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biology-sl | ib
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1. Unity and Diversity
2. Continuity and Change
2.1.1 Concept of water potential in plants
2.1.2 Osmosis and water movement in cells
2.1.3 Importance in plant hydration and transport
2.2.1 Mitigation strategies and global agreements
2.2.2 Causes and effects of climate change
2.2.3 Greenhouse gases and global warming
2.3.1 Asexual vs sexual reproduction
2.3.2 Sexual cycles in animals and plants
2.3.3 Reproductive strategies
2.4.1 Mendelian inheritance
2.4.2 Punnett squares and genetic ratios
2.4.3 Genetic disorders and gene linkage
2.5.1 DNA structure and replication mechanism
2.5.2 Enzymes involved in DNA replication
2.5.3 Replication errors and repair mechanisms
2.6.1 Mechanisms of temperature, pH, and water balance
2.6.2 Regulation of blood glucose levels
2.6.3 Feedback loops in homeostasis
2.7.1 Transcription and translation processes
2.7.2 Role of mRNA, tRNA, and ribosomes
2.7.3 Post-translational modifications
2.8.1 Mechanisms of natural selection
2.8.2 Adaptations and evolutionary fitness
2.8.3 Evidence for evolution
2.9.1 Sustainable agricultural practices
2.9.2 Renewable resources and ecological conservation
2.9.3 Impact of human activity on the environment
2.10.1 Types of mutations: Point, frameshift, chromosomal
2.10.2 Causes and effects of mutations
2.10.3 Gene editing technologies (e.g. CRISPR)
2.11.1 Mitosis vs meiosis
2.11.2 Phases of mitosis and meiosis
2.11.3 Chromosome number and genetic variation
3. Interaction and Interdependence
3.1.1 Enzyme kinetics and inhibition
3.1.2 Metabolic pathways: Anabolism and catabolism
3.1.3 ATP synthesis and energy transfer
3.1.4 Enzyme structure and function
3.2.1 Vaccines and immunity
3.2.2 Disorders of the immune system (e.g. autoimmune diseases)
3.2.3 Immune response: Innate vs adaptive immunity
3.2.4 Antigens and antibodies
3.3.1 Population dynamics: Growth models, carrying capacity
3.3.2 Community interactions: Competition, predation, mutualism
3.3.3 Succession in ecosystems
3.4.1 Glycolysis, Krebs cycle, and oxidative phosphorylation
3.4.2 Aerobic vs anaerobic respiration
3.4.3 Role of mitochondria in energy production
3.4.4 Energy yield in cellular respiration
3.6.1 The light-dependent and light-independent reactions
3.6.2 Chloroplast structure and function
3.6.3 Photosystem II and I
3.6.4 Calvin cycle and carbon fixation
3.7.1 Structure of neurons
3.7.2 Action potential and neurotransmission
3.7.3 Synaptic transmission
3.7.4 Nervous system coordination and reflex arcs
3.8.1 Homeostasis and feedback mechanisms
3.8.2 Endocrine and nervous system integration
3.8.3 Coordination of digestive, excretory, and circulatory systems
4. Form and Function
4.1.1 Compartmentalization of processes in eukaryotic cells
4.1.2 Role of organelles in cellular function
4.1.3 Endoplasmic reticulum, Golgi apparatus, mitochondria, and nucleus
4.2.1 Stem cells and differentiation
4.2.2 Types of specialized cells (e.g. muscle, nerve, blood)
4.2.3 Tissue types and their functions
4.3.1 Respiratory structures in animals and plants
4.3.2 Mechanisms of gas exchange: Diffusion, active transport
4.3.3 Human respiratory system
4.3.4 Plant gas exchange: Stomata, leaf structure
4.4.1 Structure and function of carbohydrates and lipids
4.4.2 Metabolism of glucose
4.4.3 Role of lipids in energy storage
4.4.4 Membrane structure: Phospholipid bilayer
4.5.1 Circulatory systems: Open vs closed
4.5.2 Blood circulation in mammals (cardiovascular system)
4.5.3 Role of the heart and blood vessels
4.5.4 Transport of gases and nutrients in animals and plants
4.6.1 Structure of proteins: Primary to quaternary structure
4.6.2 Enzymes as biological catalysts
4.6.3 Protein folding and denaturation
4.6.4 Functions of proteins in cells
4.7.1 Structural, behavioral, and physiological adaptations
4.7.2 Adaptation to extreme environments (e.g. deserts polar regions)
4.7.3 Adaptations in animals and plants to survive in specific habitats
4.8.1 Structure and properties of cell membranes
4.8.2 Passive transport: Diffusion, osmosis
4.8.3 Active transport: Pumps, endocytosis, exocytosis
4.8.4 Membrane potential and action potentials
4.9.1 Ecological niche concept
4.9.2 Role of organisms in ecosystems
4.9.3 Competition, predation, and symbiosis
4.9.4 Fundamental vs realized niches
5. Experimental Programme
Energy flow in ecosystems: Trophic levels and food webs

