Unit 4 revision cards


Ecology definitions

Ecology - The study of inter-relationships between organisms and their environment

Abiotic – non living components

Biotic – living components - .

Ecosystems - Made up of all the interacting abiotic and biotic features of a specific area

Populations - Species are made up of many groups of individuals called populations.

Community - A community is made up of all the different populations of different species living and interacting in a given place at a given time.

Habitat - A habitat is a place where a community of organisms live.

Ecological niche  - Describes how an organism fits into its environment Refers to where an organisms lives and what it does there Includes all biotic and abiotic requirements for an organism to live No two species will occupy the exact same niche

1 of 57


Due to time constraints and collateral damage, only small areas within a habitat are studied in detail; these samples represent the population as a whole.

The larger the number of samples, the more representative of the community the results will be.

Random sampling – Quadrat Systematic sampling – Transect Size of quadrate – Larger quadrats are used to measure larger species. If the species occurs in groups, a large number of small quadrats should be used.

Number of quadrats – Greater number of species, greater number of quadrats

Position of quadrats – Random - create co ordinates using a random number generator

2 of 57

Investigating populations + mark release recapture

Systematic Sampling:

Line transect – used to illustrate a transition along which communities of plants/animals change.

Abundance – Number of species in a given space

Frequencychance of a particular species occurring within a quadrat

Percentage cover – Estimate of the area within a quadrat that a species occupies To measure the abundance of a mobile species:

Estimate of population = no. individuals caught in first sample x no. caught in second sample

                                                                                     No. recaptured


Proportion of marked/unmarked individuals is the second sample is the same for the whole population

The population has a definite boundary. (no immigration/migration)

Birth/Death is low                                        Marking method is not toxic/ conspicuous  

3 of 57

Variations in population

Population growth curves Growth curves of populations usually have three main phases:

1.)A period of slow growth due to the fact that there is only a limited number of interbreeding species

2.)A period of rapid growth,caused by the ever increase in organisms that are able to reproduce. For each interval of time the population size doubles

3.)Population size begins to level off as there are limiting factors on the population growth such as availability of resources.

Population size

No population growth will continue indefinitely. This is because in time there will eventually be limiting factors that will limit the population size. The various factors that limit population size can be of two types, abiotic and biotic

4 of 57

Variation in populations size


Temperature – Each species has an optimum temperature at which they survive best at. The further a group of organisms are away from this temperature, the smaller there growth rate will be. If they are below the temperature, metabolic rate maybe lower if they are cold blooded. However if they are mammals, they will produce heat during respiration, at low temperatures more energy is used to maintain a stable body temperature and less is used for growth.

Light – Light is the ultimate source of energy for an ecosystem. If light intensity is greater in plants, the more energy they can use to create spores and seeds and so they reproduce quicker.

pH – Affects the function of enzymes. Enzymes work best at different pH levels and so if an organism exists somewhere where there are more appropriate pH levels then they will likely have a larger population.

Water and humidity – humidity affects transpiration rates in plants and the rate of evaporation of water from animals.

5 of 57

Competition + Predation

Competition between members of the same species is intraspecific

  • Competition between members of different species is called interspecific Intraspecific competition:
  • Populations that undergo intraspecific competition are often limited by the number of resources available. It is the avaliability of resources that etermines population size. E.g. robins competing for territory as females only attracted to males with established territory.
  • Interspecific competition: The competitive exclusion principle states that where two species are competing for limited resources the one that uses these resources most effectively will ultimately eliminate the other one. Resources avaliable will hence be reduced, this will limit populations hence there is less energy for reproduction - population size will decrease
  • Predation – As predetors have evolved, they have become better adapted to capture prey, e.g. more protective camoflage, faster movement etc. Prey also become equally more adapted. If either of them have not met the improvements of the other it would brcome extinct.  
6 of 57

Predetor - prey relationships

  • Predators eat there prey, thereby reducing the population of the prey.
  • With fewer prey available, the predators are in competition with one another for the prey that is still left
  • Predator population decreases due to some predators not being able to catch enough prey
  • With fewer predators around, fewer prey are consumed
  • Prey population increases
  • More prey available, predator population also increases

In reality, there is normally more than one food source available so population size fluctuations are rarely so severe. Disease and climatic factors also play a part

Periodic population crashes create selection pressures that only allow certain individuals with the alleles to survive adverse conditions.

7 of 57

Human populations

There are two major factors that have caused an increase in the size of the human population:

The development of agriculture The development of manufacturing that created the industrial revolution

Factors affecting growth and size of human populations

It is the balance between the birth and death rate that ultimately determines whether or not the population is increasing, decreasing or remaining the same.

