Photosynthesis

Chapter 10

Photosynthesis

• Light energy stored as chemical energy for future use

• Original source of energy for other organisms

• Except for a few species of bacteria, all life depends on the energy-storing reactions of photosynthesis

Discoveries Leading to the Understanding of Photosynthesis

Until 17th century, scholars believed that plants derived the bulk of their substance from soil humus.

Discoveries Leading to the Understanding of Photosynthesis

• Joannes van Helmont– Disproved idea that plants get bulk of

substance from soil humus• Planted 5 lb. willow in 200 lbs. of dried soil• Over 5 year time span, only watered plant with

rainwater• At end of 5 years

– Plant grew from 5 lbs. to 169 lbs.– Soil only lost 2 oz. during the 5 years

• Reasoned plant substance must have come from water

Discoveries Leading to the Understanding of Photosynthesis

• Joseph Priestly– 1772– Reported sprig of mint could restore air that

had been made impure by a burning candle– Plant changed air so mouse could live in it– Experiment not always successful

• Sometimes didn’t provide adequate light for plant

Discoveries Leading to the Understanding of Photosynthesis

• Jean Senebier– 1780– pointed out that “fixed air,” carbon dioxide was

required for photosynthesis

• Antoine Lavoisier– Stated that green plants use carbon dioxide

and produce oxygen

Discoveries Leading to the Understanding of Photosynthesis

• Jan Ingen-Housz– 1796– Found that carbon went into the nutrition of

the plant

• Nicolas de Saussure– 1804– Observed that water was involved in the

photosynthetic process

Discoveries Leading to the Understanding of Photosynthesis

• Julius von Sachs– Between 1862 and 1864 observed

• Starch grains are present in chloroplasts of higher plants

• If leaves containing starch are kept in darkness for some time, starch disappears

• If same leaves are exposed to light, starch reappears in chloroplasts

• First person to connect appearance of starch (carbohydrate) with both fixation of carbon in the chloroplasts and the presence of light

Discoveries Leading to the Understanding of Photosynthesis

• Cornelis van Niel– 1930s– Compared photosynthesis in different groups of

photosynthetic bacteria• Green and sulfur bacteria use H2S instead of H2O to

reduce CO2

• Found that sulfur was liberated instead of O2

• Since sulfur could only come from H2S, van Niel reasoned that O2 liberated by higher plants comes from H2O not CO2

Discoveries Leading to the Understanding of Photosynthesis

• Cornelis van Niel– His general equation for photosynthesis

6CO2 + 12H2A C6H12O6 + 6H2O + 12Alight

carbohydrateHydrogen donor

Carbon dioxide

water A

H2A could be H2O, H2S, H2 or any molecule capable of donating an electron. Reaction requires energy input. When H2A gives up electrons, it is oxidized to A.

Specific Photosynthetic Reactions

• T.W. Engelmann– Between 1883 and 1885– Demonstrated which colors of light are used in

photosynthesis– Found that red and blue light were trapped by

algal photosynthetic organelles

Specific Photosynthetic Reactions

• J. Reinke– Studied effect of changing the intensity of light on

photosynthesis– Observed rate of photosynthesis increased

proportionally to increase in light intensity at low-to-moderate light intensities

– At greater light intensities, rate of photosynthesis was not affected by changing light intensities

– Indicated reaction was already proceeding at maximum rate

Specific Photosynthetic Reactions

• F.F. Blackman– 1905– Reasoned photosynthesis could be divided

into two general parts• Photochemical reactions (light reactions)• Temperature-sensitive reactions (previously called

dark reactions)

Specific Photosynthetic Reactions

• Photochemical reactions– Light reactions– Insensitive to temperature changes

• Temperature-sensitive reactions– Previously called dark reactions– Enzymatic reactions– Do not depend directly on light– Chloroplast proteins, thioredoxins, regulate

activities of some dark reactions

Chloroplast Research

• Robin Hill– 1932– Demonstrated chloroplasts isolated from cell

could still trap light energy and liberate oxygen

• Daniel Amon– 1954– Proved isolated chloroplasts could convert

light energy to chemical energy and use this energy to reduce CO2

Chloroplast Structure

• Double-membrane envelope

• Two types of internal membranes– Grana (singular, granum)– Stroma lamella – interconnect grana

• Stroma– Made up of grana and stroma lamella

Division of Labor in Chloroplasts

• Research has shown that– Intact chloroplasts carry out complete process

of photosynthesis– Broken plastids

• Carry out only part of photosynthetic reactions• Will liberate oxygen

Division of Labor in Chloroplasts

• Division of labor– Green thylakoids

• Capture light

• Liberate O2 from H2O

• Form ATP from ADP and phosphate• Reduce NADP+ to NADPH

– Colorless stroma• Contain water-soluble enzymes

• Captures CO2

• Uses energy from ATP and NADPH in sugar synthesis

Characteristics of Light

• Two models describing nature of light– Interpret light as electromagnetic waves– Light acts as if it were composed of discrete

packets of energy called photons

Characteristics of Light

• Light is small portion of electromagnetic energy spectrum that comes from sun

• Longest waves– Cannot see– Infrared and radio waves– Longer than visible red wavelength

