GCSE Science₈₄₆₅

Describe how it is thought an oxygen-rich atmosphere developed over time.

Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.

One theory suggests that during the first billion years of the Earth’s existence there was intense volcanic activity, which released gases that formed the early atmosphere and water vapour that condensed to form the oceans. At the start of this period the Earth’s atmosphere may have been like the atmospheres of Mars and Venus today, consisting mainly of carbon dioxide with little or no oxygen gas.

Volcanoes also produced nitrogen, which gradually built up in the atmosphere, and there may have been small proportions of methane and ammonia.

When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of carbon dioxide in the atmosphere.

Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis.

Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.

Photosynthesis by algae and plants also decreased the percentage of carbon dioxide in the atmosphere. Carbon dioxide was also used up in the formation of sedimentary rocks, such as limestone, and fossil fuels such as coal, natural gas and oil.

WS 1.1

Given appropriate information, interpret evidence and evaluate different theories about the Earth’s early atmosphere.

WS 1.3

Explain why evidence is uncertain or incomplete in a complex context.

MS 1c

Use ratios, fractions and percentages.

Recall that many different materials cycle through the abiotic and biotic components of an ecosystem.

Explain the importance of the carbon cycle to living organisms.

Describe photosynthetic organisms as the main producers of food and therefore biomass for life on Earth.

Explain the role of microorganisms in the cycling of materials through an ecosystem.

The element carbon is found as carbon dioxide in the atmosphere, dissolved in the water of the oceans, as calcium carbonate in sea shells, in fossil fuels and in limestone rocks, and as carbohydrates and other large molecules in all living organisms. Carbon cycles through the environment by processes that include photosynthesis, respiration, combustion of fuels and the industrial uses of limestone.

Life depends on photosynthesis in producers such as green plants, which make carbohydrates from carbon dioxide in the air. Animals feed on plants, passing the carbon compounds along food chains. Animals and plants respire and release carbon dioxide back into the air.

Decay of dead plants and animals by microorganisms returns carbon to the atmosphere as carbon dioxide and mineral ions to the soil.

WS 1.2

Draw and interpret diagrams to represent the main stores of carbon and the flows of carbon between them in the cycle.

This topic links with Ecosystems and biodiversity .

Describe the greenhouse effect in terms of the interaction of radiation with matter.

(HT only) Recall that different substances may absorb, transmit or reflect these waves in ways that vary with wavelength.

Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. They allow short-wavelength radiation from the Sun to pass through the atmosphere to the Earth’s surface but absorb the outgoing long-wavelength radiation from the Earth’s surface, causing an increase in temperature. Water vapour, carbon dioxide and methane are greenhouse gases that increase the absorption of outgoing, long-wavelength radiation.

WS 1.2

Interpret and draw diagrams to describe the greenhouse effect.

Evaluate the evidence for additional anthropogenic causes of climate change, including the correlation between change in atmospheric carbon dioxide concentration and the consumption of fossil fuels, and describe the uncertainties in the evidence base.

Human activities that involve burning fossil fuels (coal, oil and gas) for generating electricity, transport and industry all add carbon dioxide to the atmosphere. These activities have led to a large rise in the concentration of carbon dioxide in the air over the last 150 years. Over the same time the average temperature of the surface of the Earth has risen. The scientific consensus is that this is more than correlation and that the rise in greenhouse gas concentrations has caused the rise in temperature.

Climate describes the long-term patterns of weather in different parts of the world. Climate change is shown by changes to patterns in measures of such things as air temperature, rainfall, sunshine and wind speed.

Scientists analyse data on climate change using computer models based on the physics that describes the movements of mass and energy in the climate system. Many complex changes on Earth affect the climate, and detailed data about the scale of the changes is not available from all over the world. Also, when predicting climate change, scientists have to make assumptions about future greenhouse gas emissions. This means that there are uncertainties in the predictions.

WS 1.6

Explain the importance of scientists publishing their findings and theories so that they can be evaluated critically by other scientists.

Understand that the scientific consensus about global warming and climate change is based on systematic reviews of thousands of peer reviewed publications.

WS 1.3

Explain why evidence is uncertain or incomplete in a complex context.

MS 2c, 4a

Extract and interpret information from charts, graphs and tables.

MS 2h

Use orders of magnitude to evaluate the significance of data.

Describe the potential effects of increased levels of carbon dioxide and methane on the Earth’s climate and how these effects may be mitigated, including consideration of scale, risk and environmental implications.

Consequences of global warming and climate change include:

• sea-level rise
• loss of habitats
• changes to weather extremes
• changes in the amount, timing and distribution of rainfall
• temperature and water stress for humans and wildlife
• changes in the distribution of species
• changes in the food-producing capacity of some regions.

