Hodder Science catalogue 2024 - Flipbook - Page 43
Pearson Edexcel A Level
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Biology: Martin Rowland, Ed Lees, C.J. Clegg
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1
Introducing the chemis
try of life
Table 1.1 Most common elements
Earth’s crust
Element
Look inside
47.0
Silicon
e
pl
m
sa
% of atoms
63.0
25.5
9.5
1.4
0.3
0.2
Ions Charg
Charged particles
formed when
wh atoms
gain or los
lose electrons.
Cations ar
are positively
charged, whereas
w
anions
are negatively
negativ charged.
Acid
A
cid A com
compound that
releases hydrogen
hy
ions in solu
solution. Acidic
solutions have
ha a pH
value below 7.
2
ions
The fundamental unit of
chemical structure is the
atom. Atoms group together
molecules and molecules
olecules are the sm
to form
smallest part of most elements
exist alone under
or compounds that can
er normal condi
conditions. For example,
both oxygen and nitrogen
combine with
h another atom of the same
naturally
type to form a molecule
(O2 and N , respectively).
If an atom
2
om gains or loses an electron,
an ion is formed. Depending
migrate to the poles
on their charge, ions
pole of an electric oeld.
Positively charged ions migrate
pole (cathode)
de) and so are called cations.
to
In contrast, negatively charged the negative
the positive pole (anode)
ions migrate to
and so are called anions.
Available as
eBooks on
Acids and bases
An acid is a compound
that releases hydrogen ions
in solution. We are familiar
the sharp taste that acids
with
such as lemon juice or vinegar
give to the tongue. These
relatively weak acids, weak
are
enough to use on foods.
The stronger the acid the
dangerous and corrosive
more
it is, and the more hydrogen
ions it releases. An example
strong acid is hydrochloric
of a
acid. In water, this acid
dissociates completely. The
dissociate means 8separates
word
into its constituent ions9:
HCl
ĺ
H+
+
Cl−
hydrochloric acid
1
Calcium
Phosphorus
e
pl
m
sa
Atoms, molecules
les and
Key te
terms
Atom The smallest part
of an elem
element that can
take part in a chemical
change.
Some basic concepts
Some basic concepts
Carbon
Nitrogen
3.5
2.5
In fact, about 16 elements
are required to build up
all the molecules of the
therefore essential for life.
cell, and are
Consequently, the full list
of essential elements is a
short one. Furthermore,
relatively
about 99 per cent of living
matter consists of just four
carbon, hydrogen, oxygen
elements:
and nitrogen.
The elements carbon, hydrogen
and oxygen predominate
large quantities of water,
because living things contain
and also because most other
molecules present in cells
organisms are compounds
and
of carbon combined
d with hydrogen and oxygen,
the carbohydrates and lipids.
including
We will examine the struc
structures and roles of carbohydrates
and lipids shortly.
The element nitrogen is
combined with carbon,
hydr
hydrogen
called amino acids, from
and oxygen in compounds
which proteins are constructed
cons
introduce some inorganic
(Chapter 2). First, we will
ions essential
sential for o
organisms, and then discuss
water.
Test yourself on prior knowledge
wle
Chemical elements are
the units of pure substance
that make up our world.
Earth is composed of about
The
92 stable elements; living
things are built from some
of them. Table 1.1 shows
a comparison between the
most common elements
Earth9s crust and in us.
in the
You can see that the bulk
of the Earth is composed
elements oxygen, silicon,
of the
aluminium and iron. Of
these, only oxygen is a
component of our cells.
major
Oxygen
7.9
4.5
Calcium
Sodium
Human body
Element
Hydrogen
28.0
Aluminium
Iron
1 Give one way in which
the
e composition of a carbohydrate
that of a lipid and one
molecule is similar to
e way in which it is d
different from that of a
lipid.
2 Name one type of carbohydrate
used for energy storage
used for energy
in an animal and one
gy storage in a plant.
pla
3 How do animal
mal lipids differ from plant
lipids at room temperature?
4 Give two
o functions of lip
lipids.
5 Calcium
um ions are ess
essential for healthy growth
in
animals
function
and plants. Give one
ion of calcium
calci
ions in humans and one
function of calcium ions
6 In cells, water is a reactant
in plants.
in hydrolysis reactions
and in condensation reactions.
Describe the difference
between these two types
of reaction.
View sample material
from Edexcel A Level
Biology Year 1 and 2
% of atoms
Oxygen
Prior knowledge
In this chapter you will need
to recall that:
➜ carbohydrates contain
the chemical elements carbon,
hydrogen and oxygen in the
➜ carbohydrates include
ratio Cx (H2O)
simple sugars, such as glucose,
y
and complex carbohydrates,
cellulose and starch
such as
➜ simple sugars are used
in respiration; complex carbohydrates
might be glucose stores (starch)
structural components of
or
cells (cellulose)
➜ lipids also contain the
chemical elements carbon,
hydrogen and oxygen, but
oxygen in lipids than in carbohydrates
there is much less
➜ lipids can be useful storage
compounds and can also
be structural components
➜ inorganic ions are present
of cells
in the cytoplasm of cells and
in body nuids; each type of
specioc function
ion has a
➜ water is essential for life
and most of the mass of an
organism comprises
c
➜ water is a reactant in many
water
cell reactions; these reactions
also occur
o
in solution in water.
1 Introducing
In
the chemistry
of life
hydrogen
ion (proton)
chloride
ion
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