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Energy Flow in Ecosystems: Trophic Levels and Food Webs

Introduction

Energy flow is a fundamental concept in understanding ecosystems, illustrating how energy moves through different organisms and trophic levels. In the context of the International Baccalaureate (IB) Biology SL curriculum, comprehending energy transfer and food web dynamics is essential for analyzing ecological interactions and the sustainability of biological communities.

Key Concepts

Energy in Ecosystems

Energy is the driving force behind all biological processes in an ecosystem. It originates from the sun and is captured by producers through photosynthesis, converting solar energy into chemical energy stored in organic molecules. This energy is then transferred through various trophic levels as organisms consume one another.

Trophic Levels

Trophic levels represent the hierarchical positions in a food chain, indicating the number of steps an organism is from the primary energy source, typically the sun. Understanding trophic levels is crucial for mapping energy flow and assessing the efficiency of energy transfer within an ecosystem.

  • Producers (Autotrophs): These are primarily green plants and algae that produce their own food through photosynthesis, serving as the base of the food chain.
  • Primary Consumers (Herbivores): Organisms that feed directly on producers. Examples include rabbits, deer, and caterpillars.
  • Secondary Consumers (Carnivores and Omnivores): These eat primary consumers. Examples include snakes, frogs, and some birds.
  • Tertiary Consumers: Predators at the top of the food chain that consume secondary consumers, such as eagles and lions.
  • Decomposers: Organisms like bacteria and fungi that break down dead matter, recycling nutrients back into the ecosystem.

Food Chains vs. Food Webs

A food chain is a linear sequence of organisms where each is eaten by the next. In contrast, a food web is a complex network of interconnected food chains, illustrating multiple pathways through which energy and nutrients flow in an ecosystem. Food webs provide a more realistic depiction of ecological interactions.

Energy Transfer and Efficiency

Energy transfer between trophic levels is governed by the 10% rule, which states that only about 10% of the energy at one trophic level is transferred to the next. The remaining 90% is lost primarily through metabolic processes as heat, and through incomplete consumption and digestion.

$$ \text{Energy transferred to next level} = \text{Energy at current level} \times 0.10 $$

This low efficiency limits the number of trophic levels within an ecosystem, as energy becomes insufficient to support higher levels.

Ecological Pyramids

Ecological pyramids visually represent the distribution of energy, biomass, or number of organisms across trophic levels.

  • Pyramid of Energy: Displays the flow of energy at each trophic level, always decreasing from producers to top consumers due to energy loss.
  • Pyramid of Biomass: Shows the total mass of living organisms at each trophic level. Typically, producers have the greatest biomass.
  • Pyramid of Numbers: Represents the number of individual organisms at each trophic level, which can vary based on species and ecosystem dynamics.

Keystone Species

Keystone species play a critical role in maintaining the structure of an ecosystem. Their presence influences the population sizes of other species, thereby affecting the overall energy flow and stability of the ecosystem. The removal of a keystone species can lead to significant disruptions in trophic levels and food webs.

Example Food Web

Consider a forest ecosystem where:

  • Sunlight is captured by trees (producers).
  • Herbivores like deer consume the trees.
  • Carnivores such as wolves prey on the deer.
  • Scavengers and decomposers break down dead organisms, returning nutrients to the soil.

This web demonstrates multiple interactions and energy pathways, highlighting the complexity of ecosystems.

Ecological Efficiency

Ecological efficiency refers to the percentage of energy transferred from one trophic level to the next. It is a critical factor in determining the number of trophic levels an ecosystem can support.

$$ \text{Ecological Efficiency (\%)} = \left(\frac{\text{Energy at next level}}{\text{Energy at current level}}\right) \times 100 $$

For instance, if producers capture 1000 kcal of energy, primary consumers receive approximately 100 kcal, and secondary consumers receive about 10 kcal.

