Topic 5 - Energy transfer (photosynthesis, respiration, / AQA A-level Biology, Study notes of Biology

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Photoautotrophs utilise photosynthesis to convert energy supplied by absorption of visible light photons from the sun to

chemical energy in the form of intracellular ATP. ATP supplies used to drive metabolic processes through phosphorylating cycles

• occurs in chloroplast stroma + thylakoid membrane system suspended in it.
• consists of light dependent (photon + electron transfers) and light independent phase (biosynthesis of carbohydrates)

The net result of photosynthesis is generation of sugars via fixation of atmospheric carbon dioxide

Sugars synthesised are stored for use as substrates used in respiration

• photolysis of water contributes electrons to replace excited chlorophyll electrons lost to redox cascade and liberates H+
• Electrons reduce coenzymes used in subsequent carbon fixation
• H+ actively transported to establish gradient across thylakoid membrane
• H+ passage back across membrane coupled with ATP synthesis
• oxygen functions as the terminal electron acceptor in the respiratory cascade released as a by-product

Photosynthesis occurs during periods where light incidence on leaf surface is sufficient, plants most photoactive

LEAF MORPHOLOGY

Cell bodies of mesophyll elongated and contain few other

organelles, maximum volume for occupation by chloroplasts

▪

▪ Large thin + flat leaf

⇢ maximises absorption surface

⇢ minimises diffusion distance for gas exchange

▪ Transparent cuticle/epidermal layer permits passage of sunlight

guard cells control stomatal aperture - responsive to diurnal

change in light intensity

▪

▪ Starch granuales allow storage of carbohydrate - energy reserves

▪ Air spaces in spongey mesophyll - efficient passage in gas phase

CHLOROPLAST ADAPTATIONS

▪ Dual thylakoid membrane

Creates compartments between stroma and lumen, provides membrane to cause ion separation

⇢ allows proton gradient to be established across membrane of thylakoid between lumen and stroma

embedded complexes form redox chain

⇢ transfer of electrons from NADH to O 2

, shuttling down series with higher reduction potentials

⇢ Small steps, less exothermic energy loss .energy instead stores in proton gradient

▪

Membrane pumps

• Selective permeability to ions, establishment of proton gradients generates charge separation
• many ATP-ase pumps embedded for efficient chemiosmotic coupling,

▪

▪ High phytopigment content, maximal light capture

contain own biosynthetic machinery

Distinct DNA and ribosomes

⇢ fast manufacture of proteins

⇢ Allows surplus of RuBIsCo to be generated

highly bioavailable enzyme allows fixation via

more energetically favourable route

▪

SPECIALISATION

Photosynthetic tissue specialised to maximise efficiency of chloroplast function. Photosynthetic tissues dominant upper leaf

surfaces, Highest chloroplast abundances in parenchyma cells in the upper mesophyll layer found in aerial parts of plant where

sunlight exposure is maximised

3.5.1 - PHOTOSYNTHESIS

28 April 2022 21:

3.5.1 - PHOTOSYNTHESIS Page 1

PIGMENTS

Aggregates of photosensitive pigments contained in light harvesting complexes of PSII absorb and redirect energy

towards reaction centres which contain the final chlorophyll a which undergoes excitation.

CAROTENOIDS

Broad class of accessory pigments which encompasses range

of colours and absorb in blue-green range of spectrum.

400 - 500 nm range absorbance maxima

⇢ Non overlapping absorption spectra broaden range

absorbable in photosynthesis

• orange colouration

CHLOROPHYLLS

Depends on the interplay of two photosystems, linked by common intermediates.

