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3.5.1 Photosynthesis

What is the equation for photosynthesis?

6CO₂ + 6H₂O -----> C₆H₁₂O₆ + 6O₂

Carbon dioxide + water -> Glucose + Oxygen


What makes leaves good at photosynthesis?

Large surface area
- Absorbs maximum light.
Leaf Arrangement
- Minimises leaves overlapping.
- Most light absorbed by the first few micrometres.
- Efficient gas exchange
Transparent cuticle and epidermis
- Allows light into upper mesophyll.
Palisade cells are long, thin, and tightly packed.
- Max light absorption
Many stomata
- Efficient gas exchange, short diffusion distance
Stomata open and close in response to light intensity
- Reduces transpiration.
Many air spaces in lower mesophyll
- Allows rapid gas exchange.
Xylem and Phloem
- Allow transpiration and translocation


What is the law of limiting factors?

When a process depends on two or more factors, the rate of that process is limited by the factor which is in shortest supply


What are the limiting factors of photosynthesis?

Light intensity
Availability of water


How are chloroplasts adapted to their function?

Granal membranes
- Provide a large surface area for chlorophyll, electron carriers and enzymes to attach.
Network of proteins in grana
- Hold chlorophyll in specific way to allow maximum light absorption.
Fluid in stroma contains enzymes.
- Enzymes are needed for light-independent stage of photosynthesis.
- As the chloroplast is membrane-bound it allows it to maintain the optimum enzyme concentration
Chloroplasts contain DNA and ribosomes.
- They can quickly and easily manufacture proteins needed for photosynthesis.
Granal membranes have ATP synthase channels.
- Catalyse the production of ATP.
- Also, selectively permeable
Stroma surrounds the grana.
- Products from the LDR easily diffuse into the stroma for the LDR


Where does the light dependent reaction of photosynthesis take place?

The thylakoid membranes of chloroplasts


Describe the first stage of light dependent reaction (Photoionisation)

Light energy is absorbed by Photosystem II.
The light energy excites electrons in the chlorophyll.
The electrons move to a higher energy level and are released from the chlorophyll and move down the electron transport chain to Photosystem I


Describe the second stage of light dependent reaction (Photolysis of water)

As the excited electrons move from chlorophyll leave PSII to move down the electron transport chain, they must be replaced.
Light energy splits water into protons (H+ ions), electrons and oxygen
Reaction is:
2H₂O  O₂ + 4H⁺ + 4e⁻


Describe the third stage of light dependent reaction (chemiosmotic theory)

The excited electrons lose energy as they move down the electron transport chain.
This energy is used to transport protons into the thylakoid, so that the thylakoid has a higher concentration of protons than the stroma.
This forms a proton gradient across the thylakoid membrane.
Protons move down their concentration gradient, into the stroma, via the enzyme ATP synthase, which is embedded in the thylakoid membrane.
The energy from this movement combines ADP and inorganic phosphate to form ATP


Describe the final stage of light dependent reaction (reduced NADP)

Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level.
Finally, the electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP (NADPH)


What are the products of non-cyclic photophosphorylation?

Reduced NADP


What is cyclic photophosphorylation?

Only uses PSI.
The electrons from the chlorophyll are not passed onto NADP but are passed back to PSI via electron carriers.
This means the electrons are recycled and can repeatedly flow through PSI.
This process does not produce any reduced NADP or Oxygen.
It only produces small amounts of ATP


Where does the light independent reaction take place?

In the stroma of chloroplasts


What is needed for the light independent reaction?

Products of LDR
- NADPH (protons and electrons) to form simple sugar
- ATP as the hydrolysis of ATP provides the energy for the reaction


What are the three sections of the Calvin Cycle?

1. Carbon Fixation
- CO2 combines with RuBP to make 2x GPs (glycerate 3-phosphate)

2. Reduction
- ATP and NADPH used to reduce GP to TP (triose phosphate)

3. Regeneration
- RuBP is regenerated


Describe the steps of the Calvin cycle

1. CO2 diffuses through the stomata and into the stroma of the chloroplast
2. The CO2 (1 carbon compound) reacts with a 5-carbon compound ribulose bisphosphate (RuBP)
3. This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (Rubisco)
4. This reaction between CO2 and RuBP forms a very unstable 6-carbon compound
5. The 6-carbon compound is immediately broken down into 2x 3C GPs
6. NADPH (from LDR) reduces 3 carbon TP using ATP (from LDR)
7. NADPH is reformed (OILRIG) and returns to the LDR
8. TP has 2 uses
- 5/6 carbons are used to regenerate RuBP using ATP (from LDR)
- 1/6 carbons form organic substances such as carbohydrates (glucose), lipids and amino acids


How many times must the calvin cycle turn to make 1 glucose molecule?

