Biology A2- Energy Flow, Nitrogen Cycle, Carbon Cycle

A2 level for AQA

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Energy is transferred through Ecosystems

Main route energy enters an ecosystem is through photosynthesis

Some energy enters sea ecosystems when bacteria use chemicals from deep sea vents as an energy source

During photosynthesis, plants convert sunlight energy into a form that can be used by other organisms. Plants are called producers and they produce organic molecules

Energy is transferred through ecosystems when organisms eat other organisms

Producers >Primary Consumers >Secondary Consumers >Tertiary Consumers

Each of the stages are called trophic levels

Food chains and food webs show how energy is transferred. Food chains show simple lines of transfer and food webs show lots of chains and how they overlap

Energy locked in things that can't be eaten e.g. bones/ faeces are recycled back into the ecosystem by microorganisms called decomposers- break down dead/ undigested material. 

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Not all energy transferred to next trophic level:

90% of the total energy available is lost in various ways

60% is never taken in by the organisms in the first place:

- Plants can't use all the light energy that reaches leaves, some is reflected, some is the wrong wavelength and some passes straight through the leaves

-Some sunlight hits parts of the plant that can't photosynthesis e.g. bark

- Some foods e.g. roots/ bones aren't eaten by organisms so energy isn't taken in

- Some food is indigestible so passes through organisms and come out as waste

40% is absorbed, called gross productivity. Not all moves to next level:

-30% total energy available (75% gross) lost to environment through respiratory loss- energy from respiration used for movement or body heat 

-10% total (25% gross) becomes biomass (stored for growth)- net productivity

Net productivity is the energy available to the next trophic level 

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Net productivity = gross productivity - respiratory loss

e.g. rabbits recieve 20 000 kJm-2 yr-1 of energy but don't take in 12 000 of it so gross productivity = 8000 kJm-2 yr-1. They lose 6000 kJm-2 yr-1 from respiration.

Net productivity = 8000-6000 = 2000 kJm-2 yr-1

% efficency of energy transfer between trophic levels =

(net productivity of a level / net productivity of previous level) x 100

e.g. rabbits receive 20 000 kJm-2 yr-1, and net productivity is 2000 kJm-2 yr-1 so percentage efficiency = 2000/ 20 000 = 0.1 ... 0.1 x 100 = 10%

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FYI- Producers are always on the bottom 

Pyramids of numbers:

Show number of organisms in each trophic level. Bigger block = more organisms. Not always pyramid shape- small numbers of big organisms e.g. trees and large numbers of small organisms e.g. parasites change things

Pyramids of biomass:

Show the amount of biomass in each trophic level, dry mass of organisms in kgm-2) at a single moment in time. Bigger the block= more biomass. Nearly always pyramid shaped. (exception is when the base is plant plankton (photosynthesise) amount of plankton is small at a given time, but because they have a short life span and reproduce quickly, there is a lot round over  period of time).

Pyramids of energy:

Show the amount of energy available in each trophic level, in kilojoules per square metre per year (kgm-2yr-1) aka the net productivity of each trophic level. Bigger the block= more energy there is. Pyramids always pyramid shaped

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Intensive Farming

Natural ecosystem= ecosystem that hasn't been changed by human activity

Energy input of natural ecosystem= amount of sunlight captured by producers

Intensive farming changes an ecosystem by controlling biotic/ abiotic conditions to make it more favourable for crops or livestock. Can manipulate transfer of energy.

Intensively farmed stuff have greater net productivity (greater amount of biomass) than organisms in natural ecosystems

Energy input might be greater in intensive farming e.g. cows given food higher in energy, or might be the same as a natural ecosystem e.g. crop in field has the same amount of sunlight.

Farming methods increase productivity in different ways:

1. Increase the efficiency of energy conversion- more of the energy organisms have is used for growth and less for movement/ recovering from disease

2. Remove growth-limit factors- so more of the energy available used for growth

3. Increase energy input- more energy added= more energy for growth

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Intensive farming practice 1 = killing pest specie

Pests reduces productivity + energy available for growth,so crops less efficient at converting energy. Ways farmers reduce pest numbers:

1. Using chemical pesticides

- Herbicides kill weeds that compete for energy. Less competition = more energy for crops, so they grow faster and become larger.

-Fungicides kill fungal infections,crops use energy for growth not recovering.

- Insecticides kill insect pests that eat and damage crops, killing pests= less biomass is lost from crops so they grow faster, larger and increase productivity. 

2. Using biological agents

-Reduces number of pests so crops lose less energy and biomass

- Natural predators in ecosystem eat pest species e.g. ladybirds eat greenfly

- Parasites live in or lay eggs on pest insect. Either kills or reduces function

- Pathogenic bacteria and viruses used to kill pests

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1. Killing pests and 2. Using fertilisers

3. Using integrated systems

Uses both chemical pesticides and biological agents

- Combined effect reduces pests and increses productivity even more

- Reduces costs as more expensive methods can be used less as its not alone

- Reduces environment impact of things like pesticides as less is used

Farmers use fertilisers

Chemicals that provide crops with minerals for growth e.g. nitrates. 