Individual populations are affected by migration Immigration – joining a population from outside Emigration – leaving a population

Population growth = (Births + immigration) – (deaths + emigration) %

growth rate in a given period = population change during a period x 100

                                                        Population at the start of the period

8 of 57

Human populations

Factors affecting birth rates Economic conditions – less developed countries tend to have higher birth rates

Cultural/religious backgrounds – some countries/religions encourage larger families

Social pressures – in some countries, a larger family improves social standing Birth control – the extent at which contraception/abortion is available affects birth rate

Political factors – governments can influence birth rates through education and taxation

Birth rate = number of births per year x 1000                   

                    Total population in the same year

Factors affecting death rate Age profile – the greater the proportion of elderly, the higher the death rate Life expectancy at birth – Residents of more developed countries tend to live  longer Food supply – Poor nutrition will cause an increase in death rate Safe drinking water – poor quality drinking water will cause an increase in water born diseases Medical care – access to medical care will reduce death rate Natural disasters – the more prone a region is to drought/famine, the higher the death rate War – War will cause an increase on death rate

9 of 57

Energy and ATP

Both plants and animals breakdown organic molecules to make ATP

 Why do organisms need energy?

Living organisms are highly organised systems that require a constant input of energy to prevent them from becoming disordered

. Metabolism – chemical processes

Movement (inside/outside)

Active transport

Production of enzymes/hormones

Maintaining body temperature

10 of 57

Energy and ATP

How does ATP store energy?

The bonds between phosphate groups are unstable and have low activation energies. Water is used to covert ATP into ADP (ATP + H2O ADP + Pi + ENERGY) This is a hydrolysis reaction The reaction is reversible when ADP reacts with Pi in a condensation reaction.

Roles of ATP

ATP is an intermediate energy substance used to transfer energy.

Cells maintain just a few seconds supply of ATP

It is a better immediate energy source than glucose because the energy is more manageable in small quantities.

The hydrolysis of ATP is a single step reaction

11 of 57

Light dependant reaction

The making of ATP

  • Chlorophyll absorbs light energy 2 electrons move to high energy levels and leave the chlorophyll molecule.
  • Electrons are taken up by electron carriers
  • Electrons loose energy at each stage, which is used to make ATP
  1. ETC releases energy
  2. Used to pump H+ from stroma into thylakoid space
  3. By active transport and electrochemical gradient
  4. H+ conc in thylakoid space > stroma so H+ ions pass back from space between two mitochondrial membranes
  5. Through pores which are associated with the enzyme ATP synthetase
  6. Diffuse down conc gradient across thylakoid membrane and Produces ATP by photophosphorylation
  • Photolysis
  • The electrons that are lost from the chlorophyll are replaced by electrons released during the photolysis of water where oxygen is released as a bi-product.
12 of 57

Light independant reaction

The products of the light dependent reaction are ATP and reduced NADP,used to reduce carbon in the LID reaction. This stage does not require light, however it does require the products form the light dependent reaction.  

Calvin cycle

  • 1.Carbon dioxide from the atmosphere diffuses into the leaf through the leaf stomata, in to the cell wall, then into the cytoplasm, and finally into the chloroplast stroma.
  • 2.In the stroma, the carbon dioxide combines with a 5 carbon compound called ribulose biphosphate (RuBP) using the enzyme Rubisco 
  • 3.The combination of the carbon dioxide and the RuBP produces two new molecules of a 3 carbon compound called glycerate 3-phosphate (GP)
  •  4.ATP and reduced NADP  reduce the 3-phosphate to triose phosphate (TP).
  • 5.The NADP is reformed and returns to the light dependent reaction cycle
  • 6.Some triose phosphate molecules are converted to useful organic substances such as glucose.
  • 7.Most triose phosphate molecules are used to regenerate ribulose biphosphate using ATP from the light dependent reaction.
13 of 57

Light independant reaction

Site of the light-independent reaction The light independent reaction takes place in the stroma of the chloroplasts.

The chloroplast is adapted to carrying out the light independent reaction in the following ways:

The fluid from the stroma contains all the necessaryenzymes to carry out the light independent reaction. (Reduction of carbon dioxide).

The stroma fluid surrounds the grana and so the products of the light dependent reaction in the grana can readily diffuse into the stroma.

14 of 57

Limiting factors of photosynthesis

Limiting factors

The rate of photosynthesis is always restricted by just one factor. This is called a limiting factor. Changing the levels of other factors will not affect the rate of photosynthesis.

If light is a limiting factor, increasing the temperature for example will not affect the rate of photosynthesis.   However this will not continue indefinitely. Photosynthesis will eventually be limited by a different factor.

