Chapt08 Lecture Photosynthesis 4 1

8/12/2019 Chapt08 Lecture Photosynthesis 4 1

1/61

Copyright (c) The McGraw-Hill

Companies, Inc. Permission requiredfor reproduction or display. 1

CHAPTER 8

PHOTOSYNTHESIS

Prepared by

Brenda Leady, Universi ty of Toledo


2/61

2

PHOTOSYNTHESISSWBAT:

Describe overall equation of photosynthesis

Distinguish: autotrophs, heterotrophs, photoautotrophs andchemoautotrophs

Describe stages of photosynthesis (light-dependent reactionsand Calvin Cycle)

Outline chemiosmosis in photophosphorilation

Describe alternative ways of Carbon fixation (C4 and CAM

plants)


3/61

3

PHOTOSYNTHESISSWBAT:

Describe overall equation of photosynthesis

Distinguish: autotrophs, heterotrophs, photoautotrophs andchemoautotrophs

Describe stages of photosynthesis (light-dependent reactionsand Calvin Cycle)

Outline chemiosmosis in photophosphorilation

Describe alternative ways of Carbon fixation (C4 and CAM

plants)


4/61

4

Trophic organization

Heterotroph

Must eat food, organic molecules from their

environment, to sustain lifeAutotroph

Make organic molecules from inorganicsources Photoautotroph

Use light as a source of energy

Green plants, algae, cyanobacteria


5/61

5

Photosynthesis

Energy within light is captured and used to

synthesize carbohydrates

CO2+ H2O + light energy C6H12O6+ O2+ H2O

CO2is reduced H2O is oxidized

Energy from light drives this endergonic

reaction


6/61

6

Link between Photosynthesis and Respiration:


7/61

7

Who does it?

Plants

Algae

Cyanobacteria

Photosynthetic bacteria


8/61

8


9/61

9

Biosphere

Regions on the surface of the Earth and in

the atmosphere where living organisms

exist Largely driven by the photosynthetic

power of green plants

Cycle where cells use organic moleculesfor energy and plants replenish those

molecules using photosynthesis

Plants also produce oxygen


10/61

10

Chloroplast

Organelles in plants and algae that carry

out photosynthesis

Chlorophyll- green pigment

Majority of photosynthesis occurs in

leaves in central mesophyll

Stomata- carbon dioxide enters and

oxygen exits leaf


11/61

11

Chloroplast anatomy

Outer and inner membrane

Intermembrane space

Thylakoid membrane contains pigmentmolecules

Thylakoid membrane forms thylakoids

Enclose thylakoid lumen

Granum- stack of thylakoids

Stroma- fluid filled region betweenthylakoid membrane and inner membrane


12/61

12
http://localhost/var/www/apps/conversion/tmp/scratch_7/chapt04_lecture-2.ppt


13/61

1313


14/61

14

2 stages of photosynthesis

Light reactions

Take place in thylakoid membranes

Produce ATP, NADPH and O2

Calvin cycle

Occurs in stroma

Uses ATP and NADPH to incorporate CO2into organic molecules


15/61

15


16/61

16

Light energy

Type of electromagnetic radiation

Travels as waves

Short to long wavelengths

Also behaves as particles- photons

Shorter wavelengths have more energy


17/61

17


18/61

18

Photosynthetic pigments absorb some lightenergy and reflect others

Leaves are green because they reflect green

wavelengths

Absorption boosts electrons to higher energylevels

Wavelength of light that a pigment absorbs

depends on the amount of energy needed to

boost an electron to a higher orbital


19/61

19


20/61

20

After an electron absorbs energy, it is anexcited state and usually unstable

Releases energy asHeatLight

Excited electrons in pigments can be

transferred to another molecule orcaptured

Captured light energy can be transferredto other molecules to ultimately produce

energy intermediates for cellular work


21/61

21

Pigments

Chlorophyll a

Chlorophyll b

Carotenoids


22/61

22

Spectrophotometers are used tomeasure light absorption

White

light

Refracting

prism

Chlorophyll

solution Photoelectric

tube

The low transmittance

(high absorption)reading indicates that

chlorophyll absorbs

most blue light.

Slit moves to

pass light

of selectedwavelength

Blue

light


23/61

23

Absorption vs. action spectrum Absorption spectrum

Wavelengths that are absorbed bydifferent pigments in the plant

Action spectrumRate of photosynthesis by wholeplant at specific wavelengths

The color of the pigment comes from the

wavelengths of light reflected (in other words, those

not absorbed). Chlorophyll, the green pigment

common to all photosynthetic cells, absorbs all

wavelengths of visible light except green


24/61

24

The color of the pigment comes

from the wavelengths of light

reflected (in other words, those

not absorbed). Chlorophyll, the

green pigment common to all

photosynthetic cells, absorbs all

wavelengths of visible light

except green


25/61

25

Photosystems

Thylakoid membrane:

o Photosystem I (PSI)

o Photosystem II (PSII)

Each of them has a light harvesting complex

(antenna complex) and a reaction center


26/61

26

Photosytem II (PSII)

2 main components Light-harvesting complex or antenna complex

Directly absorbs photons

Energy transferred via resonance energy transfer to P680in the reaction center

Reaction center P680 P680* (Relatively unstable)

P680* actually releases its high-energy electronto theprimary electron acceptor and becomes P680+(more stable).

