"Without this flow of energy from the sun, channeled largely through the eukaryotic cells, the pace of life on this planet would swiftly diminish and then, following the inexorable second law of thermodynamics would virtually cease altogether." Raven, P. H. et al. 1992.

I. INTRODUCTIONCHAPTER 7

PHOTOSYNTHESIS

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PHOTOSYNTHESISOVERVIEW

Photosynthesis - summary CARBON DIOXIDE + WATER + LIGHT CARBOHYDRATE + OXYGEN

6 CO2 + 6 H2O C6H12O6 + 6 O2 + H2O

• This reaction captures energy from sunlight• Light energy is used to reduce carbon (removes hydrogen

from water and adds it to carbon)• Energy is stored in plant tissues as starch, sugars and in

cellulose (wood, coal, gas, oil)• Stored energy can be released by burning, or by metabolism

IMPORTANCE OF PHOTOSYNTHESIS

•AUTOTROPHS (Can make own food, i.e., can manufacture organic compounds from inorganic compounds)•HETEROTROPHS (Eat others to get organic compounds)

PHOTOSYNTHETIC ORGANISMS ARE AT THE BASE OF THE FOOD CHAIN

Photosynthesis is the source of Oxygen in the atmosphere

Earth's atmosphere is a layer of gases surrounding the planet and retained by the Earth's gravity.

78% nitrogen 20.95% oxygen 0.93% argon 0.038% carbon dioxide (up from 0.028%) a variable amount (average around 1%) of water vapor

Oxygen is reactive with inorganic compounds and does not stay in the atmosphere. Thus, our current atmosphere is a balance of oxygen produced by plants and loss to geological processes

THE NATURE OF LIGHT

• THE VISIBLE SPECTRUM CAN BE SEEN WHEN WHITE LIGHT IS PASSED THROUGH A PRISM.

• THE SPECTRUM IS SEEN AS DIFFERENT

COLORS OF LIGHT

Spectrum

A “SPECTRUM” IS A RANGE OF WAVELENGTHS OF ELECTROMAGNETIC RADIATION

One wavelength

Ultraviolet light, X-rays and gamma rays have too much energy for biological systems. They break bonds, knock electrons out of orbit and generally cause cell damage. We are protected from UV by the ozone layer

Infrared light is heat and has too low an energy level to be used for photosynthesis

• Pigments are molecules that absorb light. We see them as being the color of the light that is reflected

• In order to trap light, light must be absorbed. (If light is reflected, it has no value)

Pigments - absorption of light

PIGMENTS

T. W. Engelmann (1883)

• Designed an elegant experiment with spirogyra, an

algae with a single long coiled chloroplast. • He combined the algae with an oxygen-requiring

bacterium and exposed the algae to light coming

through a prism. • He showed that the bacteria clustered in the blue

and red areas suggesting that these areas were

producing oxygen.

T. W. Englemann’s experiment 1882

Spirogyra

•Chlorophyll a •Chlorophyll b• Chlorophyll c

(replaces Chlorophyll b in some algae)•Carotenoids

•Carotene•Xanthophylls

•Phycobilins (photosynthetic bacteria) •Phycoerythrin•Phycocyanin

PIGMENTS OF PLANTSPIGMENTS OF PLANTSPIGMENTS OF PLANTSPIGMENTS OF PLANTS

Chlorophyll a

CarotenoidsCarotenoids•Absorb maximally between 460 and 550 nm, absorb in the blue range and therefore appear red, orange and yellow

Carotenoids

The third major class of pigments, the phycobilins, are found in the cyanobacteria and red algae only

REACTIONS OF PHOTOSYNTHESIS

A. LIGHT vs DARK REACTIONS

F. F. Blackman (1905)

English Plant Physiologist: Concluded that there were two reactions, one of which was light dependent (photochemical reactions) and one of which wastemperature dependent (biochemical reactions or dark reactions)

Stroma

Grana (stacked granum thylakoids)

