- Cells are composed of a nucleus surrounded by a nuclear membrane and cytoplasm surrounded by a plasma membrane. The plasma membrane is made mainly of proteins and lipids arranged in a bilayer that allows some substances to pass through, while preventing others. - There are two main types of transport across the plasma membrane: passive transport such as simple diffusion of lipid-soluble substances and facilitated diffusion through membrane proteins; and active transport using membrane proteins that require energy to move substances against a concentration gradient. - Osmosis is the diffusion of water across a semipermeable membrane according to the concentration of solutes on each side. It allows water to move from an area of high water concentration and low solutes to an area

1. Objectives:
-Cell structure.
-Cell membrane.
-Transport mechanism through cell membrane.

2. ORGANIZATION OF THE CELL
• cells consist of a nucleus surrounded by a nuclear
membrane, and a cytoplasm surrounded by plasma
membrane.
• cellular components (protoplasm) are made of :
1- water
2- Ions
3- proteins
4- lipids
5- carbohydrates

3. Cell membrane
• Thin, pliable and almost entirely made of
proteins and lipids
the lipid bilayer impedes water penetration
cell membrane proteins are of two types:
integral and peripheral, these proteins
perform various functions (channels, carriers,
receptors, enzymes, controllers)

4. TRANSPORT THROUGH CELL
MEMBRANE
• The lipid bilayer prevents mixing of ECF & ICF  water
and water-soluble substances cannot move freely
through it.
• lipid-soluble substances move freely through the
plasma membrane
• Water, ions and water-soluble substances are
transported through proteins (channel proteins &
carriers)

5. Transport pathways through cell
membrane

6. • Constant motion of molecules, ions, colloid
particles through cell membrane
• Two types:
1-Simple diffusion.
2-facilitated.
diffusion

7. Diffusion

8. Simple diffusion
• Diffusion of a matter through the lipid bilayer is
determined by its lipid solubility (O2, N2 ,CO2 move
easily through the membrane)
• Water and some water-soluble substances (urea) pass
through channels “pores” in proteins that penetrate all
the way through the membrane
• Diffusion through channels is characterized by:
 1-selective permeability (charge) , (size)
 2-controled by opening and closing gates (voltage,
ligand)

9. • Carrier-mediated diffusion
• the rate of diffusion reaches a maximum (Vmax)
• According to Fick’s law, the rate of diffusion
depends on:
1-the magnitude of the concentration gradient
2-the permeability of the plasma membrane to a
substance.
3-the surface area of the membrane across which
diffusion takes place
4-the molecular weight of a substance
5-the distance through which diffusion takes place
Facilitated diffusion

12. Fig. 3-14a, p. 56
Step 1
Conformation X of
carrier (binding sites
exposed to ECF)
Molecule to be
transported binds to
carrier
Molecule to be
transported
Concentration
gradient
Plasma
membrane
Carrier molecule
(Low)
(High)ECF
ICF

13. Fig. 3-14b, p. 56
Step 2
On binding with
molecules to be
transported, carrier
changes its
conformation
Conformation X of carrier
ConformationY
of carrier

14. Fig. 3-14c, p. 56
Step 3
ConformationY of
carrier (binding sites
exposed to ICF)
Transported molecule
detaches from carrier
Direction of
transport
ECF
ICF

15. Fig. 3-14d, p. 56
Step 4
ECF
ICF
Conformation X of
carrier (binding sites
exposed to ECF)
After detachment,
carrier reverts to
original shape

16. II. ActiveTransport:
Primary : - Secondary
Na-K pump Co- & counter-tr.
Ca - pump Na - Gl Na -aa
H - pump Na -H Na - Ca

18. Fig. 3-16, p. 58
Phosphorylated
conformationY
of carrier
Step 1
Phosphorylated conformationY of
carrier has high affinity for passenger.
Molecule to be transported binds to
carrier on low-concentration side.
Molecule to be
transported Step 2
Dephosphorylated conformation X
of carrier has low affinity for
passenger. Transported molecule
detaches from carrier on high-concentration side.
= phosphate
Direction of
transport
Concentration
gradient
(High)
(Low)
Dephosphorylated
conformation X
of carrier
ICF
ECF
Na+