P680 has to be regenerated, by replacing the electron so

P680 can work again: : This missing electron of P680+isreplaced with a low-energy electronfrom water which yieldsoxygen gas


27/61

27


28/61

28

Photosystem II (PSII)

Redox machine

Recent research in biochemical

composition of protein complex and role ofcomponents

3 dimensional structure determined in

2004 using x-ray crystallography


29/61

29

D1 and D2, contain the reaction center that carries out the redox reactions

CP43 and CP47, bind the pigment molecules that form the light-harvesting complex wraparound D1 and D2 so that pigments in CP43 and CP47 can transfer energy to P680 byresonance energy transfer.Pp: primary electron acceptor: a chlorophyll molecule lacking Mg2+, called pheophytin (Pp).QA:plastoquinone molecule, designated QAQBanother plastoquinone molecule which can accept two high-energy electrons and bind

two H+

. QBcan diffuse away from the reaction center.


30/61

30

The oxidation of water occurs in a region called the manganese cluster. This site is

located on the side of D1 that faces the thylakoid lumen.The manganese cluster has four Mn2+, one Ca2+, and one Cl.Two water molecules bind to this site.D1 catalyzes the removal of four low-energy electrons from the two water moleculesto create four H+and O2.Each low-energy electronis transferred, one at a time, to an amino acid in D1 (a

tyrosine, Tyr) and then to P680+to produce P680.


31/61

31

Electrons accepted by primary electron acceptorin PSII are transferred to a pigment molecule in

the reaction center of PSI

Electron releases some of its energy along the

way

Establishes H+electrochemical gradient

ATP synthesis uses chemiosmotic mechanism similar

to mitochondria


32/61

32

Photosystem I (PSI)

Key role to make NADPH

Light striking light-harvesting complex ofPSI transfers energy to a reaction center

High energy electron removed from P700and transferred to a primary electronacceptor

NADP+reductaseNADP++ 2 electrons + H + NADPH

P700+replaces its electrons fromplastocyanin

No splitting water, no oxygen gas formed


33/61

33

Summary1. O2produced in thylakoid lumenby oxidation of

H2O by PSII

2 electrons transferred to P680+

2. ATP produced in stroma by H+electrochemicalgradient(chemiosmosis) due to:

1. Splitting of water places H+in the lumen

2. High-energy electrons move from PSII to PSI,pumping H+into the lumen

3. Formation of NADPH consumes H+in the stroma

3. NADPH produced in the stromafrom high-energyelectrons that start in PSII and boosted in PSI

NADP++ 2 electrons + H + NADPH


34/61

34

QBcan diffuse away from the reaction center PSII carrying a pair of electronsEach electron enters an electron transport chain: a series of electron carriers located in the thylakoidmembrane. The electron transport chain functions similarly to the one found in mitochondria.The electrons go from QB, to a cytochrome complex, then to plastocyanin (Pc), a small protein; and then toPSIAlong its journey from PSII to PSI, the electron releases some of its energy at particular steps and is transferred tothe next component that has a higher electronegativity. In the cyochrome complex the energy released is harnessedto pump H+into the thylakoid lumen. One result of the electron movement is to establish a H+electrochemical gradient

From FSII to FSI


35/61

35

When light strikes the light-harvesting complex of PSI, this energy is also transferred to a reactioncenter, where a high-energy electron is removed from a pigment molecule, designated P700, andtransferred to a primary electron acceptor.

A protein called ferredoxin (Fd) can accept two high-energy electrons, one at a time, from the primaryelectron acceptor.

Fd then transfers the two electrons to the enzyme NADP+reductase. This enzyme transfers the twoelectrons to NADP+and together with a H+creates NADPH. The formation of NADPH results in fewerH+in the stroma and thereby contributes to the formation of a H+electrochemical gradient across thethylakoid membrane.

FSI


36/61

36

Synthesis of ATP in chloroplasts is achieved by a chemiosmotic mechanism similar to that used tomake ATP in mitochondria.

ATP synthesis is driven by the flow of H+from the thylakoid lumen into the stroma via ATPsynthase

A H+gradient is generated in three ways:(1) the splitting of water, which places H+in the thylakoid lumen;(2) the movement of high-energy electrons from photosystem II to photosystem I, which pumps H+into

the thylakoid lumen(3) the formation of NADPH, which consumes H+in the stroma.

Chemiosmosis


37/61

37

Cyclic and noncyclic electron flow

Noncyclic

Electrons begin at PSII and eventually

transfer to NADPHLinear process produces ATP and NADPH in

equal amounts

Cyclic photophosphorylation

Electron cycling releases energy to transportH+into lumen driving synthesis of ATP


38/61

38

Cyclic photophosphorylation

Electron cycling releases energy to transport H+into lumen driving synthesis of ATP.

