notes photosynthesis 2010

8/6/2019 Notes Photosynthesis 2010

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Cellular Energy

Photosynthesis


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How do plants make energy & food?

Plants use the energy from the sun

to make ATP energy to make sugars

glucose, sucrose, cellulose, starch, & more

sun

ATP

sugars


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Overview of Photosynthesis

Photosynthesis involves 2

energy conversions: Conversion of light energy into

chemical energy (The LightReactions)

Storage of chemical energy in

the form of sugars (The Calvin

Cycle)


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Chloroplasts organelles ofphotosynthesis

Leaves

Chloroplastscontain

Chlorophyll

Chloroplastsin cell

sun

Pigments in chloroplast absorb

sunlight. (All colors?)


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Chloroplasts- organelles ofphotosynthesis

Thylakoids:

Membranes containing

pigments

Granum: a stack of

thylakoids

Stroma: Fluid

around Grana


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Chloroplasts- organelles of

photosynthesis

Pigments light absorbing

substances. Found in thylakoidmembranes of chloroplasts. Absorbwaves of visible light, reflect thecolor seen by your eye

Chlorophyll a and b Major pigments

involved in photosynthesis. Greenpigments, absorb all colors exceptgreen.

Accessory pigments absorb green.

Visible in autumn, when chlorophyll

breaks down.


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Any Questions?

H2Osugars

ATP

enzymes

CO2

sun


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Photosynthesis: 2 Stages

Light Reactions: in

thylakoid membranes Sunlight ATP +

NADPH

Calvin Cycle: in

stroma

NADPH + ATP Sugar


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The Light Reactions

Converts visible light into chemical energy carried by

electrons in high-energy molecules (ATP and NADPH) Splits an H2O. Uses H, releases O.

Next, we zoom in to look more

closely at the thylakoids!


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1. Light strikes the first photosystem (PSI), causing it to transfer

excited e- to the primary electron acceptor. These e- are replaced

by splitting H2O, which releases O2 as a product.

1

2

3

4


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1. Light strikes the first photosystem

(PSI), causing it to transfer excited e-

to the primary electron acceptor. These

e- are replaced by splitting H2O, which

releases O2 as a product.

2. The excited e- travel down and e-transport

chain. This process pumps H+ ions across the

membrane into the thylakoid.

1

2

3

4


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2. The excited e- travel down

and e-transport chain. This

process pumps H+ ions across

the membrane into the

thylakoid.

3. Light-excited e-

in PSII are

transferred to

NADP+. These e-are replaced by

those coming from

e- transport chain.

1

2

3

4


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2. The excited e- travel down

and e-transport chain. This

process pumps H+ ions across

the membrane into the

thylakoid.

3. Light-excited e- in PSII are

transferred to NADP+. These e- are

replaced by those coming from e-

transport chain.

4. Thebackflow of

hydrogen ions

out of the

thylakoid pass

through ATP

synthase,powering ATP

production.

1

2

3

4


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The Light Reactions Another View


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The Light Reactions - Summary

Light absorbed into thylakoidmembrane by pigments in

photosystems PSI and PSII (see figure4.10)

Light energy is transferred to thereaction center within thephotosystem.

H2O splits apart at PSII (2H2O 4H+ +

4e- + O2) Oxygen diffuses out of the plant

Protons (H+): Transported to the thylakoid

Electrons (e-): electron transport chain toPSI


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The Light Reactions Summary cont.

Formation of NADPH (use e- from H2O splitting)

Electrons reach PSI

Used to join NADP+ with H+ NADPH

This high-energy molecule will be used later for the Calvin

cycle.

Build-up of H+ ions

Energy electrons received from reaction center is used inactive transport to pump protons into thylakoid

H+ build up inside thylakoid, result in potential energy

difference (like a battery)


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Review: Light Reaction

H2O + sunlight O2 + ATP +

NADPH To see this in action, check out the

YouTube video:Light Dependent

Reactions

2 Unstablecompounds

formed. Must be

converted to

C6

H12

O6

for

storage!


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The Calvin Cycle (Dark Reactions)

Occurs in stroma

of chloroplast. Energy from ATP

and NADPHmoleculesconverted tochemical bondswithin glucose forstorage.


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The Calvin Cycle

1

2

3

1. Carbon Fixation:

CO2 taken in

through leaves.

Combines with

RuBP, a 5-Carbon

sugar-phosphate,

by enzyme rubisco.

CO2 gas

is fixed

into anorganic

molecule.

6-Carbon

sugar

formed,

immediate

ly splits

into two

3-C PGA

molecules

4


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The Calvin Cycle

1

2

3

1. Carbon Fixation:

CO2 taken in through

leaves. Combines with

RuBP, a 5-Carbon

sugar-phosphate, by

enzyme rubisco.

CO2 gas is

fixed into

an organic

molecule.

6-Carbon

sugar

formed,

immediately

splits into

two 3-C PGA

molecules2. ATP and

NADPH

used torearrange 6

PGA

molecules

into 6 G3P

(PGAL).

4


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The Calvin Cycle

1

2

3

1. Carbon Fixation:



RuBP, a 5-Carbon

sugar-phosphate, by

enzyme rubisco.

CO2 gas is

fixed into

an organic

molecule.

6-Carbon

sugar

formed,

immediately

splits into

two 3-C PGA

molecules2. ATP and NADPH

used to rearrange 6

PGA molecules into6 G3P (PGAL).

3. One PGAL released,

remaining 5 stay in Calvin

cycle.

