Transport of molecules across cell
membrane
Controls what comes in and out of the
cell. Does not let large, charged or polar things through without
help.
SELECTIVELY PERMEABLE:


Is one of the most important functions of cell
membrane (Plasma membrane)
A steady traffic ‫ العبور المنتطم‬of small molecules
and ions move across the plasma membrane in
both directions.
 For
example, sugars, amino acids, and other nutrients
enter a muscle cell and metabolic waste products leave
it.
 The cell absorbs O2 and expels CO2.
 It also regulates concentrations of inorganic ions, like
Na+, K+, Ca2+, and Cl-, by passing them across the
membrane.




However, substances do not move across the
membrane barrier indiscriminately (randamly)
‫عشوائيا‬.
Hydrophobic molecules, like hydrocarbons,
CO2, and O2, can dissolve in the lipid bilayer
and cross easily.
Ions and polar molecules, like water, glucose
and sugars pass through with difficulty.
Proteins can assist and regulate ‫ يساعد و ينظم‬the
transport of ions and polar molecules.
***
Transport
Passive
Active
Large
molecules
(Membrane)
Exocytosis
Small
Molecules/ions
(T. protein)
Facilitated
diffusion
(T. protein)
Osmosis
Diffusion
(Membrane)
(Membrane)
Endocytosis
Phagocytosis
Pinocytosis
Receptor-mediated
endocytosis
Cellular eating
Cellular drinking
Selective eating
Using specific
receptors (coated
vesicles) that bind to
specific legends and
engulf it. It is specific

Passive transport
is a kind of transport
molecules
move
along
by which ions or
a
concentration
gradiant , ( means movement from an area
of
higher
concentration
of lower concentration)
to
an
area
Passive Transport
A process that does not require energy to move
molecules from a HIGH to LOW concentration
 Diffusion
 Facilitated Diffusion
 Osmosis

Simple Diffusion: ‫اإلنتشار‬
is movement ( spread ) of molecules
1) randomly
2) from areas of high concentration to areas
of low concentration
3) Requires no energy
For example, in case of presence of a permeable membrane
separating a solution containing dye molecules from pure
water, the dye molecules will cross the membrane randomly.
The dye will cross the membrane until both solutions have
equal concentrations of the dye (dynamic equilibrium ‫التعادل‬
‫)الديناميكى‬.
Osmosis:
is a passive transport in
which water diffuses across
a selectively permeable
membrane from the
hypotonic solution to the
hypertonic solution until the
solutions become isotonic
Osmosis
• Hypertonic solution: ‫عالى التركيز‬
contains high concentration of solute ‫ ُمذاب‬molecules.
• Hypotonic solution: ‫منخفض التركيز‬
contains low concentration of solute molecules.
• Isotonic solution: ‫متعادل‬
contains equal concentrations of solute molecules
Biological
Membrane
H2O
Hypertonic
Hypotonic
Semi-permeable membrane is permeable
to water, but not to sugar

A cell in a hypotonic
solution will gain
water, swell, and
burst.

The cell in a
hypertonic
environment will
loose water, shrivel
‫تنكمش‬, and die.

Organisms without rigid walls have
osmotic problems in either a
hypertonic or hypotonic
environment and must have
adaptations for osmoregulation to
maintain their internal
environment.

Example, Paramecium have a
specialized organelle ( the
contractile vacuole), that functions
as a pump to force ‫ يطرد‬water out of
the cell.




Another form of passive transport
Many molecules and ions diffuse passively
through the lipid bilayer with the help of
transport proteins
The passive movement of molecules down its
concentration gradient via a transport protein is
called facilitated diffusion
Used for molecules that are too large to cross
the membrane by diffusion (i.e. glucose)


Carrier proteins bind
to larger molecules,
and change their
shape so molecules
can diffuse through.
Channel proteins
provide water filled
pores for charged ions
to pass through


Molecules move against the concentration
gradient (low to high)
Energy must be provided via ATP (even when we
are resting, 40% of our energy is spent on active
transport!)

Uses specialized transport proteins and
protein pumps
Maintains internal cell environments (i.e. cell’s
electrical gradient, roots pull in minerals from
soil, filtering blood in your kidneys)
A- small molecules
Sodium potassium pump

The sodium-potassium pump maintains the transport of
sodium (Na+) and potassium ions (K+) across the
membrane.

The animal cell has higher concentrations of K+ and lower
concentrations of Na+ inside the cell.

The sodium-potassium pump (T. protein) uses the energy of one
ATP to pump 3 Na+ ions out and 2 K+ ions in.
1ATP
T. protein
High conc.
+
of K
Low conc.
+ 2
of K
High conc.
+
of Na
3
Low conc.
+
of Na
***
Two roles of
membrane protein
Both diffusion and facilitated diffusion are forms of passive transport of molecules down
their concentration gradient, while active transport requires an investment of energy to
move molecules against their concentration gradient.
B)- Large molecules (macromolecules)

Large molecules, such
as polysaccharides and
proteins, cross the
membrane by vesicles
‫الحويصالت‬

This occurs by
Exocytosis and
Endocytosis
***
Exocytosis ‫اإلخراج الخلوى‬:

a transport vesicle
budded from ‫ ينشأ من‬the
Golgi apparatus is moved
by the cytoskeleton to the
plasma membrane.

When the two
membranes come in
contact ‫تالمس‬, the bilayers
fuse ‫ يندمج‬and spill ‫ يُفرع‬the
contents to the outside.
***

Endocytosis ‫اإلدخال الخلوى‬, the cell brings in
macromolecules and particulate matter by
forming new vesicles from the plasma
membrane and include the following types:
1. Phagocytosis (solids)
2. Pinocytosis (liquids)
3. Receptor mediated
A)- Phagocytosis ‫اإلبتالع الخلوى‬:

Called “cellular eating”.
The cell engulfs a particle by extending pseudopodia
‫ أقدام كاذبة‬around it and packaging it in a large vacuole.

The contents of the vacuole are digested when the
vacuole fuses with a lysosome.
Phagocytosis
B)- Pinocytosis, ‫الشرب الخلوى‬
“cellular drinking”,
a cell creates a vesicle around droplets of

extracellular fluid ‫السائل الموجود خارج الخلية‬.

This is a non-specific process ‫عملية غير متخصصة‬.
C)- Receptor-mediated endocytosis: ‫اإلدخال الخلوى عن طريق المستقبالت المتخصصة‬

It Is called (Selective eating)

It is highly specific in substances are being transported.


It is triggered ‫ تـُستـَحث‬when extracellular substances bind to special
receptors, on the membrane surface. This triggers the formation of a
vesicle
It enables a cell to take large quantities of specific materials that may be
in low concentrations in the environment.




http://www.youtube.com/watch?v=STLAJH7
_zkY diffusion of food coloring
http://www.youtube.com/watch?v=HIGEmTl
NAT4 artistic
http://www.youtube.com/watch?v=SSS3EtK
AzYc egg cell osmosis
http://www.teachersdomain.org/asset/tdc02_
int_membraneweb/