Saturday, April 9, 2011

Transport mechanism across cell membrane

  • cell membrane:

The cell membrane (also called the plasma membrane or plasmalemma) is the biological membrane separating the interior of a cell from the outside environment


COMPOSITION OF CELL MEMBRANE:
Cell Membrane is composed of
1.Lipid bilayer
Composed 2 layers of phospholipids molecule, having hydrophilic heads and hydrophobic tails.
 2.proteins
They are glycoproteins. Two types (a) integral proteins (b) peripheral proteins
 3.carbohydrates or cell glycocalyx





TRANSPORT ACROSS CELL MEMBRANE:
Transport through the cell membrane, either directly through the Lipid bilayer or through the Proteins (channel or carrier proteins) , occurs by following basic processes.
(A)SOLUTE FLOW--Physical principles governing movement of solutes
            1. Diffusion or Passive transport
       2.. Active transport
(B) SOLVENT FLOW --Physical principles governing movement of solvents
            1. osmosis
(C) VESICULAR TRANSPORT
1.Phagocytosis of bacteria, dead cells, etc.
2.Receptor-mediated endocytosis and exocytosis



A. SOLUTE FLOW:

1. DIFFUSION:
Diffusion means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with carrier proteins
 The energy that causes diffusion is the energy of normal Kinetic Motion of matter
 IT IS OF TWO TYPES
(A).Simple diffusion
(B).facilitated diffusion


SIMPLE DIFFUSION: means kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in membrane
THE RATE OF DIFFUSION IS DETERMINED BY
1.
  • Amount of substance available 
2.
  •  velocity of kinetic motion 
3.
  • The number and sizes of opening in cell membrane 
Simple Diffusion can occur through the cell membrane by two path ways
1.
  • Through the interstices of lipid bilayer ( if the diffusing substance is lipid soluble) 
2.
  •  Through watery channels that penetrate all the way through some of the large transport proteins 
DIFFUSION THROUGH LIPID BILAYER
>Lipid solouble substances can easily diffuse through lipid bilayer E.g: O2, N2, Alcohol, CO2.
>Water, although insoluble, can diffuse through lipid bilayer due to its small size and high Kinetic energy.
DIFFUSION THROUGH PROTEIN CHANNELS
(1)SELECTIVE PERMIABILITY:
(a) NA+ CHANNELS: -vely charged channels, attracts NA ions only
(b) K+ CHANNELS: smaller and uncharged, allows only smaller hydrated K+ ions and rejects bigger hydrated NA+ ions, thus selective for K+
(2) GATES OF PROTEIN CHANNELS:
(a)For NA+ channels– outside of membrane
(b) For K+ channels– on inside of membrane

(3) GATING OF PROTEIN CHANNELS:
Opening and closing of protein channels is called Gating, two types
(a)VOLTAGE- GATING: gates are  open or closed due to electrical potential changes across cell membrane E.g.
>NA GATES– open when inside of the membrane becomes negative
>K+ GATES– when inside of the membrane becomes positive
(b) LIGAND- GATING: gates are open or closed due to binding of another molecule with channel protein, and binding substance is known as “Ligand”
E.g: ACH binds with ACH-channels and opens it


FACILIATATED DIFFUSION:
qIt is also called carrier mediated diffusion
q A substance transported in this manner usually cannot pass through the membrane without assistant of a specific Carrier Protein
Facilitated diffusion involves the following two steps:
1.The molecule to be transported enters a blind-ended channel and binds to specific receptor
2. A conformational change occurs in the Carrier protein, so the channel now opens to opposite side of membrane.
Among the most important substances that cross cell membrane thorough facilitated diffusion are  GLUCOSE and most of the AMINO ACIDS.



FACILITATED DIFFUSION V/S SIMPLE DIFFUSION
>The rate of Simple Diffusion increases proportionaly with the concentration of the diffusing substance
>The rate of Facilitated Diffusion approaches a maximum (Vmax) as the concentration of substance increases. This maximum rate is dictated by the rate at which the carrier protein molecule can undergo the conformational change.


