ht

tp
s
://
br
iti
sh
stu
de
nt
ro
mo
.c o
iAS Biology

m
/
Unit 1

Molecules, Diet, Transport

and Health
WBI11/01

By Dr. Eman Salama

https://www.facebook.com/EmanSalamaIGCSE
+201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
Unit 1 Syllabus Overview

.c o
m
/
Topic 1: Molecules, transport and health
A. Chemistry for biologists
1.The chemistry of life
2. Carbohydrates 1: Monosaccharides and disaccharides
3. Carbohydrates 2: Polysaccharides
4. Lipids
5. Proteins

B. Mammalian transport systems

1. The principles of circulation
2. The roles of the blood
3. Circulation in the blood vessels
4. The mammalian heart
5. Atherosclerosis

C. Cardiovascular health and risk

1. Risk, correlation and cause
2. Investigating the causes of CVDs
3. Risk factors for cardiovascular disease
4. Diet and cardiovascular health
5. Dietary antioxidants and cardiovascular disease
6. Using the evidence
7. The benefits and risks of treatment

Topic 2: Membranes, proteins, DNA and gene expression

A. Membranes and transport
1. Cell membranes
2. Cell transport and diffusion
3. Osmosis: A special case of diffusion
4. Active transport
5. The need for gas exchange surfaces
6. The mammalian gas exchange system

-2-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
B. Proteins and DNA

.c o
m
/
1. Enzymes
2. How enzymes work
3. The structure of DNA and RNA
4. How DNA works
5. The genetic code
6. DNA and protein synthesis

C. Gene expression and genetics

1. Gene mutation
2. Patterns of inheritance
3. Sex linkage
4. Cystic fibrosis: A genetic disease
5. Genetic screening

Practical Skills:

-3-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
Unit 1 Assessment Overview

.c o
m
/
Relationship of Assessment Objectives to Unit

-4-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
Topic 1: Molecules, transport and health

.c o
m
/
A. Chemistry for Biologists
1. The chemistry if life:

The chemical bonds within and between molecules affect the properties of the compounds they
form. This affects their functions within the cell and organism.
Types of chemical bonds:
1.Ionic bonds: strong forces of attraction holding oppositely charged ions together (anion and
cation).
2. Covalent bonds: strong bonds that are formed between atoms that share electrons together.
3. Hydrogen bonds: weak electrostatic attraction between slightly charged atoms.

To understand the chemistry of water; we need to understand chemical bonds and how dipoles are
created within molecules.
Covalent compounds, in which atoms share their electrons, are either:
a. Non-polar compounds: where the molecule formed is neutral; the electrons are evenly shared
b. Polar compounds: where the electrons are not quite evenly shared so the molecules are slightly
polarized.

Dipole: is the separation of charge in a polar molecule where the molecule has a slightly
negative part and a slightly positive one.

The chemistry of water: H2O

In a water molecule, the electrons are held closer to the oxygen atom than to the hydrogen atoms.
So, water is a polar and nonlinear molecule

-5-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
A major effect of this polarity is that water molecules form hydrogen bonds. The slightly negative

.c o
m
/
oxygen atom of one water molecule will attract the slightly positive hydrogen atoms of other water
molecules (cohesive forces). Each hydrogen bond is weak but there are many of them so water
molecules tend to “stick together” and move as an unbroken column. Water has a relatively high
melting and boiling points as it takes a lot of energy to break all the hydrogen bonds that hold the
molecules together.

Importance of water:
1. Water is an excellent solvent. Due to its polarity, it dissolves ionic compounds. Water also
dissolves all polar covalent substances that do not dissolve in other covalently bonded solvents
such as ethanol.

