Lecture 5
PLASMA MEMBRANE
Dr.Sarab Daoud AL-Shamaa
Plasma Membrane
Boundary that separates the living cell from it’s
non-living surroundings.
Phospholipid bilayer
Amphipathic - having both:
hydrophilic heads
hydrophobic tails
• ~8 nm thick
Phospholipid
Phosopholipid Bi-Layer
-polar heads are hydrophilic “water loving”
-tails (fatty acids) are hydrophobic “water fearing” and face inward
3
Selective Permeability
The property of biological membranes which
allows some substances to cross more easily
than others.
1972 - