Cell Membrane Transport

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Cell Membrane Transport
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Cell Membrane Transport

• Interstitial fluid
• Surrounds cells
• Contained within tissues.
• Part of the extra-cellular water compartment
• Derived from blood plasma
• Contains amino acids, vitamins, hormones, salts,
  waste products, etc.
• Cells must be able to transport materials across
  the cell membrane
• Movement of materials is controlled by the plasma
  membrane, that is selectively permeable so that
  it allows only certain materials pass in and out
  of the cell to maintain differences between ICF
  and ECF.

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Mechanisms of transport across cell membrane are

• Passive transport (Diffusion)
• Active transport
• Bulk transport

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Factors Affecting the Direction of Transport

• The cell membrane is.
• Impermeable to most water-soluble substances
  (substances that dissolve in water)
• Closely controls passage of materials in and out
  of the cell.
• Passive Transport versus Active Transport
• Passive Transport
• Movement of substances through the membrane
  without the use of energy from the cell is a
  physical or passive process.
• Does not require ATP
• Includes simple diffusion, facilitated diffusion,
  osmosis, and filtration.
• Active Transport
• Movement of material through the membrane that
  requires metabolic energy (ATP) is called an
  active physiological process.
• Includes Primary and Secondary Active Transport

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Driving Forces Acting on Molecules

• Driving forces affect the direction of movement
  of molecules
• Gradient
• A difference in driving force (chemical or
  electrical energy) across a cell membrane that
  tends to push molecules in one direction or
  another
• Always from higher to lower energy if allowed to
  move spontaneously
• There are three types of driving forces
• Chemical, Electrical, and Electrochemical

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Driving Forces Acting on Molecules

• Chemical driving force
• Difference in energy due to a concentration
  gradient that causes a molecule to move from high
  to low concentration
• Electrical driving force
• Difference in energy due to a separation of
  charge that acts to move ions from high energy to
  low energy
• Electrochemical driving force
• Sum of the chemical and electrical driving forces

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Simple diffusion

• Routs of diffusion
• Through lipid bilayerthe rate of diffusion is
  directly proportional to lipid solubility of
  substances
• Lipid soluble substances(O2, N2)
• Water molecules as they are small have high
  kinetic energy
• Lipid insoluble if they are small and unchrged

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• Through protein channels
• For transport of ions mainly
• They have
• Selective permeability
• Gates their opening and closing are controlled
  by
• Voltage gating
• Ligand gating acetylecholine receptors

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Facilitated Diffusion Passive Transport Through
Membrane Proteins

• Particles must be helped through the membrane
  with the use of transmembrane proteins
  (carriers/transporters, channels/pores).
• Requires a concentration gradient
• Example
• Glucose
• Important substance that is lipid insoluble and
  is too large to pass through membrane pores.
• Glucose molecules combine with a protein carrier
  molecule on the surface of the plasma membrane.
  The carrier changes shape and releases the
  glucose inside the cell then returns to its
  original shape to bring in another glucose on the
  outside of the membrane.

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Transport Proteins in Facilitated Diffusion

• Carriers
• A transmembrane protein that binds to a molecule
  on one side of the membrane
• Conformational change
• The carrier flips to bring the transported
  molecule to the other side of the membrane
• Transport is limited by the number of carriers
  available on the membrane.

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Features of facilitated diffusion

1. Specificity of the carrier
2. Competition between similar substances
3. Its rate ? with concentration gradient upto
   certain maximum rate
4. It is more sensitive to temperature

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Osmosis Passive Transport of WaterAcross
Membranes

• The flow of water across a selectively permeable
  membrane
• Always from an area of high water concentration
  to an area of low water concentration.
• A special case of diffusion of water across a
  selectively permeable membrane, such as the
  plasma membrane.
• A semi-permeable membrane is freely permeable to
  water but not to solutes.
• It is a very important process because water is
  found throughout cells and extra-cellular areas
  of the body.