Photosynthesis is made up of a series of small reactions. It is the slowest of these reactions that determines the overall rate of photosynthesis.

The law of limiting factors – At any given moment, the rate of photosynthesis is limited by the factor that is at its least favourable value.

15 of 57

Limiting factors of photosynthesis

light intensity

The rate of photosynthesis can be measured by the volume of O given off or CO2 used up in a given time. When light is a limiting factor, the rate of photosynthesis is proportional to light intensity. The compensation point is the point at which O2 used up in respiration is equal to the O given off in photosynthesis. There is therefore no net gas exchange.

carbon dioxide on the rate of photosynthesis

The optimum CO concentration for photosynthesis is 0.1% whereas the CO2 concentration in the atmosphere is 0.04%. High CO2 concentrations can effect the enzyme catalysed reactions that combine ribulose biphosphate with CO2. The effect of temperature on the rate ofphotosynthesis Between 0 - 25oc the rate of photosynthesis approximately doubles for each 10oc rise in temperature.

Higher temperatures often cause the rate of photosynthesis to decrease since enzymes become denatured.

16 of 57

Photosyntheic pigments

  • Acts as an electron donor
  • Green light is usually reflected -> plants dont use that colour
  • Red and blue light is usually absorbed -> increases rate of photosynthesis

How can a plant benefit from having more that one photosynthetic pigment?

  • More wavelengths of light can be absorbed
  • Hence more photosynthesis can occur

E.g seaweeds adapt to wavelengths of light

  • Have a selection of photosynthetic pigments
  • Red absorbed by seaweed living close to the surface.
  • When the tide comes in it will change
  • The number of photosynthetic pigments means that change in water depth does not affect their ability to photosynthesise
17 of 57

Respiration - Glycolysis


Common to both aerobic and anaerobic respiration

Occurs in the cytoplasm

Glucose (6 Carbon) is split into pyruvate (3 Carbon) 4Stages of Glycolysis

1.Glucose is activated by phosphorylation - Two ATP molecules are used so that two inorganic phosphate molecule can bind onto the glucose molecules making it more reactive, since its activation energy is lowered for the enzyme catalysed stage.

2.The phosphorylated glucose molecules split into two (3C) trios phosphate molecules.

3.Triosphosphate is oxidised -Hydrogen is removed from each triosphosphate molecules to the hydrogen carrier NAD to make NADH (reduced NAD)

4.Production of ATP – Triosphosphate is converted into Pyruvate (another 3 carbon molecule). As this occurs 2 molecules of ATP are regenerated from ADP and Pi.

18 of 57

Glycolysis + Link

Yield from glycolysis

  • 2 ATP
  • 2 NADH
  • 2 pyruvate

The link reaction

Pyruvate from Glycolysis is actively transported into the matrix of the mitochondria Pyruvate undergoes a series of reactions

Pyruvate is oxidised by removing hydrogen to from NADH, COand a two carbon molecule called an acetyl group.

The acetyl group reacts with an enzyme called coenzyme A. (CoA)

This forms acetyl coenzyme A. (acetyl CoA)

Pyruvate + NAD + CoA -----> Acetyl CoA + NADH + CO

19 of 57


  • Acetyl CoA is too large to bind to oxaloacetate (4C) hence CoA splits off
  •  Acetyl CoA (2C) reactions with a (4C) molecule to form a (6C) molecule
  • The (6C) molecule loses COand and NAD+ is reduced.
  • This produces a 5C molecule that gets reduced by 2 NAD+ molecules and one FAD molecule. It loses CO2
  • produces one ATP molecule as a result of substrate level phosphorylation.

Called a cycle because the 4C molecule is the starting and ending material

For one glucose:

3NADH + 1ATP+ 1FADH2         PER ACETYL  (+3CO2)

6NADH + 2ATP+ 2FADH2         PER glucose molecule  (+6CO2)


20 of 57


Takes place in the inner membrane of the mitochondria

Stages of the electron transport chain

  • 1.NADH and FADH2 are oxidised, thus releasing a proton and an electron.
  • 2.The protons are actively transported into the intermembranous space. (between the inner and outer membrane)
  • 3.The electron is taken up by an electron carrier
  • 4.The electron-carrier is therefore reduced.
  • 5.The electron from the reduced carrier is oxidised again by passing the electron to a new carrier which in turn also becomes reduced.
  • 6.By passing the electron down a chain of electron carriers through oxidation/reduction reactions the electron loses energy in the process. It is this lost energy that is used to combine ADP with Pi to form ATP by ATPase
  • 7.Protons accumulate in the intermembranous space and so diffuse back into the cell through special protein channels
  • 8.At the end of the chain, electrons combine with the proton as well as oxygen to form water.
  • 9.Oxygen is therefore the terminal electron acceptor in the electron transport chain.
21 of 57

Anaerobic respiration

Since oxygen is the final acceptor of electrons in the electron transport chain, when it is not present, ATP cannot be produced in this way.