• Shortest waves– Cannot see– Ultraviolet waves, X-rays, gamma rays– Shorter than violet

Characteristics of Light

• White light (visible light)– Separate into component colors to form

visible spectrum– Visible wavelengths range from

• Red (640 – 740 nm)• Violet (400 – 425 nm)

Photons

• Packet of energy making up light

• Contains amount of energy inversely proportional to wavelength of light characteristic for that photon– Blue light has more energy per photon than

does red light

Photons

• Only one photon is absorbed by one pigment molecule at a time

• Energy of photon is absorbed by an electron of pigment molecule– Gives electron more energy

Absorption of Light Energy by Plant Pigments

• Spectrophotometer– Instrument used to measure amount of

specific wavelength of light absorbed by a pigment

– Absorption spectrum• Graph of data obtained

• Chlorophyll– Reflects green light– Absorbs blue and red wavelengths

• Wavelengths used in photosynthesis

Absorption of Light Energy by Plant Pigments

• Chlorophyll– Two major types of chlorophyll in vascular

plants• Chlorophylls a and b• In solution absorb much of red, blue, indigo, and

violet light• In thin green leaf

– Absorption spectrum similar to but not identical to that of chlorophyll in solution

Absorption of Light Energy by Chlorophyll

• Chlorophyll molecule absorbs or traps photon

• Energy of photon causes electron from one of chlorophyll’s atoms to move to higher energy state

• Unstable condition

• Electron moves back to original energy level

Absorption of Light Energy by Chlorophyll

• Absorbed energy transferred to adjacent pigment molecule– Process called resonance

• Energy eventually transferred to chlorophyll a reception center– Series of steps drives electrons from water to reduce

NADP+

– Formation of NADPH represents conversion of light energy to chemical energy

– NADPH reduces CO2 in enzymatic reactions leading to sugar formation

Two Photosystems

• Robert Emerson– 1950s– Made observations that led to realization that

there are two light reactions and two pigment systems

• Photosystem I• Photosystem II

Two PhotosystemsPigments

Reaction Center

Description

Photosystem I Chlorophyll a and b P700

Greater proportion of chlorophyll a than b in *light-harvesting complex, sensitive to longer wavelength light

Photosystem IIChlorophyll a and b, carotene

P680

Equal amounts of chlorophyll a and b, *light-harvesting complex sensitive to shorter wavelength light

* light-harvesting complex – functional pigment units that act as light traps

Adenosine Triphosphate Synthesis

• Photophosporylation– Light-driven production of ATP in chloroplasts

• Two types– Cyclic photophosphorylation– Noncyclic photophosphorylation

Adenosine Triphosphate Synthesis

• Cyclic Photophosphorylation– Electrons flow from light-excited chlorophyll

molecules to electron acceptors and cyclically back to chlorophyll

– No O2 liberated

– No NADP+ is reduced– Produces H+ gradient that leads to energy

conservation in ATP production– Only photosystem I involved

Adenosine Triphosphate Synthesis

• Noncyclic photophosphorylation– Electrons from excited chlorophyll molecules

are trapped in NADP+ to form NADPH– Electrons do not cycle back to chlorophyll– Photosystems I and II are involved– ATP and NADPH are formed

• Energy drives CO2 reduction reactions of photosynthesis

Enzymes of Light-Independent Reactions

• All enzymes participating directly in photosynthesis occur in chloroplasts– Many are water-soluble– Many found in stroma

• Ribulose biphosphate carboxylase/oxygenase (rubisco)– Catalyzes first step in carbon cycle of

photosynthesis

Enzymes of Light-Independent Reactions

rubisco

Carbon dioxide + ribulose biphosphate 2 phosphoglyceric acid

*(RuBP)

•RuBP 5-C sugar present in plastid stroma, spontaneous reaction

Photosynthetic Carbon Reduction Cycle

• Methods used to isolate carbon compounds formed during enzymatic reactions– Used radioactive carbon (14C) in CO2 to trace

each intermediate product– Two-dimensional paper chromatography

Photosynthetic Carbon Reduction Cycle

• Melvin Calvin– 1950s

– Used radioactive C (14C) in CO2 to trace intermediate products of carbon reduction cycle

– Nobel Prize

C3 Pathway

• First product PGA contains 3 Cs• Calvin cycle (in honor of discoverer, Melvin

Calvin)• Key points

– CO2 enters cycle and combines with RuBP produced in stroma

• 2 molecules of PGA are produced

– Energy stored in NADPH and ATP transferred into stored energy in phosphoglyceraldehyde (PGAL)

C3 Pathway

– PGAL may be enzymatically converted to 3-C sugar phosphate, dihydroxyacetone phosphate

– Two molecules of dihydroxyacetone phosphate combine to form a sugar phosphate, fructose 1,6 - biphosphate

