Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PHOTOSYNTHESIS.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PHOTOSYNTHESIS


Overview: The Process That Feeds the Biosphere

• Photosynthesis is the process that converts solar energy (light energy from the sun) into chemical energy


• Autotrophs sustain themselves without eating anything derived from other organisms

• Producers

• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide (inorganic)

Autotrophs – self feeders


Producers of the biosphere

• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes

• These organisms feed not only themselves but also the entire living world

Examples of Producers

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 µm

1.5 µm

40 µm


• Heterotrophs obtain their organic material from other organisms

• Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen

Heterotrophs - Consumers


The Basics of Photosynthesis

• Chloroplasts: Sites of Photosynthesis

LE 10-3

Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

Photosynthesis takes place in the chloroplasts/found in several organisms

1.Leaves are the major location for photosynthesis

2.Green color is from chlorophyll-the pigment in chloroplasts – absorbs light energy

3.Through microscopic pores (stomata) CO2 enters and O2 leaves (stoma means “mouth”)

4.Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf.

•A typical mesophyll has 30-40 chloroplasts

•6. In the inner membrane encloses a second compartment which is filled with thick fluid called stroma.

•Suspended in the stroma are the thylakoids, which contains the thylakoids space.

•Thylakoids can be concentrated in stacks called grana

•5.The chlorophyll is in the membranes of thylakoids


Equation of Photosynthesis

• Photosynthesis can be summarized as the following equation:

6 CO2 + 6H2O + Light energy C6H12O6 + 6 O2

What do you notice about this equation compared to respiration?

Requires a lot of energy that comes from sunlight• Chloroplasts split water into hydrogen and oxygen, incorporating the electrons

of hydrogen into sugar molecules, releasing oxygen.


Photosynthesis is a Redox Process, like cellular respiration

• Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

• The redox process takes the form of hydrogen transfer from water to CO2


Photosynthesis Equation

• Water molecules are split apart, yielding O2, and are oxidized. They lose electrons along with hydrogen ions.

• CO2 is reduced to sugar as electrons and hydrogen ions are added to it.

• The food-producing redox reactions of photosynthesis involve an uphill climb

• The light energy captured by chlorophyll molecules in the chloroplast provides the boost for the electrons.


The Two Stages of Photosynthesis: A Preview

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

LE 10-5_1

H2O

LIGHTREACTIONS

Chloroplast

Light

Light energy is absorbed by chlorophyll and drives the transfer of electrons and hydrogen from water to the electron acceptor….

The Light Reactions – converts solar energy to chemical energy

Take place in the thylakoids

Split water

LE 10-5_2

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

Stores energized electrons

Electron acceptor is NADP+

Reduced to NADPH

Oxygen is released

ATP is formed

NADPH is an electron carrier.

LE 10-5_3

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)

The Calvin Cycle

•The 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


Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

• Chloroplasts are solar-powered chemical factories

• Their thylakoids transform light energy into the chemical energy of ATP and NADPH


The Nature of Sunlight

• Sunlight is a form of electromagnetic energy, also called radiation

• Like other electromagnetic energy, light travels in rhythmic waves

• Wavelength = distance between crests of waves

• Light also behaves as though it consists of discrete particles, called photons


Visible radiation

• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation

• Visible light consists of colors we can see, including wavelengths that drive photosynthesis

LE 10-6

Visible light

Gammarays

X-rays UV Infrared Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

(109 nm) 103 m

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy

Shorter wavelength

Higher energy

The amount of energy in a photon is inversely related to its wavelength


Photosynthetic Pigments: The Light Receptors

• 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

Animation: Light and Pigments

LE 10-7

Chloroplast

Light

Reflected light

Absorbed light

Transmitted light

Granum


• Chlorophyll a is the main photosynthetic pigment

– Participates directly in the light reactions

• Accessory pigments, such as chlorophyll b,

– Absorbs mainly blue and orange light (reflects yellow-green)

• Broadens the range of light that a plant can use

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

– Absorb mainly blue-green light (reflect yellow-orange)

Chloroplast Pigments


Excitation of Chlorophyll by Light

• Light behaves as discrete packets of energy called photons

– A photon is a fixed quantity of light energy

• When a pigment absorbs a photon, the electron has been raised from a ground state to an excited state, which is very unstable

• When excited electrons fall back to the ground state, photons are given off (electrons lose energy), and emit an afterglow called fluorescence


A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

• A photosystem consists of a reaction center surrounded by light-harvesting complexes

• The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center


Photosystem

• A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a

• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

LE 10-12

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

ylak

oid

mem

bra

ne

Pigment molecules include chlorophyll a and b.