Instead, ATP is produced anaerobically.

Ones produced in Glycolysis, products such as pyruvate and hydrogen must be constantly removed. Furthermore, the hydrogen from NAD must be released so that it can be used again.

In order to do this the pyruvate with react with reduced NAD in plants and some microorganisms, pyruvate is converted into ethanol and water, whereas in mammals and other organisms it is converted into lactate.  

22 of 57

Anaerobic respiration

Production of ethanol in plants/some organisms

Occurs in organisms such as yeast and root hair cells for example that are in waterlogged soil The reaction for the production of ethanol and CO2 is as follows: Pyruvate (3C) + reduced NAD -> ethanol (2C) + carbon dioxide (1C) + NAD

Production of lactate in animals

 As with any other form of anaerobic respiration, the reduced NAD must be converted back to NAD for the process to continue, and so it reacts with pyruvate.

The reaction is as follows Pyruvate (3C) + reduced NAD lactate (3C) + NAD Lactate being acidic will cause pain and cramps to be experienced in muscle tissue. It must therefore be removed quickly by oxidising it with O2 to release more energy or taken to the liver by the blood to be stored as glycogen.

23 of 57

Food Chains and food webs

  • The ultimate source of energy in an ecosystem comes from sunlight
  • This energy is converted to an organic form using photosynthesis which is then passed between organisms Producers – photosynthetic organisms that obtain their energy through the photosynthesis of sunlight Consumers – Organisms that feed off of other organisms.
  • They do not produce their own food by photosynthesis. Consumers can be primary, secondary, etc depending on which stage of the food chain they are at.
  • Decomposers – When producers/consumers die, the energy that they contain can be accessed by decomposers that will break down the larger more complex molecules that they are made of into smaller simple components again. The simple components are recycled as they are taken up again by plants.
  • Consumers include, fungi and bacteria and to a lesser extent animals such as detritivores.
  • Food chains - Describes the feeding relationships between organisms each stage of the chain is referred to as being a “trophic level”
  • Food webs - most animals do not rely upon a single food source. Within a single habitat there may be many food chains linked together to form a food web
24 of 57

Energy transfers through an ecosystem

Only 1 - 3% of the energy available to plants is converted into organic matter

This is because:

Over 90% of the suns energy is reflected back into space by the atmosphere

Not all wavelengths of light can be absorbed by plants in photosynthesis

Light may not actually fall of the chlorophyll molecule

Limiting factors may slow down photosynthesis The rate at which energy is stored is called “net production”

Net production = gross production – respiratory losses  

25 of 57

Energy transfers through an ecosystem

Only approximately 10% of the energy stored in plants is passed on to primary consumers. Secondary and tertiary consumers however are more efficient, transferring approximately 20% of the energy available to them. The low amount of energy absorbed at each stage is due to:

Some of the organism not being eaten Some parts can be eaten but not digested Some of the energy is lost in excretion

Some of the energy is lost via respiration that is used to maintain a high body temperature. This is especially the case in mammals Most food chains have only 4/5 trophic levels since there is not enough energy to support a large breeding population at trophic levels higher than these   Calculating the efficiency of energy transfers (UNITS = kj-2 year-1)

The formula used to calculate the energy transfer is:

Energytransfer = Energy available after the transfer x 100       

                           Energy available before the transfer                                                             

26 of 57


they do not provide any quantitative information.

Pyramids of number Usually the higher up in trophic levels you go the fewer organisms there are. There are however significant drawbacks to this method. These include:

  • No account is taken for size. For example 1 tree will count the same as one piece of grass.
  • The number of individuals can be so great it can be almost impossible to count them for example all of the grass in a field.

Pyramids of biomass

  • This method is more reliable than the last as it does take size into account.
  • Biomass is the total mass of plants/animals of species in a given place.
  • Biomass can be unreliable however as there are various different amount of water than can be stored in an organism.

Dry mass is therefore measured instead. However, to do this, organisms must be killed Both pyramids of biomass and numbers can be unreliable as they do not account for seasonal differences in the amount of organisms present.

27 of 57


Pyramids of energy

  • The most accurate representation of energy flow in a food chain
  • Collecting data can be difficult/complex
  • Data is usually collected in a given area for a given period of time (e.g. a year)

This is more accurate than using biomass since different organisms may have the same mass but one may have more fat for example than the other and so will have more energy

The energy flow in this type of pyramid is usually measured in kjm-2 year-1

28 of 57

Agricultural ecosystems

What is an agricultural ecosystem?

animals/plants used to made food for humans. Agriculture tries to ensure that as much of the energy available from the sun is transferred to humans as possible which Increases productivity

What is productivity?

