unit 4 revision notes
TRANSCRIPT
Biology A2 Unit 4 Revision Notes
AQA
Biology Unit 4 – 3.4.1 Definitions Word Definition Ecosystem An ecosystem is made up of all the interacting biotic
(living) and abiotic (non-‐living) factors in a specific area
Population Is a group of interbreeding organisms of one species in a habitat
Community All the populations of different organisms in a given area at a given time
Habitat Place where a community of organisms lives (e.g. decaying log)
Ecological Niche How an organism fits into the environment, refers to both where an organism lives and what it does there. No two species occupy the same niche
Investigating populations
Quadrats 3 factors to consider:
1. The size of quadrat to use – this will depend on the size of whatever you’re sampling and how they are distributed within the sample area
2. The number of quadrats to record within the sample area – for questions asking about this 10 or more should be used within each area to get a big enough sample size, basically more is better
3. The position of each quadrat within the sample area – random sampling should be used
Random sampling • This is used to prevent bias – as someone might pick to place a quadrat where there is a large
amount of clover for example but this may not be representative.
You should:
• Place two tape measures at right angles along two sides of the area you’re studying • Obtain co-‐ordinates using a random numbers table • Place quadrats at the intersection of each pair of co-‐ordinates
Systematic sampling using transects • Transect is a line or tape • This can be used more effectively than quadrats for measuring things such as abundance of species
as you enter a forest or measuring the abundance of species comared with how far away from the sea you are
They can be used in two ways:
• Any organism over which the line passes is recorded • You can put a quadrat down every however many meters down the transect
Measuring abundance Sampling is used to get a measure of abundance. This is the number of individuals of a species within a given space. Two ways of measuring this:
• Frequency – counting individual animals or plants • Percentage cover – an estimate of the area within the quadrat that one species covers
Mark-‐release-‐recapture This is carried out to determine the population size. This method is used:
• A known number of animals are caught • These are marked in some way • These are then released back into the community • Later more animals are caught and the number of marked individuals are recorded
Then this formula is used to determine population size (needs to be remembered for exam):
𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑠𝑖𝑧𝑒
= 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 1 × 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 2
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑎𝑟𝑘𝑒𝑑 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒 2
This technique relies on a number of assumptions:
• The marked individuals distribute themselves evenly amongst the other individuals in the population.
• No deaths or births in the population • No emigration or immigration effecting the population • The method of marking is non-‐toxic to the individual and it doesn’t make them more liable to
predation • The mark isn’t rubbed off or lost during the investigation
Population growth curves
1. Lag phase – slow growth – small numbers initially so reproduction slow – organisms are getting used to the conditions
2. Log phase – rapid growth and optimum conditions 3. Stationary phase – stable state – no population growth – small fluctuations due to changes in
factors such as food supply
Abiotic factors • Temperature – each species has an optimum – the further away from this you go the smaller the
population that can be supported • Light – ultimate source of energy for ecosystems – rate of photosynthesis increases as light
intensity increases – this allows a larger primary consumer population to be supported • pH – this effects the action of enzymes – a population of organisms is larger where the optimum pH
is • water and humidity – where water is scarce populations are small and only well adapted organisms
survive – humidity effects the later loss from plants (transpiration) and animals in dry air conditions only those individuals with adaptations to this will survive
Competition
Intraspecific • Individuals of the same species • Competing for resources such as food, space, light etc. • Availability of the resources that determines population size • Lower the availability smaller the population size and vice versa
Interspecific • Individuals of different species • Competing for resources such as food, space, light etc. • Competitive advantage determines which population will grow
• If conditions remain the same this will lead to the complete removal of one species as they can’t compete in this niche
Predation • Occurs when one organism is consumed by another
Data can be inaccurate on this as it has to be measured in the wild by sampling which is only as good as the methods used. None of these methods guarantee complete accuracy so caution is advised with any data produced this way