Flow of Matter vs. Flow of Energy

While energy flows unidirectionally through ecosystems, matter cycles in a cyclical manner. Energy enters the ecosystem through producers and leaves primarily as heat. In contrast, matter is recycled through various processes, maintaining the ecosystem's nutrient balance.

Comparison Table

Aspect Food Chain Food Web
Definition A linear sequence of organisms each dependent on the next as a source of food. A complex network of interconnected food chains within an ecosystem.
Complexity Simple and straightforward. Highly complex with multiple interactions.
Representation One path of energy flow. Multiple paths of energy flow.
Stability Less stable; disruption affects the entire chain. More stable; alternative pathways can compensate for disruptions.
Realism Less realistic representation of ecosystems. More accurately reflects natural ecosystems.

Summary and Key Takeaways

  • Energy flows through ecosystems via trophic levels, starting from producers to various consumers.
  • The 10% rule demonstrates low energy transfer efficiency between trophic levels.
  • Food webs provide a comprehensive view of the complex interactions within ecosystems.
  • Ecological pyramids illustrate the distribution of energy, biomass, and organism numbers across trophic levels.
  • Keystone species are crucial for maintaining ecosystem stability and energy flow.

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Examiner Tip
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Tips

Use the mnemonic PROPP to remember the trophic levels: Producers, R primary consumers, O secondary consumers, P tertiary consumers, and P decomposers. Additionally, practice drawing food webs to visualize complex interactions and reinforce your understanding of energy flow. For exam success, focus on understanding the 10% rule and being able to explain ecological pyramids clearly.

Did You Know
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Did You Know

In some deep-sea ecosystems, such as hydrothermal vent communities, chemosynthesis replaces photosynthesis as the primary energy source. Instead of sunlight, these producers utilize chemical energy from volcanic emissions to produce organic matter. Additionally, certain ecosystems like rainforests can have up to 10 different trophic levels, showcasing remarkable energy transfer complexity.

Common Mistakes
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Common Mistakes

Misunderstanding Trophic Levels: Students often confuse trophic levels by placing decomposers within the main food chain. Incorrect: Including decomposers as a separate trophic level. Correct: Recognizing decomposers as part of the recycling process rather than a distinct trophic level.

Overlooking Energy Loss: Assuming energy transfer is 100% efficient. Incorrect: Believing that all energy is passed to the next level. Correct: Understanding that only about 10% of energy is transferred, with the rest lost as heat.

