BIO 2, Lecture 14

BIO 2, Lecture 14BIO 2, Lecture 14FIGHTING ENTROPY III:FIGHTING ENTROPY III:

PHOTOSYNTHESISPHOTOSYNTHESIS

• Autotrophs sustain themselves without eating anything derived from other organisms

• Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

• Almost all plants are photoautotrophs, using the energy of sunlight to make complex organic molecules from H2O and CO2

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

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

(a) Plants

(c) Unicellular protist10 µm

1.5 µm

40 µm(d) Cyanobacteria

(e) Purple sulfur bacteria

(b) Multicellular alga

• Heterotrophs obtain their organic material from other organisms

• Heterotrophs are the consumers of the biosphere

• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

• In plants, the work of photosynthesis is done by organelles called chloroplasts

• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

• In plants, leaves are the major locations of photosynthesis

• Their green color is from chlorophyll, the green pigment within chloroplasts

• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

• CO2 enters and O2 exits the leaf through microscopic pores called stomata

• Chloroplasts are found mainly in cells of the mesophyll, an interior tissue of the leaf• A typical mesophyll cell has 30–40

chloroplasts

• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

• Chloroplasts also contain stroma, a dense fluid

5 µm

Mesophyll cell

Stomata CO2 O2

Chloroplast

Mesophyll

VeinLeaf cross section

1 µm

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

• Photosynthesis can be summarized as the following equation:

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

• Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

• Photosynthesis is a redox process in which H2O is oxidized (to O2) and CO2 is reduced (to C6H12O6)

Reactants: 6 CO2

Products:

12 H2O

6 O26 H2OC6H12O6

Reduced CO2

Oxidized H2O

Photosynthesis consists of two parts:

1.“Photo” part = light-dependent reactions• Require light; only occur in the daytime

2.“Synthesis” (of sugar) part = light independent reactions• Also called the Calvin cycle• Occur both in the daytime and at

night• Plant switches most energy to

Calvin Cycle at night

• The light reactions– Occur in the thylakoid membrane and

thylakoid space (inside the thylakoid)– Split H2O into H+ and O2 (gas)– Reduce NADP+ to NADPH– Generate ATP from ADP and Pi

• The Calvin cycle • Occurs in the stroma• Forms sugar from CO2, using the ATP and

NADPH generated in the light reactions• Begins with carbon fixation,

incorporating CO2 into organic molecules

Light

H2O

Chloroplast

LightReactions

NADP+

PADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O](sugar)

“Dark” (light-independent)

reactions (occur in the presence and absence of light)

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

• Like other electromagnetic energy, light travels in rhythmic waves

• Wavelength is the distance between crests of waves• Wavelength determines the type of

electromagnetic energy• Shorter wavelength = higher energy

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

• Visible light consists of wavelengths (including those that drive photo-synthesis) that produce colors we can see

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

UV

Visible light

Infrared Micro-waves

RadiowavesX-raysGamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nmLonger wavelengthLower energyHigher energy

Shorter wavelength

• Pigments are substances that absorb visible light• Different pigments absorb different

wavelengths

• Wavelengths that are not absorbed are reflected back (and seen by observers)• Leaves appear green because chlorophyll

reflects green light back to our eyes • It is actually the combined wavelengths

not absorbed by chlorophyll that collectively appear green

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

• Chlorophyll a is the main photosynthetic pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

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

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

• When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

Ener

gy o

f ele

ctro

n e–

Chlorophyllmolecule

• Photosynthesis begins at photosystems located in the thylakoid membrane

• A photosystem consists of a reaction-center complex (comprised of several proteins) surrounded by light-harvesting complexes

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

• There are two types of photosystems in the thylakoid membrane:• Photosystem II (PS II) functions first

(the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

• Photosystem I (PS I) functions second and is best at absorbing a wavelength of 700 nm

• At the beginning of photosynthesis, a primary electron acceptor in the reaction center of Photosystem II accepts an electron from chlorophyll a that has been excited by a photon

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

• The chlorophyll molecule that is now missing an electron is a very strong oxidizing agent

• It grabs an electron from H2O (in the thylakoid space) and is reduced

• O2 is released as a by-product of this splitting of H2O

• H+ is also formed, which begins to build up in the thylakoid space (sound familiar?)

• The electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

• Energy released by the fall is used by proteins in the electron transport chain to drive H+ ions from the stroma into the inner space of the thylakoid

• Thus, there are two sources of build-up of H+ ions in the inner space: from the splitting of water and from the electron transport chain

• ATP synthase, embedded in the thylakoid membrane, coverts ADP + Pi to ATP using the energy generated by the rush of the H+ ions in the inner compartment out into the stroma (with their concentration gradient)

• Meanwhile, the electrons passing through the electron transport chain of Photo-system II are eventually dumped off at Photosystem I• When photons hit chlorophyll molecules in

Photosystem I, electrons are kicked off chlorophyll to a second electron transport chain

• Chlorophyll molecules in Photosystem I (now strong oxidixing agents) grab electrons dumped dumped off from Photosystem II to return to their reduced state

• Water is not split at Photosystem I

• Electrons excited by photons at Photosystem I “fall” down a second electron transport chain and are eventually dumped onto NADP+, reducing it to NADPH

• The electrons of NADPH are available for the reactions of the Calvin cycle (that drive the endergonic reactions that create sugar and starch)

• ATP produced through the proton motive force are also used in the Calvin Cycle

Millmakes

ATP

e–

NADPH

Phot

on

e–e–

e–

e–

e–

Phot

on

ATP

Photosystem II Photosystem I

e–

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I4 H+

4 H+

Pq

Pc

Light NADP+

reductaseNADP+ + H+

NADPH

+2 H+

H2O O2

e–e–

1/21

2

3

• Chloroplasts and mitochondria 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, which is used to produce food

• Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities

• In mitochondria, protons are pumped out of the inner space (matrix) and into the intermembrane space• ATP synthesis occurs as they diffuse back

into the mitochondrial matrix

• In chloroplasts, protons are pumped into the inner thylakoid space and out of the stroma• ATP synthesis occurs as they diffuse back

out of the inner thylakoid space (into the stroma)

Key

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

MITOCHONDRIONSTRUCTURE

Intermembranespace

Innermembrane

Electrontransport

chain

H+ Diffusion

Matrix

Higher [H+]Lower [H+]

Stroma

ATPsynthase

ADP + PiH+

ATP

Thylakoidspace

Thylakoidmembrane

• 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• The ATP and NADPH come from the light-

dependent reactions

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

• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

• The Calvin cycle has three phases:– Carbon fixation (catalyzed by rubisco)– Reduction– Regeneration of the CO2 acceptor

(RuBP)

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

InputCO2

P

3 6

3

3

P

PPP


3-Phosphoglycerate




Rubisco

Input CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P61,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)Glucose andother organiccompounds

CalvinCycle


3-Phosphoglycerate




Rubisco

Input CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P61,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)Glucose andother organiccompounds

CalvinCycle

3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADPP i+

RuBP 3-PhosphoglycerateCalvinCycle

G3PATP

NADPH Starch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

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BIO 2, Lecture 14