molecular organization of cell membrane

صدق الله العظيم 58االسراء اية

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Dr. Abdel Aziz M. HusseinDr. Abdel Aziz M. HusseinLecturer of Medical PhysiologyLecturer of Medical Physiology

Organization of Human BodyOrganization of Human Body• The human body is made up of different systems

e.g. digestive system, whose functions are coordinated and integrated together.

• Each system consists of many organs that made up of many tissues of complementary functions.

• Each tissue consists of millions of similar cells.• The cell is the basic unit of structure and function

in the body• Each animal cell is surrounded by a cell membrane

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Cell (Plasma) MembraneCell (Plasma) Membrane

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Cell Membrane Cell Membrane

• It is a very thin elastic semi-permeable membrane (allowing some substances to pass through it and prevent others) that surrounds the cell

• It is about 7.5 nm (75 Ao = Angstrom = 10-10 of meter) in thickness.

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Functions of Cell Membrane Functions of Cell Membrane 1. It separates the cytoplasm from ECF.2. It maintains cell's internal environment.3. It transports of macromolecules into and out of the

cell.4. It controls distribution of ions e.g. Na, K extracellular

ICF and ECF.5. It generates transmembrane potentials.6. It is contains receptors for hormones and transmitter

substances which are important for signal transduction

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Components of Cell Membrane Components of Cell Membrane • It is formed of 3 components; 1.Lipids 42%.2.Proteins 55%.3.Carbohydrates 3%.

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Cell Membrane Lipids Cell Membrane Lipids

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Cell Membrane Lipids Cell Membrane Lipids

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•Form the basic structure of the membrane.

•Include mainly phospholipids, cholesterol, and glycolipids.

•Phospholipids are arranged into 2 layers (Lipid bilayer)

•Phospholipids molecules resemble the clothes-pin shape

•It consist of;

•Head (PO4 group) (face the ICF and ECF sides) relatively H2O

soluble (polar, hydrophilic).

•Tails (face each other in the interior of the bilayer) → relatively H2O

insoluble (non-polar, hydrophobic).

Cell Membrane ProteinsCell Membrane Proteins

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Functions of Cell Membrane Integral Functions of Cell Membrane Integral ProteinsProteins

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1.1. Integral or intrinsic Integral or intrinsic proteins→proteins→bind to

hydrophobic center of the lipid bilayer

2.2. Peripheral or extrinsic Peripheral or extrinsic proteinsproteins→ bind to the

hydrophilic polar heads of the lipids or to the

integral proteins


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i)Transmembrane proteins i)Transmembrane proteins → span the entire bilayer which act

as:

a. Channels → for diffusion of small, water–soluble substances.

b. Carriers actively transport materials across the bilayer.

c. Pumps actively transport ions across the bilayer.

d.Receptors initiate intracellular reactions when activated.


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ii)Present only on one side of the membraneThey act as enzymes that activate or inactive metabolic processes.

Functions of Cell Membrane Peripheral Functions of Cell Membrane Peripheral ProteinsProteins

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The peripheral protein may be:

i) Peripheral proteinsi) Peripheral proteins: : bind to

the intracellular surface of the

membrane and contribute to the

cytoskeleton.

ii) Peripheral proteinsii) Peripheral proteins: : bind to

the extracellular surface of the

membrane and contribute to the

glycocalyx.

Glycocalyx

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2. It acts as an attachment matrix for

other cells (cell adhesion

molecules).

•It is composed of glycolipids and

glycoproteins and covers the cell

membrane.

•Functions Functions

1. 1. It binds to the extracellular Ca to

stabilize membrane structures.