6 times as requires 6 carbons since only one carbon released from TP each cycle
- It requires 18 ATP, 12 NADPH and 6 CO2


3.5.2 respiration

What is the equation for respiration?

C6H12O6 + 6O2 --> 6CO2 + 6H2O

Glucose + Oxygen --> Carbon dioxide + water


What is respiration?

A series of reactions in which energy is transferred from organic compounds, such as carbohydrates, to the temporary energy store ATP


What are the four stages of respiration?

1. Glycolysis
- Splitting 6C glucose into 2x 3C pyruvates

2. Link Reaction
- Converts 3C pyruvate into CO2 and 2C acetyl coenzyme A

3. Krebs Cycle
- Acetyl coenzyme A goes through redox reactions that produces ATP and reduced NAD and reduced FAD

4. Oxidative Phosphorylation
- Uses electrons associated with rFAD to synthesise ATP (and waste water)


Where do each of the four stages of respiration happen?

Glycolysis = Cytoplasm of cell
Link Reaction = Matrix of mitochondria
Krebs Cycle = Also in matrix
Oxidative Phosphorylation = Utilises proteins found in the membrane of the crista


What are the stages of glycolysis?

1. Activation of glucose by phosphorylation
- Before splitting glucose is made more reactive by adding 2 phosphate molecules (phosphorylation)
- Phosphate molecules come from hydrolysis of 2 ATP molecules to ADP
- This provides energy to activate glucose and lowers activation energy for enzyme-controlled reactions

2. Splitting of phosphorylated glucose
- Each glucose molecule split into 2x 3C molecules (TP)

3. Oxidation of TP to Pyruvate
- Hydrogen removes from each of 2 x TP molecules
- Hydrogen transferred to NAD (hydrogen carrier)
- This forms reduced NAD

4. Production of ATP
- Enzyme-controlled reactions convert each TP into another 3C molecule called pyruvate
- 2 ATPs regenerated from ADP


What is the total yield of glycolysis?

2 Pyruvate


What does glycolysis being anaerobic prove?

As the process is carried out by all living things it provides evidence for common ancestry


What are the stages of the link reaction?

1. Pyruvate produced in glycolysis actively transported into matrix of mitochondria

2. Pyruvate is oxidised to acetate. To do this it is decarboxylated (loses 1 CO2 molecule) and is dehydrogenated (2 hydrogen molecules removed). This produces acetate

3. The 2 hydrogens removed are accepted by NAD to form reduced NAD which is later used to produce ATP

4. Coenzyme A combines with acetate to form acetyl coenzyme A


What is the overall equation for link reaction?

Pyruvate + NAD + CoA-> acetyl CoA + reduced NAD + CO2


How many times does the link reaction have to occur for every glucose molecule?


- For each glucose molecule used in glycolysis, 2 pyruvate molecules are made
- Link reaction only uses one pyruvate molecule so link reaction happens twice for every glucose molecule going through glycolysis


What are the products of the link reaction for each glucose molecule?

2 acetyl coenzyme A (go into Krebs cycle)
2 CO2 (released as waste products)
2 reduced NAD (go to oxidative phosphorylation)


What are the stages of the Krebs Cycle?

1. Acetyl CoA from link reaction joins with oxaloacetate (4C) to form citric acid/citrate

2. Coenzyme A goes back to link reaction to be reused

3. 6C citrate loses CO2 (decarboxylation) and hydrogen (dehydrogenation)

4. Hydrogen reduces NAD

5. 5C compound is formed which is then converted into 4C molecule

6. Decarboxylation and dehydrogenation occur which forms 1 reduced FAD and 2 reduced NAD

7. ATP is produced (substrate level phosphorylation)

8. 4C oxaloacetate then joins with acetyl CoA (whole cycle starts again)


How many times does Krebs cycle go around per glucose molecule?

From one glucose molecule you get 2 pyruvate molecules so krebs cycle twice


What are the products of the Krebs Cycle for each acetyl CoA entering?

2 CO2 molecules (plus one from link reaction)
1 ATP molecule
Reduced coenzymes NAD and FAD (later used in oxidative phosphorylation to produce ATP)


What is substrate level phosphorylation?

