Aqa a2 biology unit 4

AQA A2 BIOLOGY

Unit 4

POPULATIONS KEY WORDS:• Abiotic – non-biological factor that makes up part of an ecosystem• Biotic – biological factor that makes up part of an ecosystem• Biodiversity – the range and variety of living organisms within a particular area• Biomass – total mass of living material in a specific area at a given time. (usually dry mass as

amount of water varies)• Climax Community – the organisms that make up the final stage of ecological succession• Community – the organisms of all species that live in the same area.• Conservation – method of maintaining ecosystems and the living organisms that occupy them• Consumer – any organism that obtains energy by ‘eating’ another• Ecological Niche – all conditions and resources required for an organism to survive, reproduce

and maintain a viable population• Ecosystem – self-contained functional unit made up of all the interacting biotic and abiotic

factors in a specific area• Habitat – the place where an organism lives• Limiting Factor – a variable that limits the rate of a chemical reaction• Population – a group of individuals of the same species that occupy the same habitat at the

same time• Producer – an organism that synthesises organic molecules from simple inorganic ones.

(photosynthetic, 1st trophic level etc.)• Species – a group of similar organisms that can breed together to produce fertile offspring

• Fill in words to check understanding of key words:

The study of the inter-relationships between organisms and their environment is called ____. The layer of land, air and water that surrounds the Earth is called the ____. An ecosystem is a more or less self-contained functional unit made up of all the living or ____ features and non-living or _____ features in a specific area. Within each ecosystem are groups of different organisms called a _____, which live and interact in a particular place at the same time. A group of interbreeding organisms occupying the same place at the same time is called a _____, and the place where they live is know as a _____.

ANSWER:• The study of the inter-relationships between

organisms and their environment is called ecology. The layer of land, air and water that surrounds the Earth is called the biosphere. An ecosystem is a more or less self-contained functional unit made up of all the living or biotic features and non-living or abiotic features in a specific area. Within each ecosystem are groups of different organisms called a community, which live and interact in a particular place at the same time. A group of interbreeding organisms occupying the same place at the same time is called a population, and the place where they live is know as a habitat.

Investigating Populations

• Quadrats• Random Sampling• Transects and Systematic Sampling• Measuring abundance• Mark-release-recapture• Analysing data• Ethics and Fieldwork

Quadrats• 3 factors to consider when using quadrats:

- size to use. When studying larger species, you will require larger quadrats and vice versa. Although when the species in bunched in groups in random places in the area, (ie. the species are not evenly distributed throughout the area), a large number of small quadrats will give more representative results than a small number of large quadrats.

- number of sample quadrats to record within area. The more sample quadrats, the more reliable the results will be. However it can be time consuming if too many samples are taken, so there must be a balance between result reliability and time available. The greater the number of the different species present in the area being studied, the more quadrats required to produce valid results.

- position of each quadrat within the area. Random sampling is used, in order to produce statistically significant results.

• Point quadrats• Frame quadrats

Back to Investigating Populations Next to Population Size

Point Quadrats

• A horizontal bar supported by 2 legs.• At set intervals along the bar are ten holes,

through each of which a long pin may be dropped.

• Each species that the pin touches is recorded.

• Frame quadrats


Frame Quadrats

• A square frame divided by string or wire into equal size subdivisions.

• The quadrat is randomly placed in different locations within the area being studied.

• The abundance of each species is recorded.

• Point quadrats


Random Sampling

• It is important to sample randomly in order to avoid bias.

• METHOD:1. Lay out 2 tap measures at right angles along two sides of the study area.2. Obtain a series of coordinates by using random numbers taken from a table/random number generator.3. Place a quadrat at the intersection of these coordinates and record the species within it.


Transects and Systematic Sampling

• Systematic sampling is when samples are taken at fixed intervals, usually along a line.

• A line transect uses a tape measure stretched across the ground in a straight line. Any species that touches the line are recorded.

• A belt transect uses 2 lines, and all the species present between the 2 lines are recorded.

• An interrupted belt transect uses quadrats that are placed at intervals, and species within them are recorded.

• Transects are used across areas where there are clear environmental gradients.


Measuring Abundance• How abundance is measured depends upon the size of the

species being counted and the habitat.• Frequency – the likelihood of a particular species occuring in

a quadrat. Useful when a species is hard to count, e.g. Grass. For example, if grass occurs in 15/30 quadrats, the frequency of it’s occurence is 50%.

• Percentage cover – an estimate of the area within a quadrat that a particular plant species covers. Again useful if a species is hard to count. This method enables data to be collected rapidly.

• To ensure reliable results, a large sample size is necessary. The larger the sample size the more representative of the community the results will be.


Mark-Release-Recapture• A known number of animals are caught,

marked in some way, and then released back into the community. Later, a number of individuals are collected randomly and the number of marked individuals is recorded.


