Biological Molecules

• Water is a polar molecule; unevenly distributed charge.

• Hydrogen end of the molecule is slightly positive and oxygen end is slightly negative due to greater share of electrons.

• Water is a dipole à charge at both ends.

• Hydrogen bonds are created between +ve and –ve ends which holds water molecules together à why water is a liquid at room temperature.

• Water has cohesion and adhesion.

• Able to defy gravity and be pulled up.

Carbohydrates:

• Examples are sugars and starch.

• Group of substances that are important in many biological processes.

• Provide energy and are used to build body structures.

• Contain carbon, hydrogen and oxygen.

• Follow formula C_x (H₂O) _y – x and y are variable.

• E.g. glucose C₆H₁₂O₆.

Sugars are either monosaccharides, single sugar units, or disaccharides, in which two single sugar units have combined in a condensation reaction. Long straight or branched chains of sugar units form polysaccharides.

Monosaccharides:

• Single sugar units.

• (CH₂O) n (n= no of carbon atoms in the molecule).

• Between 3 – 7 carbon atoms.

• The monosaccharides glucose, galactose and fructose all contain six carbon atoms, and are known as hexose sugars.

• A hexose sugar molecule has a ring structure formed by 5 carbons and 1 oxygen atom; the sixth carbon projects above or below the ring.

• Provide a rapid source of energy – readily absorbed and require little change before being used in cellular respiration.

• Glucose and fructose are found naturally in fruit, vegetables and honey.

Glucose:

• Main sugar used by all cells in respiration.

• Starch and glycogen are polymers made up of glucose subunits joined together. When either is digested, glucose is produced. This can be absorbed and transported in the bloodstream to cells. This is known as alpha glucose.

• Two forms: alpha glucose and beta glucose.

   

Galactose:

• Occurs in our diet mainly as part of the disaccharide sugar lactose, which is found in milk. The –OH groups on carbon 1 and carbon 4 lie on the opposite side of the ring compared with their position in glucose.

Fructose:

• Occurs naturally in fruit, honey and some vegetables. Its sweetness attracts animals to eat the fruits and so help with seed dispersal.

Disaccharides:

• Two single sugar units can join together and form a disaccharide in a condensation reaction releasing a water molecule.

• The bond that forms between the two sugar units is known as a gylcosidic bond or link.

  

• Two glucose molecules à condensation reaction à maltose disaccharide.

• The bond in maltose is known as a 1,4-glycosidic bond.

• Carbon atoms are numbered anti-clockwise from the oxygen.

• Common disaccharides found in food are sucrose, maltose and lactose.

Sucrose:

• Formed from glucose and fructose.

• Usual form in which sugar is transported around the plant.

Maltose:

• Formed from two glucose molecules.

• The disaccharide produced when amylase breaks down starch.

• It is found in germinating seeds such as barley as they break down their starch stores to use for food.

Lactose:

• Formed from galactose and glucose.

Polysaccharides:

• Monosaccharides joined by glycosidic bonds to form long chains.

• Three main types found in food: starch and cellulose in plants, and glycogen in animals.

• Although all three are polymers of glucose molecules – they do not dissolve easily (insoluble) and do not taste sweet.

• Starch and glycogen act as energy storage molecules within cells – because they are compact molecules with low solubility in water \ they do not affect the concentration of water in the cytoplasm or osmosis.

Starch:

• Storage carbohydrate found in plants.

• Made up of a mixture of two molecules; amylose and amylopectin.

• Amylose – composed of a straight chain of between 200 and 5000 glucose molecules with 1,4-glycosidic bonds between adjacent glucose molecules. Position of bonds = chain coils into a spiral shape.

• Amylopectin – polymer of glucose but has side branches. A 1,6-glycosidic link holds each side branch onto the main chain. Increased contact points for hydrolysis.

Glycogen:

• Used by bacteria, fungi and animals instead of starch as an energy store.

• Another polymer of glucose molecules.

• Has numerous side branches = rapidly hydrolysed = easy access to energy stores.

• Stored in the liver and muscles.

Cellulose:

• Main sugar in plants.
• Made of glucose.
• Straight chain, no branches.
• Indigestible to humans, but good for digestion as fibre.
• Extremely strong due to crosslinks.

Lipids:

• General term for fats and oils.

• Supply twice as much energy as carbohydrates.

• Insoluble in water but soluble in organic solvents e.g. ethanol.

• Most of the lipids we eat are triglycerides which are used as energy stores in plants and animals.

• Triglycerides are made up of three fatty acids and one glycerol molecule linked by condensation reactions.

• The bond that forms between each fatty acid and the glycerol is known as an ester bond.

• Each is formed in a condensation reaction with the release of a water molecule.

• Hydrolysis breaks up triglycerides.

Saturated fats:

• Saturated fats are ‘saturated’ with hydrogen. They are all C-C single bonds.
• E.g. animal fats from meat and dairy products.
• Straight saturated fats can pack closely together – strong intermolecular bonds between triglycerides= solid at room temperature – high melting point.
• Saturated fats are not good for us.
• They increase the amount of low-density lipoprotein (LDLs) in our blood.
• LDLs secrete cholesterol onto our artery walls.
• This can cause atherosclerosis.

Unsaturated fats:

• Monounsaturated = one double carbon-carbon bond between two of the carbons in each fatty acid chain; polyunsaturated = many double bonds between carbon atoms.

• Double bond = kink in hydrocarbon chain.

• The kink in the chain prevents them packing closely together, so unsaturated fats have a low melting point and are oils that are liquid at room temperature.

• E.g. olive oil, nuts, and fish.

• Unsaturated fats produce high-density lipoproteins, which reduce the amount of cholesterol in our blood, therefore reducing the risk of CVD.

DNA:

• DNA – deoxyribonucleic acid.

• A long polymer chain made of many units called nucleotides (mononucleotides).

• A nucleotide contains three molecules linked together by condensation reactions. They are deoxyribose (a pentose sugar), a phosphate group, and an organic base containing nitrogen.

• Mononucleotides link together by condensation reactions between the sugar of one mononucleotide and the phosphate of the next one, producing a polynucleotide.
• The bond that forms between the two nucleotides is known as a phosphodiester bond.
• In a DNA molecule, there are two polynucleotide strands twisted around each other to form a double helix.

• The sugars and phosphates form the backbone of the molecule and are on the outside.

• The bases point inwards horizontally and are held together in pairs by hydrogen bonds.