Theme 3- THE DETERMINATION OF THE SIZES OF LIPID MOLECULES BY THE APPLICATION OF MONOLAYERS Flashcards
What is the cell membrane
-The membrane can be described as the boundary which defines a specific region from the rest of the surroundings.
-It also controls the composition of the enclosed space.
Properties of the cell membrane
-In living systems, each cell is covered by a 5 to 10nm wide layer of phospholipids and proteins, which encloses the whole cell and protects
cell cytoplasm from the extracellular environment
-It has a semi-permeable nature, through
which selective transport and retention of cell nutrients takes place and toxic or unwanted substances cannot pass into the cell
Basic components of the cell membrane
-Lipids, proteins and carbohydrates are the main components of a plasma membrane.
-Their compositions and relative proportions in membranes may vary in different types of cells
-A common feature of all biomembranes is the presence of a bilayer of phospholipids
Lipids- the main constituents of cell membrane
-The lipids components of membranes are further divided into glycerophospholipids,
sphingolipids, glycolipids and sterols
-In general, glycerophospholipids and
sphingolipids constitute the largest proportions of lipids in all biological membranes
Phospholipids- the main components of biomembranes
-They make up 40-90% of all types of the membrane lipids.
-By structure, they are diesters of phosphoric acid, in the molecule in which one of the hydroxyl groups is esterified with glycerol or
sphingosine
Characteristics of the lipid molecule
-Lipid molecules make up about 50% of the mass of most animal cell membranes.
-These amphiphilic (amphipathic) molecules are arranged into a two-dimensional sheet-like bilayer structure.
-Dual characteristics exhibited by the lipids
in cell membranes can be attributed to the presence of a hydrophilic polar end and a
hydrophobic nonpolar end in these molecules.
What is a phospholipid?
-They are compound/complex lipids containing lipid molecules attached to a phosphate group. —Phospholipids are derivatives of glycerol-3 phosphate in which the glycerol backbone is attached to two fatty acids on one end and
esterified phosphoric acid and an organic alcoholic group on the other.
Structure of the phospholipid bilayer
-The phospholipid bilayer consists of phospholipids arranged in two layers with
exterior facing hydrophilic polar heads and interior hydrophobic non-polar tails.
-This imparts the amphiphilic nature to phospholipids.
2 types of phospholipids
1) Glycerophospholipids (phosphoglycerides)/ Glycerol phospholipids
2) Phosphatidylethanolamine
Glycerophospholipids (phosphoglycerides)/ Glycerol phospholipids
-Glycerol serves as the backbone.
-Depending upon the nature of its head group,
different phosphoglycerides exist, they are:
Phosphatidylcholine (PC), Phosphatidylserine (PS) and Phosphatidylinositol (PI)
Phosphatidylethanolamine
-The phosphate group, when bound to other head molecules such as hydrogen, and
ethanolamine, it is known as phosphatidic acid and phosphatidylethanolamine.
-Phosphatidic acid is considered to be the
precursor to many phospholipids. Thus, it is
the most fundamental one.
-Phosphatidylethanolamines are found in all living cells, composing 25% of all phospholipids. –In human physiology, they are found particularly in nervous tissue such as the white matter of the brain, nerves, neural tissue, and spinal cord, where they make up 45% of all phospholipids.
Plasmalogens
-The members of phosphoglycerides that consist of one hydrocarbon chain attached to glycerol by an ester bond.
-In contrast, the other hydrocarbon chain is
attached to glycerol by an ether linkage.
-These are highly abundant in the human
heart and brain tissue.
Sphingophospholipids
-Sphingosine acts as the backbone.
-Sphingosine is an amino alcohol with a long
hydrocarbon chain.
Sphingomyelins
-Phospholipids whose overall structure is quite
similar to that of phosphatidylcholine.
-In sphingomyelin, phosphocholine is attached
to the terminal hydroxyl group of the sphingosine backbone.