Predator-‐prey relationship
• Predators eat their prey therefore reducing the population size of the prey • With fewer prey available the predators are in greater competition with each other for the
remaining prey • The predator population is reduced as some individuals are unable to consume enough prey to
survive and reproduce • With fewer predators left less prey is consumed • The prey population increases • More prey are now available so predator population increases
This is a cycle and carries on and as the graph shows
The human population
Factors effecting population size • Birth rate • Death rate • Immigration • Emigration
𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑔𝑟𝑜𝑤𝑡ℎ = 𝑏𝑖𝑟𝑡ℎ𝑠 + 𝑖𝑚𝑚𝑖𝑔𝑟𝑎𝑡𝑖𝑜𝑛 − (𝑑𝑒𝑎𝑡ℎ𝑠 + 𝑒𝑚𝑖𝑔𝑟𝑎𝑡𝑖𝑜𝑛)
𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑔𝑟𝑜𝑤𝑡ℎ = 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑐ℎ𝑎𝑛𝑔𝑒 𝑑𝑢𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑝𝑒𝑟𝑖𝑜𝑑𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑡𝑎𝑟𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑒𝑟𝑖𝑜𝑑 ×100%
Factors effecting birth rates • Economic conditions • Cultural and religious backgrounds • Social pressures and conditions • Birth control • Political factors
𝐵𝑖𝑟𝑡ℎ 𝑟𝑎𝑡𝑒 = 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑖𝑟𝑡ℎ𝑠 𝑝𝑒𝑟 𝑦𝑒𝑎𝑟𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑦𝑒𝑎𝑟×1000
Factors effecting death rate • Age • Life expectancy • Food supply • Availability of safe drinking water and effective sanitation • Access to medical care • Natural disasters • War
𝐷𝑒𝑎𝑡ℎ 𝑟𝑎𝑡𝑒 = 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑎𝑡ℎ𝑠 𝑝𝑒𝑟 𝑦𝑒𝑎𝑟𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑦𝑒𝑎𝑟×1000
Demographic Transition
(© Nelson Thornes A2 AQA Biology text book)
This summarises a pattern that different countries have gone through as they have developed
Age population pyramids These fit into the demographic transition stages based on the shape of the pyramid:
Stage Pyramid Shape explanation 1
Bigger bars at the bottom and less at the top as high birth rate so lots of children but also high death rate so less older people
2
Still high birth rate so big bars at the bottom but decreasing death rate so more in the middle and more people living longer.
3
Low death rate so bigger bars in the middle and more people reaching the top of the pyramid. Birth rate is decreasing so less at the bottom of the pyramid
4
Low birth rate and death rate so smaller bars at the bottom and bigger bars in the middle also more people reaching the top so larger bars there
Biology Unit 4 – Section 3.4.2
Why do organisms need energy • Metabolism – all the reactions that take place in living organisms involve energy • Movement • Active transport • Maintenance, repair and division of cells • Production of enzymes and hormones • Maintenance of body temperature
Synthesis of ATP from ADP This requires the addition of a phosphate to ADP. There are 3 ways this occurs:
• PHOTOPHOSOHORYLATION – This occurs in chlorophyll containing plant cells during photosynthesis • OXIDATIVE PHOSPHORYLATION – Which occurs in the mitochondria of plant and animal cells during
the process of the electron transport chain • SUBSTRATE – LEVEL PHOSPHORYLATION -‐ Occurs in plant and animal cells when donor molecules
donate phosphate to the ADP to make ATP like in the formation of pyruvate
ATP IS AN IMMEDIATE ENERGY SOURCE AND ISN’T A GOOD LONG TERM STORE OF ENERGY. IT RELEASES ENERGY IN MANAGABLE QUANTITIES FOR CELL REACTIONS AND CAN QUICKLY BE REFORMED MAKING IT A GOOD IMMEDIATE ENERGY SOURCE
ATP ADP + Pi
Hydrolysis (ATP + Water)
Energy from respiration
Biology Unit 4 – Section 3.4.3 Overview of photosynthesis
1. Capturing of light energy by chloroplast pigments 2. Light dependent reaction 3. Light independent reaction
Structure of a chloroplast
(image from passscience.blogspot.com)
• Light dependent reaction takes place in the thylakoids • Light independent reaction takes place in the stroma
Light dependent reaction Two purposes:
• Add phosphate to ADP making ATP (photophosphorylation) • Split water into H+ ions, electrons and Oxygen (photolysis)
Oxidation and reduction Oxidation – The loss of electrons, the loss of hydrogen or the gain of oxygen
Reduction – The gain of electrons, the gain of hydrogen or the loss of oxygen
Wordy Explanation of what happens • When chlorophyll molecules absorb light energy a pair it boosts the energy of a a pair of electrons
which raises them to a higher energy level • These electrons have so much energy that they leave the chlorophyll molecule • They are taken up by an electron carrier • The pair of electrons are now passed along a series of electron carriers in oxidation reduction
reactions (located in the membrane of the thylakoids) • Each carrier is at a slightly lower energy level than the last and the electrons lose energy • This energy is used to add a phosphate to ADP to make ATP • The photolysis of water also happens • This is due to chlorophyll molecules losing two electrons so these need to be replaced
• These replacement electrons are provided by the splitting of water molecules as shown below:
2𝐻!𝑂 → 4𝐻! + 4𝑒! + 𝑂!
