2. • The plasma membrane defines the cell and separates the inside from outside.
• In eukaryotes, membrane also defines the intracellular organelles such as
nucleus,mitochondria and lysosomes.
• In prokaryotes-surrounded by a single plasma membrane .Single plasma membrane
contains hundreds of different types of proteins that are integral to function of cell.
• Some of proteins catalyze ATP synthesis and initiation of DNA replication.
• Some are membrane transport proteins- enable specific ions ,sugars,amino acid and
vitamins to cross the impermeable membrane and allow specific metabolic products
to exit.
• Receptors are proteins that allow cells to recognize chemical signals and adjust its
metabolism or pattern of gene expression in response.
• Eukaryotes -also have plasma membrane with multitude of proteins that perform
variety of functions including membrane transport ,cell signaling and connecting cells
unto tissues.
3. Eukaryotes cells have : Internal membrane bound
orgnelles
Unique complement
of proteins
That enable it to carry out
its cellular functions : ATP
generation and DNA
synthesis
The Lipid Bilayer:
(Composition and structural organisation)
-Most common phospholipids are Phosphogylcerides.
-All phospholipids are amphipathic.
-Biomembranes also contain small amount of other amphipathic lipids , such as
glycolipids and chloesterol.
-Amphipathic nature of phospholipids Critical to structure of
biomembranes.
4. Phospholipids aggregate into one of 3 forms:
Spherical micelles
Liposomes
Phospholipid bilayer
These structures depends on several factors:
• Including length of fatty acyl chain in hydrophobic
tail.
• their degree of saturation
• temperature
It effect cause the fatty acyl chains to aggregate & exclude water molecules from
the ‘core’.
• They are rarely formed from natural phospholipids .
• These are formed if one of the 2 fatty acyl chains that make
the tail of phospholipids is removed by hydrolysis.
5. • Each phospholipids layer in this lamellar structure is called leaflet.
• Hydrophobic fatty acyl chains -- minimize their contact with water by aligned
themselves tightly together in center of bilayer forming a thick hydrophobic core.
• Inonic and H-bond stabilize interaction of phospholipid polar head groups with
another and with water.
• Phospholipid layer -- does not allow many water soluble solutes --- includes salts ,
sugars and small hydrophilic molecule including water.
The hydrophobic and vander
walls interaction between F.A.
chain
Maintain the integrity of bilayer
structure.
All phospholipid bilayer- form sealed
closed compartments - inside is separated
from outside.
These are much more energetic that the outside part of phospholipid bilayer
with hydrocarbon core of bilayer exposed to an aqueous solution - is unstable.
6. 1. Phosphoglycerides:
• derivative of glycerol 3-phosphate
• Consists of hydrophobic tail composed of 2 F.A. based chain esterified to 2 hydroxyl
groups in glycerol phosphate & a polar head group attachedd to phosphate group.
• Phosphotidycholines --- Head groups consists of choline , +vely charged alcohol,
esterified to -vely charged phophate.
• Biomembranes contain 3 classes of amphipatheic lipids:
Sterols/Cholesterol Sphingolipids
Phosphoglycerides
Phospholipases act on
phosphoglycerides
Produce lysophospholipides
• Important signalling molecule
• released from cell and recognised by specific receptors
• their presence also affect physical properties of mem
2. Sphingolipids:
• Derived from sphingosine
• Have phosphate based polar head. Phosphocholine is attached to terminal hydroxyl
group of sphingosine.
7. • So sphnigomylein is a phospholipid and it is similar to phosphotidylcholine.
• Other sphingolipids are amphipathic glycolipids whose polar head groups are sugars
that are linked via phosphate group
• Glycocerebroside --- Simplest glycosphingolipid
* Contains a single glucose unit attached to sphingosine
Gangliosides --- one or 2 branched sugar chains containing sialic acid groups are
attached to sphinosine.
3. Cholesterol:
• Analogs constitute the third impo. class of mem. lipids - the sterols.
• Sterol - 4 ring isoprenoid -based hydrocarbon.
• cholesterol - has a hydroxyl substituent on one ring.
It is amphipathic because hydroxyl
group interact with water.
Major animal sterol
8. • Cholestrol ---- Abundant in p.m. of mammalian cells but present in prokaryotic
and all plant cells.
• Cholestrol --- too hydrophobic to because to from a bilayer structure.
• Sterols --- provide structural support to membranes , preventing too close
packing of phospholipids acyl chains to maintain a significant measure of
membrane fluidity.
• Cholestrol is precursor for several important bioactive molecules.
Include bile acids (made in liver)
help in emulsify dietary fats for digestion and absoption in intestines.
