AP Biology

Ms. Haut

Light energy

enzymes

Photosynthesis is the process that converts solar energy into chemical energy

Directly or indirectly, photosynthesis nourishes almost the entire living world

Photosynthesis—process in which some of the solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers).

6CO2 + 6H2O C6H12O6 + 6O2

Glucose is the main source of energy for all life. The energy is stored in the chemical bonds.

Cellular Respiration— process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities.

C6H12O6 + 6O2 6CO2 + 6H2O + energy

enzymes

Autotroph —organisms that synthesize organic molecules from inorganic materials (a.k.a. producers)

–Photoautotrophs —use light as an energy source (plants, algae, some prokaryotes)–Chemoautotrophs —use the oxidation of inorganic substances (some bacteria)

• Heterotroph —organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)

Sunlight = electromagnetic energy•Wavelike properties•Particlelike properties (photon)

Thylakoids trap sunlight

Light may be reflected, transmitted, or absorbed when it contacts matter

Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected

or transmitted Leaves appear green because chlorophyll

reflects and transmits green light

Absorb light of varying wavelengths and transfer the energy to chlorophyll a

Chlorophyll b -yellow-green pigment Carotenoids -yellow and orange

pigments

An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Ab

sorp

tio

n o

f lig

ht

by

chlo

rop

last

pig

men

ts

400 500 600 700

Endergonic redox process; energy is required to reduce CO2

Light is the energy source that boosts potential energy of electrons (e-) as they are moved from water to CO2

When water is split, e- are transformed from the water to CO2, reducing it to sugar

6CO2 + 6H2O

reduction

oxidation

C6H12O6 + 6O2

Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

Light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH

Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

Light reactions —convert light energy to chemical bond energy in ATP and NADPH

Occurs in thylakoids in chloroplasts NADP+ reduced to NADPH—temporary

energy storage (transferred from water) Give off O2 as a by-product Generates ATP by phosphorylating ADP

Calvin Cycle —carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate

Occur in the stroma of the chloroplast Do not require light directly, but requires products of

the light reactions Incorporates into existing organic molecules and

then reduces fixed carbon into carbohydrate NADPH provides the reducing power ATP provides chemical energy

Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released

Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced

Interdependent Reactions

Photosystem: assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane

form a light gathering antennae that absorb photons and pass energy from molecule to molecule

Photosystem I —specialized chlorophyll a molecule, P700

Photosystem II —specialized chlorophyll a molecule, P680

• Light drives the light reactions to synthesize NADPH and ATP• Includes cooperation of both photosystems, in which e- pass continuously from water to NADP+

1. When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor.

• This leaves a hole in the P680

2. To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center

• A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2

3. Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain.

• e- are transferred to plastoquinone (Pq) and plastocyanin (Pc) (e- carriers)

4. As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP (PHOTOPHOSPHORYLATION)

• This ATP is used to make glucose during Calvin cycle

5. When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I.

• Pre-existing hole was left by former e- that was excited

6. When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary e- acceptor.

• e- are transferred to ferredoxin (Fd) (e- carrier)• NADP+ reductase transfers e- from Fd to NADP+,

storing energy in NADPH (reduction reaction)• NADPH provides reducing power for making glucose in

Calvin cycle

Only photosystem I is used Only ATP is produced

ChemiosmosisChemiosmosis

Energy released from e- transport chain is used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid. Creates concentration gradient across

thylakoid membrane Process provides energy for chemisomostic

production of ATP

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2O O21

1⁄2

2

Photosystem ILight

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

Figure 10.17

The light reactions and chemiosmosis: the organization of the thylakoid membrane

Carbon enters the cycle in the form of CO2 and leaves in the form of sugar (glucose)

The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar

For the net synthesis of this sugar, the cycle must take place 2 times

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

3

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3 P P

Ribulose bisphosphate(RuBP)

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P

1,3-Bisphosphoglycerate

P

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)

Output

Phase 2:Reduction

Glucose andother organiccompounds

Calvin CycleCalvin Cycle

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P

1,3-Bisphosphoglycerate

P

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)

Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

P5

G3P

1. Carbon Fixation: 3 CO2 molecules bind to 3 5-Carbon sugars, ribulose bisphosphate (RuBP) using enzyme called RuBP carboxylase (rubisco)

• Produces 6 molecules of a 3-carbon sugar, 3-phosphoglycerate

1. Carbon Fixation2. Reduction: 6 ATP molecules transfer

phosphate group to each molecule of 3-phos. to make 1,3-diphosphoglycerate

• 6 molecules of NADPH reduce each molecule of 1,3-diphosph. to make glyceraldehyde 3-phosphate (G3P)

3. One of the G3P exits the cycle to be used by the plant; the other 5 molecules are used to regenerate the CO2 acceptor, RuBP: 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP

• 3 more CO2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle.

• 2 G3P molecules can be used to make glucose

Interdependent

Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis

On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis

The closing of stomata reduces access to CO2

and causes O2 to build up These conditions favor a seemingly wasteful

process called photorespiration

• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound

• In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2

Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

Corn

Crab Grass

PEP carboxylase-high affinity to CO2 and no affinity for O2, thus no photorespiration possible

C4 Plants

CAM plants open their stomata at night, incorporating CO2 into organic acids

Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close

Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

http://ecology.botany.ufl.edu/ecologyf02/graphics/saguaro.GIF

Bundle-sheathcell

Mesophyllcell Organic acid

C4

CO2

CO2

CALVINCYCLE

Sugarcane Pineapple

Organic acidsrelease CO2 toCalvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Organic acid

CAM

CO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night

Are similar in that CO2 is first incorporated into organic intermediates before it enters the Calvin cycle

Differ in that the initial steps of carbon fixation in C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times

Regardless of whether the plant uses C3, C4, or CAM pathway, all plants use the Calvin Cycle to produce sugar from CO2

The CAM and C4 pathways: