Light and photosynthesisPlant Phys and Biotech

Biol 3470Lecture 6, Tues. 24 Jan. 2006

Chps. 3 & 4

Rost et al., “Plant Biology”, 2nd ed.

What is light?Is it a

Particle – quantifiable in discreet units (mol, photons)

or a

Wave – has a frequency (s-1) and wavelength ()

frequency (v) = c/and is proportional to 1/light energy is proportional to v is proportional to 1/higher wavelength less energy

Fig. 3.1

Light interactions with plant pigments power photosynthesis

• Light can interact with matter – i.e., with plant pigments in photosynthesis

• Pigments absorb the energy in light and use it to excite electrons

• These electrons then flow through electron transport chains in the thylakoid membranes and are used to make ATP

Pigments are required to convert radiant energy into chemical potential energy

• They accept energy from triplet state electrons (are reduced) then pass on electrons to photosynthetic electron transport chain (are oxidized)

ENERGY

Ground State Light (hv)

Excited/singlet state Triplet state Pigment e- transport

Make ATP, NADPHFuel the plant!

SHORT LIFETIME

LONGER LIFETIME

Heat (entropy)

Pigments define the portion of the electromagnetic spectrum useable by plants

• Since the pigments useful in photosynthesis interact with light, they are photoreceptors (perception of information)

• Typical pigment = chlorophyll

•Pigments have action spectra•are able to absorb energy from only certain wavelengths of light

•Should be same as (light) absorption spectrum•These are unique to each pigment

–i.e., chlorophyll

•Pigments must be able to absorb light energy in order to excite electrons and drive the photosynthetic E.T.C.

More light absorbed

Less light absorbed

Red Green Blue

Photosynthesis action spectrum

Plant pigment extract absorption spectrum (mostly chlorophyll)

Fig 3.4

Chlorophyll’s structure permits its role in absorbing

light energy

• Chlorophyll has a porphyrin ring (like heme)– Long hydrocarbon tail:

hydro_________– So it is membrane-bound in

thylakoids– Almost all chlorophyll is also bound

to membrane proteins involved in harvesting light energy

• These are the photosystems

Fig. 3.7

Other pigments also contribute to the collection and transduction of light energy

• Light absorption is not just dependent on chlorophyll

• Accessory light-harvesting pigments collect light energy and pass it to chlorophyll to excite electrons for entry into photosynthetic E.T.C.

• Note how many of these have absorption spectra that complement that of chlorophyll!

Fig. 3.10: Phycocyanin and phycoerythrin

Compare the absorption spectra of these different plant pigments

Fig. 3.8: Chlorophyll

Fig. 3.12: Carotenes

Fig. 3.15: Pelargonin

These accessory pigments are often called “antenna pigments”

• Fill in action spectrum of photosynthesis more light useable for E.T.C. (not just red and blue of chlorophyll)

• Are important in human nutrition– Directly – -carotene – Vitamin A (metabolic

cofactor)– Indirectly – as antioxidants – quench excess

photosynthetically generated electrons in chlorophyll and reactive O2 (singlet excited O2)

Photosynthesis is composed of light-dependent and –independent

reactions(see chapter 4)• As we have discussed, this process

conserves energy found in light-excited electrons as ATP and NADPH

• Involved in the conversion of less stable energy more stable energy– Metabolically useful– Leaves are structured to maximize light

absorption

Chloroplasts are predominantly found in the leaf palisade mesophyll cells

Location is just underneath the top surface of leafThe structure of leaves favors light absorption

– Incoming photons directly excite chlorophyll in chloroplasts

– Sieve effect – epidermis does not absorb light passes through layers until absorbed

– Epidermal cells lenses redirecting light to palisade

– Palisade cells are light guides – allocate photosynthesis to spongy mesophyll save energy!

Fig. 4.2

A

C

B

D

• These reactions take place in the autotrophic tissues of the plant

• Again, predominantly (80%+) in chloroplasts of mesophyll cells of leaves

Light energy + H2O O2 + ATP + NADPH

• These products are then used in the light-independent (dark) reactions to convert CO2 to sugar

The light-dependent reactions of photosynthesis generate oxygen

and chemical energy

Chemiosmoticsynthesis

Note that the equation is not stoichiometric!

Like oxidative phosphorylation, the light-dependent reactions are a series

of redox reactions• It thus involves giving up (oxidation) and

accepting (reduction) electrons• Defined as the “photochemical reduction of

CO2”

– CO2 accepts electrons and thus H+ and forms the reducing sugar glucose

– CO2 C6H12O6 – not energetically favoured

– Highly positive G°’

• Use the photosynthetic E.T.C to generate ATP and NADPH

The structure of the photosynthetic electron transport chain can be conceptualized linearly

• 3 major multiprotein complexes embedded in thylakoid membranes

• The electrons flow noncyclically from an e- source (water) to an e- sink (oxidized reducing power, NADP+)

• The electrons pass through– Photosystem II– Cytochrome complex– Photosystem I

Rost et al., “Plant Biology”, 2nd ed.

