2 Energy, Ecosystems and Nutrient Cycles

Question 1

Define ecosystem.

Answer 1

An ecosystem includes all the organisms living in a particular area and all the abiotic conditions.

Question 2

How do plants synthesise organic compounds?

Answer 2

In all ecosystems there are producers- organisms that make their own food.
During photosynthesis plants use energy and CO2 (from the atmosphere in land based ecosystems, or dissolved in water in aquatic systems) to make glucose and other sugars.

Question 3

What happens to sugars produced during photosynthesis?

Answer 3

Some of sugars produced used in respiration, to release energy for growth. Rest of glucose used to make other biological molecules such as cellulose. These biological molecules make up plants biomass. Biomass can also be thought as the chemical energy stored in plant.

Question 4

How is energy transferred through living organisms of an ecosystem?

Answer 4

When organisms eat other organisms, eg. Producers eaten by organisms called primary consumers. Primary consumers eaten by secondary consumers, secondary consumers consumers eaten by tertiary consumers. This is a food chain.

Question 5

What is biomass?

Answer 5

Biomass can be measured in terms of mass of carbon that an organism contains or the dry mass of its tissue per unit area per unit time

Question 6

How is biomass measured?

Answer 6

• dry mass is mass of organism with water removed.
• to measure dry mass, sample of organism is dried, often in an oven set to low temp.
• sample weighed at regular intervals. Once mass becomes constant you know all water has been removed
• results from sample can be scaled up to give dry mass of total population or are being investigated
• mass of carbon present is generally taken to be 50% of the dry mass
• biomass changes overtime so typical units for biomass might be kg/m^2/yr

Question 7

How can the chemical energy store in dry biomass can be estimated using calorimetry?

Answer 7

• sample of dry biomass burnt and energy released is used to heat a known volume of water.
• change in temperature of water used to calculate to chemical energy of the dry biomass.

Question 8

What is gross primary production (GPP) ?

Answer 8

The chemical energy store in plant biomass, in a given area or volume.

Question 9

What is net primary production (NPP)?

Answer 9

The chemical energy store in plant biomass after respiratory losses to the environment have been taken into account.
NPP = GPP - R
R represents respiratory losses to the environment.
NPP available for plant growth and reproduction. It’s also available to other trophies levels in the ecosystem, such as herbivores and decomposers.

Question 10

What’s the formula for net production for consumers?

Answer 10

N = I - (F+R)
I represents chemical energy store in ingested food
F represents chemical energy lost to the environment in faeces and urine.
R represents respiratory losses to the environment

Question 11

Net production for consumers.

Answer 11

• consumers store chemical energy in their biomass
• consumer get energy from investing plant material or animals that have eaten plant material
• not all chemical energy stored in consumers’ food is eaten so energy it contains is not taken in. Then, of the parts that are ingested: some are indigestible, so are egested as faeces. Some energy is also lost to the environment through respiration or excretion of urine.
• the energy’s that left after all this is stored in the consumers’ biomass and is available to the next trophies level. This energy is the consumers net production.

Question 12

What is primary and secondary productivity?

Answer 12

the rate of primary or secondary production, respectively. It is measured as biomass in a given area in a given time eg kJ/ha/year–1.

Question 13

What are food chains?

Answer 13

Show how energy is transferred through an ecosystem and show simple lines of energy transfer. Each of stages in a food chain called a trophic level.

Question 14

What are food webs?

Answer 14

Show how energy is transferred through an ecosystem and show lots of food chains in an ecosystem and how they overlap. Decomposers are also part of food webs, they break down dead or undigested material, allowing nutrients to be recycled.

Question 15

How can farmers reduce pest numbers using chemical pesticides and so reduce energy losses to non-human food chains?

Answer 15

• insecticides kill insect pests that eat + damage crops. Killing insect pests means less biomass lost from lost from crops, so they grow to be larger, means productivity is greater
• herbicides kill weeds. Killing weeds can remove direct competition with the crop for energy from sun. Can also remove preferred habitat or food source of insect pests, helping to further reduce their numbers and simplify the food web.

Question 16

How are biological agents used to reduce the number of pests and so reduce energy losses to non-human food chains?

Answer 16

• parasites live in or lay their eggs on a pest insect. Parasites either kill insect or reduce its ability to function.
• pathogenic bacteria and viruses used to kill pests, eg bacterium Bacillus thuringiensis produces toxin that kills wide range of caterpillars

Question 17

How is energy lost through respiration reduced?

Answer 17

• farmers control conditions that livestock live in, so more of their energy used for growth and less is lost through respiration eg
  
  • movement increase rate of respiration, so animals may be kept in pens to restrict movement.
  • pens often indoors and kept warm so less energy wasted by generating body heat
• benefits are that more food can be produced in shorter space of time, often at lower cost however, could raise ethical issues

Question 18

Define natural ecosystem.

Answer 18

An ecosystem that hasn’t been changed by human activity. Nutrients recycled through the food webs, but human activity often disrupts the cycling of nutrients.

Question 19

What is the role of saprobionts in decomposition.

Answer 19

Saprobionts secrete enzymes and digest their food externally, then absorb nutrients they need. This is known as extra cellular digestion. During this process, organic molecules are broken down into inorganic ions obtaining nutrients from dead organic matter using extra cellular digestion is known as saprobiotic nutrition

Question 20

What’s the role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants?

