AQA A2 Biology Unit 4 Notes

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AQA BIOLOGY Unit 4
1. Populations
The number of organisms of one species in a habitat
Populations and Ecosystems
Ecosystem: All the organisms living in a particular area and all the abiotic (non
living/biological) conditions.
Biosphere: The life-supporting layer of land, air and water that surrounds the Earth.
Community: The organisms of all species that live in the same area (*group of
species/populations).
Ecological Niche: Comprises of all the conditions and resources required for an organism to
survive, reproduce and maintain a viable population.
Investigating Populations
Large and Random Sampling,
To reduce roll played by chance
So data is representative of the population
To avoid bias
Making results more valid
To carry out statistical tests
Achieved by (1) creating a grid of survey land, (2) use calculator to generate random number,
(3) use numbers as coordinates, (4) place centre of quadrat on coordinate then (5) count
species or percentage cover or abundance scale inside quadrat.
Systematic sampling along transects e.g. useful for succession, (1) set up transect/ tape
measure across area, (2) place quadrats at regular intervals along the line then (5) count
species or percentage cover or abundance scale inside quadrat.
Abundance: number of individuals of a species within a given space
Mark-release-recapture techniques
*Capture sample, mark with appropriate method that does not harm individual (e.g. paint,
tags) and release back to community. After a period of time, take second sample and count
number of marked organisms.

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This technique relies on the following assumptions:
Mark does is not lost/rubbed off
Population size does not change i.e. due to deaths or births
Population has a definite boundary so no immigration or migration
Capture and mark process did not effect individuals likeliness to be captures, e.g.
more afraid of humans making them avoid capture or vice versa
Released individuals have time to distribute themselves evenly among population
Variation in Population Size
Population growth curves
Abiotic factors e.g. temperature, light, pH, water and humidity.…read more

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Human Populations
There was a large increase in human population due to development of agriculture,
manufacturing and trade being able to sustain a large population (industrial revolution).
Population growth = (births + immigration) ­ (deaths + emigration)
*Birth rate: Births per thousand per year.
Death rate: Deaths per thousand per year.
Factors affecting birth rate
Economic conditions ­ developing countries often have high birth rates.…read more

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ATP is useful as an immediate energy supply because:
Energy release only involves a single reaction (hydrolysis)
Energy released in small manageable quantities
ATP is the source of energy for:
Metabolic processes ­ energy needed to build macromolecules from basic units
Movement ­ energy needed for muscle contraction (sliding filaments)
Active transport ­ energy needed to change shape of carrier proteins in plasma
membranes
Secretion ­ energy needed to from lysosomes for secretion of cell products
Activation of molecules ­ when a phosphate molecule is…read more

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Site of light-dependent reaction (thylakoids)
Thylakoid membranes provide a large surface area for attachment of chlorophyll,
electron carriers and enzymes that carry out the LDR
Networks of proteins in the grana hold the chlorophyll very precisely to optimise light
absorption
Granal membranes have enzymes attached to them to help manufacture ATP
Contain DNA and ribosomes to quickly manufacture proteins needed for LDR
Site of light-independent reaction (stroma)
Stroma fluid contains enzymes needed to carry out LIR
Stroma fluid surrounds grana so products of LDR can…read more

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NADP and ATP will enter the grana to be used in the light independent stage of
photosynthesis.…read more

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The Light-Independent Reaction (LIR) / The Calvin Cycle
CO2 from the atmosphere diffuses into the leaf through the stomata and dissolves in water
around the walls of the mesophyll cells. It then diffuses through the plasma membrane,
cytoplasm and chloroplast membranes into the stroma of the chloroplast.
In the stroma, the CO2 combines with the 5C compound ribulose biphosphate (RuBP) using
an enzyme.
The combination of CO2 and RuBP produces two molecules of the 3C glycerate 3-phosphate
(GP).…read more

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Factors Affecting Photosynthesis
CO2 concentration
CO2 is needed in the LIR to be combined with RuBP to form GP, if there is a low
concentration of carbon dioxide then GP cannot be formed, this causes RuBP to
build up and the reaction then stops.…read more

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Glycolysis
The splitting of the 6C glucose molecule into two 3C pyruvate molecules, which is an
anaerobic process that occurs in the cytoplasm of the cell.
1. Activation of glucose by phosphorylation
Glucose is first phosphorylated (2 phosphate molecules are added) ­ Pi
comes from hydrolysis of 2 ATP molecules to ADP and Pi ­ releasing energy.
This energy is used to catalyse the next enzyme-controlled reactions by
activating (lowering the activation energy of) glucose.
2.…read more

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The conversion of the 3C pyruvate molecule into carbon dioxide and a 2C molecule called
acetylcoenzyme A. Takes place in the mitochondrial matrix. The link reaction occurs twice for
each glucose molecule since 2 pyruvate molecules are formed in glycolysis.
1. Oxidation
Pyruvate is oxidised by removing hydrogen, this hydrogen is then accepted by
NAD to form NADH (used in ETC)
2. Decarboxylation
1 carbon atom is removed in the form of CO2 (waste product)
3.…read more

Comments

Hannah

thanks 

JV31

the unit 4 notes are amazing!

Do you have any unit 5 notes?

L.B

Great notes- thank you so much

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