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Biology

Chapter 7: Photosynthesis - Light Reactions

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Lecture

Photosynthesis: Light Reactions

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Energy and Light

Energy from Light

We looked last week at the way cells break down carbohydrates (in particular, glucose) into carbon dioxide and water, releasing energy in the process. This week, we look at the way the chloroplasts in plant and certain photosynthetic single-celled organisms store energy from light in chemical bonds, or, to put it another way, make sugar from sunshine.

Green plants have cells with 20-100 chloroplasts, organelles with many similarities to the mitochondria. The chloroplasts have multiple sets of membranes. Inside an inner membrane are the enzymes necessary to produce the carbohydrates. Inside another set of membranes are the thylakoids, flat structures with sacs called grana which all contain chlorophyll, a light-sensitive chemical (study the diagram in 7.2). It is inside these grana that photosynthesis takes place.

The simplified equation for photosynthesis is the reverse of that for aerobic respiration:

6 CO2 + 6H2O --> C6H12O6 + 6O2

This equation hides all the many individual chemical steps actually involved in the overall reaction. In considering these steps, we break the reaction process into two parts: the light-dependent reactions that take place in the chloroplasts, and the carbon-fixation reactions. During the light-dependent reactions, sunlight is used to split water, produce ATP and convert NADP+ to NADP. You may recall that ATP is not very long-lived; in fact, your body has only about a 5-minute store of ATP. So it must constantly manufacture ATP in order to replace what is used up. For longer term energy storage, cells use carbohydrates created during the carbon-fixation reactions.

A light course in physics

To understand what happens when plants store light energy, we need to digress briefly into an area usually reserved for physicists: the nature of light. Most of the energy on earth available to life forms reaches the earth as sunlight. We can think of an individual "beam" light as a wave traveling through space, much the way a wave of energy travels through water. The longer the wave, the less the energy it carries. With light, the wavelength translates to color: the longer the wavelength, the redder it is; the shorter the wavelength, the bluer it is. Actually, "light" is a general term for all electromagnetic radiation, which includes many wavelengths our eyes cannot see: from radio waves and the waves that heat your tea water in the microwave to the doctor's x-rays and gamma radiation from exploding galaxies now long gone. Each "beam" of light has a particular wavelength, and that wavelength is a specific amount of energy and a specific color.

Now we jump back to chapter two and atoms: remember those electrons "circling" the atom at discrete distances? Electron orbital
Suppose we want to move an electron from an inner orbital to an outer one. We can't just pump any amount of energy in: the electron needs a specific amount to move outward to the next orbital. Electron absorbs energy
Think of seats at the opera: the orchestra seats are $80 a performance, the third balcony rear are $12. The opera sales office will take checks for $80 or for $12, and put in you the right seat. But if you send them a check for $50, they do not make change: they return the check with a politely-worded letter asking you to remit the correct amount for the seat preferred.
In the same way, the electron can absorb or emit a specific amount of energy that will let it jump to a particular energy level and orbital. Having absorbed the energy, when it emits the same amount of energy, it falls back to the original orbital.
electron jumps orbitals
Now the electron is in a lower, more stable orbit. It can not move to the higher energy level without absorbing energy again. Note that absorbing or emitting energy doesn't change the electron itself; it only changes the electron's energy level, which dictates how fast it moves and how far it can be from the nucleus. So an electron can absorb and emit energy over and over. Electron low energy
From light theory, we know that the amount of energy that an electron can absorb corresponds to a given wavelength, which in turn we interpret as a specific color. So we can think of the process this way: the electron absorbs a beam of green light, and jumps up and away from its home atom. In the case of the chemicals in the chloroplast (chlorophylls and carotenoids), the electrons absorb blue and red light. They don't respond to yellow and green light, so those colors are reflected by the cell, and the plant leaf made up of those cells looks green or greenish-yellow to us. Electron transitions

Photosystems

Many plants actually have several light-sensitive pigments; the two primary ones are different forms of chlorophyll (chlorophyll a and chlorophyll b). Plants use a number different mechanism for photosynthesis; each mechanism requires specific chemicals and processes, which taken together make up a single photosystem.

In each process, several of chlorophyll molecules are grouped together as a reaction center in a cluster of enzymes and proteins. When light energy strikes the chlorophyll molecule, one of the chlorophyll's electrons jumps to a high orbit, and then passes to a protein which can act as a primary electron receptor. This protein holds on to the electron for a very short time, then passes it to another receptor, which moves it on again, like a molecular game of hot potato, or a ball bouncing down a flight of stairs. At each transfer or step, some of the energy in the excited electron is used to make ATP from ADP (that means a phosphate is added to the ADP; the process, remember, is phosphorylation, so the light-driven process is photophosphorylation. (It's a useful term to know if you ever need to think of a single word with three ph pairs in it).

In noncyclic photophosphorylation, electrons from water molecules replace those which have left the chlorophyll B molecule in a previous reaction. Light strikes the chlorophyll B and dislodges an electron. After the excited electron from has passed through several receptors and contributed to the production of ATP and the release of oxygen, it winds up in a molecule of NADPH, which also can be used for short term energy storage and is especially useful in making carbohydrates from CO2.

In cyclic photophosphorylation, the electrons released by the light energy bounce through a series of electron acceptors, contributing to ATP production, but not to the release of oxygen or to the creation of NADPH; the electron eventually returns to the chlorophyll atom.

Photosynthesis Summary

Chlorophyll molecule

Photosynthesis involves two chlorophyll molecules that differ only by the presence of a double-bonded oxygen in chlorophyll β substituted for two hydrogens in chlorophyll α. Light striking chlorophyll β (and other pigments in the plant mesophyll tissues) releases high-energy electrons that transfer light energy to chlorophyll α molecules. This ability means that light striking other molecules can be efficiently redirected to the chlorophyll α molecules that can make use of them, and very high energy light that could disturb or destroy the chlorophyll is absorbed and reduced to a less dangerous amount before it is transmitted elsewhere in the leaf.

Two kinds of chlorophyll α molecules exist, identical in structure but differing in their links to other molecules in the thylakoids, in such a way that their ability to absorb light energy differs slightly. Light with a wavelength of 680nm enters the P680 chlorophyll molecule, exciting electrons in the magnesium central atom until they leave the molecule entirely. The electrons collide with a nearby membrane, cascade back through a sequence of electron acceptor molecules in the membrane, and return to replenish the P700 molecules. As they cascade, the energy released is used to synthesize ATP. When the P700 molecules are excited by light at 700nm, the electron released this time cascades through a shorter set of molecules, releasing energy that is used to reduce NADP+ to NADPH. The energy stored in the ATP and NADPH is available for use in the sequence of reactions that make up the Calvin cycle.

Study the steps in the image below. Note which direction the electrons and H+ ions are traveling, and where energy is harvested from each. Photosynthesis is a "harvesting" operation -- the energy isn't actually stored in sugars and starches until the Calvin cycle process starts.

Thylakoid membrane