monosaccharide question Watch

Death_Chronic
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#1
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hey guys, hope you're all fine, just wanted to know which monosaccharide is more permeable to membrane of yeast cell from the given monosaccharide molecules (glucose, galactose, fructose, or sorbose) and why. if possible, list them in their permeability order, thanks for your help! :cool:
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Cirsium
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Well the reason will be something to do with active transport - how many pumps there are in the yeast membrane, and how fast the monosaccharide is removed from the outer cytoplasm - as this will influence the concentration gradient. I would imagine glucose, simply because yeast respires glucose much faster but I couldn't give you a definite answer
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Death_Chronic
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ok to make things more confusing, my teacher goes that fructose would be the fastest respiratory substrate simply because it is common isomer in carbohydrates and yeast cells have more carrier proteins for that.
but yeast cells come in variation, how can this be true and assumed on all, surely glucose has the same probability of diffusing?
another thought: what about the shape of the monosaccharide molecules glucose is an aldehyde--yes shocking but true , according to books! so what imact would that have on carrier proteins?
please help guys, don't simply ignore this!
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belis
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Galactose is an aldehyde as well. There is notjing special or shocking about that, just the fact that it contains aldehyde functional group. I don't think it has any special impact on carrier proteins.
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Ramez
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glucose is phosphorolated into a phosphorolated hexose, which breaks down into two molecules of triosephosphate. the triosephosphate made from different sugars form different isomers of triosephosphate. e.g. you can have a triosephosphate which is a ketone or a aldehyde depending on the structure it has inhereted from the monosaccharde that was broken down.

the next stage is the oxidation stage. (it is important to note that aldehydes (e.g. glucose) can be oxidised into carboxylic acids more easily than can ketones ---> the carboxylic acid being pyruvate or pyruvic acid as some of you may have learnt. This process is carried out by the coenzyme NAD+.... which takes two hydrogens away from the triosephosphate to make 2NADH and 2pyruvate molecules, more easily when it be an aldehyde triosephosphate than a ketone triosephosphate.

the reason why aldehydes can be oxidised into carboxylic acid more easily than ketones is all due to the fact that aldehyde group allows for the easy removal of the hydrogen. but i dont think the examiners marking your coursework would want you to go that far.......

aldehydes, such as glucose can be oxidised into pyruvate more easily. pyruvate is turned into alcohol and carbon dioxides by the enzyme zymase.

this results in the continious falling of the concentration of the aldehyde (glucose) inside the cell because they are constantly oxidised into pyruvate ----> alcohol & CO2. Consequently a high a high concentration gradient accross the plasma membrane of the cell continually exists. soo a fast rate of active intake of this aldehyde occurs into cell.

ketones do not oxidise at all from what i have learned.... but i think we may come across a sugar that for example has an aldehyde isomer and a ketone isomer// i'm not sure!!! dont rely on that..

thats probably the reason why we get any fermentation at all with ketone sugars.

i havn't said much about the carrier proteins when talking about monosaccharides// only because cant see how the shape of the monosaccharide sugars can affect the efficiency of the protein carrier transporting it (especially when almost all of the structures are alike! except for fructose) // i think that the only reason why fructose is fermented faster than any disaccharide is because it is a monosaccharide.

a disaccharide is too large to be transported across through the protein carriers. disaccharides are broken down into monomers (monosaccharides) by special enzymes (hydrolysing enzymes) first to become small enough to be transported accross the plasma membrane..... and this takes time. therefore slower rate of fermentation

i havn't thought about differences between monosaccharides in detail. so i can't explain why a fructose would have a faster rate of fermenation than glucose or galactose if thats what you guys hav said.

sorry for the lecture// i get told by my teachers i have a habit of over complicating things. keep that in mind. dont write all of that if you are not needed to.... clarify it with ur teacher first. THE ABOVE WAS ALL MY OWN PERSONAL ASSUMPTION. I MAY BE WRONG!

let me know too/// coz i'm doing the same coursework in A2

i clarify too many things with my teachers... // they'v threatend to deduct marks if i askd for too many clarifications!!

thanks people!
gud luck

i hope someone could return the favour.
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Death_Chronic
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thanks for your comments guys! however, my experiment is aerobical, so fermentation is assumed not happening.
an extra detail: in my prelimenary work, i have noted that glucose is a far better substrate than fructose, which produces CO2 bubbles more efficiently and longer compared to fructose that releases hardly any bubbles. sorbose and galactose are sadly untested. this evidence is totally contradicting what my teacher had said! any comments please?
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Comp_Genius
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Well there are many factors which contribute to the difference in rate of oxidation of the two monosaccharides. To get to glyceraldehyde-3-phosphate, glucose goes through 4 steps whereas fructose goes through only 3; but this probably doesn't matter, what matters is the type of enzyme expressed in the cell, and the concentration of the different enzymes which are present. Unless you find out the different concentrations of different enzymes, rate determining step, their kinetics you simply can't know which is the better substrate.

What carrier proteins is your teacher talking about?

P.S. ketones can be oxidised in biochemical pathways; in metabolism the sugar is not just simply oxidised (i.e. by burning in the presence of O2) but modified step-by-step by a series of enzymes.
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Death_Chronic
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#8
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my teacher simply generalised the idea of yeast having more enzymes for fructose.
moving on, when you said that the monosaccharides go through steps before ending up as triose diphosphate, or more specifically glyceraldehyde-3-phosphate (for aldehyde monosaccharides), where can i get data on how many steps my given monosaccharides (glucose, galactose, fructose, or sorbose) would go through before being susceptable to go through krebs cycle and beyond? a link is very much appreciated. i know this goes beyond a-level, but it seems quite fascinating i haven't had any luck finding such info anywhere

another thing: are all the monosaccharides transported by the same yeast protein across the membrane?
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