Total number of individuals in the first sample xEstimated population size = Total number of individuals in the second sample Number of marked individuals recaptured

( )

Analysing Data1. Present data in a table/graph. This makes it easier to

compare data (e.g. 2 different locations.)2. Statistical analysis of data.3. Check whether results are due to chance.4. Data analysed for possible correlations and causes.5. Statistical tests can be used to calculate the strength of

the correlation.

However, the two factors may correlate very well, but it is possible that both of them are affected by the same environmental factor.


Ethics and Fieldwork• Where possible, the organisms should be studied in situ.

(Where they are found) If it is necessary to remove them, the numbers taken should be kept to the absolute minimum.

• Any organisms removed from a site should be returned to their original habitat as soon as possible. This applies even if they are dead.

• A sufficient period of time should elapse before a site is used for future studies.

• Disturbance and damage to the habitat should be avoided. Trampling, overturning stones, permanently removing organisms etc. Can all adversely affect a habitat.

• There must be an appropriate balance between the damage done and the value of the information gained.Back to Investigating Populations Next to Population Size

Population Size

No population continues to grow indefinitely because certain factors limit growth, for example; availability of food, light, water, oxygen and shelter, the accumulation of toxic waste, disease and predators.

Population Growth Curves• The usual pattern of growth for a natural population has 3 phases.1. Slow growth. Initially small numbers of individuals reproduce.2. Rapid growth. The ever-increasing number of individuals continue to

reproduce. The population size doubles during each interval of time.3. Stable state, no growth. Growth is limited by factors such as increased

predation or food supply. The graph therefore levels out, some cyclic fluctuations due to variations in factors.

p

Abiotic Factors• The non-living part of the environment conditions that influence the size

of a population:- Temperature; Each species has a different optimum temperature at

which it is best able to survive. Cold-blooded animals + plants: temperatures below optimum, enzymes work more slowly and so their metabolic rate is reduced. Populations therefore grow at a slower rate. Temperatures above optimum, enzymes undergo denaturation and therefore they work less efficiently. Again the population grows more slowly. Warm-blooded animals: can maintain a relatively constant body temperature. However, the further the temperature of the external environment gets from their optimum temperature, the more energy these organisms expend in trying to maintain their normal body temperature.This leaves less energy for growth and so they mature more slowly, and their reproductive rate slows. The population size therefore decreases.

Abiotic Factors cont.- Light; is a source of energy for ecosystems. The rate of photosynthesis

increases as light intensity increases. The greater the rate of photosynthesis, the faster plants grow and the more seeds they produce. There population size is therefore potentially greater and in turn the populations size of the animals that feed on the plants is too greater.

- pH; Enzymes operate most effectively at their optimum pH. Population size is larger where the appropriate pH exists and decreases/becomes non-existent where the pH is too far from optimum.

- Water and humidity; Where water is scarce, populations are small and consist of species well adapted to living in dry conditions. Humidity affects the transpiration rates in plants and the evaporation of water from the bodies of animals. The population of species adapted to low humidity will be larger than those without these adaptations.

Practice questions

1. An ecologist was estimating the population of sandhoppers on a beach. 100 sandhoppers were collected, marked and released again. A week later 80 sandhoppers were collected, of which five were marked. Calculate the estimated size of the sandhopper population on the beach.

2. When using mark-release-recapture technique, explain how each of the following might affect the final estimate of a population:a.) The marks put on the individuals captured in the first sample make them more noticable to predation and so proportionally more are eaten than unmarked individuals.b.) Between the release of marked individuals and the collection of a second sample an increased ‘birth’ rate leads to a very large increase in the population.c.) Between the release of marked individuals and the collection of a second sample, disease kills large numbers of all types of individual.

Answers

1. 100 x 80 =1600 5

2. a.) Population over-estimated as there will be proportionally fewer marked individuals in the second sample.b.) Population over-estimated as there will be proportionally fewer marked individuals in the second sample because all the ‘new’ individuals will be unmarked.c.) No difference because the proportion of marked and unmarked individuals killed should be the same.

Application Questions

1. Suggest a reason why even dead organisms should be returned to the habitat from which they came.

2. Suggest why it is beneficial to a habitat that further investigations are not carried too soon after an initial study.

3. In the study of a seashore, students turn over large stones to record the numbers of different organisms on their underside. Suggest reasons why it is important that these stones are replaced the same way up as they were originally.

4. In the case of experienced ecologists obtaining data that enables habitats to be conserved, the benefits usually outweigh any damage that they cause to the habitats. This makes their work ethically justifiable. It might be said that the same is not true of school or college students performing field studies. Give reasons for and against A-level students carrying out ecological investigations in this field.

Answers

1. They can be eaten by other organisms and so provide energy and nutrients to the ecosystem.

2. It allows the habitat to recover from any disturbance/removal of organisms. The results of a further study carried out too soon after may result in data that are not typical of the habitat under ‘normal’ conditions.

3. The organisms live beneath stones so they remain moist when not covered by the tide. If the stone is left upside down the organism may become desiccated and die.

