SNAB biology unit 4 topic 5 key points

Key recall points needed for the first half of the Unit 4 A2 exam in SNAB EdExcel biology.

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Ecology Key Terms

Abiotic - an ecological factor that is part of the non-living or physical environment of an organism. Abiotic factors include climatic features (such as rainfall and temperature), solar energy input and edaphic factors (relating to the soil).

Biotic - an ecological factor that is part of the living environment of an organism. Biotic factors are determined by living organisms and include predation and competition. Biotic factors are usually density-dependent as their effects are related to the population density of the organism concerned.

Habitat - the particular place where a community of organisms is found. Woodlands, coral reefs and cultivated fields are all habitats for particular communities of organisms.

Population - a group of individuals belonging to one species. Members of a population are generally found in one place at a particular time and are able to breed with one another. All the oak trees in a wood, or all the frogs in a pond are examples of populations.

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Community - all the living organisms found in a particular place at a particular time. The community found on a rocky seashore, for example, consists of all the seaweeds, together with all the different species of animals as well as all the bacteria and single-celled organisms.

Producer - an organisms that can make its own organic compounds from inorganic compounds either by photosynthesis or by using the energy released from chemical reactions.Green plants, algae and some bacteria are producers. Producers are at the start of all food chains.

Primary Consumer - an organism that consumes plant material for its food; also called a herbivore.

Secondary Consumer - an organism that feeds on primary consumers; also called a carnivore.

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Autotroph - an organism that builds up the organic molecules it requires from small inorganic molecules such as carbon dioxide and water. In order to do this, a source of energy is necessary. In photoautotrophs, the source of energy is light. In chemoautotrophs, this energy comes from another chemical reaction. Autotrophs are producers in food chains.

Heterotroph - an organism that gains its nutrients by feeding on other organisms. The complex organic molecules in its food are broken down by enzymes into simple soluble substances before being built up agian to form the complex organic substances that the organism requires. Heterotrophs are the consumers in food chains.

Trophic level - the position an organism occupies in a food chain; e.g. producer, primary consumer, secondary consumer or tertiary consumer. Energy is transferred from one trophic level to the next trophic level in a food chain. Since energy is lost to the environment at each stage in the food chain, it is rare for food chains to have more than five trophic levels.

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The progressive replacement of one dominant type of species or community by another in an ecosystem until a stable climax community is established.

Primary succession - in a newly formed habitat where there has never been a community before. Pioneer species are the first organisms to colonise a bare habitat.

Secondary succession - on bare soil where an existing community has been cleared e.g. after a forest fire.

Deflected succession - a community that remains stable only because human activity prevents succession from running its full course.

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Carbon Dioxide + Water --> Oxygen + Glucose

6CO2 + GH2O --> 602 + C6H12O6


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Electron transport chains - electrons move along protein carriers releasing energy at each protein carrier.

ATP: Adenosine triphosphate - ATP is the energy storage component in cells. It is an energy transfer molecule.The energy relerased in electron transport chains forms ATP. After energy has been released, ATP becomes Adenosine Diphosphate (ADP). Plants use ATP to make organic food molecules.

Coenzymes - Coenzymes are molecules that work alongside enzymes, transferring chemical groups or atoms from the active site of one enzyme to the active site of another. In photosynthesis, coenzymes easily accept electrons and then give them up again. Coenzymes are involved in Redox reactions.

Energy transfer - When bonds are broken in reactions, energy is given out.

Oxidation and Reduction - Oxidation is loss of electrons or hydrogen ions, Reduction is gain of electrons or hydrogen ions. Redox reactions are when something is both reduced and oxidised.

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Light-Dependent Reaction

- Happens in the thylakoid membrane
- Electron transport chain
- ADP + Phosphate --> ATP
- NADP = Coenzyme

1. Light energy provided by the sun.
2. Light absorbed by chlorophyll (pigment in chloroplasts).
3. Light energy excites electrons in the cholorphyll.
4. The electrons pass along protein carriers in an electron transport chain.
5. During this electron transport chain, the enrgy released form the oxidation and reduction of carriers enables ATP to form from ADP and a phosphate. (Photophosphorylation).
6. Electrons and hydrogen ions from the water move along a series of carriers and are accepted by the coenzyme NADP. The NADP becomes reduced NADP (NADPH).
7. As well as two hydrogen ions being taken from the water (H2O), two electrons are also taken to replace those originally from the chlorophyll. The point of the electron transport chain is to split water (photolysis).
8. The oxygen from the water is released through the leaf (waste product).