19. Fig. 3-17, p
= Sodium (Na+) = Potassium (K+) = Phosphate
When open to the ECF, the carrier drops off Na+ on its high-concentration
side and picks up K+ from its low-concentration side
Phosphorylated conformationY
of Na+–K+ pump has high affinity
for Na+ and low affinity for K+
when exposed to ICF
When open to the ICF, the carrier picks up Na+ from its low-concentration
side and drops off K+ on its high-concentration side
Dephosphorylated
conformation X of Na+–K+
pump has high affinity for
K+ and low affinity for Na+
when exposed to ECF
ICF
ECF

22. Transport of big particles
 - Endocytosis
 - Exocytosis
 - Pancytosis

23. •Basic principles of osmosis:
water diffuses from high [H2O] to
region of lower [H2O] . (Kinetics).
Osmosis

25. OSMOTIC PRESSURE
•the exact amount of pressure required to stop osmosis.
•osmotic pressure is determined by the number of particles
rather than their mass.
•osmole: 1 gram molecular weight of osmotically active
solute
“ a solution having 1 osmole of solute
dissolved in each Kg of water is said to have
an osmolality of 1 osmole/Kg”
• 1 milliosmole/liter  19.3 mmHg osmotic pressure at
normal body temperature

26. Osmosis across selectively
permeable membranes

27. Fig. 3-9, p. 53
100% water concentration
0% solute concentration
90% water concentration
10% solute concentration
= Water molecule = Solute molecule

28. Fig. 3-10, p. 53
Membrane
Higher H2O
concentration,
lower solute
concentration
Lower H2O
concentration,
higher solute
concentration
= Water molecule = Solute molecule
H2O

29. Fig. 3-11, p. 53
Membrane (permeable to both water and solute)
Side 1 Side 2
Higher H2O concentration,
lower solute concentration
Lower H2O concentration,
higher solute concentration
H2O moves from side 1 to side 2
down its concentration gradient
Solute moves from side 2 to side 1
down its concentration gradient
• Water concentrations equal
• Solute concentrations equal
• No further net diffusion
• Steady state exists
Side 1 Side 2
= Water molecule
= Solute molecule
H2O
Solute

30. Fig. 3-12, p. 54
= Water molecule
= Solute molecule
Membrane (permeable to H2O but impermeable to solute)
Higher H2O concentration,
lower solute concentration
Lower H2O concentration,
higher solute concentration
H2O moves from side 1 to side 2
down its concentration gradient
• Water concentrations equal
• Solute concentrations equal
• No further net diffusion
• Steady state exists
Solute unable to move from side 2 to
side 1 down its concentration gradient
Side 1 Side 2
Side 1 Side 2
Original
level of
solutions
H2O

31. Fig. 3-13, p. 54
= Water molecule
= Solute molecule
Membrane (permeable to H2O but impermeable to solute)
Pure water Lower H2O concentration,
higher solute concentration
H2O moves from side 1 to side 2
down its concentration gradient
Solute unable to move from side 2 to
side 1 down its concentration gradient
Side 1 Side 2
Side 1 Side 2
Original
level of
solutions
H2O
• Water concentrations not equal
• Solute concentrations not equal
• Tendency for water to diffuse by
osmosis into side 2 is exactly
balanced by opposing tendency for
hydrostatic pressure difference to
push water into side 1
• Osmosis ceases
• Opposing pressure necessary to
completely stop osmosis is equal
to osmotic pressure of solution
Hydrostatic
(fluid)
pressure
difference
Osmosis
Hydrostatic
pressure

32. 1 osmole = 1 mole of solute particles (6.02 x10).
1 mole glucose = 1 osm.
1 mole NaCl = 2 osm.
1 mole Na2SO3 = 3 osm.
Relation between moles and
osmoles

33. - Osmolality and osmolarity in
human fluids are equal.

34. •Osmotic pressure : pressure that prevents
the osmosis .
•The higher the osmotic pressure of a
solution, the lower its [H2O] but the higher its
[solute].
•According to van’t hoff’s law:
π = CRT
π= 19300 mm Hg for 1 osmole/liter at body
temp.
π(osmotic pr.) C(solute con. In osmole/liter)
R (ideal gas const.) T(absolute temp.)

35. • osmotic pr. = osmolarity(mOsm/L) X 19.3 mmHg
• the calculated value is not 100% correct due to intraionic
and intermolecular interactions between the particles and
it has to be multiplied by the “osmotic coefficient” of the
particles to reach the true value.
• the osmolarity of the body fluids is around 300 mOsm/L,
the plasma being 1mOsm/L higher because of the
osmotic effect of plasma proteins