Energy Transfer in Ecosystems

Question 1

Producers

Answer 1

Photosynthetic organisms that manufacture organic substances using light energy, water and CO2, mineral ions

Question 2

Consumers

Answer 2

• Organisms that obtain their energy by feeding on other organisms rather than using energy of sunlight directly
• Those that directly eat producers are primary consumers
• Animals eating primary consumers are secondary consumers
• Those eating secondary are tertiary consumers
• Secondary and tertiary are usually predators but can be scavengers or parasites

Question 3

Biomass

Answer 3

• Total mass of living material in a specific area at a given time
• Fresh mass is quite easy to assess, but presence of varying amounts of water makes it unreliable
• Measuring mass of carbon or dry mass overcomes this problem but because organisms must be killed, only get a small sample (may not be representative)
• Energy in biomass measured using calorimetry
• Mass of living material
• Biomass can also be thought of as the chemical energy stored in the plant

Question 4

What is an Ecosystem?

Answer 4

• Includes all the organisms living in a particular area and all the non-living (abiotic) conditions
• In all ecosystems, there are producers-organisms that make their own food

Question 5

How is Biomass made?

Answer 5

• During photosynthesis plants use energy (from sunlight) and carbon dioxide (from the atmosphere in land based ecosystems, or dissolved in water in aquatic ecosystems) to make glucose and other sugars
• Some of the sugars produced during photosynthesis are used in respiration, to release energy for growth
• Rest of glucose is used to make other biological molecules such as cellulose (component of plant cell walls)
• These biological molecules make up the plant’s biomass

Question 6

Food Chain

Answer 6

Energy transferred through the living organisms of an ecosystem when organisms eat other organisms

Question 7

Measuring Biomass

Answer 7

• Biomass can be measured in terms of the mass of carbon that an organism contains or the dry mass of its tissue per unit area per unit time
• Dry mass is the mass of the organism with the water removed
• Water content of living tissue varies, so dry mass is used as a measure of biomass rather than wet mass
• To measure dry mass, sample of organism is dried, often in an oven set to a low temperature
• Sample is then weighed at regular intervals
• Once the mass becomes constant you know that all water has been removed
• Mass of carbon present is generally taken to be 50% of the dry mass
• Once you’ve measured the dry mass of a sample, you can scale up the result to give the dry mass (biomass) of the total population or the area being investigated (typical units are kg m-2)
• Biomass changes overtime- means it’s useful to give biomass over a particular time period (typical units are kg m-2 yr-1)

Question 8

Calorimetry

Answer 8

• Burnt in pure oxygen
• Can estimate amount of chemical energy stored in biomass by burning the biomass in a calorimeter
• Amount of heat given off tells you how much energy is in it
• Energy is measured in J or kJ

Question 9

Gross Primary Production (GPP)

Answer 9

Total amount of chemical energy converted from light energy by plants, in a given area, in a given time

Question 10

Respiratory Loss (R)

Answer 10

Approximately 50% of the GPP is lost to the environment as heat when the plant’s respire

Question 11

Net primary production (NPP)

Answer 11

• The remaining chemical energy is called the NPP
• NPP= GPP-R
• The NPP is the energy available to the plant for growth and reproduction- the energy is stored in the plant’s biomass
• It is also the energy available to organisms at the next stage in the food chain (next trophic level)
• These include herbivores (animals that eat the plants) and decomposers

Question 12

Why isn’t photosynthesis 100% efficient?

Answer 12

Not all of the light energy absorbed by a plant will be converted to chemical energy

Question 13

Source of energy for ecosystems?

Answer 13

Sun

Question 14

Net production in consumers

Answer 14

• Consumers also store chemical energy in their biomass- they get energy by ingesting plant material or animals that have eaten plant material
• Not all chemical energy stored in the consumers’ food is transferred to the next trophic level (most energy lost)
• Not all food is eaten (plant roots, bones) so energy it contains is not taken in
• Some are indigestible, so are egested as faeces (chemical energy stored in this is therefore lost to the environment)
• Some energy is also lost to the environment through respiration or excretion of urine
• Energy that’s left after all this is stored in the consumers’ biomass and is available to the next trophic level
• This energy is the consumers’ net production
• N= I - (F+R)
• N= Net production
• I= Chemical energy in ingested food
• F= Chemical energy lost in faeces and urine
• R= Energy lost through respiration

Question 15

Efficiency of energy transfer

Answer 15

• net production of trophic level/net production of previous level x 100
• or… energy available after transfer/energy available before transfer
• As you move up food chain, energy transfer becomes more efficient
• This is because plants (producers) contain more indigestible matter than animals (consumers)

Question 16

Why isn’t most of the sun’s energy converted to organic matter?

