Biology - Chapter 13: Energy and Ecosystems

What is the ultimate source of energy for all organisms?

Sunlight

How do farming practices increase the productivity of agricultural crops? (4)

1. Fertilisers added to soil: adds ions like nitrates, which may be used to make proteins

2. Selective breeding: breed particular crops to have highest yield + withstand environmental conditions

3. Ploughing: aerates soil to allow nitrification and decrease denitrification

4. Crop rotation: prevent depletion of soul nutrients

What do plants synthesise organic compounds from?

Atmospheric / aquatic CO2 and energy from sunlight

What are the sugars synthesised by plants used for? (2)

- As respiratory substrates
- Make other groups of biological molecules i.e. the biomass of the plant

Give two ways biomass is measured in terms of

1. Mass of carbon
2. Dry mass of tissue per given area

3 groups of organisms

Producers, consumers and saprobionts

Producers

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

Consumers

Organisms that obtain their energy by feeding on other organisms rather than using the energy of sunlight directly

3 types of consumers

1. Primary consumers: those that directly eat producers
2. Secondary consumers: those that eat primary consumers
3. Tertiary consumers: those that eat secondary consumers

Saprobionts

A group of organisms that break down the complex materials in dead organisms into simple ones, releasing valuable materials and elements in a form that can be absorbed by plants and so contribute to recycling

Give two examples of saprobionts

Fungi and bacteria

Trophic level

Each stage on a food chain

Food chain

A description of the feeding relationship in which producers are eaten by primary consumers, which in turn are eaten by secondary and tertiary consumers

What do the arrows on food chains represent?

The direction of energy transfer

Food web

Multiple food chains for the animals in a single habitat linked together

Biomass

The total mass of living material in a specific area at a given time

What is a problem with measuring the biomass?

The organisms must be killed first, so usually a small sample is used - this may not be representative

How can you estimate the chemical energy store in dry mass?

By using calorimetry

How is calorimetry used to estimate the chemical energy store in dry mass?

- Sample of dry material is weighed, burnt in pure oxygen in bomb (sealed container)
- Bomb is surrounded by water bath; combustion causes temperature of water bath to rise
- energy = mass x specific heat capacity x change in temp

What proportion of the Sun's energy do plants convert to organic matter?

1 - 3%

Give 4 reasons why most of the Sun's energy is not converted to organic matter by photosynthesis

1. Light is reflected
2. Light is wrong wavelength so cannot be absorbed by chlorophyll pigments
3. Light misses chlorophyll
4. A limiting factor (CO2 or temperature) is limiting the rate of photosynthesis

Gross primary production (GPP)

The total quantity of the chemical energy store in plant biomass in a given area or volume, in a given time

How much of the GPP do plants use in respiration?

20-50%

Net primary productivity (NPP)

The chemical energy store in plant biomass after respiratory losses in environment have been taken into account

How do you calculate NPP?

NPP = GPP - R
(R = respiratory losses)

Why is a low percentage of energy transferred at each stage of a food chain? (4)

1. Some of the organism isn't consumed
2. Some parts of the organism are consumed but cannot be digested so are lost in faeces
3. Some of the energy is lost in excretory materials
4. Some energy losses occur as heat from respiration and lost to the environment

Why do mammals / birds lose a lot of energy due to heat from respiration?

They have high body temperatures, which need much energy to maintain as heat is constantly being lost to the environment

How do you calculate net production of consumers?

N = I - (F + R)
(where N = net production, I = chemical energy store of ingested food, F = energy lost in faces/urine, R = energy lost in respiration)

Why do most food chains only have 4/5 trophic levels?

There is insufficient energy available to support a large enough breeding population at trophic levels higher than these

Primary / secondary productivity and give its units

The rate of primary / secondary production, measured as biomass in a given area in a given time (e.g. kJ ha-1 year-1)

Saprobiontic microorganisms

xx

How are mineral ions absorbed into plant roots?

By active transport

Give the 4 stages of the nitrogen cycle

Nitrogen fixation, ammonification, nitrification, denitrification

Give some examples of organic nitrogen-containing compounds

Urea, proteins, nucleic acids, vitamins

What do nitrifying bacteria need in order to carry out the nitrification reaction and what should farmers do?

Oxygen, so they need soil with many air spaces - farmers should keep soil light and well-aerated + use good drainage so soil doesn't fill with water

Describe the process of the phosphorus cycle (5)

1. Rocks containing phosphate are weathered due to harsh weather conditions, slowly releasing phosphate ions into soil and water sources due to the weathering and erosion of rocks
2. Phosphate ions are taken up from the soil by plants via their roots or from water by algae and are used to make DNA or ATP
3. Phosphate ions are transferred to consumers by feeding and are also used to make DNA or ATP
4. Phosphate ions in waste products and dead organisms are released into the soil/water during decomposition by saprobionts
5. Phosphate ions now taken up by producers again or are trapped in sediments, which may turn into phosphorus-containing rock again

Give some examples of phosphorus-containing organic compounds

ATP, nucleic acids, phospholipids

Describe the role of mycorrhizae in nutrient cycles (3)

- Fungi act as extensions of the plant's root surface, increasing the total surface area for the absorption of water and minerals
- Mycorrhiza holds water and minerals in the neighbourhood of its roots
- Enables plant to better resist droughts and to take up inorganic ions more readily, improving their uptake of relatively scarce ions

Mycorrhizae

Associations between certain types of fungi and the roots of the vast majority of plants

How is the mycorrhizal relationship between plants and fungi mutualistic?

