Chloroplasts

CHLOROPLASTS

In those organisms that have chloroplasts, this is where the energy gathering occurs. The structure of a chloroplast is shown in figure 26:

One of the main structures inside chloroplasts are called thylakoids. These are folded cylindrical sheets of membranes that have a very large surface area so as to gather the most light possible. Bacteria that have no chloroplasts either use the cell membrane itself or bunched up pieces of cell membrane that are also called thylakoids.

In algae and plants, there are ten to a hundred chloroplasts per cell. These are membranous structures that have an internal stroma that is filled with stacks of thylakoids, where photosynthesis takes place. Each thylakoid is membranous and has its own lumen, called the thylakoid space. Within the membrane, similar to the idea that inside mitochondrial cristae there are membrane-bound enzymes, there are membranebound protein complexes in these thylakoids that participate in the process of photosynthesis.

There is a variety of pigments used in absorbing light as part of photosynthesis. By far the most important and abundant is chlorophyll. Other pigments used in photosynthesis are xanthophylls, carotenes, fucoxanthin, phycoerythrin, and

phycocyanin . These will be the pigments seen in brown algae, green-blue algae, and red algae. The pigments are imbedded in complexes known as light-harvesting complexes.

Light-dependent reactions happen in the chloroplast membrane (in the thylakoids). The chlorophyll pigment absorbs a photon of light, losing an electron in the process. This electron flows from one molecule to another in a complex that leads to the production of NADPH from NADP. There is a proton gradient produced by this process—the same proton gradient in mitochondria that is necessary to make ATP using ATP synthase. The chlorophyll molecule gets its electron back by splitting water to make oxygen gas (O2) as a byproduct.

Not all wavelengths of light work for photosynthesis. This is completely dependent on the pigment used in the organism. The part of the light spectrum that is not absorbed is “given off” in a sense as the visible color of the organism. In plants, the light not absorbed is mostly green so that the color of the organism is green. In red algae and other non-green organisms that participate in photosynthesis, different spectrums of light are absorbed and not absorbed than is seen in plants.

There are cyclic and non-cyclic forms of light-dependent reactions in the thylakoid membranes of the chloroplast. The non-cyclic forms involve the capture of light by chlorophyll that frees up an electron from the pigment, shuttling the electron down an electron transport chain called a Z-scheme. This generates a chemiosmotic gradient across the thylakoid membrane that charges the ATP synthase molecule to make ATP. The final electron acceptor in this case is NADP, which takes it to make NADPH.

The cyclic photosynthetic reaction is similar to the non-cyclic form but makes only one ATP molecule and does not create an NADPH molecule. The electron that is passed down the electron chain and is ultimately returned to the system to start over again. This is, of course, why it is called cyclic.

When chlorophyll loses its electron (when light is absorbed by it) it is oxidized and needs an electron back again. In most plants and in cyanobacteria, the electron donor in this case is water. Two water molecules are split as part of photosynthesis to yield O2 and four hydrogen ions that have electrons to share. These are shared through a series

of reactions to reduce chlorophyll again. They also are used in part to create the chemiosmotic membrane potential that drives ATP synthase.

The Calvin cycle is a light-independent or dark reaction series in plants. There is an important enzyme called RuBisCO that takes the CO2 from the atmosphere and fixes it to make three-carbon sugars that are later combined to make things like sucrose and starch. It relies on both ATP and NADPH to drive these reactions. Fortunately, these molecules are in abundant supply because of the light-dependent reactions.

The end product made from the Calvin cycle is called triose phosphate, a 3-carbon sugar. Ribulose-1,5-bisphosphate is the five-carbon sugar in the cycle that takes on CO2 and splits to make glyceraldehyde-3-phosphate, also referred to as triose phosphate. Most of these will go on to regenerate the cycle again, similar to what goes on with oxaloacetate in the Krebs cycle; however, some will not do this and will join to make a six-carbon sugar outside of the cycle. Other molecules are subsequently built from these six-carbon sugars with the storage molecule in plants being starch (chains of glucose molecules).