Photosynthesis in Detail

Light Reactions and Calvin CycleHonors Biology

The Two Stages of Photosynthesis: A Preview

• Photosynthesis consists of two processes1) The Light reactions

•NEEDS LIGHT• Light Dependent Reactions

2) The Calvin cycle•A.k.a- Dark Reactions or Light Independent Reactions •DOES NOT NEED LIGHT

The Light ReactionsOccur in the grana (& thylakoids)

Convert solar energy to chemical energy

Chlorophyll absorbs solar energy Split water release oxygen gas (a by-product) produce ATP (using chemiosmosis) Forms NADPH from NADP+ (an e- acceptor)

Temporarily stores high energy e-’s“Electron shuttle bus”

The Calvin Cycle

Occurs in the stromaForms SUGAR from carbon dioxideCarbon fixation occurs (CO2

fixed carbon) using ATP for energy and NADPH

for reducing power (adding e-s to fixed carbon) Fixed carbon carbohydrate (glucose)

An overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

GLUCOSE(sugar)

NADPH

NADP

ADP+ P

O2

ATP

G3P

Starch

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• Occurs in the chloroplast• Divided into 2 sets of reactions:

- Light Dependent Reaction- Calvin Cycle (Dark Reaction)

Photosynthesis

Photosynthesis

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• Requires Light!!• Occurs in the thylakoid/grana (in chloroplast)• Electron Transport Chain (ETC) makes ATP and

NADPH sends it to the Calvin Cycle WASTE PRODUCT: O2 Gas

Light Reaction

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Part 1 – Light Reactions You need to be able to draw this picture!

1. Photosystem II2. ETC3. Photosystem I

H2O Splits

What are the main steps in the light reactions?

• Uses energy from sunlight• Splits water into H+ and O2

• Converts ADP into ATP and NADP+ into NADPH– Use sunlight and water– Produces Oxygen, NADPH, and ATP

WRITELight Rxn Sequence of Events…

1) Photosystem II2) ETC3) Photosystem I

How is light absorbed in the thylakoid?

• Photosystems absorb light– Clusters of chlorophyll and proteins trap energy

from the sun– Energy is transferred to electrons makes

“excited” electrons

What are Electron Carriers?• Compound (NADP+) that can accept a pair of

high-energy electrons and transfer them to another molecule

• NADP+ grabs/carries 2 electrons and a H+ becomes NADPH

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Photosystem II• Photosystem II Steps →1) Chlorophyll and other light-

absorbing pigments absorb energy from the sun

• Energy transferred to e- high energy e- leave the chlorophyll and go to the ETC (electron transport chain)

2) H2O molecules are broken down –- Electrons come from H2O–- O-, H+ and e- are separated –For every TWO H2Os we get 4e-,

4H+, and 1 O2

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Photosystem II (cont’d)• 3) The excited e- jump from protein to protein and

enter the ETC (electron transport chain) on the thylakoid membrane• As e- jump through the ETC, they lose energy• This energy pumps/pulls H+ from the outside (stroma) to

the inside (lumen - aka - thylakoid space)• H+ ions start to build up inside the thylakoid

• Next, e- move to Photosystem I

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Photosystem I• Photosystem I → 4) More energy is absorbed from sunlight– Energy is added to e- from Photosystem II– Excited e- leave the molecules

5) E- are added to NADP+ which then becomes NADPH

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Photosystem I (cont’d)• Now, remember…those H+ ions are building up inside the

lumen6) H+ ions diffuse through a protein channel embedded in the

thylakoid membrane– The protein channel is part of an enzyme called ATP Synthase

7) As the H+ ions flow through the protein channel, the enzyme (ATP Synthase) makes ATP by adding a P group to ADP

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Light Reaction Summary…

• Summary: – Chemiosmosis The process that uses the proton gradient and ATP

synthase to make ATP– e- travels from PS II PS I added to NADP+ makes NADPH– Both ATP (via ATP Synthase) and NADPH are produced

During the light reaction thesethen enter theCalvin Cycle

NADP+ is the final e- acceptor in photosynthesis

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Calvin Cycle!

