HMM1414 Chapter 6

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HMM/SCM1414_Biology 1

CHAPTER 6

PHOTOSYNTHESIS

Life on Earth is solar powered.

Chloroplasts capture light energy from sun

and convert it to chemical energy stored in

sugars and other organic molecules.

6.1Types of NutritionPhotosynthesis nourishes almost all the

living world directly or indirectly.

All organisms use organic compounds for

energy and for carbon skeletons.

Obtain organic compounds by autotrophic

or heterotrophic nutrition.

6.1.1 Autotrophic NutritionAutotrophs produce organic molecules fromCO2 and other inorganic raw materialsobtained from environment.

Autotrophs are ultimate sources of

organic compounds for all heterotrophic

organisms.

Producersof the biosphere.

Two groups based on energy source that

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drives their metabolism.

Photoautotrophs use light as energy

source to synthesize organic compounds.

Example, plants, algae, some other

protists, and some prokaryotes.

Chemoautotrophs harvest energy from

oxidizing inorganic substances, such as

sulfur and ammonia.

Unique to prokaryotes.

6.1.2 Heterotrophic NutritionHeterotrophs live on organic compoundsproduced by other organisms.

Consumersof the biosphere.

Most feeds on other organisms.

Example, animals.

Others decompose and feed on dead

organisms or organic litter, like feces and

fallen leaves.

Example, most fungi and many

prokaryotes.

Almost all heterotrophs completely

dependent on photoautotrophs for food

and for oxygen, a by-product of

photosynthesis.

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6.2 The biophysics of lightThylakoids convert light energy into chemical

energy of ATP and NADPH.Light is a form of electromagnetic radiation.

Light travels in rhythmic waves.

Distance between crests of electromagnetic

waves = wavelength. Wavelengths range from less than a

nanometer (gamma rays) to more than a

kilometer (radio waves).

Entire range of electromagnetic radiation =

electromagnetic spectrum.(Figure 10.6, Campbell, page 186)

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The most important segment for life is

narrow band between 380 to 750 nm =

visible light.Light is composed of small particles, or

packets of energy, the photons. Photons have fixed quantities of energy.

Amount of energy packaged in a photon is

inversely related to its wavelength.

The shorter the wavelengths, the more

the energy.

Atmosphere selectively screens out most

wavelengths, allowing only visible light to

pass in significant quantities.

Visible light drives photosynthesis.

When light meets matter, it may be reflected,

transmitted, or absorbed.

Different pigments absorb photons of

different wavelengths.

Wavelengths that are absorbed

disappear.

A leaf looks green because chlorophyll

absorbs red and blue light, while

transmitting and reflecting green light.(Figure 10.7, Campbell, page 186)

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It beams narrow wavelengths of light

through solution containing pigment and

measures fraction of light transmitted at

each wavelength.

An absorption spectrum plots apigments light absorption versus

wavelength.(Figure 10.9 (a), Campbell, page 187)

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Light reaction can perform work with those

wavelengths of light that are absorbed.

Thylakoid contains several pigments that

differ in their absorption spectra.

Chlorophyll a , the dominant pigment,absorbs best in red and violet-blue

wavelengths and least in green.

Other pigments have different absorption

spectra.

Collectively, these photosynthetic pigments

determine an overall action spectrum forphotosynthesis.

(Figure 10.9 (b), Campbell, page 187)

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Action spectrum measures changes in

some measure of photosynthetic activity

(for example, O2 release) as wavelength

is varied.

Action spectrum of photosynthesis was first

demonstrated by Thomas Engelmann (1883):

Different segments of filamentous alga

were exposed to different wavelengths of

light.

Areas receiving wavelengths favorable to

photosynthesis produced excess O2.

Most aerobic bacteria clustered along

segment of algal filament emitting most

O2, i.e., segments in red and blue portion

of spectrum. (Figure 10.9 (c), Campbell, page 187)

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Action spectrum of photosynthesis does not

match exactly absorption spectrum of any

one photosynthetic pigment, includingchlorophyll a.