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Light-Independent Reaction

- Happens in the stroma
- Also called the Calvin Cycle
- Carbon Dioxide is "fixed" into glucose
- ATP --> ADP + Phosphate
- RuBISCO = Coenzyme

Ribulose Bisphosphate is a 5-carbon structure present in the chloroplast to be used in the light-independent reaction. It is used at the begining of the light-independent reaction and re-made at the end.

1. Ribulose Bisphosphate joins with the carbon from CO2 to form a 6-carbon compound. This is speeded up by an enzyme called RuBISCO (Ribulose Bisphosphate Carboxylase). O2 is released as a waste product.
2. The 6-carbon compound is unstable and breaks down into two 3-carbon compounds called glycerate 3-phosphate (GP).
3. ATP provides the energy for the reduction of the 3-carbon compound. (ATP turnes back into ADP and is used again in the light-dependent reaction.)

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4. NADPH reduces the 3-carbon compound. GP is converted into glyceraldehyde 3-phosphate (GALP). The oxidised NADP is used in the light-dependent reaction. The hydrogen ions from NADPH goes into the GALP. GALP is restructured, reduced GP.
5. Some GALP is used to regenerate Ribulose Bisphosphate, and some is removed from the cycle. ATP is used (more of it) in the regeneration of Ribulose Bisphosphate.

Each turn of the cycle produces one net carbon that can be removed. This is used to produce glucose. When they are needed, the glucose is used to produce nucleic acids, lipids, carbohydrates and proteins.

The Calvin Cycle happens 6 times in order to make the amount of glucose needed by the plant. CO2 x 6 = 12 GALPS. 10 out of these 12 GALPS go back to make Ribulose Bisphosphate.

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Biomass - the dry mass of an organism.

Gross Primary Productivity (GPP) - the rate at which energy is built into organic molecules (glucose) by an ecosystem. GPP is measured in units of energy per unit area per year.

Net Primary Productivity (NPP) - the rate at which the GPP energy is made into new plant biomass (proteins, lipids, nucleic acids).

NPP = GPP - Respiration

Energy reduces along a food chain, it can't keep going continuously.

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Evidence for Climate Change

Temperature Records

Some termperature records go back 2 or 3 centuries. These can be looked at to see whether the climate has changed. However, the accuracy of thermometers has changed over time.

Studying Peat Bogs

Peat is an accumulation of partially decayed organic matter, mainly the remains of dead plants. The anaerobic and acid conditions of the bog mean that the decay rate is slowed or stopped altogether.

Pollen grains are particularly well preserved in peat. Plants produce pollen in vast amounts and each plant species has a distinctive type of pollen. Peat forms in layer (deeper = older) and carbon-14 dating allows the age of each layer to be established. Each species of plant has a particular set of conditions in which it flourishes best. If we find pollen from a species favouring warmer conditions, we can infer that the peat was laid down when the climate was warmer.

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Tree-Ring Analysis - Dendrochronology

Every year, trees produce a new layer of xylem vessels by the division of cells underneath the bark. The diameter of new xylem vessels varies according to the season when they were produced: wide vessels in the spring when the tree grows quickly, followed by narrow vessels in summer. The different widths of vessels create a pattern of rings across the trunk, which can easily be seen when the tree is cut down, with a ring for each year of tree growth.

Instead of cutting down a tree to see the rings, a core samples can be taken and examined.

The wider the ring, the better conditions for growth (warmer/wetter).

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The Greenhouse Effect

The sun radiates energy (light) and the Earth absorbs some of these energy. The Earth warms up and radiates energy back into space (infrared radiation). Some of this energy warms up gases in the atmosphere, trapping energy. The gases in the atmosphere that stop the infrared radiation from escaping are called greenhouse gases. They create the greenhouse effect, which keeps the Earth warm.

<--> Some visible solar radiation is reflected by the Earth.

<--> Some visible solar radiation is reflected by the clouds.

----|  Most ultraviolet solar radiation is absorbed by the ozone in the stratosphere.

----> Most solar radiation is absorbed by the Earth's surface, which warms up.

|----  Some infrared radiation emitted by the Earth is absorbed by the greenhouse gases, warming the troposphere.

