Carbohydrates

• A single monomer is called a monosaccharide; all monosaccharides have the general formula:

(CH₂O)_n

where n = number of carbon atoms

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• Two monosaccharides bonded together form a disaccharide
• The bond formed (by condensation reaction) is a glycosidic bond
• Many monosaccharides joined together form a polysaccharide

Monosaccharides

Alpha-D-glucose

Beta-D-glucose

Beta-D-ribose

Condensation reaction = glycosidic bond

Disaccharides

Produced in sugar cane/beet

Produced in mammalian milk

Produced in germinating seeds

Starch (plants)

• Function = storage of glucose

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• Compact so many glucose molecules stored in small space
• Insoluble so does not affect osmotic balance of cell (i.e. water does not move in by osmosis)
• Large molecule so does not move out of cell

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• Amylose (α helix straight chain) – 20% of starch
• Amylopectin (branched) – 80%

Amylose (plants)

• Unbranched: α-glucose molecules joined by α1-4 glycosidic bonds
• Forms an helix structure

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• Few branch ends and highly compact so good for storage of glucose

http://www.biotopics.co.uk/jsmol/amylose.html

Amylopectin (plants)

• Highly branched: α-glucose molecules joined by α1-4 glycosidic bonds with some α1-6 branches

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• More branch ends so can be hydrolysed by amylase more quickly than amylose

Glycogen (Humans)

• Highly branched: α-glucose molecules joined by α1-4 glycosidic bonds with α1-6 branches every 20-30 monomers

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• Function = storage of glucose
• Similar reasons to starch

Cellulose (plants)

• Unbranched: β-glucose molecules joined by β1-4 glycosidic bonds
• Alternate glucose molecules are inverted.
• Forms hydrogen bonds between chains producing microfibrils

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• Function = main structural component in plant cell walls
• Very strong because thousands of chains link together
• Fully permeable so allows movement of water and numerous substances to and from membrane

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Why can’t we digest cellulose?

• We don’t produce Cellulase
• Ruminants, like cows, have symbiotic bacteria in their stomachs

Question

• Compare and contrast the structures of glycogen and cellulose, showing how each molecule’s structure is linked to its function.

[10 marks]

Markscheme

[1] Gycogen is a chain of α-glucose molecules

[2] Cellulose – chain of β-glucose molecules

[3] Glycogen’s chain is compact but very branched, whereas

[4] Cellulose’s chain is very long, straight and unbranched

[5] and these chains in cellulose are bonded to form fibres

[6] Glycogen’s structure makes it a good food store in animals

[7] The branches allow enzymes to access the glycosidic bonds

[8] to break the food store down quickly

[9] Cellulose’s structure makes it a good structure in cell walls

[10] The fibres/ H bonds provide strength

Lipids

Objectives

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Lipids

• Made up of C, H and O
• Three main types:
• Triglycerides = three fatty acids + glycerol
• Phospholipids = two fatty acids + glycerol + phosphate group
• Steroids = four fused rings

Fatty acids

• Each fatty acid consists of:
• a carboxylic acid group COOH
• a long hydrocarbon chain
• A methyl group CH₃
• Fatty acids can be:
• Saturated - no double bonds (i.e. saturated with hydrogen)
• Monounsaturated - one double bond between carbon atoms
• Polyunsaturated - more than one double bond between carbon atoms

Fatty acids

• Unsaturated fatty acids can be:
• Cis - hydrogen atoms are on the same side of a double bond
• Trans - hydrogen atoms are on opposite sides of a double bond

Triglycerides

• 3 fatty acid molecules joined to a glycerol
• Fatty acids bond to glycerol by ester bonds formed by condensation reactions

Triglyceride

Lipids - function

• Store energy
• Floatation
• Insulation

Fats vs carbohydrates

• Draw an α-D-glucose molecule and a saturated fatty acid

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• Energy is released when C-C bonds are broken by oxidation. Which contains more energy?

Fatty acids

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• Which contains more energy per g?

Fatty acids

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• Which will have less effect on cells when stored?

Fatty acids

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Lipid function

• Protection of vital organs
• To insulate the body
• As a source of energy
• As a component of cell membranes
• They form the myelin sheath around some neurones
• To prevent evaporation in plants & animals (i.e. waxes)

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Emulsion test

• Add a few drops of the liquid food sample to a dry test tube.
• Add 2 cm³ ethanol and shake it thoroughly
• Add 2 cm³ of deionised water.
• Make observations.

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Results?

A layer of cloudy white suspension forms at the top of the solution.

