photosynthesis project 10

8/8/2019 Photosynthesis Project 10

http://slidepdf.com/reader/full/photosynthesis-project-10 1/47

Photosynthesis

Light energy from the sun is

converted to chemical energy that isstored in sugar



The sun powers all life on earth

directly or indirectlyAutotrophs (producers) obtain energy directly from

the sun, while heterotrophs (consumers) obtain

energy by eating autotrophs or other heterotrophsthat ate autotrophs.



Our plan

1st: general information about photosynthesis

2nd: two stages of photosynthesis

- light reactions- Calvin cycle

3rd: evolutionary adaptations of plants



Chloroplasts

Every square millimeter of leaf tissue contains

about half a million chloroplasts.

Chloroplasts are found mainly in cells of themesophyll, the interior of the leaf.

Carbon dioxide enters (and oxygen leaves) the leaf

through stomata on the underside of the leaf.



2005-2006



2005-2006

A Closer Look at Stomata

http://www.tutorvista.com/content/biology/biology-iv/plant-water-relations/opening-

closing-stoma.phphttp://academic.kellogg.edu/herbrandsonc/bio111/animations/0021.swf



Chloroplasts

A double membrane encloses fluid called stroma.

Membranous sacs called thylakoids contain chlorophyll

and separate the thylakoid space from the rest of thechloroplast.

Thylakoids may be stacked in columns called grana.



Photosynthesis as a Redox Reaction

6 CO2 + 6 H2O C6H12O6 + 6 O2

- Water is split and electrons (along with H ions)are transferred to carbon dioxide, reducing it

to glucose.

- Energy (from sunlight) is required to energize

the electrons as they move from water to

sugar.



Photosynthesis and Free Energy

Anabolic = complex products are made by assemblingsimpler reactants

Enderg

onic =energy is required

± ( G is positive

± Reaction is uphill

± Reactants contain less energy than the product



Light (electromagnetic energy)

The amount of energy in light is inversely proportional to

the wavelength of light.



Why are plants green?

When sunlight (whitelight) hits an object,some wavelengths are

absorbed and othersare reflected.

± Pigments absorb lightand those wavelengths

disappear. ± We see the color of

the light that isreflected.



Chlorophylls Absorption Spectrum

* Chlorophyll is a

pigment that

absorbs red, blue,and violet light,

but reflects green

light.



Overview of Photosynthesis

1) Light reactions

- Convert solar energy to chemical energy

- Light absorbed by chlorophyll energizes

electrons which are transferred to NADP+ toform NADPH

- Splits water; oxygen is produced

- Some ATP is made by photophosphorylation

Light energy NADPH and ATP

No sugar has been made yet




2) Calvin Cycle (dark reactions; light

independent reactions; carbon fixation)

- Uses NAD

PH and the energy inAT

P to reducecarbon dioxide to glucose

Carbon dioxide, NADPH and ATP Glucose



Preview of Light Reactions

Purpose: Convert solar energy into chemicalenergy (ATP and NADPH) that will later be used

to convert carbon dioxide into glucose

1st: sunlight excites chlorophylls electrons

2nd: the excited electrons give off energy as they

pass through electron transport chains3rd: the energy released from the electrons is used

to make ATP and the electrons are picked up byNADP+ to make NADPH



Pigment Molecules are organized into Photosystems

that are embedded in the thylakoid membranes

- Reaction centersurrounded by light-harvesting complexes

- Reaction center contains 2special chlorophyll amolecules and a primaryelectron acceptor

- Each light-harvesting complex contains otherpigment molecules thatfunnel energy to thereaction center (antenna)Two Photosystems work in order: Photosystem

II works first, and then Photosystem I



Light Reactions begin in

Photosystem II

1) Sunlight excites the electrons of the pigments inthe light-harvesting complexes

2) The energy is funneled to the chlorophyll amolecules in the reaction center

3) Chlorophyll as electrons are excited andtransferred to the primary electron acceptor

REDOX REACTION: chlorophyll a is oxidized and the

primary electron acceptor is reduced



High energy electrons pass through an electron

transport chain from Photosystem II to Photosystem I

4) As the electrons from Photosystem II travel through theelectron transport chain, they release a little bit of energythat is used to make ATP by photophosphorylation (verysimilar to oxidative phosphorylation in respiration)

5) Electrons in the reaction center of Photosystem I are alsoexcited by sunlight and transferred to the primary electronacceptor

6) Energized electrons from Photosystem I travel down asecond electron transport chain and are picked up byNADP+ to form NADPH

7) The electrons from Photosystem II arrive at the reactioncenter of Photosystem I to replace the lost electrons



Uh-Oh! Photosystem II has lost

electrons!

The light reactions usually progress by noncyclicelectron flow. ± Electrons from Photosystem II replace the electrons

lost by Photosystem I to NADPH

± Buthow are Photosystem IIs electrons replaced?

Enzymes in Photosystem II split water into 2electrons, 2 hydrogen atoms, and an oxygen atom ± The electrons replace those lost in the reaction center

± The oxygen atom combines with another and isreleased



Sometimes, cyclic electron

flow occurs

Only Photosystem I is used ± Electrons leave

Photosystem I, enter theprevious electron

transport chain, andreturn to Photosystem I

± ATP is made, but NADPH isnot

This happens because theCalvin cycle needs moreATP than NADPH to makeglucose



Electron Transport Chain in

Photosynthesis



Electron Transport Chain in

Photosynthesis

Photophosphorylation is almost exactly the same asoxidative phosphorylation in cellular respiration

± A proton-motive force is generated across a membrane

± H+ ions can only cross the membrane through ATP

synthase ± Chemiosmosis: energy of H+ ions flowing down their

concentration gradient powers the addition of aphosphate group to ADP

One spatial difference ± In cellular respiration, H+ ions were pumped out of the

matrix and into the intermembrane space

± In photosynthesis, H+ ions are pumped into the thylakoidspace from the stroma



Dont lose focus!

