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South Tuen Mun Government Secondary School
Biology Revision Note 2
Fluid mosaic model
Characteristics
Functions
Phospholipid bilayer
makes the cell membrane permeable to fat-soluble substances, simple and
small substances; but impermeable to water, ions and water-soluble
substance
Protein (embedded half-way)
acts as enzymes
Channel protein
makes the cell membrane permeable to certain ions, water and water-soluble
(penetrate through the bilayer)
substances
Carrier protein (penetrate
through the bilayer)
carries out active transport
Glycoprotein (embedded
half-way)
acts as receptors to substances in the blood e.g. hormones.
acts as recognition substances e.g. antigens for recognize foreign cells
Transport across the membrane
Diffusion
the net movement of substances from a region of higher concentration to that of lower
concentration
Osmosis
the net movement of water from a region of lower solute concentration/higher water
potential to that of higher solute concentration/lower water potential through a
semi-permeable membrane
Active transport
the movement of substance through a membrane with the expense of energy, this method
can move substance against a concentration gradient
Phagocytosis
large substance is engulfed, it is digested and useful substances are absorbed; energy is
required e.g. white blood cells kill bacteria
Osmosis and cells
In hypertonic solution, water moves out of a plant cell by osmosis, the plant cell becomes flaccid, this is known
as plasmolysis and the cell is plasmolysed.
In hypertonic solution, water moves out of an animal cell by osmosis, the cell becomes crinkled.
In isotonic solution (equal water potential) there is no net movement of water in or out of a cell.
In hypotonic solution (higher water potential) – water moves in a plant cell by osmosis, the plant cell is turgid.
In hypotonic solution – water moves in an animal cell by osmosis, the cell will burst, this process is known as
haemolysis and the cell is said to be haemolysed.
Enzyme
 acts as a catalyst to increase the speed of chemical reaction at 37oC
A+ B C + D
(A and B are substrates, C and D are products)
 speed of chemical reaction can be measured by the rate of disappearance of substrate and / or the rate of
making products
 words ended with –ase, e.g. catalase : catalyse the break down of hydrogen peroxide to form oxygen
 enzyme is specific
 enzyme is not used by the chemical reaction, it remains unchanged after a reaction
 enzyme works best at 37oC, at low temperature, enzyme is inactive; at high temperature, enzyme (protein
in nature) denatures and lose it function forever

different enzymes work best at different optimum pH, enzyme, not at its optimum pH, denatures
Lock and Key hypothesis
Inhibitors
 Competitive inhibitor has a similar shape to substrate. It competes with the substrate for the active site.
When it binds with the enzyme, at the active site, the substrate cannot bind with enzyme. Thus the rate of
reaction is lower. Under high concentration of substrate, the rate of reaction can be increased.

Non-competitive inhibitor does not has a similar shape to substrate. It binds with the enzyme, not at the
active site. It changes the shape of the active site. The substrate cannot bind at the active site. Thus the rate
of reaction is lower. Under high concentration of substrate, the rate of reaction cannot be increased.
Application of enzymes in daily life
 Biological washing powder : protease and lipase to remove protein (blood) stain and grease

Meat tenderizer : protease in fresh fruit (e.g. pineapple, papaya) is used to break down protein and makes
meat soft