Electrons are given up in the oxidation stage (PSII) and participate in series of reductions in the electron transport chain,

shuttle down membrane before acceptance at the PSI

LDR LIR

OUTCOME Convert light energy into chemical energy Use stored chemical energy to fix CO 2

and create a

product that can be converted into glucose

SITE thylakoids Stroma

INPUT Sunlight, H 2

O, NADP

, ADP CO

2

, NADPH, ATP

OUTUT NADPH, ATP, O 2 NADP

, ADP, G3P (2 G3P ⇢ 1 C 6 H 12 O 6 )

LIGHT DEPENDENT STAGE - THYLAKOID

PHOTOABSORPTION

Initial absorption of photons occurs in antenna complex

Photosynthetic pigments in granal membrane-embedded protein matrices absorb and funnel energy from incident sunlight

Cascade through antennae complex between pigments terminates at reaction centre chlorophyll-a,

A sufficient quanta of energy is absorbed to cause photoinduced electron excitation

⇢ Electron enters redox series.

Each antenna complex has 250 - 400

accessory pigment molecules.

Diversity of accessory pigments with different

absorption optima broaden the range of

absorbable wavelengths.

In vascular plants accessory pigments include

chlorophylls, xanthophylls, and carotenes.

Chlorophylls constitute primary class of light-absorbing pigment present in green plants

Absorption maxima occur in regions corresponding to and red

• Chlorophyll a - 430 (violet) + 662 (red) nm
• Chlorophyll b - 450 (Blue) + 640 (orange) nm

•

• diffusive reflection of green wavelengths gives it its characteristic colour.
• inserted into the thylakoid membranes, head of chlorophyll associated with magnesium ion coordinated by ring
• chlorophylls A:B found in 3:1 ratio

Photoionization occurs when chlorophyll a molecules at the reaction centre is raised to higher energy state

⇢ phytolysis of water supplies electrons which reduce chlorophyll and restore it to ground state.

3.5.1 - PHOTOSYNTHESIS Page 2

ELECTRON TRANSPORT

Photoinduced Electron transfer - transfer of excited electron from donor to acceptor generates charge separation.

ETC is comprised by a series of transmembrane embedded protein complexes, involves reduction of carriers at

successively lower energy levels,

electron transfer is exergonic, energy release during transfer exploited to drive

photolysis catalysed by PSII (H 2

O photodissociates into H+ released into the thylakoid lumen + O 2

○ )

Shuttling of protons into thylakoid space via active transport to generate gradient

⇢ movement of proton back across membrane coupled with atp synthase pumps

○

Terminal acceptance of electrons in the PSI by activated carrier NADP+, NADPH formed

NADP

+ 2H

• 2e → NADPH + H

⇢ NADPH exported to stroma for ue in LIR

light + 2H

2

O + 2ADP + 2P

i

• 2NADP

+

O

2

• 2 ATP + 2NADPH + 2H

+

G3P ⁐ RuBP

( REGENERATION)

RELEASED INTO ATMOSPHERE

ANTENNAE COMPLEX - 4 PHOTONS

ABSORPTION FROM SOIL

3.5.1 - PHOTOSYNTHESIS Page 3

LIGHT INDEPENDENT STAGE - STROMA

Light independent stage concludes with the biosynthesis of organic carbohydrates via fixation of atmospheric CO

LIR involves 3 intermediate phases - carboxylation, reduction and RuBP regeneration.

no direct conversion of CO 2

• to sugars. Direct sugar synthesis energetically unfavourable.

15 out of 18 carbons from the 6 G3P used to regenerate RuBP

1 G3P exported to cytosol for sugar synthesis

⇢ 1 glucose molecule made from 2 triose sugar molecules requires 6 cycles.

STROMA ADAPTATIONS

Compartmentalization, plasma membrane allows distinct environments to be maintained between cytoplasm and stroma.

