2. Plasma Membrane
Plasma membrane is outer membrane of a cell, which is composed of two layers of
phospholipids and embedded with proteins. It is a thin semi permeable membrane layer,
which surrounds the cytoplasm and other constituents of the cell. The cell membrane also
known as the cell membrane or cytoplasmic membrane, and historically referred to as the
plasma lemma.
Definition of semipermeable. : partially but not freely or wholly permeable; specifically :
permeable to some usually small molecules but not to other usually larger particles.
a semipermeable membrane.
Role of the plasma membrane
All cells are surrounded by a plasma membrane. The membrane is composed of a
phospholipid bilayer arranged back-to-back. The membrane is also covered in places with
cholesterol molecules and proteins. The plasma membrane is selectively permeable and
regulates which molecules are allowed to enter and exit the cell.
3.
4. Structure of plasma membrane
(I) Lipid Bilayer Hypothesis: In Overton (1895) suggested that the cell membranes contain
lipids. This conclusion was based on the fact that fat solvents dissolved the membrane easily
and fat soluble substances passed easily through the cell membranes. Gorter and Grendel
(1925)’ were the first to suggest a possible structure of the cell membrane.
5. (ii) Unit Membrane Model:
In( 1950) J. David Robertson studied the cell membranes from electron micrographs of
sectioned material. The unit membrane is 75 Å thick with a 35 Å thick phospholipid layer
between two 20 Å thick protein layers. The plasma membrane surrounding the cell is thicker
at the free surfaces of the cell than where it is in contact with other cells. In unit membrane
model the protein layers are asymmetrical. On the outer surface it is mucoprotein while on
the inner surface it is non-mucoprotein.
6. (iii) Davson and Danielli : In (1935) The model describes a phospholipid bilayer that lies
between two layers of globular proteins and It is trilaminar and lipoprotinious. In 1935,
Davson and Danielli proposed that biological membranes are made up of lipid bi-layers that
are coated on both sides with thin sheets of protein and they simplified their model into the
"pauci-molecular" theory.
7. (iv) Fluid mosaic model: In (1972), S. Jonathan Singer and Garth Nicolson developed
new ideas for membrane structure. Their proposal was the fluid mosaic model, which is the
dominant model now. It has two key features—a mosaic of proteins embedded in the
membrane, and the membrane being a fluid bi-layer of lipids. The lipid bi-layer suggestion
agrees with previous models but views proteins as globular entities embedded in the layer
instead of thin sheets on the surface. According to the model, membrane proteins are in three
classes based on how they are linked to the lipid bi-layer:
Integral Proteins: Immersed in the bi-layer and held in place by the affinity
of hydrophobic parts of the protein for the hydrophobic tails of phospholipids on interior of
the layer.
Peripheral proteins: More hydrophilic, and thus are non-covalently linked to the polar heads of
phospholipids and other hydrophilic parts of other membrane proteins on the surface of the
membrane.
Lipid anchored proteins: Essentially hydrophilic, so, are also located on the surface of the
membrane, and are covalently attached to lipid molecules embedded in the layer and have
two parts 1."Head": hydrophilic → attracts and mixes with H2O
2.Two "fatty acid tails": hydrophobic
8.
9. Function of plasma membrane-
Protein
Carrier (change shape for different molecules) for water-soluble molecules such as
glucose
Channels for ions (sodium and chloride ions)
Pumps use energy to move water-soluble molecules and ions
Adhesion molecules for holding cells to extracellular matrix
Receptors enable hormones and nerve transmitters to bind to specific cells
Recognition sites, which identify a cell as being of a particular type
Enzymes, which speed up chemical reactions at the edge of the membrane
Adhesion sites, which help some cells to stick together
E.g. glycoprotein acts as a receptor and recognition site
Diffusion
Substances move down their conc. gradient until the conc. are in equilibrium
Microvilli are extensions of the plasma membrane
o They increase the surface area of the membrane, therefore
o They accelerate the rate of diffusion
10. Facilitate diffusion
Trans membrane proteins form a water-filled ion channel
o Allows the passage of ions (Ca2+, Na+, Cl-) down their conc. gradient //passive - no
ATP required
o Some channels use a gate to regulate the flow of ions
o Selective permeability - Not all molecules can pass through selective channels
How do molecules move across the membrane?
o Substrate (molecule to move across the membrane) binds to carrier protein
o Molecule changes shape
o Release of the molecule (product) at the other side of the membrane
Example
o If you want to move a muscle a nerve impulse is sent to this muscle
o The nerve impulse triggers the release of a neurotransmitter
o Binding of the neurotransmitter to specific trans membrane proteins
o Opens channels that allow the passage of Na+ across the membrane
o In this specific case, the result is muscle contraction
o These Na+ channels can also be opened by a change in voltage
11. Osmosis
Special term used for the diffusion of water through a differentially permeable cell
membrane
Water is polar and able to pass through the lipid bilayer
Trans membrane proteins that form hydrophilic channels accelerate osmosis, but water is
still able to get through membrane without them
Osmosis generates pressure called osmotic pressure
o Water moves down its concentration gradient
o When pressure is equal on both sites net flow ceases (equilibrium)
o The pressure is said to be hydrostatic (water-stopping)
Passive Transport-Uses energy from moving particles (Kinetic Energy)
Active Transport
• Movement of solute against the conc. gradient, from low to high conc.
• Involves materials which will not move directly through the bilayer
• Molecules bind to specific carrier proteins / intrinsic proteins
• Involves ATP by cells (mitochondria) / respiration
Direct Active Transport - transporters use hydrolysis to drive active transport
Indirect Active Transport - transporters use energy already stored in gradient of a directly-
pumped ion.
12. Endocytosis and Exocytosis
Substances are transported across plasma membrane in bulk via small vesicles
Endocytosis
o Part of the plasma membrane sinks into the cell
o Forms a vesicle with substances from outside
o Seals back onto the plasma membrane again
Phagocytosis: endocytosis brings solid material into the cell
Pinocytosis: endocytosis brings fluid materials into the cell
Exocytosis
o Vesicle is formed in the cytoplasm
o Moves towards plasma membrane and fuses with plasma membrane
o Contents are pushed outside cell
o Insulin is secreted from cells in this way