Steps can be taken to mitigate the effects of climate change by reducing the overall rate at which greenhouse gases are added to the atmosphere. Examples of mitigation include:

• using energy resources more efficiently
• using renewable sources of energy in place of fossil fuels (see Resources of materials and energy )
• reducing waste by recycling
• stopping the destruction of forests
• regenerating forests
• developing techniques to capture and store carbon dioxide from power stations.

WS 1.4

In the context of climate change, evaluate associated economic and environmental implications; and make decisions based on the evaluation of evidence and arguments.

Describe the major sources of carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates in the atmosphere and explain the problems caused by increased amounts of these substances.

The combustion of fuels is a major source of atmospheric pollutants that can be harmful to health and the environment.

Carbon monoxide is formed by the incomplete combustion of hydrocarbon fuels when there is not enough air. Carbon monoxide is a toxic gas that combines very strongly with haemoglobin in the blood. At low doses it puts a strain on the heart by reducing the capacity of the blood to carry oxygen. At high doses it kills.

Sulfur dioxide is produced by burning fuels that contain some sulfur. These include coal in power stations and some diesel fuel burnt in ships and heavy vehicles. Sulfur dioxide turns to sulfuric acid in moist air.

Oxides of nitrogen are produced by the reaction of nitrogen and oxygen from the air at the high temperatures involved when fuels are burned.

Sulfur dioxide and oxides of nitrogen cause respiratory problems in humans and cause acid rain. Acid rain damages plants and buildings. It also harms living organisms in ponds, rivers and lakes.

Particulates in the air include soot (carbon) from diesel engines and dust from roads and industry. The smaller particulates can go deep into people’s lungs and cause damage that can lead to heart disease and lung cancer.

WS 1.4

Describe, explain or evaluate ways in which human activities affect the environment.

Explain the importance of the water cycle to living organisms.

Water is found in the solid state in glaciers and ice sheets, in the liquid state in the oceans, rivers, lakes and aquifers and in the gas state in the atmosphere. Water cycles through the environment by processes that include melting, freezing, evaporation and condensation. Precipitation of water from the atmosphere can take the form of rain, sleet or snow.

Life on Earth depends on water, on land and in the seas. Water acts as the solvent for chemical reactions in cells. It also helps transport dissolved compounds into and out of cells. Water is either a reactant or a product of biochemical changes such as respiration, photosynthesis and digestion. Rivers, lakes and seas provide habitats for many living organisms.

WS 1.2

Draw and interpret diagrams to represent the main stores of water and the flows of water between them in the cycle.

Describe the principal methods for increasing the availability of potable water in terms of the separation techniques used, including ease of treatment of waste, ground and salt water.

Describe, explain and exemplify the processes of simple distillation.

Water that is safe to drink is called potable water. Potable water is not pure water in the chemical sense because it contains dissolved substances.

The methods used to produce potable water depend on available supplies of water and local conditions. In the UK, rain provides water with low levels of dissolved substances (fresh water) that collects in the ground and in lakes and rivers and most potable water is produced by:

• choosing an appropriate source of fresh water
• passing the water through filters
• sterilising.

Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.

If supplies of fresh water are limited, desalination of salty water or sea water may be required. Desalination can be done by distillation or by processes that use membranes such as reverse osmosis. Energy resources have to be used to run these processes.

Urban lifestyles and industrial processes produce large amounts of waste water that require treatment before being released into the environment. Sewage and agricultural waste water require removal of organic matter and harmful microbes. Industrial waste water may require removal of organic matter and harmful chemicals.

Sewage treatment includes:

• screening and grit removal
• sedimentation to produce sewage sludge and effluent
• anaerobic digestion of sewage sludge
• aerobic biological treatment of effluent.

WS 1.4

Explain everyday and technological applications of science; evaluate associated personal, social, economic and environmental implications with reference to the sources of potable water and treatment of waste water.

Describe different levels of organisation in an ecosystem from individual organisms to the whole ecosystem.

An ecosystem is made up of all the living organisms in a particular environment together with the non-living components such as soil, air and water. A habitat is where a particular organism lives in an ecosystem. A population is made up of all the individuals of the same species in a habitat. A community is made up of all the populations of different organisms that live in the same habitat.

Feeding relationships within a community can be represented by food chains. All food chains begin with a producer that synthesises molecules. This is usually a green plant, which absorbs light to make glucose.

A food web can be used to understand the interdependence of species within an ecosystem in terms of food sources.

Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers and then tertiary consumers.

Consumers that kill and eat other animals are predators, and those eaten are prey. In a community the numbers of predators and prey rise and fall in cycles.