⇢ maintains specific enzymatic and chemical composition

•

• Stroma fluid inundates grana, allows easy diffusion of LDR products into stroma for use in LIR
• distinct chloroplast DNA and ribosomes, own cellular machinery allows fast assembly of proteins

CALVIN CYCLE

3 CO 2

• 3 RuBP + 6 NADPH + 6 H

+

• 9 ATP 3 - GP + 6 NADP

+

• 9 ADP + 8 P i

PSI - FINAL ELECTRON ACCEPTANCE

NADP

• 2H

• 2e → NADPH + H

G3P ⁐ RuBP

( REGENERATION)

PSII- PHOTOLYSIS

3 H

2

O → 6 H

+ 1.5 O

2

ATMOSPHERE

SUGAR SYNTHESIS - CYTOSOL

(2 G3P

⇢

C

6

H

12

O

6

)

RETURN TO PSI

3.5.1 - PHOTOSYNTHESIS Page 4

LIGHT

Dependency on sun arises from photolytic function of light.

The light compensation point defines when carbon dioxide intake balances oxidation of fixed carbon compounds during cellular respiration.

⇢ net carbon dioxide assimilation is zero

• Rate of photosynthesis directly proportional to intensity from irradiances above LCP until saturation occurs.
• The point at which increased light intensity has no further effect is due to another limiting factor causing a stagnation in rate.
• Low light intensities below LCP limit RuBP regeneration

TEMPERATURE

Temperature increases within the range of 10-35 °C

have positive effect on rate of photosynthesis due to

particle kinetics.

When temperature increases in range between

minimum to optimum ( 0 - 25 °C), rate doubles

every 10° increase.

▪

Above optimum rate exhibits continued

increase for short duration but later declines as

the temperature cause enzymes to denature,

loss of function

▪

Temperatures too far below optimum correspond to

kinetic energy content of system being too low to

allow enzymes to function at appreciable enough rate

to match sugar consumption

CO 2

consumption in darkness when plant is not photoactive

▪ Pigment density

▪ Type of pigment

▪ Leaf anatomy (stomata, surface area)

accumulation of carbohydrates ▪

▪ hormones

rate increases

linearly as a function

of light intensity.

CO 2

constitutes about 0.041% of atmosphere,

when not under artificially controlled conditions

theoretical maximum rate for photosynthesis

often never achieved due to insufficient CO 2

intense illumination results in radiation

damage to photosynthetic apparatus.

CO 2

influx = respiratory consumption rate

PHOTOSYNTHETIC RATES

The rate limiting factor determines f the maximum rate at

which a reaction can occur. When a metabolic process is dependent

on multiple factors, rate is limited by the variable nearest its minimum

value or experiences the greatest insufficiency

ENVIRONMENTAL

▪ light

▪ carbon dioxide

▪ Temperature

▪ Water/ion absorption

▪ Atmospheric pollutant

rate stagnates and further increase in light

intensity fails to increase rate

INTERNAL

Light saturation point occurs at light intensity

of around 10% of full sunlight (10,800 lux)

LIGHT LIMITED CO

2

LIMITED

PHOTOINHIBITION

3.5.1 - PHOTOSYNTHESIS Page 5

ETHANOLIC PATHWAY

Pyruvate reduced to ethanol + CO 2

▪ pathway restricted to prokaryotes

Alcohol dehydrogenases catalyse

alcoholic fermentation

▪

NAD+ REGENERATED

LACTIC ACID PATHWAY`

Pyruvate reduced to lactate

Mechanism employed in respiring tissues during strenuous exercise

⇢ causes lactic acid build up in tissue

pH in the cytoplasm quickly drops when hydrogen ions accumulate in

the muscle, eventually inhibiting glycolytic enzymes

⇢ process is unsustainable, suitable only for short term energy

requirements

NAD+ REGENERATED, lactate broken down by liver back into glucose

Net conversion of glucose to pyruvic acid. Yield per glucose:

▪ 2 pyruvate molecules

• Reduction of 2 NAD+
• 2 ATP generated

HEXOSE ENTRY

sugars enter glycolysis pathway via 2 paths:

• Dietary saccharides – direct absorption through GI

Glycogenolysis – glycogen stores expended

OXIDATION

2 pyruvate molecules formed when G3P

undergoes oxidation through losing electrons

to oxidising agent NAD+

+ 4ATP

+ 2 PYRUVATE

+ 1 NADH

PHOSPHORYLATION

diphosphorylation of glucose to 6c intermediate

fructose-1,6-bisphosphate formed

- 2 ATP

HYDROLYSIS

Phosphate hexose ⇢ glyceraldehyde- 3 - phosphate

+ 2 G3P

REGENERATION

Coenzyme regeneration via electron donation to

acceptor molecule

Acceptor molecule differs depending on oxygen

availability. If no oxygen is present then NAD

regeneration necessitates consumption of pyruvate.

g

NAD

• 2e

+ 2H

⇌ NADH + H

ANAEROBIC

AEROBIC

ACETYL COA

Pyruvate shuttled to

mitochondria,

converted to acetyl-

coA for use in the

citric acid cycle

The low-yield reaction replenishes cellular NAD+ supplies and phosphorylates 2 ADP

In mammals, greater quantity of ATP only is accessible through pyruvate entering the Krebs cycle in presence of O 2

Cytosolic pathway that constitutes initial stage of aerobic

respiration and principle ATP generation in anaerobic conditions

3.5.2 - GLYCOLYSIS

09 July 2021 17:

3.5.2 - RESPIRATION Page 1

Pyruvate entering from glycolytic pathway converted to acetyl-coA via dehydrogenase

Intermediary conversion, no net ATP generation during this stage.

As glycolysis splits glucose into two pyruvate molecules, link reaction occurs twice per molecule of glucose..

1. Pyruvate pumoed from cytosol into mitochondrial matrix

pyruvate loses COO- group (decarboxylation) via oxidation (-CO 2

2. )

3. Acetyl group reacts with CoA via acetylation to form acetyl-coA

reduction of coenzyme NAD

• (⇢NADH for later use in generative pathways)

pyruvate ⇢ e-

Citrate (6C) compound undergoes decarboxylation twice to 4C,

yielding 2 CO2 molecules.

▪ dehydrogenases catalysing decarboxylation NAD+ linked

multiple oxidation reactions result in reduction of coenzymes

▪

⇢ deposit electrons into electron transport chain

FAD (flavin adenine dinucleotide)

Dinucleotide coenzyme which functions as an electron

donor or acceptor, cycling between 4 redox states.

TCA - FAD accepts 2 electrons, reduced to FADH

FADH 2 then reverts to FAD, sending its two high-

energy electrons through the electron transport chain.

energy in FADH 2

is enough to produce 1.

equivalents of ATP via oxidative phosphorylation

▪

OXIDATION

COENZYME ACTION

NAD (nicotinamide adenine dinucleotide)

NAD

2H

• 2e

NADH + H

REDUCTION

glucose → 2 pyruvate

↓

• CoA + NAD

→ Acetyl-CoA + CO 2

+ NADH

+ H

+ 3 NADH

+ 1 FADH

2

Coenzymes function as secondary substrates, either energy currencies themselves (such as

NADH and ATP) or carriers of chemical groups such as coenzyme A

LINK STAGE

GLYCOLYSIS

LINK

OXIDATION

REDUCTION

+ NAD

⇢ NADH + H

KREBS CYCLE

TCA - NAD

is reduced to NADH through

electron acceptance. Electrons contributed into

ETC where they terminally reduce O 2

Dinucleotide coenzyme, continuously cycles between:

KREBS CYCLE (CITRIC ACID CYCLE)

Reaction series occuring within the mitochondrial matrix providing

reduced forms of coenzymes NADH and FADH 2 for participation in

later stages of respiratory chain (oxidative phosphorylation)

O 2 not used directly as a substrate but still

required to proceed (obligately aerobic)

▪

⇢ functions as terminal electron acceptor

Requires acetyl CoA, generated from

decarboxylation of pyruvate

▪

OXIDISING AGENT

REDUCING AGENT

3.5.2 - RESPIRATION Page 2

Terminal stage in the metabolic pathway, facilitates the controlled release of free energy that was stored in reduced cofactors.

cells release majority of the chemical energy captured by activated carriers at earlier stages.

