Membrane Structure and Function

Membrane Structure and Function

Eva Socorro Estrada Aranas-Angel, MD, FPSGMDepartment of Biochemistry

Davao Medical School Foundation

28 July 201028 July 2010 ESEAngelESEAngel

Biomedical ImportanceBiomedical Importance

Plasma Membranes – selective Plasma Membranes – selective permeabilities, channels, pumps, permeabilities, channels, pumps, cell-cell interactions, trans-cell-cell interactions, trans-membrane signalingmembrane signaling

Specialized Compartments – Specialized Compartments – enzymes, excitation-response enzymes, excitation-response coupling, energy transductioncoupling, energy transduction


Compartmentalization of Internal Body WaterCompartmentalization of Internal Body Water Intracellular Fluid –Intracellular Fluid –- 2/3 of the total body water- Provides environment for the cell to - make, store and utilize energy - repair itself - replicate - perform special functions

Extracellular FluidExtracellular Fluid

- - 1/3 of total body water


Ionic Compositions of ICF and ECFSubstance Extracellular Intracellular Na+ 140 10 mmol/L K+ 4 140 Ca++ 2.5 0.1umol/L

Cl- 100 4 HCO3- 27 10 PO4- 2 60Glucose 5.5 0-1Protein 2 16 g/dL


MMembranes Are Complex Structures

Asymmetric sheet-like enclosed structures with distinct inner and outer surfaces

Sheet-like structures are non-covalent assemblies that are thermodynamically stable


MMembranes Are Complex Structures Major Lipids in Mammalian

Membranes

A. Phospholipids- More common- Consist of glycerol backbone to

which are attached two fatty acids in ester linkage and phosphorylated alcohol

- FA constituents are usually even numbered, 16-18 carbons

- Unbranched, saturated or unsaturated

O IIR1-C- O- 1CH2 I R2-C- O- 2CH O- I I 3CH2- O-P-O-R3 II O

Fatty Acids

Glycerol Alcohol



O IIR1-C- O- 1CH2 I R2-C- O- 2CH O- I I 3CH2- O-P-O-R3 II O

Fatty Acids

Glycerol Alcohol

Major Lipids in Mammalian Membranes

A. Phospholipids

- Phosphatidic acid is simplest phosphoglyceride

- 1,2-diacylglycerol 3-phosphate, a key intermediate in the formation of all other phosphoglycerides




B. Glycosphongolipids

- Sugar containing lipids built on a backbone of ceramide

- Include galactosyl and glucosyl ceramide

(cerebrosides) and the gangliosides




C. Sterols

- Most common sterol in membranes is cholesterol

- Intercalates among the phospholipids of the membrane, with its hydroxyl group at the aqueous surface and the remainder of the molecule within the leaflet



Membrane Lipids are Amphipathic

- Contain both hydrophobic and hydrophilic region

S U S S

POLARHEAD GROUP

Apolar HydrocarbonTails



Straight tails – saturated fatty acids

Kinked tails – unsaturated fatty acids, make membrane become less compact, more fluid

S U S S

POLARHEAD GROUP

Apolar HydrocarbonTails



Membrane Lipids Form BilayersMicelle – a structure where hydrophobic regions

are shielded from water while the hydrophilic polar groups are immersed in the

aqueous environmentBilayers- key structures in bilogic membranes. A

bilayer exists as a sheet in which the hydrohobic regions of the phospholipids are protected from the aqueous environment while the hydrophilic regions are immersed in water



Membrane Lipids Form Bilayers

1. How many biologic materials are lipid soluble?

- gases such as oxygen, CO2, nitrogen, small molecules with little interaction with solvents

- steroids




2. How are the transmembrane concentration gradients for non-lipid soluble maintained

- Membranes contain proteins, and proteins are also amphipathic molecules that can be inserted into the correspondingly amphipathic lipid bilayer




Proteins form channels for the movement of ions and small molecules and serve as transporters for molecules that otherwise could not pass the bilayer



Membrane Proteins Are associated with the Lipid Bilayers

Phospholipids –act as solvent for membrane proteins creating an environment in which the latter can function


MMembranes Are Complex Structures Membranes Are Asymmetric Structures- Inside-Outside asymmetry

-Attributed to irregular distribution of proteins within the membranes.

- provided by the external location of carbohydrates attached to membrane proteins

- inside-outside asymmetry of phospholipids


MMembranes Are Complex Structures Membranes Are Asymmetric Structures- Inside-Outside asymmetry

- choline containing PPL (PPC and sphingomyelin, outer molecular layer

- aminophospholipids (PPS and PPE) are preferentially located in the inner membrane

- GSL’s and glycoproteins whose sugar moieties protrude outward from plasma membrane

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Membrane Structure and Function