Edexcel A level Biology B Notes
BIOLOGICAL
MOLECULES
,Water and Inorganic Ions
Structure of Water
Water molecules have covalent bonds with electrons that are not evenly shared, creating a
dipole. The H is d+ while O is d- as electrons are closer to oxygen. Polar molecules are
molecules that have a dipole.
Due to the dipole nature of water molecules, they form intermolecular bonds called
hydrogen bonds (oxygen bonds with hydrogen from another water molecule). These are
weaker than covalent bonds; however, in large quantities, they require a lot of energy to
overcome
Properties of Water
High BP and MP: Due to hydrogen bonds requiring large amounts of energy to overcome,
water is a liquid at biological temperatures.
Good solvent: Water pulls salts apart as it is hydrophilic and can store dissolved substances;
therefore, it acts as a transport media.
High specific heat capacity: More energy is needed for molecules to start moving, making
water increase in temperature slowly, therefore more stable for body temperatures.
High surface tension: Attraction to other water molecules is greater than the attraction to
air, creating a barrier.
Solid form is less dense: Maximum density at 4 degrees as ice has a lattice structure making
it more spaced out and less dense. Ice floats and insulates the water beneath.
Incompressible: Water is a liquid and therefore supports the shape of plants.
Cohesive and adhesive: Sticks together due to hydrogen bonds and sticks to other
molecules, allowing transpiration to work.
Inorganic Ions
Anions: Nitrates (formation of amino acids and DNA), Phosphates (formation of ATP, ADP,
DNA and RNA), Chlorides (nerve impulses and sweating), Hydrogen carbonates (buffering
the blood)
Cations: Sodium (nerve impulses), Calcium (calcium pectate for middle lamella and bones),
Hydrogen (cellular respiration ATP synthesis), Magnesium (chlorophyll), Iron (binding oxygen
to haemoglobin), Potassium (nerve impulses)
Monosaccharides and Disaccharides
Structure
Carbohydrates are the primary energy source, consisting of carbon, hydrogen, and oxygen
atoms. There are 3 types of carbohydrates: mono-, di-, and polysaccharides. Many
monomers join to form polymers which are macromolecules.
Monosaccharides
Monosaccharides are simple sugars with a low number of carbon atoms (triose, pentose,
and hexose sugars). Examples of hexose sugars are glucose, fructose, and galactose.
Ribose and deoxyribose are both pentose sugars for RNA and DNA respectively (deoxyribose
is missing oxygen on 2nd carbon).
, There are 2 isomers of glucose: alpha-glucose and beta-glucose, which affects the polymers
made.
Disaccharides
Made from two monosaccharides joined together in a condensation reaction where a
molecule of H2O is released. This forms a glycosidic bond which is a covalent bond. Numbers
are used to show which carbon atoms the bond is on. An example is a 1,4-glycosidic bond.
Common disaccharides: Sucrose (glucose and fructose), lactose (glucose and galactose and
maltose (glucose and glucose)
Test for Mono and Disaccharides (GCSE)
Different monosaccharides form different disaccharides. All monosaccharides and some
disaccharides are reducing sugars, while polysaccharides are non-reducing sugars. It can be
tested using Benedict’s solution (brick red= positive for glucose). A quantitative test shows
blue to green to yellow to orange to red as the concentration of glucose increases.
Polysaccharides
Properties of Polysaccharides
Oligosaccharides: Molecules with 3-10 monosaccharides. Polysaccharides: 11+
monosaccharides.
Polysaccharides are good storage molecules: compact, glycosidic bonds are easily broken
down for cellular respiration, not very soluble so they have little effect on water potential.
Test for Polysaccharides (GCSE)
Add iodine solution and observe colour change. Blue/black for the presence of starch.
Energy Store in Plants: Starch
Starch is made of long chains of alpha glucose and consists of amylose and amylopectin.
Amylose: An unbranched straight polymer made of 200 to 5000 glucose molecules, all
connected by 1-4 glycosidic bonds. As it gets longer, the chain starts to spiral for storage.
Amylopectin: A branched polymer with 1-4 glycosidic bonds and 1-6 glycosidic bonds. As it
has more branches, more enzymes can break it down at once; therefore, it can be broken
down faster than amylose.
Energy Store in Animals: Glycogen