-It is a member of both phospholipids and sphingolipids.
Arrangement of phospholipids in memebranes
-Arranged in the bilayer structure with hydrophobic tails inside and hydrophilic heads outside the bilayer in an aqueous environment.
-The hydrophilic head molecule possesses charged or uncharged polar groups, which form
electrostatic interactions or hydrogen bonds with water to make them readily dissolve
in it.
-However, hydrophobic fatty acyl chains in the tail region are uncharged and non-polar, which doesn’t let them interact with water.
How are the spherical micelles/biomolecular sheets of bilayer formed
-To minimize the cost of free energy
required to reorganize water molecules when phospholipids are dispersed in water, the
phospholipids aggregate in such a way that they expose their hydrophilic heads to
water and hide their hydrophobic tails in the interior. This results formation of
spherical micelles or bimolecular sheets or bilayers.
Properties of the phospholipid bilayer
-Phospholipids are amphiphilic.
-Phospholipids can self-assemble.
-The hydrophobic core of the lipid bilayer prevents the diffusion of hydrophilic
solutes across it.
-Phospholipids in the lipid bilayer show either rotation or lateral movement in
one bilayer, while transverse movement between bilayers in a “flip-flop” manner.
-Membrane fluidity is increased by phosphoglycerides while decreased by
sphingolipids and cholesterol.
-The fluid nature of phospholipids allows several cellular processes, such as pinocytosis and endocytosis.
-Phospholipids are structural units of biological membranes and facilitate anchoring the membrane proteins.
-Phospholipids exhibit asymmetrical distribution in the two leaflets of the bilayer
Biological functions of the phospholipid bilayer
- Some phospholipids, when degraded by an enzyme, produce products that act as
second messengers in signal transduction. - Phospholipids participate in a wide range of cellular activities like apoptosis, phagocytosis, and regulation of mitochondrial physiology.
- Phospholipids contribute to cellular membrane fluidity and flexibility. Excessive alcohol intake decreases the number of phospholipids in hepatic cells, thereby reducing the flexibility of hepatic cells and leading to liver injury or cell damage.
- Phospholipids are necessary for lipid metabolism and absorption.
Glycolipids
-The sub-group of sphingolipids and generally present approximately 2% of the total lipid content of plasma membranes.
-These sugar-containing lipids have
a sphingosine-derived backbone and mono- or oligosaccharide bound to the Csn-3
position.
-These are generally found at the outer leaflet of the plasma membrane and their carbohydrate moieties are exposed on cell surface
Sterols
-Sterols are present in the cell membrane for important hormones and signalling
molecules.
-In general, all steroids contain four hydrocarbon rings.
-Cholesterol is the major steroidal constituent of plasma membranes
Cholesterol
-Accounts for near-about 20% and 30-50% of the
of total lipid content in animal cell and plant cell, respectively.
-However, cholesterol is generally absent in prokaryotic membranes and mitochondrial membrane.
-Cholesterol has the entire hydrocarbon structure, but also has a hydroxyl substituent on one ring which makes this molecule as amphipathic.
-Cholesterol maintains the
rigidity and stability of the membrane.
Membrane proteins
-Proteins also contribute to the major proportion (50% of the total mass of membrane) in biological membranes and these proteins participate in several biological activities
like transport, enzymatic actions, communications etc.
-Structurally, membrane proteins may be present as single proteins or as multimeric protein subunits.
-They may also get conjugated with carbohydrates and lipids and present in the form of glycoprotein and lipoprotein, respectively.
How are membrane proteins classified
-Location-based
-Function-based
Location-based classification
-One of the classifications of membrane proteins is based on their location in the membrane.
-Membrane proteins are categorized as integral membrane proteins and peripheral membrane proteins
Location-based classification (integral membrane proteins)
-The proteins which are present in between the phospholipid bilayer or its major part penetrate or span the bilayer.
