slides diffusion

§ Important to the life of a cell
§ Due to lipid bilayer which serve as barrier

§ Few substances can penetrate this bilayer
§ Continuity interrupted only by penetrating proteins
§ Transport thru the cell membrane
1. thru lipid bilayer
2. protein channel

§ May occur as diffusion or active transport
Simple diffusion
§ Random molecular movement of substances thru
-intermolecular spaces in the membrane
-in combination with a carrier protein

§ Energy from normal kinetic motion of matter

§ Active transport – combines with protein and moves
against energy gradients
diffusion thru lipid bilayer
§ A. lipid soluble substances
--O2, N, CO2
§ B. non-lipid soluble substances
--water – due to : size
concentration
kinetic energy
§ C. ions– electrical charge
- impedes movement due to –hydrated ions
reaction with cell membrane

protein channel
§ Watery pathways in protein molecules
§ Characteristics:
1. selective – selective properties due to
diameter
shape
electrical charge
2. presence of gates
§ Sodium channel– negatively charged inner
surface

§ Potassium channel– smaller channel
not negatively charged
§ Gating of protein channel
- control of permeability
-extension of transport of transport
protein molecules
2 ways of control
§ 1. voltage gating – responds to the electrical
potential across the cell
membrane

§ 2. ligand gating – binding of another molecule
resulting to conformational
changes
- acetylcholine
Facilitated diffusion
§ Carrier mediated diffusion

§ Dependent on specific carrier
Factors affecting net diffusion
§ 1.permeability of the membrane

§ 2.difference in concentration of diffusing
substance
§ 3.pressure difference

§ 4.Electrical potential across the cell membrane
Facilitated diffusion
§ Carrier mediated diffusion

§ Needs carrier protein

§ Difference with simple diffusion
- rate of diffusion reaches maximum as
concentration increases

mechanism of facilitated diffusion
§ Carrier protein has receptor for specific
molecule
§ Binding causes conformational changes

§ Opening of the channel

§ Glucose and amino acid
Factors affecting permeability of membrane
§ 1.thickness of the membrane
§ 2.lipid solubility
§ 3.number of protein channel
§ 4.temperature
§ 5.molecular weight of the substance
§ 6.area of the membrane
§ Concentration difference…

§ --rate of diffusion inward is proportional to the concentration of the substance

§ Net diffusion = D(Co –Ci)
§ Effect of electrical potential…
§ --if electrical potential is applied across cell
membrane, ions move thru the membrane
even with no concentration difference
existing

Nearnst equation emf=61 log _C1
C2
§ Effect of pressure difference…
§ Pressure – sum of all forces of different
molecules striking a unit surface
at a given time
§ Capillary membrane – 20 mmHg greater in
pressure than outside

§ Movement of molecules from high pressure
side to low pressure side
osmosis
§ Net movement of water caused by pressure
difference

§ Water – most abundant substance diffusing
in and out of cell, RBC

Osmotic pressure – amount of pressure required to
to stop osmosis
active transport
§ No substance can diffuse against electrochemical gradient

§ Needed for the survival of cell

§ Sodium and potassium ions
Primary and secondary active transport
§ Differ in the source of energy

§ ATP and energy from ionic concentration difference

§ Both are dependent on carrier proteins
Primary active transport
§ Primary active transport – derived its energy from
ATP

§ Secondary transport derived its energy from –
energy is derived secondarily from ionic
concentration gradient that had been created
by primary active transport
sodium pump
§ Maintain sodium concentration difference
§ Maintain electrical potential inside the cell
§ Basic component of the pump
--carrier protein-2 separate globular proteins
-- features of the protein
1. 3 receptor sites for Na
2. 2 receptors sites for K
3. presence of ATPase activity
Sodium pump
§ 3 Na ions bind with the inside of carrier protein
§ 2 K ions bind on the outside of the protein
§ ATPase activated – Na ions extruded out
- K ions thrown inside

§ Important in control of cell volume
§ Electrogenic pump
Primary pump
§ CALCIUM PUMP –
§ Calcium ions are normally maintained at low
concentration intracellularly

§ 2 calcium pumps – in cell membrane
- in membrane of organelles
Secondary transport
§ Co-transport – concentration gradient created from
primary active transport can pull
other substance : glucose,a.a.

§ Counter-transport – exchange of ions in different
direction: calcium, H ion
active transport thru sheets of cells
§ Occurs through – intestinal epithelium
renal tubular epithelium
exocrine gland epithelium
gallbladder epithelium

§ Basic mechanism for transport through cellular sheets
1. active transport on one side of the membrane
2. simple or facilitated diffusion of the opposite side




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