Biology Unit 4 AQA


Populations and Ecosystems


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

Abiotic - non living components

Biotic - living components

The supporting layers of land, air and water that surrounds the earth is called the biosphere.

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

Populations - a group of individuals from the same species that interbreed in a habitat

- Can be difficult to define boundaries of populations

Community - populations of different species that live and interact within the same habita

Ecological niche - how an organism fits into its environment, where it lives and what it does. This includes all biotic and abiotic requirements for an organism to live. No two species will occupy the exact same niche

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Investigating Populations

Due to time and the amount of damage it would cause, only small area within a habitat are studied; 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 - quadrats are used

Quadrats - size of quadrat is based on size of species being measured

- use a random number generator to find coordinates to place quadrat to avoid bias

Systematic sampling - transects are used

Line transect (species along line are counted) - used to illustrate a transition along which communites of plants/ animals change

Belt transect (species between two lines are counted) - provides information on the density of a species

(no. individuals caught in first sample x no. caught in second sample) / no. recaptured 

= Estimate of population

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Investigating Populations


  • Proportion of marked to unmarked individuals in the second sample is the for the whole population
  • Individuals in the first sample distribute themselves evenly
  • The population has a definite boundary
  • No immigration or emigration
  • Birth rate and death rate are low
  • Marking non-toxic and conspicuous
  • Marking is not lost
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Variation in Population Size

Population Growth Curves

Growth curves of populations usually have three main phases:

1) A period of short growth due to the fact there is only a limited number of interbreeding individuals; this is known as the lag phase

2) A period of rapid growth, caused by the increase in organisms that are able to reproduce; this is known as the exponential phase

3) Population size begins to level off as there are limiting factors on the population growth such as the availablity of resources; this is known as the stationary phase


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Variation in Population Size

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 factors are either biotic or abiotic

Abiotic factors include:

Temperature - each species has an optimum temperature at which it survives best. The closer a group of organisms is to this temperature the faster the growth rate.

Light - light is the ultimate source of energy for an ecosystem. For plants, the greater the light intensity the more energy they have to produce spores and seed for reproduction.

pH - affects the function of enzymes. Enzymes work best at different pH levels.

Water and humidity - humidty affects transpiration rates in plants and the rate of evaporation of water from animals

Biotic factors include:

Intraspecific or interspecific competition


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Competition and Predation

Interspecific cometition - between members of different species

When two species are competiting for limited resources the one that uses these resources the most effectively will ultimately eliminate the other. 

Intraspecific competition - between members of the same species

Populations that undergo intraspecific competition are often limited by the number of resources available. 

Predation - occurs when one organism consumes another

Effect of predators on population size:

Predators eat their prey thereby reducing the population of prey. With fewer prey available the preadtors compete with one another for the prey that is left. Predator population decreases due to some predators not being able to catch enough prey. With fewer predators around, fewer prey are consumed and population of prey increases again. More prey available, predator population also increases.

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Human Populations

Human Population Size and Growth Rate

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

- The development of agriculture

- The development of manfacturing (Industrial Revolution)

Factors that affect this:

Death Rate and Birth Rate

Immigration (joining a population from outside) and Emigration (leaving a population)

Population growth = (Births + Immigration) - (Deaths + Emigration)

% growth rate = (Population change during a period / Population at start of period) x 100

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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 to 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

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Human Populations

Factors Affecting Death Rates

Age profile - the greater the proportion of elderly people the higher the death rate

Life expectancy at birth - residents of more developed countries live longer

Food supply - poor nutrition will cause an increase in death rate

Safe drinking water - poor quality of drinking water will cause an increase in water born diseases thus increasing death rate

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 - will cause an increase in death rate

Death rate = (Number of deaths per year x 1000) / Total population the same year

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Human Populations

Population Structure


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Energy and ATP

ATP is an immediate energy source. Both animals and plants breakdown organic molecules to make ATP.

What is energy?

  • Energy is ability to do work
  • It can take a variety of forms, including light, thermal, electrical, kinetic, etc
  • It can change from one form to another 
  • It cannot be created or destroyed
  • It is measured in joules (j)

Why do organisms need energy?

  • Metabolism - chemical processes 
  • Movement
  • Active transport
  • Production of enzymes
  • Maintaining body temperature

Water is used to convert ATP into ADP, in a hydrolysis reaction:

(ATP + H20 --> ADP + Pi + Energy)

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Photosynthesis - Light Dependent Reaction

Three main stages of photosynthesis:

1) Capturing of light energy

2) LDR - splitting of water, products are reduced NADP, ATP and O2 

3) LIR - CO2 is reduced to produce sugars and other organic molecules

Leaves are adapted to bring together the 3 raw materials of photosynthesis.

