Transcript Plants to feed the world

Plants to feed the world
(Chapter 11)
Plants to feed the world
• Hunger, starvation, and malnutrition are endemic in many
parts of the world today.
• Rapid increases in the world population have intensified
these problems!
• ALL of the food we eat comes either directly or indirectly
from plants.
• Can’t just grow more plants, land for cultivation has
geographic limits
– Also, can destroy ecosystems!
Plants to feed the world
• At the latest count there are between 250,000 and 400,000
plant species on the earth.
• But three - maize, wheat and rice - and a few close runnersup, have become the crops that feed the world. All produce
starch, helping to provide energy and nutrition, and all can
be stored.
• Maize converts the sun’s energy into sugar faster, and
potentially produces more grains, than any of the other
major staples.
Plants to feed the world
• The term Green Revolution is used to
describe the transformation of
agriculture in many developing
nations that led to significant increases
in agricultural production between the
1940s and 1960s
• Scientists bred short plants that
converted the sun’s energy into grain
rather than stem, so preventing the
mass starvation in the developing
world predicted before the 1960s, at a
cost of higher inputs from chemical
fertilizers and irrigation.
Plants to feed the world
• Disease-resistant wheat varieties
with high yield potentials are now
being produced for a wide range of
global, environmental and cultural
conditions.
• The Green Revolution has had
major social and ecological
impacts, which have drawn intense
praise and equally intense criticism.
Plants to feed the world
• The Green Revolution is sometimes
misinterpreted to apply to present
times.
• In fact, many regions of the world
peaked in food production in the
period 1980 to 1995, and are
presently in decline, since
desertification and critical water
supplies have become limiting
factors in a number of world
regions.
A few of the many medicinal plants
Energy flow through an ecosystem
• Energy enters as sunlight
• Producers convert sunlight to
chemical energy.
• Consumers eat the plants (and each
other).
• Decomposer organisms breakdown
the organic molecules of producers
and consumers to be used by other
living things
• Heat is lost at every step – So Sun
must provide constant energy input
for the process to continue!
Photosynthesis
•Very little of the Sun’s energy gets
to the ground
gets absorbed by water vapor
in the atmosphere
•The absorbance spectra of
chlorophyll.
Absorbs strongly in the blue
and red portion of the
spectrum
Green light is reflected and
gives plants their color.
•There are two pigments
•Chlorophyll A and B
Photosynthetic pigments
•
•
•
•
Two types in plants:
Chlorophyll- a
Chlorophyll –b
Structure almost identical,
– Differ in the composition of a
sidechain
– In a it is -CH3, in b it is CHO
• The different sidegroups 'tune'
the absorption spectrum to
slightly different wavelengths
– light that is not significantly absorbed by
chlorophyll a, will instead be captured
by chlorophyll b
Photosynthetic pigments
• Chlorophyll has a complex
ring structure
– The basic structure is a
porphyrin ring, co-coordinated
to a central atom.
– This is very similar to the heme
group of hemoglobin
• Ring contains loosely bound
electrons
– It is the part of the molecule
involved in electron transitions
and redox reactions of
photosynthesis
The Chloroplast
• Membranes contain chlophyll
and it’s associated proteins
– Site of photosynthesis
• Have inner & outer membranes
• 3rd membrane system
– Thylakoids
• Stack of Thylakoids = Granum
• Surrounded by Stroma
– Works like mitochondria
• During photosynthesis, ATP
from stroma provide the energy
for the production of sugar
molecules
General overall reaction
6 CO2 + 6 H2O
C6H12O6 + 6 O2
Carbon dioxide Water
Carbohydrate
Oxygen
Photosynthetic organisms use solar energy to synthesize carbon
compounds that cannot be formed without the input of energy.
More specifically, light energy drives the synthesis of carbohydrates
from carbon dioxide and water with the generation of oxygen.
The chemical reaction of
photosynthesis is driven by light
• The initial reaction of
photosynthesis is:
– CO2 +H2O
(CH2O) + O2
– Under optimal conditions (red
light at 680 nm), the
photochemical yield is almost
100 %
– However, the efficiency of
converting light energy to
chemical energy is about 27 %
• Very high for an energy
conversion system
The chemical reaction of
photosynthesis is driven by light
– Quantum efficiency: Measure
of the fraction of absorbed
photons that take part in
photosynthesis.
