3.1.2 - carbohydrates

what are monosaccharrides

the molecules from which larger carbs are made

3 examples of common monosaccharrides

• glucose

• galactose

• fructose

what does a condensation reaction between 2 monosaccharides form

glycosidic bond

what is formed by the condensation of two monosaccharides

disaccharides

EXAMPLES OF FORMATIONS OF DISACCHARIDES:

• what is formed by the condensation of two α - glucose molecules

maltose

EXAMPLES OF FORMATIONS OF DISACCHARIDES:

• what is formed by the condensation of a glucose molecule and a fructose molecule

• sucrose

EXAMPLES OF FORMATIONS OF DISACCHARIDES:

• what is formed by the condensation of a glucose molecule and a galactose molecule

• lactose

what is an isomer

a molecule that has the same chemical formula (in this context, as glucose (C₆H₁₂O₆)) but a different arrangement of atoms

• how many isomers does glucose have

• what are these called

• diagram

• 2

• α-glucose and β-glucose

what are polysaccharrides formed by

the condensation of MANY glucose units

give 4 polysaccharides

• starch

• glycogen

• galactose

• amylose

glycogen and starch are formed by the condensation of…

α-glucose

cellulose is formed by the condensation of…

β-glucose

KNOW THE BASIC STRUCTURE AND FUNCTIONS OF CELLULOSE, and the relationship of structure to function of these (in animal and plant cells)

starch is a c________ p___________________ made up of 2 s_________ p_____________________: a___________ and a_______________.

starch is a complex polysaccharride made up of 2 simpler polysaccharrides: amylose and amylopectin.

what monomer is amylose and amylopectin only made up of?

α-glucose monomers

• structure of amylose

• bonding of amylose

• unbranched, coiled helix (helical structure)

• monomers joined only by α-1,4 glycosidic bonds

• structure of amylopectin

• bonding of amylopectin

• branched structure

• monomers joined by both α-1,4 and α-1,6 glycosidic bonds

how is the structure of starch suited to its function? (reference both amylose and amylopectin)

• amylose = helical structure makes it compact, so it can store lots of glucose in a small space

• amylopectin = branched chain, so makes it easier for enzymes (e.g. amylase) to access and act on the branched ends simultaneously and break starch down —→ gluc. monomers (energy) can be released very rapidly when needed

how is starch being insoluble suited to its function?

doesn’t dissolve / create osmotic pressure in cells —→ could damage them

keeps starch as a stable, non-reactive energy source

what would happen if starch were soluble?

• would dissolve in the cytoplasm, increasing solute conc.

• would cause water to enter the cell by osmosis

• Too much water = pressure builds up → cell could swell and burst (esp. in animal cells without a strong cell wall)

how is starch being a large molecule suited to its function?

• ideal for long-term storage

• lots of gluc. can be stored in one molecule

• prevents it from diffusing out the cell

• what organisms is glycogen found in

• in animals, glycogen is stored as s______ g__________ mainly in the m_______ and l______

• in animals and bacteria (but never plant cells)

• in animals, glycogen is stored as small granules mainly in the muscles and liver

glycogen has a similar structure to starch, but how does it differ?

• has shorter chains

• is more highly branched

how does the structure of glycogen suit its function?

• compact —→ a lot can be stored in a small place

• insoluble —→ doesn’t draw water into the cells by osmosis + doesn’t diffuse out of cells

• more highly branched than starch —→ more ends that can be acted on simultaneously by enzymes —→ is more rapidly broken down into gluc. monomers

in terms of monomers, how does cellulose differ from starch and glycogen

made of monomers of β-glucose (not α-glucose)

in terms of structure, how does cellulose differ from starch

• cellulose = has straight, unbranched chains

• (unlike starch, which has a coiled, helical structure)

describe the cellulose structure in more detail

• straight, unbranched chains run parallel to one another

• which allows H bonds to form cross linkages between adjacent chains

what do the extra H bonds do for strengthening the molecule

• while each individual H bond adds v. little to the strength of the molecule…

• sheer volume of them makes a considerable contribution to strengthening cellulose

where is the -OH on C1 in the β-glucose molecule

where is the -H on C1 in the β-glucose molecule

(it’s opposite for α-glucose molecules)

up, above the ring

down, below the ring

because of this structure, forming glycosidic bonds is difficult.

how do we overcome this?

(add diagram)

alternate gluc. need to be flipped 180 degrees to ensure the OH groups are next to each other.

what are reducing sugars

carbohydrates that can donate electrons (or H atoms) to another chemical, typically Benedict’s reagent (which is used to test for the presence of reducing sugars)

• test for reducing sugars? (2 steps)

• result?

(1) add benedict’s reagent to sample in a test tube

(2) heat the mixture in water bath at ~80℃ for 2-5 mins

• if reducing sugars = present, a brick-red precipitate should form

• which monosaccharides are reducing sugars?

• give examples

• all of them!

• glucose, galactose and fructose

which disaccharides are reducing sugars?

• maltose

• lactose

• what does this mean sucrose is?

• a non-reducing sugar

define a non-reducing sugar

does not donate electrons to a chemical, and therefore does not give a positive result when reacted with Benedict’s reagent

• how would you test for a non-reducing sugar (i.e. if the reducing sugar test was negative?) (4 steps)

(before all this would be the 2 steps for the Benedict’s reagent test, but obvi it would have failed bc no reducing sugar is present !!)

1. hydrolyse the non-reducing sugar (NRS) by adding dilute HCl to the new sample

2. heat in a water bath for 5 min to break the NRS into reducing sugars (monosaccharides)

3. neutralise the acid by slowly adding a weak alkali, like sodium hydrogen carbonate (NaHCO₃), until it is neutral. test pH to make sure solution is neutral 7 (Benedict's test requires alkaline conditions)

4. repeat Benedict’s test (add Ben’s solution again and heat for 2-5 mins)

give the positive and negative result, and what they indicate

POSTIVE RESULT → a brick-red precipitate forms → NRS was present

NEGATIVE RESULT —→ sample remains blue —→ no sugar was present at all (NRS or reducing)

what is the biochemical test for reducing sugars and non-reducing sugars

Benedict’s solution

what is used for the biochemical test for starch

iodine or potassium iodide