Fig. 4.3

The electrons pass through the “Z-scheme” from water to the final e-

acceptor NADP+ Fig. 4.8• So-named because

of the shape• The photosystems

are excited by light energy

• This increases their energy level and redox potential

– + = accepts e-– - = passes on e-

• Thus they become very good at passing on their e- to other e- carriers

The Z-scheme shows the order of e- excitation and flow in the light-dependent reactions of photosynthesis

Cytochrome complex

First, PSII oxidizes water to produce oxygen and electrons

• H2O is oxidized on inside the thylakoid (in the lumen)

• This produces O2 + H+ important to generate proton gradient

• e- pass through the cytochrome complex on their way to PSI

• PSII needs energy from 8 photons to evolve one O2

Rost et al., “Plant Biology”, 2nd ed.

1

• More light is used to increase energy level of electrons in PSI at P700 (reaction centre)

• Excited reaction centre P700 reduces ferridoxin (Fd)

• Fd reduces NADP+ NADPH

Rost et al., “Plant Biology”, 2nd ed.

2

Second, PSI re-excites electrons in the transport chain, allowing them to reduce

NADP+

E.T.C function is to extract low energy electrons from water and use light to convert them to high energy electrons

• These are then used to generate a strong reductant (NADPH) that is used in the subsequent light-independent reactions of photosynthesis

• The efficiency of the light dependent reactions is “only” 32%– 32% of energy from excited

photons is conserved in reducing NADP+ NADPH

• However, the proton gradient generated in lumen drives chemiosmotic ATP synthesis (photophosphorylation)

• This substantially increases the energy yield!

Rost et al., “Plant Biology”, 2nd ed.

The photosystem physical structure is designed to maximize its ability to extract

energy from light• Light excites electrons in

antenna chlorophyll• The antenna chlorophyll

surrounds the reaction centre

• The reaction centre is the site where e- are swapped between donor (A) and acceptor (Q) proteins

• Recall that only the more stable triplet state electron is passed to the e- acceptor protein, reducing it

• This occurs in both photosystems! – P700 (PSI) – P680 (PSII)

Fig. 4.4The sum total of these components is called a light harvesting complex

• 90%+ thanks to large number of antenna chlorophylls per reaction centre

• Both LHCs together contain ~70% of total chlorophyll in plant

• Do not memorize the names of the electron carrier proteins in the membrane

• Do note the large number of proteins involved in this process!

Fig. 4.6

The PS design allows a high efficiency of energy collection from light

ATP can also be generated by cyclic electron flow

• This process involves passing e- between PSI and certain e- carrier proteins– Generates more H+ !– Chemiosmotically synthesize

more ATP

• Purpose: supplies energy for chloroplast metabolism beyond that needed for dark reactions (oxygen fixation)

Fig. 4.9

NADP+

Cyclic electron flow is possible because plants can adapt their metabolic needs to

their environment• ATP is synthesized when H+ built up inside the

lumen by the splitting of water pass through ATP synthases in thylakoid membrane

• Precise stoichiometry difficult to calculate because plant metabolism is very plastic– Cyclic and noncyclic transport importance vary– Electrons supply can be adjusted to modulate ATP

and NADPH levels

• Generally– 2 ATP per NADPH– 3 H+ through ATP synthase per ATP

The plasticity of the light-dependent reactions extends to their location

• The three main E.T.C. components (2 PS, 1 cytochrome complex) are NOT:– Fixed in membrane (movement)– Present in equal (stoichiometric) numbers– Physically next to each other – spatial

segregation

• All of these factors affect ATP and NADPH yield

The light harvesting complexes are essential parts of the photosystems

• We have seen that they are required to gather energy from light and transduce it to E.T.C.

• The amount and activity of the LHC proteins can be varied to modulate the electron flow through the photosystems– Sun plants: less– Shade plants: more– Long-term adjustment: changes [protein]

• Sun stress via sunflecks– Reversible phosphorylation of LHCII: +PO4, ↓ ability of

LHCII to pass energy to PSII therefore downregulate E.T.C.

– Short-term adjustment: changes protein activity

Photoinhibition occurs when the supply of light-excited e- exceeds the demand

from the E.T.C.• Too much light damages PSII reaction

centre (P680) = photoinhibition• Electron transport saturated excess

energy damages reaction centre protein– Electrons passed to oxygen forms reactive

oxygen species (ROS)

• Carotenoids – accessory pigments– Photoprotect by dissipating this energy (-

carotene)– Activated forms release energy as heat

Herbicides act by stealing e- away from the photosynthetic E.T.C.

• Herbicides are thus alternative electron acceptors in photosynthetic E.T.C. (viologen dyes)

• They can then pass these e- to oxygen to form superoxide and other ROS– Damage chlorophyll and membranes– Block protein binding in reaction centres necessary

for electron transfer

We will see next lecture how the products of the light-dependent reactions (NADPH and ATP) are used to fix atmospheric carbon in the light-independent reactions