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• Osmosis depends on
• A concentration gradient for water
• Relative permeability of dissolved solutes
• Osmosis occurs when
• There is more water and less solute on one side
  of the membrane
• A high concentration of water or a low
  concentration of solute
• And less water and more solute on the other side
• A low concentration of water or a high
  concentration of solute
• The concentration gradient is for water

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• Osmolarity
• Total solute concentration
• Unit is osmole (Osm) or milliosmole (mOsm)
• Normal osmolarity (concentration) of body fluids
  is 300 mOsm
• Total solute concentration is 300 milliosmoles
  per liter
• Depends on the total concentration of dissolved
  solutes
• Example
• 150 mOsm NaCl
• Dissolved in water the molecule separated into
  two particles, so osmolarity is doubled, 300 mOsm

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Tonicity

• It is the osmolaritity of a solution relative to
  the osmolarity of plasma

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• Tonicity
• Refers to the relationships between body cells
  and the surrounding fluids.
• A measure of the ability of a solution to cause a
  change in cell tone (volume or pressure) by
  promoting the osmotic flow of water.
• Dependent upon concentration and diffusibility of
  the dissolved solutes
• Impermeant solutes
• Cannot cross cell membrane
• Permeant solutes
• Can move across cell membrane and add to the
  total solutes within cell

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Types of solutions

• Iso-tonic
• Same osmolarity as plasma
• Hyper-tonic
• Solution has greater osmolarity than plasma
• Hypo-tonic
• Solution has lower osmolarity than plasma

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• Isotonic
• A solution that has the same concentration of
  solute (osmotic pressure) as body fluids.
• Fluid surrounding a cell has the same
  concentration of solute as that inside of the
  cell.
• No osmosis occurs.
• Hypotonic
• A solution that has a lower concentration of
  solute (osmotic pressure) than body fluids.
• Hypotonic extra-cellular fluid has a lower
  concentration of solute than the concentration
  inside cell and causes water to move into the
  cell following its concentration gradient (more
  water outside, less inside).
• Too much water moving into the cell membrane may
  cause the cell to burst.

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• Hypertonic
• A solution has a higher concentration of solute
  (osmotic pressure) than the concentration found
  in body fluids.
• Hypertonic extra-cellular fluid will cause water
  to leave the cell following its concentration
  gradient (more water inside, less outside)
  producing a shrunken or crenated cell.

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• Isotonic saline solution
• A solution that is .9 saline because the body's
  red blood cells are .9 salt or NaCl.
• Therefore, when an isotonic saline solution is
  introduced into the body, fluid equilibrium will
  be maintained.
• Remember
• The key to understanding the above terms and
  process is to understand that hyper and hypo
  refer to the solute in the solution, not to the
  water.
• Water will move toward the greater amount of
  solute because the concentration of water there
  is less.

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• Osmotic Pressure (p)
• The membrane is selectively permeable in that it
  does not allow the solute to pass, it is not
  permeable to certain molecules, particles, or
  solute.
• Remember that high solute concentration means low
  water concentration (requires more water to
  reach equilibrium) and low solute concentration
  means high water concentration (requires water to
  leave to reach equilibrium).
• Osmosis will continue to occur or the water will
  continue to move until
• Equilibrium for water is reached so that the
  concentration of water and solute is equal on
  each side of the membrane.

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• Osmotic pressure stops the movement of water.
• Osmotic pressure is the amount of pressure
  required to prevent further water movement.
• The ability of osmosis to generate enough
  pressure to lift a volume of water.
• A potential pressure due to the presence of
  non-diffusible solute particles.
• The greater the amount of non-diffusible solute,
  the greater the gradient attracting water across
  the membrane and the greater the osmotic pressure
  produced.
• Example
• NaCl is a very osmotically active particle
  because when it dissociates it produces two ions,
  or double the osmotic activity
• Water movement changes the volume of water in the
  container or cell.

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Passive Transport Filtration

• Particles forced through a filter or a membrane
  by hydrostatic pressure.
• Hydrostatic pressure
• Fluid pressure of the blood generated by the left
  ventricle
• Opposed by osmotically active particles in the
  blood (plasma proteins).
• Example
• Blood pressure generated by the heart and blood
  vessels forces tissue fluid out of tiny openings
  in the capillary wall and leaving larger
  particles of blood cells and protein molecules
  inside the capillary.
• Coffee filters work by the pressure from weight
  of the water above the coffee grounds forcing the
  flavored water through the filter and leaving the
  large particles of coffee grounds on the filter
  paper.
• Filtration and osmosis are the major processes in
  the capillaries of tissues and the kidney.