Transport through the Cell Transport through the Cell MembraneMembrane

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Transport through the Cell MembraneTransport through the Cell Membrane

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Transport through the Cell MembraneTransport through the Cell Membrane

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There are 3 mechanisms for transport of substances through cell membranes;

a) Passive transport (Diffusion):a) Passive transport (Diffusion):It is movement of substances across the cell membrane down its electrochemical gradient.

b) Active transport: b) Active transport: It is movement of substances across the cell membrane against its electrochemical gradient.

c) Vesicular transport: c) Vesicular transport: It is the process by which large sized substances are engulfed by the cell membrane to be either pushed inside the cell (endocytosis) or pushed outside the cell (exocytosis)

DiffusionDiffusion

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It is divided into 3 subtypes:1.Simple diffusion.2.Facilitated diffusion.3.Osmosis (water only)

Simple DiffusionSimple Diffusion

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It is movement of substances across the cell membrane down its electrochemical gradient by simple movement without the necessity of binding with carrier proteins

Mechanisms:Mechanisms:It occurs either

through;•The interstices of the

lipid bilayer.•The watery proteins

channels.

Simple Diffusion Simple Diffusion

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Characters:Characters:It is characterized by;

a) It occurs down an electro-chemical gradient.

b) It is passive i.e. no external energy is required.


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Characters:Characters:It is characterized by;

c) It is not rate-limiting i.e. linear with concentration gradients.

d) The diffusion process is not saturable.

Osmosis or Diffusion of WaterOsmosis or Diffusion of Water

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Water diffuses through lipid bilayer of the membrane at high rate like bullets due to;

Its small size.

Its very high kinetic energy

Water moves from high to low concentrations of water

Facilitated DiffusionFacilitated Diffusion

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It is the diffusion of substances across the cell membrane (down electrochemical gradient) which needs the presence of carrier proteins→ carrier mediated diffusion

Characters

i) It occurs down an electrochemical gradientii) It is passive i.e. no energy is required.iii) It requires the presence of a transport carrier protein.iv) It is rate-limiting and saturable because it depends on the availability of a definite number of carrier or channel proteins

II) Active TransportII) Active Transport

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Def.,It is the movement of substances across the cell membranes against an electrochemical gradient.

Types:

There are 2 subtypes of active transport:i) Primary active transport → obtain its energy directly from the hydrolysis of ATP e.g. Na-K Pump, Ca ATPase, H-K ATPase

ii) Secondary active transport → use the energy stored in the Na concentration gradient e.g. Na-glucose co-transport and Na-Ca exchange

II) Active TransportII) Active Transport

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Characters

1. It occurs against the electrochemical gradient

2. It is active i.e. energy is required

3. It requires the presence of a transport carrier protein4. It exhibits stereospecificity, saturation, and competition

1ry Active Transport1ry Active Transport

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(Na-K ATPase)

Na-K pump

Ca-ATPase

(Ca-pump)

K-H-ATPase

(proton pump)

Site It presents in the

cell membranes.

It is present in

sarcoplasmic

reticulum and cell

membranes.

It presents in

stomach parietal-

cells and renal

distal tubular cells.

Importance maintains low

intracellular Na

and high

intracellular K.

It maintains the

intracellular Ca

ions concentration

low.

It transports H ions

into the lumen

against its

electrochemical

gradient.

2ry Active Transport2ry Active Transport

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Na-glucose co-

transport

Na-Ca Exchange

Site It presents in the cell

membranes.

It is present in ventricular

ms cells

Importance Transport of Na &

glucose into the cells

It transports Ca uphill from

low ICF Ca to high ECF

Ca

Differences ( ) simple diffusion, facilitated diffusion and active transport

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Simple diffusion Facilitated diffusion Active transport

Electrochemical

gradient

Downhill Downhill Uphill

Energy Doenot need energy Doenot need energy Needs energy

Rate Not limited Limited Limited

Saturation Not Saturable Saturable Saturable

Carrier Doenot need carrier Needs carrier Needs carrier

Competition Not show competition Shows competition Shows competition

III) Vesicular TransportIII) Vesicular Transport

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It is the mechanism by which the large sized substances can cross the cell membranesa) Endocytosisa) Endocytosis

It is active process which uses energy

The extracellular material is trapped within vesicles that are formed by invagination of the cell membrane

It is 3 types

III) Vesicular TransportIII) Vesicular Transport

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a) Exocytosisa) Exocytosis

It is active process which uses energy

The intracellular material is trapped within vesicles, then the vesicles fuse with the cell membrane and release their contents to the ECF

e.g. release of hormones, digestive enzymes, and synaptic transmitters

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molecular organization of cell membrane

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