biology topic 5a

energy and ecosystems

what is biomass?

the total mass of carbon or dry mass of tissue per given area at a given time. measured in gm^-2(area) or gm^-3(volume)

in an ecosystem, what do plants synthesise organic compounds from?

atmospheric or aquatic carbon dioxide

what are most sugars synthesised by plants used as? what are the rest used for?

respiratory substrates. rest are used to make other biological molecules (DNA, RNA, phospholipids…) which form biomass

why is dry mass or carbon measured and not fresh mass?

fresh mass includes water, which varies so its unreliable

whats a problem with using carbon or dry mass to measure biomass?

organism must be killed, so only small sample which may not be representative

how can you measure the chemical energy store in dry mass? describe method

calorimetry

1. dry to constant mass in oven to reove water

2. measure mass of dry mass before

3. burn dry biomass in pure oxygen in bomb calorimeter

4. measure volume of water

5. measure temperature change

6. use q=mct

where is all energy for ecosystem derived from?

the sun

how do producers asborb light energy?

during process of photosynthesis they fix it into chemical energy in the form of organic molecules which is passed up the trophic levels.

whats a problem with energy transfer in ecosystems?

energy transfer into ecosystems and up trophic levels is nowhere near 100%

what is gross primary production?

the chemical energy store in plant biomass, in a given area or volume ie result of photosynthesis

what may be a problem with the suns energy which causes lots of energy to be lost up trophic levels?

not all the suns energy is converted into organic molecules → over 90% of suns light never reaches the plant, not all wavelengths absorbed (green light reflected back), light not falling on chlorophyll molecule etc. energy that is converted to organic molecules is the GPP

what are trophic levels?

each stage in food chain

what is net primary production? (NPP)

the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account (amount of energy that can be passed onto the next trophic level)

how can energy be lost from energy that is fixed into organic molecules (GPP)?

some of it (20-50%) is used in respiration. glucose made in photosynthesis is used in respiration and the energy contained is lost as heat therefore not available to be made into other organic molecules

what are examples of organic molecules?

DNA, RNA, proteins, phospholipids

how do you calculate NPP?

NPP=GPP-R

R is respiratory losses

what is NPP available for?

plant growth and reproduction (growth via cell divison, where DNA needs to be replicated, new organelles etc)

how do you calculate the net production of consumers?

N= I - (F+R)

I is chemical energy store in indigested food

F is energy lost to environment in faeces and urine

R is respiratory losses to environment

as a summary, why is so much energy lost at each trophic level?

• in plants, some light energy is reflected/transmitted through leaves/not right wavelength → efficiency of photosynthesis is low

• some energy is lost to surroundings as heat from respiration

• some energy is lost in faeces and urine

• some parts of organisms are not eaten, not transferred to next trophic level

how is energy lost when a consumer eats a producer?

less than 10% of NPP is incorporated into primary consumers biomass hence only 10% passed up to next trophic level

due to:

• some organism not being consumed

• some parts lost as faeces or urine

• some energy lost as heat via respiration like plants

why are food chains often limited to 4-5 trophic levels?

• if energy is lost at each trophic level then efficiency of energy transfer is low

• total biomass is less at higher tropic levels

• therefore insufficient energy available to support a large breeding population if more than 4-5

how do you calculate percentage efficiency?

=(energy available after transfer/energy available before transfer) x 100

whats the difference between production and productivity?

productivity includes over a certain period of time, production doesnt

how are farming practises designed to increase the efficiency of energy transfer?

1. simplify food webs ie reduce energy losses to nonhuman food chains, increasing NPP by eg using pesticides so energy isnt lost when pests feed on crops

2. reducing respiratory losses within a human food chain ie control conditions livestock are kept in eg restrict movement so less energy lost as heat from respiration or keep them warm so less energy used to generate body heat from respiration

what are primary and secondary productivity?

the rate of primary and secondary production respecitvely. its measured as biomass in a given area in a given time eg kJha-1year-1