1. Ecosystems Important Definitions Ecosystem: the living and non-living components of a distinct environment and their interactions. Species: organism that share biochemical, behavioural and morphological similarities and are able to breed together to produce fertile offspring. Population: all of the members of a particular species living in the ecosystem at the same time and are able to breed together. Community: the collection of all the living organisms in a particular ecosystem – a number of different species interacting in a particular place. Biotic factor: living components that affect an ecosystem Abiotic factor: non-living components which affect an ecosystem Niche: the role of an organism in an ecosystem Trophic level: an organism’s position in the food chain Producer: can convert inorganic energy into organic energy – it must be an autotroph Primary consumer: an organism that eats the producer to gain its energy Secondary consumer: an organism that eats the primary consumer to gain its energy Detritivores: animals which consume dead and waste matter, extracting nutrition. Saprotrophs: organisms that feed by secreting enzymes onto food and absorbing digested nutrients across their outer walls. Nitrogen–fixation : conversion of inorganic N 2 into organic NH 4 +, NO 2 - or NO 3 - Nitrification: chemoautotrophic bacteria absorb ammonium ions and oxidise them to NO 2 - or NO 3 - Denitrification: organic nitrogen converted back to nitrogen gas, N 2.

2. Understanding Food Chains All of the energy in an ecosystem comes from the sun via photosynthesis All of the plants energy comes form the sun; all of the zebra’s energy came from the plants; all of the lions energy came from the zebra, so… These arrows therefore not only show what is eaten by what, but also the direction of energy transfer! In the example above, the producer is the plant, the primary consumer is the zebra and the secondary consumer is the lion. If we look at the values of energy on the arrows, we would see that each trophic level has less energy flowing through it than the previous trophic level. This is because:  Some food is not eaten: some animals lack the enzyme cellulose – so cellulose walls of plants can’t be digested  Some energy is lost in excretion  Energy lost in respiration  Energy lost as heat in movement and metabolism 700,000 7000 700 Because of this, there is less energy available at higher levels of the food chain so less living tissue can be kept alive.

3. Energy Transfer Very little of the sunlight energy reaching Earth is used for photosynthesis:  Some light is reflected off the leaf  Some is transmitted straight through the leaf  Some of the light is the wrong wavelength and is not captured by chlorophyll.  The total amount of energy that is captured by leaves for photosynthesis is the gross primary productivity.  Some of this energy will be used by the plant and lost as respiratory heat.  The net primary productivity (NPP) is therefore the gross primary productivity minus the energy that is lost in respiration. This is the amount of energy that is available to the primary consumer in the ecosystem. Net Primary Productivity = Gross primary productivity – respiratory heat loss Human activities can manipulate the flow of energy through ecosystems. We can increase the net primary productivity, making energy conversion more efficient, reducing energy loss and increasing crop yields.  Light levels can limit rate of photosynthesis and hence NPP. Plants can be grown under light banks or planted earlier to give a longer growing season to harvest more light  Temperature can limit the speed of a plant’s chemical reactions: greenhouses can provide a warmer temperature for growing plants and therefore increase NPP.  Lack of nutrients slows the rate of photosynthesis. Including a nitrogen-fixing crop that replenishes levels of nitrate in the soil can help.  Using herbicides to kill weeds to reduce competition for light, water and nutrients can improve NPP.

4. Measuring Energy Transfer  Pyramids of number: the area of each bar in the pyramid is proportional to the number of individuals a that trophic level. This is not always an accurate measure of how much living tissue exists at each level.  Pyramids of biomass: area of each bar is proportional to the dry mass of all the organisms at that trophic level. The dry mass is found by collecting the organisms and placing them in an over at 80° until all of the water has evaporated. Wet biomass is often used because dry biomass is a far more destructive method.  Pyramids of energy can be created. This involves burning the organisms in a calorimeter and working out how much heat is released per gram – this is done by calculating the temperature rise of a known mass of water. Productivity is the rate at which energy passes through each trophic level in a food chain. Productivity gives an idea of how much energy is available to the organisms at a particular trophic level per unit area in a given amount of time. Pyramid of Number Pyramid of Biomass

