BIOL 101

Chapter 10

Photosynthesis

Rob Swatski Associate Professor of Biology

HACC – York Campus

The Process That Feeds the Biosphere

Photosynthesis:

- converts solar energy into chemical energy

- directly or indirectly feeds entire living world

- in plants, algae & other protists, some prokaryotes

Autotrophs: survive without eating anything derived from other organisms

- Producers: make organic molecules from inorganic molecules (H2O and CO2)

- Photoautotrophs: use solar energy to make organic molecules

(a) Plants

(c) Unicellular protist

10 µm

1.5 µm

40 µm (d) Cyanobacteria

(e) Purple sulfur bacteria

(b) Multicellular alga

Heterotrophs: obtain their organic material from other organisms

- Consumers of the biosphere

- heterotrophs depend on photoautotrophs for food and O2

Can an organism be both autotrophic and heterotrophic?

Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

- their structure enables the chemical reactions of photosynthesis to occur – how so?

Leaves: the major organs of photosynthesis

- chlorophyll: green pigment inside chloroplasts

- absorbs light energy that powers the synthesis of organic molecules

CO2 enters and O2 exits the leaf through stomata

Chloroplasts: found in mesophyll cells in the leaf’s interior

- 30-40 chloroplasts per mesophyll cell

Chlorophyll is in the thylakoid membranes of chloroplasts

- thylakoids are stacked into grana (granum)

- stroma

5 µm

Mesophyll cell

Stomata CO2 O2

Chloroplast

Mesophyll

Vein

Leaf cross section

1 µm

Thylakoid space

Chloroplast

Granum

Intermembrane space

Inner membrane

Outer membrane

Stroma

Thylakoid

Photosynthesis summary equation:

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

Photosynthesis is a redox process:

- H2O is oxidized

- CO2 is reduced

Chloroplasts split H2O into hydrogen and oxygen, incorporating the e- of H into glucose

Reactants: 6 CO2

Products:

12 H2O

6 O2 6 H2O C6H12O6

The Two Stages of Photosynthesis: A Preview

Light reactions: the “photo” part

Calvin cycle: the “synthesis” part

Light Reactions (in thylakoids):

- split H2O

- release O2

- reduce NADP+ to NADPH

- generate ATP from ADP by photophosphorylation

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

ATP

NADPH

O2

Calvin cycle (in stroma)

- forms glucose from CO2

- uses ATP and NADPH

Begins with carbon fixation

- incorporate CO2 into organic molecules

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

ATP

NADPH

O2

Calvin Cycle

CO2

Light

H2O

Chloroplast

Light Reactions

NADP+

P

ADP

i +

ATP

NADPH

O2

Calvin Cycle

CO2

[CH2O] (sugar)

The Nature of Sunlight

Light is a form of electromagnetic energy (radiation)

- travels in rhythmic waves

Wavelength: distance between crests of waves

- determines the type of electromagnetic energy

Electromagnetic spectrum: the entire range of electromagnetic radiation

Visible light: the wavelengths that produce colors we can see

- includes wavelengths that drive PSN

- behaves as though it consists of discrete energy “particles” (photons)

UV

Visible light

Infrared Micro- waves

Radio waves X-rays Gamma

rays

103 m 1 m

(109 nm) 106 nm 103 nm 1 nm 10–3 nm 10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy Higher energy

Shorter wavelength

Photosynthetic Pigments: The Light Receptors

Pigments: chemicals 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

Reflected light

Absorbed light

Light

Transmitted light

Granum

Spectrophotometer: measures a pigment’s ability to absorb different wavelengths

- sends light through pigments

- measures the fraction of light transmitted at each wavelength

Galvanometer

Slit moves to pass light of selected wavelength

White light

Green light

Blue light

The low transmittance (high absorption) reading -chlorophyll absorbs most blue light

High transmittance (low absorption) reading -chlorophyll absorbs very little green light

