Ch10_Photosynthesis

8/7/2019 Ch10_Photosynthesis

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Chapter 10

Photosynthesis


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Photosynthesis Occurs in Chloroplasts

Stoma GasExchange in Leaves

Chloroplasts are found mainly

in mesophyll cells


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Photosynthesis

Occurs inChloroplasts

Chloroplasts

Inner and OuterMembrane

Stroma

Fluid surrounding

thylakoidsGrana

Stacks of thylakoid

membranes


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Photosynthesis

Overall equation:

6CO2 + 6H20 C6H12O6 + 6O2

Glucose is made from CO2 (air)Oxygen given off during splitting of water


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Light Reaction

Light and water taken in

NADP+ and ADP needed Reduced to ATP and

NADPH

O2 given off as byproduct

Calvin Cycle

CO2 taken in

ATP and NADPHneeded

Oxydized to NADP+ and

ADP

Sugar produced (G3P)

2 Stages


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The Light Reactions

Overview

Light reactions convertsolar energy to

chemical energy

Water is splitElectrons from H2O travel

down electron transport

chainATP produced by

Chemiosmosis

NADP+ reduced NADPH

O2 given off


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Calvin Cycle Overview

CO2 is assimilated

into organic molecule

(carbon fixation)Fixed carbon is

reduced to

carbohydrate

Energy from ATP

Electrons (reducing

power) from NADPH


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Pigments in Chloroplasts Absorb Visible

LightChlorophyll A Blue

Green reflected

Chlorophyll B

Yellow Greenreflected

Accessory Pigments

other wavelengthsCarotenoids

Yellow/Orange


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Photosystems in the

Thylakoids

Harvest Light Energy

Photosystems

Chlorophyll, proteins

Antenna for reaction center

Collect and focus light

energy, pass from

chlorophyll

chlorophyll

reaction center

Reaction Center

2 molecules of chlorophyll a

Primary electron acceptor

Photon of light hits pigment,

energy is passed along to

reaction center. At RC and excited

e- from special chlorophyll iscaptured by primary e- acceptor


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Photosystem II (Comes First)

1. Photon strikes pigmentmolecule. Energypassed along.

2. Electron passed toprimary electron

acceptor.3. Water molecule is split

2e- + 2H + O1. Electrons replace lost

electrons in special

chlorophyll2. 2 oxygens combine, lost as

gas

3. H contribute toconcentration gradientacross thylakoid membrane


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Non-Cyclic Electron Flow

What happens after Photosystem II?


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Non-Cyclic

Electron Flow

4. Photoexcited e- move

from PSII to PSI down an

electron transport chain

5. This fuels ATP synthesis

6. Light hits and excites e- in PSI. When electron from

PSII reaches PSI, it fills a hole caused by light

exciting RC of PSI

7. Photoexcited e- from PSI falls down a second

transport chain

8. This electron is used to reduce NADP+ (final e-

acceptor)


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Cyclic Electron Flow

Occurs when

NADPH builds

up in stroma

Negative

feedback

Produces

more ATP,but no

NADPH


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In chloroplasts and mitochondriaProteins in a membrane arranged from low high

electronegativity

Electrons fall down toward a final e- acceptorH+ ions pumped across membrane as e- fall

H+ diffuse down a concentration gradient, through ATPsynthase enzyme

ATP synthase performs oxydative phosphorylation to reduceADP ATP

Chemiosmosis A review


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Chemiosmosis Using membranes to

couple redox to ATP synthesis

In MitochondriaEnergy and e- come

from food molecules

H+ pumped into IMS,diffuse into matrix

In chloroplastsEnergy from Sun, e-

from water

H+ pumped intothylakoid, diffuse intostroma (location ofCalvin cycle)


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Chemiosmosis in Light Reactions

Be the narrator: explain this picture to your neighbor

Summarize LR


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Calvin Cycle

CO2 taken in

Fixed to RuBP by

enzyme Rubisco

Major steps

Carbon fixation

Reduction 3 CO2 1G3P

Regeneration of

RuBP


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Calvin Cycle

EnergyAccounting

To make 1 G3P

9 ATP6 NADPH

The fate of G3P

Sugars used for energy

Synthesis of organic molecules(carbon skeleton, building blocks)

Cellulose

Fatty acids, amino acids

Extra sugars stored as starch


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PhotorespirationHot dry days, plants partly close stomata

Minimize water loss via transpiration

Also reduces intake of CO2, and removal

of O2In high concentrations, O2 can be fixed

into Calvin cycle by rubisco.

Results in photorespiration: Uses energy

to make a 2-C compound that is broken

down, CO2 released as waste- Carbon lost

- Energy used

- no known benefit to plants

- possibly an evolutionary relic

when atmosphere had little O2, no

benefit for Rubisco differentiating


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C4 Plants Adaptationsagainst

photorespiration

To avoid fixation of O2 by rubiscoCalvin cycle takes place in bundle

sheath cells

Carbon fixed in mesophyll cells by

another enzyme (non rubisco)Forms a 4-C molecule: oxaloacetate

4-C product is then moved into bundle

sheath cell through plasmodesmata

CO2 released, enters the Calvin Cycle


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Adaptations Against Photorespiration

C4 Plants separatecarbon fixation from the

Calvin Cycle by location

CAM plants also have

an adaptation to avoid

photorespiration; they

separate these 2

phases by time.Carbon is fixed at night

when stomata are open

Calvin cycle occurs during

the day


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Photosynthesis Summary

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