5. Energy remains stored in dead organisms and waste material. This is only available to decomposers. Decomposers recycle dead and waste matter, releasing it back into the soil or food chain to be continually cycled. Detritivores are animals which consume dead and waste matter, extracting nutrition. Worms and woodlice are common detritivores, eating only dead plant and animal matter and waste. The detritivores themselves are a vital source of energy and nutrition for other organisms like birds and badgers, so this means that energy in the dead and waste material is recycled into the food chain. Saprotrophs feed saprotrophically. They secrete digestive enzymes onto dead and waste material. The enzymes digest the material into small molecules which are then absorbed into the body of the saprotroph. Digestion

6. mutualistic Nodules on the roots of leguminous plants like peas and beans contain rhizobium bacteria. The relationship between the plant and the bacteria can be described as mutualistic - both benefit from each other! The legume provides the rhizobium bacteria with energy in the form of sugars. The rhizobium provides the plant with a supply of organic nitrogen. The organic nitrogen that the rhizobium provides is used by the plant to form amino acids and nucleotides that are used to form polypeptides and DNA. The legume plant must provide anaerobic conditions in order for the rhizobium to fix nitrogen. It does this by forming a protein called leghaemoglobin which binds strongly to oxygen. The rhizobium therefore are free to convert nitrogen to organic nitrogen in an oxygen free environment! Sugars Organic Nitrogen

7. The Nitrogen Cycle Nitrogen fixation: conversion of inorganic N 2 into organic NH 4 +, NO 2 - or NO 3 -. There are 3 different ways that nitrogen can be fixed: 1.Lightening – causes N 2 and O 2 in the atmosphere to react to form NO 3 - 2.Nitrogen fixing bacteria – Rhizobium: these are found free in the soil or in the root nodules of legumes: these convert N 2 from the air into NH 4 +, 3.The Haber process reacts N 2 gas and H 2 to form ammonium containing fertilisers that farmers use on crops. Nitrification: chemoautotrophic bacteria absorb ammonium ions and oxidise them to NO 2 - or NO 3 -. The bacteria nitrosomonas converts ammonium (NH 4 + ) to nitrite (NO 2 - ). The bacteria nitrobacter converts nitrite (NO 2 - ) to nitrate (NO 3 - ). Denitrification: organic nitrates converted back to inorganic nitrogen in the atmosphere. This is done by denitrifying bacteria in in anaerobic bacteria Organic nitrogen compounds are absorbed by plants and used to form DNA and proteins. Animals then eat the plants. The proteins are digested, and excess amino acids are excreted as urea in the urine. Decomposers in the soil digest the urea and release NH 4 + into the soil.

8. Explain how nitrogen is cycled in ecosystems (9)  Nitrogen fixation is the conversion of inorganic N2 to organic NH 4 +, NO 2 - and NO 3 -  Rhizobium are nitrogen fixing bacteria. They are found free in the soil or within the root nodules of leguminous plants  Rhizobium convert N 2 to NH 4 +  Lightning causes N 2 to react with O 2 forming NO 3 -  Nitrification converts one form of organic nitrogen to another; from NH 4 + to NO 2 - to NO 3 -. This process is carried out by nitrifying bacteria.  NH 4 + to NO 2 - by nitrosomonas bacteria  NO 2 - to NO 3 - by nitrobacter bacteria  Plants absorb organic nitrogen compounds to produce amino acids  Plants are then consumed by animals  When animals die, decomposers convert proteins, amino acids and urea to NH 4 +  Denitrifying bacteria convert organic nitrogen NO 3 - to atmospheric nitrogen N 2 Exam-Style Questions

9. Explain how energy is transferred through food chains and food webs in ecosystems. Refer to the efficiency of this transfer in your answer.  The sun is the energy source for the system  Producers (photoautotrophs) absorb the sun’s energy  Photosynthesis converts the light energy form the sun to chemical energy  Not all of the sun’s energy is trapped by the producer; some is reflected and some is transmitted straight through the leaf  Energy is used in plant metabolism so this energy is not available to the primary consumer  Some energy is used for plant growth so this energy is available to the primary consumer  Primary consumer eats the producer  Some of the producer is not edible. Cellulose is not digested in animals as they lack the cellulose enzyme required to break it down  Secondary consumer eats the primary consumer  Some parts of the animal are not edible  Energy used by secondary consumer to move, and energy is lost through excretion  Transfer of energy to decomposers