• ATP synthesised in large volumes, net yield approximately 32 per cycle

In eukaryotes process occurs along the inner membrane infoldings of the mitochondria (cristae).

Sequential redox reactions result in free energy release at each stage of an electron transfer cascade. Electrons flow in direction

of compounds with higher reduction potentials

⇢ terminal electron acceptor is molecular oxygen

▪ Greater quantity of energy release per step results in exothermic heat transfer to surroundings.

▪ Energy conserved through release in multiple steps with smaller, more manageable quantities

CHEMIOSMOSIS

ALTERNATE SUBSTRATES

Glycerol is subsequently phosphorylated and converted to triose phosphate which enters the glycolysis pathway.

The fatty acid component is oxidised to acetyl-CoA in the mitochondria and subsequently enters the Krebs cycle.

The free energy liberated at each stage is

used to generate proton motive force.

Reduction of coenzymes deposits protons

which are moved into perimitochondrial

space via active transport, the ion gradient

established creating difference in

transmembrane electrical potential

Diffusion of H+ ions back across the

mitochondrial membrane down gradient

into matrix through transmembrane

protein generates a proton-motive force

exploited by ATP synthases.

ATP synthases couple phosphorylation of ADP return of protons across the membrane down an electrochemical gradient.

• Potential energy converted to mechanical energy which rotates stalk
• Phosphorylation of ADP yields ATP

The final acceptor of the electrons is elemental oxygen, which becomes reduced to water.

G3P

LIPIDS

PROTEIN - Excess amino acids are converted into molecules that can enter the pathways of glucose catabolism.

OXIDATIVE PHOSPHORYLATION

triglycerides ⇢ 3 fatty acids + glycerol

PHOSPHORYLATION

OXIDATION

acetyl CoA

⇢ GLYCOLYSIS

⇢ KREBS CYCLE

HYDROLYSIS

3.5.2 - RESPIRATION Page 3

ECOSYSTEM PRODUCTIVITY

Energy transfer between trophic levels is inefficient

Approximately 10% of the net production

value at the preceding trophic level retained

•

food chains rarely extend beyond 5 levels

due to low efficiencies.

•

Pathways traced by energy through

communities are determined by three energy

transfer efficiencies: consumption, assimilation

and production.

Chemical energy stored in biomass of

organisms at different trophic levels within a

pyramid exhibits a unidirectional decrease as

more transfers occur.

Successive loss as food web complexifies during

transfer between systems.

Loss of energy means chain cannot support many organisms with high nutritive demands, corresponding to fewer

organisms in higher trophic levels and an uneven distribution between number of individuals in species.

BIOMASS

Biomass content provides indicator of the quantity of fixed carbon existing within bodies of organic matter. provides

estimate of molecular stock contained within each successive group lower down the energy pyramid.

Sugars not used as substrates are converted into different classes of organic compounds which constutute bulk of organic

matter in carbon-based life forms

Largest contributant to bulk biomass are plants, constitute basal trophic level

QUANTIFICATION

sample collection ⇢ fresh weight measurement ⇢ dry mass measurement

Water content of organisms is highly variable, removal of water allows for greater accuracy of comparison.

Sample mass measured and then dehydrated through heating.

Sample heated until 'constant mass', indicating total desaturation.

Once fully dehydrated the final value for dry mass expressed per unit area occupied by plants can be obtained

⇢ Further calorimetric analysis can occur in which the chemical energy in dry biomass can be measured by burning

it and calculating the amount of heat energy released

ECOLOGICAL EFFICIENCY

Measures availability and accessibility of energy for use by next highest trophic level.