NADPH IS NOT produced

H2O is not splitter and O2 is not produced


39/61

39

Photosystems II and I work together to produce

ATP and NADPH


40/61

40

Z scheme (energy curve) Robin Hill and Fay Bendall


41/61

41

Photosynthesis animations

Photosynthetic Electron Transport and ATP Synthesis
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesishttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis


42/61

The cytochrome complexes of mitochondria and

chloroplasts have evolutionarily related proteins in

common

Homologous genes are similar because theyare derived from a common ancestor

Comparing the electron transport chains ofmitochondria and chloroplasts revealshomologous genes

Family of cytochrome b-type proteins plays

similar but specialized roles


43/61


44/61

44

Calvin cycle

ATP and NADPH used to make

carbohydrates

Somewhat similar to citric acid cycle

CO2incorporated into carbohydrates

Precursors to all organic molecules

Energy storage


45/61

45

Overview of Calvin Cycle


46/61

46

CO2incorporation

Also called Calvin-Benson cycle

Requires massive input of energy: For every 6CO

2

incorporated, 18 ATP and 12 NADPH used

Glucose is not directly made. Instead, moleculesof glyceraldehyde-3-phosphate, which areproducts of the Calvin cycle, are used as startingmaterials for the synthesis of glucose and othermolecules, including sucrose. After glucose molecules aremade, they may be linked together to form a polymer of glucose called starch, whichis stored in the chloroplast for later use. Alternatively, the disaccharide sucrose maybe made and transported out of the leaf to other parts of the plant.


47/61

47

3 phases

1. Carbon fixation CO2incorporated in ribulose bisphosphate (RuBP)

using RuBP carboxylase/oxygenase (rubisco)

6 carbon intermediate splits into 2 3PG

2. Reduction and carbohydrate production ATP is used to convert 3 phosphoglycerate (3PG)

into 1,3-bisphosphoglycerate (1,3 BPG)

NADPH electrons reduce it to glyceraldehyde 3 P(G3P)

6 CO2 12 G3P 2 for carbohydrates

10 for regeneration

3. Regeneration of RuBP

10 G3P converted into 6 RuBP using 6 ATP


48/61

48


49/61

49

Photosynthesis animations

calvinCycle.swf
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/calvinCycle.swf::calvinCycle.swfhttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/calvinCycle.swf::calvinCycle.swf


50/61

The Calvin cycle was determined by isotope

labeling methods

14C-labeled CO2injected into cultures of greenalgae

Allowed to incubate different lengths of time

Separated newly made radiolabeled moleculesusing two-dimensional paper chromatography

Autoradiography- radiation from 14C-labeled

molecules makes dark spots on the film Identified 14C-labeled spots and the order they

appeared

Calvin awarded Nobel Prize in 1961


51/61


52/61

52

Variations in photosynthesis

Certain environmental conditions can

influence both the efficiency and way the

Calvin cycle worksLight intensity

Temperature

Water availability


53/61

53

Photorespiration

RuBP + CO22 3PGRubisco functions as a carboxylase

C3plants make 3PG

Rubisco can also be an oxygenaseAdds O2to RuBP eventually releasing CO2

Photorespiration

Using O2

and liberating CO2

is wasteful

More likely in hot and dry environment

Favored when CO2low and O2high

Rubisco: RuBP carboxylase/oxygenase


54/61

54Wheat plants Oak leaves

C3 plants

90% of plants are C3


55/61

55

C4plants: To avoid photorespiration

C4plants make a 4 carbon compound inthe first step of carbon fixation

Hatch-Slack pathway

Leaves have 2-cell layer organizationMesophyll cells

CO2enters via stomata and 4 carbon compoundformed (PEP carboxylase does not promote

photorespiration)Bundle-sheath cells

4 carbon compound transferred that releasessteady supply CO2


56/61

56

C4 plants

In warm and dry climates, C4plants have an advantage. During the day, they cankeep their stomata partially closed to conserve water. Furthermore, they can avoidphotorespiration. C4plants are well adapted to habitats with high daytimetemperatures and intense sunlight.

Examples of C4plants are sugarcane, crabgrass, and corn.


57/61

57

CAM plants

Some C4plants separate processes using time

Crassulacean Acid Metabolism

CAM plants open their stomata at night CO2enters and is converted to malate

Stomata close during the day to conserve water

Malate broken down into CO2to drive Calvincycle


58/61

58

Process in separated cells Process at different times

C4 CAM


59/61

59

Adaptations for hot weather: C4 and CAM

plants

C4: corn, sugarcane andsorghum

CAM: succulents (aloe, jade),pineapple, cactiCAM = crassulasean acid

metabolism


60/61

60

C4 and CAM compared

Both fix CO2into a C4 compound

Then CO2is transferred to the Calvin cycle

In C4 plants there is a spatial separation (2 celltypes)

In CAM plants there is a temporal separation (C4accumulates at night, Calvin cycle during theday)


61/61

What is better? C3 or C4 strategy?

In warm dry climates C4plants have the

advantage in conserving water and preventing

photorespiration

In cooler climates, C3plants use less energy to

fix CO2

90% of plants are C3

Chapt08 Lecture Photosynthesis 4 1

Documents

Transcript of Chapt08 Lecture Photosynthesis 4 1