Only chlorophyll aparticipates directly in the

light reaction, but accessory photosynthetic

pigments absorb light and transfer energy to

chlorophyll a.

Chlorophyll b has slightly different10

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absorption spectrum and funnels energy

from these wavelengths to chlorophyll a.

Carotenoids funnel energy from otherwavelengths to chlorophyll aand also

participate in photoprotectionagainst

excessive light:

Absorb and dissipate excessive light

energy that would damage chlorophyll.

Also interact with oxygen to formreactive oxidative molecules that could

damage the cell.

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6.3How the two photosystems of plantswork together

When a molecule absorbs a photon, one of itselectrons is elevated to an orbital with more

potential energy.

The electron moves from its ground state

to an excited state.

Excited electrons are unstable.

Generally, they drop to their ground state in a

billionth of a second, releasing heat energy.

In the thylakoid membrane, chlorophyll is

organized with proteins and smaller organic

molecules into photosystems.Photosystem is composed of a reactioncenter surrounded by a light-harvestingcomplex. (Figure 10.12, Campbell, page 189)

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Each light-harvesting complex consists ofpigment molecules (chlorophyll a, chlorophyll

b, and carotenoid molecules) bound to

particular proteins.

Light-harvesting complexes act like light-

gathering antenna complexes for reaction

center.When antenna molecule absorbs photon, it is

transmitted from molecule to molecule until

it reaches a particular chlorophyll a

molecule, the reaction center.At the reaction center is a primaryelectron acceptor, which accepts an

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excited electron from chlorophyll a.

The solar-powered transfer of an electron

from chlorophyll a to the primary

electron acceptor is the first step of light

reactions.

Two types of photosystems in thylakoid

membrane.

1.Photosystem I (PS I) has a reactioncenter chlorophyll athat has

absorption peak at 700 nm.

2.Photosystem II (PS II) has a reactioncenter chlorophyll athat has

absorption peak at 680 nm.

The differences between these reactioncenters (and their absorption spectra) lie

not in the chlorophyll molecules, but in

the proteins associated with each

reaction center.

Both photosystems work together to use

light energy to generate ATP and NADPH.

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6.4 Light dependent reactionTwo possible routes for electron flow: cyclic

and non-cyclic.

6.4.1 Non-cyclic Electron Flow(Non- Cyclic Photophosphorylation)

(Figure 10.13, Campbell, page 190)

The predominant route - produces both ATP

and NADPH:

1. Photosystem II absorbs a photon of

light. One of the electrons of P680 is

excited to a higher energy state.

2. Electron captured by primary

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electron acceptor, leaving reaction

center oxidized.

3. An enzyme extracts electrons from

water and supplies them to oxidized

reaction center. Water split into two

H+ and an oxygen atom that

combines with another oxygen atom

to form O2.

4. Photoexcited electrons pass alongan electron transport chain before

ending up at an oxidized

photosystem I reaction center.

(Electron transport chain made up

of plastoquinine, Pq, a cytochrome

complex, and plastocyanin, Pc.)5. As these electrons fall to a lower

energy level, their energy is used to

produce ATP.

6. Meanwhile, light energy excited an

electron of PSIs P700 reaction

center.7. Photoexcited electron is captured

by PSIs primary electron acceptor,

creating an electron hole in P700.

8. Hole is filled by electron from PS II.

9. Photoexcited electrons are passed

from PSIs primary electron

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acceptor down a second electron

transport chain through the protein

ferredoxin (Fd).

10. NADP+ reductase transfers

electrons from Fd to NADP+.

11. Two electrons are required for

NADP+s reduction to NADPH.

12. NADPH will carry reducing power of

these high-energy electrons toCalvin cycle.