The purpose of the light reactions is to use

solar energy to generate ATP and NADPH,

which will later provide the energy needed to

reduce carbon dioxide to glucose



Summarize the Light Reactions



Preview of Calvin Cycle

Purpose: to use the energy and reducing power stored in ATP

and NADPH to form glucose from carbon dioxide

CO2 has very little chemical energy

± fully oxidized

C6H12O6 contains a lot of chemical energy

± reduced

Reduction of CO2

p C6

H1

2

O6

proceeds in many small steps

± each catalyzed by specific enzyme

± using energy stored in ATP and NADPH

Needs products of light reactions to drive synthesis of glucose



Calvin Cycle



Carbon Fixation

1) RuBP is a 5-carbon sugar present at thebeginning of the Calvin cycle

2) Rubisco adds one molecule of CO2 to RuBP to

form a 6-carbon sugar

3 molecules of CO2 enter, one at a time, and each is

added to a different molecule of RuBP

3) This 6-carbon sugar is very unstable and splits inhalf immediately to form 2 molecules of

phosphoglycerate (each contains 3 carbons)



Reduction

4) All 6 molecules of 3-phosphoglycerate arephosphorylated by ATP and reduced by NADPH toform 6 molecules of glyceraldehyde-3-phosphate

(G3P)

F or every 3 molecules of CO2 that enter, 6 molecules

of G3P are made.

5) 1 molecule of G3P leaves to become glucose andthe other 5 molecules of G3P remain in the Calvincycle



Regeneration of RuBP

6) The 5 molecules of G3P are re-organized into

3 molecules of RuBP

- requiresAT

P

7) 3 molecules of RuBP are ready to accept 3

more molecules of CO2



Rubisco

Enzyme which fixes carbon from atmosphere

± ribulose bisphosphate carboxylase

± the most important enzyme

± the most abundant enzyme



Accounting in the Calvin Cycle is

complicated

3 turns of Calvin cycle yield 1 G3P

± 3 CO2p1 G3P (3C)

6 turns of Calvin cycle yield 1 C6H12O6 (6C)

± 6 CO2p1 C6H12O6 (6C)

±

18 ATP + 12 NADPHp 1 C6H12O6

* See the need for cyclic electron flow in the light

reactions?



Dont lose focus!

The purpose of the Calvin cycleis to use the

energy made inthe lightreactions tocombine

molecules of carbon dioxideinto glucose



Summarize the Calvin Cycle



Summarize all of Photosynthesis



Photorespiration

Problem plants face: ± Carbon dioxide needed for photosynthesis enters through leaf

pores called stomata

± Water is lost when stomata are open (transpiration)

How plants react: ± Close stomata on hot, dry days to conserve water

± Now produce less glucose (less CO2 enters leaf)

± Rubisco adds O2 to Calvin cycle and a 2-carbon molecule is

produced (rubisco has an affinity for oxygen!!!!

) ± Uses ATP, doesnt make glucose

Is this evolutionary baggage or is it productive?



Adaptation of C4 plants

(a better way to capture CO2

)

Two distinct types of photosynthetic leaf cells

± 1st: PEP carboxylase in the mesophyll cells adds carbon dioxideto a 3-carbon molecule forming a 4-carbon molecule

PE

P carboxylase has a much higher affinity for CO2 and a much loweraffinity for O2 than rubisco

PEP carboxylase can fix carbon more efficiently when the stomata areclosed and CO2 levels in the leaf are low

± 2nd: the 4-carbon molecule travels through plasmodesmata tobundle-sheath cells, breaks down, and release CO

2for use in the

Calvin cycle

Keeps CO2 levels high enough in bundle-sheath cells to avoidphotorespiration



C4 pathway



C4 pathway

Physically separates carbon fixation from Calvin cycle

Outer cells (mesophyll)

-near stomata- fix carbon

- pump CO2 to inner cells

- keep O2 away from inner cells (Rubisco)

Inner cells (Bundle-sheath)

- Make glucose from CO2 being pumped in

- Glucose enters veins easily



Adaptation of CAM plants

Stomata open at night and closed during day ± Closing stomata during the day helps them conserve water,

but also cuts off their CO2 supply

Take in CO2 at night and fix it into organic acids that arestored in the mesophyll cells

During the day when the light reactions produce ATP

and NAD

PH, these acids break down to release CO2

Also separates carbon fixation from the Calvin cycle, butbased on time, not location (like C4 plants)



C4 and CAM plants

Carbon dioxide enters plant

leaves through the

stomata, while oxygen (the

photosynthetic

waste product) and water

from the leaves exit

through the stomata.

Plants must constantly

balance both water loss and

energy gain(photosynthesis).

This has led to the evolution

of various modifications of

C3 photosynthesis.



Summarizer: Plant Adaptations



Summarizer: Plant Adaptations

C3 C4 CAM

How and when does

carbon dioxide get

into each leaf?

How do these plants

avoid

photorespiration? (In

other

words, how do they

concentrate CO2 for

the Calvin

cycle?)

Where does the

Calvin cycle

occur?

photosynthesis project 10

Documents