• Combines net efficiencies of resource acquisition and assimilation in an ecosystem
• discounts energy loss through exothermic reactions such as respiration, defecation, non-predatory death

CARBON FIXATION

Conversion into organic form via photosynthesis driven by incident solar radiation. Total carbon influx corresponds to its

use by respiring autotrophs.

CO 2 efflux by respiration of organisms

Ecosystems form from interactions of biotic and physicochemical components within an environment.

interactions can be quantified in terms of ecological efficiency based on productivity of organisms comprising a community.

100%

10%

1%

0.1%

Energy quantified in temrs of

ecological efficiency (energy making it to the next level) J m

2

• )

dry biomass content (mass of organic matter) (t ha

1

• )

carbon fixed (C m

2

• )

3.5.3 - ENERGY TRANSFER

11 July 2021 08:

3.5.3 - ENERGY AND ECOSYSTEMS Page 1

PRODUCTIVITY

PRIMARY PRODUCTION

Initial absorption of photons occurs in antenna complex

Carbon entry into biosphere occurs via fixation of carbon dioxide by primary producers.

Gross primary productivity (GPP) - total amount of energy directly acquired by autotrophic energy harvesting

⇢ Photosynthetic efficiency averages 5.5% in terrrestrial ecosystems.

Of the energy fixed, 20-50% is used in respiration or other biochemical operations within the plant, accounted for through

net primary productivity (NPP) which deducts respiratory and thermodynamic loss

SECONDARY PRODUCTION

Quantifies generation of biomass through heterotrophic nutrition.

Expresses quantity of new tissue created through assimilation of

energy consumed through food.

Consumption includes:

primary consumption of plant matter (herbivorous/omnivorous)

⇢ herbivorous assimilation efficiencies 20-50%

▪

Higher consumption levels of animals (omnivorous/carnivorous)

⇢ carnivore assimilation efficiency level of 80%

▪

Ecosystem processes involve complex interactions between detrtitus and predatory food webs, nutrient cycling between

trophic cascades maintains flow and allows continuous generation

Autotrophs absorb organic minerals and fix them in usable organic forms for abbsorption by autotrophs through the soil

Nutrients that are not assimilate return to the ecosystem via two main pathways:

▪ Consumers release mineral nutrients via excretion, direction deposition and return

Defecant and unassimilated organic matter from non-predatory death form detritus. mineralisation by microbes

reintroduces nutrients to soil

▪

AGRICULTURAL PRODUCTIVITY

Minimising respiratory losses in animals reared for farming increases efficiency of biomass conversion through:

▪ Restricting movement, higher stock density in enclosed compact spaces limits respiratory losses

Keeping ambient environment warm to minimise use of energy for body heat

• endotherms maintain constant body temperature higher than ambient environment, lose energy through radiance

▪

▪ Controlled feeding so animals recieve optimum nutrients for growth

▪ Exclusion of predators limits losses through predatory deaths

Simplification of food webs - removal of competitive species such as weeds which compete for resources. achieved through:

• Biologial pest control (use of predatory insects which are natural predators to invasive species)
• Chemical Pesticides (toxic to pests)

▪

GSP = I - F

NSP = GSP - R

H

PHOTOSYNTHETIC EFFICIENCY LIMITANTS:

▪ Non-absorbable light wavelengths refracted

▪ Reflection by particulate matter e.g dust, clouds

▪ Light incidence on non chlorophyllic region

▪ Climate/ diurnal changes

▪ carbon dioxide concentrations

▪ Resource scarctiy

The factors that limit terrestrial primary productivity are

solar energy (and particularly its inefficient use by plants),

water and temperature.