Light reactions use solar power of photons

absorbed by both photosystem I and

photosystem II to provide

(i) chemical energy ATP; and

(ii) reducing power - in the form of

electrons carried by NADPH.

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6.4.2 Cyclic Electron Flow(Figure 10.15, Campbell, page 101)

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Under certain conditions, photoexcited

electrons from photosystem I can take an

alternative pathway, cyclic electron flow. Excited electrons cycle from reaction

center to a primary acceptor, along an

electron transport chain, and return to

the oxidized P700 chlorophyll.

As electrons flow along electron

transport chain, they generate ATP by

cyclic photophosphorylation. No NADPH produced and no release of

oxygen.

What is the function of cyclic electron flow?

Noncyclic electron flow produces ATP and

NADPH in roughly equal quantities.

However, Calvin cycle consumes more ATP

than NADPH.

Cyclic electron flow allows the chloroplast to

generate enough surplus ATP to satisfy the

higher demand for ATP in Calvin cycle.

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6.4.3 Chemiosmosis(Figure 10.17, Campbell, page 193)

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Formation of ATP in light reaction as a

result of a pH gradient across thylakoid

membrane.

1. Photon strikes pigment molecule in

PSII.

2. Excited electron is passed along

electron carriers.3. Water is split O2 is released and H

+

remains in thylakoid space.

4. Energy released as electron passes

through electron carriers between PSII

and PSI is used by proton pump to pump

H+ into thylakoid space.

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5. A pH gradient develops between inside

and outside of thylakoid sac.

6. Inside high H+ concentration (lower pH).

7. A strong diffusion gradient is set up.

8. H+ cross back across membrane

through ATPase synthetase complex.

9. Energy released as H+ flow down their

gradient is used for synthesis of ATP

from ADP and P.10. 3 H+ pass through ATPase synthetase

complex to make 1 ATP.

11. NADP is the final electron acceptor

producing NADPH

Figure: Electron Transport and Chemiosmosis during

Photosynthesis

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Light-reaction produces ATP and NADPH on

stroma side of thylakoid, where Calvin cycle

reactions take place.

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6.5 Light independent reaction6.5.1 Calvin cycle/C3 cycle

Cycle regenerates its starting material after

molecules enter and leave cycle.

Cycle is anabolic - uses energy to build sugar

from smaller molecules.

1. Carbon enters cycle as CO2 and leaves

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as sugar.

2. Cycle uses energy of ATP and

reducing power of electrons carried by

NADPH to make sugar.

3. Actual sugar product of cycle is not

glucose, but glyceraldehyde-3-phosphate (G3P).

4. Each turn of cycle fixes one carbon.

5.

Net synthesis of one G3P molecule,requires three cycles, fixing three

molecules of CO2.

6. One glucose molecule requires six

cycles and fixation of six CO2

molecules.

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7. Three phases.(Figure 10.18, Campbell, page 194)

Phase 1: Carbon fixation

CO2 is attached to a 5C sugar, ribulose

bisphosphate (RuBP).

Catalyzed by RuBP carboxylase or

rubisco. Most abundant protein in chloroplasts

and on Earth.

6C intermediate formed is unstable and

splits in half to form two 3C molecules of

3-phosphoglycerate for each CO2.

Phase 2: Reduction

Each 3-phosphoglycerate receives another

phosphate group from ATP to form 1,3-

bisphosphoglycerate.

A pair of electrons from NADPH reduces each

1,3-bisphosphoglycerate to G3P (3C).

Electrons reduce a carboxyl group to

aldehyde group of G3P, which stores

more potential energy.

For every three CO2 (3C) and three RuBP

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(15C), six molecules of G3P (18C) are formed.

One of these six G3P (3C) is a net gain of

carbohydrate. Out of the 6 molecules of

G3P, one will be converted tosugar/carbohydrate.

This molecule exit cycle to be

used by plant cell.

