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Revision lesson 2
• Enzymes are biological catalysts – they speed
up a biological reaction.
• Each enzyme is specific to a substrate. The
substrate molecules are changed into product
• Enzyme controlled reactions are affected by
pH and temperature.
• Optimum pH or temperature – the pH /
temperature where the reaction works best.
• Lock and key theory- each enzyme has a unique
sequence of amino acids – therefore each
enzyme has a different shape. Within this shape
is a structure called an active site. Only one type
of substrate can fit into the active site, this makes
enzymes specific to a reaction. Once the
substrate is attached to the active site it is turned
into a product. The enzyme is like a lock and the
substrate is like the key.
• Optimum – the best temperature and
pH that enzymes work at.
• Denatured – when the shape of the
enzyme’s active site is changed
• Substrate – the chemical that is
• Active site – the area on the enzyme
that the substrate fits into.
Denaturing enzymes
• If the shape of an enzyme changes it can no
longer catalyse a reaction because the
substrate can no longer fit into the active site.
The enzyme has become denatured.
• Enzymes can be denatured by:
• Extremes of pH
• High temperatures
• Diffusion – the movement of a substance from a region of
high concentration to a region of low concentration.
• Moving substances in the body
• Different substances diffuse in and out of the cells across
the cell membranes.
• Oxygen – moves from the lungs into the red blood cells. It
then moves from the red blood cells into the body tissue.
• Carbon dioxide – moves from the body tissue into the
blood, then from the blood into the lungs.
• After eating – digested food molecules move from the small
intestine into the blood. They then leave the blood and go
into body tissue
Surfaces Across Which Gases Are
Exchanged Are Often Specialised in
Humans by Having Large Surface Areas
This is to
increase the
rate at which
diffusion can
Good examples in the
Human Body are:1. Alveoli
2. Villi
3. Placenta
4. Neurones
Cells use diffusion to swap
the oxygen they need for
the carbon dioxide they
no longer want:
diffuses in
Out goes
waste CO2
Other examples of where diffusion happens in humans:
Alveoli in the lungs
Villi in the intestines
Changing the rate of diffusion
It can be increased by:
Increasing the surface area
Decreasing the diffusion distance
A greater concentration difference
Surfaces adapted for diffusion
• Diffusion takes place in the villi in the small intestine and
the alveoli in the lungs. Both are adapted to increase the
rate of diffusion.
• Villus – produce a large surface area, villi wall have folds
(microvilli). Surface area of small intestine is approx 9 m2.
• One cell thick, food does not have to far to diffuse into the
• Good blood supply – means digested food is quickly taken
away from villus so more can diffuse across to replace it
• Membrane of villi is permeable, this means food molecules
can pass through the membrane
Alveoli – in the lungs
• Breathing makes sure there is always a high
concentration of oxygen in the alveoli.
• Good blood supply makes sure as oxygen diffuses into
the blood it is replaced with blood containing very little
• Alveolus is only one cell thick so the gases do not have
far to travel
• Large numbers of alveoli – this helps to increase the
surface area, so more molecules can move across at
any time.
• Alveoli membrane is permeable to gases and is also
moist; this helps to speed up diffusion.
Other substances adapted for
• Placenta
• To move substances across the placenta as quickly as possible. To
speed up movement, the placenta has:
• A very large surface area
• A very thin wall so substances only have a short distance to diffuse
• The leaf
• To increase the rate of gas exchange, the leaf has a large surface
area. The under-surface of the leaf also has many stomata through
which gases can diffuse.
• Synapses
• The gap between two neurones (nerve cells). The synapse releases
a chemical that can diffuse across the gap between the two
neurones. A large surface area and short diffusion distance is
The Alveoli in the Lungs Provide a
Massive Surface Area for Exchange
of Oxygen & Carbon Dioxide