Chapter 15 Optimizing food production

• Golden rice: two European scientists created a new strain of rice enriched in β-carotene, which the body uses to make vitamin A. This is a kind of transgenic plant (转基因植物 ).

15.1 Humans eat at all trophic levels

• Only about 1 percent of the solar energy reaching the Earth’s surface is used in photosynthesis, producing 170 billion tons of organic material per year.

• Trophic structure (food chains)

• No more than 10 percent of the energy contained in the organic material of one trophic level is incorporated into the next higher level.

• The higher the trophic level, the smaller the possible population of organisms.

• Eating meat is a luxury: The amount of biochemical energy people will obtain from eating the chickens is minuscule compared with the amount of biochemical energy in raising the chickens. In the US, more than 70 percent of grain production is fed to livestock.

• If people in US ate 10 percents less meat, the savings in resources could feed 100 million people.

15.2 Plants require nutrients

Element From available to plants Relative number of ions in dry plant material

Macronutrients

Nitrogen, N NO3-, NH4

+ 1,000,000

Potassium, K K+ 250,000

Calcium, Ca Ca2+ 125,000

Magnesium, Mg Mg2+ 80,000

Phosphorus, P H2PO4-, HPO4

2- 60,000

Sulfur, S SO42- 30,000

Micronutrients

Chlorine, Cl Cl- 3000

Iron, Fe Fe3+,Fe2+ 2000

Boron, B H2BO3- 2000

Manganese, Mn Mn2+ 1000

Zinc, Zn Zn2+ 300

Copper, Cu Cu+, Cu2+ 100

Molybdenum, Mo MoO42- 1

table15.1 essential elements for most plants

Measured relative to molybdenum=1Source: Salisbury and ross, plant physiology. Belmont, CA: Wadsworth, 1985

• Plants utilize nitrogen, phosphorus, and potassium• Plants need nitrogen to build proteins and a variety of oth

er biomolecules, such as chlorophyll (叶绿素 ).• Nitrogen fixation

Soil bacteria N2+8H++6e- 2NH4

+

nitrogenase

lightening

N2+3O2+2e- 2NO3-

fig 15.4 two pathways for nitrogen fixation, a source of nitrogen for plants. (a) both free-living bacteria in the soil and microorganisms in root nodules produce ammonium ions. (b) lightning provides the energy needed to form nitrate ions from atmospheric nitrogen.

Root nodule

• Plants need phosphorus to build nucleic acids, phospholiqids, and ATP.

• Potassium ions activate many of the enzymes essential for photosynthesis and respiration.

• Plants also utilize calcium, magnesium, and sulfur• Calcium ions are essential for building cell walls.• Magnesium ions are essential for the formation of chlorophyll.

15.3 Soil fertility is determined by soil structure and

nutrient retention • Topsoil usually

contains sand, silt, and clay.

• Fertile topsoil is a mixture of at least four components: mineral particles, water, air, organic matter.

Soil readily retains positively charged ions

humus

Positively charged nutrient ions

Plant root

mineral

Mineral particle

phenolate

Fig 15.10 the negatively charged surfaces of soil mineral particles and humus help retain positively charged nutrients

• The pH value of soil is largely a function of carbon dioxide present. A healthy soil may have enough carbon dioxide as a product released from to give a pH value of 4-7.

• Hydronium ions are able to displace nutrient ions held to mineral particles and humus.

Fig15.11 by releasing carbon dioxide, a plant guarantees a steady flow of nutrients from the soil to its roots

① Root releases CO2

② CO2 Reacts with H2O, forming H2CO3

③ H2CO3 reacts with H2O, forming HCO3

- and H3O+

④ H3O+ displaces nutrient ion (K+ shown), which is then available to root

Soil particle

root

15.4 Natural and synthetic fertilizers help restore soil fertility

• Natural occurring fertilizers are compost (堆肥 ) and minerals.• The natural occurring minerals are limited.• In 1913, a German scientist, Fritz Haber, developed a process fo

r producing ammonia from hydrogen and nitrogen.

N-P-K system

Fig 15.13 between 1956 and 1972 world crop yields grew in tandem with increases in the use of nitrogen fertilizers

Kilo

gram

per

hec

tare

Year

15.5 Pesticides kill insects, weeds, and fungi

• Insecticides kill insects• The most widely used insectici

des are chlorinated hydrocarbons, organophosphorus compounds and carbamates.

• The chlorinated hydrocarbons have a remarkable persistence, killing insects for months and years on treated surface since they are not degradable and water soluble.

• DDT: The early increased crop yields resulting from DDT use were not sustainable.

Fig 15.15

the chemical name for DDT is dichlorodiphenytrichloroethane

Silent Spring and bioaccumulation

Fig15.17 the DDT concentration in a food chain can be magnified from 0.000003 parts per million (ppm) as a pollutant in the water to 25 ppm in a bird at the top of the chain

Alternatives to DDT

C

H

C ClCl

Cl

O OH3C CH3 EnzymeH2O

H3C OH C

H

C ClCl

Cl

-O O CH3 ++ H+

ethergroup

methoxychlor

Polar products(water-soluble)Fig 15.18 methoxychlor is one of many alte

rnatives to DDT, enzymes in the liver can clean the ether groups to produce polar products. Look back to Fig15.15, and you will see that DDT lacks ether groups

Phosphorous compounds and carbamates

CH3O P

OCH3

S

S

CHC

CH2

COCH2CH3O

O

OCH2CH3

phosphorus

OC

N

H

CH3

Ocarbamate group

Malathion(an organophosphorus compound)

CarbarylA carbamate

Fig 15.20 the widely used pesticides malathion and carbaryl

Herbicides kill weeds

OCH2COHCl

Cl

O

2,4-D

Cl

Cl

Cl

OCH2COH

O

2,4,5 -T

O

OCl Cl

ClCl

TCDD

CH2HO CCH2NH

O

P

O

OH

OH

glyphosate

+N N+ CH3H3C

Cl-Cl-

paraquat

N N

N Cl

NHCH2CH3

CH3CHNH

CH3

Atrazine

Fig 15.21 the herbicides 2,4-D and 2,4,5-Tand the dioxin contaminant TCDD

Fig 15.22 the herbicides atrazine, paraquat, and glyphosate

Fungicides kill fungi

CN(H3C)2 S

S

S C

S

N(CH3)2

Fig 15.23

the fungicide thiram

thriam

15.6 There is much to learn from past agricultural practice

• Pesticides are toxic.• Fertilizers damage the ecosystem. (Blue-baby

syndrome caused by excessive amount of nitrate in water).

• Over-use of fertilizers also hardens the top-soil, which would be much easily to be removed.

15.7 High agricultural yields can be sustained with proper practice

• Organic farming is environmentally friendly (concern for environment and a commitment to using only chemicals that occur in nature)

• Crop rotation• Compost• Organic farming is energy effective.

• Integrated crop management is a strategy for sustainable agriculture

• Multi-cropping• Integrated pest management• Pheromones (信息素 )

Fig 15.34 female gypsy moths emit the pheromone disparlure (top) to entice male gypsy moths (bottom left) into mating. The males are so sensitive to one molicule in 1017 molecules of air. This astounding sensitivity enables them to respond to a female who may be more than 1 kilometer away. However, they can also be tricked into responding to insecticide traps laced with synthetic disparlure (bottom right)

• 15.8 A crop can be improved by inserting a gene from another species

• Transgenic organisms

• Bovine growth hormone (牛生长激素 )

• 15.9 World hunger is not inevitable