Phase 3: Regeneration

The other five G3P (15C) remain in cycle to

regenerate three RuBP. Carbon skeletons offive molecules of G3P are rearranged in a

series of reactions to regenerate threemolecules of RuBP.

Three ATP used.

For net synthesis of one G3P molecule, the

Calvin cycle consumes nine ATP and six

NADPH.

G3P from Calvin cycle is the starting material

for metabolic pathways that synthesize other

organic compounds, including glucose and

other carbohydrates.

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6.5.2 PhotorespirationOne major problem facing terrestrial plants is

dehydration.Stomata - major route for gas exchange (CO2

in; O2 out), and for evaporative loss of water.

On hot, dry days, plants close stomata to

conserve water.

In C3 plants, initial fixation of CO2 occursvia rubisco, forming 3C compound, 3-

phosphoglycerate.

C3 plants - rice, wheat, and soybeans.

When stomata partially close on a hot, dry

day, CO2 levels drop as CO2 is used in Calvin

cycle.

At same time, O2 levels rise as light reaction

converts light to chemical energy.

Rubisco normally accepts CO.

But, when O2:CO2 ratio increases (on a hot,

dry day with closed stomata), rubisco can

add O2 to RuBP.

RuBP then splits into a 3C piece (3-

phosphoglycerate) and a 2C piece

(phosphoglycolate) in a process called

photorespiration.

2C fragment(phosphoglycolate) is

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exported from chloroplast and degraded

to CO2 by mitochondria and peroxisomes.

1. Unlike normal respiration, this process

produces no ATP, but consumesATP.

2. Unlike photosynthesis,

photorespiration does not produceorganic molecules, but decreases

photosynthetic output by siphoning

organic material from Calvin cycle.

Hypothesis for the existence of

photorespiration metabolic relic from a

much earlier time:

When rubisco first evolved, atmosphere

had far less O2 and more CO2 than it does

today.

Inability of active site of rubisco to

exclude O2 would have made little

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O2

+

RuBP

Rubisco 3-Phosphoglycerate

+

Phosphoglycolate

CO2

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difference.

Today it does make a difference.

Photorespiration can drain away as much

as 50% of carbon fixed by Calvin cycle on

a hot, dry day.

6.4.3 C4 / Hatch-Slack pathway

Certain plant species have evolved alternate

modes of CO2 fixation to minimize

photorespiration.

C4 plants first fix CO2 in a 4C compound.

Example of C4 plant: sugarcane and corn.

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A unique leaf anatomy is correlated with

mechanism of C4 photosynthesis.(Figure 10.19, Campbell, page 196)

C3 plants

C4 plants

Two types of photosynthetic cells in C4

plants:

(a) bundle-sheath cells(b) mesophyll cells.(a) Bundle-sheath cells - arranged into

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tightly packed sheaths around veins

of leaf.

(b) Mesophyll cells - more looselyarranged cells located between

bundle sheath and leaf surface.

Calvin cycle C3:-

confined to chloroplasts ofbundle-sheath

cells. (Figure 10.19, Campbell, page 196)

Mesophyll Cell :- C4

Phosphoenolpyruvate carboxylase (pepco),

adds CO2 to phosphoenolpyruvate (PEP) to

form oxaloacetate (OAA) in mesophyll cells.

PEP carboxylase has very high affinityfor CO2 and can fix CO2 efficiently when

rubisco cannot (i.e., on hot, dry days

when stomata are closed).

OAA converted to malate (4C).

Malate exported into bundle-sheath cells.

Malate is decarboxylated forming

pyruvate, releasing CO2.

Rubisco starts Calvin cycle, using

abundant supply of CO2.

Pyruvate returns to mesophyll cells and

regenerated to PEP. ATP is used.

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By pumping CO2 into bundle sheath cells, CO2

levels are kept high for rubisco to accept CO2

and not O2.

Photorespiration is minimized; sugar

production is enhanced.

C4 plants thrive in hot regions with intense

sunlight.