4. FOR:- Practical experience aids learning/better than theoretical study- Experienced ecologists have to start somewhere- Students may become ecologists and so aid conservation in the long term AGAINST:- Students are inexperienced and therefore more likely to damage habitats- Informations could be provided by theory/videos- Large number of A-level students puts pressure on/increase damage on popular

sites

Competition• Intraspecific – individuals of same species compete for the

same resources. (food, water, breeding sites etc.) Lower the availability, the smaller the population and vice versa. e.g. Limpets competing for algaeOak trees competing for resources (some grow larger and restrict light availability)Robins competing for breeding territory. (females only attracted to those who have established breeding territories)

• Interspecific – individuals of different species compete for resources such as food, light, water etc. Where 2 species occupy the same niche, the competitive exclusion principle applies. This principle states that where 2 species are competing for limited resources, the one that uses these resources most effectively will ultimately eliminate the other.

Predation• A predator is an organism that feeds on

another organism, known as their prey.• In laboratory, the prey is usually exterminated

by the predator. This is because the situation in their natural environment is different. In their own habitat, the area over which the population can travel is greater and there are more potential refuges. In these circumstances, some of the prey can escape predation.

Predator-Prey Relationship + Population Size

• Predators eat prey, prey population decreases.• Fewer prey available for predator food and

limits chances of survival for some predators, predator population decreases.

• Fewer predators, so fewer prey eaten. Prey population increases.

• More prey available as food, so predator population increases.

Human Populations

• Recent explosion in human population size:- the development of agriculture.- the development of manufacturing and trade that created the industrial revolution.

• We are currently in the exponential phase, in which our population grows rapidly rather than gives way to the stationary phase in which the population stabilises.

Factors affecting growth

• Immigration (Individuals join from outside.)• Emigration (Individuals leave a population.)

Population growth = (births + immigration) – (deaths + emigration)

% Population growth rate = population change during period -------------------------------------------- x100 population at the start of the period

Factors affecting birth rate• Economic conditions – countries with a low per capita income tend to

have higher birth rates.• Cultural and religious backgrounds – Some countries encourage larger

families and some religions oppose birth control.• Social pressures and conditions – In some countries a large family

improves social standing.• Birth control – The extent to which contraception and abortion are

used markedly influences the birth rate.• Political factors – Governments influence birth rates through education

and taxation policies.

number of births per yearBirth rate = total population in the same year x 1000

Factors affecting death rate• Age profile. The greater the proportion of elderly people in a

population, the higher the death rate is likely to be.• Life expectancy at birth. The residents of economically developed

countries live longer than those of economically less develooped countries.

• Food supply. (Nutrition.)• Safe drinking water. Reduces the risk of water-borne diseases such as

cholera.• Medical care. • Natural disasters. How prone a region is to drought, famine or disease.• War. Deaths during war produces an immediate drop in population and

a longer term fall as a result of fewer fertile adults.

number of deaths per yearDeath rate = total population in the same year x 1000

Population Structure

• Demographic transition – A change in population from those where life expectancy is short and birth rates are high, to those where life expectancy is long and birth rates are low.

• Stable population – Birth and death rate are same, so there is no change in population size.

• Increasing population – High birth rate. Typical of economically less-developed countries.

• Decreasing population – Low birth rate + low mortality rate. E.g. Japan.

Average life expectancy = age at which 50% of population are still alive.

ATP + Respiration

• Anaerobic Respiration• ATP Synthesis• Summary of aerobic respiration• Glycolysis• Link Reaction• Krebs Cycle• Electron Transport Chain

Anaerobic Respiration

Glycolysis

Pyruvate

Lactic acid Ethanal

Ethanol

ATP + Respiration Photosynthesis

ANIM

ALS

PLAN

TS

CO2

2HNADH

NAD

2HNADH

NAD

-PRODUCES 2 ATP AS AN EMERGENCY SHORT BURST OF POWER-ETC ALONE PRODUCES 38 ATP, SO THIS IS MUCH LONGER LASTING THAN ANAEROBIC RESPIRATION

ATP Synthesis• ADP + Pi ATP• Photophosphorylation (During photosynthesis)• Oxidative phosphorylation (During respiration)• Substrate-level phosphorylation (When phosphate groups are transferred from donor

molecules to ADP to make ATP.)Role of ATP:The instability of its phosphate bonds makes ATP a good energy donor, but also stops it

from being a good long term energy store. ATP is better as an immediate energy source (than glucose) because ATP can be broken down to ADP + Pi in a single reaction releasing energy. The breakdown of glucose is a long series of reactions and therefore the energy release takes longer.

ATP Is the Source of Energy for:- Metabolic processes- Movement- Active transport- Activation of molecules- SecretionATP + Respiration Photosynthesis

Aerobic Respiration• C6 H12 O6 + 6O2 energy + 6H20 + 6CO2• requires oxygen and produces carbon dioxide, water and lots

of ATP. Aerobic Respiration can be summarised into four stages -

1. Glycolysis - splitting of a 6 carbon molecule into two 3 carbon pyruvates

2. Link Reaction - conversion of pyruvate into C02 and 2 carbon molecule Acetyl coA.

3. Krebs Cycle - intro of Acetyl coA into a cycle of oxidation-reduction reactions that creates some ATP and a large number of electrons.

