Transport through plasma membraneTransport through plasma membrane

Physiology -IPhysiology -I

PHL 215PHL 215

Dr/Gamal GabrDr/Gamal Gabr Pharmacy CollegePharmacy College

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MembranesMembranes

Membranes may be:Membranes may be:

1- Impermeable membrane: 1- Impermeable membrane:

Membrane through which nothing can pass.

2- Freely permeable membrane: 2- Freely permeable membrane:

Membrane through which any substance can pass.

3- Selectively permeable membrane: 3- Selectively permeable membrane:

Membrane through which some substance can pass while others can not.

Plasma membrane is selectively permeablePlasma membrane is selectively permeable

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Mechanisms of transport through plasma membraneMechanisms of transport through plasma membrane

3 basic mechanisms:

I) Passive Transport:

passive means no ATP energy is required

II) Active Transport:

ATP energy is required

III) Vesicular Transport:

NOTE: There are 2 types of transport through cells: one requires energy and the other

does not

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I) Passive transportI) Passive transport

o Passive transport is the movement of molecules through a cell membrane without expending any energy.

o In passive transportpassive transport

1- Energy is not needed to move molecules across the membrane.

2- Molecules move from high to low concentration.

Types:

1: Simple Diffusion.

2: Facilitated Diffusion.

3: Osmosis.

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1: Simple Diffusion1: Simple Diffusion It is the passage of molecules directlydirectly through the lipid

layer of plasma membrane (between phospholipid molecules) according to the concentration gradientaccording to the concentration gradient

i.e: from high to low concentration . Molecules diffuse from high to low concentration, so NO

ATP energy is needed for this process to occur. Diffusion continues until the concentration of the

molecules on both sides of the membrane is equal (equilibrium state).

What two elements are required for diffusion? Concentration gradient and membrane

permeability

NOTES:o A cell that suddenly stopped producing energy

could still carry out diffusion.o In the process of simple diffusion, No pores or openings

are needed.

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o What does the term Equilibrium mean?

1- The concentration of molecules is equal on both sides of a membrane.

2- The rate of movement of molecules across a membrane is equal in both directions.

o What does the term Concentration gradient mean?

It is the difference in the concentration of molecules on both sides of the membrane.

Factors affecting the rate of diffusion:Factors affecting the rate of diffusion:

1) 1) Concentration gradient:Concentration gradient: Increased concentration gradient increases the

rate of diffusion

2) Molecular size: 2) Molecular size: Molecules of small size diffuse more easily

through lipid layer of the membrane.

3) Lipid solubility: 3) Lipid solubility: Lipid soluble molecules (Non-polar) diffuse more

easily than water soluble molecules (polar) because Non-polar molecules are easily soluble in

the lipid layer of the membrane.

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Molecules that transport through plasma membranes by simple diffusion:

1) Hydrophobic (Non-polar) molecules as O2, CO2, Nitrogen and steroids

diffuses very quickly.

2) Polar molecules of very small size as H2O, glycerol, urea, ethanol diffuses freely, but more slowly than non-polar molecules.

NOTES:Water molecules are polar, but are small enough to pass through cell membranes.Because O2 and CO2 are soluble in lipids, they can diffuse directly through the phospholipid bilayer of the cell membrane. Because the size and polarity of Glucose and sucrose, they cannot diffuse directly through the lipid bilayer of the cell membrane

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22: : Facilitated diffusionFacilitated diffusion

Facilitated diffusion is a type of passive transport.

It is the passage of large molecules or ions through the plasma membrane with the aid of integral proteins (Channel proteins), according to the concentration gradient.

Because the process occurs from high to low concentration, NO energy is required.

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Molecules that transport through plasma membrane by facilitated diffusion:Molecules that transport through plasma membrane by facilitated diffusion:

Glucose molecules diffuse through the cell membrane by simple diffusion very

slowly because glucose is not easily soluble in the phospholipid bilayer.

However, glucose diffuses very quickly across a cell membrane by facilitated

diffusion because the Channel proteins help the glucose molecule to cross into

the cell.