6.5.4 Crassulacean Acid /CAM pathway(Figure 10.20, Campbell, page 197)CAM plants - cacti, pineapples, and several

other plant families.

CAM - crassulacean acid metabolism.

Open stomata at night and

close stomata during the day. Temperatures lower at night,

and humidity is higher.

Night - plants fix CO2 into a variety of

organic acids in mesophyll cells.

Organic acids stored in vacuoles.

Day - light reactions supply ATP and

NADPH; CO2 is released from the organic

acids and enters Calvin cycle.

Both C4 and CAM plants add CO2 into organic

intermediates before it enters Calvin cycle.

In C4 plants, carbon fixation and Calvin

cycle are spat ia l l y separated.33

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In CAM plants, carbon fixation and Calvin

cycle are tempora l l y . . pemi sahanm a s a = s i a n g m a l a m separated.

Both eventually use Calvin cycle to make

sugar from CO2.

6.6 Factors affecting photosynthesisrate

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Rate of photosynthesis may be measured

by quantity of CO2 consumed per unit time.

or O2 released per unit time

(1) Light intensity

As light intensity increases, photosynthetic

rate increases until a point is reached where

rate begins to level off.

At low light intensity, photosynthesis occurs

slowly because only a small quantity of ATP

and NADPH is created by the light dependentreactions.

As light intensity increases, more ATP and

NADPH are created, thus increasing the

photosynthetic rate.

At high light intensity, photosynthetic rate

levels out, not due to light intensity but due

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to other limiting factors, e.g., competition

between O2 & CO2 for active site on RUBP

carboxylase.

(2) CO2 concentration

As CO2 concentration increases, rate ofphotosynthesis increases.

At high concentrations, rate of

photosynthesis begins to level out due to

factors not related to carbon dioxide

concentration.

Example, some enzymes of

photosynthesis might be working at

their maximum rate.

In general, CO2 is found in low concentration

in atmosphere.

So, atmospheric CO2 levels may be a major

limiting factor on photosynthesis when at low

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levels.

(3) Temperature

As temperature increases above

freezing, rate of photosynthesis

increases.

This occurs because molecules

are moving more quickly and there is

a greater chance of a collision

resulting in a chemical reaction.

At some point, a temperature isreached that is an optimum

temperature.

Photosynthetic reaction rate is at its

quickest rate at this point.

Above that temperature, enzymes begin

to denature (as in RUBP carboxylase),

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slowing rate of photosynthesis until a

temperature is reached where

photosynthesis does not occur at all.

(4) Light duration

Photosynthesis is only affected by light

duration in that it only occurs during periods

of light.

(5) Level of air pollution

Low levels of O3 and SO2 are very damaging

to some plant leaves.

Soot can block stomata and prevent light

from reaching chloroplasts by coating leaf.

(6) Oxygen

Competes with CO2 for active site of RuBP

carboxylase.

Relatively high concentrations of O2, for

example the 21% in our atmosphere, inhibit

photosynthesis.

O2 does not inhibit CO2 fixation in C4 plants.

(7) Water

Slight lack of water can lead to severe loss

of carbohydrate yield.

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(8) Chlorophyll concentration

Can become a limiting factor if

chlorophyll levels are abnormally low.

May be caused by disease, mineral

deficiency, and senescence.

Iron, magnesium, nitrogen, and sunlight

are necessary for chlorophyll

production - lack of any one of these

can lead to yellowing of leaves.

Compensation Point Point during photosynthesis where

rate of photosynthesis exactly matches

rate of respiration, i.e., input of CO2 or O2 issame as output of the other by a plant.

Point is reached during early

mornings and late evenings.

Summary of compensation point:

Rate of photosynthesis = rate of

respiration. Amount of O2 produced by plant is equal

to amount used up at that point in time.

Thus, there is no net output of O2 by the

plant.

There is nil effective photosynthesis.

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