Stroma thylakoids

Plant cell wall

Chloroplast double membrane

Light reactions take place in the grana thylakoids

Light reactions

Closeup of grana and stroma

Stack of granum

thylakoids

Stroma

A stack of folded membranes is referred to as a granum. The stroma is a watery matrix

Light reactions

Dark or biochemical rxns

REACTIONS OF PHOTOSYNTHESIS

B. LIGHT REACTION

Light-trapping pigments float in the thylakoid membranes

The pigment aggregates have been given the name antenna complex because it acts as an antenna to trap light and transfer the absorbed energy

Antenna complex = Chlorophyll a, chlorophyll b and carotenoids

e-

When light has the correct energy level, it is absorbed by an electron. The electron must move to a new orbital whose energy level corresponds to the electron’s new energy load

Photon

Absorption of light

e-

The excited state is unstable. The atom can become stable if the electron drops back into lower orbit. When this happens the excess energy may be released as visible light (fluorescence) or heat (infared light)

Heat or light

Absorption of light

Alternately, the electron can move to a more stable orbital on an entirely different atom. This electron transfer is the critical step in photosynthesis

Absorption of light

Reaction center: Within the antenna complex, there is a special pair of chlorophyll A molecules and associated proteins called a reaction center

Pigment complexes

Function of the reaction center: accepts electrons transferred from the antenna complex and boosts them into higher orbit where they can be transferred to another electron acceptor molecule

Pigment complexes

There are two different types of reaction center/antenna complexes in the thylakoid membranes (light reaction).

• The reaction center of Photosystem I absorbs maximally at 700 nm. The reaction center is called P700

• The reaction center of Photosystem II absorbs maximally at 680 nm. The reaction center is called P680

Energy depiction of Photosynthesis, the “Z” scheme

Photophosphorylation = LIGHT driven production of ATP 1. Electrons are passed along a series of electron carriers

When electrons reach transmembrane pumps, protons are pumped into the lumen of the thylakoid membrane

This proton gradient drivessynthesis of ATP into the stroma (cytoplasm) of thechloroplast

2.Similar in principle to oxidative phosphorylation in mitochondria

Photophosphorylation

• Requires light• Independent of temperature (an indication that enzymes are not involved)

• Light is used to make ATP and to reduce electron carrier molecules

Light reactions - summary

Produce 3 ATPsReduce 2 molecules of NADP+ to NADPHOxygen is produced from splitting water

Summary of light reactionEight photons used in the light (photochemical) reaction

CYCLIC ELECTRON FLOWCYCLIC ELECTRON FLOW

In photosynthetic prokaryotes, only part of the reaction is present (photosystem 1). This is the older system. Green plants have added a second system to increase energy capture. •No water is split•No NADPH is made

a. Cyclic phosphorylation alone may have existed for a billion years in bacteria who used sulfur as an electron donor but modern plants use water as a source of electrons and that required a major change because the energy in P700 isn’t high enough to remove an electron from water.b. Plants overcame this problem by grafting on a second, more powerful photosystem which could harvest shorter, higher energy wavelengths of light (Z scheme)

from

ATP sy

nthas

e

REACTIONS OF PHOTOSYNTHESIS

B. DARK REACTIONS

PHOTOSYNTHESIS IS COMPOSED OF TWO SETS OF REACTIONS

LIGHT DEPENDENT = PHOTOCHEMICALTEMPERATURE INDEPENDENTLIGHT DEPENDENT

DARK REACTIONS = CARBON FIXATION RXNS = BIOCHEMICAL RXNS

TEMPERATURE DEPENDENTLIGHT INDEPENDENT

Berkeley scientist, Melvin Calvin (1950) was the first to elucidate the steps in the dark reaction of photosynthesis

Dark dependent stage• Occurs in the light but does not require light• Temperature dependent or sensitive reactions. Reactions

increase up to 30C then decrease. This temperature profile is an indication that enzymes are involved

• Energy products of the light reaction are used to reduce carbon from CO2 to a sugar (carbon fixation)

RUBISCO

Ribulose-1,5-bisphosphate or RuBP8 large subunits (gene in chloroplast)8 small subunits (gene in nucleus)

