Objectives:

-Cell structure.

-Cell membrane.

-Transport mechanism through cell membrane.

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

Cell membrane

• Thin, pliable and almost entirely made of proteins and lipids

the lipid bilayer impedes water penetrationcell membrane proteins are of two types:

integral and peripheral, these proteins perform various functions (channels, carriers, receptors, enzymes, controllers)

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)

Transport pathways through cell membrane

• Constant motion of molecules, ions, colloid particles through cell membrane

• Two types:1-Simple diffusion.2-facilitated.

diffusion

Diffusion

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)

• 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 gradient2-the permeability of the plasma membrane to a substance.3-the surface area of the membrane across which diffusion takes place4-the molecular weight of a substance5-the distance through which diffusion takes place

Facilitated diffusion

Fig. 3-14a, p. 56

Step 1

Conformation X ofcarrier (binding sitesexposed to ECF)Molecule to betransported binds tocarrier

Molecule to betransported

Concentrationgradient

Plasmamembrane

Carrier molecule

(Low)

(High)ECF

ICF

Fig. 3-14b, p. 56

Step 2

On binding withmolecules to betransported, carrierchanges itsconformation

Conformation X of carrierConformation Yof carrier

Fig. 3-14c, p. 56

Step 3

Conformation Y ofcarrier (binding sitesexposed to ICF)Transported moleculedetaches from carrier

Direction oftransport

ECF

ICF

Fig. 3-14d, p. 56

Step 4

ECF

ICF

Conformation X ofcarrier (binding sitesexposed to ECF)After detachment,carrier reverts tooriginal shape

II. Active Transport:

Primary : - Secondary

Na-K pump Co- & counter-tr.

Ca - pump Na - Gl Na -aa

H - pump Na -H Na - Ca

Fig. 3-16, p. 58

Phosphorylatedconformation Yof carrier

Step 1

Phosphorylated conformation Y of carrier has high affinity for passenger. Molecule to be transported binds to carrier on low-concentration side.

Molecule to betransported Step 2

Dephosphorylated conformation X of carrier has low affinity for passenger. Transported molecule detaches from carrier on high-concentration side.

= phosphate

Direction oftransport

Concentrationgradient

(High)

(Low)

Dephosphorylatedconformation Xof carrier

ICF

ECF

Na+

Fig. 3-17, p. 59= 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 conformation Yof Na+–K+ pump has high affinityfor 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

Dephosphorylatedconformation X of Na+–K+

pump has high affinity forK+ and low affinity for Na+

when exposed to ECF

ICF

ECF

Transport of big particles

- Endocytosis

- Exocytosis

- Pancytosis

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

Osmosis

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

Osmosis across selectively permeable membranes

Fig. 3-9, p. 53

100% water concentration 0% solute concentration

90% water concentration 10% solute concentration

= Water molecule = Solute molecule

Fig. 3-10, p. 53

Membrane

Higher H2Oconcentration,lower soluteconcentration

Lower H2Oconcentration,higher soluteconcentration

= Water molecule = Solute molecule

H2O

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 2down its concentration gradient

Solute moves from side 2 to side 1down 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

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 2down its concentration gradient

• Water concentrations equal• Solute concentrations equal• No further net diffusion• Steady state exists

Solute unable to move from side 2 toside 1 down its concentration gradient

Side 1 Side 2

Side 1 Side 2

Originallevel ofsolutions

H2O

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 2down its concentration gradient

Solute unable to move from side 2 toside 1 down its concentration gradient

Side 1 Side 2

Side 1 Side 2

Originallevel ofsolutions

H2O

• Water concentrations not equal• Solute concentrations not equal• Tendency for water to diffuse by

osmosis into side 2 is exactlybalanced by opposing tendency forhydrostatic pressure difference topush water into side 1

• Osmosis ceases• Opposing pressure necessary to

completely stop osmosis is equalto osmotic pressure of solution

Hydrostatic(fluid)pressuredifference

Osmosis

Hydrostatic pressure

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

- Osmolality and osmolarity inhuman fluids are equal.

•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 = CRTπ= 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.)

• 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