Summary Topic 1; Molecules, Transport and Health, Edexcel AS and A Level short notes

Topic 1: Molecules, transport and health
1A CHEMISTRY FOR BIOLOGISTS
1. The chemistry of life
 In some covalent compounds, the molecules are slightly polarized meaning electrons
in covalent bonds are not evenly shared. So, it has a slightly negative part and a
slightly positive part. This separation of charge is called a dipole and tiny charges are
represented as S+ and S-. Molecule is described as a polar molecule. This is common
if Hydrogen is present.
 As electrons are held closer to oxygen molecule than to the hydrogen atoms, water is
a polar molecule. Due to this, they form hydrogen bonds. Slightly negative oxygen
atom attracts the slightly positive hydrogen atoms of other water molecules in a weak
electrostatic attraction called a hydrogen bond. These bonds are weak but there are
many so water molecules ‘stick together’. Water has a higher boiling point as more
energy is needed to break all these bonds.
 Properties of water:
1. Polar solvent = Many ionic substances dissolve in it so chemical reactions occur
in water in cells.
2. Dipole nature = An excellent transport medium as water enables many different
substances to dissolve in it.
3. Reaches maximum density at 4℃ = Molecules become more widely spaced as it
cools further so ice is less dense than water and floats forming an insulating layer
which prevents the water underneath it from freezing. This enables organisms to
live in water in cold countries.
4. High specific heat capacity = Hydrogen bonds between the molecules need a lot
of energy to separate them. So lakes stay at the same temperature throughout the
year making them good animal habitats.
5. Liquid = Cannot be compressed so an important factor in many hydraulic
mechanisms in organisms.
6. Cohesive water molecules = Forces between molecules mean they stick together.
This is very important for the movement of water from roots to leaves of plants.
7. Adhesive water molecules = Attracted to other different water molecules which
is important in plant transport systems and surface tension.
8. High surface tension = Because the attraction between water molecules,
including hydrogens bonds, is greater than the attraction between the water
molecules and air. As a result, water molecules hold together forming a thin ‘skin’
of surface tension. Important in plant transport systems and also affects life at the
surface of lakes, ponds, etc.

, 2. carbohydrates
 There are three main groups of carbohydrates; Monosaccharides, Disaccharides,
and Polysaccharides.


MONOSACCHARIDES
 Simple sugars in which there is one oxygen and two hydrogen molecules for each
carbon.
 General formulae is (CH2 O)n
 EXAMPLES:
1. Triose sugars = Three carbon atoms and formula is C3 H6 O3 . Important in
mitochondria, where respiration process breaks down glucose into triose sugars.
2. Pentose sugars = Five carbon atoms and the general formula C5 H10 O5 . Ribose
and deoxyribose are important in the nucleic acids deoxyribonucleic acid (DNA)
and ribonucleic acid (RNA) which makes up the genetic material.
3. Hexose sugars = Six carbon atoms and the general formula C6 H12 O6 . They are
the best known monosaccharides, often taste sweet and includes glucose,
galactose and fructose.




RIBOSE STRUCTURE α-GLUCOSE STRUCTURE


DISACCHARIDES
 Two monosaccharides join in a condensation reaction to form a disaccharide and a
molecule of water is released. The link between the two monosaccharides results in a
covalent bond known as a glycosidic bond.
EXAMPLES:
Disaccharide Source Monosaccharides
Sucrose Stored in plants like sugarcane Glucose + Fructose
Lactose Milk sugar (Main carbohydrate in milk) Glucose + Galactose
Maltose Malt sugar (Found in germinating seed like Glucose + Glucose
barley

, POLYSACCHARIDES
 Many monosaccharide units joined together by condensation reaction that create
glycosidic bonds
 Molecules with 3-10 sugar units are called oligosaccharides
 Can form very compact molecules that take up little space.
 Physically and chemically inactive, so they don’t interfere with other cell functions.
 Not very soluble in water so have almost no effect on water potential within a cell and
cause no osmotic water movements.
EXAMPLES:
1. STARCH
 Sugars produced by photosynthesis are rapidly converted to starch, which is
insoluble and compact but can be broken down rapidly to release glucose
when needed.
 Long chain of α-glucose, though this is a mixture of both amylose and
amylopectin.
2. GLYCOGEN
 Found in animals.
 Similar to amylopectin molecules in starch but has many α-glucose units.
 Compact like starch but has more 1,6-glycosidic bonds giving it many side
branches, so it can be broken down rapidly.
 Glycogen has more branches than amylopectin.


AMYLOSE AND AMYLOPECTIN
 Amylose = An unbranched polymer of between 200 and 5000 glucose units. As the
chain lengthens the molecule spirals, making it more compact for storage.
Amylose has only 1,4-glycosodic bonds which is why molecules are long
unbranched chains.

 Amylopectin = A branched polymer of glucose units. The branching chain have
many terminal glucose units that can be broken off rapidly when energy is needed.
Amylopectin has many glucose molecules formed by 1,6-glycosidic
bonds resulting in branched chains.

 As starch has both, carbohydrate foods like rice and pasta are good when doing
sports. The amylopectin releases glucose for cellular respiration rapidly when needed.
Amylose releases glucose more slowly over time, keeping the body going longer.