Most abundant protein on earth

PGAL is an intermediate in respiration during the oxidation of glucose to pyruvate

Sugar

C3 PLANTS

PLANTS WHICH USE ONLY THE CALVIN CYCLE TO FIX CO2 INTO SUGARS ARE CALLED C3 PLANTS (BECAUSE A THREE-CARBON MOLECULE IS THE FIRST STABLE MOLECULE IN THE CYCLE)

MOST PLANT SPECIES ARE C3 PLANTS CEREALSPEANUTSCOTTONSOYBEANSMOST TREESMOST LAWN GRASSES

REACTIONS OF PHOTOSYNTHESIS

Photorespiration

•When CO2 levels in the leaf get too low, RUBISCO starts binding oxygen instead of CO2

•Happens when stomates close to prevent water loss. (This prevents CO2 diffusion into the leaf)•Forces another pathway O2 + RuBP PGA + phosphoglycolic acid

•Reactions called photorespiration because they occur in the light, use oxygen and release CO2

•Unlike normal respiration, it produces no ATP•It is a wasteful salvage pathway

PHOTOSYNTHESIS IS NOT PERFECT:PHOTORESPIRATION

RUBISCO

•RUBISCO binds CO2and O2 in the same site

O2

CO2

•If oxygen is bound, the Calvin cycle cannot take place

•Instead, photorespiration takes place

waste

d

REACTIONS OF PHOTOSYNTHESIS

The C4 pathway

• Some plants have developed mechanisms to bypass the RUBISCO carboxylase/oxygenase problem

• These plants are called C4 plants because they fix carbon into a 4-carbon sugar

• Plants out-compete C3 plants in hot dry weather only.

C4 PLANTS

C4 carbon fixation has evolved on up to 40 independent occasions in different groups of plants, making it an example of convergent evolution. Plants which use C4 metabolism include sugarcane, maize, sorghum, finger millet, amaranth, and switchgrass. C4 plants arose around 25 to 32 million years ago

Calvin cycle Malate

C4 plants have a specific kind of leaf anatomy. Carbon is fixed into malate in mesophyll cells. The Calvin cycle occurs in bundle sheath cells that surround the vascular system (few thylakoid grana and protected from oxygen). Thus, the two processes are separated in space.

Bundle sheath cellChloroplasts lack grana

Mesophyll cell with granna

Calvin cycle-note few grana

Malate

Plasmodesmata

REACTIONS OF PHOTOSYNTHESIS

CAM Plants

Crassulacean acid metabolismSeparates CO2 intake and the Calvin cycle by time rather than by space

Crassula ovata – Jade Plant

Crassulacean acid metabolism

The majority of plants possessing Crassulacean Acid Metabolism are either epiphytes (e.g. orchids, bromeliads) or succulent xerophytes (e.g. cacti, cactoid Euphorbias).

16,000 species (7% of plants). Most are angiosperms but includes some gymnosperms, ferns and quillworts

REACTIONS OF PHOTOSYNTHESIS

Carbon Cycle

Global Warming

Another important carbon “sink” is the arctic tundra which is now decomposing and releasing CO2 as the arctic thaws.

Carbon Dioxide in the atmosphere

GREENHOUSE EFFECT – HUMANS ARE RELEASING STORED CARBON WHICH INCREASES ATMOSPHERIC CO2

The present level is higher than at any time during the last 800 thousand years,[6] and likely higher than in the past 20 million years.[7]

6.Amos, Jonathan (2006-09-04). "Deep ice tells long climate story". BBC News. http://news.bbc.co.uk/2/hi/science/nature/5314592.stm. Retrieved 2010-04-28.7. Climate Change 2001: The Scientific Basis

TRENDS•TOTAL GHG EMMISIONS INCREASED 14.2% SINCE 1990

•DOMINANT GAS EMITTED IS CO2, MOSTLY FROM FOSSIL FUEL

COMBUSTION (oxidation of sugars produced by fossil plants)

SummerNOAA CO2 tracker

WinterNOAA CO2 tracker

Red=high CO2 emissionsBlue = low CO2 emissions

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