PLASMA MEMBRANE

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PLASMA MEMBRANE

• M.Prasad Naidu
• MSc Medical Biochemistry, Ph.D,.

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Plasma Membrane

• Separates different parts of cell , So that
  cellular activities are compartmentalised.
• Highly selective permeability

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Fluid Mosaic Model Plasma membrane
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• Fluid Mosaic Model
• Phospholipids are arranged in bilayers with
  polar head groups oriented towards extra cellular
  or cytoplasmic side with a hydrophobic core .

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• Made up of carbohydrates - glycoproteins and
  glycolipids
• Phospholipids are most common lipids
• which are amphipathic nature.
• Lipidbilayer shows free lateral movement of its
  components, hence it is fluid in nature.
• Fluidity depends on the cholesterol content as
  well as nature of fatty acids.

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• Membrane proteins
• 1.Integral proteins
• Rhodopsin of retinal cell
• Receptors for hormones,growth factors
• Immunoglobulins on cell membrane of
  leukocytes
• 2.Peripheral proteins
• Secretory proteins
• Peptide hormones

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• Transfer of molecules across membrane
• Small molecules
• Passive Transport
• It is the movement of molecules across the cell
  that doesnt require expenditure of energy.
• They are of 3 types.

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Simple diffusion

• It is the passive movement of molecules from
  area of high concentration to an area of low
  concentration.
• Factors affecting net diffusion of a substance
• Steepness of concentration gradient
• Temperature Increased temperature will increase
  particle motion and increased diffusion.

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• Charge presence of oppositely charged particles
  on the other side will increase diffusion. More
  toward the solution that has opposite charge.
  The inside of the cell usually has a negative
  charge.
• Diameter of diffusing molecule smaller the
  molecule faster the diffusion
• Rate of diffusion of lipophilic substances is
  directly proportional to its lipid solubility and
  diffusion coefficient.

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• Fatty acids , steroids diffuse rapidly but water
  soluble substances sugars, inorganic ions diffuse
  very slowly.
• Simple diffusion not saturable.
• eg Diffusion of Pentoses , gases like O2,
  N2, CO2, NO.(Nitric oxide)

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Facilitated diffusion

• Facilitated diffusion / Transport
• It is the passive movement of molecules down the
  concentration gradient with the aid of a
  transport protein ( carrier protein)
• Also called passive mediated transport
• Present only in eukaryotes
• No need of energy.
• Transport proteins undergo a conformational
  change (shape change) so that the molecule can
  pass through the membrane by Ping pong mechanism.

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• Facilitated diffusion is saturable
• Factors determining facilitated diffusion
• Concentration gradient across membrane
• Amount of carrier protein available
• Rapidity of solute carrier interaction.
• eg Glucose entry from intestinal cell to
  blood via GLUT2.

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Osmosis

• It is the passage of WATER across a membrane
  from an area of higher to lower concentration.
• Osmosis is a diffusion but it specifically
  refers to the H2O molecules. Water passes from
  hypotonic to hypertonic solution across membrane.

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Active transport

• Transport of molecules against concentration
  gradient with expenditure of energy.
• Requires transporters
• Operates Unidirectionally
• Saturable at higher concentration of solute.25
  to 30 of total energy requirements in a cell is
  used for active transport system.

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• Active transport
• Primary Secondary
• Primary Active transport Requires energy
  directly eg Na pump.
• Secondary active transport Requires energy
• indirectly
• eg glucose transport into intestinal mucosal
  cell

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Sodium Potassium Pump

• The Sodium Potassium ATPase is an integral
  protein and requires phospholipids for its
  activity.
• This system is an oligomeric protein containing
  2a subunits(larger) and 2ßsubunits (smaller).
• ß subunits are glycoproteins.
• Binding sites for Na ATP on the inside
  (cytoplasmic side) and for K on the outside.

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• Extracellular side
• ß a a Cytosolic side
• ß

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• For each ATP hydrolysed 3 Na move out of cell
  and 2 K enter into the cell.
• Note In nerve and muscle 60 70 of energy
• synthesized is used for this pump.
• In resting cells 35 of energy maybe used.
• Kidney cortex contain more activity of sodium
  potassium
• ATPase.

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Na - K Pump
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Functions of Sodium Potassium Pump

• Control cell volume
• Renders nerve and muscle cells electrically
  excitable.
• Active transport of aminoacids and sugar

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Clinical Aspect

• Sodium potassium ATPase is inhibited by digitalis
  (digoxin) which increases force of contraction
  of heart muscle by altering the excitability.
• OUABAIN is another inhibitor

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Transport systems

• Symport
• A cotransport system in which transporter
  carries two solutes in same direction across
  membrane.
• eg sodium glucose transport into mucosal cell
  (useful in ORT).
• Sodium amino acid transporters in mammalian
  cells..

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Antiport

• A co-transport system in which two solutes are
  transported in opposite directions,
  simultaneously.
• eg Na - K pump.
• Cl- - Hco3 exchange in RBC membrane.

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• Mitochondria contains several antiport mechanisms
• 1. ATP / ADP
• 2.Aspartate / Glutamate
• 3. Phosphate / OH-
• 4. Malate / Phosphate

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Uniport

• Involves transport of a single solute through
  the membrane bi-directionally .
• Glucose permeases
• Present in mammalian cell membrane. six members
  have been identified, named as GLU T1, GLU T2 etc
• These permeases are active with D-glucose

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• Transport of large molecules
• Exocytosis
• Endocytosis Phagocytosis
• Pinocytosis
• Receptor mediated endocytosis

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Endocytosis

• Molecules are transported into the cell by the in
  
• folding of cell membrane around the substance
• internalising it.
• 1.Egpolysaccharides,proteins,polynucleotides
  etc.
• 2.Also helps in regulating the a number of
  hormone receptors
• Endocytosis requires ATP, Calcium in ECF, and
  contractile elements in cell.(microfilaments).

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• Phagocytosis --- Cell eating
• is a process by which bacteria , viruses ,dead
  tissue etc are engulfed by leukocytes

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• Pinocytosis - cell drinking
• cellular uptake of fluid and fluid contents.
• eg Protein molecules.

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• Receptor mediated endocytosis
• Substance to be transported bind with a receptor
  which is then engulfed by endocytosis.
• eg Transport of LDL Cholesterol.
• Also responsible for some diseases
• 1. Viral (eg Hepatic cells)
• 2. Poliomyelitis (affecting motor neuron)
• 3. AIDS (affecting T Cells)

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• Exocytosis-
• Most cells release macro molecules to the
  exterior by exocytosis.
• This process helps in membrane remodelling
  when the components synthesized in golgi are
  carried in vesicles to the Plasma membrane.
• Calcium triggers exocytosis

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• Molecules released by exocytosis may
• 1.Attach to cell surface and become peripheral
  proteins. eg antigens
• 2. Become part of extracellular matrix .
• eg collagen
• 3. They can enter extracellular fluid and signal
  other cells . Eg Insulin , parathormone etc