2. Water is an excellent transport medium due to its dipole nature. Other than the substances that
water can dissolve; it also carries other substances such as starch (whose molecules are bigger
than water molecules so form a colloid). Accordingly, most of the chemical reactions within
cells occur in water (aqueous solution).
-6-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
3. Low density of Ice: As water cools to 4 ℃, it reaches its maximum density. As it cools more, the

m
.c o
m
/
solid ice is least in density and floats over liquid water forming an insulating layer, thus
preventing water underneath from freezing and keeping marine life.
4. High specific heat capacity: water is slow to absorb and release heat. So, temperatures of lakes
and seas do not change much throughout the year. Thus, being good habitats for living
organisms.
5. Water is a liquid so it can’t be compressed. This is important in hydraulic mechanisms in living
organisms.
6. Water molecules are cohesive – the forces between the molecules mean they stick together. This
is how water moves from the roots to leaves of plants.
7. Water molecules are adhesive – they are attracted to other different molecules. This is important
in plant transport systems and in surface tension.
8. Water has a very high surface tension because the attraction between the water molecules (by
hydrogen bonds) is greater than the attraction between water molecules and air. So water
molecules hold together forming a thin “skin” of surface tension. This is important in plant
transport systems and life at the surface of water masses.
9. Water is amphoteric as it can act as a proton donor (forms H+ ions) or a proton acceptor (forms
OH- ions) thus keeping pH constant during metabolic reactions; acts a “buffer”

-7-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
2. Carbohydrates 1: Monosaccharides and disaccharides

.c o
m
/
3. Carbohydrates 2: Polysaccharides

Organic molecules:
✓ Organic compounds contain Carbon atoms together with
Hydrogen and Oxygen. Some of them contain
Phosphorus and/or Sulphur.
✓ Carbon atoms are tetravalent. The four bonds are
arranged in a tetrahedral shape thus carbon compounds
can be any 3D shape.
✓ Molecular formula: it shows the chemical symbol of
each atom and its number in a molecule. Example: a
molecule of glucose has the molecular formula C6H12O6
✓ Structural formula: it shows the arrangement of atoms
within a molecule.
✓ Macromolecule: a large molecule formed by linking together many subunits called
monomers.
✓ Monomer: subunits that can be linked together by condensation reactions to form larger
molecules (macromolecules or polymers)
✓ Isomer: molecules having the same molecular formula but different structural formulae.
✓ Polymer: a macromolecule made of similar or identical monomers.

-8-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
✓ Condensation reaction: removing water by a chemical reaction to join two molecules

m
.c o
m
/
together.
✓ Hydrolysis: adding water by a chemical reaction to breakdown a macromolecule into
smaller molecules.
✓ All polymers are macromolecules; but not all macromolecules are polymers.
✓ Poly saccharides and polypeptides are polymers while lipids are macromolecules but
not polymers.

The are 3 main groups of carbohydrates:

monosaccharides, disaccharides and poly saccharides.
A. Monosaccharides: (CH2O)n

• Triose sugars (n=3) only 3 carbon atoms so C3H6O3 During respiration

process in the mitochondria, a glucose molecule is broken down into 2
triose molecules.

• Pentose sugars (n=5) have 5 carbon atoms so C5H10O5. Examples are:

ribose sugar in RNA and deoxyribose sugar in DNA.

• Hexose sugars (n=6) have 6 carbon atoms so C6H12O6. Examples are:

glucose galactose fructose.

Glucose is used in respiration, fructose is found in fruits & galactose forms lactose of milk.

• Glucose has 2 isomers: α-glucose and β-glucose

-9-
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
B. Disaccharides: double sugars CnH2n-2On-1

.c o
m
/
• Made up of two monosaccharides (monomers) joined together.

• Monomers are joined together by condensation reaction (where a molecule of water is

removed) and a split apart by hydrolysis reaction (where a molecule of water is added).

• The link between the two monosaccharides is a covalent bond known as glycosidic bond.

• When different monosaccharides join together, different disaccharides are made and these
have different properties:

- 10 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
• Monosaccharides and disaccharides have low molecular weight.

m
.c o
m
/
• They are both water soluble (hydrophilic/polar) so they affect the water balance of the cells.

• They are both have sweet taste.

• They cannot be used to store energy because they are chemically active.

- 11 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
.c o
m
/
C. Polysaccharides: large number of monemers joined together to form complex carbohydrate
by condensation reaction Cn(H2O)n-1

• Oligosaccharides: molecules between 3 and 10 sugar units.