Answer 20

The mutualistic relationship between fungi and plant roots are known as mycorrhizae. The fungi made up of long, thin strands called hyphae, which connect to plants roots. Hyphae increase SA of plants root system, helping plant to absorb ions from soil that are usually scarce. Hyphae also increase uptake of water by plant. In turn fungi obtain organic compounds such as glucose from plant.

Question 21

Why do plants and animals need nitrogen?

Answer 21

To make proteins and nuclei acids.

Question 22

What’s the first step of the nitrogen cycle?

Answer 22

Nitrogen fixation.

• when nitrogen gas in atmosphere turned into nitrogen contains compounds. Biological nitrogen fixation carried out by bacteria such as Rhizobium. They turn nitrogen into ammonia which can be used by plants.
• Rhizobium found inside root modules of leguminous plants. Eg peas
• they form form mutualistic relationship with plants, they provide plant with nitrogen compounds and plant provides them with carbohydrates.

Question 23

What’s the second step of the nitrogen cycle?

Answer 23

Ammonification

• when nitrogen compounds from dead organisms turned into ammonia by saprobionts, which goes on to form ammonium ions.
• animal waste contains nitrogen compounds. These also turned into ammonia by saprobionts and go in to form ammonium ions.

Question 24

What’s the third step of the nitrogen cycle?

Answer 24

Nitrification

• when ammonium ions in soil are changed into nitrogen compounds that can be used by plants.
• first nitrifying bacteria called Nitrosomonas change ammonium ions into nitrites
• other nitrifying bacteria called Nitrobacter change nitrites into nitrates.

Question 25

What’s the last step of the nitrogen cycle?

Answer 25

Denitrification

• when nitrates in soil converted into nitrogen gas by denitrifying bacteria, they use nitrates in soil to carry out respiration and produce nitrogen gas
• happens under anaerobic conditions eg in waterlogged soil.

Question 26

Stage 1 of phosphorus cycle.

Answer 26

phosphate ions in rocks released into soil by weathering.

Question 27

Stage 2 of phosphorus cycle.

Answer 27

Phosphate ions takes into plants through roots. Mycorrhizae increase rate at which phosphorus can be assimilated.

Question 28

Stage 3 of phosphorus cycle.

Answer 28

Phosphate ions transferred through food chain as animals eat the plants and are in turn eaten by other animals.

Question 29

Stage 4 of phosphorus cycle.

Answer 29

Phosphate ions lost from animals in waste products.

Question 30

Stage 5 of phosphorus cycle.

Answer 30

When plants + animals die, saprobionts are involved in breaking down the organic compounds, releasing phosphate ions into soil for assimilation by plants. These microorganisms also release the phosphate ions from urine and faeces.

Question 31

Stage 6 of phosphorus cycle.

Answer 31

Weathering of rocks also releases phosphate ions into seas, lakes and rivers. This is taken up by aquatic producers, such as algae, and passed along food chain to birds.

Question 32

Stage 7 of phosphorus cycle.

Answer 32

Waste product by sea birds is known as guano and contains high proportion of phosphate ions. Guano returns significant amount of phosphate ions to soils. Often used as a natural fertiliser.

Question 33

How are nutrients lost when crops are harvested?

Answer 33

• crops take in minerals from soil as they grow and use them to build their own tissue
• when crops harvested, they’re removed from field where they’re grown rather than being allowed to die and decompose there. This means mineral ions they contain not returned to soil by decomposers in nitrogen and phosphorus cycles.
• phosphates + nitrates also lost from system when animals or animal products removed from land. Animals eat grass + other plants, taking in their nutrients. When they’re slaughter or transferred to different field, nutrients are replaced through their remains or waste products.

Question 34

Define artificial fertiliser.

Answer 34

Are inorganic and contain pure chemicals as powders or pellets.

Question 35

Define natural fertilisers.

Answer 35

Are organic matter, include manure, composted vegetables, crop residue and sewage sludge.

Question 36

How does using fertiliser raise environmental issues?

Answer 36

• sometimes more fertiliser applied than plants need or are able to use at particular time.
• can lead to fertilisers leaching into waterways.
• this can lead to eutrophication
• inorganic ions in chemical fertilisers are relatively soluble so excess minerals that aren’t used immediately more likely to leach into waterways. In natural fertilisers, nitrogen and phosphorus still contained in organic molecules that need to be decomposed by microorganisms before can be absorbed by plants. This means their release into soil for uptake by plants is more controlled and leaching less likely.
• leaching of phosphates less likely than leaching of nitrates because phosphates less soluble
• using fertiliser changes balance of nutrients in soil, too much can cause crops and other plants to die.

Question 37

Define leaching.

Answer 37

When water-soluble compounds in soil are washed away eg by rain or irrigation systems. They’re often washed into nearby ponds and rivers. It’s more likely to occur if fertilisers applied just before heavy rainfall.

Question 38

Describe process of eutrophication.

Answer 38

• mineral ions leached from fertilised fields stimulate rapid growth of algae in ponds and rivers (algal bloom)
• large amounts of algae block light from reaching plants below
• plants below unable to photosynthesis so die
• saprobionts break down dead plants. Increased number of bacteria reduce oxygen concentration in water by carrying out aerobic respiration
• fish and other aquatic organisms die because there isn’t enough dissolved oxygen.