– Energy efficiency: Measure of
how much energy in the
absorbed photons is stored as
chemical products
• ¼ energy from photons stored –
the rest is converted to heat
The light reactions
• Step 1 – chlorophyll in vesicle
membrane capture light energy
• Step 2 – this energy is used to
split water into 2H and O.
• Step3 – O released to
atmosphere. Each H is further
split into H+ ion and an
electron (e-).
• Step 4 – H+ ion build up in the
stacked vesicle membranes.
The light reactions
• Step 5 – The e- move down a
chain of electron transport
proteins that are part of the
vesicle membrane.
• Step 6 – e- ultimately
delivered to the molecule
NADP+ - forming NADPH
• Step 7 - Some membrane
proteins pump H+ into the
interior space of the vesicle
– Stored energy
• Step 8 – These make ATP!
Summary of light reactions
• Plants have two reaction centers:
– PS-II
• Absorbs Red light – 680mn
• makes strong reductant (& weak oxidant)
• oxidizes 2 H2O molecules to 4 electrons, 4 protons & 1 O2
molecule
• Mostly found in Granum
– PS-I
•
•
•
•
Absorbs Far-Red light – 700nm
strong oxidant (& weak reductant)
PS-I reduces NADP to NADPH
Mostly found in Stroma
The Carbon
reactions
• The NADPH and ATP move
into the liquid environment of
the Stroma.
• The NADPH provides H and
the ATP provides energy to
make glucose from CO2.
• The Calvin cycle thus fixes
atmospheric CO2 into plant
organic material.
Overview of the carbon reactions
• The Calvin cycle:
• The cycle runs six times:
– Each time incorporating a new
carbon . Those six carbon
dioxides are reduced to glucose:
– Glucose can now serve as a
building block to make:
• polysaccharides
• other monosaccharides
• fats
• amino acids
• nucleotides
Photorespiration
• Occurs when the CO2 levels inside a leaf become
low
– This happens on hot dry days when a plant is forced to
close its stomata to prevent excess water loss
• If the plant continues to attempt to fix CO2 when its
stomata are closed
– CO2 will get used up and the O2 ratio in the leaf will
increase relative to CO2 concentrations
• When the CO2 levels inside the leaf drop to around
50 ppm,
– Rubisco starts to combine O2 with Ribulose-1,5bisphosphate instead of CO2
The C4 carbon Cycle
• The C4 carbon Cycle occurs in 16 families of both
monocots and dicots.
–
–
–
–
Corn
Millet
Sugarcane
Maize
• There are three variations of the basic C4 carbon
Cycle
– Due to the different four carbon molecule used
The C4 carbon Cycle
• This is a biochemical pathway
that prevents photorespiration
• C4 leaves have TWO chloroplast
containing cells
– Mesophyll cells
– Bundle sheath (deep in the leaf so
atmospheric oxygen cannot diffuse easily to
them)
• C3 plants only have Mesophyll cells
• Operation of the C4 cycle requires the
coordinated effort of both cell types
– No mesophyll cells is more than
three cells away from a bundle
sheath cells
• Many plasmodesmata for
communication
How the rest of plant works
• Roots – absorb water from the soil
as well as many mineral nutrients
• Xylem – transports water from the
roots to the rest of the plant
• Phloem – transports sugars made in
the leaves via photosynthesis to the
pest of the plant
• Leaves – Site of gas exchange CO2
brought in and O2 out. Have
structures called Stomata which also
control water loss.
Water across plant membranes
• There is some diffusion of
water directly across the bilipid membrane.
• Auqaporins: Integral
membrane proteins that form
water selective channels –
allows water to diffuse faster
– Facilitates water movement in
plants
• Alters the rate of water flow
across the plant cell
membrane – NOT direction
Water transport in Plants
• Xylem:
– Main water-conducting tissue of
vascular plants.
– arise from individual cylindrical
cells oriented end to end.
– At maturity the end walls of
these cells dissolve away and
the cytoplasmic contents die.
– The result is the xylem vessel,
a continuous nonliving duct.
– carry water and some dissolved
solutes, such as inorganic ions,
up the plant
Water transport in Plants
• Phloem:
– The main components of phloem are
• sieve elements
• companion cells.
– Sieve elements have no nucleus and only
a sparse collection of other organelles .
Companion cell provides energy
– so-named because end walls are
perforated - allows cytoplasmic
connections between vertically-stacked
cells
.
