Nutrients for cellular respiration

[asdfgh123]

Hello

I´m a bit confused by the concept of cellular respiration. I know that primarily, we start out with glucose - but I know that other nutrients can be used. Which are these? Does these molecules need to be transformed into glucose before they can start cellular respiration?

And in the case of glucose: As I have understood it, only monomeric (monosaccharides) carbohydrates can pass from the intestine to the epithelial lining and on to the blood stream. But how is that possible without some sort of membrane protein like the kind H2O uses? Carbohydrates are generally very polar molecules. And why can monosaccharides diffuse across membranes, but not disaccharides and polysaccharides?

Thank you for your help. Sorry for all the questions - these are though not homework questions. I´m just a hobby biologist.

 

[Smachine]

And in the case of glucose: As I have understood it, only monomeric (monosaccharides) carbohydrates can pass from the intestine to the epithelial lining and on to the blood stream. But how is that possible without some sort of membrane protein like the kind H2O uses?

Glucose is a polar molecule so without any protein it cannot cross membrane. In case of intestine there is also an electrochemical gradient that drives the transport. There is less Na⁺ in intestine epithelial cells due to the presence of Na⁺/K⁺ pumps in basal cell membrane. Glucose is transported together with 2 Na⁺ ions to the inside of cells using symporter protein against its concentration gradient.

Carbohydrates are generally very polar molecules. And why can monosaccharides diffuse across membranes, but not disaccharides and polysaccharides?

They can only diffuse in facilitated diffusion, as they are polar as you said and the interior of plasmatic membrane is hyrdophobe. Monosaccharides have transporters like GLUT proteins, other molecules don't so they cannot pass.

 

[Andy Resnick]

Hello

I´m a bit confused by the concept of cellular respiration. I know that primarily, we start out with glucose - but I know that other nutrients can be used. Which are these? Does these molecules need to be transformed into glucose before they can start cellular respiration?

<snip>

Just to add a bit, the citric acid cycle does involve glucose, but the body can use other sugars (fructose, maltose, etc). It's just that they have to be broken down into glucose first.

http://en.wikipedia.org/wiki/Citric_acid_cycle
http://en.wikipedia.org/wiki/Oxidative_phosphorylation

What is interesting (to me), is that if I ate 10 bizillion calories of pure glucose, my body would store it *as fat*.

 

[Ygggdrasil]

There are three main classes of nutrients the body can use to create energy: carbohydrates, fat and proteins. Carbohydrates generally enter our metabolic processes via conversion to glucose (e.g. in the case of galactose) or at some early stage of glycolysis (e.g. as a fructose-6-phosphate for the case of fructose).

Fat is metabolized by oxidation into acetyl-CoA, producing NADH which can be used to generate energy via the electron transport chain. Furthermore, the product of fatty acid oxidation, acetyl-CoA, feeds directly into the citric acid cycle to produce more energy. The liver is also able to convert acetyl-CoA into glucose via a process called gluconeogenesis. (Gluconeogenesis, in essence, siphons off products from the citric acid cycle for use as building blocks to construct glucose. Thus, any metabolic pathway that supplies the citric acid cycle can contribute material to gluconeogenesis.) In this way, the body can convert fat into carbohydrate in cases where the body lacks carbohydrates. This is particularly important because brain cells can metabolize only carbohydrates for energy.

Proteins are broken down into amino acids and the amino acids are then converted to numerous intermediates of glycolysis and the citric acid cycle. The exact pathway and mechanism depends on the particular amino acid. Because these intermediates feed directly or indirectly into the citric acid cycle, they can be used as substrates for gluconeogenesis.

Regarding Andy's comment about fat, fat is a great medium for storing energy as, of the three nutrients I mentioned, it gives the most stored energy per unit mass. It is no wonder than our primary fuel of choice in the modern world (petroleum) resembles nature's primary choice for long term energy storage (fatty acids).

 

[Andy Resnick]

Yes, but my point is that mammals (and reptiles, I guess) convert excess carbohydrates into fats (plants don't). Seems like a logical pathway to target drugs.

 

[alxm]

In this way, the body can convert fat into carbohydrate in cases where the body lacks carbohydrates. This is particularly important because brain cells can metabolize only carbohydrates for energy.

No, they can metabolize ketone bodies (from transamination of amino acids) as well, and do - once you run out of liver glycogen and your blood sugar bottoms out. It doesn't all go via gluconeogenesis.

It is no wonder than our primary fuel of choice in the modern world (petroleum) resembles nature's primary choice for long term energy storage (fatty acids).

Ah, well I don't really think so. Petroleum is good but not extraordinary in terms of energy density. I'd say it's got a lot more to do with the production costs being little more than what it takes to pump it out of the ground.

Yes, but my point is that mammals (and reptiles, I guess) convert excess carbohydrates into fats (plants don't).

Sure plants do. Where would vegetable fats come from, otherwise?
They don't have fat tissue or produce fat for quite the same reasons (but rather in ripening, usually in the seeds). And they mostly just make triglycerides. But they produce lipids from carbohydrates anyway.

Seems like a logical pathway to target drugs.

Could be. Seems a bit risky? You've always got a bunch of metabolic pathways competing with each other. I'm not sure you want to throw a wrench into that particular machinery.. Stopping nutrient uptake at the gut (as they seem to be trying to do) seems less risky to me. That's our real problem anyway: Eating too much.

 

[Andy Resnick]

No, plants store excess carbohydrates as starch- potatoes, for example.

 

[Ygggdrasil]

One structure where plants do use fats for energy storage is in the seed. The reason plants have evolved this way likely has to do with the need for mobility. Plants are non-motile so they do not need to store their energy very efficiently in terms of space and weight. Seeds, however, generally need to be moved through the environment (e.g. through the wind) so they have evolved to store their energy in fats in order to save space and weight.

This may also explain why animals, also motile creatures, have evolved to store excess energy as fats.

 

[Andy Resnick]

I hear what you are saying, and sure- many types of algae also generate hydrocarbons. But, fruit and vegetable plants (were selected for) the vast quantity of sugars they store. Avocados and olives might be good counterexamples, I think. It just seems like an interesting difference, that's all.