Membranes

Biochemistry of Extracellular & Biochemistry of Extracellular & Intracellular CommunicationIntracellular Communication

Membrane Structure & Membrane Structure & DynamicsDynamics

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

AMNA MEDANI,2015AMNA MEDANI,2015

• 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.


• 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


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


Membrane proteins:• Distinctive membrane functions such as

transport, communication & energy transduction are mediated by proteins

• There are 2 types of membrane proteins:

– Integral

– Peripheral


Fluid Mosaic Model


Membrane selectivity allows Membrane 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.


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


• 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 processes


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.


Exocytosis:Exocytosis:

A process by which most cells release A process by which most cells release macromolecules to the exteriormacromolecules to the exterior

• 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


Membranes

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