Second part of PS Occurs in the stroma of the chloroplast Can occur with or without light

Three turns shown above

Calvin Cycle

Summary of Steps:• ATP and NADPH used for

energy• CO2 is taken into the plant• High-energy sugars are

made • Also known as the sug ar

factory

Carbon dioxide & Calvin Cycle• Enters cycle from the atmosphere• Carbon from the carbon dioxide molecule is

used to make glucose (C6H12O6)• Plants then use the glucose for:

• Food!• Plant energy! – Stored in bonds of the

glucose molecule

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Calvin Cycle!Main job:

• Incorporates CO2 and rearranges the C’s to form sugar using the energy (ATP/NADPH) from the Light Reaction

• The CC “fixes” CO2

- Carbon enters as CO2 and leaves as sugar

• Goal is to produce a sugar (G3P)

• Each turn “fixes” one molecule of carbon, so one G3P takes 3 turns of the Calvin Cycle

INPUT: Each turn needs:

- 3 ATP- 2 NADPH- 1 CO2

Three turns shown above

Carbon “fixing” – process of turning CO2 into a solid compound

The Calvin cycle has three phases

1. Carbon fixation2. Reduction3. Regeneration of the CO2

acceptor (RuBP)

The Calvin Cycle Steps

CARBON FIXATION1. CO2 enters cycle and attached to a 5-

carbon sugar called ribulose biphosphate (RuBP) forming 6-C molecule (unstable)

Enzyme RUBISCO catalyzes reaction

2. Unstable 6-C molecule immediately breaks down to 3 3-C molecules called 3-phosphoglycerate (3-PGA)

REDUCTION3. Each 3-phosphoglycerate (3-PGA) gets an

additional phosphate from ATP (from LIGHT RXN) becomes 1,3 phosphoglycerate

4. NADPH reduces 1,3 phosphoglycerate to Glyceraldehyde-3-phosphate (G3P)– G3P = a sugar that stores

potential energy – Every 3 CO2 yields 6 G3P’s BUT only 1

can be counted in net gain for carbohydrate (GLUCOSE) production

REGENERATION OF CO2 ACCEPTOR (RuBP)

5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules

–ATP is used !!!!

The Calvin cycle

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

Also known as PGAL

NOTE:

MORE ATP is needed

than NADPH!!

Calvin Cycle Overview

For 1 G3P molecule made 9 ATP molecules are used 6 NADPH molecules are used

G3P (aka PGAL)= starting material to make other organic molecules (glucose, starch, etc.)

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Calvin Cycle Steps:1) Enzyme Rubisco

attaches CO2 to RuBP creates 6 x 3-PGA

2) 6 x PGA are reduced to = 6 x G3P (sugar) via ATP/NADPH

3) 1 x G3P exits as glucose

4) 5 x G3P remain in the cycle. ATP rearranges the atoms 3 x RuBP

Note: RuBP and Rubisco are NOT the same thing!

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6CO2 + 6H2O + sunlight C6H12O6 + 6O2

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Rate of Photosynthesis

• The rate of PS is affected by 3 factors: – Light intensity– Amount of CO2

– Temperature

• Why wouldn’t roots need chloroplasts?

Root cells don't have chloroplasts, because chloroplasts catch sunlight! Since roots are underground, they are not exposed to the sun!

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Alternate Pathways• If it is too hot out, the plant will close the stomata so that

it doesn’t lose too much water and become dehydrated– However this eliminates the gas exchange!!

• SO the levels of CO2 drop and the levels of O2 increase– This results in…. PHOTORESPIRATION

• Photorespiration adds oxygen to the Calvin Cycle instead of carbon dioxide

- This makes NO sugar or ATP- This wastes all of the plants resources!

• Two types of plants: 1) CAM 2) C4 plants

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Alternate Pathways

• Therefore, certain plants, (cacti, pineapple, etc.) only open their stomata at NIGHT! – This prevents them from drying out and still lets

them get CO2!

What is wrong with this picture?

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For the test, you will need to know these pictures and labels:

• Flower Reproductive • Leaf Structure• Chloroplast • Light Reactions• Calvin Cycle

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Exit Slip-April 9, 2014