Differences amongst biomes, depends on proportion of

the biomass photosynthetically active

DETRITOVORES

fragmentation of plant and animal debris, saprophagous

microorganisms complete decomposition and mineralization

▪ responsible for clearance of organic waste

⇢ In the absence of detritivores, detritus would accumulate,

creating a bottleneck in nutrient cycling and inhibiting further

primary production

DECOMPOSERS (saprobionts)

Saphrophytic organism participate in

extracellular digestion

▪ enzyme secretion through cell membrane

occurs external to a digestive tract, absorption

and metabolism at molecular level

▪

terrestrial : earthworms

aquatic : sea cucumbers, crabs

Energy is lost during conversion into new animal

mass, only fraction of the material ingested (I)

absorbed and assimilated contributing to growth of

organism + reproductive activity.

Most used for respiration (Rh), remainder defecated

Energy loss in secondary production due to:

▪ Inedible parts of organism not consumed

▪ Indigestible parts excreted as undigested waste

⇢ faecal matter replenishes nutrient supply

through saphrophytic nutrition

Decomposition of dead organic matter by microorganisms returns constituent

molecules in the form of detritus to basal pool of an ecosystem

NPP = GPP - R a

SAPHROPHYTIC NUTRITION

3.5.3 - ENERGY AND ECOSYSTEMS Page 2

Interconversion between allotropes of nitrogen as it circulates between the atmosphere, terrestrial and marine ecosystems.

Plants must secure nitrogen in its fixed form, which requires it to be incorporation in compounds

atmosphere is composed of 79% nitrogen gas N 2

, however this major nitrogen reservoir is not available for use in

biological systems, leading to resource scarcity in many ecosystems.

⇢ N2 (g) bond enthalpy is high, meaning low reactivity and high energy requirements associated with fixation

▪

ALLOTROPE FORMULA PRESENCE IN NATURE

Diatomic nitrogen N 2

Air

Ammonia NH 3

urea, rainwater, manure,

rotting plants

Ammonium NH 4

• ammonium salts in

fertile soil + seawater

Nitrite NO 2

• ionic salts

Nitrate NO 3

• salt deposits such as

sodium nitrate or in

mineral forms

Nitrous oxide N 2

O gas emissions

Nitric oxide NO thunderstorms

ORGANISMS

Nitrogen fixing bacteria and

root nodule symbionts in

association with plants fix

nitrogen and cooperate with

host to supply nitrogen

Ecosystems recycling occurs locally through interconversion of mineral nutrients during biomass transfer in addition to

larger scale participation in a global system of inputs and outputs where matter is exchanged in biogeochemical cycles.

• Nutrient flow is non-linear unlike energy flow through food chains, instead involves in cyclic transfer

PROCESS conversion of inert nitrogen gas present in the

atmosphere into nitrogenous compounds capable of entering into

terrestrial and aquatic ecosystems from their deposit forms.

Conversion of nitrogen between its various allotropes

#predominantly results from microbial activity, either

in chemoautotrophic energy acquisition or to access

nitrogen required for their growth.

FIXATION

LIGHTNING STRIKES

NO + NO

2

formation is catalysed by lightning which provides

sufficient energy to overcome high bond enthalpy.

⇢ HNO

3 ,

enters soil via precipitation

INDUSTRIAL FIXATION

conversion of H 2

+ N

2

into

NH

3

under high pressures

and temperatures in Haber

process. Used in fertilizer

formation

DIAZOTROPHY

Diazotrophic microorganisms associate with plant rhizosphere to form nodules which

increase surface area of roots. Nitrogenase enzymes fix N 2

into absorbable form.

Association is mutual, symbiosis provides the diazotroph with a steady influx of carbon

sources + regulated oxygen supply tightly bound by leghaemoglobin

RHIZOBIA associate with legume species, infecting root nodules.

