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Biological molecules- AQA A-level biology (following spec)

-3.1.1 Monomers and Polymers
All life has a similar biochemical basis of life.
Monomers- Individual units that repeat to make up chains (polymers). E.g. monosaccharides, amino acids and nucleotides.
Polymers- Long chains of monomers that have been joined together.
Amino acids and nucleotides are monosaccharides that are examples of monomers.
When monosaccharides join, a molecule of water is removed and the reaction is called a condensation reaction. The bond formed is called a glycosidic bond.
When water is added to a disaccharide, the glycosidic bond is broken. This is called hydrolysis.


-3.1.2 Carbohydrates
Monosaccharides are soluble molecules that have the general formula of (CH2O)n where n is 3-7. E.g., glucose, galactose and fructose. A condensation reaction between two monosaccharides forms a glycosidic bond.
Disaccharides are formed by the condensation of two molecules. E.g.
-glucose+ glucose-> maltose
-glucose+ fructose -> sucrose
-glucose+ galactose -> lactose.
Glucose has two isomers: α-glucose and β-glucose.
Polysaccharides- polymers, formed by the condensation of many glucose units. E.g., Starch, glycogen and cellulose.
Starch
Starch is found in plants. It’s made up of alpha glucose monosaccharides linked by glycosidic bonds. The chains may be branched or unbranched. The unbranched chain is wounded into a tight coil making starch very compact. The structure makes it well suited for many roles:
It is insoluble so doesn’t affect the water potential of the cell. Water isn’t drawn into cells by osmosis.
As it’s large it cannot diffuse out of cells.
It’s very compact so a lot of it can be stored in a small space
When hydrolysed forms alpha glucose which can be easily transported and used for respiration.
The branched chains have many ends so enzymes can work simultaneously, allowing for rapid release of fucose monomers.
Starch is never found in animal cells, however, a similar polysaccharide called glycogen is.
Glycogen
Glycogen is found in animals and bacteria. Glycogen is made up of alpha glucose monomers, however, has shorter more branched chains than starch. It’s structure suits for its storage because:
It is insoluble so doesn’t draw water into cells by osmosis
As it’s insoluble it doesn’t diffuse out of cells
It’s compact so a lot can be stored in a small space
It’s more highly branched than starch so can be acted on simultaneously by enzymes. It’s therefore more rapidly broken down into glucose monomers. This is important in animals as they have a higher metabolic rate as they are more active than plants.
Cellulose
Cellulose is a polysaccharide formed by the condensation of beta glucose monomers. This forms a whole new structure. Cellulose has straight unbranched chains that run parallel to each other allowing for H-bonds to form cross-linkages between the adjacent chains. The overall number of H-bonds strengthens the cellulose molecule- making it an affective structural material.
Cellulose molecules are grouped together to form microfibrils which are arranged in parallel groups called fibres. Cellulose provides rigidity to the plant which prevents it from bursting when water enters by osmosis. This maintains leaves and the stem in a turgid state which provides the maximum surface area for photosynthesis. It’s structure is suited for its function because:
The beta cells form long straight unbranched chains
The cellulose molecules run parallel to each other allowing for H-bonds to form which add collective strength
The molecules are grouped to form microfibres which then are grouped to for fibres which provides more strength.

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