Concept discussion

[Yacoubb]

Hey guys I've started this topic where everyone can jump on and we can just discuss some concepts in Bio 3+4, sort of like revision + just so we can learn things/advise others, etc.

Shall we start off by talking about macromolecules? Feel free to start any discussion you like!

[Bad Student]

What makes a macromolecule organic? Does it need to have carbon? Or is it carbon and hydrogen? I haven't been able to find a satisfactory definition because they all have exceptions.

[alondouek]

What makes a macromolecule organic? Does it need to have carbon? Or is it carbon and hydrogen? I haven't been able to find a satisfactory definition because they all have exceptions.

Right, so the broadest definition of an organic molecule is a molecule which has a carbon 'backbone' (where carbon is generally the structural basis of the molecule) bonded to other elements (leading to compounds such as alkanols, amines, etc) or radicals (atoms, molecules, or ions with unpaired electrons - but this is more chemistry).

You may have heard humans referred to as 'carbon-based life-forms' - as science-fictiony as this may seem, it's pretty much correct. All our biological macromolecules have carbon backbones;

• Carbohydrates - composed of C, H, O
• Proteins - composed of C, H, O, N, (S)
• Lipids - C, H, O, P
• Nucleic Acids - C, H, O, N, P

As you can tell, carbon is common to all of these biomacromolecules, so they are defined as organic.

[Bad Student]

But if you define organic molecules as carbon based, then carbon dioxide would be organic when its not.

[alondouek]

You're right; I should have been more specific

Organic compounds are ones that contain carbon covalently bonded to hydrogen. This is the case with all biomacromolecules.

Any questions?

[pi]

There isn't really a fixed definition for what an "organic compound" is. To see the discussion/exceptions -> http://en.wikipedia.org/wiki/Organic_compound

most carbon-containing compounds are organic, and most compounds with a C-H bond are organic. Not all organic compounds necessarily contain C-H bonds (e.g., urea).

[paulsterio]

I believe that all carbon containing compounds are organic apart from oxides (i.e. carbon dioxide), cyanides, carbides, diamond and graphite.

And yes, not all organic compounds contain C-H bonds, urea is one, I guess you could have tetra-fluoro-methane and stuff like that as well.

[Bad Student]

So basically, organic compounds:

• are carbon containing compounds
• except for oxides, cyanides, carbides and allotropes of carbon
• often contain C-H bonds

[pi]

I believe that all carbon containing compounds are organic apart from oxides (i.e. carbon dioxide), cyanides, carbides, diamond and graphite.

Line of wikipedia:

a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon (such as CO and CO2), and cyanides, as well as the allotropes of carbon such as diamond and graphite, are considered inorganic

...

[Yacoubb]

I agree that carbon is definitely present in all macromolecules, making them organic compounds. Remember though that although lipids and polysaccharides are composed of Carbon, Hydrogen and Oxygen, the levels of oxygen in lipids is less than that of polysaccharides. Hence, lipids are more dense than water (ie lipids like oils float on water) and more energy-dense.

[Yacoubb]

What makes a macromolecule organic? Does it need to have carbon? Or is it carbon and hydrogen? I haven't been able to find a satisfactory definition because they all have exceptions.

I think that what VCAA would like you to know is what chemical composition makes up the macromolecules polysaccharides, proteins, lipids and nucleic acid an exam question may ask you to say whether C6H12O6 is glucose or the enzyme maltase. By recognising that it is made up of Carbon, Hydrogen and Oxygen, it makes it a monosaccharide, hence glucose. The absence of Nitrogen sufficiently indicates that it is not a protein, or in this case, the enzyme Maltase

[alondouek]

although lipids and polysaccharides are composed of Carbon, Hydrogen and Oxygen, the levels of oxygen in lipids is less than that of polysaccharides. Hence, lipids are more dense than water (ie lipids like oils float on water) and more energy-dense.

haha so many U3 chem lessons, oh the nostalgia

Summarised depiction of the above quote:

Arachidic acid, a saturated lipid has the chemical formula CH₃(CH₂)₁₈CO₂H. Note that there are only 2 oxygen molecule present, despite the relative complexity of the overall molecule.

Conversely, the monosaccharide glucose (C₆H₁₂O₆) is very simple structurally, yet contains three times the amount of oxygen. Side note - chemical formulae of carbohydrates follow the C:H:O of 1:2:1

Now for a slightly tricky question for anyone keen: Outline the differences between white and brown adipose tissue, and explain why brown adipose tissue is prevalent in thermogenesis. (This is beyond VCAA standards, but it never hurts to have some extra comprehension and knowledge in biol )

[Bad Student]

I agree that carbon is definitely present in all macromolecules, making them organic compounds. Remember though that although lipids and polysaccharides are composed of Carbon, Hydrogen and Oxygen, the levels of oxygen in lipids is less than that of polysaccharides. Hence, lipids are more dense than water (ie lipids like oils float on water) and more energy-dense.

Did you mean lipids are less dense than water? Why does having less oxygen molecules make it less dense?

[pi]

I think that what VCAA would like you to know is what chemical composition makes up the macromolecules polysaccharides, proteins, lipids and nucleic acid an exam question may ask you to say whether C6H12O6 is glucose or the enzyme maltase. By recognising that it is made up of Carbon, Hydrogen and Oxygen, it makes it a monosaccharide, hence glucose. The absence of Nitrogen sufficiently indicates that it is not a protein, or in this case, the enzyme Maltase

VCE Biology isn't really that dense in chemistry. I'm with alondouek on this one.

Hence, lipids are more dense than water (ie lipids like oils float on water)

Check this please

[Yacoubb]

VCE Biology isn't really that dense in chemistry. I'm with alondouek on this one.


Check this please

Oops gosh how can I do that! Water is more dense than H2O. That is obviously why it floats. Daft moment sorry.