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THE CELL III) Cell membrane (Plasma membrane)

Physiology -I PHL 215

Dr/ Gamal Gabr Pharmacy College

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III) Plasma membrane

Plasma membrane is like a gate

It allow certain substances to enter or leave the cell.

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• Plasma membrane is a thin, flexible outer barrier that separates the interior of the cell from the outside environment.

o Thickness: 8–10 nanometers (nm).

1 nm = 1/1 000 000 mm

• Plasma membrane is a selectively permeable membrane = It controls the movement of substances in and out of cells.

o What chemical materials are plasma membranes made of?

It made of proteins floating in a double layer of phospholipids, which are fluid at body temperature (like vegetable oil).

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Composition of plasma membrane

• The plasma membrane is composed of different types of macromolecules.

• The most common molecules are:

❶ Lipids

❷ Proteins

❸ Carbohydrates.

Lipids Protei

ns

Carbohydrat

es

72 % 25% 3 %

Phospholipids

: 55%

Cholesterol:

13%

Glycolipids:

4%

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Fluid-mosaic model

The fluid-mosaic model describes the plasma membrane of animal cells.

Fluid:

Because the plasma membrane has the consistency of vegetable oil at body temperature .

So, the molecules of lipids and proteins can move side-to-side within the same layer.

Mosaic:

Because proteins and other substances such as cholesterol are distributes in the bilayer, giving the membrane the look of a mosaic.

o What factors determine fluidity of the cell membrane?

Temperature, lipid composition and cholesterol content

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❶ Membrane lipids

• Lipid molecules = 3 major classes: • Phospholipids (55%) + Cholesterol (13%) + Glycolipids (4%).

So, the major lipid molecules are Phospholipids.

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Phospholipids

o Each phospholipid molecule has:

a) Phosphorylated glycerol head that is attracted to water. i.e. hydrophilic OR water soluble.

b) 2 fatty acid tails that repels water. i.e. hydrophobic OR fat soluble

NOTES:o Hydrophilic means water loving = Water soluble OR Polar.o Hydrophobic means water fearing = Fat soluble OR nonpolar.

o Molecules having both hydrophilic and hydrophobic properties are called amphipathic. Accordingly, the phospholipid molecules are considered as amphipathic molecules.

Which part of a phospholipid molecule is polar and thus considered hydrophilic?

the phosphate glycerol head

Which part of a phospholipid molecule is nonpolar and thus is considered hydrophobic?

the 2 fatty acid tails

Phosphorylated

glycerol head

Two fatty acid

tails

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Phospholipids

Lipids bilayer …… Why?

1- Heads of the phospholipid molecules are hydrophilic, So attracted to water and directed into:

A- ICF (inner surface of the membrane)

B- ECF (outer surface of the membrane).

2- Tails of the phospholipid molecules are hydrophobic , So repelled by water and arranged tail to tail forming a hydrophobic region in the center of the membrane.

• The result = Lipid bilayer

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Phospholipids

Functions: (The lipid bilayer)

1- Forms a barrier so protects the cell

2- Selectively permeable, so controls the movement of substances into and out of the cell:

a) Hydrophobic substances as oxygen, carbon dioxide, and alcohol can diffuse easily.

b) Hydrophilic substances as ions, glucose, and urea can not diffuse easily.

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❷ Membrane Proteins

• The major protein molecules are Glycoproteins.

NOTE:

At physiological temperatures, the lipid bilayer is fluid, so proteins can move side-to-side within the same layer, but can’t flip from one layer to the other

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Membrane proteins = 2 major types:

A- Integral proteins OR transmembrane proteins

B- Peripheral proteins

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A) Integral proteins:

They crosses the lipid bilayer of the membrane with one end outside the cell and the other end inside.

Integral proteins are not fixed, so they moves through the membrane very quickly.

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Each integral protein has 3 types of amino acids:

1- Hydrophilic cytosolic amino acids, in contact with

ICF (Inner surface).

2- Hydrophobic central amino acids in contact with

the fatty acids in the centre of the membrane.

3- Hydrophilic extracellular amino acids in contact

with ECF (Outer surface).

Integral proteins = 3 types:

1- Transport proteins

a) Channel proteins

b) Carrier proteins

2- Marker proteins

3- Receptor proteins.

Hydrophilic region of protein

Hydrophobic region of protein

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1- Transport proteins: • They regulate the movement of substances through the plasma

membrane.

Types:

a- Channel proteins: Channel proteins transport water soluble substances, especially the

ions or very small molecules from one side of the membrane to the other side according to concentration

this process is known as diffusion

b- Carrier proteins: Carrier proteins change their shape to transport certain molecules from

one side of the membrane to the other side against concentration

this process is known as active transport.

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2- Marker proteins: Marker proteins are specific for each organism =

fingerprints. They identify cells to the body's immune system. They play an important role in organ transplantation.

3- Receptor proteins: They have specific binding sites that respond to

hormones, antigens and neurotransmitters. They play an important role in intercellular

communication.

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B) Peripheral proteins:• They does not penetrate the plasma membrane.• Usually: found on the inside of the membrane. • They attach the membrane to the cytoskeleton, so keep the shape of the cell.• They function as enzymes catalyze some metabolic reactions which occur near

the plasma membrane.

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Functions of membrane proteins:• Control the movement of substances through cell membranes

Diffusion & Active transport• Identification of cells to the body's immune system. • Act as receptors.• Act as enzymes.• Attach the membrane to the cytoskeleton, so keeping the shape of the cell.

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❸ Membrane Carbohydrates:

• Carbohydrate chains attach to the outer surface of the plasma membrane of each cell forming a loose coat called glycocalyx.

• They attached to :

Proteins Glycoproteins OR

phospholipids Glycolipids.

• Glycocalyx of some cells attaches to glycocalyx of other cells, thus attaching the cells to each other.

• Many of the carbohydrates act as receptors, as they bind with some hormones as insulin.

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Function of Carbohydrates:

• Attaches the cells to each others.• Act as receptors for some hormones.

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Functions of plasma membrane:

1- Protects the cell

2- Controls the movement of substances into and out of the cell (Selectively permeable)

3- Identification of cells to the body's immune system (marker protein).

4- Communication with other cells and the outside world (receptor protein).

5- keep the shape of the cell.