Higher Biology
Unit 2
7: METABOLISM AND ENZYMES
Cell metabolism
What is a metabolic pathway?
 Catabolic pathways – bring about the
breakdown of complex molecules to simpler
ones, releasing energy.
 Give an example of such a reaction
 Anabolic pathways – bring about the
biosynthesis of complex molecules from simpler
building blocks, require energy
 Give an example of such a reaction

The two types of metabolic
pathway
Protein
Glucose +
oxygen
catabolic
pathway
CO2 +
water
energy
anabolic
pathway
Amino acids
Reversible and irreversible steps
Enzyme A
Glucose
Intermediate 1
IRREVERSIBLE



Glucose diffuses from a high concentration outside the
cell to a low concentration inside the cell, then
converted to intermediate 1.
This reaction is irreversible
It is of benefit to the cell as it maintains a low
concentration of glucose outside the cell and therefore
allows glucose to diffuse constantly into the cell.
Reversible and irreversible steps
Glycogen (mammals)
Enzyme B
Intermediate 1
Intermediate 2
REVERSIBLE
Starch (plants)


The conversion of Intermediate 1 to Intermediate 2 is
reversible.
If more intermediate 2 if formed than the cell needs
for the next step hen some can changed back into
intermediate 1 and used in al alternative pathway,
e.g to build glycogen in animal cells or starch in
plant cells.
Reversible and irreversible steps
Enzyme C
Intermediate 2
Intermediate 3
IRREVERSIBLE
 The
conversion of Intermediate 2 to
Intermediate 3 is reversible.
 Intermediate 3 will always be
converted to pyruvate (through
many further steps).
Summary
glucose
Enzyme A
Glycogen
(mammals)
intermediate 1
Starch
(plants)
Enzyme B
intermediate 2
Enzyme C
intermediate 3
Many steps, many enzymes
pyruvate
Alternative routes
glucose
Enzyme A

Pathways can be
modified and
contain alternative
routes, so steps can
be bypassed.
intermediate 1
Alternative
route
Enzyme B
intermediate 2
Enzyme C
intermediate 3
Many steps, many enzymes
pyruvate
sorbitol
Several
enzyme
controlled
steps
Membranes




This mitochondrion has a
double membrane – an
inner and outer
membrane.
The inner membrane is
folded so that metabolic
activity is localised –
particular sites are
responsible for particular
chemical reactions.
Here the metabolites are
kept close to the enzymes
that are required for the
reactions to proceed.
Which reactions occur n
the mitochondrion?
Membranes

In this chloroplast the
enzymes needed for ATP
generation are bound
together on flattened
sacs containing
chlorophyll

The Calvin cycle occurs in
the fluid outside the sacs
where the required
enzymes are also present.
Lysosomes

Lysosomes contain strong
digestive enzymes.

If these enzymes were free
in the cytoplasm they would
destroy organelles in the
cytoplasm.

The lysosome ensures the
enzymes remain
compartmentalised until
they are needed.
Surface area to volume ratio

The smaller the cube
the larger the surface
relative to its volume.

Membranes, folds
and compartments
within the cell present
a larger surface are
upon which
metabolic reactions
can take place.
Membrane transport
Write
a definition of diffusion
Does diffusion require energy?
What size of molecules can
freely diffuse across the cell
membrane?
Role of protein pores

Larger molecules depend
on protein carrier
molecules to allow them
to move across the
membrane.

These transport proteins
contains pores or
channels.

These channels only allow
specific substances to
diffuse across the
membrane.
pore
Active transport
What
is active transport?
Does it require energy?
Sodium/potassium pump
Certain proteins on the cell
membrane act as carrier
molecules which transfer
specific ion across the
membrane.
 These proteins are
sometimes called pumps.
 They can have a dual role,
e.g. the sodium/potassium
pump actively pumps
sodium ions out of the cell
and potassium ions into the
cell, both against their own
concentration gradient.

Conditions required by protein
pumps
The
active transport of molecules
across the membrane requires
energy from respiration.
Therefore what conditions will
affect the rate of active
transport?
Activation energy an enzyme
action
Activation energy and enzyme
action





A chemical reaction may involve the joining together of
simple molecules into more complex ones or the splitting
of complex molecules into simpler ones.
Either way, energy is required to initially break the bonds
(activation energy) in the reactants to form an unstable
compound. The molecules are in the transition state.
When bonds are made to form the product energy is
released.
Enzymes serve to lower the activation energy needed for
a reaction to proceed.
Thus biochemical reactions are able to proceed rapidly
at relatively low temperatures.
Enzymes are specific

What are enzymes made of?

Enzymes are specific – describe what this means.

Draw and label an enzyme and substrate fitting
together. Label the active site.
Induced fit



The active site is not a rigid structure, it is flexible.
When the substrate enters the active site, the enzyme
molecule and the active site change slightly making the
active site fit very closely round the substrate molecule.
The induced fit ensures that the active site comes into very
close contact with the molecules of substrate and
increases the chance of a reaction taking place
Orientation
of reactants

When the reaction involves two (or more) substrates, the shape of
the active site helps orientate the reactants in the right position so a
reaction can take place.
1.
The active site holds the reactants together in an induced fit.
2.
The chemical bonds in the reactants are weakened, the activation
energy is reduced.
3.
The products now have a low affinity for the active site and are
released from the active site
4.
The enzyme is free to repeat the process.
Factors affecting enzyme action

Describe the effect of temperature on enzyme
action. Sketch a graph

Describe the effect of pH on enzyme action.

Enzyme action can also be affected by an
inadequate supply of substrate or the presence
of an inhibitor.
Effect of substrate concentration
on enzyme activity
Low concentration – too few
substrate molecules present to make
use of all the active sites on the
enzyme molecules.
 Increasing substrate leads to an
increase in reaction rate as more
active sites are involved.
 Further increase in substrate
concentration does not increase rate
of reaction further (graph levels off)
since all the active sites are
occupied.
 The enzyme concentration has
become a limiting factor.

Direction of enzyme action
Enzyme 2
Enzyme 1
Metabolite W
Metabolite X
Enzyme 3
Metabolite Y
Metabolite Z

The diagram above is metabolic pathway.

Once metabolite W becomes available enzyme 1 is activated
and converts W to X.

When metabolite X becomes available enzyme 2 is activated
and converts X to Y and so on……

Most enzyme reactions are reversible, the actual direction of the
reaction depends on the relative concentrations of reactants
and products.

Thus a balance of metabolites is is always maintained.