– conducts sugars and amino acids - from
the leaves, to the rest of the plant
Osmosis and Tonicity
• Osmosis is the diffusion
of water across a plasma
membrane.
• Osmosis occurs when
there is an unequal
concentration of water on
either side of the
selectively permeable
plasma membrane.
• Remember, H2O
CAN cross the plasma
membrane.
• Tonicity is the osmolarity of
a solution--the amount of
solute in a solution.
• Solute--dissolved
substances like sugars and
salts.
• Tonicity is always in
comparison to a cell.
• The cell has a specific
amount of sugar and salt.
Tonic Solutions
• A Hypertonic solution has more solute than the cell. A cell
placed in this solution will give up water (osmosis) and
shrink.
• A Hypotonic solution has less solute than the cell. A cell
placed in this solution will take up water (osmosis) and
blow up.
• An Isotonic solution has just the right amount of solute for
the cell. A cell placed in this solution will stay the same.
Plant cell in hypotonic solution
• Flaccid cell in 0.1M sucrose solution.
• Water moves from sucrose solution to cell – swells up –becomes
turgid
• This is a Hypotonic solution - has less solute than the cell. So
higher water conc.
• Pressure increases on the cell wall as cell expands to equilibrium
Plant cell in hypertonic solution
• Turgid cell in 0.3M sucrose
solution
• Water movers from cell to
sucrose solution
• A Hypertonic solution has
more solute than the cell. So
lower water conc
• Turgor pressure reduced and
protoplast pulls away from
the cell wall
Plant cell in Isotonic solution
• Water is the same inside the
cell and outside
• An Isotonic solution has the
same solute than the cell. So no
osmotic flow
• Turgor pressure and osmotic
pressure are the same
Water transport
• Transpiration
• Evaporation of water into the
atmosphere from the leaves and stems
of plants.
• It occurs chiefly at the leaves while
their stomata are open for the passage
of CO2 and O2 during photosynthesis.
• Transpiration is not simply a hazard of
plant life. It is the "engine" that pulls
water up from the roots to:
– supply photosynthesis (1%-2% of the
total)
– bring minerals from the roots for
biosynthesis within leaf
– cool the leaf.
Stomatal control
• Almost all leaf transpiration
results from diffusion of water
vapor through the stomatal pore
– waxy cuticle
• Provide a low resistance
pathway for diffusion of gasses
across the epidermis and cuticle
• Regulates water loss in plants
and the rate of CO2 uptake
– Needed for sustained CO2 fixation
during photosynthesis
Stomatal control
• When water is abundant:
• Temporal regulation of stomata is
used:
– OPEN during the day
– CLOSED at night
• At night there is no photosynthesis,
so no demand for CO2 inside the
leaf
• Stomata closed to prevent water
loss
• Sunny day - demand for CO2 in leaf
is high – stomata wide open
• As there is plenty of water, plant
trades water loss for photosynthesis
products
Stomatal control
• When water is limited:
– Stomata will open less or
even remain closed even on
a sunny morning
• Plant can avoid dehydration
• Stomatal resistance can be
controlled by opening and
closing the stomatal pores.
• Specialized cells – The Guard
cells
Stomatal guard cells
• Guard cells act as hydraulic valves
• Environmental factors are sensed by guard cells
– Light intensity, temperature, relative humidity,
intercellular CO2 concentration
• Integrated into well defined responses
– Ion uptake in guard cell
– Biosynthesis of organic molecules in guard cells
• This alters the water potential in the guard cells
• Water enders them
• Swell up 40-100%
Relationship between water
loss and CO2 gain
• Effectiveness of controlling water loss and allowing
CO2 uptake for photosynthesis is called the
transpiration ratio.
• There is a large ratio of water efflux and CO2 influx
– Concentration ratio driving water loss is 50 larger than
that driving CO2 influx
– CO2 diffuses 1.6 times slower than water
• Due to CO2 being a larger molecule than water
– CO2 uptake must cross the plasma membrane, cytoplasm,
and chloroplast membrane. All add resistance
water status of plants
• Cell division slows down
• Reduction of synthesis of:
– Cell wall
– Proteins
• Closure of stomata
• Due to accumulation of the
plant hormone Abscisic acid
– This hormone induces closure
of stomata during water stress
• Naturally more of this
hormone in desert plants
Plants and water
• Water is the essential medium of life.
