2. CHAPTER 3 :
MOVEMENT OF
SUBSTANCES ACROSS THE
PLASMA MEMBRANE

3. SUBSTOPICS
3.1 - Movement of Substances Across the
Plasma Membrane
3.2 – Understanding the Movement of
Substances Across the Plasma
Membrane in Everyday Life
3.3 – Appreciating the Movement of
Substances Across the Plasma
Membrane

4. LEARNING OUTCOMES
To state the substances required by living
cells
To state the substances that have to be
eliminated from cells
To explain the necessity for movement of
substances across the plasma membrane
To describe the structure of the plasma
membrane
To describe the permeability of the
plasma membrane

5. NECESSITY FOR MOVEMENT OF
SUBSTANCES ACROSS THE PLASMA
MEMBRANE
To provide nutrients for metabolism &
growth;
To supply oxygen for respiration;
To regulate solute concentration &
suitable pH for maintaining a stable
internal environment for optimal
enzymatic activities
To maintain an ion concentration
gradient required for nerve & muscle
cell activities;

6. To secrete useful substances, for
example, digestive enzymes &
hormones;
To eliminate toxic waste products
such as urea & carbon dioxide

7. Substances can move into or out
of a cell by :
Passive transport
Simple diffusion
Osmosis
Facilitated diffusion
Active transport

8. Movement of substances across
the plasma membrane would
depend on :
Selectivity of the partially
permeable membrane;
The difference in concentration
between the cell & extracellular
fluid

10. Structure of the Plasma
Membrane

11. Structure of the Plasma
Membrane
All cells are covered by a thin plasma
membrane.
It separates the cell contents from the
surrounding
1972, S.J. Singer & G.L. Nicolson
proposed the fluid-mosaic model of
plasma membrane.

12. The plasma membrane is dynamic &
fluid. The phospholipid molecules can
move thus giving the membrane its
fluidity & flexibility
The proteins are scattered in the
membrane giving it a mosaic
appearance
Thickness : 7.0 – 8.0 nm.

13. Structure of the Plasma
Membrane
The membrane consists of
a phospholipid bilayer (2
molecules thick)
The polar hydrophilic heads
– outer layer face
outwards, chemically
attracted to the watery
surrounding
The non-polar hydrophobic
hydrocarbon fatty acid tails
– face inwards, away from
water.

14. Structure of the Plasma
Membrane
There are proteins on the outer & inner
surfaces of the plasma membrane.
Some proteins penetrate partially
through the membrane, others
penetrate completely.
The phospholipid bilayer is permeable
to diffusion of small uncharged
molecules such as O2 & CO2.

15. Structure of the Plasma
Membrane
Two types of transport protein :
Channel / pore proteins – have pore to facilitate
diffusion of particular ions / molecules across the PM.
Some carrier proteins – have binding sites that bind to
specific molecules such as glucose @ amino acids 
alter their shape to facilitate the diffusion of solutes.
Other carrier proteins – function in active transport 
an energized carrier protein actively pumps the solute
across the cell membrane against the concentration
gradient.

17. Structure of the Plasma
Membrane
Cholesterol molecules stabilise the
structure of PM.

18. MECHANISM OF
MOVEMENT OF
SUBSTANCES ACROSS
THE PLASMA MEMBRANE
PERMEABILITY
A semipermeable @ partially permeable
membrane = selectively permeable to small
molecules such as water & glucose.
Does not permit large molecule to move through
it.
Examples : egg membrane, plasma membrane
of living cells & cellaphone membrane of the
Visking tubing.

19. MECHANISM OF
MOVEMENT OF
SUBSTANCES ACROSS
THE PLASMA MEMBRANE
A permeable membrane – permeable to the
many solvent (water) & solute molecules 
diffusion can occur.
Example : cellulose cell wall of plant cell
An impermeable membrane – not allow
substances to diffuse through it.
Example : the impermeable polythene
membrane.

