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The lipid bilayer
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The document describes the lipid bilayer of cells in detail.
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Membrane Structure 1
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Some examples from this set of practice questions
1.
Which phospholipids are the most abundant membrane lipids?
Answer: Phospholipids
2.
Which are the major phospholipids in the animal cell membranes?
Answer: Phosphoglycerides
3.
What are the three abundant phosphoglycerides in mammalian cell membranes?
Answer: Phosphatidylethanolamine, Phosphatidylserine and Phosphatidylcholine
4.
Which enzyme catalyzes flip-flop of phospholipids from one monolayer to the other?
Answer: Flippases
5.
What are the specialized domains formed by lipids in the plasma membrane?
Answer: Lipid rafts
Content preview
Contents1. Introduction
2. Functions of the Plasma Membrane
3. Components of the Plasma Membrane
4. Major lipids
5. Bilayer Formation by the phospholipids
6. Fluidity of the bilayer
7. Domains of varying compositions within the bilayer
8. Lipid droplets
9. Functional importance of the bilayer’s asymmetry
10. Types of Membrane Proteins
11. Integral Membrane Proteins
12. Peripheral Membrane Proteins
13. Lipid-anchored membrane proteins
14. Action of detergents on membrane proteins
15. The Bacteriorhodopsin
16. Large protein complexes
17. Diffusion of membrane proteins
18. Protein and lipid domains within the membrane
19. The cortical cytoskeleton
20. Bilayer deformation by membrane proteins
21. RBC’s Plasma Membrane.
,1. INTRODUCTION
The Plasma Membrane (PM) is the cell’s skin.
It surrounds the cells and the organelles enclosed within them.
PM distinguishes the cells from their external environment
The PM that surrounds the organelles differentiates them from the cytosol.
PM is around 5-10 nm wide and is extremely thin.
PM has differing functions depending on its location.
Despite the functional differences, the PMs share a fundamental feature.
They are all composed of a lipid bilayer.
This lipid bilayer provides the basic fluid structure of the PM and serves as a
relatively impermeable barrier to the passage of most water-soluble molecules.
The proteins present in the PM span this lipid bilayer and are responsible for the
majority of the functions mediated by the PM.
2. FUNCTIONS OF THE PLASMA MEMBRANE
Compartmentalization
PMs are continuous sheets that form distinct compartments within the cell (For
e.g. Mitochondria and Golgi are different compartments of the cell. These compartments
are formed by the PMs that surround them).
Compartmentalization facilitates interference-free specialized cellular activities.
Scaffold for biochemical activities
Membranes carry the proteins and other components that are crucial for certain
biochemical activities such as ATP synthesis, Ion transport etc.
Therefore, PMs serve as a scaffold for carrying out these biochemical activities.
Selective Permeability
PMs prevent unrestricted exchange of molecules between the 2 sides of the
membrane.
At the same time, it also enables cell-cell interaction.
Hence, PMs are responsible for the movement of selective molecules.
Solute transportation
PMs carry the machinery for solute transportation (From a region of high
concentration to a region of low concentration).
, They allow the cells to accumulate substances needed for forming biomolecules
such as sugars, amino acids, lipids etc.
They also establish ion gradients needed for the functioning of some cells such as
the nerves.
Signal Transduction
The plasma membrane plays a pivotal role in signal transduction.
The PM possess receptors that combine with specific molecules (ligands) or
respond to other types of stimuli such as light or mechanical tension.
Different types of cells have membranes with different receptors and are,
therefore, capable of recognizing and responding to different environmental
stimuli.
The interaction of a plasma membrane receptor with an external stimulus may
cause the membrane to generate a signal that stimulates or inhibits internal
activities. (For example, signals generated at the plasma membrane may tell a cell to
manufacture more glycogen, to prepare for cell division, to move toward a higher
concentration of a particular compound, to release calcium from internal stores, or
possibly to commit suicide (apoptosis).
Intercellular Interaction
The PM allows cells to recognize and signal one another, to adhere when
appropriate, and to exchange materials and information.
Proteins within the plasma membrane may also facilitate the interaction between
extracellular materials and the intracellular cytoskeleton.
Energy Transduction
PMs are involved in the processes by which one type of energy is converted to
another type (energy transduction).
The most fundamental energy transduction occurs during photosynthesis when
energy in sunlight is absorbed by membrane‐bound pigments, converted into
chemical energy, and stored in carbohydrates.
Membranes are also involved in the transfer of chemical energy from
carbohydrates and fats to ATP.
In eukaryotes, the machinery for these energy conversions is contained within
membranes of chloroplasts and mitochondria.
, 3. COMPONENTS OF THE PLASMA MEMBRANE
Lipids
Lipid molecules constitute about 50% of the mass of most animal cell membranes.
These lipids form the lipid bilayer of the PM.
The lipid bilayer serves primarily as a structural backbone of the membrane and
provides the barrier that prevents random movements of water‐soluble materials
into and out of the cell.
An important property of lipid bilayers is that they behave as two-dimensional
fluids in which individual molecules are free to rotate and move laterally.
Fluidity is a critical property of the PM and is determined by both temperature and
lipid composition.
There are approximately 5 × 106 lipid molecules in a 1 μm × 1 μm area of lipid bilayer,
or about 109 lipid molecules in the plasma membrane of a small animal cell.
All of the lipid molecules in cell membranes are amphiphilic—that is, they have a
hydrophilic (“water-loving”) or polar end and a hydrophobic (“water-fearing”) or
nonpolar end.
Proteins
In PMs almost 50% is made up of proteins.
Different PMs carry different proteins.
The lipid bilayer provides the structure for the PM but the major functions of the
cells are carried out by the proteins embedded in the PM.
Therefore, proteins provide each PM with characteristic properties.
Each membrane protein has a defined orientation relative to the cytoplasm, so
that the properties of one surface of a membrane are very different from those of
the other surface.
This asymmetry is referred to as membrane “sidedness.”
In the PM for e.g. those parts of membrane proteins that interact with other cells
or with extracellular substances are exposed to the extracellular space, whereas
those parts of membrane proteins that interact with cytoplasmic molecules are
exposed to the cytosol.
The amounts and types of proteins in a membrane are highly variable.
In the myelin membrane, which serves mainly as electrical insulation for nerve-
cell axons, less than 25% of the membrane mass is protein.
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