The rate at which something is produced The rate at which plants for example assimilated energy from the sun into chemical energy is called the gross productivity and is measured in Kjm-2 year-1 Some of the chemical energy that is assimilated by plants is used for respiration, the remainder is called the net productivity.  

Net productivity = gross productivity – respiratory losses

Net productivity is affected by two main things:

  • 1.)The efficiency of the crop carrying out photosynthesis. This can be improved if the limiting factors are reduced.
  • 2.)The area of the ground covered by the leaves of the crop
29 of 57

Comparisons of natural and agricultural ecosystems

Energy Imput

To maintain an agricultural ecosystem it is important to prevent the climax community from forming by excluding the other species in that communityIt takes an extra input to do this seeing as it requires removing pests, diseases, feeding animals and removing weeds.

The energy to do this comes from two sources:

  • 1.)Food – farmers use energy to do work on the farm. Energy comes from the food that they eat.
  • 2.)Fossil fuels – Farms have become mechanised. The energy that powers these machines comes from fossil fuels.


  • Productivity in natural ecosystems is relatively low
  • Energy input in agricultural ecosystems removes limiting factors to improve productivity
  • Other species are removed to reduce competition for light and other nutrients
  • Fertiliser is added to the soil to reduce the limiting factor of nitrate concentration on growth.
30 of 57

Pests and Pesticides

What are pests and pesticides?

  • A pest is an organism that competes with humans for food/space
  • Pesticides are poisonous chemicals that kill pests
  • Herbicides kill plants, insecticides kill insects, fungicides kill fungi, etc

An effective pesticide should:

Be specific – only kills the organism it is directed at. Should not kill humans, natural predators of the pest, earthworms, and to pollinators such as bees

Biodegrade – once applied should break down into harmless molecules.

Be cost effective – pesticides can only be used for a limited amount of time until the pest develops resistance Not accumulate – does not build up in parts of an organism or food chain

31 of 57

Biological control

Biological control - Uses other organisms to maintain a low population of pests

  • The surviving pest would be able to then multiply rapidly
  • Specific (to one pest)
  • Maintains low population of the pest
  • Pests dont develop resistance
  • cost effective seeing as the organism can reproduce itself
  • No bioaccumulation

Disadvantages of biological control include:

  •  Acts more slowly, interval of time between introducing the biological control and actually seeing its effect
  • The control organism its self may become a pest

  • Does not eradicate the pest completely
32 of 57

chemical control

Involves using chemicals to get rid of pests

  • Herbicides - Kills weeds that compete with agricultural crops for energy - reducing competition hence crops recieve more energy which, in turn increases productivity.
  • Fungicides - Kill fungi infections that damage crops - crops use more energy for growth and less to fight infection - increases productivity
  • Insecticides - Kill insect pests that kill and damage crops - less biomass is lost - less energy and resources needed to repair it - increases productivity


  • Fast acting


  • Will have an effect on non-pest species
  • Pests can develop resistance -> new pesticides have to be constantly developed
  • Must be reapplied regularly -> very expensive
33 of 57

Integrated pest control

  • Managing the environment and ensuring there are nearby habitats for predators
  • Regulating the crops so early action can be taken
  • Removing the pest mechanically (by hand)
  • Using biological agents if necessary
  • Using pesticides as a last resort

Uses both biological and chemical agents

  • Combined effect of both biological and chemical agents can reduce pest numbers even more than either method alone, increasing productivity
  • Pests cant become resistant to the biological control
  • Reduces environmental impact from pesticides
  • Can reduce cost if one method is particularly expensive
34 of 57

Intensive rearing and energy conversion

As you move down a food chain, energy is gradually lost to respiratory losses. This is because in mammals, the rate of respiration high since the organism needs to maintain a high body temperature as well as move around to avoid predators and catch prey. This leaves little energy to be converted into biomass. To ensure that farming of animals is efficient, respiratory losses must be decreased.

This can be done as follows:

  • Movement is restricted so little energy is lost in muscle contraction
  • The environment can be kept warm so less energy is required to maintain a high body temperature
  • Nutrition is carefully controlled to ensure organisms receive the optimum amount and type of food so that there is maximum growth and little wastage
  • Predators are excluded and so there is no loss to other organisms

Other means may also include:

  •  selectively breeding animals that are more efficient in converting the food into biomass
  • Using hormones to increase growth rate
35 of 57


  • Chemicals that provide crops with minerals for growth e.g. nitrates.
  • Crops use up minerals in the soil as they grow, so their growth is limited when there arent enough minerals.
  • Adding fertiliser replaces the lost minerals, so more energy can be used for growth - increasing efficiency of energy conversion and productivity.