• True polysaccharides: molecules containing 11 or more
monosaccharides.
• Hydrolysis takes place in the gut during digestion and in liver
& muscle cells when carbohydrate stores are broken down to
release sugars needed for cellular respiration.
• Polysaccharides have high molecular weight.
• They are water-insoluble (hydrophobic/non-polar)
• They have non-sweet taste.
• Polysaccharides are starch, cellulose and glycogen.
• Starch: is composed of α-glucose monomers joined by condensation reaction.
It is a mixture of two compounds:

Amylose 30% Amylopectin 70%

➢ Unbranched polymer chain of α-glucose units ➢ Branched polymer chain of α-glucose joined by
joined by α 1,4-glycosidic bonds. α 1,4-glycosidic bonds.
➢ As the chain lengthens its coils up into a helix. ➢ Branches arise every 20-30 glucose subunits by
α 1,6-glycosidic bonds at the branching point.
➢ The branching chains have many terminal
glucose units that can be broken off rapidly
when energy is needed.

- 12 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
• Glycogen: “animal starch” composed of α-glucose monomers.

m
.c o
m
/
It is similar to amylopectin but branches are more frequent (every 10 glucose
subunits) so it can be broken down very rapidly.

❖ The structure of polysaccharides makes them ideal as ideal as

energy storage molecules within the cell (or adaptations of
starch and glycogen for their storage function):

✓ They form compact molecules, so a large number of glucose

molecules can be stored in small space.
✓ Physically and chemically inactive (non-reactive) so they do not
interfere with any metabolic reactions in the cell.
✓ Water insoluble so they do not affect osmotic pressure of cells.
✓ Rapidly hydrolyzed by enzymes due to many terminal glucose
units in the side branches.

- 13 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
4. Lipids:

.c o
m
/
• They are formed of C, H and O atoms like carbohydrates but the ratio of H:O is not 2:1 (much
less oxygen than carbohydrates).
• Fats and oils are chemically similar but fats (having saturated fatty acids) are solids at room
temperature while oils (having unsaturated fatty acids) are liquids at room temperature.
• Fats and oils contain fatty acids and glycerol that are combined by ester bonds to form
triglycerides (C3H8O3).

Triglycerides:
▪ Act as energy store (more than carbohydrates)
▪ Thermal insulator to prevent heat loss in animals.
▪ Electrical insulator; myelin sheath around nerves to conduct electrical impulses at high speed
▪ Have low density so body fat helps animals to float over water (buoyancy).
▪ Supporting organs in our body (fatty tissue surrounding some organs).
▪ Hydrophobic: they do not dissolve in water. Lipids that are transported in blood plasma are
converted into hydrophilic lipoproteins.
▪ They only dissolve in organic solvents so they do not affect any water-based reactions in the
cell.
▪ Each triglyceride molecule is formed of 1 glycerol molecules and 3 fatty acids molecules
joined by ester bonds.
▪ Fatty acids differ in the length of the carbon chain. A fatty acid may be saturated or
unsaturated.

- 14 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
❖ Contrast saturated and unsaturated fatty acids:

m
.c o
m
/
Saturated fatty acids Unsaturated fatty acids

- CnH2nO2 - For each double bond the number of hydrogen

- The fatty acid chain contains the max. possible atoms decrease by 2.
number of hydrogen atoms. - Fatty acid chain contains less than max. number
- No carbon-carbon double bonds. of hydrogen atoms.
- Straight structure - If 1 C=C then it is monounsaturated fatty acid
- Solid at room temperature (e.g. animal fats) If C=C is more than 1 then it is poly
- Causes cardiovascular diseases. unsaturated fatty acid.
- Have high melting points. - Has kinky/non-straight structure.
- Are tightly packed with strong intermolecular - Liquid at room temperature (plant oils).
forces. - Do not cause cardiovascular diseases.
- High energy is needed to break these forces. - Have low melting points.

This type of condensation reaction is called esterification where an ester bond is formed between
carboxyl group (-COOH) of fatty acid and one of the hydroxyl groups (-OH) of glycerol.
Phospholipids:
▪ They are the same as triglycerides but one of the fatty acid chains is replaced by hydrophilic
phosphate group.