20. LEARNING OUTCOMES
To explain the movement of substances across
the plasma membrane through the process of
passive transport
To explain the movement of water molecules
across the plasma membrane by osmosis,
To explain the movement of substances across
the plasma membrane through the process of
active transport,
To explain the process of passive transport in
living organisms using examples

21. PASSIVE
TRANSPORTThe movement of particles (molecules/ions)
within a gas or liquid across the plasma
membrane from a region of higher
concentration to a region of lower
concentration & does not require expenditure
of energy from ATP.
The substances move down their
concentration gradient through different
ways :
Phospholipid bilayer
Pore protein/ channel protein
Carrier protein

23. PASSIVE
TRANSPORT
SIMPLE
DIFFUSION
OSMOSIS
FACILITATED
DIFFUSION

24. SIMPLE DIFFUSION
The net movement of molecules /
ions from a region of higher
concentration to a region of lower
concentration until an equilibrium
is reached.
Substances :
Small non-polar molecules – O2 & CO2
Lipid-soluble substances – vitamins ADEK,
steroids & alcohols
Water molecules

25. The bigger the concentration gradient
the faster the rate of diffusion.
These substances will diffuse down the
concentration gradient if there is a
concentration gradient. (until an
equilibrium is reached).
Examples : gaseous exchange between
the alveolus & the blood capillaries,
blood capillaries & body cells.
SIMPLE DIFFUSION

30. The diffusion of water molecules (solvent) from
a region of higher water concentration (diluted
solution) to a region of lower water
concentration (concentrated solution) through a
semi-permeable membrane until an equilibrium
is reached.
A special type of diffusion.
Examples :
Absorption of water from soil solution by plant root
hairs
Reabsorption of water by kidney tubules
osmosis

32. The movement of molecules / ions down
their concentration gradient assisted by
transport proteins (channel protein / pore
protein) across the plasma membrane
without using energy.
The transport proteins facilitate & increase
the rate of diffusion across the plasma
membrane.
Not require energy
FACILITATED DIFFUSION

33. The rate of facilitated diffusion depends on the
number of transport protein molecules in the
membrane & how fast they can move their
specific solute.
Only allows small charged molecules such as
mineral ions to pass through the pore protein.
Carrier protein : allows larger uncharged
polar molecules – glucose & amino acids to
cross the membrane.
FACILITATED DIFFUSION

35. THE MECHANISM
1. The solute moves to the binding site of
the specific carrier protein.
2. The solute binds to the carrier protein
at the binding site & triggers the
carrier protein to change its shape.
3. The carrier protein changes its shape
& moves the solute across the
membrane.
4. The carrier protein returns back to its
original shape.

36. The solutes can be transported by
carrier proteins in either direction but
the net movement is always down the
concentration gradient.
Examples : the transportation of
glucose, amino acids & mineral ions
across the membrane of the vilus at the
ileum & body cells.
FACILITATED DIFFUSION

37. The movement of substances across the
plasma membrane from a region of low
concentration to a region of high
concentration (against the concentration
gradient) by using metabolic energy.
The substances move across a membrane
against the concentration gradient, using
metabolic energy
Perform by a specific protein embedded in the
plasma membrane.
ACTIVE
TRANSPORT

38. ACTIVE
TRANSPORT
Require energy to change the shape of the
protein such that the substance can be
pumped across the membrane.
Example : absorption of potassium ions from
pond water by algae Nitella sp. against a
concentration gradient, the intake of mineral
ions by the plant root hairs, Na+/ K+ protein
pumps in the plasma membrane of neurones
transport Na+ & K+ against their
concentration gradients.

39. ACTIVE
TRANSPORT

40. ACTIVE
TRANSPORT

41. COMPARISON BETWEEN PASSIVE & ACTIVE
TRANSPORT
PASSIVE
TRANSPORT
SIMILARITIES ACTIVE
TRANSPORT
Transport of substances across the plasma membrane
Need a difference of concentration gradient between extracellular environment
& the cell
DIFFERENCES
Follow Concentration
gradient
Against
Does not expend energy Cellular energy Need to expend energy
Until an equilibrium is
reached
Outcome of the
process
Depends on the cells
requirement (no need to
reach an equilibrium)
Non-living & living
organisms
Occurs in Living organisms only
Simple diffusion, osmosis,
facilitated diffusion
Name of process Active transport
Molecules of water
(osmosis)
Type of molecules Molecules of glucose and
mineral salt