4. Electron Transport Chain - electrons from the Krebs cycle are used to synthesise ATP with water produced as a by product.


Glycolysis

• Occurs in cytoplasm• 2 ATP go in as activation energy• 4 ATP produced, net gain of 2 ATP1. Glucose (6C) is phosphorylated.2. Breaks down into 2 x triose phosphate (3C).3. Phosphate donated to ATP.4. NAD reduced.5. 3+4 happen twice per glucose molecule, as 2 triose

phosphate molecules are produced6. This creates 2 x pyruvate (3C).


Link Reaction

• Occurs in mitochondrial matrix1. Co-enzyme A combines with pyruvate (3C).2. CO2 is removed.3. 2H removed, donated to NAD, produces

NADH.4. Resultant molecule is Acetyl CoA (2C)


Krebs Cycle

• Occurs in mitochondrial matrix1. 4C compound combines with Acetyl CoA to

produce 6C compound.2. CoA is removed and recycled.3. NAD + FAD are reduced.4. ATP created.5. 2 CO2 removed, regenerating 4C compound


Electron Transport Chain• Occurs in mitochondria cristae1. NAD and FAD are reduced.2. They then donate electrons to the first molecule in the electron

transport chain.3. This releases protons which are then actively transported across the

inner mitochondrial membrane.4. Meanwhile, the electrons pass along the chain of electron transport

carrier molecules in a series of oxidation-reduction reactions. The electrons lose energy as they travel down the chain and some of this energy is used to combine ADP and Pi to form ATP. The rest is released as heat.

5. Protons accumulate in intermembrane space before they diffuse back into the mitochondrial matrix through special channel proteins.

6. At the end of the chain, the electrons combine with the protons and oxygen to make water. O2 is the final acceptor of electrons in ETC.


Photosynthesis• CHLOROPLAST STRUCTURE:

Photosynthesis takes place in the chloroplast of the plant cell. They consist of a double membrane, thylakoids which are stacked up into grana, lamellae which link grana molecules, photosynthetic pigments, stroma and starch grains.

• Photosynthetic pigments (e.g chlorophyll and carotene) are coloured substances that absorb light energy for photosynthesis, they are found in the thylakoid membrane and are attached to proteins. The protein and pigment are called a photosystem.

• Plants use two types of photosystems to capture light energy, 1 which absorbs light at 700nm and number 2 which absorbs light at 680nm.

• Stroma is the gel liquid which fills the rest of the chloroplast and surrounds the thykaloids. It contains some sugars, enzymes and organic acids.

• Carbs produced by photosynthesis and not used up straight away are stored as starch grains (round blobs) in the stroma.

The equation for Photosynthesis is -• 6 CO2 + 6 H2O ------------> C6H12O6 + 6 02

Light Dependent Reaction• Photosynthesis takes place in two stages - light dependant and light

independent.• The light dependant stage takes place in the thykaloid membranes of the

chloroplast.• The light energy is used to create ATP (adding another phosphate to ADP),

and to reduce NADP to form Reduced NADP.• The products of the light dependant stage (ATP, Reduced NADP) are used

in the light independent stage. The ATP is used as energy and the reduced NADP transfers a hydrogen.

• During the light dependant stage, photolysis of water occurs (use of light to break water). Water is split into proton (H+), electron (e-) and oxygen, which is released into the atmosphere. The e- is used to replace the excited electron emitted from chlorophyll, the H+ goes back to the e- (after it has passed through electron carriers) to reduce NADP.

Light Independent Reaction

• The light independent reaction takes place in the stroma of the chloroplast. This reaction is also known as The Calvin Cycle. Carbon fixation takes place in the Calvin Cycle, this is where carbon is 'fixed' into an organic molecule.

• The products of the light independent stage are triose phosphate which can be used to make glucose and other useful organic substances.

Limiting Factors

There are optimum conditions for photosynthesis:

• High light intensity of a certain wavelength• temperature around 25 degrees • C02 at 0.4%. All of these factors can limit photosynthesis, if

one or any of the factors are too high or too low it will slow down photosynthesis

ENERGY AND ECOSYSTEMS KEYWORDS• Producer – Photosynthetic organisms that manufacture

organic substances.• Consumer – Organisms that obtain their energy by feeding

on other organisms rather than using the energy of sunlight directly.

• Decomposer - An organism who recycles nutrients by performing decomposition as it feeds on dead or decaying organisms.

• Detritivore - An organism that feeds on detritus or organic waste.

• Trophic level – The position of an organism in a food chain.• Net production = gross production – respiratory losses.

Energy transfer between trophic levels

• Plants normally convert 1-3% of the Sun’s available energy into organic matter. This is because:

- 90%+ is reflected back into space by clouds and dust or absorbed into the atmosphere.

