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Cambridge International AS & A Level Sciences
Biology (9700), Chemistry (9701) and
Physics (9702)
Student’s Books
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Biology: C.J. Clegg Chemistry: Peter Cann and Peter Hughes
Physics: Mike Crundell and Geoff Goodwin
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scienti昀椀c skills are applied, and analytical skills developed.
Biology • Paperback • 9781510482876
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Chemistry • Paperback • 9781510480230
Boost eBook • 2 year access • 9781398369627
Physics • Paperback • 9781510482807
Boost eBook • 2 year access • 9781398333949
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Biology • Paperback • 9781510482869
Chemistry • Paperback • 9781510482852
Physics • Paperback • 9781510482845
3
synthesis of complex
molecules used in growth
and development and in
metabolic processes,
e.g. proteins,
polysaccharides,
lipids, hormones, growth
factors, haemoglobin,
chlorophyll
3 ENZYMES
nutrients
sugars,
amino acids,
fatty acids,
i.e. smaller
organic
molecules
catabolism
energy-releasing
reactions,
i.e. exergonic
reactions
Look inside
release of simple
substances,
e.g. small inorganic
molecules, CO2, H2O,
mineral ions
▲ Figure 3.1 Metabolism:
an overview
View sample material
from our Student’s Books.
» they are effective in small amounts
» they remain unchanged at the end of the reaction.
Enzymes as biological catalysts
A catalyst is a molecule that speeds up a chemical reaction but remains
unchanged at the end of the reaction. Most chemical reactions do not occur
spontaneously. In a laboratory or in an industrial process, chemical reactions
may be made to occur by applying high temperatures, high pressures, extremes
of pH and by keeping a high concentration of the reacting molecules. If
these drastic conditions were not applied, very little of the chemical product
would be formed. In contrast, in cells and organisms, many of the chemical
reactions of metabolism occur at exactly the same moment, at extremely
low concentrations, at normal temperatures and under the very mild, almost
neutral, aqueous conditions we find in cells.
The presence of enzymes enables reactions to occur at incredible speeds, in an
orderly manner, yielding products that the organism requires, when they are
needed. Sometimes reactions happen even though the reacting molecules are
present in very low concentrations.
The ‘right’ conditions happen so rarely that the reaction does not happen to a
significant extent normally. If we introduce extreme conditions, such as those
listed above, we can cause the reaction to happen. On the other hand, if we
introduce an enzyme for this particular reaction, then the reaction occurs at
great speed. Enzymes are amazing molecules in this respect.
Where do enzymes operate?
Some enzymes are exported from cells, such as the digestive enzymes. Enzymes
like these, that are parcelled up, secreted and work externally are called
extracellular enzymes. However, many enzymes remain within the cells and
work there. These are intracellular enzymes. They are found inside organelles,
in the membranes of organelles, in the fluid medium around the organelles (the
cytosol) and in the cell surface membrane.
Enzymes control metabolism
There is a huge array of enzymes that facilitate the chemical reactions of
the metabolism. Since these reactions can only take place in the presence of
specific enzymes, we know that if an enzyme is not present then the reaction it
catalyses cannot occur.
Many enzymes are always present in cells and organisms but some enzymes are
produced only under particular conditions or at certain stages. By making some
enzymes and not others, cells can control what chemical reactions happen
in the cytoplasm. Sometimes it is the presence of the substrate molecule
CH2OH
1 The reaction: hydrolysis of sucrose
to form glucose and fructose
from water
oxygen bridge
O
CH2OH
1
OH
O
HO
How is this brought about?
For a reaction between two molecules to occur, there must be a successful
collision between them. The molecules must collide with each other in the right
way and at the right speed (Figure 3.2). If the angle of collision is not correct,
the molecules bounce apart. Alternatively, if the speed of the collision is wrong
or the impact is too gentle, for example, then there will be insufficient energy
for the rearrangement of electrons. Only if the molecules are lined up and
collide with the correct energies does a reaction occur.
3
How enzymes work
So, enzymes are biological catalysts made of protein. They speed up the rate of
a chemical reaction. The general properties of catalysts are:
3.1 Mode of action of enzymes
anabolism:
energy-requiring
reactions, i.e.
endergonic
reactions
that triggers the synthesis of the enzyme. In Chapter 4 we see how protein
synthesis (and therefore enzyme production) is directly controlled by the cell
nucleus.
Enzymes as globular proteins
In Topic 2 we saw that the tertiary structure of globular proteins is typically
spherical. In these molecules their linear chain of amino acids (primary
structure) is precisely folded and held in a globular, three-dimensional shape
containing a-helices and b-sheets. Also, the R-groups of the amino acids
present on the exterior of the molecule are hydrophilic groups, making the
protein water soluble. Remember, this structure contrasts with that of the
fibrous proteins, such as collagen.
HO
O
CH2OH
+
OH
HOH 2C
HO
H O
water H
sucrose
OH
O
CH2OH
OH
HOH 2C
HO
OH
glucose
fructose
OH
sucrose
HO
O
2
OH
sucrose
sucrose
2 Random collision possibilities:
when sucrose and water collide at
the wrong angle
water
water
water
sucrose
sucrose
sucrose
when sucrose and water collide at
the wrong speed
These events are what happens at
most random collisions
water
water
water
sucrose
For the reaction to occur, sucrose
and water must collide in
just the right way – glucose and
fructose are formed
Under normal conditions this
happens so infrequently that
it is an insignificant event
glucose
water
fructose
3 In the presence of one molecule of the enzyme sucrase (invertase),
approximately 3.0 × 104 molecules of sucrose are hydrolysed each minute!
▲ Figure 3.2 Can a reaction occur without an enzyme?
7
6
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