Lecture 35 Summary

The basic equation of photosynthesis as carried out by green plants was revealed by experiments carried out during the 18th and 19th centuries by investigators including Priestley, Ingenhousz, and de Saussure. This equation is as shown below.

At the beginning of the 20th century, it was believed that light energy directly caused the reduction of carbon dioxide to carbohydrates, but we now understand photosynthesis as a two-stage process. In the first stage, light energy is used by green plants to oxidize the oxygen in water, splitting it into molecular oxygen and electrons, with energy left over to spare:

( N.B. By convention, we associate a standard reduction potential with a half-reaction written as a reduction. The above equation is written as an oxidation of the oxygen in water, so photosynthesis in green plants must create an oxidant species with an even more positive reduction potential than molecular oxygen.)

The equation above represents what is referred to as the light reaction(s) of photosynthesis in green plants. The oxygen evolved is derived from water, not carbon dioxide. In the second stage of photosynthesis, green plants use the energy and electrons (reducing power) derived from the light reactions to incorporate, or "fix", carbon dioxide in the form of carbohydrate.

Interaction of matter with light

The specialized pigment molecules of photosynthesis - principally chlorophyll (see figures below) - are highly conjugated molecules with absorption frequencies in the visible range. The Planck relationship states that the energy of a photon of electromagnetic radiation is proportional to its frequency, and inversely proportional to its wavelength. When this energy corresponds to the difference between energies of electronic molecular orbitals, the photon may be absorbed and its energy then appears as an excited state of the molecule - that is, an electron is promoted to a higher energy molecular orbital.

But this excited state is short-lived. Four potential fates of the excited state concern us here.

(1) Internal conversion - the energy is dissipated into molecular rotational and vibrational modes. This is basically entropic loss.

(2) Fluorescence - the molecule may emit a photon, typically a longer wavelength than the absorbed photon.

(3) Photooxidation - the molecule in its excited state holds on to that electron a little less tightly, making this species - if it has a long enough lifetime - a better reducing agent. If the electron in the higher energy state is transferred to an acceptor species and away from the original excited molecule, that latter molecule has been oxidized.

(4) Exciton transfer (also called resonance energy transfer) - the energy of the excited state can be transferred to a nearby molecule. This is also an important process in photosynthesis, as there are many more pigment molecules than the number that actually undergo photooxidation.

Together, proceses (3) and (4) are the essence of the efficient harvesting of light energy in photosynthesis.

Chloroplast physiology

Just as in mitochondrial electron transport, a transmembrane "proton-motive force" is generated in photosynthesis that in turn is used to synthesize ATP. The photosynthetic organelle is the chloroplast. Here we consider some basic features of chloroplasts in order to better understand photosynthesis. An examination of chloroplast physiology and membrane properties will serve to make both the analogy with mitochondria and its limitations more clear.

Like mitochondria, chloroplasts have a relatively porous outer membrane. While in mitochondria, the inner membrane is highly invaginated and is the site of electron transport, this is not true of chloroplasts - they have an additional membrane within the inner membrane called the thylakoid membrane.

The thylakoid membrane encloses a third compartment within the chloroplast called the thylakoid space. Between the inner membrane and the thylakoid membrane is the stroma. Thus, the thylakoid membrane - where the protein-pigment complexes that harvest light energy are located - separates the thylakoid space from the stroma. The stroma can be considered as analogous to the mitochondrial matrix, while the thylakoid membrane is like the inner mitochondrial membrane. By this scheme, thylakoid space would correspond to the intermembrane space, which is essentially equivalent to the cytosol. This may at first seem counterintuitive or confusing, but it makes heuristic sense. Furthermore, the thylakoid membrane forms in developing chloroplasts by the invagination and ultimate "budding" of the inner membrane. These invaginations would have originally surrounded the intermembrane compartment, so it seems physiologically and topologically correct to think of the thylakoid space as a compartment related to the intermembrane space. Of course, in mature chloroplasts, the thylakoid membrane and its enclosed compartment have quite different compositions and properties; for example, the unique lipid composition and the high proportion of protein in the thylakoid membrane. The latter characteristic is consistent with the central role of the thylakoid membrane in photosynthetic energy transduction.

References