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 runners-up, 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 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-1,5-bisphosphate 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 bi-lipid 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, 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 andsalt. Osmosis and Tonicity
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.
Figure 11.8 (1) How do plants get their nitrogen? • 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
Figure 11.12 (2) How do plants get their nitrogen? • The Venus fly trap has an “active 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
Figure 11.13 Increasing crop yields • To feed the increasing 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
Figure 11.13 Increasing crop yields • Algal blooms - a relatively rapid 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.