# Topic 5B- Energy transfer and nutrient cycles

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• Created by: DBaruch
• Created on: 26-10-16 10:40

## Ecosystem basics, measuring biomass, calorimetry

• An ecosystem involves all the organisms living in a particular area. In all ecosystems there are producers. During photosynthesis plants use energy and carbon dioxide to make glucose and sugars. The sugars are used for different processes like respiration or growth. Biological molecules like cellulose make up the plants biomass. Energy is transfereed through the living organisms of an ecosystem when organisms eat other organisms.
• Biomass can be measure in terms of the mass of carbon that an organism contains or the dry mass of its tissue per unit area per unit time. The mass of carbon present is generally taken to be 50% of the dry mass. Once you have measure the dry mass of a sample, you can scale up the result to give the dry mass of the total population or the area being investigated.
• You can estimate the amount of chemical energy stored in biomass by burning the biomass in a calorimeter. The amount of heat given off tells how much energy is in it. A sample of dry biomass is burnt and the energy released is ued to heat a known volume of water. The change in temperature of the water is used to calculate the chemical energy of the dry biomass
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## Primary production, net production in consumers

• Gross primary production (GPP) is the total amount of chemical energy converted from light energy by plants, in a given area, in a given time. Around 50% of the GPP is lost to the environment as heat when the plants respire. This is known as respiratory loss(R). The remaining chemical energy is called net primary production (NPP). NPP=GPP-R
• Consumers also store chemical energy in their biomass. Consumers get energy by ingesting plant material, or animals that have eaten plant material. However, not all the chemical energy stored in the consumers food is transferred to the next tropic level, around 90% of the total available energy is lost in various ways. Not all the good is eaten so the energy is not taken in. Some parts are indigestable and some is lost to the environment. the energy thats left is stored in the consumers biomass and is available to the next trophic level. This energy is the consumers net production. N= net production, I= chemical energy in ingested food, F= chemical energy lost in faeces and urine, R= energy lost through respiration. N=I-(F+R)
• %Effciency of energy transfer= net production of trophic level / net production of previous trophic level X100
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## Food chains, food webs and increasing effciency

• Food chains and webs show how energy is transferred through an ecosystem. Each stage of the food chain is called a trophic level.
• Most farming practices aim to increase the amount of energy that is available for human consumption. This means increasing the net primary production (NPP) of crops and the net production(NP) of livestock. The energy lost to other organisms and the energy lost through the respiration of livestock can be reduced
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## Increasing effciency: simplifying food webs, reduc

• Pests can reduce the amount of energy available for crop growth and therefore the NPP of crops and this lowers the amount of energy available for humans by simplifying food webs, energy losses will be reduced and the NPP of the crop will increase. To get rid of pests farmers can use chemical pesticides. Biological agents also reduce the numbers of pests, so crops lose less energy and biomass, increasing the effciency of energy transfer to humans. Farmers use both chemical and biological methods. Using both can reduce pest numbers even more and increase NPP
• One way farmers can increae the net production of their livestock is by controlling the conditons that they live in, so that more of their energy is used for growth and less is lost through respiration. This means that more biomass is produced and more chemical energy can be stored, increasing net production and the effciency of energy transfer to humans. The benefits are that more food can be produced in a shorter space of time often at lower cost. However increasing net prodcution can raise ethical issues.
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## The role of microorganisms

• Saprobionts do 2 things, they feed on the remains of dead plants and animals and on their waste products, breaking them down. This makes saprobionts a type of decomposer and it allows important chemical elements in the remains and waste to be recycled. They secrete enzymes and digest their food externally, then absorb the nutrients they need. This is called extracellular digestion. During this process, organic molecules are broken down into inorganic ions. Obtaining nutrients from dead organic matter and animal waste during extracellular digestion is known as saprobiotic nutrition.
• Some fungi form symbiotic relationships with the roots of plants and these relationships are known as mycorrhizae. The fungi are made up of long, thin strands called hyphae, which connect to the plants roots. The hyphae increase the surface area of the plants root system, helping the plant to absorb ions from the soil that usually scarce. Hyphae also increase the uptake of water by the plant.
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## The nitrogen cycle

• Plants and animals use nitrogen to make proteins and nucleic acids. Plants and animals are unable to use the nitrogen gas in the atmosphere, so they need bacteria to convert it into nitrogen-containing compounds
• Nitrogen fixation- is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds. Biological nitrogen fixation is carried out by bacteria such as rhizobium. It turns nitrogen into ammonia which goes on to form ammonium ions in solution that can be used by plants
• Ammonification- is when nitrogen compounds from dead organisms are turned into ammonia by saprobionts, which goes on to form ammonium ions.
• Nitrification- is when ammonium ions in the soil are changed into nitrogen compounds that can then be used by plants. First nitrifying bacteria called nitrosomonas change ammonium ions into nitrites. Then other nitrifying bacteria called nitrobacter change nitries into nitrates.
• Denitrification- is when nitrates in the soil are converted into nitrogen gas by denitrifying bacteria- they use nitrates in the soil to carry out respiration and produce nitrogen gas. This happens under anaerobic conditions.
• Other ways nitrogen gets into the ecosystem are by lightning or by artificial fertilisers
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## The phosphorus cycle

• Plants and animals need phosphorus to make biological molecules. Phosphorus is found in rocks and dissolved in oceans to form phosphate ions. Phosphate ions dissolved in water in the soil can be assimilated by plants and other producers.
• Phosphate ions in rocks are released into the soil by weathering
• Phosphate ions are taken into the plants through the roots. Mycorrhizae greatly increase the rate at which phosphorus can be assimilated
• Phosphate ions are transfereed through the food chain and they are lost from animals in waste products
• When plants die saprobionts break down the organic compounds, releasing phosphate ions into the soil for assmilation by plants.
• Weathering of rocks also releases phosphate ions into seas, laes and rivers and they are passed along the food chain
• The waste produced by sea birds is known as guano and contains a high proportion of phosphate ions. guano returns a significant amount of phosphate ions to soils and is used as a natural fertiliser.
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## Fertilisers, environmental issues and eutrophicati

• Adding fertiliser replaces the lost minerals, so more energy from the ecosystem can be used for growth, increasing the effciency of energy transer. Artificial fertilisers are inorganic and contain pure chemicals in the form of powders or pellets. Natural fertilisers are organic matter and they are normally made from manure and composted vegetables.
• Sometimes more fertiliser is added to plants that cannot use them. This can lead to leaching in the waterways and leaching is when water-soluble compounds in the soil are washed away and this can lead to eutrophication
• Eutrophication is caused by excess nutrients.
• Minerals ions leached from fertilised fields stimulate the rapid growth of algae in ponds and rivers
• Large amounts of algae block light from reaching plants in the pond or river
• Bacteria feed on the dead plant matter. The increased numbers of bacteria reduce the oxygen concentration in the water by carrying out aerobic respiration
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