Direct phosphorylation of ADP with a phosphate group using energy obtained from a coupled reaction


What is oxidative phosphorylation?

Production of ATP from oxidised NADH and FADH


What are the stages of oxidative phosphorylation?

1. As the electrons pass along the chain they release energy which causes the active transport of protons across the inner mitochondrial membrane into the inter-membranal space
2. The protons gather in the area between the mitochondrial membranes
3. The electrons pass along the ETC in a series of redox reactions
4. Hydrogen atoms from glycolysis and Krebs join with coenzymes NAD and FAD
5. Reduced NAD and FAD donate electrons of the hydrogen atoms they are carrying to the first electron transport molecule
6. They then diffuse back into the mitochondrial matrix through ATP synthase channels in the inner mitochondrial membrane
7. At the end of the chain the electrons combine with the protons and oxygen making water
8. Oxygen is the final acceptor of the electrons in the transport chain


What is the importance of oxygen in respiration?

Oxygen is crucial in respiration as it is the final acceptor
Also important so hydrogen atoms produced in glycolysis and Krebs cycle can be converted to water and drive production of ATP


What would happen if there was no oxygen?

Without its presence to remove protons and electrons, they would block the flow along the chain and respiration would stop
Krebs and ETC can't take place so pyruvate biulds up in cell


Why is oxidative phosphorylation a step by step approach?

If a lot of energy was released in one step. a lot is lost as heat and therefore energy is wasted
However if it is released slowly over number of steps, more energy is available for use of the organism
This is why NAD and FAD transfer their electrons gradually


What other substances aside from glucose are able to undergo oxidation to release energy?



Describe the process of oxidation of lipids

1. Lipids are hydrolysed into glycerol and fatty acids
2. Glycerol is phosphorylated and converted into TP
3. TP then converted to pyruvate and enters the link reaction and then Krebs
4. The fatty acids are hydrolysed in 2C fragments which are converted into acetyl CoA
5. This then also joins Krebs cycle


What is the advantage of oxidation of lipids producing lots of hydrogen atoms?

The hydrogen atoms are used to produce ATP during oxidative phosphorylation
This means lipids release twice as much energy as carbohydrate


Describe the process of oxidation of proteins

1. Proteins are hydrolysed into amino acids
2. They are then deaminated (amino group removed)
3. They then enter the respiratory pathway at different points depending on their number of carbon atoms
4. 3C compounds = converted to pyruvate
4C compounds = converted to intermediates in Krebs


Why is anaerobic respiration important?

NAD must be replenished by pyruvate
Pyruvate must accept hydrogen from NADH
The newly oxidised NAD can then be reused in glycolysis to allow the process to continue


How does anaerobic respiration occur in plants and microorganisms

Occurs in bacteria and certain fungus (yeast)

1. Pyruvate is decarboxylated (loses CO2)
2. This forms ethanal
3. Ethanal is reduced by hydrogen atoms supplied by NADH
4. This forms ethanol

Pyruvate (3C) + NADH (reduced) --> ethanol (2C) + CO2 (1C) + NAD (oxidised)


What are the uses of anaerobic respiration in plants and microorganisms?

Yeast in the brewing industry
Yeast grown anaerobically
Ferments natural carbohydrates from plant products into ethanol
- Grapes into wine
- Barley seeds into beer


Why does anaerobic respiration occur in animals?

To overcome a temporary oxygen shortage
Gives a survival advantage e.g. immediately after birth, when water has low O2 levels or to escape predators
Most common in skeletal muscles


Describe the process of anaerobic respiration in animals

During an oxygen shortage, NADH from glycolysis can accumulate and must be removed
To get rid of it, pyruvate (C3H4O3) takes up 2 hydrogen atoms from NADH
This forms lactate (C3H6O3)
Therefore NAD is regenerated from NADH by the reduction of pyruvate to lactate
When oxygen is available again, the lactate is oxidised back to pyruvate
The pyruvate can be further oxidised to release energy or converted back to glucose


What are the problems associated with lactate?

Muscle cramps and fatigue
Lactate is an acid so causes pH changes that effect enzyme action
- It is therefore removed by the blood and taken to the liver where it is converted to glycogen
This regeneration requires lots of ATP (produced by aerobic respiration)
Therefore regeneration leads to oxygen debt, where the athlete is continuing a high level of oxygen consumption post-exercise


3.5.3 Energy and Ecosystems

What is a food chain?

