Cell membrane

• Biological membranes are sheet–like structures that are

composed of lipid and protein molecules held together by non-

covalent interactions.

• The membrane is a barrier to prevent the contents of the cell

from escaping and mixing with the surrounding medium.

Structure of cell membrane

Cell membrane composed of:

a) lipid bilayer

b) protein

c) carbohydrates in a small ratio

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a) Lipid bilayer: The most abundant lipids in the cell

membrane are the phospholipids.

• The phospholipids are amphipathic.

• The lipid bilayer is arranged in which the non-polar

(hydrophobic tail) towards the interior and the polar

(hydrophilic heads) towards the surface.

• Cholesterol molecules are embedded in the hydrophobic

interior of the bilayer.

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b) Protein: There are two types

1) Integral proteins (intrinsic proteins)

2) Peripheral proteins (extrinsic proteins)

Integral proteins:

• They are tightly bound within the membrane.

• They are embedded in the membrane.

• They are large molecules extend across the lipid bilayer

and protrude from both surfaces of the membrane.

• Most of the integral proteins act as channels.

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Peripheral proteins:

• Loosely attached to the membrane surface.

• Not penetrate the membrane.

• They are almost entirely in the inner surface (cytoplasmic

surface).

• Most of them act as enzymes.

c) carbohydrates: Membranes also contain carbohydrates

in a small ratio (2- 10 %) that are linked to lipids (form

glycolipids) and proteins (form glycoproteins). They

found on the extracellular side of the plasma membrane.

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Functions of the plasma membrane:

The main role of the plasma membrane is to regulate the

exchange of materials between the cell and its environment.

Function:

1) Transport

2) Endocytosis

3) Cell recognition and adhesion

4) Hormone receptor

5) Oxidative phosphorylation

6) Transmission of nerve impulses

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Transport:

• Lipid bilayers are highly impermeable to all ions and

charged molecules.

• Small nonpolar molecules (e.g. O2 and CO2) readily

dissolve in lipid bilayers and rapidly diffuse across them.

• The cell membrane is selectively permeable (i.e the

membrane permits the passage of some substances while

blocking the passage of others.

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A) Passive transport:

Diffusion:

The movement of a substance across a membrane from

high concentration to low concentration, down its

concentration gradient.

a) Simple diffusion:

The passive flow of a solute from a higher to a lower

concentration across a membrane due to random thermal

movement without a protein carrier and not require

energy.

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b) Facilitated diffusion:

Movement of substances across a membrane from high

concentration region to low concentration region by a

transport protein without the use of energy.

B) Active transport

Active transport is movement of a solute across a

membrane against a concentration gradient, and thus

requires energy (frequently derived from the hydrolysis of

ATP); a specific transporter (pump) is involved.

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• http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles

%20of%20Life2e/Animated%20Tutorials/pol2e_at_0502_Passi

ve_Transport/pol2e_at_0502_Passive_Transport.html

• http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles

%20of%20Life2e/Animated%20Tutorials/pol2e_at_0503_Acti

ve_Transport/pol2e_at_0503_Active_Transport.html

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Isotonic solution:

A solution with a concentration of solutes outside the cell

equal to the concentration of solute inside the cell cell

size will not change and no movement of water to or out

the cell.

Hypotonic solution:

A solution with solutes concentration outside the cell lower

than the solute concentration inside the cell water

will move in by osmosis cell will swell burst

(lysis).

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Hypertonic solution:

A solution with solute concentration outside the cell higher

than the solute concentration inside the cell water

will move from inside to outside by osmosis shrinking

of the cell.

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