-The membrane-spanning portions of transmembrane proteins are usually
composed of hydrophobic amino acids which make α-helical secondary confirmations except porins (pore-forming proteins in bacteria).
-In porins beta-sheet structures are predominant.
-In general, these proteins are tightly bound to the membrane through hydrophobic interactions and their terminal portions may exposed on one or both sides of the membrane.
Location-based classification (peripheral membrane proteins)
-The proteins present outside the membrane are considered peripheral membrane proteins.
-These proteins do not interact with the hydrophobic core of the bilayer and are loosely
associated with the membrane through non-covalent interactions (generally ionic bonds with the polar head groups of the phospholipids).
-Therefore, it is easier to isolate peripheral membrane proteins and this can be done by using solutions of high ionic strength/salt concentration on the membrane.
Function-based classification
-Proteins present in the membrane are responsible for the vital functions of cells and therefore these proteins may also be differentiated based on their following functions. These include:
1) Transport or Channel Proteins
2) Carrier Proteins
3) Receptor Proteins
4) Cell Recognition Proteins
5) Enzymatic Proteins
Function-based classification (Transport or Channel Proteins)
-Gaseous molecules e.g. O2 and CO2 and small
hydrophobic molecules can pass through the lipid bilayer, but, water and other hydrophilic nutrients generally can’t pass through the lipid bilayer or at a very slow pace.
-The movement of the hydrophilic molecules is done by the transport proteins which are generally embedded in the membrane.
-These transport proteins show specificity
for the selection of the molecules to be transported.
-On several occasions, these proteins
transport nutrients against their concentration gradient and the energy of ATP is utilized to
catalyze passage.
Function-based classification (Carrier proteins)
These proteins are generally present inside the membrane with some part surface exposed and the binding site on the protein surface “grabs” certain molecules and pulls them into the cell e.g. gated channels.
Function-based classification (Cell recognition proteins)
-Several proteins are present as glycoproteins, where attached carbohydrates are localized on the outer exo-plasmic membrane face of
eukaryotic cells.
-These proteins act as ID tags for identification of specific cells by the immune system of the body.
Function-based classification (Receptor proteins)
-The cell membrane also contains some specific proteins which act as sensors (or molecular triggers) for external signals and allow the cell to produce a response due to changes in the external environment of the cell.
-These protein sensors are also termed cell protein receptors.
-Their main function is to transfer information e.g. release of hormones due
to receptor proteins.
Function-based classification (Enzymatic proteins)
Several proteins present on the membrane surface show enzymatic activities and carry out metabolic reactions.
The Fluid Mosaic Model
The model states that the cell membrane exists as a fluid mosaic, two-dimensional in nature of freely diffusing lipids with embedded proteins.
Major features of the fluid mosaic model
-The phospholipids and glycolipids molecules are arranged as bilayers regarded as two-dimensional solutions. The polar heads are oriented towards the aqueous environment while the long hydrocarbon chains face outwards.
-Membrane lipids interact specifically with specific proteins which is essential for their
function.
-For the membrane to exist as a fluid-structure the right ratio of saturated to unsaturated fatty acids is important. The lipid molecules show free lateral diffusion whereas the flip-flop movement i.e. transverse movement occurs once in several
hours.
-The protein molecules show lateral diffusion but no transverse movement.
Brief characteristics of oleic acid
-Oleic acid is a fatty acid that occurs naturally in various animal and vegetable fats and oils.
-It is an odourless, colourless oil, although commercial samples may be yellowish.
-It has the formula:
CH3−(CH2)7−CH=CH−(CH2)7−COOH
Benefits of oleic acid
1) It strengthens the walls of blood vessels, breaks down harmful cholesterol, strengthens immunity, stabilizes blood pressure, and removes free radicals and toxins from the body.
2) It is part of various cosmetic products and helps to:
- increases skin elasticity
- slows down ageing processes
- restores the hair structure
- moisturizes the skin and reduces skin irritations