- Not all wavelengths can be absorbed and used for photosynthesis.

- Light may not fall on a chlorophyll molecule.- A factor e.g. Low CO2 levels, may limit the rate

of photosynthesis.


• The total quantity of energy that the plants in a community convert to organic matter is called the gross production.

• However plants use 20-50% of this in respiration.

• The rate at which they store the remaining energy is called net production.

• Net production = gross production – respiratory losses.


• 10% of energy stored in plants is used by primary consumers for growth.

• Secondary and tertiary consumers are slightly more efficient (transferring 20%). Low % energy transfer is due to:

- Some of organism is not eaten.- Some parts are eaten but not digested and therefore lost in

faeces.- Some of energy is lost in urine.- Some energy losses are due to heat from the body to the

environment and heat loss during respiration. These losses are high in mammals and birds because they require more energy to maintain their high body temperature.


• Because energy transfer between trophic levels is inefficient. It explains why:

- Most food chains have only 4/5 trophic levels. Insufficient energy to support a large enough breeding population at trophic levels higher than these.

- The biomass is less at higher trophic levels.- Total amount of energy stored is less at each

level as you go up the food chain.

Calculating the efficiency of energy transfer

• Energy transfer = energy available after the transfer ----------------------------------------------- X 100

energy available before the transfer

Ecological pyramids

• Pyramids of number• Pyramids of biomass• Pyramids of energy• Comparison

Pyramids of Number• An ecological pyramid of numbers shows graphically the population of

each trophic level in a food chain.• No account is taken of size – this sometimes results in an inverted shape,

or a shape that does not resemble a pyramid at all.• The number of individuals is so great that it is not possible to represent

them accurately on the same scale as other species in the food chain. E.g. 1 tree may accommodate for millions of greenfly.

Ecological Pyramids

Agricultural Ecosystems

Pyramids of Biomass• Biomass is the total mass of the plants/animals in a

particular place.• Measured in gm-2 for an area (grassland etc.), gm-3 for a

volume (sea etc.)

Ecological Pyramids


Pyramids of Energy• A representation of the energy flow through a

food chain, measuring the energy stored in organisms at each level.

Ecological Pyramids


ComparisonNumber Biomass EnergyDoesn’t take into account size of organisms.

Does take into account size of organisms.

Collecting the data can be difficult and complex.

Number of individuals are so great that it is impossible to represent them accurately.

Fresh mass easy to access and measure, however varying amounts of water make it unreliable.

More reliable data than biomass, because 2 organisms of the same dry mass may store different amounts of energy.

Dry mass is harder to access, organisms must be killed and so it is usually only made on a small sample – which may be unrepresentative of the population.

Seasonal differences not taken into account, as only organisms at a particular are shown.

Seasonal differences not taken into account, as only organisms at a particular are shown.

Data collected for a year and so can take into account seasonal differences.

Ecological Pyramids


Agricultural Ecosystems• Net productivity = gross productivity – respiratory losses

Natural Ecosystem Agricultural Ecosystem

Solar energy only – no additional energy input

Solar energy + energy from food (labour) and fossil fuels (machinery and transport)

Lower productivity Higher productivity

More species diversity Less species diversity

More genetic diversity Less genetic diversity

Nutrients recycled naturally within the ecosystem with little addition from outside

Natural recycling is more limited and supplemented by the addition of artificial fertilisers

Populations are controlled by natural means, e.g. Competition and climate

Population are controlled naturally and by use of pesticides and cultivation

A natural climax community An artificial community prevented from reaching its natural climax community.

Pest and Pesticides• A pest is an organism that competes with humans

for food/space/could be a danger to health.An effective pesticide must be:- Specific (toxic to the pest only.)- Biodegradable (once applied, it will break down into

harmless substances in the soil.)- Cost-effective (development costs are high and new

pesticides remain useful only for a limited time as pests can develop genetic resistance.)

- Not accumulate (so that it does not build up in an organism or as it passes along food chains.)

Biological control

• Controlling pests using organisms that are either predators or parasites of the pest organism.

• The aim is to control, not eradicate. If the pests were eradicated, the predators would die out from lack of food and therefore the pests could reinstate themselves. The pest and control agent should exist in a balance at a level where the pest causes little damage.

Pesticides vs. Biological controlBiological Control Pesticides

Very specific Not as specific – often have an effect on non-target species.

Once introduced the control organism reproduces itself.

Must be reapplied at regular intervals, this means they are expensive.

Pests do not become resistant. Pests develop genetic resistance and new pesticides have to be developed.

Does not leave chemical in environment / on crop / no bioaccumulation.

Can bioaccumulate.

May become a pest itself. Does not become a pest itself.

Does not get rid of pest completely. Aim is to eradicate pest completely.

Takes time to reduce pest population. Fast effect.

Integrated pest-control systems• Integrates all forms of pest control rather than being reliant on one type.• Aim is to reduce pest population to an acceptable level. Eradication is costly,

counterproductive and almost impossible to achieve.• Integrated control involves:- Choose animal/plant varieties suitable for the area and as pest-resistant as

possible- Managing the environment to provide suitable habitats close to the crops for

natural predators- Regularly monitor crop for signs of pests and take early action if need be- Removing the pests mechanically (hand-picking/erecting barriers...) if pest

exceeds acceptable population level- Use biological agents if necessary and available- Pesticides are to be used as a last resort, when pest population is starting to

get out of control

Pest control and productivity

• Pests reduce productivity, any resources taken by the pest means less is available for crop.

• One may become a limiting factor in photosynthesis, reducing the rate and subsequently productivity.

• Pests also eat leaves of crops, limiting photosynthesis and leaving less crop for harvest.

• Many crops are grown in MONOCULTURE. This means insect and fungal pests as well as diseases can spread rapidly. Animals may not grow as rapidly, die, be unfit for human consumption – due to pests and therefore would lead to reduced productivity.

Intensive Rearing of Domestic Livestock

• Aim: to convert the smallest amount of food energy into the greatest quantity of animal mass.

• How to increase energy-conversion rate:- Restrict movement, less energy used by muscle contraction- Environment kept warm in order to reduce heat loss as most

livestock are warm-blooded- Feeding can be controlled so that the animals receive

optimum amount and type of food for maximum growth- Predators are excluded- Selective breeding to produce varieties that are more

efficient at converting the food they eat into body mass- Use hormones to increase growth rates

Features of Intensive Rearing of Domestic Livestock

• Efficient energy conversion• Low cost products• Less space used• Safety, smaller concentrated units are easier to regulate• Disease, living in close proximity results in easily spread infection• Use of drugs (over use of antibiotics has led to antibiotic

resistance)• Animal welfare, better healthcare but confined space can cause

distress• Pollution, large concentration of waste in small area• Use of fossil fuels to heat buildings• Reduced genetic diversity due to selective breeding, resulting in

the loss of genes that may have been beneficial

Nutrient Cycles

• Basic Sequence• Carbon Cycle• Nitrogen Cycle• Fertilisers• Consequences of Nitrogen fertilisers

Basic Nutrient Cycle SequenceInorganic molecule

or ion

Organic molecules in

Producers

Organic molecules in Consumers

Organic molecules in Decomposers

AbsorptionFeeding and digestion

Death

Decomposition

Nutrient Cycles Succession

Carbon Cycle

CO2 in atmosphere

C containing compounds in

Producers

Organic molecules in Consumers


Photosynthesis

Feeding

Death

Decomposition

Respiration

Combustion

Decay Prevented

Fossil Fuels


Nitrogen Cycle

Nitrate in atmosphere

Ammonium containing

molecules in Producers

Ammonium containing

molecules in Consumers


Absorption

Feeding and digestion

Death

Ammonification

Ammonium Ions

Nitrate Ions

NitrificationNitrate Ions

NitrificationDenitrification

Nitrogen fixation by free-living bacteria

Death and excretion


Ammonification

• The product of ammonia from organic ammonium containing compounds

• These compounds include urea, proteins, nucleic acids and vitamins

• Saprobiotic microorganisms feed on these materials, releasing ammonia, forming ammonium ions in the soil.

Nutrient Cycles SuccessionNitrogen Cycle

Nitrification• The conversion of ammonium ions to nitrate ions. The

conversion occurs in 2 stages:1. Oxidation of ammonium ions to nitrite ions (NO2-)2. Oxidation of nitrite ions to nitrate ions (NO3-)• This is an oxidation reaction, and so releases energy• It is carried out by free-living soil microorganisms called

nitrifying bacteria• The bacteria require oxygen to carry out this conversion and so

they need soil with many air spaces• To raise productivity, it is important for farmers to keep soil

structure light and well aerated by ploughing.• Good drainage prevents air spaces from being filled with water


Nitrogen Fixation

• Nitrogen gas is converted into nitrogen-containing compounds.

• It is carried out by 2 different microorganisms:1. Free-living Nitrogen-fixing bacteria – these reduce

gaseous nitrogen to ammonia, which they then use to manufacture amino acids. When they die and decay, they release nitrogen-rich compounds.

2. Mutualistic Nitrogen-fixing bacteria – these live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant, and the plant acquires amino acids from the bacteria.


Denitrification

• When soils become waterlogged, the conditions become anaerobic.

• Therefore fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and more anaerobic denitrifying bacteria.

