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.

•
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

• Two types in plants :
• Chlorophylla
• 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

• 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
• 3 rd 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

6 CO
2
+ 6 H
2
O C
6
H
12
O
6
+ 6 O
2
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 initial reaction of photosynthesis is:
– CO
2
+H
2
O (CH
2
O) + O
2
– 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

–
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!

• Plants have two reaction centers:
–
PS-II
• Absorbs Red light – 680mn
• makes strong reductant (& weak oxidant)
• oxidizes 2 H
2
O molecules to 4 electrons, 4 protons & 1 O
2 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 CO
2
.
• The
Calvin cycle thus fixes atmospheric CO
2 organic material.
into plant

• 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

• Occurs when the CO
2 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 CO
2 when its stomata are closed
–
CO
2 will get used up and the O
2 ratio in the leaf will increase relative to CO
2 concentrations
• When the CO
2 levels inside the leaf drop to around
50 ppm,
–
Rubisco starts to combine O
2 with Ribulose-1,5bisphosphate instead of CO
2

4
• The C
4 carbon Cycle occurs in 16 families of both monocots and dicots.
–
Corn
–
Millet
–
Sugarcane
–
Maize
• There are three variations of the basic C
4 carbon
Cycle
– Due to the different four carbon molecule used

4
• This is a biochemical pathway that prevents photorespiration
• C
4 leaves have TWO chloroplast containing cells
– Mesophyll cells
– Bundle sheath ( deep in the leaf so atmospheric oxygen cannot diffuse easily to them )
• C
3 plants only have Mesophyll cells
• Operation of the C
4 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 CO
2 brought in and O
2 out. Have structures called Stomata which also control water loss.

• 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 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, H
2 O
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.

• 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.

• 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

• 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

• 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 CO
2 and O
2 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.

• 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 CO
2 uptake
–
Needed for sustained CO
2 fixation during photosynthesis

• 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 CO
2 inside the leaf
• Stomata closed to prevent water loss
• Sunny day - demand for CO
2 in leaf is high – stomata wide open
• As there is plenty of water, plant trades water loss for photosynthesis products

• 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

• Guard cells act as hydraulic valves
• Environmental factors are sensed by guard cells
–
Light intensity, temperature, relative humidity, intercellular CO
2 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%

2
• Effectiveness of controlling water loss and allowing
CO
2 uptake for photosynthesis is called the transpiration ratio .
• There is a large ratio of water efflux and CO
2 influx
– Concentration ratio driving water loss is 50 larger than that driving CO
2 influx
– CO
2 diffuses 1.6 times slower than water
•
Due to CO
2 being a larger molecule than water
– CO
2 uptake must cross the plasma membrane, cytoplasm, and chloroplast membrane. All add resistance

• 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

• 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 CO
2 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 CO
2 uptake
– 5: guard cells – control stomata .

• 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

• Present in many forms
• 78% of atmosphere is N
2
– Most of this is NOT available to living organisms
• Getting N
2 for the atmosphere requires breaking the triple bond between N
2 gas to produce:
• Ammonia (NH
3
)
• Nitrate (NO
3
-
)
• So, N
2 has to be fixed from the atmosphere so plants can use it

• This occurs naturally by:-
Lightning :
–
8%: splits H
2 attack N
2
O: the free O and H
– forms HNO
3
(nitric acid) which fall to ground with rain
•
Photochemical reactions :
– 2%: photochemical reactions between NO gas and O
3
HNO
3
•
Nitrogen fixation : to give
–
90%: biological – bacteria fix N
2 to ammonium (NH
4
+ )

• Once fixed in ammonium or nitrate :-
– N
2 enters biochemical cycle
– Passes through several organic or inorganic forms before it returns to molecular nitrogen
– The ammonium (NH
4
+ ) and nitrate (NO
3
) 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
• Some plant species are nitrogen?
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 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 NH
3 which can be used directly by the plant
• The plant gives the bacteria organic nutrients.

How do plants get their 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
• 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 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
•
Algal blooms
Figure 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
• 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
• 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
• 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
Figure 11.20
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
•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 (CaCl
2
).
– 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
•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?

Any Questions?