Shows the feeding relationship between producers and consumers


What is a food web?

More accurate demonstration of feeding relationships. Link food webs of one habitat together


What is a producer?

Photosynthetic organisms that produce organic substances using light, energy, water, CO2, and mineral ions – plants


What is a primary consumer?

Organism in the food chain that directly obtains its energy by feeding on the producer. First consumer/animal in food chain


What is a secondary consumer?

Second consumer in food chain. Feeds on primary consumer


What is a tertiary consumer?

Third consumer in food chain. Feeds on secondary consumer


What are saprobionts?

Decomposers (bacteria and fungus mainly). Break down material from dead organisms into simple molecules. They release essential nutrients which are absorbed and thus recycled by plants


What is a trophic level?

Position of a food chain occupied by a producer or consumer


What is an herbivore?

Plant eating consumer (sheep, cow)


What is a carnivore?

Meat eating consumer (lion, shark)


What is an omnivore?

Consumer that eats both plants and animals (crow, bear, human)


What is a habitat?

Place where an organism normally lives, and which is characterised by physical conditions and the types of other organisms present


What is an ecosystem?

All the living and non-living components of a particular area


What is biomass?

The total mass of living material, normally measured in a specific area over a time period


Why is biomass only an estimate?

Biomass can only be measured on a sample of a killed organism. The sample may not be representative


How is biomass measured?

Dry mass per given area in a given time.
Terrestrial environments measured in gm-2.
Aquatic environments measured in gm-3


What is a calorimeter and how is it used?

A calorimeter is used to estimate the chemical energy store in dry mass.

A sample of dry material is weighed and then burnt in pure oxygen within a sealed container called a bomb.
The bomb is surrounded by water bath and heat of combustion causes a small temperature to rise in this water.
As we know how much heat (energy)is required to raise 1g of water by 1˚C, if we know the volume of water and temperature rise, we can calculate the energy released from the mass of burnt biomass in units KJKg-1


Why is most of the sun’s energy not converted to organic matter by photosynthesis?

Not all light will hit a chlorophyll molecule.
Not all wavelengths of light can be absorbed and used for photosynthesis.
Over 90% of sun’s energy is reflected back into space by clouds/dust or absorbed by atmosphere.
Limiting factors e.g., low temperature, may reduce the rate of photosynthesis


What is Gross Primary Production (GPP)?

The total quantity of energy that plants in an area/volume convert to organic matter (biomass) in a given time


What is Net Primary Productivity (NPP)?

Respiration uses 20-50% of energy in GPP.
Energy left over to be stored is NPP.

NPP = GPP – Respiratory Losses


Why does GPP vary between habitats?

Temperature variations
Constant/no/little seasonal change
Higher plant density
Higher water availability/rainfall
More light (energy)/intensity


What is NPP available for?

Plant growth and reproduction
It can also be passed along the food chain to other trophic levels


How is energy lost in food chains?

Not all of the organism is eaten.
Not all is digested and is lost as faeces.
Some lost in excretory materials e.g., urine.
Heat loss to the environment


What is the calculation for Net Production of Consumers?

N = I – (F + R)

N = Net Production
I = Chemical energy store of ingested food
F = Energy lost in faeces and urine
R = Energy lost in respiration


Why can food chains only support up to 5 trophic levels?

Food chains are extremely inefficient.
Insufficient energy available to support a large population at higher trophic levels.
The total biomass is less at each level.
Therefore, the amount of energy available is less


What is the relationship between farming and energy transfer?

Intensive rearing can be hugely profitable.
Farmers aim to produce the maximum yield with the minimum output.
The focus of intensive rearing is to convert the smallest amount of food energy into the greatest quantity of animal mass.
Minimising energy loss helps this.
This allows more food energy to be taken in by the animals to be converted to body mass.
This is then passing along the food chain to humans (and this means more profit)


What measures may farmers take to improve the energy-conversion rate in the livestock (animals they rear)?

1. Limit movement
- Less movement = less energy lost during exercise

2. Heat the environment around the animals (in barns)
- Do not waste energy keeping themselves warm.

3. Control Diet
- Food is high in protein and low in fibre to build muscle mass.

4. Remove predators and pests.
- Sick animals use energy fighting infections.


What are the benefits of intensive rearing?

1. Efficient energy conservation
- Restricting wasteful energy loss means more energy is passed on to humans along the food chain.

