Photosynthesis

*Calvin cycle

These organisms use light energy to drive the synthesis of organic molecules from carbon dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs).

(a) Plants

(b) Multicellular algae

(c) Unicellular protist10 m

40 m(d) Cyanobacteria

1.5 m(e) Pruple sulfurbacteria

Figure 10.2

• Overview: This process creates the nutrient source for the BIOSPHERE.

• Photosynthesis – Is the process that converts solar energy into chemical

energy

– Plants are photoautotrophs, using energy to make organic molecules from water and carbon dioxide.

– Photosynthesis also occurs in algae, some other protists, and some prokaryotes.

Major sites of photosynthesis - leaves

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

• Sites of photosynthesis

• Organelle:

– Double membrane

– Thylakoids

– Grana/granum

– Stroma

– Lamella

Guard cells: Stomata/Stoma

• Stomata are created by pair of guard cells.

• Guard cells take in water by osmosis, become turgid, and swell

– Increases gap between cells – stoma open

• Guard cells lose water, become flaccid, and shrink

– Decreases gap between cells – stoma closed

• Changes in turgor pressure due to reversible uptake of potassium ions - K+

• When stomata are open – guard cells get K + from neighbouring epidermal cells.– Increase solute conc. in guard cells – osmosis follows

– Water in

• When stomata are closed - guard cells lose K +, diffuses out.– Decrease solute conc. in guard cells – osmosis follows

– Water out

• Movement of K +

occurs passively, in response to pumping of H+

across membrane.

• In general stomata are open during day, and closed at night.

• Why?• Cues for stomatal opening? (How do plants “know”

when its time?)– Blue-light receptor in guard cells in plasma membrane –

stimulates activity of ATP-powered pumps – Photosynthesis begins in guard cell chloroplasts – making

ATP available– Depletion of CO2 in air spaces within leaf– Internal clock – plant in dark continues roughly 24 hour

cycling of stomatal opening – circadian rhythm.

• Evolutionary background:– Land plants evolved from aquatic plants 425 million

years ago, adapting to problem of dehydration.

– Early environment had different atmosphere than present.

• Trade off between prevention of excessive water loss, and photosynthesis

• If plants close stomata during the day, they reduce the amount of CO2 available, and increase amount of O2 in leaf air spaces.

Photosynthesis• Summarized as:• 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

• Chloroplasts split water into

– Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

6 CO2 12 H2OReactants:

Products: C6H12O6 6 H2O 6 O2

Figure 10.4

Photosynthesis PreviewPhotosynthesis is broken into two processes

• Occur in grana

• Split water,

• Release oxygen,

• Produce ATP, and

• Form NADPH

• Occurs in the stroma

• Forms sugar from carbon dioxide,

• Using ATP for energy and NADPH for reducing power

The Light Reactions The Calvin Cycle

Overview of Photosynthesis

Light Reactions

• Light reactions convert solar energy to the chemical energy of ATP and NADPH

• Light – form of electromagnetic energy, travels in waves

• Wavelength

• The distance between the crests

• Determines the type of electromagnetic energy

• The electromagnetic spectrum

– Is the entire range of electromagnetic energy, or radiation

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energyFigure 10.6

LightReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight Figure 10.7

• The visible light spectrum

– Include the colours of light we can see

– Includes the wavelengths that drive photosynthesis

• Pigments are

– substances that absorb visible light

– Reflect light, which include the colours we see

The Spectrophotometer

• Machine that sends light through pigments and measures the fraction of light transmitted at each wavelength.

• The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths driving photosynthesis.

• An absorption spectrum– Is a graph plotting light absorption versus wavelength

Figure 10.8

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

Slit moves topass lightof selectedwavelength

Greenlight

The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

The low transmittance(high absorption) readingchlorophyll absorbs most blue light.

Bluelight

1

2 3

4

0 100

0 100

• The absorption spectra of three types of pigments in chloroplasts

Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.

EXPERIMENT

RESULTSA

bso

rpti

on

of

ligh

t b

ych

loro

pla

st p

igm

ents

Chlorophyll a

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

Wavelength of light (nm)

Chlorophyll b

Carotenoids

Figure 10.9

• The action spectrum of a pigment– Profiles the relative effectiveness of different

wavelengths of radiation in driving photosynthesis

Rat

e o

f p

ho

tosy

nth

esis

(mea

sure

d b

y O

2re

leas

e)

Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

(b)

• The action spectrum for photosynthesis– Was first demonstrated by Theodor W. Engelmann

400 500 600 700

Aerobic bacteria

Filamentof alga

Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.

(c)

Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.CONCLUSION

• Chlorophyll a

– Is the main photosynthetic pigment

• Chlorophyll b

– Is an accessory pigment

C

CH

CH2

CC

CC

C

CNNC

H3C

C

C

C

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2 CH3

H

CH3C

HH

CH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Figure 10.10

Accessory pigments• Absorb different

wavelengths of light and pass the energy to chlorophyll a

• When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable.