MYCORRHIZAE are fungi colonisers of plant roots, form arbuscular tubules, hyphal

extensions increase surface area for uptake of water + minerals

NITROGEN CYCLE

3.5.4 - NUTRIENT CYCLE

12 July 2021 17:

3.5.4 - NUTRIENT CYCLES Page 1

AMMONIFICATION

PROCESS conversion of nitrogenous waste from decomposing

organic matter (dead animals/ faeces) into ammonium

Saprobionts mineralize waste via aerobic or anaerobic

decomposition, proteolytic enzymes catalyze formation of

inorganic NH 4

⇢ ammonium salt compounds form in soil + aqueous reserves,

forming deposits which can be nitrified

SOIL DETRITUS

AMMONIUM NH

4

PROCESS conversion of ammonium/ammonia to nitrites

via oxidation, undergo further oxidation to form nitrates.

▪ process utilized by chemoautotrophs

Oxygen required in ammonium and nitrite oxidation,

both nitrifying and nitrite-oxidizing bacteria are aerobes.

▪

NITRATE NO

3

NITRITE NO

2

(⇢ absorption)

AMMONIUM NH

NITRIFICATION

DENITRIFICATION

PROCESS anaerobic denitrifying bacteria facilitate

conversion of ions in soil into gaseous nitrogen (N 2

)

▪ returns bioavailable nitrogen to atmosphere,

▪ typically occurs in anoxic environments

⇢ O

2

unavailable for use as terminal electron acceptor in

electron transport, N 2

substituted in anaerobic respiration

▪ Increased usage in chemical fertilizers in agriculture alters nitrogen cycle

Agricultural and industrial nitrogen inputs currently exceeds inputs from

natural fixation, accounting for an increase in nitrogenous compound

deposition to marine + terrestrial ecosystems (+ 200% since 1600)

▪

SYNTHETIC - ammonia derived compounds

generated through Haber process

NATURAL - compost, animal manure,

harvested minerals, crop rotations

Denitrification can be considered an ecologically beneficial and

detrimental process, depending upon when and where it occurs.

(+)

useful in water-treatment processes, removal of

nitrogen from water supplies prior to discharge into

natural bodies prevents eutrophication

(-)

by-products include NO + N 2 O, greenhouse gases which

contribute to global warming

(-)

decrease soil fertility, removing nitrogen from soil means

loss from basal mineral pool

DEPOSITION - FERTILISER USE

EUTROPHICATION

ECOSYSTEM EFFECTS

▪ Soil acidification

▪ Toxic compounds can accumulate in soils or in animals and pass throughout food chain

▪ Ozone degradation due to NO production, acceleration of global warming

▪ Reduction of species diversity as nitrogen rich soils favour fast-growth plants, leading to out competition of other species

▪ Decreased atmospheric visibility

3.5.4 - NUTRIENT CYCLES Page 2

PHOSPHORUS CYCLE

Involves cycling of phosphoric allotropes through the lithosphere

hydrosphere and biosphere. Phosphorus is predominantly found in

sedimentary deposits or in aqueous dissolved phosphate reserves.

lacks gaseous phase, entry of orthophosphates into soils occurs through:

gradual accumulation over geological time as a consequence of

weathering or uplifting / erosion of rocks.

-

breakdown of manure, chemical fertilisers, biosolids or

plant/animal debris.

-

▪

Sedimentation of dissolved particulate phosphorus creates layer

of seabed phosphorus rocks which gradually erode

Necessary for plant growth, deficiency

causes decolouration and stunted growth

Uses of orthophosphates in plants:

▪ ATP

▪ Nucleic acids

▪ Buffering agent

▪ Phospholipid bilayer

Tectonic uplift exposes phosphorus-

bearing rocks to weathering, physical

erosion of bare rock contributes

dissolved and particulate phosphorus

to soils, lakes and rivers.

PHOSPHORUS LOSS

Soils have a finite capacity to bind P. When saturated with P, desorption is accelerated, increasinsg soil

leaching and runoff. As too much phosphate is present for all to be absorbed

▪

▪ Loss through removal of crop, phosphates not returned to soil through decomposition

3.5.4 - NUTRIENT CYCLES Page 3