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IB Biology HL

Allen

Biological Macromolecules:

CarbohydratesSee Chap 2 in your text: Biology

Carbohydrates• Major source of energy from our diet

• Most abundant organic compounds in nature

• Serve both as structural compounds and as

energy reserves to fuel life processes

• Alpha (α) glucose, a six carbon sugar (C6H12O6)

is the immediate energy source to cells.

• Composed of the elements C, H and O in a

1:2:1 ratio. Empirical formula is (CH2O)n

• Millions of tons produced by plants & algae

every year by via photosynthesis

• saccharide and the suffix ose refer to sugar. i.e.

glucose or monosaccharide.

Types of Carbohydrates

• Monosaccharides

• Oligosaccharides

Contain 2 or 3 monosaccharide units

• Polysaccharides

Contain many monosaccharide units

Monosaccharides

• Carbohydrate monomers are called monosaccharides.

• distinguished by the carbonyl group they posses and the

number of atoms in their carbon backbone.

• carbonyl groups: aldehyde or ketone

aldehyde

ketone

…Monosaccharides

• Distinguished by the number of atoms in their carbon backbone.

• Three carbons = Triose

• Four carbons = Tetrose

• Five carbons = Pentose

• Six carbons = Hexose

aldehyde

ketone

Aldoses are

monosaccharides that

contain an aldehyde

group

Ketoses are

monosaccharides

that contain a

ketone group

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Stop & Think

• When fats are catabolized (broken down)

for energy, they produce ketones. Excessive amounts of ketones in the blood

is called ketoacidosis which lowers pH.

• Make a connection between Diabetes

ketoacidosis.• http://www.diabetesnet.com/diabetes_complications/diabetes_eye_whydamage.php

…Monosaccharides

• The two simplest monosaccharides

are…

– dihydroxyacetone (a triose with a

ketone group) and…

– glyceraldehyde (a triose with an

aldehyde group)

…Monosaccharides

• Aldoses are monosaccharides with an aldehyde

group and many hydroxyl (-OH) groups.

• Ketoses are monosaccharides with a ketone group

and many hydroxyl (-OH) groups.

Learning Check

Identify each as tetrose, pentose or hexose…

and as aldose or ketose

A B

C

C

CH2OH

H OHC

OHH C

OHH

H OH

C OH

C

C

CH2OH

OHH C

HHO

O

CH2OH

…Monosaccharides:linear vs. ring structures

• Monosaccharides with five or more carbons are linear molecules in the dry state, but readily form ring structures when dissolved in water.

…Monosaccharides: linear vs. ring structures

• When glucose dissolves in water, the hydroxyl group on carbon 5reacts with the aldehyde group at carbon 1 to form a closed, six-membered ring.

• When this occurs, there is a 50% chance that the hydroxyl group at carbon 1 will end up below the plane of the ring.

– Glucose with hydroxyl group below ring � α-glucose (α = alpha).

– Glucose with hydroxyl group above ring � β-glucose is formed (β = Beta).

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Learning Check

Is this α-glucose or β-Glucose?

Oligosaccharides

• sugars containing two or three simple sugars

attached to one another by covalent bonds called

glycosidic linkages.

• Oligosaccharides include disaccharides, a sugar

made of two monosaccharides.

– Some disaccharides include..

• Sucrose � glucose + fructose

• Lactose � glucose + galactose

• Maltose � glucose + glucose

these monosaccharides are monomers

Glycosidic linkages• Glycosidic linkages are bonds formed by condensation

reactions, in which the H atom comes from a hydroxyl group

on one sugar and the –OH group comes from a hydroxyl

group on the other. Both the examples below are 1,4

glycosidic linkages because the bond occurs between the 1st

and 4th carbons.

Polysaccharides

• AKA complex carbohydrates

• Several hundred to several thousand

monosaccharide subunits held together by

glycosidic linkages.

• Some are chains, some are branched

…Polysaccharides

• Amylose is an unbranched α-glucose polymer held

together by α-glucose 1,4 glycosidic linkages

• Amylopectin is a branched α-glucose polymer

composed of a main chain with glucose molecules

attached by α 1–4 glycosidic bonds and branch points

formed by α 1–6 glycosidic linkages

… Polysaccharides

• Starch is a polysaccharide made of 20% amylose and

80% amylopectin by mass.

• Plants produce more carbohydrates than they need.

They use enzymes to link excess glucose molecules

and store it as insoluble starch granules.

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… Polysaccharides

• Humans and other heterotrophs

break down starch into glucose and

use it for energy in cellular

respiration.

• A small amount of the glucose can

be converted to glycogen, another

polysaccharide, and stored in the

liver and muscles.

• Glycogen is similar to amylopectin

but it has more branches.

… Polysaccharides

• Cellulose is a straight chain polymer of β-glucose

held together by β 1–4 glycosidic linkages.

• Hydroxyl groups at the 1 and 4 positions in β-glucose

cause every other monomer to be inverted for the

glycosidic linkage to form.

Learning Check

• Why is every other monomer inverted in

cellulose?

… Polysaccharides

• Cellulose has a straight

shape. This allows

hydrogen bonds to form

between the hydroxyl

groups of parallel

molecules. Why does

this make cellulose

suitable for cell walls?