Adaptations - air spaces, waxy cuticle, xylem, stomata, thin upper epidermis, palisade layer

The making of ATP (Light Dependent Reaction)

Chlorophyll absorbs light energy 

2 electrons in the chlorophyll molecule are boosted to higher energy levels and passed on to electron carriers

Electrons transferred along an electron transfer chain

Electrons lose energy at each stage, which is used to make ATP from ADP + Pi

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Photosynthesis - Light Dependent Reaction


The electrons that are lost from the chlorophyll are replaced by electrons released during the photolysis of water.

2H2O --> 4H+ + O2 + 4e-

Light is needed for the reaction to take place

The H+ ions create reduced NADP

O2 is used in respiration

The electrons return to chlorophyll

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Light Independent Reaction

The products from the light dependent reaction are ATP and reduced NADP are used in the LDR

The Calvin Cycle

1) CO2 from the atomsphere diffuses into the leaf through the stomata and makes it way to the stroma

2)  CO2 combines with ribulose biphosphate (RuBP) using an enzyme and produces two molecules of glycerate-3-phosphate (GP)

3) ATP and reduced NADP from the LDR are used to convert the GP into triose phosphate (TP)

4) The NADP is reformed

5) Some TP is converted into useful organic substances, but most is regenerated into RuBP using ATP from the LDR

Stroma contains enzymes for reaction to take place

Stroma fluid surrounds grana, so products can readily diffuse into stroma

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Cellular Respiration

Conversion of glucose to ATP

  • Aerobic respiration - requires O2 and produces CO2, H2O and lots of ATP
  • Anaerobic respiration - O2 absent and produces ethanol in plants and lactate in animals along with little ATP


1) Glucose is phosphorylated twice to make phosphorylated glucose (6 carbon compound). 2 ATPs are used to supply the Pi groups

2) The phosphorylated glucose breaks down to form triose phosphate (which is a 3 carbon compound)

3) Hydrogen is then removed from these molecules and reduces NAD. 2 ATPs are made from the conversion of TP into pyruvate, using ADP and Pi.

2 ATP molecules are yielded

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Link Reaction and Kreb's Cycle

The Link Reaction

Pyruvate + NAD + Coenzyme A --> Acetyl coenzyme A + Reduced NAD + CO2

The Krebs Cycle

1) Acetyl coenzyme A reacts with a 4 carbon compound to form a 6 carbon compound

2) The 6 carbon compound loses CO2 and hydrogen twice to produce one ATP molecule and two reduced NAD molecules

3) FAD and NAD are reduced regenerating a 4 carbon compound

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Kreb's Cycle

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Electron Transport Chain

Takes place in the inner membrane of the mitochondria

Reduced NAD and reduced FAD from the Krebs cycle are used 

Stages of the electron transport chain

1) Reduced NAD and reduced FAD are oxidised, releasing a proton and an electron 

2) The protons are actively transported into the intermembranous space

3) The electron is taken up by an electron carrier, the carrier becoming reduced

4) The electron is passed on, so old electron carrier is oxidised and new one is reduced

5) By passing it down a chain of electron carriers the electron loses energy. It is this energy that is used to combine ADP + Pi to form ATP

6) Protons accumulate and diffuse back into the cell through special protein channels and combine with the electrons and water to from water

7) Oxygen is therefore the final acceptor of electrons in the electron transport chain

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Anaerobic Respiration

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

Once produced in glycloysis, products such as pyruvate and hydrogen must be constantly removed. Furthermore, the hydrogen from NAD must be released so it can be used again. In order to do this the pyruvate will react with reduced NAD.