2. Low-cost food production
- Foods such as meat, eggs and milk can be produced more cheaply than by other methods. With more families now reliant on foodbanks this could be helpful

3. Less space required.
- Intensive rearing uses less land with efficient production meaning that less of the countryside is required for agriculture, leaving more as natural habitats.

4. Safety – easier to control as its small area.
- Small, concentrated units are easier to control and regulate. It is easier to prevent infections being introduced from outside and to isolate the animals if this happens.

5. Food is essential for life.
- With an ever-expanding human population, there is pressure to produce more food intensively


What are the disadvantages of intensive farming?

1. Quality of food
- The taste of foods produced by intensive rearing is inferior to food produced less intensively.

2. Disease
- Large numbers of animals living in close proximity means that infections can spread easily amongst them. To control this the animals are regularly given antibiotics.

3. Use of drugs
- Over-use of antibiotics to prevent disease in animals has led to evolution of antibiotic resistance. This resistance can be transferred to bacteria that cause human diseases, making their treatment with certain antibiotics ineffective. Other drugs may be given to animals to improve growth or reduce aggressive behaviour. These may alter flavour of food or pass into foods and then into humans, affecting health.

4. Animal welfare
- Animals are kept unnaturally, and this may cause stress, resulting in aggressive behaviour. This may cause them to harm each other or themselves, which is why battery chickens are debeaked. Restricted movement can lead to osteoporosis and joint pain.

5. Pollution
- Intensively reared animals produce large amounts of waste in small area. Rivers and ground water may be polluted. Pollutant gases can be dangerous and may smell. Large intensive farms ay have own disposal facilities that enable them to treat waste more effectively than smaller farms.

6. Reducing genetic diversity
- Selective breeding is used to develop animals with high energy conversion rates and a tolerance of confined conditions. This reduces the genetic diversity of domestic animals, resulting in loss of genes that might have been beneficial.

7. Use of fossil fuels.
- High energy conversion rates are possible because fossil fuels are used to heat the building that house the animals I, in the production of materials in buildings (especially cement) and in production and transportation of animal feeds. The carbon dioxide emitted increases global warming


3.5.4 Nutrient Cycles

What are the key stages of a nutrient cycle?

Nutrient absorbed as simple inorganic molecules by the producer.
Producer converts it into complex organic molecules.
Nutrient passed during feeding relationships.
After death, consumer broken down by saprobionts.
Nutrient released back into the soil in its simplest form


Why do living things need nitrogen?

Amino Acids
Few organisms use gaseous nitrogen as it has triple bond so is very stable.
Plants use nitrates from soil taken up via active transport as cannot use nitrogen straight from air as nitrogen gas is unreactive so not easily converted into other compounds.
Nitrate ions being soluble is problematic to plants as they are easily washed away from plant roots by rainwater.
Animals get nitrogen from consuming plants


What are the four stages of the nitrogen cycle?

1. Nitrogen fixation
2. Ammonification
3. Nitrification
4. Denitrification


What are the three ways in which nitrogen fixation can occur?

1. Nitrogen gas can be ‘fixed’ into ammonia by humans using Haber process which is then used in production of fertilisers.
2. A small amount of nitrogen is fixed to nitrate by lightning.
3. Microorganisms that carry out this process (free living and mutualistic)


What is nitrogen fixation?

Nitrogen gas in the atmosphere is converted into nitrogen containing compounds.
Biological fixation is carried out by bacteria e.g., rhizobium.
They turn nitrogen into ammonia.
Rhizobium is found inside root nodules of leguminous plants. They form mutualistic relationship with plants as provide plant with nitrogen compounds and plant provides them with carbohydrates.


What is ammonification?

When nitrogen compounds from dead organisms are turned into ammonia by saprobionts, which goes on to form ammonia ions.
Animal waste (urine and faeces) also contains nitrogen compounds. These are also turned into ammonia by saprobionts and go on to form ammonia ions


What is nitrification?

When ammonium ions in the soil are changed into nitrogen compounds that can be used by plants (nitrates)
First nitrifying bacteria called Nitrosomonas change ammonium ions into nitrites.
Then other nitrifying bacteria called Nitrobacter change nitrites into nitrates.
This is an oxidation reaction so releases energy.
Nitrifying bacteria require oxygen to carry out these conversions so the soil needs many air spaces which can be achieved by ploughing or good drainage.


What is denitrification?