Cluster of pigmented molecules functions as a light-gathering antenna.

Photons jump from pigment to pigment and arrive at the reaction center

The reaction center is a chlorophyll a molecule and a primary electron acceptor molecule

Acceptor traps light excited electron from reaction center chlorophyll.

Trapped energy

converted to ATP and NADPH


The Two Photosystems in the light reactions

• Water Splitting (photosystem II)

– Uses light energy to extract electrons from water

– Releases O2 as a waste product

• NADPH producing (photosystem I)

– Produces NADPH by transferring light-excited electrons from chlorophyll to NADP+

• An electron transport chain connecting the two systems releases energy that the chloroplast uses to make ATP


PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

LE 10-14

ATP

Water splittingsystem

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

NADPH producing system

Ph

oto

n

NADPH


Generating ATP in the Light Reactions

Light reactions take place in the thylakoid membrane

• The mechanism of ATP production is similar to ATP production in cellular respiration

• An electron transport chain pumps hydrogen ions (H+) across the thylakoid membrane

• ATP synthases use the energy stored by the H+ gradient to make ATP


The Light Reactions

•

Electrons are stored at a high state of potential energy in NADPH. This and ATP will move on to the Calvin Cycle


Review of light reactions

http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html

http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html


The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

• The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. It requires more ATP than NADPH

• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

• For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2


The Calvin Cycle

• The Calvin cycle has three phases:

– Carbon fixation (catalyzed by rubisco)

– Reduction

– Regeneration of the CO2 acceptor (RuBP)

• Takes place in the stromaPlay

LE 10-18_1

[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)

CO2 is added to a five-C sugar. The reaction is catalyzed by the enzyme Rubisco

splits into 2 molecules of

LE 10-18_2

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light Input

CO2


Rubisco

3 P P



6 P


6 ADP

3

P P


3

6 NADP+

6

6 NADPH

P i

6 P

1,3-BisphosphoglycerateP

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

Each molecule of 3… is then phosphorylated by ATP to form

2 electrons from NADPH reduce this compound to create

The cycle must turn 3 times to create a net gain of one molecule of G3P - glucose

LE 10-18_3

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light Input

CO2


Rubisco

3 P P



6 P


6 ADP

CALVINCYCLE

3

P P


3

6 NADP+

6

6 NADPH

P i

6 P

1,3-BisphosphoglycerateP

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

The rearrangement of five molecules of G3P into the 3 molecules of RuBP requires 3 more ATP

9 molecules of ATP and 6 of NADPH are required to synthesize one G3P


Calvin Cycle review

•

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html


C4 and CAM plants

• All photosynthetic plants need CO2 to build sugar

– CO2 binds to RuBP (in Calvin Cycle)

– This is called the C3 pathway because it produces 2 – 3carbon molecules that continue in the cycle

• The enzyme that catalyzes this reaction is RUBISCO

– Rubisco is not efficient at grabbing CO2

– If CO2 levels fall too low, rubisco grabs O2 and sugars are burned up.

– This process is called photorespiration


Photorespiration - problems

• Problem during hot, dry days – or in these climates, plants keep stomates closed to prevent water loss

• Different systems evolved to deal with this problem

– So stomates don’t need to be open too often


Comparing C4 and CAM plants

C4

• During the day

• Takes place in mesophyll cells

• OAA pumped to bundle sheath (which are other cells)

• CO2 is then released to Calvin Cycle

• Ex. Corn, sugar cane, tropical grass

CAM

• Happens at night

• OAA stored in vacuoles within the cell

• Stomates can be open in evening when cooler avoiding water loss

• CO2 released to Calvin Cycle in day and driven by suns energy

• More common: ex: cacti and succulents

Each use a different enzyme to bind CO2Harvest more CO2 without opening stomates too oftenEnzyme is PEP carboxylase instead of Rubisco (4 –C-molecule, hence the name)Forms OAA (oxaloacetate)CO2 comes from this to use in the Calvin Cycle

LE 10-20

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

No structural separation like C4 plants: instead, the two processes are separated in time.


The Importance of Photosynthesis: A Review

• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

• In addition to food production, photosynthesis produces the oxygen in our atmosphere

LE 10-21

Light

CO2H2O

Light reactions Calvin cycle

NADP+

RuBP

G3PATP

Photosystem IIElectron transport

chainPhotosystem I

O2

Chloroplast

NADPH

ADP+ P i

3-Phosphoglycerate

Starch(storage)

Amino acidsFatty acids

Sucrose (export)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PHOTOSYNTHESIS.

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Transcript of Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PHOTOSYNTHESIS.