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Active Transport Processes

• Movement of particles or solutes against a
  concentration gradient
• Requires energy or cellular action with ATP
• Primary Active Transport
• Direct transport of substances using ATP
• Secondary Active Transport
• Movement of substances driven by concentration or
  electrochemical gradients created by Primary
  Active Transport mechanisms

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Primary Active Transport

• Solute pumping
• Pump or protein carrier
• An enzyme-like protein carrier that pumps or
  carries solutes such as ions of sodium,
  potassium, and calcium, into or out of the cell
  against their concentration gradients.
• ATPase
• The enzyme on the protein carrier or pump that
  catalyzes the breakdown or phosphorylation of ATP
  producing energy that drives the pump.
• This action may require up to 40 of a cells
  supply of ATP

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• Sodium-potassium pump
• (Na/K ATPase Pump)
• Maintains the resting membrane potential of nerve
  and muscle cells
• Sodium
• Primary extra-cellular ion that is constantly
  leaking into cells.
• Potassium
• Primary intracellular ion that is constantly
  leaking out of cells.
• The sodium/potassium pump constantly pumps 3
  sodium ions out and 2 potassium ions into the
  cell, maintaining the relative negativity inside
  the cell.
• All cells have a negative charge inside because
  of this mechanism.

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Solute Pumping to Maintain the Membrane Potential

• Pumps
• Transport proteins that use energy from ATP
  hydrolysis to transport specific molecules
  against the electrochemical gradient across a
  membrane
• Sodium-Potassium pump (Na/K ATPase Pump)
• Transports Na/K ions in opposite directions
  across cell membranes
• Move 3 Na ions out of the cell for every 2 K
  ions into cell
• Specific for Na/K and unidirectional
• Phosphorylation of the pump protein causes a
  conformational change that turns the binding
  sites outward to expel Na
• Also decreases affinity for Na and increases its
  affinity for K
• Critical in maintaining resting membrane
  potential for nerve and muscle impulse conduction

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Secondary Active Transport

• Movement of a molecule that is coupled to the
  active transport of another molecule
• One substance moves down its electrochemical
  gradient and releases energy in the process
• This energy is then used to drive the movement of
  another substance against its electrochemical
  gradient

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Cotransport (Symport)

• Movement of 2 substances in the same direction
• Example
• Sodium-linked glucose transport
• Couples the inward flow of sodium with the inward
  flow of glucose
• Sodium movement with its electrochemical gradient
  releases energy that drives the movement of
  glucose against its concentration gradient

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Countertransport (Antiport or Exchange)

• Movement of 2 substances in opposite directions
• Example
• Sodium proton exchange
• Couples the inward flow of sodium with the
  outward flow of protons (H)
• Energy released from the inward flow of sodium
  along its electrochemical gradient is used to
  drive the outward flow of protons against its
  electrochemical gradient

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Pumps and Leaks

• Differences in composition of intra- and
  extra-cellular fluid are maintained by pumps
• Substances are constantly, passively leaking
  across cell membrane in the opposite direction
  and at the same rate that they are actively
  pumped across the cell membrane
• Net flux across the cell membrane is zero

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Bulk transport

• Endocytosis
• Exocytosis

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Endocytosis

• Endocytosis is the process by which cells ingest
  materials. The cellular membrane folds around the
  desired materials outside the cell. The ingested
  particle is trapped within a pouch, vacuole or
  inside the cytoplasm. Often enzymes from
  lysosomes are then used to digest the molecules
  absorbed by this process.

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Types

• pinocytosis and phagocytosis.
• In pinocytosis, cells engulf liquid particles (in
  humans this process occurs in the small
  intestine, cells there engulf fat droplets)
• In phagocytosis, cells engulf solid particles.

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Exocytosis

• Exocytosis is the process by which cells excrete
  waste and other large molecules from the
  protoplasm