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Biology library
Course: Biology library > Unit 12
Lesson 6: Variations on cellular respirationConnections between cellular respiration and other pathways
How molecules other than glucose enter cellular respiration. Use of cellular respiration intermediates for biosynthesis.
Introduction
So far, we’ve spent a lot of time describing the pathways used to break down glucose. When you sit down for lunch, you might have a turkey sandwich, a veggie burger, or a salad, but you’re probably not going to dig in to a bowl of pure glucose. How, then, are the other components of food – such as proteins, lipids, and non-glucose carbohydrates – broken down to generate ATP?
As it turns out, the cellular respiration pathways we’ve already seen are central to the extraction of energy from all these different molecules. Amino acids, lipids, and other carbohydrates can be converted to various intermediates of glycolysis and the citric acid cycle, allowing them to slip into the cellular respiration pathway through a multitude of side doors. Once these molecules enter the pathway, it makes no difference where they came from: they’ll simply go through the remaining steps, yielding NADH, FADHstart subscript, 2, end subscript, and ATP.
In addition, not every molecule that enters cellular respiration will complete the entire pathway. Just as various types of molecules can feed into cellular respiration through different intermediates, so intermediates of glycolysis and the citric acid cycle may be removed at various stages and used to make other molecules. For instance, many intermediates of glycolysis and the citric acid cycle are used in the pathways that build amino acidsstart superscript, 1, end superscript.
In the sections below, we’ll look at a few examples of how different non-glucose molecules can enter cellular respiration.
How carbohydrates enter the pathway
Most carbohydrates enter cellular respiration during glycolysis. In some cases, entering the pathway simply involves breaking a glucose polymer down into individual glucose molecules. For instance, the glucose polymer glycogen is made and stored in both liver and muscle cells in our bodies. If blood sugar levels drop, the glycogen will be broken down into phosphate-bearing glucose molecules, which can easily enter glycolysis.
Non-glucose monosaccharides can also enter glycolysis. For instance, sucrose (table sugar) is made up of glucose and fructose. When this sugar is broken down, the fructose can easily enter glycolysis: addition of a phosphate group turns it into fructose-6-phosphate, the third molecule in the glycolysis pathwaysquared. Because it enters so close to the top of the pathway, fructose yields the same number of ATP as glucose during cellular respiration.
How proteins enter the pathway
When you eat proteins in food, your body has to break them down into amino acids before they can be used by your cells. Most of the time, amino acids are recycled and used to make new proteins, not oxidized for fuel.
However, if there are more amino acids than the body needs, or if cells are starving, some amino acids will get broken down for energy via cellular respiration. In order to enter cellular respiration, amino acids must first have their amino group removed. This step makes ammonia left parenthesis, start text, N, H, end text, start subscript, 3, end subscript, right parenthesis as a waste product, and in humans and other mammals, the ammonia is converted to urea and removed from the body in urine.
Once they’ve been deaminated, different amino acids enter the cellular respiration pathways at different stages. The chemical properties of each amino acid determine what intermediate it can be most easily converted into.
For example, the amino acid glutamate, which has a carboxylic acid side chain, gets converted into the citric acid cycle intermediate α-ketoglutarate. This point of entry for glutamate makes sense because both molecules have a similar structure with two carboxyl groups, as shown belowcubed.
How lipids enter the pathway
Fats, known more formally as triglycerides, can be broken down into two components that enter the cellular respiration pathways at different stages. A triglyceride is made up of a three-carbon molecule called glycerol, and of three fatty acid tails attached to the glycerol. Glycerol can be converted to glyceraldehyde-3-phosphate, an intermediate of glycolysis, and continue through the remainder of the cellular respiration breakdown pathway.
Fatty acids, on the other hand, must be broken down in a process called beta-oxidation, which takes place in the matrix of the mitochondria. In beta-oxidation, the fatty acid tails are broken down into a series of two-carbon units that combine with coenzyme A, forming acetyl CoA. This acetyl CoA feeds smoothly into the citric acid cycle.
Cellular respiration: It's a two-way street
We've thought a lot about how molecules can enter cellular respiration, but it's also important to consider how they can exit. Molecules in the cellular respiration pathway can be pulled out at many stages and used to build other molecules, including amino acids, nucleotides, lipids, and carbohydrates.
To give just one example, acetyl CoA (mentioned above) that's produced in cellular respiration can be diverted from the citric acid cycle and used to build the lipid cholesterol. Cholesterol forms the backbone of the steroid hormones in our bodies, such as testosterone and estrogens.
Whether it's better to "burn" molecules for fuel via cellular respiration or use them to build other molecules depends on the needs of the cell—and so does which specific molecules they're used to build!
Want to join the conversation?