- 15 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
▪ Accordingly, a phospholipid molecule has a polar/hydrophilic head which is the phosphate

m
.c o
m
/
group and two non-polar/hydrophobic tails which are two the fatty acids.
▪ Phospholipid molecules are a major constituent of the cell membrane known as phospholipid
bilayer.

Cholesterol:
▪ Different from structure of triglycerides as they are short lipids.
▪ Examples are sex hormones, in bile salts and integral parts of the cell membranes.

** just for your reference, not required.

- 16 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
5. Proteins:

.c o
m
/
• They are formed of C, H, O and N atoms (with or without phosphorus and Sulphur)
• They are polymers of amino acids (monomers) linked together by peptide bonds in
condensation reaction.
• All amino acids have the same basic structure consisting of a central carbon attached to a
hydrogen atom, carboxyl group (-COOH) on one side and amino group (-NH2) on the other
side together with a variable R group.
• The R groups are not involved in the reactions
which join the amino acids together.
• The R group affect the way the amino acid interacts
with others within the protein molecule.
• R groups may be polar or non-polar.
• There are only 20 different amino acids, the kind of protein differ according to the kinds,
sequence and number of amino acids forming the protein.
• Humans can only make 12 amino acids in their bodies, therefore the other ones must be
supplied through diet “essential amino acids”.
Forming proteins from amino acids:
➢ One amino acid loses OH from its carboxylic group while the other amino acid loses H from its
amino group in a condensation reaction where a peptide bond is formed and water is removed.
➢ Peptide bonds may be broken down by addition of water in hydrolysis reaction.
➢ A dipeptide is formed when two amino acids join together. When more than two join together,
it’s a polypeptide.
➢ Since R groups are the only different thing between amino acids, therefore they:
- Determine the properties of amino acids.
- Determine the function of the formed proteins.
- Determine types of bonds that hold proteins in the tertiary and quaternary structures.
- 17 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
.c o
m
/
Protein Structure:
1. Primary structure “linear sequence”
It is the number, type and sequence of amino acids in the polypeptide chain, held together by
peptide bonds.
2. Secondary structure
It is the arrangement of the polypeptide chain into a regular, repeating 3D structure, held
together by Hydrogen bonds (formed between slightly charged atoms in the backbone of amino
acids) producing an α-helix or β-pleated sheets.

- 18 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
α-helix: a spiral coil formed by Hydrogen bonds between amino acids of one turn and those of

m
.c o
m
/
the next turn while the R groups are protruding in all directions. Example: Keratin of nails and
hair.
β-pleated: polypeptide chain folds into regular pleats held together by hydrogen bonds between
the amino and carboxyl ends of the amino acids of two adjacent chains. Example: fibroin in
silk.
Most fibrous proteins have this type of structure.
N.B.
✓ R groups are not involved in the secondary structure.
✓ Backbones of polypeptide are similar, so the secondary structure is not specific for
particular proteins.
✓ Sometimes there is no regular secondary structure and the polypeptide forms a random
coil.

3. Tertiary structure
• It is another level of 3D organization in addition to the secondary structure.
• The amino acids chain (including α-helices and β-pleated sheets) is folded further into
more complicated shapes through formation of bonds between the R groups of nearby
amino acids.
• These bonds that hold the 3D shapes in place are: hydrogen bonds, disulfide bonds and
ionic bonds. Also, hydrophobic interactions exist to make the chain bend or fold.
Types of bonds:
Hydrogen bonds Ionic bonds Disulfide bonds

1. formed between partial 1. formed between the 1. Formed by oxidation reaction

positive charge of H and charged amino group and between R-groups of amino
partial negative charge of charged carboxyl group of acids that contain sulfur
the O found in R-groups. R-groups of the amino (cysteine).
acids.
2. Very weak bonds but are 2. They are stronger than 2. The strongest of all bonds.
very frequent. Hydrogen bonds but Occur less frequent than
weaker than disulfide hydrogen bonds.
bond. They are not
commonly found.
3. Break easily and reform if 3. Broken by pH changes. 3. Only broken by reducing
pH and temperature agents (chemicals such a
conditions change. perming treatments)

- 19 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
➢ Hydrophobic interactions: they are not real bonds. Their presence between hydrophobic R-

m
.c o
m
/
groups makes the polypeptide chain bend and fold.