Refracting prism Photoelectric

tube

Chlorophyll solution

TECHNIQUE

1

2 3

4

Absorption spectrum: graph plotting a pigment’s light absorption vs. wavelength

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

Action spectrum: profiles the relative effectiveness of different wavelengths of radiation in driving PSN

Wavelength of light (nm)

(b) Action spectrum

(a) Absorption spectra

(c) Engelmann’s experiment

Aerobic bacteria

RESULTS

Filament of alga

Chloro- phyll a Chlorophyll b

Carotenoids

500 400 600 700

700 600 500 400

Chlorophyll a: the main photosynthetic pigment

Accessory pigments:

- Chlorophyll b: broadens the PSN spectrum

- Carotenoids: absorb excess light that would damage chlorophyll

Porphyrin ring: light-absorbing “head” of molecule -Mg atom at center

in chlorophyll a CH3

Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts (H atoms not shown)

CHO in chlorophyll b

Excitation of Chlorophyll by Light

When a pigment absorbs light, it goes from a stable ground state to an unstable excited state

When excited e- fall back to the ground state, photons are given off (fluorescence)

- if illuminated, a chlorophyll solution will fluoresce, giving off light and heat

(a) Excitation of isolated chlorophyll molecule

Heat

Excited state

(b) Fluorescence

Photon Ground state

Photon (fluorescence)

Ene

rgy

of

ele

ctro

n

e–

Chlorophyll molecule

e–

Photosystem consists of:

- Reaction-center complex surrounded by

- Light-harvesting complexes (pigments bound to proteins)

Funnels energy of photons to the reaction center

Primary electron acceptor (in reaction center complex)

- accepts an excited e- from chlorophyll a

- the 1st step of the light reactions

THYLAKOID SPACE (INSIDE THYLAKOID)

STROMA

e–

Pigment molecules

Photon

Transfer of energy

Special pair of chlorophyll a

molecules (P680 or P700)

Thyl

ako

id m

emb

ran

e

Photosystem

Primary electron acceptor

Reaction-center complex

Light-harvesting complexes

Two Types of Photosystems:

Photosystem II (PS II) functions first:

- best at absorbing wavelengths of 680 nm

P680: the reaction-center chlorophyll a of PS II

Photosystem I (PS I) functions second:

- best at absorbing wavelengths of 700 nm

P700: the reaction-center chlorophyll a of PS I

Linear Electron Flow

Two possible routes for e- flow during the light reactions: cyclic and linear

Linear electron flow: the primary pathway

- involves both photosystems I and II

- produces ATP and NADPH using light energy

A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

- an excited e- from P680 is transferred to the primary electron acceptor

Pigment molecules

Light

P680

e– 2

1

Photosystem II (PS II)

Primary acceptor

e-

P680+ = P680 with one less e-

- is a very strong oxidizing agent

- H2O is split by enzymes

- its e- are transferred from H atoms to P680+

- reduces P680+ to P680

- O2 is released as a by-product of this reaction

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

Photosystem II (PS II)

e-

e-

e-

Each e- “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

- energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

- the diffusion of H+ (protons) across the membrane drives ATP synthesis

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome complex

5

ATP

Photosystem II (PS II)

e-

e-

e-

e-

e- e-

e-

e-

e-

e-

e-

In PS I (like PS II), transferred light energy excites P700, which loses an e- to an electron acceptor

P700+: P700 that is missing an e-

- accepts an e- passed down from PS II via the electron transport chain

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome complex

5

ATP

Photosystem II (PS II)

e-

e-

e-

e-

e- e-

e-

e-

e-

e-

e-

Photosystem I (PS I)

Light

Primary acceptor

e–

P700

6

e-

e-

Each e- “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

- the e- are then transferred to NADP+ and reduce it to NADPH

- the e- of NADPH are available for the reactions of the Calvin cycle

Pigment molecules

Light

P680

e–

Primary acceptor

2

1

e–

e–

2 H+

O2

+

3

H2O

1/2

4

Pq

Pc

Cytochrome complex

5

ATP

Photosystem I (PS I)