Proteins

Objectives

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The amino acid rule…

• 20 different amino acids used to build polypeptides
• Found in all organisms
• Coded for by universal sequences (codons) in DNA

Build an amino acid

• Amino acids are made up of an amino group (NH₂), acidic group called a carboxyl group (COOH) and an R group, which varies in different amino acids.
• The amino group consists of nitrogen atom (N) with single bonds to hydrogen atoms (H).
• The carboxyl group consists of a carbon atom (C) that has formed a double bond with one oxygen atom (O) and a single bond with another oxygen atom.
• The oxygen atom in the carboxyl group that only has a single bond with the carbon atom also has a bond with a hydrogen atom.
• The carbon atom in the carboxyl group is bonded to another carbon atom. This same carbon atom also has a bond with the carbon atom in the carboxyl group.
• The carbon atom that links the amino and carboxyl group also has a bond with a hydrogen atom and a variable group (R).

Amino acid structure

C

H

R-GROUP (Variant)

Dipeptide structure

Glycine

Cysteine

Peptide bond

+H₂O

Protein primary structure

= the sequence of amino acids

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Protein secondary structure

= formation of α-helix or β-pleated sheet

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(Hydrogen bonds – between N-H and C=O groups on amino acids)

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Protein tertiary structure

= 3D structure is formed by further bonds

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(Ionic bonds – between charged amino acids

Hydrogen bonds – between polar amino acids

Disulphide bonds – between cysteine amino acids)

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Tertiary structure modelling

Tertiary structure modelling

• Connect them as follows
• Blue-blue-blue-blue-blue-pink-red-red-yellow-blue-blue-green-blue-blue-pink-blue
• Write the primary structure of your polypeptide
• Assume the polypeptide was placed in water, it would fold as follows
• Polar groups on the outside
• Non-polar groups on the inside
• Positive near negative
• Cysteine besides other cysteine
• Describe the types of bonds that form in the polypeptide between different groups

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Protein quaternary structure

= when two or more polypeptide chains join together, sometimes with an inorganic component, to form a protein

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Examples

• Collagen (3 polypeptide chains)
• Insulin (2 polypeptide chains linked together)
• Haemoglobin (four polypeptide chains (2 alpha and 2 beta chains), each of which is linked to a heme group)

Globular proteins

• Soluble in water
• Have a more rounded, three dimensional shape
• Generally have metabolic functions
• Examples include haemoglobin (binds to oxygen in lungs and transports it to respiring tissues) and pepsin (an enzyme that digests proteins in the stomach)

Fibrous proteins

• Insoluble in water
• Long and narrow
• Generally have structural functions
• Examples include collagen (structural protein which strengthens bones, tendons and skin) and keratin (found in hair and nails)

Functions

• Enzymes: Catalyse biochemical reactions, e.g. pepsin breaks down protein in to polypeptides
• Cell membrane proteins: Transport substances across the membrane for processes such as facilitated diffusion and active transport.
• Hormones: are passed through the blood and trigger reactions in other parts of the body e.g. insulin regulates blood sugar.
• Antibodies: are made by lymphocytes and act against antigenic sites on microbes.
• Structural proteins: give strength to organs, e.g. collagen makes tendons tough.
• Transport proteins: e.g. haemoglobin transports oxygen in the blood.
• Contractile proteins: e.g. actin and myosin help muscles shorten during contraction
• Storage proteins: e.g. aleurone in seeds helps germination, and casein in milk helps supply valuable protein to babies.
• Buffer proteins: e.g. blood proteins, due to their high charge, help maintain the pH of plasma.

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Questions

• What bonds hold together the secondary structure of a polypeptide? Why is the secondary structure regular in shape?
• Which bonds form between R groups to form the 3D shape of the protein (tertiary structure)? Write them in order of decreasing strength.
• When a protein is in a watery environment, which R groups will be on the outside and which on the inside of the folded protein?

Homework

http://goo.gl/Ia32dm

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Question

• Write out a step-by-step method for the Benedict’s test AND state what a positive and negative result will look like [4]
• Add sample and Benedict's reagent to boiling tube
• Mix and heat at 80-100^oC for 5 minutes
• Positive (reducing sugar present) = red/orange colour
• Negative (no reducing sugar present) = remains blue

Question

• Write out a step-by-step method for the biuret test AND state what a positive and negative result will look like [4]
• Add sample and biuret solution to boiling tube
• Mix gently
• Positive (peptide bonds present) = purple colour
• Negative (no peptide bonds present) = remains blue