AQA A Level Biology - Energy and Ecosystems

define ecosystem

all the organisms living in a particular area and all the abiotic conditions

how do plants make biomass?

• plants synthesis organic compounds like glucose from atmospheric or aquatic CO2

• most of the sugars synthesised are used as respiratory substrates

• the rest are used to make other groups of biological molecules which make up the plants biomass

define biomass

dry mass of carbon or dry mass of tissue in kg/m²

define dry mass

• mass of an organism with the water removed

• as water content of living tissues varies

• mass of carbon is roughly 50% dry mass

• can be found for one organism then scaled up for an entire population or area

how is dry mass measured?

• drying a sample of living tissue in an oven

• regularly weighing it until constant mass is achieved

• this indicates all the water has been removed

how would you estimate the amount of chemical energy in biomass?

• burning it in a calorimeter

• done by heating a known mass of water and measuring temperature change

• measured in KJ/T

define gross primary production (GPP)

• chemical energy store in plant biomass in a given area/volume

• amount of energy fixed by producers, i.e. converted from light energy

define net primary production (NPP)

• chemical energy store in plant biomass after respiratory losses (R) due to heat have been taken into account

• amount of energy available to the next tropic level and to the plant for growth and reproduction

what is the equation for NPP?

NPP = GPP - R

define net primary productivity

rate of primary production in biomass/area/time (i.e. Kg/m²/year)

define net production of consumers (N)

• chemical energy store in a consumer’s biomass that is available to the next tropic level

• after respiratory losses, egestion and excretion

• can also be called secondary production/productivity

what is the equation for N?

• N = I - (F+R)

• where I is chemical energy in ingested food

• F is chemical energy lost in faeces and urine

• R is respiratory energy losses

why does NPP not equal N?

• not all of the food/producer is eaten

• some food is indigestible (like cellulose) so is egested as faeces

• some energy is lost via respiration/urine

what is the equation for efficiency of energy transfer?

= net production of tropic level/net production of previous trophic level * 100

how does efficiency change up a food chain?

energy transfer becomes more efficient

why is the sun→ producer the least efficient energy transfer?

• lots of sunlight is made up of wavelengths unsuitable for photosynthesis

• 1/5 of usuable light is reflected by or transmitted through the leaf or is absorbed by parts of the leaf that aren’t chloroplasts

• ¾ of the remaining light is lost due to the inefficiency of photosynthesis and a lack of available CO2 - remainder is the GPP

what do food chains/webs show?

• show how energy is transferred through an ecosystem

• includes decomposers

what is the difference between food chains and food webs?

• food chains show simple lines of energy transfer

• food webs show lots of food chains and their overlap

why do farming practices want to make energy transfers more efficient?

• increase energy available for human consumption

• means increasing NPP of crops and NP of livestock

• two main ways are simplifying food webs and reducing respiratory losses

how do farming practices increase efficiency by simplifying food webs?

• get rid of food chains that don’t involve humans to reduce energy loss so that the NPP of crops increases

• pest control - prevents loss of crops to pests which would decrease NPP, so total biomass increases and GPP so NPP will increase

• herbicides can kill weeds, which reduces competition with crops for minerals, water and sunlight; also removes habitats for pests

• can also use biological control like parasites, pest pathogens or natural predators, although this doesn’t simplify food chains much

how do farming practices increase efficiency by reducing respiratory losses?

• done by controlling conditions for livestock so more energy is able to be used for growth and less for respiration

• e.g. keeping animals in pens which restrict movement, reducing respiration

• keeping animals in heated indoor environments so less energy is wasted via thermoregulation

• increases biomass so N increases so more energy available to humans

• selective breeding - producing animals that grow bigger and faster, so more biomass in shorter time, increasing I and reducing F and R so increasing N

• however controversial as may be ethical issues as restrict natural behaviour of animals