In plants pyruvate is converted into ethanol and water:

Pyruvate + Reduced NAD --> Ethanol + Carbon dioxide + NAD

In animals pyruvate is converted into lactate:

Pyruvate + Reduced NAD --> Lactate + NAD

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Food Chains and Food Webs

The ultimate source of energy in an ecosystem comes from sunlight

The energy is converted into an organic form using photosynthesis which is then passed between organisms

Producers - photosynthetic organisms that obtain 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 in the food chain they are

Decomposers - when producers/consumers die, the energy they contain can be accessed by decomposers that will break down the larger more complex molecules that they are made up of into smaller simple components again

Food Chains

  • Describes the relationships between organisms
  • Each stage of the chain is known as a "trophic level"

Food Webs 

  • In reality most animals do not rely upon a single food source
  • Within a single habitat there may be many food chains linked together
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Energy Transfer Between Trophic Levels

Energy losses in food chains

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 atomsphere
  • Not all wavelengths of light can be absorbed by plants in photosynthesis
  • Light may not actually fall on 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

Only about 10% of the energy stored in plants is passed on to primary consumers. Secondary and tertitary consumers are moe efficient at transferring energy. But energy is still lost because of:

  • Some of the organism not being eaten
  • Some parts can be eaten but not digested
  • Some parts lost in excretion 
  • Some of the energy lost in respiration 
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Energy Transfer Between Trophic Levels

Calculating the efficiency of energy transfers

Energy transfer = (Energy available after the transfer / Energy available before) x 100

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Ecological Pyramids

Pyramids of Number:

As you go up the trophic level the number of organisms gets fewer e.g Grass -> Rabbit -> Fox

No account is taken for size and the number individuals can be so great it can be almost impossible to count them all though, so this method has significant drawbacks.

Pyramids of Biomass:

This method does take size into account

Biomass is the total mass of plants/animals of species in a given place 

Biomass can be unreliable as the amount of water stored in an organism can vary

Dry mass is therefore measured instead, but this means organisms must be killed

Pyramids of Energy:

Most accurate representation of energy flow in a food chain but collecting data is complex

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Agricultural Ecosystems

What is an agricultural ecosystem?

Made up of plants and animals used to make food for humans. Agriculture tries to ensure that as much of the energy available from the sun is transferred to humans as possible. 

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 gross productivity. Some of this is used for respiration, the remainder is called net productivity.

Net productivity = Gross productivity - Respiratory losses

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Agricultural Ecosystems

Comparisions of Natural and Agricultural Ecosystems

Energy Input

To maintain an agricultural ecosystem it is important to prevent a climax community from forming by excluding other species in that community. Pests, diseases and weeds have to be controlled. This involves energy:

  • Food - farmers use energy to do work on the farm
  • Fossil fuels - farms have become mechanised and so many different machines are used


  • In natural ecosystems this is low
  • Energy input in agricultural ecosystems removes limiting factors to improve productivity
  • Other species removed to reduce competition 
  • Fertiliser is added to the soil
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Chemical and Biological Control of Pests

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, insectides kill insects etc. An effective pesticide should:

  • Be specific
  • Biodegrade
  • Be cost effective
  • Not accumulate

Biological Control

Uses other organisms and does not eradicate pest but simply controls it. 


  • Acts more slowly, interval of time between introducing the control and seeing effect
  • The control organism could become a pest


  • Pests do not become resistant 
  • Very specific and cost effective
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Chemical and Biological Control of Pests

Integrated Pest Control System

This uses all forms of pest control with the aim to determine an accepted level of the pest rather than trying to eradicate it

  • Choosing animal/plant varieties that are as pest resilient as possible
  • Managing the environment and ensuring there are nearby habitats for predators
  • Regulating crops
  • Removing pests by mechanically
  • Biological agents
  • Pesticides as last resort
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Intensive Rearing of Domestic Livestock

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 is high since the organism needs to maintain a high body temperature. This leaves a little amount of energy to be converted into biomass. Respiratory losses in farming are decreased by:

  • Movement being restricted so little energy is lost in muscle contraction
  • The environment is warm so less energy needed to maintain body temperature
  • Nutrition is carefully controlled to ensure organisms recieve 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
  • Selectively breeding animals that are more efficient in converting the food they eat into biomass
  • Using hormones to increase growth rate
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The Carbon Cycle

Nutrients are recycled

Nutrient Cycle:

1) Nutrients taken up by producer as simple inorganic molecules

2) The molecule is incorporated into more complex molecules with producer

3) When the producer is eaten, the nutrient passes to the consumer

4) Passes through food chain and when organism dies the complex molecules are broken down by saprobiotic organisms

Variations in the rates of respiration and temperature give rise to brief fluctuations of oxygen and carbon dioxide in the air. CO2 has dramatically increased in recent years due to:

  • The combustion of fossil fuels, which releases previously locked up carbon
  • Deforestation -  reduces large amounts of photosynthesising biomass that can remove CO2 from the air

The sea allows for large amounts of CO2 from the air to dissolve thus lowering the concentration.

