Dr Pradeep Kumar
Professor, Physiology
KGMU
The Plasma Membrane – a Phospholipid
Bilayer
Components of Plasma Membrane
Lipids
~50%
1) Phospholipids
(75%)
2) Cholesterol
(20%)
3) Glycolipds
(5%)
Proteins
~50%
1) Peripheral
(Associated)
2) Integral
(Membrane Spanning)
3) Glycoproteins

Isolate the cell’s contents from the external
environment

Regulate traffic in and out of the cell

Communicate with other cells
The phospholipid bilayer
1.
Impermeable to water-soluble and polar molecules, ions
2.
Permeable to small and nonpolar molecules
3.
Lipids oriented with polar heads facing out
tails
(hydrophobic)
head
(hydrophilic)
extracellular fluid
(watery environment)
phospholipid
hydrophilic
heads
hydrophobic
tails
bilayer
hydrophilic
heads
cytoplasm
(watery environment)
 Membranes
are “fluid mosaics” with proteins
embedded in or attached to the membrane
 Proteins
can move within the fluid lipid bilayer
extracellular fluid (outside)
recognition protein
receptor protein
transport protein
binding site
phospholipid
bilayer
carbohydrate
phospholipid
cholesterol
protein filaments
cytoplasm (inside)
1.
Transport proteins
◦ regulate the movement of water-soluble
molecules across the membrane
 Ion Channel proteins, Pumps, Receptors
 Carrier proteins, Enzymes, Cell adhesion
Molecules
2. Receptor Proteins
◦
trigger cellular response when specific
molecules bind to them
Nervous system
Endocrine system

Passive transport is a function of molecular
size, lipid solubility, and size of the
concentration gradient
1. Simple diffusion
1 A drop of dye is
placed in water.
drop of dye
pure water
2 Dye molecules
diffuse into the
water; water
molecules diffuse
into the dye.
3 Both dye molecules
and water molecules
are evenly dispersed.
(a) simple diffusion
(extracellular fluid)
(cytoplasm)
Passive transport…(cont.)
◦
2.
•
•
•
Osmosis
a. Isotonic
b. Hypertonic
c. Hypotonic
10 micrometers
(a) isotonic solution
equal movement of water
into and out of cells
(b) hypertonic solution
net water movement
out of cells
(c) hypotonic solution
net water movement
into cells
Passive transport…(cont.)
◦
3. Facilitated diffusion
(b) facilitated diffusion through a channel
ions
proteins forming
permanent
hydrophilic channel
channel
protein
(c) facilitated diffusion through a carrier
amino acids,
sugars,
small proteins
carrier
protein
Carrier protein
has binding
site for
molecule.
(extracellular fluid)
(cytoplasm)
Molecule
enters binding
site.
Carrier protein changes
Carrier protein
shape, transporting molecule resumes original
shape.
across membrane.
Energy-requiring transport
1. Active transport
• Ion gradients and energy production
2. Endocytosis
3. Exocytosis
Primary Active Transport: The Na+/K+ Pump
Antiport
Secondary Active
(Indirect):
e.g.,
Na+/Glucose transporter
Symport
(extracellular fluid)
transport protein
ATP ATP
Transport protein
recognition
Transport protein uses
binding Transport protein
resumes original
site
energy from ATP to
site
binds ATP and
Ca2+
change shape and move shape.
2+
Ca .
(cytoplasm)
ion across membrane.
(a)
pinocytosis
(extracellular fluid)
1
2
3
vesicle containing
extracellular
fluid
(cytoplasm)
cell
(b)
phagocytosis
food particle
pseudopod
1
2
3
particle
enclosed in vesicle
Thank You