FACTORS INCREASING DIFFUSION THROUGH PROTEINS CHANNELS:
  • Permeability 
  • Temperature 
  • Concentration gradient 
  • Pressure gradient


ACTIVE TRANSPORT:
“When cell membrane moves molecule or ions uphill against a Concentration gradient ( or uphill against an Electrical or Pressure gradient), the process is called ACTIVE TRANSPORT”
                                                               OR
“Movement of molecules from a region of lower to higher concentration with expenditure of energy
ACTIVE TRANSPORT IS DIVIDED INTO TWO TYPES ACCORDING TO SOURCE OF ENERGY USED TO EFFECT THE TRANSPORT.
In both the instances, transport depends on carrier proteins that penetrate membrane.
1.1.PRIMARY ACTIVE TRANSPORT
2.2.SECONDARY ACTIVE TRANSPORT

PRIMARY ACTIVE TRANSPORT:
In this transport the energy is derived directly from breakdown of ATP or some high energy phosphate compound
EXAMPLES:
1.NA+,K+-ATPase PUMP (or NA+_K+ PUMP)
oPresent in all the cells of the body, maintains low intracellular [NA+] and high intracellar [K+]
oBy transporting 3 NA+ from intracellular to extra cellular fluid
oAnd 2 K+ from extra cellular to intracellular fluid
oBoth NA+ and K+ are transported against their electrochemical gradients


CARRIER PROTEINS OF NA+_K+ PUMP------- has two globular proteins, smaller one has unknown function, Larger one has following 3 functions
1. 3 receptors for NA+ ions on inside
2 .2 receptors for K+ ions outside
3. Inside portion has ATP-ase activity
>When 2 K+ ions binds on outside the carrier proteins and 3 NA+ ions on inside, the ATPase becomes activated and liberating energy from ATP
>This energy causes conformational change in pump
>Expelling 3 NA+ to outside and 2 K+ to inside
IT IS KNOWN AS ELECTROGENIC PUMP --- since this pump , pumps 3 NA+ outside and 2 K+ inside, it causes negativity inside the cell membrane.

2.2. CALCIUM PUMP:
CA+ conc. inside is about 10,000 times less than that is ECF
 It is maintained by 2 calcium pumps
(A)In cell membrane – pumps Ca+ outside the  cell
(B)Other pumps Ca+ ions into one or more vesicular organelles of the cell SUCH AS:
ØSarcoplasmic reticulum– of muscle cell
ØMitochondria of all cells
3. HYDROGEN PUMP:
Present in
(A)Gastric glands of stomach (basis for secreting HCL in stomach digestive secretions)
(B)Late distal tubules and cortical collecting ducts of the kidneys (secrets H+ from blood into Urine for the purpose of secreting

SECONDARY ACTIVE TRANSPORT:
Ø In secondary active transport, the energy is derived secondarily from the energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances b/w the two sides of the cell membrane, created originally by primary active transport
Ø The transport of two or more solutes is coupled
ØOne of the solutes (usually NA+) is transported downhill and provides energy for the uphill transport of other solute
Ø As the energy is not provided directly, but indirectly from NA+ gradient, thus inhibition of NA+_K+ATPase pump will decrease transport of NA+ out of cell, decrease the transmembrane NA+ gradient, and eventually inhibit the active transport.

EXAMPLES:
1. CO-TRANSPORT OF GLUCOSE AND AMINO ACIDS ALONG WITH SODIUM IONS:
For NA+ to be pull another substance along with it, a coupling mechanism is required
It is achieved by a Carrier Protein, having two binding sites on outside, one for NA+ and other for Glucose or Amino acid
Once they both are attached, conc. gradient of NA+ causes conformational change in Carrier protein
And hence Glucose or Amino acid CO-transported with NA+ to inside
 FOUND IN: epithelium of GIT and Renal Tubule


(a)
2. SODIUM COUNTER-TRANSPORT OF CALCIUM AND HYDROGEN ION:
This transport occurs in opposite direction
 Na+ is transported to inside and other substances to outside  Na+ binds to outside binding site of carrier protein and other substance binds to interior projection of carrier protein  conformational change occurs. Energy released by Na+ ions moving to interior causes the other substance to move to exterior.
(a)

 B. SOLVENT FLOW:
OSMOSIS:
§
  • It is a physical process in which a solvent moves, without input of energy, across a semi-permeable membrane
§
  • The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent.
§
  • Osmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration).


C. VESICULAR TRANSPORT:
Phagocytosis of bacteria, dead cells, ETC
Receptor-mediated endocytosis and exocytosis

PHAGOCYTOSIS:
  • Phagocytosis is involved in the acquisition of nutrients for some cells
  • Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytosed.
ENDOCYTOSIS:
Receptor-mediated endocytosis (RME), also called clathrin-dependent endocytosis, is a process by which cells internalize molecules (endocytosis) by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized.
EXOCYTOSIS:
 it is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.





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