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The Greenhouse Effect and Global Warming

When solar radiation reaches the earth, some has been reflected back into space and some has been absorbed by the atomsphere. The radiation that reaches the earth is absorbed and then reemitted back into space. However, some of this radiation is absorbed by clouds and greenhouse gases that reflect the radiation back to earth. This causes a heating effect known as the greenhouse effect.

Greenhouse gases

CO2 - increase due to humans

Methane - Produced by the breakdown of organic molecules of which other organisms are made of

Global Warming 

The mean average temperature has increased, cannot say for certain how much we have to do with this.


  • Affects niches available in a community, leading to alterations in species distribution 
  • Melting of polar ice caps
  • High temperatures might mean crop failure
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The Nitrogen Cycle

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

1) Ammonification - production of ammonia from organic compounds containing ammonium, saprobiotic bacteria feed on the materals releasing ammonia which converts to ammonium into the soil.

2) Nitrification - carried out by saprophytic bacteria in the soil, converting ammonium into nitrite ions and then into nitrate ions. Oxygen is required for this.

3) Nitrogen Fixation - nitrogen gas is converted into nitrogen containing compounds. 

- Free living bacteria reduce gaseous nitrogen into ammonia, which they then use for amino acids.

- Mutualistic nitrogen-fixing bacteria provide plants with amino acids in return for carbohydrates

4) Denitrification - denitrifying bacteria convert soil nitrates into gaseous nitrogen when little oxygen is present.

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Use of Fertilisers

The need for them:

  • All plants need mineral ions, especially nitrogen, from the soil
  • Specific areas of land are often used to grow crops
  • Plants use up all the nitrogen containing compounds in the soil
  • Normally they would be returned when the plant died and was broken down by saprobiotic bacteria, however in farming they are harvested so the nitrogen is not replaced
  • The amount of nitrates in the soil decreases

Fertlisers are designed to replace what is lost:

Natural - consists of decaying/dead organisms as well as animal waste

Artifical - minerals obtained from rocks and other stuff

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Environmental Consequences of Using Fertilisers

The effects of nitrogen fertilisers:

Nitrogen containing fertilisers can have detrimental affects such as:

Reduced species diversity - nitrogen favours certain species, so other could be outcompeted

Leaching - leads to pollution of watercourses

Eutrophication - caused by leaching


  • Rain water can dissolve soluble nitrates and carry them deeper into the soil beyond the reach of plants
  • The nitrates may then find there way to water courses and into water used for human consumption
  • High levels of nitrates in water can cause inefficient transport of oxygen to the brain

EutrophicationBecause of leaching plants grow exponentially and cover upper layers of water. This means sunlight can't reach the bottom of the lake or pond. The plants on the bottom die. Saprophytic bacteria grow exponentially feeding on the decaying plant matter and using up oxygen and producing nitrates. Organisms that need oxygen, like fish die.

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Populations and Ecosystems

Succession - changes that take place within an ecosystem

Barren land such as bare rock can be formed by the eruption of a volcano or a glacier retreating.

The first stage of succession is the colonisation of a pioneer species.

Pioneer species tend to have:

  • a tolerance to extreme conditions 
  • the ability to fix nitrogen from the air 
  • the ability to photosynthesise light
  • the ability to disperse seeds across vast distances
  • seeds that repidly germinate

Pioneer species change the abiotic environment by dying and releasing nutrients such as nitrates for the production of amino acids and proteins for the organisms that follow. The growth of mosses provide habitats for insects and other animals.

At each stage of succession a certain type of species will change the environment to make it less hostile.

Climax community - consists of animals and plants which have established equilibrium

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Populations and Ecosystems

During succession there are a number of common features such as: 

  • Environment becomes less hostile - soil forms, nutrients are more plentiful, plants provide shelter from wind
  • Greater number of habitats
  • Biodiversity increases - habitats become occupied by species
  • More complex food webs due to high species diversity and therefore increased biomass
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Studying Inheritance

Genotype and phenotype

The genotype sets the limits to which characteristics can vary. Any change to the genotype is called a mutation and it will be passed on the next generation if it is present in gametes.

Phenotype is an observable characteristic of an organism. It will vary depending on the genotype and the environmental conditions.

Genes and alleles

Genes determine the proteins and compounds produced. 

An allele is one of the different forms of a gene. If both alleles are the same in a homologous pair of chromosomes it is known as homozygous and if they are different it is known as heterozygous. When there are two different alleles and one expresses itself over the other it is known as the dominant allele.

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