• The H+ is taken up by NADP to form reduced NADP (or NADPH2) this then enters the light independent reaction
Diagrammatic explanation
(© Nelson Thornes AQA A2 Biology Text Book)
Light independent reaction • Products of light dependent reaction (reduced NADP and ATP) are used • Takes place in the stroma of the chloroplasts
Wordy explanation 1. Carbon dioxide diffuses from the air eventually into the stroma of the chloroplast 2. Here it combines with the 5 carbon Ribulose Bisphosphate (RuBP) 3. This produces two molecules of glycerate-‐3-‐phosphate (GP) 4. ATP and reduced NADP from the light dependent reaction are then used to reduce GP into triose
phosphate (TP) 5. NADP (non-‐reduced) is reformed and goes back into the light dependent reaction to accept more
hydrogen 6. Some triose phosphate molecules are used to make other useful substances such as glucose 7. Most triose phosphate molecules are used to regenerate RuBP using ATP
Diagram
(© Nelson Thornes AQA A2 Biology Text Book)
Limiting factors on photosynthesis
At any given moment the rate is limited by the factor that is at its least favourable value
Factors that limit photosynthesis
• Light intensity • Temperature • CO2 Concentration
Temperature is a factor as it’s an enzyme controlled reaction so higher temperature gives the molecules more energy and therefore they collide with the right amount of energy with the enzyme and form more enzyme substrate complexes.
Biology Unit 4 – Section 3.4.4 Aerobic respiration 4 stages:
• Glycolysis • Link reaction • Krebs Cycle • Electron Transport Chain
Glycolysis • Phosphorylation of Glucose • Splitting of phosphorylated glucose • Oxidation of triose phosphate (by removal of hydrogen) • Hydrogen accepted by NAD to form NADH2 • Production of ATP • Formation of pyruvate
Link Reaction • Pyruvate oxidised by removal of hydrogen • Hydrogen accepted by NAD to form NADH2 • De-‐carboxylation occurs producing carbon dioxide • Acetyl group formed (2-‐carbon) • Combines with coenzyme A to form the 2 carbon acetylcoenzyme A
Krebs Cycle • Acetylcoenzyme A feeds into this from the link reaction • Acetylcoenzyme A combines with a 4 carbon compound to form a 6 carbon compound • This then undergoes 2 decarboxylation’s removing two molecules of carbon dioxide • It also reduces two hydrogen carriers: NAD and FAD forming reduced NAD and reduced FAD • This then is the 4 carbon compound needed to combine with another acetylcoenzyme A molecule
and so it keeps on going
Electron transport chain • Hydrogen atoms collected by coenzymes NAD and FAD are used • These are split into electrons and protons • The electrons get passed down the electron transport chain where as they are passed from carrier
to carrier they lose energy • This energy is used to combine a phosphate with ADP to make ATP • The protons are pumped into the inter-‐membrane space of the mitochondria (the space between
the cristae and the outer membrane • As they accumulate here they diffuse back through special channels • At the end of the chain the electrons combine with these protons and oxygen to form water
Anaerobic Respiration • When there is little or no oxygen neither the krebs cycle or electron transport chain can take place • Only glycolysis can • So lots of pyruvate produced in order to produce the 2 ATP molecules glycolysis yields (net yield) • NAD must be regenerated so pyruvate accepts this hydrogen • However this happens differently in animals and plants
In plants and some microorganisms
𝑝𝑦𝑟𝑢𝑣𝑎𝑡𝑒 + 𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑁𝐴𝐷 → 𝑒𝑡ℎ𝑎𝑛𝑜𝑙 + 𝑐𝑎𝑟𝑏𝑜𝑛 𝑑𝑖𝑜𝑥𝑖𝑑𝑒 + 𝑁𝐴𝐷
In animals 𝑝𝑦𝑟𝑢𝑣𝑎𝑡𝑒 + 𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑁𝐴𝐷 → 𝑙𝑎𝑐𝑡𝑎𝑡𝑒 𝑙𝑎𝑐𝑡𝑖𝑐 𝑎𝑐𝑖𝑑 + 𝑁𝐴𝐷
In both cases this is very inefficient as only 2 ATP molecules are produced by glycolysis and there is a much greater quantity produced by Aerobic respiration
Biology Unit 4 – Section 3.4.5 Food Chains and Food Webs
Producers Photosynthetic organisms that manufacture organic substances using light energy
Consumers Organisms that obtain their energy by feeding on other organisms. Those that directly eat plants are called primary consumers. The animals eating those organisms are called secondary consumers and so on. Not normally more than 4 consumers in a food chain
