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PAPER 2 BIO
Topic 5 Energy transfers in and between organisms
5.4 Nutrient Cycles
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Cards (16)
Explain the role of saprobionts in recycling chemical elements
●
Decompose
(break down)
organic
compounds eg.
proteins
/ urea /
DNA
in dead matter / organic waste
● By secreting
enzymes
for
extracellular
digestion (
saprobiotic
nutrition)
●
Absorb
soluble needed
nutrients
and release
minerals
ions eg.
phosphate
ions
Explain the role of mycorrhizae
●
Fungi
(hyphae) act as an
extension
of plant
roots
to increase
surface
area of
root
system
● To increase
rate
of
uptake
/ absorption of
water
and
inorganic
ions
● In return, fungi receive
organic
compounds eg. carbohydrates
Definition of mycorrhizae
symbiotic
association between
fungi
and plant
roots
Give examples of biological molecules that contain nitrogen
Amino
acids /
proteins
or enzymes /
urea
/ DNA or
RNA
/ chlorophyll /
ATP
or ADP / NAD or
NADP
Draw a diagram to show the key stages of the nitrogen cycle
:
Describe the role of bacteria in nitrogen fixation
●
Nitrogen
gas (N2) converted into
ammonia
(NH3), which forms
ammonium
ions (NH4+) in
soil
● By
nitrogen-fixing
bacteria (may be found in root
nodules
)
Describe the role of bacteria in ammonification
●
Nitrogen-containing
compounds eg.
proteins
/ urea from
dead
organisms / waste are broken down /
decomposed
● Converted to
ammonia
, which forms
ammonium
ions in
soil
● By
saprobionts
- secrete
enzymes
for
extracellular
digestion
Describe the role of bacteria in nitrification
●
Ammonium
ions in soil converted into
nitrites
then
nitrates
, via a two-step
oxidation
reaction
○ For uptake by plant
root
hair
cells by
active
transport
● By
nitrifying
bacteria
in
aerobic
conditions (oxygen)
Describe the role of bacteria in denitrification
●
Nitrates
in soil converted into
nitrogen
gas (
reduction
)
● By
denitrifying
bacteria in
anaerobic
conditions (no
oxygen
, eg.
waterlogged
soil)
Suggest why ploughing (aerating) soil increases its fertility
● More
ammonium
converted into
nitrite
and
nitrate
/ more
nitrification
/ more (active)
nitrifying
bacteria
● Less
nitrate
converted to
nitrogen
gas / less
denitrification
/ fewer (active)
nitrifying
bacteria
Give examples of biological molecules that contain phosphorus
Phospholipids
/ DNA or
RNA
/
ATP
or ADP / NADP /
TP
or GP / RuBP
Describe the phosphorus cycle
Phosphate
ions in
rocks
released (into
soils
/ oceans) by
erosion
/
weathering
Phosphate ions taken up by
producers
/
plants
/ algae and incorporated into their
biomass
○ Rate of
absorption
increased by
mycorrhizae
Phosphate ions transferred through
food
chain eg. as
herbivores
eat producers
Some
phosphate
ions lost from
animals
in
waste
products (excretion)
Saprobionts
decompose
organic
compounds eg.
DNA
in dead matter / organic waste,
releasing
phosphate ions
Explain why fertilisers are used
● To
replace
nitrates / phosphates
lost
when
plants
are harvested and
livestock
are removed
○ Those removed from
soil
and incorporated into
biomass
can’t be released back into the
soil
through
decomposition
by
saprobionts
● So improve
efficiency
of
energy
transfer → increase
productivity
/
yield
Describe the difference between artificial and natural fertilisers
Natural -
Organic
, eg. manure, compost, sewage →
ions
released during
decomposition
by
saprobionts
Artificial - Contain
inorganic
compounds of
nitrogen
,
phosphorus
and
potassium
Explain the key environmental issue arising from use of fertilisers
●
Phosphates
/ nitrates
dissolve
in water, leading to
leaching
of nutrients into
lakes
/ rivers /
oceans
● This leads to
eutrophication
Rapid
growth of
algae
in
pond
/ river (
algal bloom
) so
light
blocked
So
submerged
plants
die
as they cannot
photosynthesise
So
saprobionts
decompose
dead
plant matter, using
oxygen
in
aerobic
respiration
So less
oxygen
for fish to
aerobically
respire, leading to their
death
Explain the key advantage of using natural fertiliser over artificial fertiliser
●
Less
water
soluble
so less
leaching
→
eutrophication
less likely
●
Organic
molecules require
breaking
down by
saprobionts
→ slow
release
of
nitrate
/
phosphate
etc.