Topic 5: Inner workings of the cell: Carbohydrates (Szénhidrátok)

 Carbohydrates

-most common organic compounds (biomolecules)

-are energy sources for cells - broken down by cellular respiration (needs O2)-17kJ/g, half as much as lipids, but breakdown is much faster

-C,H,O

-can be mono-, di- and polyssacharides

a.  Monosaccharides

-sweet, H2O soluble, crystalizable

-general formula:  (CH2O)n

Triose sugars (n=3)

-these are all intermediate products (köztes termékek) in biochemical reactions

-most common: glyceraldehyde (oxidation product of glycerol)

Pentose sugars (n=5)

-biologically important: parts of nucleotides and nucleic acids, most common are deoxyribose and ribose

Image from http://www.layevangelism.com/bastxbk/images/deoxyribose.jpg

Hexose sugars (n=6)

-most common monosaccharides, eg. alpha-glucose, beta-glucose (szölőcukor), fructose (gyümölcscukor) and galactose are all isomers of C6H12O6 and form the monomers of most di- and polysaccharides

Image from http://www.nzetc.org/etexts/Bio14Tuat01/Bio14Tuat01_036a%28h280%29.jpg

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Image from http://www.eastchester.k12.ny.us/schools/hs/teachers/fishman/images/fructose_000.gif

Galactose

Image from https://teach.lanecc.edu/naylore/225Lectures/04A/StructureImages/2galactoseThm.jpg

b.  Disaccharides

-2 monosaccharides combine by condensation to form a disaccharide

Examples:
glucose + fructose = sucrose (table sugar) - remember this is the main form in which carbohydrates are transported in plants!

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Image from http://www.pharmas.co.uk/blog/wp-content/uploads/2009/04/glucose-fructose-and-sucrose.jpg

2 alpha glucoses = maltose - common in germinating seeds, result of hydrolysis (break down) of starch

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Image from http://www.eastchester.k12.ny.us/schools/hs/teachers/fishman/images/maltose.gif
 

glucose + galactose = lactose (tejcukor) - found in milk

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Image from http://dspace.dial.pipex.com/town/park/gfm11/nomilkgif/LACTOSE.JPG


c.  Polysaccharides
- largest carbohydrates
- long chains of monsaccharides (hexose sugars)
- can be folded
- general formula: (C6H10O5)n
- broken down into monomers (by hydrolysis) if needed for energy
- insoluble in H2O because of size
- not sweet
- cannot crystalize

TYPES of polysaccharides:

i.  Storage polysaccharides

-insoluble, therefore they won't diffuse out of cells and won't affect osmosis

Examples:
STARCH: found in plants - made of 2 types of chains

-amylose: unbranched alpha-glucose, stains blue with iodine

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Image from http://www.cheng.cam.ac.uk/research/groups/polymer/RMP/nitin/Amylose1.jpg

-amylopectin:  alpha-glucose chain with side branches, stains red-purple with iodine

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Image from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/amylopectin.gif

-the chains fold and pack together into starch grains (keményitőszemcse) - 20% amylose, 79% amylopectin, 1% fatty acids and phosphates

 GLYCOGEN: found in animals and fungi

- alpha-glucose is arranged like amylopectin (branched), but the chains are shorter and more branched.

-glycogen is found particularly in muscle and liver cells.

-human glycogen stores provide enough energy for a few hours.

ii.  Structural polysaccharides

Examples:
CELLULOSE

- makes up 50% of the plant cell wall

- each molecule is a chain of approximately 10 000 beta-glucose, unbranched

- OH-groups form H-bonds with neighbouring chains to create a lattice

- about 2000 chains mass together to form microfibrils, which are visible under an electron microscope

Image from http://www.botany.utexas.edu/facstaff/facpages/mbrown/newstat/stat38a.jpg

- the lattice is linked together by hemicellulose, which is a short polysaccharide

-cellulose is indigestible to animals (we consider it the "fiber" in our diet!)

-used to produce cotton, rayon, cellophane, celluloid and paper

CHITIN

- found in arthropod exoskeletons and fungal cell walls

- long chains of beta-glucose, but on each monomer the OH-group is substituted by a nitrogenous group (NHCOCH3)