Summary OCR A Level Biology A* Ecosystems Detailed Notes
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Course
Module 6 - Genetics, evolution and ecosystems
Institution
OCR
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6.5 Ecosystems
6.5.1 Ecosystems
Ecosystems
- Species = group of organisms with common ancestry so share the same genes and are able to reproduce to
create fer9le offspring -> reproduc9vely separate from other species
- Different species live and interact together to form communi9es
- Communi9es interact with each other and their environment to form ecosystems
- Ecosystem = rela9vely self-contained community of interac9ng organisms and their environment
- Flow of energy through ecosystem where nutrients are recycled within it through both bio9c and abio9c
components
- Ecosystems vary in size (pond vs desert) and complexity (desert vs rainforest)
- Never en9rely self-contained as organisms are linked (e.g. birds flying between)
Bio0c Factors
- Anything that influences the popula9ons within a community that is the result if another organism’s
ac9vity
1. Preda9on
2. Compe99on (inter-specific) for space/food/water/light
3. Co-opera9on between organisms
4. Parasi9sm
5. Disease -> pathogenic caused by microorganisms or carried by vectors
6. Camouflage -> organisms can escape capture and not benefit the poten9al predator species
7. Mimicry -> e.g. harmless hover fly looking like a wasp to deter poten9al predators
8. Mankind -> habitat loss/hun9ng/farming
Abio0c Factors
- Any physical/chemical factors that influences the popula9ons within a community as environmental
condi9ons can majorly impact the spread of a species
1. Availability of water
2. Light
3. Radia9on
4. Temperature
5. Turbidity - cloudiness - of water
6. Humidity
7. Atmospheric composi9on
8. PH
9. Salinity
10. Soil composi9on
6.5.2 Transfer of Biomass
Transfer of biomass
- Biomass of an organism/9ssue is the mass of living material of organism/9ssue + chemical energy storied
in the organism/9ssue, numerous way of measurement
1. Dry mass of organism/9ssue
2. Mass of carbon in organism/9ssue -> usually 50% of the total dry mass
3. Chemical energy content of the organism when burned in pure oxygen
Dry mass
- Mass of organism/9ssue aber all water has been removed
- Dry mass of sample can be used to find biomass of total popula9on/area by scaling up numbers
- Biomass can change over 9me so may be given with units of 9me too to show average biomass of an
organism within a given area over that 9me period
- E.g. trees lose leaves in autumn/winter
Calorimetry
- Used to es9mate the chemical energy stored in dry biomass by burning a dry sample in a calorimeter to
heat a known volume of water -> change in water temp provides es9mate for the chemical energy
, - Energy is a vital considera9on in ecology as every trophies level transfers energy to the subsequent level
and energy transfer is the main goal of feeding rela9onships
PRACTICAL: Finding the Dry Mass and Energy Value of Plant Biomass
Apparatus and Techniques
1. Heat proofed, open-topped container (crucible) that can withstand high oven temperatures, and moisture
can evaporate and escape
2. Oven to dry the sample slowly
3. Digital balance to monitor decreasing mass (small changes so needs high precision)
4. Calorimeter (either cheap and simple or expensive and precise – bomb calorimeter)
Method
1. Finding mass of dry sample
a) Weight crucible without sample
b) Place sample in crucible and crucible in oven on low temperature
c) Remove crucible and weight at regular intervals un9l mass becomes constant (fully dehydrated)
d) Subtract mass of crucible alone from this value to find dry mass of sample
2. Finding energy released by sample of plant biomass
a) Use calorimeter to burn sample to heat known volume of water and its change in volume
b) 4.2 kilojoules of water is needed to increase the temp of 1kg of water by 1 degree Celsius
Limita0ons
- Can take a long 9me to fully dehydrate a sample as it has to be heated at a low temp, can take several days
- Precise equipment is needed, may not be available e.g. very precise digital balance to measure mass as
sample dries, very precise digital thermometer to measure change of water temp in calorimeter
- More simple/basic calorimeter = less accurate es9mate for chemical energy as heat energy is lost and not
transferred efficiently to the water
- Bomb calorimeter ensures almost all heat energy from burning sample is transferred to water = very
accurate es9mate
6.5.3 Efficiency of Energy Transfers
Efficiency of Energy Transfers
- Large propor9on of Sun’s energy is not available to photosynthe9c plants because:
a) light falls away from plants
b) light is transmijed through leaves or reflected off
c) only certain wavelengths of light used
- Primary producers convert light energy to chemical energy in biological molecules through photosynthesis
- Storing this chemical energy as plant biomass makes a certain amount of energy available to the next
trophic level – primary consumers
- Only a small % of plant biomass becomes biomass in the primary consumer because:
a) not all plant biomass is eaten by the primary consumer
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