Photosynthesis feeds respiration

We have been studying photosynthesis up to now, and you will now find that the products of photosynthesis feed back into a process that releases energy for doing the work within the plant cell, particularly if that cell is not in light or is not one which carries out its own photosynthesis.

Respiration occurs in plants

The process in animals often called cellular respiration occurs in plants. It is very similar to the pathway used in animal cells, but does have some interesting differences as well. As you may recall respiration can be summarized simply by the following:

CH2O + O2 → CO2 + H2O + energy

Also just as in animals, this process is far more complex than that simple formula. There are four sections of critical importance in the pathway.

  1. glycolysis, an enzymatic path in the cytosol
  2. tricarboxylic acid cycle, an enzymatic path in the mitochondrial matrix
  3. electron transfer system, an electronic path in the mitochondrial inner membrane
  4. oxidative phosphorylation, a proton driven ATP synthesis between the intermembrane space and the matrix of the mitochondrion.

We shall now summarize briefly a pathway that you should already know very well from prerequisite courses, and I will add some highlight to the differences in these pathways found in plants.

Glycolysis occurs mostly in the cytosol

Glycolysis is an enzymatic path for splitting sugar into pyruvate. The reaction system is shown below. This diagram again shows the connections of glycolysis to what is happening in the rest of the cell.

Here you can see the important role in plants of sucrose, the transport form of photosynthate. For cells in the stem, the root, the flowers, the fruits, and meristems, sucrose is delivered by phloem and the role of invertase in the cytosol to cleave this sugar prior to processing in glycolysis is critical.

Also shown here is the important role of starch, the storage form of photosynthate. For plant cells the starch is stored primarily in chloroplasts or other plastids and so this must be retrieved from them enzymatically before processing in glycolysis.

The rest of glycolysis is pretty much standard...

You can see above, that there are multiple ways to get from glycolysis to the tricarboxylic acid cycle in the mitochondria! Yes, pyruvate can be processed in the usual way to load the TCA cycle, but the three-carbons of a split six-carbon sugar can be carboxylated (think C4 plants!) and the four-carbon acid can be loaded into the TCA cycle.

You might also notice the very critical role of a second pathway for putting organic acids into the mitochondrion. In that pathway, an enzyme evolved for converting phosphoenolpyruvate into oxaloacetate; this is PEP carboxylase! This enzyme evolved as an supplemental pathway...but was later harnessed by C4 and CAM plants because of its use of bicarbonate rather than dissolved CO2 to fix carbon. Wow!

Also note here that plants are capable of both well-known anaerobic pathways of fermentation...lactic acid and alcohol as final products. These mark glycolysis as an ancient pathway designed to operate in conjunction with the original anaerobic atmosphere on Earth. It was a way to recycle NAD+ so that glycolysis could serve as a way to make ATP in the absence of oxygen. The fermentation pathway is not particularly efficient in recovering the energy trapped in sugar compared to respiration, so it is no surprise that evolution has not produced enzymes for the final step that operate in oxygen-rich environments (such as the modern atmosphere). The fermentation pathway would compete with the much more efficient (though more complex) respiration pathway for pyruvate. These fermentation steps then are only activated under anaerobic or microaerobic conditions, such as in roots in flooded soils.

The glycolytic pathway is also shown below. Here the emphasis is on the chemical structures in glycolysis.

As you can see, sucrose is a disaccharide that is cleaved into glucose and fructose. The locations of phosporylations are shown. The pathway does not continue with glucose, instead all sugars go via fructose which is phosporylated and then cleaved as shown to yield a 3-carbon glyceraldehyde phosphate. This material from the investment phase is then converted to pyruvate in the harvesting phase. The intermediates and products of glycolysis and fermentation are shown up close next:

Gluconeogenesis is essentially the reverse of glycolysis

Because enzyme catalyzed reactions are essentially reversible, in most cases, plants can convert pyruvate backwards into sugars for transport and storage. The step involving phosphofructokinase is virtually irreversible however, so plants have evolved a different enzyme, fructose-1,6-bisphosphatase to catalyze the conversion of fructose-1,6-bisphosphate into fructose-6-phosphate in gluconeogenesis.

The details of how these two enzymes are allosterically regulated was covered earlier in the semester, but just to remind us of this the following image is provided here.

Gluconeogenesis is important in the metabolism of lipids in plants, but lipid metabolism is less common in plants than it is in other organisms. Where we find gluconeogenesis as a critical metabolic pathway is in the germination and early growth of oil-storing seeds. In these, the stored oils, invested into the seed by the mother plant, are converted to acetate units, maneuvered back into pyruvate, backed up into sugars, dimerized into sucrose, and transported to where growth needs to occur.