• These convert soil nitrates into gaseous nitrogen.• As this process reduces the availability of nitrogen-

containing compounds for plants.• This reduces productivity. To increase productivity, the

soils on which crops grow must therefore be kept well aerated to prevent the build-up of denitrifying bacteria


Fill in the blanks to complete the passage below in order

to check your understanding of the nitrogen cycle. A few organisms can convert nitrogen gas into compounds

useful to other organisms is a process known as (1). These organisms can be free-living or live in a relationship with certain (2). Most plants obtain their nitrogen by absorbing (3) from soil through their (4) by active transport. They then convert this to (5), which is passed to animals when they eat plants. On death, (6) break down these organisms, releasing (7), which can then be oxidised to form nitrates by (8) bacteria. Further oxidation by the same type of bacteria forms (9) ions. These ions may be converted back to the atmospheric nitrogen by the activities of (10) bacteria.

Answers

Answers:

A few organisms can convert nitrogen gas into compounds useful to other organisms is a process known as (1 – Nitrogen fixation). These organisms can be free-living or live in a relationship with certain (2 - Plants). Most plants obtain their nitrogen by absorbing (3 – Nitrate ions) from soil through their (4 – Root hairs) by active transport. They then convert this to (5 – Proteins/Amino acids/Nucleic acids), which is passed to animals when they eat plants. On death, (6 - Decomposers) break down these organisms, releasing (7 – Ammonium ions), which can then be oxidised to form nitrates by (8 - Nitrifying) bacteria. Further oxidation by the same type of bacteria forms (9 - Nitrate) ions. These ions may be converted back to the atmospheric nitrogen by the activities of (10 - Denitrifying) bacteria.


Fertilisers

• In natural ecosystems, the minerals that are removed from the soil by plants are returned when the plant is broken down by microorganisms on its death.

• In agricultural ecosystems, the crop is harvested and the dead remains are rarely returned to the same area of land. Therefore the levels of mineral ions in agricultural land falls, and in order to be replenished, fertiliser must be added to the soil.

2 types of fertiliser:1. Natural (organic) fertilisers – dead and decaying remains of

plants and animals, as well as animal waste.2. Artificial (inorganic) fertilisers – mined from rock and deposits

and converted into compounds containing nitrogen, phosphorous, potassium.


Consequences of nitrogen fertilisers

• Reduced species diversity (nitrogen-rich soils favour the species grown, which outcompetes other species.

• Leaching – Rain dissolves soluble nutrients such as nitrates, carrying them deep into soil beyond the reach of plant roots. The leached nitrates can then find their way into watercourses, such as streams and rivers. They may then pollute the source of human drinking water.

• Eutrophication – the process by which nutrients build up into bodies of water.Nutrient Cycles Succession

Eutrophication1. In most lakes and rivers there is naturally very little nitrate (a limiting factor for plant and algal

growth).2. As nitrate concentration increases as a result of leaching, it is no longer a limiting factor for the

growth of plants and algae.3. As algae mostly grow at the surface, the upper layers of water become densely populated. This is an

‘algal bloom’.4. This dense surface layer of algae absorbs light and prevents it from reaching lower depths.5. Light becomes a limiting factor for the growth of plants and algae at lower depths so they eventually

die.6. There are more dead plants and algae for the growth of saprobiotic algae which grow exponentially.7. The saprobiotic bacteria require oxygen of respiration, creating a demand for it.8. The concentration of oxygen in the water is reduced and nitrates are released from decaying

organisms.9. Oxygen then becomes the limiting factor for the population of aerobic organisms. These organisms

ultimately die as the oxygen is used up.10. Without the aerobic organisms, there is less competition for the anaerobic organisms whose

population rises exponentially.11. The anaerobic organisms further decompose dead material, releasing more nitrates and some toxic

waste, which make the water putrid.Animal slurry, organic manures, human sewage and natural leaching can all contribute to eutrophication,

but the main cause is leaching of artificial fertilisers.


Succession Key WordsKEY WORDS:• Biodiversity – the range and variety of living organisms within a

particular area• Biomass – total mass of living material in a specific area at a given time.

(usually dry mass as amount of water varies)• Climax Community – the organisms that make up the final stage of

ecological succession• Community – the organisms of all species that live in the same area.• Conservation – • Ecosystem – self-contained functional unit made up of all the interacting

biotic and abiotic factors in a specific area• Pioneer species – • Succession –

Succession

• Succession ise.g. When bare rock/barren land is colonisedThis may occur as a result of:

- a glacier retreating and depositing rock- sand being piled into dunes by wind or sea- volcanoes erupting and depositing lava- lakes or ponds being creating by land subsiding- silt and mud being deposited at river estuaries

Primary Succession

1. The hostile environment that inhospitable to other species, is colonised a pioneer species.

2. The pioneer species dies, decomposes and adds nutrients to the environment. This change in the abiotic environment caused by the pioneer species...

3. Enables other species to colonise/survive the area. 4. This results in a change in diversity/biodiversity. 5. Therefore stability increases, the environment becomes

less hostile. 6. A climax community is established.