Decomposers When producers and consumers die these organisms make the energy contained in the other organisms available for the food chain again by breaking down the producer/consumer. These nutrients can then be absorbed by plants and used in the food chain
Food webs In reality many animals don’t rely on a single food source and in a habitat many food chains link together forming a food web with lots of different organisms interacting
Energy Losses in food chains
Producers: • Over 90% of the suns energy 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 of a chlorophyll molecule • A factor such as low carbon dioxide levels may limit the rate of photosynthesis
Formula to work out NP (net production) 𝑁𝑒𝑡 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 = 𝑔𝑟𝑜𝑠𝑠 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 − 𝑟𝑒𝑠𝑝𝑖𝑟𝑎𝑡𝑜𝑟𝑦 𝑙𝑜𝑠𝑠𝑒𝑠
Losses by primary consumers: • Some of the organism isn’t eaten
• Some parts can’t be digested • Energy lost in excretory products such as urine • Energy losses due to heat loss
Why are food chains short? • Most food chains only have 4 or 5 trophic levels as there isn’t enough energy available to support a
population at another level • The total mass of the organisms in a particular place (biomass) is less at higher trophic levels • The total amount of energy stored is less at each level as you move up the food chain
Calculating efficiency of energy transfer
𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 = 𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑎𝑓𝑡𝑒𝑟 𝑡ℎ𝑒 𝑡𝑎𝑛𝑠𝑓𝑒𝑟𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑏𝑒𝑓𝑜𝑟𝑒 𝑡ℎ𝑒 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟×100%
Ecological pyramids
Numbers • The actual numbers of organisms proportional to each other • Drawbacks:
o No account is taken of size so a large oak tree is just 1 even though lots of aphids can feed on it
o This will create an inverted pyramid shape o Number of individuals can be so great that it is impossible to represent them accurately on
the same scale
Biomass • Measure of the total mass of plants/animals in a particular place • More reliable and quantitative • Only the organisms present at a particular time are measured so this doesn’t take into account
seasonal differences
Energy • Most accurate representation of energy flow through an ecosystem • This measures the energy stored in organisms • Collecting data however can be difficult and complex • Always a pyramid shape
How productivity is effected by farming practices
Fertilisers Fertilisers are substances farmers spread on the soil to replace nutrients, normally nitrogen, which plants get from the soil.
Natural fertilisers Natural fertilisers are the faeces of animals which can be spread onto the soil to increase the nutrients available to the plants. The advantages are that this is free and readily available to the farmer, however the disadvantage is that it has to be left in the soil to rot down so that the nutrients are available to the plants, which takes time. They may also contain pathogens which can be harmful to the plant
Artificial fertilisers Artificial fertilisers contain nitrogen compounds such as ammonia which are spread onto the soil normally in soluble granules to increase nitrogen, and other nutrients, concentrations in the soil. The advantages are that the nitrogen is readily available for plants to take up and use. Farmers can also buy types which have special controlled release technology which means there is a constant stable nitrogen content going into the soil. The disadvantages are the cost, as the fertiliser is very expensive whereas natural fertilisers are free. Also after long term use as the fertiliser doesn’t replace the trace mineral content in the soil, these can run out and aren’t present in the crop or the fruit so humans or animals aren’t consuming these trace elements which can be bad for health.
Fertilisers add nutrients to the soil to help increase the productivity of plants
Pesticides There are two types, chemical and biological, both do the job of killing pests which feed on food crops and fruit which ultimately can mean that farmers don’t get the maximum yield from their crops.