Natural fertilisers are organic matter - e.g. manure

Artificial fertilisers are inorganic - contain chemicals e.g. ammonium nitrate


  • can cause eutrophication by leeching into rivers or ponds
  • changes the balance of nutrients in the soil -> too much of a particular nutrient can cause crops and other plants to die
  • If too much fertiliser is used money can be wasted
36 of 57

Carbon Cycle

  1. Producers, consumers add decomposers add CO2 to the air by respiration
  2. Carbon is stored in tissues as organic matter (carbohydrates, lipids, proteins)
  3. Carbon is passed up the trophic levels by feeding
  4. Plants remove CO2 from air by photosynthesis
  5. Animals excrete carbon as waste products
  6. Decomposers decay detritus and excretory products / add carbon to soil
  7. Detrivores digest detritus to small pieces / large surface area
  8. Saprophytes digest smaller detritus by
    1. Extracellular digestion by secreting enzymes
    2. Absorb resulting nutrients across plasma membrane
    3. Releases inorganic matter (CO2, H2O, mineral ions) into soil
  9. Fossil fuels
  10. Combustion releases CO2 into air
  11. Fossilisation of carbon atoms in organic compounds in dead remains (plants, animals) and excretory products (animals)
37 of 57

Carbon Cycle

Respiration and Photosynthesis cause fluctuations in CO2 levels

  • Photosynthesis takes up more CO2 than is released by respiration
  • CO2 conc
  • Higher at night than at daylight; light-dependent reaction cannot take place
  • decreases during the day because it is being removed from the atm by photosynthesis
  • Peaks at winter time due to high oil consumption; low rate of photosynthesis due to cooler temp, shorter day length, loss of leaves.
  • Most plant growth occurs in the spring/summer (light intensity is greatest) hence more photosynthesis occurs removing more CO2 from the atmosphere.
  • Variation in a graph due to wind mixing CO2 with the surrounding air
  • Graph should show conc over whole area rather over a specific area
  • Rate of photosynthesis and respiration are balanced in a rain forest
  • Forests grow for a long time and have stored lots of carbon in their tissues, other plants have stored carbon as cellulose and lignin
38 of 57

Global Warming

When solar radiation reaches the earth, some is reflected back into space, some is absorbed by the atmosphere and some reaches the earth.

The radiation that reaches the earth is absorbed, and reemitted back into space.

However, some of this radiation is absorbed by clouds and greenhouse gases that will reflect the radiation back to earth.

This causes a heating effect known as the greenhouse effect Greenhouse gases

CO2 - Responsible for approx 50 – 70% of global warming

Remains in the atmosphere for >100 years Its increase is mainly due to human activity (burning fossil fuels)

Methane -

Remains in the atmosphere for ~ 10 years Produced when microorganisms breakdown the organic molecules of which other organisms are made (decomposers/intestinal dwellers)  

39 of 57

Global warming

The earth has always shown periodic fluctuation in temperature so we cannot say for certain that human activity is to blame. What we can say however is that the atmospheric levels of carbon dioxide have increased since the start of the industrial revolution and that these seem to be linked with increasing temperature

Consequences of global warming

Affects the niches available in a community, leading to an alteration in the distribution of species

Melting of polar ice caps and therefore increasing sea levels

High temperature may lead to crop fail

Benefits – increased rate of photosynthesis, greater rain fall, possible twice a year harvest  

40 of 57

The nitrogen cycle

Plants take up nitrates (NO3-) via active transport since they are moving against a concentration gradient. There are fourm main stages of the nitrogen cycle:  

1)Ammonification Production of ammonia from organic ammonium containing compounds Saprobiotic bacteria feed on the materials releasing ammonia which converts to ammonium in the soil (produces ammonium ions NH4+)


  • Ammonium NH4+ / nitrite NO2
  • Nitrite / nitrate
  • By nitrifying bacteria


  • Removal of oxygen from NO2- and NO3- to make N2(g)
  • By anaerobic denitrifying bacteria (in anaerobic conditions)

Oxygen is required for nitrification and so oil is kept aerated by farmers to increase productivity