<---- Some infrared radiation escapes and cools down the Earth.

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CH4 is produced by anaerobic decay of organic matter in waterlogged conditions (bogs, rice fields, domestic landfills, animal waste). Incomplete combustion of fossil fuels also releases methane.

Since pre-industrial times, the atmosphere concentration of methane has more than doubled. A molecule of methane absorbes more radiation than CO2, but it does not stay in the atmosphere for very long. After 12 years, methane reacts with oxygen to form CO2 and H2O.

Methane emissions could be reduced through better waste recycling and by using methane as a biofuel.

Carbon Dioxide:

There is a high correlation between temperature and CO2 levels in the atmosphere. However, this correlation does not prove that one causes the other.

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Other Global Warming Factors

  • Aerosols (extremely small particals or liquid droplets found in the atmosphere).
  • The degree of reflection from those parts of the Earth's surface that are free of ice and snow.
  • The fraction of the Earth's surface covered in ice and snow.
  • The extent of cloud cover.
  • Changes in the Sun's radiation.

These things affect prediction models for climate change as they are hard to predict. These therefore can cause models to be inaccurate.

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The Carbon Cycle

  • Plants take in CO2 in photosynthesis, and trees store a lot of CO2 as they gain size. Deforestation is thought to be an important cause of CO2 in the air. The large-scale planting of new trees could reduce the amount of CO2 in the atmosphere.
  • One way of reducing CO2 levels in the atmosphere would be to grow plants to use as fuel. They would only release the CO2 they had just taken in and so would be carbon neutral. However, chopping down rainforest to grow palms for oil releases more CO2 than it takes in, and using corn to make ethanol for biofuel deprives people of food.
  • Human activity has increased carbon dioxide levels in the atmosphere by adding CO2 from burning fossil fuels and decreasing the amount of CO2 that plant and trees can take in through deforestation.
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Impacts of Global Warming.

Global warming causes rising temperatures, changing rainfall patterns and changes to seasonal cycles. This can result in:

Changes to species distribution
As the average temperature rises, southern species may withdraw northwards and northern species may extend southwards. Alien species may colonise new areas, out-competing existing species, making them extinct.

Changes to development
In many reptiles, the temperature affects the sex of young that hatch from eggs. Global warming could cause a change in sex ratios in these species.

Changes to life cycles
Temperature is often an important trigger in the life cycle of many organisms. Insects may get through their life cycles more quickly and be ready to feed on plants that are not yet mature. Plants may flower earlier, but some may flower later.

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Evidence for Evolution

Natural selection - organisms change over time as they adapt to their environment.
Evolution - a change in form, or behaviour, or physiology over generations.
Gene pool - all the alleles of all the genes present in a population.

Evidence for Evolution

  • Darwin's observations
  • Fossil records
  • Gregor Mendel (pea plants)
  • Messelson & Stahl (DNA replication)
  • Watson & Crick (DNA structure)

Darwin's theory has been proved since the 1800s by:

  • Molecular evidence - DNA and protein synthesis, mutations.
  • DNA hybridisation - compare DNA of species
  • DNA profiling - genetic differences between individuals and species.
  • DNA and protein sequencing - compare bases or amino acids
  • DNA molecular clocks - compare the number of differences between species.
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Genomics - the study of DNA

Proteomics - the study of proteins

Succcession - one species follows on from another in a certain area (normally plants).

Species - a group of organisms with similar morphology, physiology and behaviour, which can interbreed and are reproductively isolated from other species.

Reproductive Isolation - can't breed with each other.

Speciation - a new species forms. Some of the species move away from the original, adapts to different conditions and becomes reproductively isolated from the original.

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Reasons for reproductive isolation:

  • Temporal isolation - the species exist in the same area, but reproduce at different times.
  • Ecological isolation - the species occupy different parts of the habitat e.g. one plant species on acidic soil, one on alkaline.
  • Behavioural isolation - the species exist in the same area, but do not respond to each other's courtship behaviour.
  • Physical incompatibility - species co-exist but there are physical reasons that prevent them from reproducing together.
  • Hybrid inviability - in some species, hybrids are produced but do not survive long enough to breed.
  • Hybrid sterility - in some species, hybrids survive to a reproductive age, but cannot reproduce (e.g. mules).
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zoe davison


These will be really useful for my students - thank you!



These are soo amazing :D thanks a bunch 



yiz dis b gr8

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