WS 1.2

Interpret graphs used to model predator–prey cycles.

Describe the importance of interdependence and competition in a community.

To survive and reproduce, organisms require a supply of materials from their surroundings and from the other living organisms in an ecosystem.

Plants often compete with each other for light and space, and for water and nutrients from the soil. Animals often compete with each other for food, mates and territory.

Within a community each species depends on other species for food, shelter, pollination, seed dispersal etc. If one species is removed it affects the whole community. A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant.

Explain how some abiotic and biotic factors affect communities.

Abiotic factors that can affect a community are:

• light intensity
• temperature
• moisture levels
• soil pH and mineral content
• wind intensity and direction
• carbon dioxide levels for plants
• oxygen levels for aquatic animals.

Biotic factors that can affect a community are:

• availability of food
• new predators arriving
• new diseases
• one species outcompeting another.

WS 1.2

Predict how a change in an abiotic, or biotic, factor would affect a given community given appropriate data or context.

MS 1c

Calculate the percentage of mass.

MS 2c, 4a

Extract and interpret information from charts, graphs and tables.

Describe how to carry out a field investigation into the distribution and abundance of organisms in an ecosystem and explain how to determine their numbers in a given area.

Ecologists use a range of investigation methods using transects and quadrats to determine the distribution and abundance of species in an ecosystem.

MS 2b

Calculate arithmetic means.

WS 3.3

Carry out and represent mathematical and statistical analysis.

MS 4a, 4c

Plot and draw appropriate graphs, selecting appropriate scales for the axes.

MS 2d

Understand the principles of sampling as applied to scientific data.

Explain some of the benefits and challenges of maintaining local and global biodiversity.

Biodiversity is greater in ecosystems that provide a bigger range of different habitats, which are home to larger populations of a variety of organisms.

Small populations are in greater danger of dying out if an ecosystem is disrupted in some way.

Ecosystems with high levels of biodiversity help to provide the resources needed to sustain life, including human life.

Ecosystems with higher biodiversity offer economic benefits by sustaining the resources needed for agriculture, fishing and forestry.

Describe negative human interactions within ecosystems and explain their impact on biodiversity.

Examples of human interactions with local ecosystems that can diminish or destroy biodiversity include:

• building, quarrying, farming, clearing woods and other activities that destroy habitats
• the destruction of peat bogs, and other areas of peat, to produce garden compost
• pollution of streams, rivers and lakes by sewage, toxic wastes and fertilisers.

An example of a global impact of human activities is global warming leading to climate change ( The Earth's atmosphere ).

WS 1.4

Evaluate given information about ways in which human activities affect the environment.

Describe positive human interactions within ecosystems and explain their impact on biodiversity.

There are programmes to reduce these negative effects on ecosystems and biodiversity. These include:

• breeding programmes for endangered species
• protecting and regenerating habitats
• reintroducing wider field margins and hedgerows in areas of monoculture
• recycling resources rather than dumping waste in landfill
• production of peat-free composts
• reducing deforestation and carbon dioxide emissions.

WS 1.4

Evaluate given information about methods that can be used to tackle problems caused by human impacts on the environment.

Explain the following terms: gamete, chromosome and gene.

Describe DNA as a polymer made up of two strands forming a double helix.

Describe the genome as the entire genetic material of an organism.

Sexual reproduction involves the joining (fusion) of male and female gametes (sperm and egg cells in animals). In sexual reproduction there is mixing of genetic information, which leads to variety in the offspring. The formation of gametes involves meiosis.

The genetic material in the nucleus of a cell is composed of a chemical called DNA contained in the chromosomes. Human body cells contain 23 pairs of chromosomes. DNA is made of very large molecules in long strands, twisted to form a double helix.

A gene is a small section of DNA on a chromosome. Each gene contains the code for a particular combination of amino acids to make a specific protein. The genome of an organism is made up of all the genes in the DNA of its body cells.

This topic has links with Cells in animals and plants .

Describe sex determination in humans.

In human cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the chromosomes are different (XY). All eggs contain an X chromosome. Sperm cells contain either an X or a Y chromosome.

Explain single gene inheritance.

Predict the results of single gene crosses.

Explain the terms allele/variant, dominant, recessive, homozygous, heterozygous.

Some characteristics are controlled by a single gene. Examples are fur colour in mice and red–green colour blindness in humans.

Each gene may have different forms called alleles.

A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).

If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.

WS 1.2

Complete a Punnett square diagram or interpret the results of a genetic cross diagram for a single gene, and understand family trees.

MS 2e

(HT only) Construct a Punnett square diagram to make predictions based on simple probability.

