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Summary 2 Biological Molecules
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EVERYTHING you need to know about "Biological Molecules" for A-level Biology CIE
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Carbohydrates10 April 2021 15:37
• Monosaccharides (the sugars)
• Disaccharides (two monosaccharides joined together, also sugars)
• Polysaccharides (many monosaccharides joined together, maybe several thousand), these molecules
do not have quite the same properties as their subunits, they are not sugars
Monosaccharides
• Have two major roles in living organisms
• They are a source of energy in respiration. This is because they contain lots of bonds between C and H.
Every time a bond is broken, energy can be harvested to make a molecule called ATP
• They are important building blocks for larger molecules such as starch and cellulose
• Monosaccharides are sugars
• They dissolve easily in water and taste sweet
• They consist of a single molecule and have the general formula (CH2O)n
• They are named according to the number of C atoms they contain, eg, trioses (3C), pentoses (5C), and
hexoses (6C)
• All sugars end with the suffix -ose
• Fructose is a pentagonal structure
• Start numbering carbons from the end with the carbon with a double covalent bond to oxygen
• When glucose (a hexose) forms a ring, carbon atom 1 joins to the oxygen on carbon atom 5
• The ring therefore contains oxygen, and carbon atom 6 is not part of the ring
Disaccharides
• Disaccharides are formed when two monosaccharides join together
• They are sugars too, so dissolve in water and taste sweet
• The three most common disaccharides are:
- Sucrose (glucose + fructose, table sugar)
- Maltose (glucose + glucose)
- Lactose (glucose + galactose, milk sugar)
• Two monosaccharides join by a process called a condensation reaction
• This results in the loss of a water molecule (hence the term 'condensation')
• The resulting bond that forms between the two monosaccharide units is called a glycosidic bond
• The opposite reaction (eg, returning sucrose to its component parts) is known as a hydrolysis reaction
• Sucrose is a non-reducing sugar
Polysaccharides
• Lots of monosaccharides joined together form a polysaccharide
• Each time another unit is joined, a condensation reaction occurs and a glycosidic bond forms
• Polysaccharides are not sugars
• The most well-known ones are starch and cellulose (in plants) and glycogen (in animals and fungi)
• Alpha glucose:
- Starch
- Glycogen
• Beta glucose:
- Cellulose
Starch and Glycogen
• Glucose is a necessary form of energy, so organisms need to store it for their energy requirements
• But glucose is soluble in water, so affects the osmosis of a cell. It's also reactive, so affects the
chemistry of the cell
• By gluing lots of glucose molecules together, they become insoluble and unreactive so make good
energy stores without affecting the function of the cell
Starch:
, • Starch is a mixture of two molecules
• Amylose and amylopectin
• Amylose is made by condensation reactions between alpha-glucose molecules, forming a long
unbranched chain of several thousand 1, 4 linked glucose molecules
• Amylopectin is also made of lots of 1, 4 linked alpha-glucose molecules, but the chains are shorter and
also branched
• The branches are caused by additional 1, 6 linkages
• Mixtures of amylose and amylopectin build up into large starch grains and are stored in cells, either in
chloroplasts or within the cytoplasm of organs, such as in potatoes
Glycogen:
• Animal cells do not make starch. They make glycogen
• This is very similar to amylopectin
• It is made of glucose molecules forming both 1, 4 and 1, 6 linkages, but it has even more branches
• Glycogen granules tend to be stored where they are needed as a quick energy store, in liver and
muscle cells
Cellulose
• By far the most abundant carbohydrate, making up more than 50% of all organic carbon
• A polymer of beta-glucose, combined by 1, 4 glycosidic links
• In order for this to work, every alternate ring has to be upside down
• To stabilise and strengthen the structure, hydrogen bonds form between adjacent strands
• In a complete cell wall, further strength is added by running the fibres in different directions
• Properties of cellulose:
- Insoluble
- Tough
- Durable
- Slightly elastic
- Can be used in industry to make paper, rayon fibres, nitrocellulose (for explosives) and cellophane
for packaging
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