Cambridge IGCSE Biology Lesson: Energy Flow and Food Chains/Food Webs

Overview

• Covers topics 19.1 energy flow and 19.2 food chains and food webs.
• Follows Cambridge syllabus for final exam preparation.

Key Learning Objectives

• Describe the flow of energy through living organisms.
• Construct and interpret food chains and food webs.
• Understand terms:
  • Producer
  • Consumer
  • Herbivore
  • Carnivore
  • Decomposer
  • Trophic level
• Impact of humans on food chains.
• Describe and interpret pyramids of numbers and biomass.
• (Extended) Describe and interpret pyramids of energy.
  • Understand inefficiency of energy transfer between organisms.
  • Efficiency of consuming crop plants vs. animal products.

Energy Flow

• Sun: Principal energy source for biological systems.
• Photosynthesis: Plants convert sunlight to chemical energy.
• Energy Transfer: Energy moves from organism to organism through food chains.
  • Lost as heat during respiration or decomposed by fungi/bacteria.

Food Chains

• Definition: Diagram showing energy transfer from one organism to the next.
• Roles:
  • Producer: Makes organic nutrients using sunlight.
  • Consumer: Feeds on other organisms (primary, secondary, tertiary, quaternary).
• Trophic Levels:
  • Producers (1st)
  • Primary Consumers (2nd) - Herbivores
  • Secondary, Tertiary, Quaternary Consumers - Carnivores
  • Decomposers: Obtain energy from dead material.
• Example: Sun → Grass → Grasshopper → Bird → Snake → Owl → Decomposers
• Arrows: Indicate direction of energy transfer.

Food Webs

• Definition: Network of interconnected food chains.
• Structure:
  • Producers at the base.
  • Consumers above (primary, secondary, etc.).
• Impact of Changes:
  • Human activities (e.g., over-harvesting, species introduction) affect populations.
  • Example: Arctic cod depletion affecting seals and polar bears.

Pyramids

• Pyramids of Numbers:
  • Show number of organisms at each trophic level.
  • Shape can vary (e.g., oak tree and caterpillars).
• Pyramids of Biomass:
  • Show mass of organisms at each trophic level.
  • Typically pyramidal shape.

Extended Content

• Pyramids of Energy:
  • Show energy flow between trophic levels.
  • Significant energy loss at each level (only ~10% transferred).
  • Biological systems rarely exceed 5 trophic levels.
• Energy Efficiency:
  • Shorter food chains are more efficient.
  • More efficient to consume plant crops than animal products.
  • Example: Direct consumption of corn/wheat vs. animal products.

Conclusion

• Complete coverage of energy flow and food chains/webs for IGCSE.
• Next topic: Nutrient cycles.