The plant benefits from improved water and inorganic ion uptake, while the fungus receives organic compounds (sugars, amino acids) from the plant

Mutualistic relationship

A nutritional relationship between two species in which both gain some advantage

Why are fertilisers needed in agricultural ecosystems? (4)

- Nitrates and phosphates absorbed by plants are lost after crops are harvested and transported for consumption (no crops there for saprobionts to decompose so not returned)
- Ions also lost when livestock are removed from land (urine/faeces/dead remains containing ions are in another area)
- Results in lower concentration of mineral ions in agricultural land so they act as a limiting factor for plant growth, causing lower productivity
- Fertilisers therefore added to soil to replace lost ions

2 types of fertilisers

Natural (organic) and artificial (inorganic)

Give 2 examples of natural fertilisers

Dead and decaying remains of plants/animals + animal wastes (e.g. manure)

Artificial fertilisers

Mined from rocks and deposits, then converted into different forms and blended together to give appropriate balance of materials (normally contain NPK)

How do fertilisers increase productivity? (3)

- Mineral ions needed for plant growth (e.g. nitrogen needed to form amino acids)
- More ions results in greater plant growth (e.g. they grow taller + leaves have greater surface area)
- Causes greater rate of photosynthesis so crop productivity increases

Give 3 environmental issues caused by the use of nitrogen-containing fertilisers

1. Reduced species diversity: nitrogen-rich soils favour growth of grasses + rapidly growing species, so they out-compete other species, which die
2. Leaching: causes pollution of watercourses
3. Eutrophication:

Leaching

The process by which nutrients are removed from the soil

How does leaching occur? (3)

1. Rainwater dissolves soluble nutrients (e.g. mineral ions)
2. They're carried deep into soil, beyond plant roots
3. The leached ions find their way to watercourses

Give 2 negative effects of leaching

1. The ions may enter sources of drinking water, which can have harmful effects on humans (e.g. very high nitrate ion concentrations are linked to stomach cancer)
2. Eutrophication

Eutrophication

The process by which nutrient concentrations increase in bodies of water

What is the main cause of eutrophication?

The leaching of artificial fertilisers

Describe the process of nitrogen fixation

Free-living bacteria + mutualistic bacteria + other natural/artificial processes (e.g. lightning) convert nitrogen gas in the air to ammonia

Describe the process of ammonification

Saprobionts break down amino acids / proteins using enzymes, producing ammonia

Describe the process of nitrifcation

Nitrifying bacteria oxidise ammonia to nitrites, then oxidise the nitrites to nitrates

In what type of conditions does nitrification occur?

Aerobic conditions

Describe the process of denitrification

Denitrifying bacteria convert nitrates in the soil to nitrogen gas

In what type of conditions does denitrification occur?

In anaerobic conditions (e.g. waterlogged soil)

Why do farmers plough the soil? (3)

Ploughing aerates the soil, so there are aerobic conditions for more nitrification to occur (so more nitrates are produced. It also prevents denitrification from taking place.

Give 3 ways crop rotation leads to high crop yields

1. Can grow crops with nitrogen-fixing bacteria
2. Different crops use different nutrients from soil (so nutrients decrease by less)
3. Different crops have different pests and pathogens

Why can adding very high concentrations of fertiliser to the soil reduce plant growth?

It causes the water potential of the soil to decrease, so water moves out of the plants into the soil by osmosis

How does a high concentration of nutrients result in greater productivity for livestock? (2)

There are more nutrients, which can be used by producers for growth - more producers results in more food for consumers, so more consumers reproduce

Name a free-living nitrogen-fixing bacteria

Azotobacter

Name a mutualistic/symbiotic nitrogen-fixing bacteria

Rhizobium

Name a nitrifying bacteria that oxidises ammonium ions to nitrite ions

Nitrosomonas

Name a nitrifying bacteria that oxidises nitrite ions to nitrate ions

Nitrobacter

Give 4 features of intensive farming

- Restricted movement: less energy used in muscle contraction
- Environment kept warm: reduce heat loss from body
- Feeding controlled: animals receive optimum amount and type of food for max growth without wastage
- Predators excluded: no loss to other organisms in the food web

Why do mammals have greater respiratory losses than insects?

Mammals have a higher body temperature

What farming processes (apart from spreading fertilisers) result in the addition of nitrogen-containing compounds to a field? (3)

1. Growing legumes - have nitrogen-fixing bacteria

2. Allow cattle/animals to graze - add dung/urine

3. Add manure - decomposed to release ammonia

What farming practices result in the removal of nitrogen-containing compounds from a field? (3)

1. Bare soil - leaching of nitrates

2. Uptake of nitrates by crop - crop is harvested

3. Animals eat plants in field - animals removed

When investigating the effect of different factors on plant growth / biomass, why should you use the dry mass of plant?

Dry mass measures the increase in biological material + water content varies

Describe what happens in eutrophication (5)

1. The increased nutrient ion concentration results in more algae growing

2. This results in an algal bloom, which blocks the light

3. This causes submerged plants to photosynthesise less, so die

4. Saprobionts aerobically respire while they break down the dead plant remains

5. So there’s less oxygen for the fish and other aquatic organisms to respire and they die

Why can increases in the biomass of the part of a plant be used to represent net primary productivity?

It represents the chemical energy store minus respiratory losses.

Why would it be more efficient to farm crops for consumption rather than animals?

Energy is lost between trophic levels as a result of respiration, as faeces etc (fewer trophic levels so less energy lost)