Light

Primary acceptor

e–

P700

6

Fd

NADP+ reductase

NADP+

+ H+

NADPH

8

7

e– e–

6

Photosystem II (PS II)

e-

Mill makes

ATP

e–

NADPH

e– e–

e–

e–

e– ATP

Photosystem II Photosystem I

e–

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

Both generate ATP by chemiosmosis, but use different sources of energy:

Mitochondria transfer chemical energy from food to ATP

Chloroplasts transform light energy into the chemical energy of ATP

Spatial Organization of Chemiosmosis:

Mitochondria: protons are pumped into the intermembrane space

- diffusion of protons back into the matrix drives ATP synthesis

Chloroplasts: protons are pumped into the thylakoid space

- diffusion of protons back into the stroma drives ATP synthesis

Key

Mitochondrion Chloroplast

CHLOROPLAST STRUCTURE

MITOCHONDRION STRUCTURE

Intermembrane space

Inner membrane

Electron transport

chain

H+ Diffusion

Matrix

Higher [H+]

Lower [H+]

Stroma

ATP synthase

ADP + P i

H+ ATP

Thylakoid space

Thylakoid membrane

ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

In summary:

- light reactions generate ATP

- they increase the potential energy of e- by moving them from H2O to NADPH

Light

Fd

Cytochrome complex

ADP +

i H+

ATP P

ATP synthase

To Calvin Cycle

STROMA (low H+ concentration)

Thylakoid membrane

THYLAKOID SPACE (high H+ concentration)

STROMA (low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

Light NADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2O O2

e– e–

1/2 1

2

3

The Calvin cycle regenerates its starting material as molecules enter and leave the cycle

- analagous to the citric acid cycle

- builds sugar from smaller molecules using ATP and the reducing power of e- carried by NADPH

Carbon enters the Calvin cycle as CO2

Carbon leaves as the sugar, glyceraldehyde-3-phospate (G3P)

For net synthesis of 1 G3P:

- the Calvin cycle must turn 3X, fixing 3 molecules of CO2

Three Phases of the Calvin Cycle

1. Carbon fixation (catalyzed by rubisco)

2. Reduction

3. Regeneration of the CO2 acceptor (RuBP)

Ribulose bisphosphate (RuBP)

3-Phosphoglycerate

Short-lived intermediate

Phase 1: Carbon fixation

(Entering one at a time)

Rubisco

Input

CO2

P

3 6

3

3

P

P P P

Ribulose bisphosphate (RuBP)

3-Phosphoglycerate

Short-lived intermediate

Phase 1: Carbon fixation

(Entering one at a time)

Rubisco

Input

CO2

P

3 6

3

3

P

P P P

ATP 6

6 ADP

P P 6

1,3-Bisphosphoglycerate

6

P

P 6

6

6 NADP+

NADPH

i

Phase 2: Reduction

Glyceraldehyde-3-phosphate (G3P)

1 P Output G3P

(a sugar)

Glucose and other organic compounds

Calvin Cycle

Ribulose bisphosphate (RuBP)

3-Phosphoglycerate

Short-lived intermediate

Phase 1: Carbon fixation

(Entering one at a time)

Rubisco

Input

CO2

P

3 6

3

3

P

P P P

ATP 6

6 ADP

P P 6

1,3-Bisphosphoglycerate

6

P

P 6

6

6 NADP+

NADPH

i

Phase 2: Reduction

Glyceraldehyde-3-phosphate (G3P)

1 P Output G3P

(a sugar)

Glucose and other organic compounds

Calvin Cycle

3

3 ADP

ATP

5 P

Phase 3: Regeneration of the CO2 acceptor (RuBP)

G3P

Light Reactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADP

P i +

RuBP 3-Phosphoglycerate

Calvin Cycle

G3P ATP

NADPH Starch

(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2