C3 Pathway

– Some fructose 1,6 – biphosphate transformed into other carbohydrates, including starch (reactions not part of C3 cycle)

– RuBP is regenerated • Free to accept more CO2

Photorespiration

• Differs from aerobic respiration– Yields no energized energy carriers– Does not occur in the dark

• Involves interaction with chloroplasts, peroxisomes, mitochondria

Photorespiration

C3 Plants

High rates of photorespiration (particularly on hot, bright days)

Produce less sugar during hot, bright days of summer, under milder conditions are more efficient because they expend less energy to capture CO2

C4 Plants

Show little or no photorespiration

Produce 2 or 3 times more sugar than C3 plants during hot, bright days of summer

Environmental Stress and Photorespiration

• Succulents– Developed methods of storing and conserving

water• Highly developed parenchyma tissue• Large vacuoles• Reduced intercellular spaces• Absorb and store water when moisture is available

Environmental Stress and Photorespiration

• Succulents– Stoma closed during the day and open at

night• Advantage

– Reduces water loss during day

• Disadvantage– Reduces CO2 uptake in daylight when photosynthesis

can occur

– Exhibit type of carbon metabolism called crassulacean acid metabolism (CAM)

Major Features of CAM

– Stomata open at night

– Leaves rapidly absorb CO2

– Enzyme phosphoenolpyruvate (PEP) carboxylase initiates fixation of CO2

– Malate, 4-C compound is usually produced– Total amount of organic acids rapidly increases in

leaf-cell vacuoles at night– Leaf acidity rapidly decreases during following day

• Organic acids are decarboxylated and CO2 released into leaf mesophyll

Major Features of CAM

– Stomata closed during the day• Prevents or greatly reduces CO2 absorption and

water loss

• C3 cycle of photosynthesis usually takes place and converts the internally released CO2 into carbohydrate

C4 Pathway

• Discovered in 1965– H.P. Kortschak, C.E. Hartt, G.O. Burr

• Extensively studied by M.D. Hatch and C.R. Slack

• Pathway also known as Hatch-Slack cycle

• Differs from C3 or Calvin cycle

– Ensures an efficient absorption of CO2 and results in low CO2 compensation point

C4 Pathway

• Compensation point– Concentration of CO2 remaining in closed

chamber at the point when CO2 produced by respiration balances or compensates for CO2 absorbed during photosynthesis

– Varies among different plants

C4 Pathway

– Example of compensation point• Place bean plant and corn plant in chamber in light• Bean plant will die before corn plant

• Corn plant has very low CO2 compensation point

• Both plants eventually die of starvation

Factors Affecting Productivity

• Only about 0.3% to 0.5% of light energy that strikes leaf is stored in photosynthesis

• Yield could be increased by factor of 10 under ideal conditions

Factors Affecting Productivity

• Breed productivity into plants– Norman Borlaug– Nobel Prize 1970– Developed high-yielding wheat strains

• Disadvantages– Strains require high levels of fertilizer

» Expensive» Create pollution

– Potential for genetic problems

Factors Affecting Productivity

• Breeding programs or use of recombinant DNA technology may lead to new C4 and C3 plants less prone to photorespiration

Environmental Fluctuations Alter Photosynthesis Rate

• To some extent, environmental factors under control of plant grower

• Water and mineral content control of soil most easily controlled

• Control of temperature, light (intensity, quality, duration), and CO2 require special equipment

Environmental Factors

• Temperature– Most plants function best between

temperatures of 10C and 25C– Above 25C

• Continuous decrease in photosynthesis rate as temperature increases

– Under low light intensity, increase in temperature beyond certain minimum does not produce increase in photosynthesis

Environmental Factors

• Light– Light intensity and wavelength affect

photosynthesis rate• Intensity to which chloroplasts are exposed affects

photosynthesis more than intensity of light falling on leaf surface

• Structural adaptations that diminish light intensity that reaches chloroplasts

– Surface hairs, thick cuticle, thick epidermis

Environmental Factors

• Light– Sunflecks

• Brief exposure to light received by plants on forest floor when breezes move upper canopy

• Contribute to majority of light used by understory vines, shrubs, and herbs

– Plants adapt to quality of light to survive• Plants growing in deep water have developed

accessory pigments to absorb blue-green wavelengths and use it in photosynthesis

Environmental Factors

• Carbon dioxide– Not possible to deplete atmospheric carbon dioxide– Continual increase in carbon dioxide contributes to

threat of global warming– Atmospheric carbon dioxide around leaves limits rate

of photosynthesis in C3 plants

– Experimentally determined an artificial increase in carbon dioxide (up to 0.6%) may increase rate of photosynthesis for limited period

• Level injurious to some plants after 10 to 15 days of exposure

Environmental Factors

• Water– Rate of photosynthesis may be changed by

small differences in water content of chlorophyll-bearing cells

– Drought reduces rate of photosynthesis in some plants

Environmental Factors

• Mineral nutrients– Poor soils can result in plants with poorly

developed photosynthetic capacities• Can increase yields by effective fertilizer programs