41 of 57

Nitrogen cycle

4) Nitroogen fixation

  1. N2(g) is converted to nitrates by lightning N2(g) + O2 NO3-
  2. By Haber process: N2(g) + H2 → ammonia NH3 //used to make fertilisers / added to soil / leakage of ions into river
  3. By Nitrogen-fixing bacteria by anaerobic nitrogenase
  4. Live free in nodules on the roots of legumes (peas, beans)
  5. NH4+ is assimilated by legumes into amino acid

symbiotic relationship bacteria get their carbon-containing compounds from the plant while the plant gets nitrates

42 of 57

Eutrophication + leeching


  • Nitrates leeched from fertilised fields which stimulates the growth of algae (algae bloom) exponentially in ponds and rivers (no limiting factor).
  • Large amount of algae block light from reaching the plants below.
  • Eventually plants die because they cant photosynthesise (light becomes the limiting factor).
  • Bacteria feed on the dead plant matter along with saprobiotic algae (grows exponentially).
  • This increased numbers of bacteria reduce the oxygen concentration in water by carrying out aerobic respiration hence it becomes a limiting factor.
  • Fish and other aquatic animals die due to the lack of dissolved oxygen.


  • Rain water can dissolve soluble nitrates and carry them deeper into the soil beyond the reach of plant roots.
  • The nitrates may then be able to find there way to water courses and into water that is used for human consumption.
  • High levels of nitrates in water can cause inefficient transport of oxygen to the brain
43 of 57


1) Section of bare land is colonised by a pioneer species

2) Changes the environment (abiotic factors) to make it less hostile

3) This enables other species to colonise and survive and increases biodiversity.

4) Outcompete pioneer species

5) Stability increases further and a less hostile environment is produced

6) Eventually a climax community

44 of 57


What is conservation?

Conservation is the act of managing the earths resources in such a way to make maximum use of them in the future. The main reasons for conservation are:

  • Ethical – Other species should be allowed to coexist. Respect for living things is preferable to disregard for them.
  • Economic – Living organisms posses a giant gene pool with a capacity to produce millions of substances
  • Cultural and aesthetic – They add variety to every day life Conserving habitats by managing succession
45 of 57

Inheritence - geontpe an phenotype

Genotype and phenotype

  • Genotype is the genetic constitution of an organism that describes all the alleles that an organism contains
  • The genotype sets the limits to which characteristics can vary
  • Any change to the genotype is called a mutation. This will be passed on to the next generation if it is present in the gametes
  • A phenotype is an on observable characteristic of an organism.
  • A phenotype will vary depending on the genotype and the environmental conditions.
  • A change to the phenotype is called a modification
46 of 57

Inheritance - definitions

  • Gene - A sequence of bases on a DNA molecule that codes for an amino acid
  • Allele - An alternative form of the same gene
  • Genotype - The genetic constituion of an organism (alleles an organism has e.g. BB or Bb)
  • Phenotype - The characteristics of an organism, often visible, reulting from both its genotype and environmental influences e.g. Brown eyes.
  • Dominant - An allele that is always expressed on the phenotype of an organism.
  • Recessive - The condition in which the effect of an allele is apparent on the phenotype of a diploid organism only in the presence of another identical allele.
  • Codominant - Alleles that are both expressed in the phenotype - niether one is recessive.
  • Locus - The fixed position of the gene on the chromosome (Alleles of a gene are found at the same locus on each chromasome in a pair.
47 of 57

Monohybrid inheritance

  • Monohybrid inheritance, is the inheritance of a single gene
  • Consider pea pods which come in two different colours:
  • When pea pods are bred only with one another until they consistently produce green coloured offspring, they are said to be pure bred.
  • The organisms in pure breeding are said to be homozygous.
  • If pure breeding green pods are crossed with pure breeding yellow pods, then all of
  • the offspring are referred to as the “first filial” or “F1” generation.
  • F1 generations are always heterozygous.
  • When you breed pure bred organisms with one another you can then deduce which alleles are dominant and which are recessive
  • For example pure bred yellow pea pods bred with green pea pods will only produce green pea pods. This is means that all of the f1 generation have a yellow allele and a green allele. From this it is clear that the green must be dominant and the yellow recessive
  • Breeding two F1 generations will produce an F2 generation. In the F2 generation there will most likely be a ratio of 1:2:1 where the first one may be homozygous dominant, the 2 heterozygous and the other 1, homozygous recessive.
48 of 57


Females have ** chromosomes whereas males have XY Sex inheritance in humans

  • Males produce both X and Y chromosomes
  • Recessive phenotypes are more likely to be present in men because they only have 1 allele where as females would need two recessive alleles for the gene to be expressed.
  • Haemophilia is for this reason almost entirely only present in men and not women
  • Males can only obtain the disease from their mother as they do not receive a Y chromosome from the father Males cannot pass the disease on to their sons but they can to their daughters
49 of 57