Excitedstate

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A

• If an isolated solution of chlorophyll is illuminated

– It will fluoresce, giving off light and heat

Figure 10.11 B

A photosystem

• Is composed of a reaction centre surrounded by a number of light-harvesting complexes.

Primary electionacceptor

Photon

Thylakoid

Light-harvestingcomplexes

Reactioncenter

Photosystem

STROMA

Thyl

ako

id m

emb

ran

e

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)Figure 10.12

e–

The light-harvesting complexes

• Consist of pigment molecules bound to particular proteins

• Funnel the energy of the photons of light to the reaction centre

• When a reaction centre chlorophyll molecule absorbs energy

– One of its electrons gets bumped up to a primary electron acceptor

The thylakoid membrane

• Is populated by two types of photosystems, I and II

There are two paths electrons travel:• Non-cyclic electron flow

– Is the primary pathway of energy transformation in the light reactions

– Route: Photosystem II to Photosystem I

Non-cyclic photophosphorylation• Produces NADPH, ATP, and oxygen

Figure 10.13Photosystem II

(PS II)

Photosystem-I(PS I)

ATP

NADPH

NADP+

ADP

CALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

Pq

Cytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

P700

Light

NADPH

NADP+

+ 2 H+

+ H+

1

5

7

2

3

4

6

8

Cyclic Electron Flow• Under certain conditions photoexcited

electrons take an alternate path

• Only photosystem I is used, ONLY ATP is produced.

Primaryacceptor

Pq

Fd

Cytochromecomplex

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

ATPFigure 10.15

Photosystem IIPhotosystem I

NADP+

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

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

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

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2OO2

1

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

Comparison between mitochondria and chloroplasts

Mitochondria

•Chemiosmosis

•Electrons from glucose

Generate ATP by :

Source of energy :

In both:

ATP synthase:

Spatial organization:

Chloroplast

• Chemiosmosis

• Energized electrons by the sun

•Redox rxns of ETC generate a H+ gradient across a membrane•Uses this proton-motive source to make ATP

Key

Higher [H+]

Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Membrance

Matrix

Electrontransportchain

H+ Diffusion

Thylakoidspace

Stroma

ATPH+

PADP+

ATPSynthase

CHLOROPLASTSTRUCTURE

• The spatial organization of chemiosmosis

– Differs in chloroplasts and mitochondria

Calvin Cycle

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

– Is similar to the citric acid cycle

– Occurs in the stroma

• Has three phases

– Carbon fixation

– Reduction

– Regeneration of the CO2 acceptor

(G3P)

Input

(Entering oneat a time)CO2

3

Rubisco

Short-livedintermediate

3 P P

3 P P

Ribulose bisphosphate(RuBP)

P

3-Phosphoglycerate

P6 P

6

1,3-Bisphoglycerate

6 NADPH

6 NADPH+

6 P

P6

Glyceraldehyde-3-phosphate(G3P)

6 ATP

3 ATP

3 ADP CALVINCYCLE

P5

P1

G3P(a sugar)Output

LightH2O CO2

LIGHTREACTION

ATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

Figure 10.18

O2

6 ADP

Glucose andother organiccompounds

• The Calvin cycle

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

Cyclic Electron Flow

• Alternative cycle when ATP is deficient

• Photosystem I used but not II; produces ATP but no NADPH

• Why? The Calvin cycle consumes more ATP than NADPH…….

• Cyclic photophosphorylation

• What causes stomata to close during the day?

– Environmental stress, water deficiency

– High temperatures (hypothesis?)

• In most plants, carbon fixation occurs via:

• Rubisco – adds CO2 to ribulose bisphosphate RuBP.

• These plants are called C3 because:

• First organic product is 3-phosphoglycerate

• Examples: rice, wheat, soybeans

• On hot, dry days stomata close and less photosynthesis occurs, meaning:

• Starvation of Calvin cycle, reduction in sugar output.

• BUT it gets worse…

• Rubisco can accept O2 in place of CO2. (Enzyme specificity?)

• O2 added to Calvin cycle instead of CO2 causes a product that splits into a 2 carbon compound that is exported from the chloroplast.

• 2 carbon compound is broken down by mitochondria to CO2.

• This is called photorespiration.

• Photorespiration – produces no ATP, and no food.

• Photorespiration decreases Calvin cycle output by reducing amount of carbons.

• Hypothesis: evolutionary baggage – early atmosphere contained very little free oxygen.

• Photorespiration drains away as much as 50% of the carbon fixed by Calvin cycle – no known benefit to plants.

• Since environment causes stomata to close –hot, dry, bright days.

• Some plants have alternate modes of carbon fixation that minimize photorespiration.

• These are C4 plants and CAM plants.

• Ex:

• Sugarcane,

• Corn, and

• Grass family

• Ex: Succulents –cacti, and pineapples

– Crassulacean acid metabolism

– How are C4 plants and CAM plants…

• Similar?

• Different?