FAQ

What is the primary source of energy for most ecosystems?
The primary source of energy for most ecosystems is sunlight, which is captured by producers through photosynthesis.
How does the 10% rule affect the number of trophic levels in an ecosystem?
The 10% rule implies that energy decreases by about 90% with each trophic level, limiting the number of trophic levels an ecosystem can support due to insufficient energy for higher levels.
What is the difference between a food chain and a food web?
A food chain is a linear sequence of organisms where each is eaten by the next, while a food web is a complex network of interconnected food chains, showing multiple pathways of energy and nutrient flow.
Why are keystone species important in an ecosystem?
Keystone species are crucial because they have a disproportionately large impact on their ecosystem, helping to maintain the structure and energy flow. Their removal can lead to significant ecological disruptions.
What are ecological pyramids and what types exist?
Ecological pyramids are graphical representations showing the distribution of energy, biomass, or number of organisms across trophic levels. The three types are the Pyramid of Energy, Pyramid of Biomass, and Pyramid of Numbers.
Can energy ever flow back from consumers to producers in an ecosystem?
No, energy flow in ecosystems is unidirectional, moving from producers to consumers and ultimately dissipating as heat. However, matter cycles within the ecosystem through processes like decomposition.
1. Unity and Diversity
2. Continuity and Change
2.1.1 Concept of water potential in plants
2.1.2 Osmosis and water movement in cells
2.1.3 Importance in plant hydration and transport
2.2.1 Mitigation strategies and global agreements
2.2.2 Causes and effects of climate change
2.2.3 Greenhouse gases and global warming
2.3.1 Asexual vs sexual reproduction
2.3.2 Sexual cycles in animals and plants
2.3.3 Reproductive strategies
2.4.1 Mendelian inheritance
2.4.2 Punnett squares and genetic ratios
2.4.3 Genetic disorders and gene linkage
2.5.1 DNA structure and replication mechanism
2.5.2 Enzymes involved in DNA replication
2.5.3 Replication errors and repair mechanisms
2.6.1 Mechanisms of temperature, pH, and water balance
2.6.2 Regulation of blood glucose levels
2.6.3 Feedback loops in homeostasis
2.7.1 Transcription and translation processes
2.7.2 Role of mRNA, tRNA, and ribosomes
2.7.3 Post-translational modifications
2.8.1 Mechanisms of natural selection
2.8.2 Adaptations and evolutionary fitness
2.8.3 Evidence for evolution
2.9.1 Sustainable agricultural practices
2.9.2 Renewable resources and ecological conservation
2.9.3 Impact of human activity on the environment
2.10.1 Types of mutations: Point, frameshift, chromosomal
2.10.2 Causes and effects of mutations
2.10.3 Gene editing technologies (e.g. CRISPR)
2.11.1 Mitosis vs meiosis
2.11.2 Phases of mitosis and meiosis
2.11.3 Chromosome number and genetic variation
3. Interaction and Interdependence
3.1.1 Enzyme kinetics and inhibition
3.1.2 Metabolic pathways: Anabolism and catabolism
3.1.3 ATP synthesis and energy transfer
3.1.4 Enzyme structure and function
3.2.1 Vaccines and immunity
3.2.2 Disorders of the immune system (e.g. autoimmune diseases)
3.2.3 Immune response: Innate vs adaptive immunity
3.2.4 Antigens and antibodies
3.3.1 Population dynamics: Growth models, carrying capacity
3.3.2 Community interactions: Competition, predation, mutualism
3.3.3 Succession in ecosystems
3.4.1 Glycolysis, Krebs cycle, and oxidative phosphorylation
3.4.2 Aerobic vs anaerobic respiration
3.4.3 Role of mitochondria in energy production
3.4.4 Energy yield in cellular respiration
3.6.1 The light-dependent and light-independent reactions
3.6.2 Chloroplast structure and function
3.6.3 Photosystem II and I
3.6.4 Calvin cycle and carbon fixation
3.7.1 Structure of neurons
3.7.2 Action potential and neurotransmission
3.7.3 Synaptic transmission
3.7.4 Nervous system coordination and reflex arcs
3.8.1 Homeostasis and feedback mechanisms
3.8.2 Endocrine and nervous system integration
3.8.3 Coordination of digestive, excretory, and circulatory systems
4. Form and Function
4.1.1 Compartmentalization of processes in eukaryotic cells
4.1.2 Role of organelles in cellular function
4.1.3 Endoplasmic reticulum, Golgi apparatus, mitochondria, and nucleus
4.2.1 Stem cells and differentiation
4.2.2 Types of specialized cells (e.g. muscle, nerve, blood)
4.2.3 Tissue types and their functions
4.3.1 Respiratory structures in animals and plants
4.3.2 Mechanisms of gas exchange: Diffusion, active transport
4.3.3 Human respiratory system
4.3.4 Plant gas exchange: Stomata, leaf structure
4.4.1 Structure and function of carbohydrates and lipids
4.4.2 Metabolism of glucose
4.4.3 Role of lipids in energy storage
4.4.4 Membrane structure: Phospholipid bilayer
4.5.1 Circulatory systems: Open vs closed
4.5.2 Blood circulation in mammals (cardiovascular system)
4.5.3 Role of the heart and blood vessels
4.5.4 Transport of gases and nutrients in animals and plants
4.6.1 Structure of proteins: Primary to quaternary structure
4.6.2 Enzymes as biological catalysts
4.6.3 Protein folding and denaturation
4.6.4 Functions of proteins in cells
4.7.1 Structural, behavioral, and physiological adaptations
4.7.2 Adaptation to extreme environments (e.g. deserts polar regions)
4.7.3 Adaptations in animals and plants to survive in specific habitats
4.8.1 Structure and properties of cell membranes
4.8.2 Passive transport: Diffusion, osmosis
4.8.3 Active transport: Pumps, endocytosis, exocytosis
4.8.4 Membrane potential and action potentials
4.9.1 Ecological niche concept
4.9.2 Role of organisms in ecosystems
4.9.3 Competition, predation, and symbiosis
4.9.4 Fundamental vs realized niches
5. Experimental Programme
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