Respiration continues in the mitochondrion

While the enzymes of glycolysis are essentially solutes in the cytosol, the rest of respiration is accomplished in the mitochondrion. I remind you here that the mitochondrion is a partially autonomous organelle believed to have originated as a prokaryotic endosymbiont. It has an outer membrane (the eukarytoic endosybiosis vesicle), and inner membrane (the prokaryotic endosymbiont cell membrane), and a matrix (the prokaryotic endosymbiont cytosol). This is diagrammed below.

Of interest is the chondriome, the genome of the prokaryotic endosymbiont. This naked, circular DNA genome includes genes for the prokaryotic tRNAs, rRNAs, and many of the proteins for the electron transfer system. Interestingly, the genes for many if not most of the TCA enzymes are located now in the nuclear genome of plant cells. It is no surprise that the ribosomes in the matrix are 70S (prokaryotic) type as their structure is essentially produced by the chondriome.

The plant chondriome is larger than most. It is around 100 kb in length versus about 16 kb in humans. The plant chondriome codes for 20 to 30 proteins while the human chondriome codes for just 13.

Another difference is that the plant chondriome follows the "universal" genetic code quite strictly whereas the human chondriome follows a modified code. On the other hand, the plant chondriome shows incredible levels of RNA editing after transcription. This modifies the RNA before it is translated...many pyrimidines are altered (C→U) prior to translation.

One critical mutation that has been found and put to use by humans in the plant chondriome is cytoplasmic male sterility. This rearrangement of the URF13 protein produces an essentially female plant. This is of great importance to plant breeding. Most plants are bisexual (hermaphroditic) and can self-pollinate. To create hybrids on a commercial scale, genetic stocks intended to serve as females can be manipulated with this gene to lose their male functions. Then we can let natural pollination from bisexual stocks in the field produce our hybrid seeds to grow crops.

This process carried out in corn had a horrible end...a fungal pathogen called Southern Corn Blight produces a toxin that interacts with the modified URF13 protein causing mitochondrial leakage and cell death. Since farmers had converted almost completely to producing hybrid corn with the modified URF13 genes in the chondriome, almost the entire national corn crop failed in the early 1970s. It seems humans learn the lessons about the dangers of monoculture only in the "school of hard knocks."

Respiration continues with the TriCarboxylic Acid cycle

Plants use a fairly standard TCA (citric acid or Krebs) cycle in the matrix of the mitochondrion. Enzymes here are mostly soluble: exceptionally succinate hydrogenase is membrane-bound. This pathway is shown below.

You might notice that many of these enzymatic steps involve the conversion of NAD+ to NADH. Here you can observe how this conversion permits this vitamin to serve as a temporary trap for protons and electrons:

One deviation in the TCA cycle for plants is the direct phosphorylation of ADP in the step from Succinyl CoA to Succinate. In other organisms, the phosphorylation there is in two steps with GTP as an intermediate.

Also one step in the TCA cycle reduces FAD to FADH and this diagram shows how that works:

Respiration continues in an Electron Transfer System

The electron transfer system in the inner membrane of mitochondria in plants is shown below.

Here you can see the usual components of the electron transfer system (ETS) that you learned about in prerequisite courses. However, added to that you can also observe some of the differences found in the ETS in plants.

  1. You can see on the intermembrane side of the inner membrane of the mitochondrion an NAD(P)H dehydrogenase.
  2. On the matrix side of the inner membrane of the plant mitochondrion has another NADPH oxidation element. This one is not sensitive to rotenone, a toxin produced by plants against herbivores. Evolution will favor plants that are resistant to their own toxins!
  3. There is an alternative oxidase (AOX not shown) to move the electrons onto oxygen before the cytochrome steps. This makes it possible for plants to store cyanogenic compounds (cyanide producing toxins) to prevent herbivory without poisoning their own metabolism. Plants can grow in the presence of cyanide. Many plants store such toxins in seeds.

It is worthy to note that while plants share ancient pathways with other organisms, plants have evolved many complications that make them interesting to study. Their evolution provides a wealth of genes to examine for potential use in genetic engineering.

Respiration is concluded in Oxidative Phosphorylation

As you observed in the previous diagram of the ETS, it is coupled to the production of ATP by proton pumping into the intermembrane space and the moving back of protons to the matrix through and ATPsynthase complex. This complex has the usual "lollipop" form and behaves in the same rotational manner. The connection of these processes is summarized below.

Respiration is a source of metabolites

While we often teach the respiration pathway as a kind of unidirectional flow of materials from carbohydrate to oxygen and ATP, this is really a gross simplification. The pathway operates in pools of the various intermediates. The cell can add to and draw from these pools of intermediates as it needs. So 2, 3, 5, and 6 carbon-units can be used for normal processes in metabolism. For example, five-carbon sugar phosphates can be drawn off for nucleic acid synthesis. Many intermediates can be aminated to produce amino acids. Conversely, amino acids can be deaminated to feed materials into the respiration pathway. Carbohydrates are NOT the only fuels for repiration. These relationships are shown below.