Answer 16

• Most of the energy is reflected back into space by clouds and dust or absorbed by the atmosphere
• Not all wavelengths of light can be absorbed and used for photosynthesis
• Light may not fall on chlorophyll molecule
• A factor, such as low CO2, may limit the rate of photosynthesis

Question 17

How can we explain relative inefficiency of energy transfer between trophic levels?

Answer 17

• Most food chains only have 4 or 5 trophic levels beacuse insufficient energy is available to support a large enough breeding population at trophic levels higher than these
• Total mass of organisms in a particular place (biomass) is less at higher trophic levels
• Total amount of energy available is less at each level as one moves up a food chain

Question 18

Food Chains

Answer 18

Simple lines of energy transfer where each stage is called a trophic level (producer to consumer)

Question 19

Food Webs

Answer 19

• Show lots of food chains in an ecosystem and how they overlap (all animals may not rely on a single food source and share a habitat with others)
• Arrows show direction of energy flow

Question 20

How to increase efficiency for NPP of crops and NP of livestock?

Answer 20

• Need to increase amount of energy available for human consumption
• Energy lost to other organisms e.g.pests can be reduced through simplication of food webs
• Energy lost through respiration of livestock can be reduced

Question 21

Simplifying food webs

Answer 21

• Pests are organisms that reduce the amount of energy available for crop growth and therefore the NPP for crops
• This ultimately reduces the amount of energy available for humans
• By simplifying food webs (getting rid of food chains that don’t involve humans), energy losses will be reduced and the NPP of the crop will increase
• To get rid of pests farmers use chemical pesticides
• Insecticides kill insect pests that eat and damage crops so less biomass is lost and NPP is greater
• Herbicides kill weeds which removes direct competition with the crop for energy from the sun (also can remove habitat or food source of pests)
• Biological agents reduce number of pests, so crops lose less energy and biomass, increasing efficiency of energy transfer to humans
• Farmers can combine both biological and chemical methods to increase NPP even more
• Natural predators can be introduced to eat pest species but this can complicate food web

Question 22

Reducing respiratory loss

Answer 22

• Can increase NP of livestock by controlling conditions they live in, so more energy is used for growth and less from respiration (movement which is why animals are kept in pens, pens kept warm so less energy wasted by generating body heat)
• Means more biomass is produced and more chemical energy can be stored, increasing NP and efficiency of energy transfer to humans
• Benefits= food can be produced in a shorter space of time, often at a lower cost
• Costs= Animals in pens raises ethical issues as it can cause them pain, distress and restricts their natural behaviour

Question 23

Role of microorganisms (saprobionts, bacteria, fungi)

Answer 23

• They feed on remains of deadplants and animals and on their waste products, breaking them down
• This makes saprobionts a type of decomposer that allows chemical elements in the remains to be recycled (saprobionts release minerals and elements in a form that plants can absorb)
• Saprobionts secrete enzymes and digest their food externally, then absorb the nutrients they need (extracellular digestion)
• During this process, organic molecules are broken down into inorganic ions
• Saprobiotic nutrition= obtaining nutrients from dead organic matter and animal waste using extracellular digestion

Question 24

Phosphorus Cycle

Answer 24

• Phosphorous present in sedimentary rock as phosphate ions and are released into soil by weathering
• Plants absorb PO₄^3- to make phospholipid/DNA/ATP
• Consumers eat plants to obtain phospholipid/DNA/ATP so transferred through food chain and waste products
• Organic material (dead plants, dead animals, animal waste) are broken down by saprobiotic decomposers, this releases phosphate ions back into the soil for assimilation by plants (microorganisms also release phosphate ions into their waste)
• Mycorrhize are fungi in the roots of plants that increase uptake of scarce minerals like phosphate ions (increase rate of phosphorus assimilation)
• Weathering can also release phosphate ions into seas and lakes which is taken up by birds when eating algae (they bring back phosphate ions when producing waste called guano which can be used as a natural fertiliser)

Question 25

Symbiotic relationship

Answer 25

When 2 species live closely together and one or both species depends on the other for survival

Question 26

Mutualistic relationship

Answer 26

Type of symbiotic relationship where both species benefit

Question 27

Mycorrhizae

Answer 27

• Some fungi form symbiotic relationships with roots of plants (known as mycorrhizae)
• Fungi are made up of long thin strands called hyphae, which connect to the plant’s roots
• Hyphae gently increase surface area of plant’s roots system, helping plants absorb ions from the soil that are usually scarce
• They also increase the uptake of water by ther plant
• In turn, fungi obtain organic compunds, such as glucose, from the plant