A pesticide should be:
• Specific – only target the plant/insect/fungus it is supposed to not the crop or any other organisms • Biodegrade – so once it has been used it doesn’t go into the soil and kill the crop, however it also
needs to have a long shelf life • Cost-‐effective – developing a pesticide costs a lot and new pesticides only are effective for a short
length of time. • Not accumulate – so it doesn’t build up in the food chain and cause problems for other organisms
Biological control This is using the predator of the pest to control the numbers of the pest. Its advantages are:
• Very specific • Once introduced the predator breeds so keeps numbers up so has a long term effect • Pests can’t become resistant
The ideal situation is for the predator to exist in balance with the pest keeping the pest at a level where it has no or little effect.
There are however some disadvantages with this method these are:
• They do not act as quickly, as the predator has to build up its numbers so there is a lag between introducing the predator and seeing a significant drop in the pest numbers
• The predator may become a pest for example if there are few natural predators to it or as the pest population decreases it may use the crops as a food source.
Integrated systems Integrated control involves:
• Choosing animal or plant varieties which are as pest resistant as possible • Managing the environment to provide habitats for natural pest controlling organisms to live • Regularly checking crops for signs of pest activity
• Removing pest mechanically (hands, vacuum, making barriers) • Using biological agents • Only using chemical pesticides as a last resort
All pests damage or compete with plants or animals leading to reduced productivity
Intensive rearing of livestock Intensive farming is about converting the smallest possible amount of food energy into the largest possible amount of animal biomass. This is achieved by minimise the energy lost by animals during their lifetime.
Ways in which this is achieved:
• Movement is restricted – less energy used in muscle contraction • Keeping the environment warm (for warm blooded animals) – reduced heat loss from body • Feeding controlled – animals receive the optimum amount and type of food for maximum growth • Predators are excluded – no loss to other organisms • Selective breeding – produces animals which are the most efficient at converting the food they eat
into biomass • Using hormones to increase growth rates
Biology Unit 4 – Section 3.4.6 See pages at the end of the carbon and nitrogen cycle diagrams, as these are the first bit of this section. Also see section 3.4.5 first as there is some overlap with the use of fertilisers
Effects of nitrogen fertilisers
Reduced species diversity This is because nitrogen rich soils favour the growth of fast growing species, these out compete many other species which causes these other species to die as a result
Leaching This is a process where nutrients are removed from the soil. Rain water will dissolve any soluble nutrients and carry them deep into the soil eventually beyond the reach of the plant roots. These eventually find their way into water courses. They can have a harmful effect on humans if they drink them and can also cause eutrophication.
Eutrophication • In most lakes and rivers there is naturally very little nitrate and so this is the limiting factor for plant
and algal growth • Ass the amount of nitrate increases due to leaching, plants and algae grow massively • As algae mostly grow on the surface massive algal blooms form and this absorbs the light and stops
it reaching the lower depths • Light can’t reach the plants at the bottom so these die • The increase in dead plant matter causes decomposers to grow • These are aerobic so require a large amount of oxygen from the water • This massively increases the BOD (biochemical oxygen demand) • Oxygen then becomes the limiting factor for aerobic organisms such as fish • If these can’t swim away (e.g. they are in a pond) they will die • There is less competition for anaerobic organisms whose populations rise massively • These organisms further digest the plant waste and this leads to more nitrates in the water and also
toxic substances such as hydrogen sulphide
Biology Unit 4 – Section 3.4.7 During succession a number of common things happen:
• The non-‐living (abiotic) environment becomes less hostile – soil forms, nutrients are more plentiful and plants provide shelter
• Greater number and variety of habitats • Increased biodiversity – as different species occupy the habitats • More complex food webs • Increased biomass – especially during the mid-‐stages
(© nelson thornes AQA Biology A text book)
Managing succession • To reach the climax community the land has gone through a lot of stages • Most of the species present in the earlier stages are no longer present • However to get grass to allow cattle to graze we need grass, this is at one of the earlier stages of
succession • To do this we manage succession by not allowing it to proceed past this point • This can be achieved by mowing or allowing cattle to eat the grass • If this was simply left shrubs would develop and then trees such as oaks to make deciduous
woodland
This is one example but there are many. Succession is managed to allow the plants to grow that we want/ are useful rather than letting everything develop to the climax community.