4. Quaternary structure

• is only found in proteins containing two or more polypeptide chains, where the chains are
linked together to form a complex functioning protein.
• Examples are: haemoglobin (4 chains), insulin hormone (2 chains) and some enzymes.
• Chains are linked together by the same types of bonds involved in the tertiary structure.

Denaturation of proteins:
It is the loss of the complex 3D shape of the protein due to breaking down of the bonds
which leads to loss of its tertiary structure and thus its function.
➢ Very high temperatures break hydrogen bonds.
➢ Severe changes of pH break hydrogen and ionic bonds.
➢ Addition of reducing agents breaks disulfide bonds.

- 20 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
Fibrous and globular proteins:

.c o
m
/
point of comparison Globular Fibrous

1. Structure Tertiary structure and sometimes Have little or no tertiary structure.

quaternary. Structure is held by hydrogen
Structure is held together by bonds between the amino acids in
Hydrogen, ionic and disulfide bonds the polypeptide backbone.
between R groups.

2. Shape Spherical/globular shapes Long parallel polypeptide chains

with cross linkages that form them
into fibers

3. Solubility Forms colloid with water where Insoluble

hydrophobic R groups are on the
inside and hydrophilic R groups on
the outside

4. Function Metabolic functions (due to its Structural functions

specific 3D shape)

5. Stability Less stable as they lose their ability More stable to changes in
to function if its shape is slightly temperature and pH as they have
changed (due to change in simpler structures
surrounding conditions)

6. Examples Haemoglobin, antibodies, enzymes Collagen, keratin and fibroin of silk

and some hormones.

- 21 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
Collagen:

.c o
m
/
✓ Collagen is the most common structural protein in animals. It gives strength to tendons,
ligaments, bones and skin.
✓ It has tensile strong fibres that are extremely strong due to the special arrangement of collagen
molecule.
✓ Each collagen molecule is made up of 3 polypeptide chains.
✓ Each polypeptide chain is made up of 1000 amino acids with the repetition of glycine and 2
other amino acids sequence.
✓ The three α-helix chains are arranged as a triple helix held together by hydrogen bonds.
✓ These collagen molecules are arranged to form fibrils that are held together to form fibres.

Haemoglobin:

✓ It is a very large globular conjugated protein made up of 4 polypeptide chains held together by
disulfide bonds.
✓ Each polypeptide chain surrounds an iron-containing haem group.

- 22 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
o
✓ The iron enables the haemoglobin to bind and release oxygen molecules.

m
.c o
m
✓ The arrangement of the polypeptide chains determines how easily oxygen binds or is released.

/
Conjugated proteins:
✓ Some protein molecules are joined with another molecule “prosthetic group” to form a
conjugated protein.
✓ This structural feature affects the performance and functions of the molecules,
✓ Haemoglobin (a globular protein) is a conjugated protein with iron as the prosthetic group.
✓ Lipoproteins:
➢ are proteins conjugated with lipids (examples: LDL and HDL)
➢ important in transporting cholesterol in blood as the lipid part of the molecule enables it to
combine with cholesterol (lipid).
✓ Glycoproteins:
➢ are proteins conjugated with a carbohydrate prosthetic group.
➢ The carbohydrate part helps the molecule to hold a lot of water so makes them slippery &
viscous (so reduces friction).
➢ The carbohydrate part makes it harder for protein-digesting enzymes (proteases) to break
them down (the mucus produced in the stomach protects the protein walls from digestion).
➢ Examples: mucus and synovial fluid in joints.

- 23 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553
 ht
tp
s
://
br
iti
sh
stu
de
nt
ro
mo
.c o
m
/

- 24 -
Edexcel iAS Biology - Dr. Eman Salama +201271111553