Question 28

Nitrogen Cycle

Answer 28

• Plants and animals need nitrogen to make proteins and DNA but they can’t use atmospheric nitrogen
• Includes how nitrogen is passed along food chain
• Plants take up most of nitrogen as nitrate ions (ions are absorbed using active transport by roots)
• Nitrate ions are very soluble and easily leach through soil
• Natural and agricultural ecosystems can restore nitrogen through recycling and adding fertilisers

Question 29

Nitrogen Fixation

Answer 29

• When atmospheric nitrogen is turned into nitrogen-containing compounds
• Biological nitrogen fixation is carried out by bacteria rhizobium (mutualistic)
• Turns nitrogen into ammonia, which goes on to form ammonium ions in solution that can be used by plants
• Rhizobium are found inside root nodules of leguminous plants
• They form a mutualistic relationship with the plants-they provide the plant with nitrogen compounds and the plant provides them with carbohydrates
• Other nitrogen-fixing bacteria are found in the soil
• Carried out by free-living (reduce gaseous nitrogen to ammonia to make amino acids and their nitrogen rich compunds are released when they die and decay) and mutualistic n.f bacteria
• Lightning fixes atmospheric nitrogen into nitrogen oxides or by artificial fertilisers (produced from atmospheric nitrogen on an industrial scale in the haber process)

Question 30

Ammonification

Answer 30

• Nitrogen compounds from dead organisms are turned into ammonia by saprobionts, which goes on to form ammonium ions
• Animal waste also contains mitrogen compounds
• These are also turned into ammonia by saprobionts and go on to form ammonium ions

Question 31

Nitrification

Answer 31

• Ammonium ions in soil are changed into nitrogen compounds that can be used by plants
• Nitrifying bacteria changes ammonium ions into nitrites and then nitrates
• Requires oxygen, so soil needs many air spaces (light soil structure and well aerated by ploughing)
• Good drainage also prevents air spaces being filled with water and so prevents air being forced out of the soil

Question 32

Denitrification

Answer 32

• Nitrates in the soil are converted into nitrogen gas by denitrifying bacteria-they use nitates in the soil to carry out respiration and produce nitrogen gas
• Happens under anaerobic conditions (waterlogged soils)
• For land to be productive, soil must be kept aerated to prevent build up of denitrifying bacteria

Question 33

Loss of nutrients

Answer 33

• When crops are harvested, they can’t die and decompose
• Means mineral ions can’t return to the soil by decomposers in the nitrogen/phosphorus cycle
• Phosphates and nitrates lost when animals or animal products are removed
• Animals eat plants and grass, so when taken away nutrients aren’t replaced through their remains or waste products

Question 34

What do fertilisers do?

Answer 34

• Replaces lost minerals, so more energy from the ecosystem can be used for growth, increasing efficiency of energy transfer
• Fertilisers increase growth as they contain nitrogen (nitrogen used to make ATP, amino acids and DNA)

Question 35

Artificial fertilisers

Answer 35

• Inorganic
• Contain pure chemicals (ammonium nitrate) as powders or pellets

Question 36

Natural fertilisers

Answer 36

• Organic
• Include manure, composed vegetables, crop residues and sewage sludge

Question 37

Environmental Issues

Answer 37

• Sometimes more fertiliser applied than the plants need or are able to use at a particular time
• This can lead to fertilisers leaching (water-soluble compounds in the soil are washed way, can lead to eutrophication) into waterways
• Inorganic ions in chemical fertilisers are relatively soluble
• Means that excess minerals that are not used can leach into waterways (more likely if fertiliser applied just before heavy rainfall)
• Leaching less likely with natural fertilisers
• This is because nitrogen and phosphorus are still contained in organic molecules that need to be decomposed by microorganisms before they can be absorbed by plants
• Means that their release into soil for uptake by plants is more controlled
• Leaching of phosphates is less likely than the leaching of nitrates because phosphates are less soluble in water
• Using fertilisers may also change balance of nutrients-too much of a particular nutrient can cause certain plants to die and reduce species diversity

Question 38

Eutrophication

Answer 38

• If large amounts of chemical fertilisers (excess) are sprayed onto fields and heavy rainfall occurs, the fertiliser may leach into local water sources
• Fertiliser will travel and build up in ponds or lakes
• Minerals will be absorbed and used by algae
• Leads to an increase growth of algae = algal bloom
• Algae grows on the upper surface of the water, this prevents light reaching the plants at the bottom of the water
• These plants cannot photosynthesise, so die
• These provide more nutrients to saprobitoic decomposers, so these increase in number
• Decomposers will aerobically respire, using up the oxygen in the water (release H2S)
• Therefore fish die as less oxygen is available