These energy transfer and nutrient cycle notes cover everything in the AQA specification. They are detailed, helping you to learn what points to write in exam questions as well as helping things stick in your mind. Made using CGP guide, AQA textbook and class notes. A* achieved with these notes.
Energy Transfer And Nutrient Cycles
An ecosystem includes all the organisms living in a particular area and all the non-living (abiotic) conditions
Organisms can be divided into three groups according to how they obtain their energy and nutrients:
Producers: Photosynthetic organisms that manufacture organic substances using light energy, water, carbon
dioxide and mineral ions e.g. plants and algae
Consumers: Organisms that obtain their energy by consuming other organisms
Saprobionts (decomposers): A group of organisms that break down the complex materials in dead organisms
into simple ones. They release valuable minerals and elements in a form that can be absorbed by plants and so
contribute to recycling. E.g. Fungi and bacteria
Biomass:
Is the total mass of living material in a specific area at a given time. The fresh mass is easy to assess, but the
presence of varying amounts of water makes it unreliable. Measuring the mass of carbon or dry mass
overcomes this problem, but because the organisms must be killed, it is usually made on a small sample, and
this sample may not be representative.
Biomass is measured in terms of the mass of carbon that an organism contains or the dry mass of its tissue per
unit area.
Measuring Dry Mass:
1. To measure the dry mass, a sample of the organism is dried, often in an oven set to a low temperature.
The sample is then weighed at regular intervals. Once the mass becomes constant, all the water has
been removed
2. If needed, the result from the sample can be scaled up to give the biomass of the total population or
the area being investigated. A typical unit for dry mass might be kg m2
3. The mass of carbon present is generally taken to be 50% of the dry mass
Using calorimetry:
1. A sample of dry biomass is burnt in pure oxygen within a sealed chamber called a bomb. The energy
released is used to heat a known volume of water
2. The change in temperature of the water is used to calculate the chemical energy of the dry biomass
Plants normally convert between 1-3% of the sun’s energy available to them into organic matter. Most of it is
not converted because:
• Over 90% of the Sun’s energy is reflected back into space by clouds and dust or absorbed by the
atmosphere
• Not all wavelengths of light can be absorbed and used for photosynthesis
• Light may not fall on a chlorophyll molecule
• A factor, such as low CO2 levels may limit the rate of photosynthesis
Gross primary production (GPP) is the total amount of chemical energy converted from light energy by plants,
in a given area
Approximately 50% of the gross primary production is lost to the environment as heat when the plants respire.
This is called respiratory loss (R).
The remaining chemical energy is called the net primary production (NPP). This is the energy available to the
plant for growth and reproduction- the energy is stored in the plant’s biomass. It is also the energy available to
organisms at the next trophic level, such as consumers and decomposers.
, Net Primary Production = Gross Primary Production – Respiratory Losses
Net production for consumers:
Consumers get energy by ingesting plant material, or animals that have eaten plant material. However, not all the
chemical energy stored in the consumer’s food is transferred to the next trophic level- around 90% of the total
available energy is lost in various ways:
• Some of the organism is not consumed
• Some parts cannot be digested and are therefore lost in faeces
• Some of the energy is lost in excretory materials, such as urine
• Some energy losses occur as heat from respiration and lost to the environment. The losses are high in
mammals and birds because of their high body temp
The net production of consumers can be calculated using the following formula:
N = I – (F + R)
N = Net production I = Chemical energy in ingested food F = Chemical energy lost in faeces and urine
R = Energy lost through respiration
This is also known as secondary production
Food chains show simple lines of energy transfer. Each of the stages in a food chain is called a trophic level
Food webs show lots of food chains in an ecosystem and how they overlap
Farming Practices:
Most farming practices aim to increase the amount of energy that is available for human consumption by:
- The energy lost to other organisms can be reduced
- The energy lost through respiration can be reduced
By simplifying the food web, energy losses will be reduced and the NPP of the crop will increase
1. Farmers can reduce pest numbers using chemical pesticides:
- Insecticides kills insects that eat and damage crops. This means less biomass is lost from crops, so they
grow to be larger, which means NPP is greater
- Herbicides kill weeds which can remove direct competition with the crop for energy from the Sun. It can
also remove the preferred habitat or food source of the insect pests, helping to further reduce their
numbers and simplify the food web
2. Biological agents also reduce the numbers of pests, so crops lose less energy and biomass
- Parasites live in or lay their eggs on a pest insect. Parasites either kill the insect or reduce its ability to
function
- Pathogenic bacteria and viruses are used to kill pests
Controlling the conditions that livestock live in:
- Movement is restricted and so less energy is used in muscle contraction, decreasing the rate of respiration
- The environment can be kept warm in order to reduce heat loss from the body
- Feeding can be controlled so that they animals receive the optimum amount and type of food for
maximum growth with no wastage
- Predators are excluded so that there is no loss to other organisms in the food web
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