understanding with example questions on nutrient transformations and population phases.