10.2 Light reactions convert light to ATP and NADPH

thylakoids transform

light into chemical energy ATP and NADPH

Electromagnetic radiation

light

Wavelength

the distance between the crests of waves, determines the type of electromagnetic energy

Electromagnetic spectrum

the entire range of electromagnetic energy or radiation

Visible light

wavelengths that can be seen by the eye

Photons

discrete particles of light

The longer the wavelength

the lower the energy

the shorter the wavelength

the higher the energy

Pigments

substances that absorb certain wavelengths of visible light

wavelengths that are not absorbed by a pigment

are reflected or transmitted

leaves only appear green because

chlorophyll does not absorb green wavelengths, it transmits and reflects them

Absorption spectrum

is a graph plotting a pigment light absorption versus the wavelength of light

Chlorophyll absorbs

violet blue and red, which mean those colors work best for photosynthesis

Action spectrum

plots the rate of a process like photosynthesis versus the wavelength of light

An action spectrum can show the relative

effectiveness of different wavelengths in driving photosynthesis

absorption spectra

the curves show the light best absorbed by the types of chloroplasts pigments

chlorophyll a

is the main photosynthetic pigment and participates directly in light reactions

Chlorophyll b

is an accessory pigment that absorbs light at different wavelengths and broadens the spectrum used for photosynthesis

carotenoids

are accessory pigments, are hydrocarbons of shades of yellow and orange because they absorb violet and blue-green light

photoprotection

carotenoids absorb dissipating excessive light energy that would otherwise cause oxidative damage to chlorophyll

CH₃

in chlorophyll a

CHO

in chlorophyll b

porphyrin ring

light absorbing head of the chlorophyll molecule, magnesium at the center

Hydrocarbon tail of the chloroplast

interacts with the hydrophobic regions of proteins inside the thylakoid membranes of chloroplasts

when a pigment molecule absorbs a photon of light

one of the pigment molecules electrons is elevated to an orbital where it has more potential energy

when the electron is in its normal orbital

the pigment is in a ground state

the absorption of light

boosts the pigment to its unstable excited state

when the excited electron falls back to the ground state

a photo of light is given off creating fluorescence

if illuminated an isolated solution of chlorophyll will

fluoresce giving off light and heat

excess energy form the electron dropping back to ground state

the energy is given off as heat and fluorescence (light)

A photosystem

consists of a reaction center complex, and a primary electron acceptor

the reaction center

a type of protein that holds a special pair of chlorophyll a li

light harvesting complexes

pigment molecules like chlorophyll a, b and carotenoids all bound to proteins, that surround the reaction center.

light harvesting complexes act like

antennas that funnel the energy of the photons from pigment molecule to pigment molecule until that energy reaches the reaction center and is transferred to the special pair of chlorophyll a molecules

the photosystem takes place

in the membrane between the stroma and the thylakoid

Two types of photosystems in the thylakoid membrane

• photosystem II (PS II)

• photosystem I (PS I)

photosystem II

functions first (the numbers reflect order of discovery) and is best at absorbing the red part of the spectrum (680)

the special reaction center chlorophyll a of a PS II is called

P680

photosystem I

is best at absorbing light with a wavelength at 7000 (far red part)

Reaction center chlorophyll a of PS I

is called P700

Light drives the synthesis of ATP and NADPH by energizing

the two photosystems embedded in the thylakoid

The key to the energy transformation

is the flow of electrons through the photosystems and the other molecular components built into the thylakoid membrane

there are two possible routes for electron flow

linear and cyclic

Linear electron flow

the primary pathway involves both photosystems and produces both ATP and NADPH using light energy

First step of linear electron flow

a photon hits a pigment and its energy is passes among pigment molecules until it excites chlorophyll P680

the second step of linear electron flow

an excited electron from P680 is transferred to the primary electron acceptor and once this happens P680 is called P680+ because it lost an electron

P680 is the

strongest biological oxidizing agent

the third step of linear electron flow

H₂O is split by the enzymes into two electrons two hydrogen ions (H+) and an oxygen atom

due to the enormous oxidizing strength of P680+

the electrons are easily transferred to P680+ and thus reducing it back to P680

during the third stage of linear electron flow the H+ are

released into the thylakoid space interior, and the oxygen atom immediately combines with another oxygen creating O₂

the fourth step of linear electron flow

each electron falls down an electron transport chain in the thylakoid membrane through redox reactions from the primary electron acceptor of PS II to the reaction center of PS I

step 5 of linear electron flow

free energy released by this electron transport chain is used to pump H+ from the stroma into the thylakoid space, creating a proton gradient across the thylakoid membrane

in linear electron flow the diffusion of H+ back across the membrane from the thylakoid space into the stroma

drives ATP sythesis

step 6 of linear electron flow

in PS I transferred light energy excites chlorophyll p700 which loses an electron to a primary electron acceptor and is now called P700+

durring linear electron flow P700 +

accepts an electron passed down from PS II via the electron transport chain reducing P700+ back to P700

step 7 linear electron flow

each photo excited electron falls down an electron transport chain in a series of redox reactions from the primary electron acceptor of PSI to the protein ferredoxin (Fd)

in step 7 the photo excited electrons falling down the electron transport chains

do not create a proton gradient and thus do not produce ATP

In step 8 of the linear electron flow

the enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+

the reduction of NADP+ to NADPH

takes two electrons

after linear electron flow the electrons of

NADPH are now available for the reactions of the calvin benson cycle, this process also removes an H+ form the stroma of the cholorplast

what is are the products of linear electron flow

ATP (PS II) and NADPH (PS I)

Chloroplasts and mitochondria both generate ATP by chemiosmosis but

they use different energy sources

Chloroplasts transform

radient energy into chemical energy of ATP, instead of food into ATP like in mitochondria

Spatial orgaization of chemiosmosis differs between

chloroplasts and mitochondria but there are similarities

In mitochondria protons are pumped

from the matrix across the inner membrane to the intermembrane space and drive ATP as they diffuse back

In chloroplasts protons are pumped

from the stroma across the thylakoid membrane into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma

what does this demonstrate

the similarities and differences of the mitochondria and the chloroplast

ATP and NADPH are produced during

the light reactions on the side of the thylakoid membrane facing the stroma

where does the Calvin benson cycle take place

the side of the thylakoid facing the stroma

the light reactions remove

electrons from the oxygen in water (PS II) and pass the electrons through the electron transport chain in the thylakoid membrane that powers chemiosmosis to generate ATP

The light reactions pass electrons

from the electron transport chain of PSII to PSI which further increases the potential energy of the electrons by moving them to NADPH

what is this demonstrating

the flow of electrons during photosynthesis

Protons that power ATP synthase

come from photosystem II, and four protons are moved across the membrane

NADPH in the light reactions comes from

photosystem II and I that then passes through NADP+ reductase where a hydrogen ion is removed from the stroma when it is taken up by NADP+ and is then reduced to NADPH