- Is there a difference in pathway between normal cells and cancerous cells?(10 votes)
- Yes, cancer cells tend to favor lactic acid fermentation and this is known as the Warburg effect.
The following is a link to get you started on learning more about this subject:
https://www.researchgate.net/figure/Metabolic-differences-between-normal-and-cancer-cells-are-shown-In-normal-cells-glucose_fig1_266086485
A recent review article on this:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4928883/
And a "webinar":
http://www.sciencemag.org/custom-publishing/webinars/metabolic-changes-cancer-beyond-warburg-effect(14 votes)
- How do lipids and proteins enter the pathway in plant cells? And is there any difference in how they are catabolized?(5 votes)
- Do nucleotides ingested from the cells we eat, say in the cells of a tomato, enter in the cellular respiration pathway? What happens to them?(7 votes)
- They may be incorporated into our own nucleic acids, used as energy carriers within cells, or enter cellular respiration.
The biochemistry is somewhat complex, but to learn more you could start with this wikipedia page:
https://en.wikipedia.org/wiki/Nucleic_acid_metabolism#Degradation_of_nucleic_acids(3 votes)
- Do molecules from a specific functional group produce more ATP or go through cellular respiration faster than another functional group? For example starches vs. proteins. Would barley (containing mainly starch), and mung beans (containing mainly protein) have a different rates of respiration? Which would be faster?(3 votes)
- Catabolism of polysaccharides is fastest of the three macromolecules. Fats are second, and probably a fairly close second at that. Catabolism of protein is going to be much slower due to being a chain of polypeptides, a more complicated molecule than polysaccharides. In fact, about 25% of the total energy gained from breaking down proteins into amino acids for energy is used simply in breaking down the protein because the process is so intensive.(8 votes)
- I'm just wonder whether the use of proteins and lipids in cellular respiration is unhealthy, or does the body needs to do this too?(4 votes)
- Proteins and lipids can also be used by the body to create ATP. However, the body prefers glucose(carbs). Therefore, glucose is first taken before lipids and proteins,(4 votes)
- Looking for a clear and concise summary of what molecules connect each of the metabolic pathways: e.g. how are the Krebs cycle and urea cycle connected? Fumarate? Others..?(3 votes)
- Fumarate forms a link between TCA and Urea cycle.
Aspartate can be converted to an intermediate of either TCA or Urea cycle.
In Urea cycle, aspartate condensates with citrulline to form argino succinic acid. Later argino succinic acid becomes fumarate.
Fumarate can be recycled to form oxaloacetate.(1 vote)
- So in my first year biology college course, we did an experiment where we used solutions of lactose, galactose, and glucose (galactose and glucose are the components of lactose) to measure how much carbon dioxide is produced through cellular respiration using duram tubes in a 40 degree celsius water bath. The glucose solution with yeast produced lots of carbon dioxide, but the galactose and lactose solutions did not. I have tried doin some research on this but I haven't found anything so far. Do you have a possible explanation for this? Please answer as soon as you can, thank you.(2 votes)
- First of all, you mentioned yeast. That yeast species probably feeds only on glucose.
Maybe lactose and galactose fermentation take place but on different environmental factors.
Either 40C was too high for enzyme degradation or yeast lacks the enzyme which would degrade lactose or galactose.
Hope this helps. :)(1 vote)
- What is hydrolytic deamination and how does it reduce adenosine to inosine? In- of the inosine reminds me of introns for some reason.. I'm trying to understand the process of polyadenylation in depth.(2 votes)
- Deamination just means the removal of an amine group.
In the case of deamination of adenosine, this means the removal of an -NH₂ to produce ammonia (NH₃) and inosine.
To understand this more, you need to look at the structures of adenosine and inosine — for example see the top part of this figure:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2946787/figure/F1/?report=objectonly
However, I wasn't aware that adenosine deamination had anything to do with polyadenylation — what is the connection?(1 vote)
- How long does it take for completion of cellular respiration and also the individual metabolic pathways (glycolysis,tca,etc) to occur?(1 vote)
- It is very hard to quantify and say exactly how long it takes for one round of cellular respiration. We do know that it occurs very quickly. Cells do not stockpile ATP which means that as soon as ATP is used up more needs to be made. Cellular respiration, more than likely, occurs in milliseconds.
But usually, it is hard to estimate how fast metabolism occurs because all reactions are connected and metabolites from anabolic reactions enter catabolic reactions. It is a big network never stopping, never making a break. Highly dynamic.(2 votes)
- Would different molecules that go through glycolysis process slower than glucose?(1 vote)
- No; it also depends on whether the other molecules finish cellular respiration. Some molecules that enter glycolysis stop cellular respiration to be branched off into a different segment and activity of the cell.(2 votes)