V. HOW THE LIGHT DEPENDENT REACTIONS CAPTURE SOLAR ENERGY

A. The nature of sunlight

1. About 120 million tons of solar matter are converted to an enormous amount of

energy each minute.

2. A very small fraction of this energy reaches earth.

3. Some of this energy strikes the leaves of plants and is absorbed by the chloroplasts.

B. Sunlight is a form of energy known as electromagnetic energy or radiation.

1. Electromagnetic energy travels in rhythmic waves which are disturbances of

electric and magnetic fields.

2. A wavelength is the distance between the crests of electromagnetic waves.

3. Wavelengths range from less than a nanometer for gamma rays to more than a

kilometer for radio waves.

4. This range of wavelengths is known as the electromagnetic spectrum.

THE MOST IMPORTANT PORTION OF THE ELECTROMAGNETIC SPECTRUM TO LIFE IS THE VISIBLE LIGHT, A NARROW BAND THAT RANGES FROM ABOUT 400 NM TO 700 NM WHICH IS DETECTABLE BY THE HUMAN EYE.

  • The atmosphere acts as a selective window that allows visible light to pass through while screening out a substantial fraction of other radiation.
  • The visible range of light is the radiation that drives photosynthesis.
  • Blue and red, the two wavelengths most effectively absorbed by chlorophyll, are the colors most useful as energy for the light reactions.

VI. PHOTOSYNTHETIC PIGMENTS

A. Light may be reflected, transmitted, or absorbed when it comes in contact with matter.

B. Pigments-substances that absorb visible light

1. Different pigments absorb light of different wavelengths

2. Wavelengths that are absorbed disappear

C. The pigments of photosynthesis are:

1. Chlorophyll a (grass-green)

2. Chlorophyll b (yellow-green)

3. Carotenoids (shades of yellow and orange)

THESE DIFFERENT PIGMENTS ABSORB WAVELENGTHS OF LIGHT THAT DIFFER. THIS BROADENS THE RANGE OF COLORS THAT CAN DRIVE PHOTOSYNTHESIS.

D. The energy absorbed by chlorophyll b and the carotenoids is transferred to chlorophyll a.

E. Only Chlorophyll a can participate directly in the light reactions.

VII. WHAT HAPPENS WHEN CHLOROPHYLL OR ACCESSORY PIGMENTS ABSORB PHOTONS?

A. Photon-a fixed quantity of energy

B. Ground state-when a pigment molecule’s electrons are in their normal orbital

C. Excited state-when a pigment molecule’s electron has been boosted to an orbital of

greater energy value (this occurs when a molecule in ground state absorbs a photon)

D. The excited state of an electron is UNSTABLE and the electron quickly FALLS BACK TO THE

GROUND STATE with excess energy being EMITTED as HEAT, LIGHT, OR BOTH as a

photon is released.

E. Fluorescence-the afterglow due to the release of a photon from a molecule

F. Pigment molecules DO NOT EMIT FLUORESCENCE when in the thylakoid membranes

due to the presence of PRIMARY ELECTRON ACCEPTOR MOLECULES which are nearby.

1. The primary electron acceptor traps the excited state electron which absorbed the

photon.

2. Chlorophyll loses an electron by the absorption of light energy and the electron

acceptor gains an electron

3. This is, therefore, a REDOX REACTION

a. Chlorophyll is oxidized

b. electron acceptor is reduced

THE TRANSFER OF THESE EXCITED STATE ELECTRONS FROM CHLOROPHYLL TO THE PRIMARY ELECTRON ACCEPTOR IS THE FIRST STEP OF THE LIGHT REACTIONS.

VIII. THE TWO PHOTOSYSTEMS

A. Photosystem-a cluster of a few hundred photosynthetic pigments within the thylakoid

membrane

1. Only one of the many chlorophyll a molecules in this group can start the light

reactions.

2. This specialized chlorophyll a molecule is located in the reaction center.

3. The other chlorophyll a, chlorophyll b, and carotenoids absorb photons and

pass the energy from molecule to molecule and finally to the reaction center.

B. There are two types of photosystems located in the thylakoid membranes

1. Photosystem I-contains the reaction center chlorophyll a called P700.

2. Photosystem II-contains P680

C. P700 and P680

1. Are the specialized chlorophyll a molecules located in the reaction centers of

Photosystems I and II.

2. P700 absorbs best at the 700 nm wavelength

3. P680 absorbs best at the 680 nm wavelength

4. These molecules are identical but each is associated with a DIFFERENT PROTEIN

which affects their electron distribution and results in a SLIGHT DIFFERENCE IN

ABSORPTION SPECTRA.

P700 AND P680 ARE DIFFERENT FROM OTHER CHLOROPHYLL A MOLECULES ONLY IN THEIR LOCATION IN THE THYLAKOID MEMBRANE, THEIR RELATIONSHIP TO SPECIFIC PROTEINS, AND THEIR LOCATION CLOSER TO PRIMARY ELECTRON ACCEPTORS.

THERE ARE TWO POSSIBLE ROUTES FOR ELECTRON FLOW DURING LIGHT REACTIONS:

  • CYCLIC FLOW OF ELECTRONS
  • NONCYCLIC FLOW OF ELECTRONS

IX. CYCLIC FLOW OF ELECTRONS

A. The simplest pathway but involves only photosystem I and generates ATP without

producing NADPH or evolving oxygen

B. Is cyclic because excited electrons that leave chlorophyll a at the reaction center return

return to the reaction center.

C. Steps of cyclic flow

1. Photosystem I absorbs two photons of light

2. Excited electrons trapped by primary electron acceptor

3. Electrons are carried along an electron transport chain in the thylakoid membrane

4. Electrons are carried back to P7oo in Photosystem I reaction center.

D. What occurs while these electrons are following cyclic flow?

1. At each reaction along the ETC the electrons lose potential energy until they return

to their ground-state orbital in P700.

Below are two different artists’ view of cyclic flow of electrons:


2. CHEMIOSMOSIS (OCCURS AS ELECTRONS ARE CARRIED DOWN THE ETC)

a. This trip is used to pump H+ ion across the thylakoid membrane.

b. An ATP synthase enzyme in the thylakoid membrane uses this to make ATP

in a process called PHOTOPHOSPHORYLATION.

c. Cyclic photophosphorylation – a form of ATP formation that occurs during the


cyclic pathway.

NOTE: ATP IS PRODUCED WITHOUT THE PRODUCTION OFNADPH OR THE RELEASE OF OXYGEN.