2 PGAL 1 Glucose

4


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The Calvin Cycle

1

2

3

1. Carbon Fixation:



RuBP, a 5-Carbon

sugar-phosphate, by

enzyme rubisco.

CO2 gas is

fixed into

an organic

molecule.

6-Carbon

sugar

formed,

immediately

splits into

two 3-C PGA

molecules2. ATP and NADPH

used to rearrange 6

PGA molecules into6 G3P (PGAL).

3. One PGAL released, remaining 5 stay in

Calvin cycle.

2 PGAL 1 Glucose

4. Remaining 5

PGAL

rearranged,

turned backinto 3 RuBP

molecules.

Calvin cycle

complete, can

start overagain. 4


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Calvin Cycle - Summary

ATP + NADPH PGAL or Glucose

Carbon fixation by rubisco

Rearrange molecules to produce a PGAL and get back to

starting molecule (RuBP)

YouTube video: The Calvin Cycle by Prentice

Hall

Any Questions??


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Quizzam!! Working with your neighbor, write a 1-sentence

summary explaining The importance of the light reactions

The importance of the Calvin cycle


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Oxidation and Reduction (Redox)Reactions

Oxidation: Removal of electrons from a molecule(decomposition)

Reduction: Addition of electrons (synthesis)

In photosynthesis: Light energy is used to split water.The water molecule looses an electron (it is oxidized).The 1st protein in electron transport chain gains the e-

(is reduced).

oxidation

2H2O O2 + 4H+ + 4e

reduction


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Redox Reactions in Photosynthesis

The oxidation of water is accompanied by a

reduction reaction resulting in the formationNADPH.

NADP+ + H20 NADPH + H+ + O

(oxidized form) (reduced form) (oxygen)


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Redox Reactions in Photosynthesis

Addition of a phosphate group (labeled, as Pi)

to ADP during the light reaction is calledphotophosphorylation.

ADP + Pi ATP


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How are products of photosynthesis

used?

PGAL produced in Calvin cycle can be used in manyways: Synthesized into larger carbohydrates: glucose, sucrose

Modified to make amino acids, glycerol

sugars used by leaf cell, or transported to other cellsin plant.

This occurs in chloroplasts, cell, or other organisms ifplant is eaten.


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Rate of Photosynthesis

Measured by CO2 consumed per unit time or

by O2 produced. Bromthymol Blue indicator. Turns yellow in the

presence of CO2.

An acid-base indicator: H2O + CO2 H2CO3 (l)

In water, CO2 creates carbonic acid.


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Photorespiration

O2 enters Calvin Cycle

Enzyme Rubisco fixes CO2 in Calvin Cycle. Due toshape, rubisco can bind with either O2 or CO2.

CO2 binds to rubisco 2 PGA molecules

O2 binds to rubisco 1 PGA and 1 2-C acid (glycolate).

Plant loses fixed carbons instead of gaining.

Benefits unknown.

Uses O2 and liberates CO2, which is why it is called

photorespiration.


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Photorespiration

Weather and Photosyntheis

In hot, dry weather, plants close leaf openings calledstomates. Helps reduce water loss. No CO2 enters.Photorespiration is favored.

Light reactions release oxygen high light intensities and

high temperatures (above ~ 30C)favor photorespiration.


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Photorespiration: C3,C4 and CAM

plants

All plants carry out photosynthesis byadding CO2 to a 5-carbon sugar.

Reaction is catalyzed by the enzymeRUBISCO.

Results in production of PGA (startingmolecule for glucose)

The process is called the Calvin cycle

and the pathway is called the C3pathway. PGA has 3 carbons. C3


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Photorespiration - C3 Plants

C3 Plants Plants that use onlytheCalvin Cycle to fix CO

2

. Make up 90% of plants on Earth

Wheat, rice, soy

Most vulnerable to high O2concentrations.

Some plants have evolved strategiesfor increasing photosynthesis, andreducing photorespiration: C4 andCAM plants.


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C4 Plants Limit Photorespiration

The C4 cycle: Structural changes in

leaf anatomy

- C4 and C3 pathways are

separated in different parts of the

leaf

- RUBISCO sequestered where the


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C4 Plants Limit Photorespiration

C4 plants are well adapted to habitats with

(1) high daytime temperatures and (2)intense sunlight.

Some examples:

crabgrass

corn (maize)

sugarcane


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CAM Plants Limit Photorespiration

How are

C4 and

CAMDifferent?


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CAM plants separate their C3 and C4 cycles by time. *Note: C4 plants separate C3 and C4 cycles by

location. CAM plants separate cycles by time of day. At night,

CAM plants take in CO2 through their stomata

CO2 is fixed into a 4-carbon molecule that accumulates in thecentral vacuole of the cells.

In the morning, the stomata close conserving moisture

The accumulated 4-C molecule leaves the vacuole andis broken down to release CO2.

The CO2 is taken up into the Calvin (C3) cycle.


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CAM plants well adapted to conditions of

(1) high daytime temperatures (2) intense sunlight, and

(3) low soil moisture.

Examples: cacti, aloe, pineapple, bryophyllum

CAM least efficient system. These plantsgrow slowly.


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Evolution ofC4 and CAM Plants

When photosynthetic organisms first evolved,the atmosphere contained no oxygen. Rubisco did not need to differentiate.

More photosynthesis occurring more O2 inthe air.

Plants adapted by developing other pathwaysto fix carbon C

4cells in C

3plants


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Photosynthesis Summary

notes photosynthesis 2010

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