MS 1c

Use direct proportion and simple ratios in genetic crosses.

Describe simply how the genome, and its interaction with the environment, influences the development of the phenotype of an organism.

Explain the terms genotype and phenotype.

Recall that most phenotypic features are the result of multiple genes rather than single gene inheritance.

All the genes present in an individual organism interact with the environment in which the organism grows and develops its observable appearance and character. These characteristics are its phenotype.

The variation in the characteristics of individuals of the same kind may be due to differences in:

• the genes they have inherited (genetic causes)
• the conditions in which they have developed (environmental causes)
• a combination of genes and the environment.

Human height is an example of a characteristic determined by many genes, each with different alleles. The set of alleles that determine the height of a person is the genotype for that characteristic. Height is also affected by diet and exercise which are part of the environment in which an individual grows up.

WS 1.2

Explain why studies involving identical twins help to separate the contribution of genes and the environment to the development of their phenotypes.

WS 1.1

Given a context and related information, discuss the potential importance for medicine of our increasing understanding of the human genome.

State that there is usually extensive genetic variation within a population of a species.

Recall that all variants arise from mutations, and that most have no effect on the phenotype, some influence the phenotype and a very few determine the phenotype.

Mutations are changes in DNA molecules that may affect genes. Mutation of a gene can alter the proteins that it contains the code for, or even prevent the protein being produced in cells.

Mutations can happen when DNA is copied during cell division or when cells are affected by environmental factors such as ionising radiation.

Describe evolution as a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of new species.

Explain how evolution occurs through natural selection of variants that give rise to phenotypes best suited to their environment.

The theory of evolution by natural selection explains the evolution of all species of living things from simple life forms that first developed more than three billion years ago.

If two populations of one species become isolated geographically or environmentally they may evolve in different ways to suit different conditions. If they become so different that they can no longer interbreed to produce fertile offspring they have formed two new species.

WS 1.2

Use the theory of evolution by natural selection in an explanation.

Describe the evidence for evolution, including fossils and antibiotic resistance in bacteria.

Evidence for evolution comes from the study of fossils that show how much or how little different organisms have changed as life developed on Earth.

Evolution of bacteria can be observed happening in a much shorter time because they reproduce so fast. Bacteria that cause disease evolve by natural selection when exposed to antibiotics; this gives rise to a resistant strain.

MS 2c, 4a

Extract and interpret information from charts, graphs and tables.

Describe the impact of developments in biology on classification systems.

In studies of evolution it is essential to be able to identify and classify living things. Traditionally living things have been classified into groups depending on their structure and characteristics.

Organisms are named by the binomial system of genus and species.

As evidence of internal structures became more developed due to improvements in microscopes and progress with the understanding of biochemical processes, new models of classification have been proposed. Modern classifications systems are based on theories about evolution developed from analysis of differences in DNA molecules.

WS 1.1

Show how new methods of investigation and new discoveries led to new scientific ideas.

Explain the impact of the selective breeding of food plants and domesticated animals.

Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic traits.

Selective breeding involves choosing parents from a mixed population with the desired characteristic. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.

The trait can be chosen for usefulness or appearance.

Selective breeding can lead to 'inbreeding' where some breeds are particularly prone to disease or inherited defects.

WS 1.3, 1.4

Evaluate the benefits and risks of selective breeding given appropriate information and consider related ethical issues.

Describe genetic engineering as a process which involves modifying the genome of an organism to introduce desirable characteristics.

(HT only) Describe the main steps in the process of genetic engineering.

Explain some of the possible benefits and risks, including practical and ethical considerations, of using gene technology in modern agriculture.

In genetic engineering, selected genes from one organism are transferred to another organism which may, or may not, belong to the same species. This process for genetic modification uses enzymes and vectors (such as bacterial plasmids or viruses) to transfer genes. It is much faster than selective breeding.

Genes can be transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with the desired characteristics.

Crops that have had their genes modified in this way are called genetically modified crops (GM crops). Crops can be genetically modified to give increased yields or to increase the amount of a vitamin in the food from the crop. Genetically modified crops also include ones that are resistant to insect attack or to herbicides. This means that farmers can cut down on the use of pesticides. They can also spray to kill weeds while leaving the crop plant unaffected.

Concerns about GM crops include the effect on populations of wild flowers and insects as a result of cross-pollination. Insects may evolve to become resistant so that the GM crops are no longer protected.

WS 1.4

Evaluate the advantages and disadvantages of GM technologies based on data or other information.

WS 1.3

Give a simple ethical argument about the rights and wrongs of a GM technology.

Recognise, in given information, the difference between a practical and an ethical argument.