Ions such as Na+ or Cl-.

Water.

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3: Osmosis3: Osmosis

It is the movement of water through the plasma membrane from high to low concentration with the aid of specific type of channel proteins called aquaporinsaquaporins.

Osmosis continue until the concentration of water on both sides of the membrane are equal.

Osmosis is a completely passive process, so ATP energy is not required.

NOTES:

1- Water moves from area of high water high water concentration to area of low water low water concentration.

OROR

2- Water moves from area of low solute low solute concentration to area of high solute high solute concentration.

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o The pressure caused by the movement of water through the membrane is called osmotic pressure.

o A solution of high osmotic pressure is the solution that contain excess solute and less water.

SO,o Water moves from area of low osmotic pressure to

area of high osmotic pressure.

o Water flows until the osmotic pressures on both sides of the membrane are equal

NOTE: The greater the concentration of solutes in a solution,

the greater the osmotic pressure of that solution.

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TonicityTonicity It is the ability of a solution to affect the volume of fluid and

pressure inside the cell

cause the cell to shrink or swell

According to tonicity, there are 3 types of solution:According to tonicity, there are 3 types of solution:

1- Hypertonic Solution (= More solute concentration)

(+ Low water concentration)

2- Hypotonic Solution (= Less solute concentration)

(+ High water concentration)

3- Isotonic Solution.

Tonicity and direction of water flowTonicity and direction of water flow

Water moves from area of hypotonic solution to the area of hypertonic solution.

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1- Hypertonic Solution1- Hypertonic Solution::

It is the solution that has higher concentration of solutes than that of the cytosol.

In this case, concentration of solutes in the ECF is higher than in the ICF.

► So, water moves from inside the cell to the outside.

►The cell lose water & shrinks (crenation).

2- Hypotonic Solution2- Hypotonic Solution::

It is the solution that has lower concentration of solutes than that of the cytosol.

In this case, concentration of solutes in the ECF is lower than in the ICF.

► So, water moves from outside the cell to the inside. ► The cell absorb water, swells and may damaged, so

release its contents (lysis).

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3- Isotonic Solution3- Isotonic Solution::It is the solution with the same solute concentration as that of

the cytosol. In this case, concentration of solutes in the ECF and ICF

are equal.► So, no net water movement ► No change in cell volume or shape .

NOTES: Solution of NaCL 0.9% and glucose 5% are isotonic

solutions, so they are important in clinical medicine because the cell neither swell nor shrink.

NaCL solution (0.9%) is called Normal saline.

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II) Active transportII) Active transport

It is the pumpingpumping of specific molecules through the plasma membrane against the concentration gradient, with the aid of carrier proteins (carrier proteins (pumps)).

i.e: from low to high concentration, therefore Energy is needed to move molecules across the membrane

The carrier proteins obtain the energy from ATP to pump the molecules.

Active transport stops if cellular respiration stops, since there is no energy.

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Examples:

1- Na+ - K+ Pump: 3 Na+ moves out of the cell and 2 K+ moves in. Both ions move from low to high

concentration. This maintain the concentration of sodium and potassium

ions on either side of the nerve cell membrane at a certain level.

2- Reabsorption of valuable molecules from the urine such as glucose, amino acids and sodium ions.

o Which energy molecule is needed to activate the sodium-potassium pump so it will work?

ATPo How many Na+ ions must be pumped out of the cell

during each cycle of the sodium-potassium pump? 3 Na+ ions o How many K+ ions must be pumped into the cell

during each cycle of the sodium-potassium pump? 2 K+ ions

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III) Vesicular TransportIII) Vesicular Transport

It is the passage of large molecules (macromolecules) such as proteins or fluid droplets through the plasma membrane in vesicles.

Types:

1: Exocytosis: Transport out of cell

2: Endocytosis: Transport into cell

a) Phagocytosis: a) Phagocytosis: Engulfing solid particles

b) Pinocytosis: b) Pinocytosis: Engulfing fluid droplets

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