Fertiliser replaces minerals lost through plant uptake so more energy from ecosystem can be used to grow, increasing the efficiency of conversion

1. Natural fertilsers are organic matter- including mnure and sweage sludge

2. Artificial are inorganic- contain pure chemicals e.g. ammonium nitrate

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3. Farmers rear livestock intensively

Involves controlling conditions they live in, so more energy is used for growth and less for other activities- efficiency of energy conversion is increased so more biomass is produced and productivity is increased

e.g. 1. Animals may be kept in warm, indoor pens where their movement is restricted. Less energy is wasted keeping warm and moving around.

e.g. 2. Animals may be given feed thats higher in energy than their natural food. This increases the energy input, so more energy is available for growth.

Benefits= more food can be produced in a shorter space of time, at a lower cost. 

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Environmental and Economical Issues


Chemical pesticides and biological agents directly affect other non-pest species

Pesticides indirectly affect food chain e.g. poisoning, passed on when organisms eaten. Reduces species diversity

Natural predators introduced to ecosystem may become a pest themselves

Fertiliser can be washed into rivers and ponds- eutrophication

Fertilisers changes the balance of nutrients in the soil- could cause crop death

Economic Issues:

Not profitable- cost may be greater than extra money made from increased productivity

Biological agents less cost effective, increase productivity less 

Farmers need to supply right amount of fertiliser, too much and its wasted and washed away, too little and productivity wont be increased. 

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Nitrogen Cycle

Plants and animals need nitrogen to make proteins and nucleic acids (DNA/RNA)

Although the  atmosphere has 78%, can't use it in that form, need saprobiotic bacteria to convert it into nitrogen compounds first.

Nitrogen Cycle includes:

nitrogen fixation, ammonification, nitrification and denitrification

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Nitrogen Cycles

1. Nitrogen Fixation

(can also happen when lightning passes through the atmosphere)

Nitrogen gas in the atmosphere is turned to ammonia by nitrogen-fixing bacteria

Free-living nitrogen fixing bacteria reduce gas to ammonia whch is then used to manufacture amino acids. Nitrogen rich compounds released when they decay

Mutualistic bacteria e.g. Rhizobium are found in root nodules of leguminous plants. e.g. peas, beans 

They form a mutualistic relationship with the plants- they provide the plant with nitrogen compounds and the plant provides them with carbohydrates

2. Ammonification

Nitrogen compounds from dead organisms are turned into ammonium compounds by decomposers

Animal waste also contains compounds and are turned into ammonium

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Nitrogen Cycles

3. Nitrification

This is the conversion of ammonium ions to nitrate ions by nitrifying bacteria, to be used by the plant. 

Firstly nitrfying bacteria oxidise ammonium ions to nitrite ions (N02-)

Secondly other nitrifying bacteria oxidise nitrite ions to nitrate ions (N03-)

The bacteria require oxygen, so soil with lots of air spaces by ploughing and good drainage so air spaces are not filled with water is needed.

4. Denitrification

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

Happens under anaerobic conditions - no oxygen e.g. waterlogged soil. 

Parts of the cycle can be carried out artificially on an industrial scale. Haber process produces ammonia from atmospheric nitrogen to make fertilisers

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Leaching and Eutrophication

Leaching- when water soluble compounds in soil are washed away, e.g. by rain / irrigation systems. Washed into nearby ponds and lakes.If nitrogen fertiliser is leached it can cause eutrophication:

1. Nitrates leached from fertilised fields stimulate growth of algae in ponds etc.

2. Large amounts of algae block light from reaching plants below

3. Plants die as they are unable to photosynthesise

4. Bacteria feed on the ded plant matter

5. Increased numbers of bacteria reduce the oxygen concentration in water by carrying out aerobic respiration

6. Fish etc. die as there isn't enough dissolved oxygen

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How to analyse data...

1. Describe the data using numbers and axis

2. Draw a conclusion using correlation etc.

3. Evaluate the methodology including variables, reliability and sample size etc.

4. Suggest an explanation for your conclusion 

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The Carbon Cycle

All organisms need carbon to make essential compounds. Carbon cycle is how carbon moves through living organisms and the non-living environment

Four processes are involved:

Photosynthesis, Respiration, Decomposition, Combustion

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The Carbon Cycle

1. Carbon (in form of CO2 in air and water) is absorbed by plants when they carry out photosynthesis- it becomes organic compounds in plant tissues

2. Carbon is passed on to primary consumers when they eat plants. It's passed on to secondary and tertiary consumers when they eat other consumers

3. All living organisms die and the carbon compounds in dead organisms are digested by microorganisms called decomposers e.g. bacteria and fungi. Feeding on dead organic matter is called saprobiontic nutrition

4. Carbon is returned to the air (and water) as all living organisms (including decomposers) carry out respiration, which produces CO2

5. If dead organic matter ends up in places where there aren't any decomposers e.g. deep oceans, the carbon compounds can be turned into fossil fuels over million of years (by heat and pressure) 

6. The carbon in fossil fuels is released when they're burnt- called combustion 

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Fluctuations in CO2 Concentration

Respiration adds CO2 to the atmosphere but photosynthesis removes CO2

Daily change:

Respiration is carried out constantly through day and night

Photosynthesis only takes place during the day

CO2 concentration falls during the day as plants remove it during photosynthesis

CO2 concentration increases at night because no photosynthesis is happening

Yearly change:

Most plant growth occurs in summer as light intensity is greatest

CO2 concentration falls during summer as more is being removed due to more plants photosynthesising

CO2 concentration increased through autumn/winter due to less photosynthesis

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On page 17 - photosynthesis carries out during the day.. you made a mistake!

but this is good!! :) heelpfull

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