When nitrates in soil are converted into nitrifying bacteria
They use nitrates in the soil to carry out respiration and produce nitrogen gas.
This happens under anaerobic conditions (no oxygen) e.g., waterlogged soil


Where is phosphorous usually found?

Usually found in the lithosphere and does not have a gaseous phase.
Usually found as phosphate ions PO₄³⁻
These are found in rocks


What are uses of phosphorous?

Nucleic acids


What are the stages of the phosphorous cycle?

1. Phosphorous is trapped in sedimentary rock.
2. The rocks are formed in the sea but are raised to surface by geological activity.
3. Weathering and erosion release phosphate ions
4. These ions are able to dissolve and are taken up by plants through roots. Mycorrhizae increase rate at which phosphorous can be assimilated.
5. They are passed to animals during feeding.
6. When organisms excrete or die, they are broken down by bacteria and fungus (saprobionts) which are able to release the phosphates.
7. These phosphates end up in the soil or dissolved in water.
8. 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.
9. Waste produced by birds (known as guano) and contains high proportion of phosphate ions. Guano returns phosphate ions to soils and is often used as natural fertiliser.
10. Some phosphates are fixed into hard tissues such as bones and shells. These take exceptionally long time to break down.
11. Some of the phosphate remaining in the water are eventually combined in sedimentary rock


Describe the process of the carbon cycle

Carbon dioxide is in the air.
Through photosynthesis it is used by plants
Animals feed on these plants, consuming the carbon.
Carbon dioxide is then released via respiration into atmosphere.
When animals die and are decomposed by saprobionts, carbon dioxide is released back into atmosphere.
The plants are also decomposed into dead vegetation. This can be burned for fossil fuels e.g., oil, coal, peat, gas. The burning releases carbon dioxide back into atmosphere.


What is a mutualistic relationship?

Mutualism is the way two organisms of different species exist in a relationship in which each individual benefit from the activity of the other


What are mycorrhizae?

Beneficial fungi growing in association with plant roots


What is the role of mycorrhizae?

1. Exist by taking sugars and amino acids from plants ‘in exchange’ for moisture and nutrients gathered from the soil by the fungi strands.
2. Mycorrhizae greatly increase the absorptive area of the plant, acting as extensions to root system.
3. They hold water close to the roots making the plant more drought tolerant.
4. Phosphorous is often is short supply in natural soils and require a vast root system for a plant to meet its phosphorous requirements unaided. Mycorrhizae are crucial in gathering phosphorous in uncultivated soils.
5. Mycorrhizas also seem to confer protection against root diseases


What are the two type of fertilisers that farmers use?

1. Organic/ Natural fertiliser
- Dead and decaying plant and animal matter
- Animal excreta e.g., manure and slurry
- Bone Meal

2. Inorganic/ Artificial fertiliser
- Mined from rock.
- Converted to suitable form.
- Blended to suit a particular crop e.g., nitrogen, phosphorous, potassium


How do fertilisers work?

The yield increases until an optimum mass is reached and then it decreases.
Increasing the nitrogen content of the soil increases the productivity of the crop.
After an optimum amount of nitrogen is supplied the yield decreases
This is due to water potential as increased nitrates reduce the water potential.
This will eventually draw water out of the plant by osmosis


What is leaching?

Rainwater dissolves soluble nutrients and washes them deep into the soil.
The plant roots are unable to reach and absorb them.
The leached nutrients find their way into watercourses and drinking water.
Excessive nitrates can prevent efficient oxygen transport in babies and has links to stomach cancer.
Can also lead to eutrophication


What is eutrophication?

Watercourses tend to contain low levels of nitrates which limits the growth of algae and bacteria.
Farmers apply fertilisers to their fields.
Rainfall dissolves the nutrient ions.
These dissolved nutrient ions are leached into watercourses.
Nutrient content is no longer a limiting factor for algae and bacteria so both populations grow.
Algae grow on the surface of the water forming algal bloom.
The algae absorb light and prevent it from reaching the bottom of the water body.
Light is now the limiting factor for plants below the surface so algae and plants below surface die.
Lack of dead organic matter is no longer limiting factor for saprobionts, so their population increases.
Their growing population requires more oxygen, so oxygen concentration reduces.
More nitrates are released from the decaying organisms.
Oxygen is now the limiting factor for organisms that aerobically respire (water becomes anoxic), and they die.
Now less competition for anaerobically respiring organism so their population increases.
They continue to decompose dead material releasing further nitrates and toxins (hydrogen sulphide)
The water becomes putrid