Food Chains & Energy Transfer (AQA A-Level Biology): Revision Notes

Food Chains & Energy Transfer

Energy in ecosystems

All organisms in ecosystems depend on energy to sustain their activities. The primary energy source for nearly all life on Earth is sunlight, which plants capture and convert into chemical energy through photosynthesis. Plants use this solar energy to create organic compounds from carbon dioxide and water, forming substances like sugars that serve as respiratory substrates.

This process establishes the foundation for energy transfer throughout ecosystems, as the organic molecules produced by plants become the biomass that other organisms consume. Energy flows between organisms through feeding relationships, creating interconnected networks that sustain entire ecological communities.

Organism classification by energy source

Living organisms can be categorised into three main groups based on how they obtain energy and nutrients:

Producers

Producers are photosynthetic organisms that manufacture organic substances using light energy, water, carbon dioxide, and mineral ions. These organisms, primarily green plants and algae, convert solar energy into chemical energy stored in organic compounds. They form the base of all food chains and are essential for supporting other life forms.

Consumers

Consumers are organisms that obtain energy by feeding on other organisms rather than directly using sunlight. They are further classified according to their position in the feeding hierarchy:

• Primary consumers are herbivores that feed directly on producers. These organisms represent the first level of consumers in any food chain.

• Secondary consumers are animals that eat primary consumers. These are typically carnivores or omnivores that occupy the second consumer level.

• Tertiary consumers feed on secondary consumers and represent the third level of consumers. Like secondary consumers, they may be carnivores or omnivores.

Secondary and tertiary consumers often function as predators, but they may also act as scavengers or parasites depending on their feeding behaviour.

Saprohionts (decomposers)

Saprohionts are organisms that break down complex organic material found in dead organisms. Through this decomposition process, they release valuable minerals and elements in forms that plants can absorb, contributing essential nutrients back to the ecosystem. Fungi and bacteria carry out the majority of this crucial recycling work.

Food chains and food webs

Food chains

A food chain represents a linear feeding relationship showing how energy flows from producers through successive consumer levels. Each stage in this sequence is called a trophic level. The arrows in food chain diagrams indicate the direction of energy flow, pointing from the organism being consumed to the consumer.

In longer food chains, tertiary consumers may themselves be consumed by quaternary consumers, adding a fourth trophic level to the sequence.

Food webs

While food chains provide a simplified view of feeding relationships, real ecosystems are far more complex. Food webs represent the interconnected nature of multiple food chains within a habitat. Most animals do not rely on a single food source, and many organisms can occupy different trophic levels depending on what they consume.

Biomass measurement

Biomass represents the total mass of living material present in a specific area at a particular time. Measuring fresh biomass can be unreliable due to varying water content between organisms, so scientists typically use dry mass measurements instead.

Measurement units

Biomass measurements use standardised units depending on the sampling method:

• When sampling a specific area (such as grassland or seashore), biomass is measured in grams per square metre ($\text{g m}^{-2}$)
• When sampling a volume (such as in a pond or ocean), biomass is measured in grams per cubic metre ($\text{g m}^{-3}$)

Limitations of biomass measurement

Since organisms must be killed to obtain dry mass measurements, scientists can usually only work with small samples. These samples may not accurately represent the entire population or ecosystem, which limits the reliability of biomass estimates.

Energy measurement through calorimetry

Scientists can estimate the chemical energy stored in dry biomass using calorimetry. This technique involves burning a weighed sample of dry biological material in pure oxygen within a sealed chamber called a bomb calorimeter.

The calorimeter sits in a water bath, and the heat released during combustion causes a measurable temperature increase in the surrounding water. Since scientists know the energy required to raise water temperature by specific amounts, they can calculate the energy content of the original biomass sample. Results are typically expressed in kilojoules per kilogram ($\text{kJ kg}^{-1}$).

This method provides valuable data for understanding energy availability at different trophic levels and calculating energy transfer efficiency between organisms in food chains.

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