Transport of Material Across Biological Membranes

1. Biochemistry of Extracellular &Biochemistry of Extracellular &
Intracellular CommunicationIntracellular Communication
Membrane Structure &Membrane Structure &
DynamicsDynamics

2. Body FluidsBody Fluids
• Water makes ~ 60% of the lean body mass & is
distributed in 2 large compartments:
– Intracellular Fluid (2/3 of water) & provides the
environment for the cell to 1) make, store & utilize
energy 2) repair 3) replicate 4) perform special
functions
– Extracellular Fluid (1/3 of water). It act as
delivery system that brings to the cell: nutrients,
oxygen, various ions & minerals & hormones. And
removes CO2, waste products, toxic & detoxified
materials from the immediate cellular environment
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3. • Intracellular environment is rich in K+
, Mg+
and
phosphate. While the extracellular fluid is
characterized by high Na+
, Ca+
and Cl-
.
• Based on the biochemical & physiological
actions; cells exchange materials with the its
environment.
• In order to maintain the internal
microenvironment, cells developed barriers
(membranes) with associated pumps.
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4. • Biological membranes are sheet-like structures,
that are composed of protein & lipid molecules
held together by non-covalent interactions
• They have selective permeability
• They create closed compartment (cell or organelle)
• Pnmps & gates regulate the molecular & ionic
compositions of these compartments
• They also control the flow of information between
cells, e.g many membranes have hormone
receptors
• Furthermore, membranes are involved in energy
conversion processes, such as oxidative
phosphorylation
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5. Membrane Lipids:
• Major classes are phospholipids, glycolipids &
cholesterol
• They are amphipathic ( hydrophilic and
hydrophobic ) molecules that spontanously
form extensive bimolecular sheets (lipid
bilayer)
• These lipid bilayers are highly impermeable to
ions & most polar molecules, but they are
quite fluid, which enables them to act as
solvents for membrane proteins
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6. Membrane proteins:
• Distinctive membrane functions such as
transport, communication & energy
transduction are mediated by proteins
• There are 2 types of membrane proteins:
– Integral
– Peripheral
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10. Fluid Mosaic Model
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11. Membrane selectivity allowsMembrane selectivity allows
specialized functionsspecialized functions
• Certain molecules freely diffuse across
membranes, but the movement of others is
restricted because of:
– Size
– Charge
– Solubility
• Plasma membranes are involved in simple &
facilitated diffusion, active transport & other
processes such as endocytosis & exocytosis.
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12. Simple Diffusion:
• Some solutes such as gases enter the cell by
diffusing down an electrochemical gradient &
is limited by:
– Thermal agitation (physics related to
particles in thermodynamic equilibrium ) of
that molecule
– Concentration gradient across the membrane
– The solubility of the solute in the
hydrophobic core of the membrane bilayer
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13. • Molecules that cannot pass freely through the
lipid bilayer membrane by themselves, do so in
association with carrier proteins. This involves 2
procesess:
– Facilitated diffusion
– Active trasnport
# Both processes involve carrier proteins &
show specificity for ions, sugars & amino
acids.
# Active transport needs energy while facilitated
diffusion is not. This is the major difference
between the two processesAMNA MEDANI,2015AMNA MEDANI,2015

14. Endocytosis (phagocytosis & pinocytosis):
• Endocytotic vesicles are generated when
segments of the plasma membrane invaginates,
enclosing a minute volume of extracellular
fluid & its contents. Then it pinches off, fuses
with other membrane structures & thus
transports its contents to other cellular
compartment.
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15. Exocytosis:Exocytosis:
A process by which most cells releaseA process by which most cells release
macromolecules to the exteriormacromolecules to the exterior

16. • Cross-membrane movement of small molecules:
– Diffusion (passive & facilitated)
– Active transport
• Cross-membrane movement of large molecules:
– Endocytosis
– Exocytosis
• Signal transmission across membranes:
– Cell surface receptors:
• Signal transduction
• Signal internalization
– Movement to intracellular receptors
Transfer of material & information across membraneTransfer of material & information across membrane
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