• Land plants faced with dehydration by water loss to
the atmosphere
• There is a conflict between the need for water
conservation and the need for CO2 assimilation
–
–
–
–
–
–
This determines much of the structure of land plants
1: extensive root system – to get water from soil
2: low resistance path way to get water to leaves – xylem
3: leaf cuticle – reduces evaporation
4: stomata – controls water loss and CO2 uptake
5: guard cells – control stomata.
Nitrogen in the environment
• Many biochemical compounds present in plant cells contain
nitrogen
– Nucleoside phosphates
– Amino acids
• These form the building blocks of nucleic acids and protein
respectively
• Only carbon, hydrogen, and oxygen are nor abundant in
plants than nitrogen
Nitrogen in the environment
• Present in many forms
• 78% of atmosphere is N2
– Most of this is NOT available to
living organisms
• Getting N2 for the atmosphere
requires breaking the triple
bond between N2 gas to
produce:
• Ammonia (NH3)
• Nitrate (NO3-)
• So, N2 has to be fixed from the
atmosphere so plants can use it
Nitrogen in the environment
• This occurs naturally by:Lightning:
– 8%: splits H2O: the free O and H
attack N2 – forms HNO3 (nitric
acid) which fall to ground with
rain
• Photochemical reactions:
– 2%: photochemical reactions
between NO gas and O3 to give
HNO3
• Nitrogen fixation:
– 90%: biological – bacteria fix N2
to ammonium (NH4+)
Nitrogen in the environment
• Once fixed in ammonium or
nitrate :– N2 enters biochemical cycle
– Passes through several organic or
inorganic forms before it returns
to molecular nitrogen
– The ammonium (NH4+) and
nitrate (NO3-) ions generated via
fixation are the object of fierce
competition between plants and
microorganisms
– Plants have developed ways to
get these from the soil as fast as
possible
•
How do plants get their
nitrogen?
Some plant species are Legumes.
• Legumes seedlings germinate
without any association to rhizobia
– Under nitrogen limiting
conditions, the plant and the
bacteria seek each other out by
an elaborate exchange of signals
• The first stage of the association is
the migration of the bacteria
through the soil towards the host
plant
•
How do plants get their
nitrogen?
Nodule formation results a finely
tuned interaction between the
bacteria and the host plant
– Involves the recognition of
specific signals between the
symbiotic bacteria and the host
plant
• The bacteria forms NH3 which can
be used directly by the plant
• The plant gives the bacteria organic
nutrients.
How do plants get their
nitrogen?
Figure 11.8 (1)
• Some plants obtain nitrogen from
digesting animals (mostly insects).
• The Pitcher plant has digestive
enzymes at the bottom of the trap
• This is a “passive trap” Insects fall
in and can not get out
• Pitcher plants have specialized
vascular network to tame the amino
acids from the digested insects to
the rest of the plant
•
How do plants get their
nitrogen?
Figure
(2)
The Venus fly trap has an “active11.12
trap”
• Good control over turgor pressure in
each plant cell.
• When the trap is sprung, ion channels
open and water moves rapidly out of the
cells.
• Turgor drops and the leaves slam shut
• Digestive enzymes take over
•
Increasing crop yields
11.13
To feed the increasingFigure
population we
have to increase crop yields.
• Fertilizers - are compounds to
promote growth; usually applied
either via the soil, for uptake by
plant roots, or by uptake through
leaves. Can be organic or inorganic
• Have caused many problems!!
• Algal blooms pollute lakes near
areas of agriculture
•
Increasing crop yields
Figure
Algal blooms - a relatively
rapid 11.13
increase in the population of (usually)
phytoplankton algae in an aquatic
system.
• Causes the death of fish and
disruption to the whole ecosystem of
the lake.
• International regulations has led to a
reduction in the occurrences of these
blooms.
•
Chemical pest control
Figure
11.17
Each year, 30% of crops
are lost to insects
and other crop pests.
• The insects leave larva, which damage the plants further.
• Fungi damage or kill a further 25% of crop plants each year.
• Any substance that kills organisms that we consider undesirable are
known as a pesticide.
• An ideal pesticide would:–
–
–
–
Kill only the target species
Have no effect on the non-target species
Avoid the development of resistance
Breakdown to harmless compounds after a short time
•
Chemical pest control
Figure
11.17
DTT was first developed
in the 1930s
• Very expensive, toxic to both harmful
and beneficial species alike.