Inheritance - Co-dominance

  • Both alleles are expressed in the phenotype
  • The snap dragon plant is a common example of co – dominance. This is shown when you observe how the plant can be of three different colours, red pink and white. If the alleles were not co – dominant only red a white plants would be able to be produced
  • If a snapdragon with red flowers is cross with a snap dragon with white flowers the offspring will have pink flowers
  • Crossing two pink flowered snap dragon plants will produce 50% pink flowered snap dragons, 25% white flowered snap dragons and 25% red flowered snap dragons
  • Remember the lowercase letter

Multiple alleles

Sometimes a gene can have many different alleles. An example of this is the human ABO blood groups. Although there are three different alleles for the blood groups, only two can be present in an organism at any one time

Multiple alleles and dominance hierarchy When there are multiple alleles, some are more likely to be more dominant than others. They are they then arranged in a hierarchy according to which alleles they are dominant over.  

50 of 57


Predicts that the frequencies of alleles wont change from one generation to the next.

Only true under certain conditions:

No mutations arise The population is isolate (no immigration emigration) There is no selective breeding The population is large Mating within the population is random

  • Can be used to predict allele frequency of a population.
  • Can also be used to predict whether the equation applies to particular alleles in certain populations ie. to test whether or not a selection pressure is taking place - if the frequencies do change then there must be a selection pressure.
51 of 57

Hardy-Weinburg equations

  • Mathematical model that is used to calculate allelic frequency
  • Let A = p and a = q. In a population that has just two alleles, p + q = 1.00 (100%)
  • As there are only 4 possible combinations of A and a (AA, Aa, aA and aa) then, p2+2pq+q2 = 1.00. You can use this equation to calculate the frequency of one genotype if you know the frequencies of the others.
52 of 57


Not all alleles are equally likely to be passed on since some organisms may have

characteristics that improve their chances of survival Reproductive success and allelic frequency

  • The difference between the reproductive success of individuals affects the allelic frequency
  • All organisms produce more offspring than can be supported by the supply of food, light, minerals etc
  • Despite too many offspring, populations stay the same
  • This means there is competition between members of the same species to survive
  • There will be a gene pool within any population
  • Some individuals will contain certain alleles that allow the to be better able to survive
  • They are therefore more likely to produce offspring

The alleles that give the best competitive advantage are most likely to be passed on

Over years the number of individuals with the advantageous alleles will increase

What is advantageous depends upon the environmental conditions

53 of 57

Types of Selection

Depending on which characteristics are favourable, selection will produce a number of different results.Selection may favour certain individuals that vary in one direction from the mean. Selection may favour average individuals that have characteristics closer to the mean

Directional selection

  • Directional selection most often occurs when there has been a change in the environmental conditions.
  • It is where individuals with alleles with certain characteristics of an extreme type are more likely to survive and reproduce. For example it makes them better at catching prey. So only the best survive and pass on their alleles to the next generation.
  • Tends to become more and more extreme over time

Stabalising selection

  • Stabilising selection often takes place where the environmental conditions have remained the same and individuals with alleles in the middle range are most likely to survive It also reduces the ramge of possible phenotypes. An example would be where temperature fluctuates throughout the year where organisms at each extreme will most likely not survive.
54 of 57


  • Occurs when a physical barrier seperating and preveting interbreeding between populations.
  • Variation due to mutation
  • Different environmental conditions mean that the separated population will experience selection pressures.
  • Selection for advantageous features / characteristics
  • The selected organisms are likely to survive and reproduce
  • Leads to a change in allele frequency accumulating in the gene pools of the seperated populations.
  • Causes changes in phenotype frequencies
  • Occurs over a long period of time
  • Eventually individuals of the seperated population will no monger be able to interbreed with the original population to produce fertile offspring hence they are now two seperate species.
55 of 57

Human populations

Factors affecting death rate

Age profile – the greater the proportion of elderly, the higher the death rate Life expectancy at birth – Residents of more developed countries tend to live  longer

Food supply – Poor nutrition will cause an increase in death rate Safe drinking water – poor quality drinking water will cause an increase in water born diseases

Medical care – access to medical care will reduce death rate

Natural disasters – the more prone a region is to drought/famine, the higher the death rate

War – War will cause an increase on death rate

56 of 57


Climax communities reach their current state by undergoing a series of successive changes.

  • Some of the organisms at previous stages are no longer present in the climax community
  • They may have been out competed by other species, or their habitat is no longer available.
  • Grazing by sheep can prevent a climax community forming since the seedlings of trees can not germinate
  • If the factor that is preventing succession taking place is removed, then succession will continue until it reaches its climax community
57 of 57


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all DNA, genetics and evolution resources »