Example of ecological succession:

- Area of bare rock- Pioneer species are Lichens as they can survive considerable drying out- Over time, weathering of the rock produces sand/soil- The lichens die and decompose releasing nutrients - These two stages change the abiotic environment and enable it to support a community of small plants- Next typical stages in succession are mosses and ferns. - More soil is built up from continuing rock erosion and more organic matter available from death of the plants- This again changes the abiotic environment, more suitable for organisms that follow e.g. Shrubs, trees

- Climax community establishes

PioneersLichens

Common features during a succession

- Non-living environment becomes less hostile (Soil forms, more nutrients, shelter provided)

- A greater number/variety of habitats (Due to less hostile environment)

- Increased biodiversity (Evident in early stages, peaks at mid-succession, decreasing when climax community is reached due to dominant species out-competing other species.)

- More complex food webs (Due to increased biodiversity)- Increased biomass (Due to the more complex food web)

Secondary Succession• Another type of succession is when land that already supports life is

suddenly altered.• This could be due to a forest fire or agriculture.• In this case, the stages are the same but they occur and reach the

climax community much more rapidly^ This is because...

Spores and seeds often remain alive in the soil and there is an influx of animals and plants through dispersal and migration in the surrounding area*This succession does not have a pioneer species as the organisms are from subsequent successional stages.

BUT as the land has been altered, the climax community will be different.

Conservation of Habitats

• Conservation is the management of the Earth’s natural resources so that use of them in the future is maximised.

Reasons for conservation:• Ethical (Respect for living things. Other species have

occupied the Earth far longer than us, therefore should be allowed to coexist with us.)

• Economic (Many species have the capacity to make substances, many of which may prove valuable in the future.)

• Cultural and Aesthetic (Variety adds interest to everyday lives and inspires writers etc.)

Inheritance and SelectionKEYWORDS• Phenotype - The observable characteristics of an organism. (Appearance.)• Genotype - The genetic composition of an organism.• Gene - A section of DNA, a sequence of nucleotide bases that usually determines a single characteristic of an organism.• Allele - Different forms of a gene.• Homologous chromosomes - A pair of chromosomes that have the same gene loci and therefore determine the same

features.• Homozygous - Two of the same alleles present.• Heterozygous - One of each allele present.• Dominant - The allele that is expressed even when present with the recessive allele.• Recessive - The allele that is not expressed. Expressed only in the presence of another identical allele.• Homozygous dominant - Two dominant alleles.• Homozygous recessive - Two recessive alleles.• Diploid - 2 copies of each allele in cells.• Co-dominant - Two alleles that both contribute to the phenotype.• Multiple alleles - A gene that has more than 2 allelic forms e.g. blood type.• Gene pool - All the alleles of all the genes of all the individuals in a population at any one time.• Allelic frequency - The number of times an allele occurs within the gene pool is referred to as the allelic frequency,• Hardy-Weinberg Principle - p + q = 1 p2 + 2pq + q2 = 1• Directional selection - Selection may favour individuals that vary in one direction from the mean of the population. It

changes the characteristics of the population.• Stabilising selection - Selection may favour average individuals. It preserves the characteristics of a population.• Speciation - The evolution of a new species from existing species. • Geographical isolation - Occurs when a physical barrier prevents two populations from breeding with one another.

Monohydrid Inheritance – Genetic cross diagram example

Parental Phenotype:Green pods Yellow pods

Parental Genotype: GG ggGametes: G + G g + g

100% green

Male gametes

Female gametes G G

g Gg Gg

g Gg Gg

Sex Inheritance/Linkage• Any gene that is carried on either the X or Y

chromosomes is said to be sex linked.• However, the X chromosome is longer than the Y

chromosome, meaning for most of the X chromosome there is no homologous equivalent portion of the Y chromosome.

• Those characteristics that are controlled by recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male.

• This is because there is no homologous portion on the Y chromosome that could possess the dominant allele, in which circumstances the recessive allele would not present itself.

Hardy-Weinberg

• p + q = 1• p2 + 2pq + q2 =1

Types of Selection

• Directional selection• Stabilising selection

Directional selection

• If environmental conditions change, so will the phenotypes needed for survival. Some individuals, which fall to the left or right of the mean, will possess a phenotype more suited to the new conditions. These individuals will be more likely to survive and breed. Over time, the mean will then move in the direction of these individuals.

Stabilising selection

• If environmental conditions remain stable, it is the individuals with phenotypes closest to the mean that are favoured. These individuals are more likely to pass their alleles on to the next generation. The individuals with extreme phenotypes are less likely to pass on their alleles. Therefore stabilising selection tends to eliminate the extremes

Speciation1. Geographical isolation occurs. A physical barrier splitting the

species into two groups, and preventing them from mixing.2. This also prevents interbreeding and forms 2 separate gene

pools.3. Variation is created between both due to mutation. 4. In each group, there are different environmental/abiotic/biotic

conditions creating selection pressures. 5. Selection favours the advantageous

features/characteristics/mutation/ allele. 6. This results in only selected organisms surviving to reproduce. 7. This leads to a change in allele frequency in the groups.This process occurs over a long period of time.

YOUTUBE LINKS

Aqa a2 biology unit 4

Technology

Transcript of Aqa a2 biology unit 4