• Over 400 insect species are now DTT
resistant.
• As with fertilizers, there are run-off
problems.
• Affects the food pyramid.
– Persist in the environment
•
Chemical pest control
DTT persists in the food
chain.
Figure
11.18
• It concentrates in fish and fisheating birds.
• Interfere with calcium metabolism,
causing a thinning in the eggs laid
by the birds – break before
incubation is finished – decrease in
population.
• Although DTT is now banned, it is
still used in some parts of the
world.
Genetically modified crops
• All plant characteristics, such as size, texture, and sweetness, are
determined on the genetic level.
•
•
•
•
•
•
Also:
The hardiness of crop plants.
Their drought resistance.
Rate of growth under different soil conditions.
Dependence on fertilizers.
Resistance to various pests and diseases.
• Used to do this by selective breeding
Genetically modified crops
•Corn plants have been Figure
selective breed11.20
to increase oil yields or protein
content for over 70 years.
•Attempts to change one trait at a time can lead to the production of an
inferior strain.
•Breeding plants with high oil content changes inherited characteristics
of a given strain
Genetically modified crops
• 1992- The first commercially grown genetically
modified food crop was a tomato - was made more
resistant to rotting, by adding an anti- sense gene
which interfered with the production of the enzyme
polygalacturonase.
– The enzyme polygalacturonase breaks down
part of the plant cell wall, which is what happens
when fruit begins to rot.
Genetically modified crops
Figure
11.21
•So to modify a plant
:
•Need to know the DNA sequence of the gene of interest
•Need to put an easily identifiable maker gene near or next to the gene
of interest
•Have to insert both of these into the plant nuclear genome
•Good screen process to find successful insertion
•Clone the genetically altered plant
Genetically modified crops
Figure 11.22 (1)
Genetically modified crops
• Particle-Gun Bombardment
– Selected DNA sticks to surface of metal pellets in a salt
solution (CaCl2).
– Loaded up into a shot gun cartridge
– Fired into plant material
• The DNA sometimes was incorporated into the
nuclear genome of the plant
– Gene has to be incorporated into cell’s DNA where it will
be transcribed
– Also inserted gene must not break up some other
necessary gene sequence
Genetically modified crops
• Agrobacterium method
– Uses the natural infection mechanism of a plant pathogen
– Agrobacterium tumefaciens naturally infects the wound
sites in dicotyledonous plant causing the formation of the
crown gall tumors.
– Capable to transfer a particular DNA segment (T-DNA)
of the tumor-inducing (Ti) plasmid into the nucleus of
infected cells where it is integrated fully into the host
genome and transcribed, causing the crown gall disease.
• So the pathogen inserts the new DNA with great success!!!
Genetically modified crops
• The vir region on the plasmid inserts DNA between the Tregion into plant nuclear genome
• Insert gene of interest and marker in the T-region by
restriction enzymes – the pathogen will then “infect” the
plant material
• Works fantastically well with all dicot plant species
– tomatoes, potatoes, cucumbers, etc
– Does not work as well with monocot plant species - corn
• As Agrobacterium tumefaciens do not naturally infect
monocots
Genetically modified crops
Figure
11.21
•So to modify a plant
:
•Need to know the DNA sequence of the gene of interest
•Need to put an easily identifiable maker gene near or next to the gene
of interest
•Have to insert both of these into the plant nuclear genome
•Good screen process to find successful insertion
•Clone the genetically altered plant
Genetically modified crops
Figure 11.22 (2)
Genetically modified crops
• Can alter nutritional content
– Potatoes with 21-22% more starch
• Resistance to pathogens
– Less damage to crops – better total yield – lower retail cost
• Herbicide-resistant plants
– Spraying the fields only kills weeds
• Longer shelf-lives
– More attractive to buy in bulk
Genetically modified crops
• Issues:
• Destroying ecosystems – tomatoes are now growing in
the artic tundra with fish antifreeze in them!
• Destroying ecosystems – will the toxin now being
produced by pest-resistance stains kill “friendly” insects
such as butterflies.
• Altering nature – should we be swapping genes between
species?
Genetically modified crops
• Issues:
• Vegetarians – what about those tomatoes?
• Religious dietary laws – anything from a pig?
• Cross-pollination – producing a super-weed
• Human health – what of the antibiotic marker gene?
The End.
Any Questions?