BIO130H1 Study Guide - Final Guide: Lipid Bilayer, Membrane Lipids, Signal Transduction

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1 Apr 2019

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Introduction to the plasma membrane
Membrane functions:
1. Compartmentalization allows specialized activities to proceed
without external interference
Enables cellular activities to be regulated independently of one another
2. Scaffold for biochemical activities components embedded in the
membrane are not floating free and can be ordered for effective interaction
3. Providing a selectively permeable barrier promotes movement of
select elements in and out of the cell
4. Transporting solutes able to transport specific molecules from one
side of the cell to another
5. Responding to external stimuli signal transduction
Membranes possess receptors that combine with ligands or respond to
other stimuli
Interaction of plasma membrane receptor and external stimulus may
cause the membrane to generate a signal that stimulates/inhibits internal
6. Intercellular interaction cell membrane allows cells to recognize
and signal one another
Proteins within the plasma membrane may also facilitate intercellular
7. Energy transduction membranes convert one type of energy into another
Membranes also involved in transfer of chemical energy from carbohydrates
fat ATP
The lipid composition of membranes
Core of membrane consists of a sheet of lipids
arranged in bilayer
Lipid bilayer serves as a structural backbone
of the membrane + provides barrier
preventing random movements of water-
soluble materials into and out of the cell
Ratio of lipid to protein in a membrane varies
depending on the type of:
o Cellular membrane (plasma, Golgi)
o Organism (bacterium, plant, animal)
o Cell (cartilage, muscle, liver)
Differences correlated with the basic functions of the membranes
Membrane lipids
o Wide diversity of lipids found in membranes
o All lipids are amphipathic contain hydrophilic and hydrophobic regions
o 3 main types of membrane lipids:
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o Phosphoglycerides phospholipids built on a glycerol backbone
Membrane glycerides are diglycerides only two of the hydroxyl groups of
glycerol are esterified to fatty acids
3rd hydroxyl is esterified to a hydrophobic phosphate group
Final molecule called phosphatidic acid absent in most membranes
Membrane phosphoglycerides have additional group linked to the phosphate
Each of these groups is small and hydrophilic
Together with the negatively charged phosphate form a water-soluble domain
head group
Fatty acyl chains are hydrophobic
Phosphoglycerides often contain one saturated and one unsaturated fatty acyl chain
o Sphingolipids derivatives of sphingosine (amino alcohol containing long
hydrocarbon chain)
Sphingosine linked to fatty acid ceramide
Sphingosine-based lipids have additional groups esterified to terminal alcohol
If substitution is phosphocholine sphingomyelin (only phospholipid without
glycerol backbone)
Is substitution is a carbohydrate glycolipid
If carbohydrate is a simple sugar, glycolipid is called cerebroside
If it is a cluster of sugars, glycolipid is called ganglioside
Overall similar in structure to phosphoglycerides, but fatty acyl chains are longer
and more saturated
Nervous system rich in glycolipids
Humans who are unable to synthesize a ganglioside suffer from serious
neurological disease characterized by seizures and blindness
Glycolipids also play a role in certain infectious diseases
o Cholesterol constitute up to 50% of the lipid molecules in plasma membrane
Cholesterol molecules are oriented with small hydrophilic hydroxyl group toward
the membrane surface
Remainder of the molecule embedded in the lipid bilayer
Hydrophobic rings of cholesterol are flat and rigid
Interfere with the movements of the fatty acid tails of
The nature and importance of the lipid bilayer
o High structural variability
Mass spectrometry used catalogue chemically distinct species of
o Lipids of a membrane can have important effects on the biological properties of a
o Lipid composition can determine the physical state of the membrane and influence the
activity of particular membrane proteins
o Membrane lipids provide precursors for highly active chemical messengers that
regulate cellular function
o Entire lipid bilayer is about 6nm thick
o Hydrocarbon chains of lipid bilayer are never exposed to the surrounding aqueous
Due to thermodynamic considerations
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o Membranes never have a free edge they are always continuous, unbroken structures
Membranes form extensive interconnected networks within the cell
o Membranes are deformable due to the flexibility of the bilayer
Overall shape can change locomotion and cell division
o Lipid bilayer facilitates regulated fusion or budding of membranes fertilization
o Lipid bilayer is able to self-assemble
If a phospholipid is dispersed in an aqueous solution, phospholipid
molecules will spontaneously form walls of fluid-filled spherical vesicle
Walls of liposomes consists of a continuous lipid bilayer, organized in the
same way as that of a natural membrane
Liposomes are very important in membrane research
Membrane proteins can be added to liposomes and
study their functions
Liposomes can be used to deliver drugs or DNA
molecules within the body
The asymmetry of a membrane lipids
o Lipid bilayer composed of two more-or-less stable, independent
monolayers with different physical and chemical properties
o All glycolipids of the plasma membrane are in the outer leaflet
serve as receptors
o PA is found in the inner leaflet as promotes curvature of the
Membrane carbohydrates
All of the carbohydrates of the plasma membrane face outward
Glycosylation most complex protein modification
o Addition of a carbohydrate
o Carbohydrate of glycoproteins is present as short, branched hydrophilic
o Oligosaccharides may be attached to several differ AAs by two major links
o Carbohydrate projections play an important role in:
Mediating the interactions of a cell with its environment
Sorting of membrane proteins to different cellular compartments
o The carbohydrates the glycolipids of the red blood cell plasma membrane determine a
person’s blood type
Variation in AB gene is a strong indicator of risk for pancreas cancer, heart
disease, and viral infection
A membrane may contain hundreds of different proteins depending on:
o The cell type
o Particular organelle within that cell
Each membrane protein has a defined orientation relative to the cytoplasm
o Properties of one surface of the membrane are very different from those of the other
surface asymmetry referred to as membrane “sidedness”
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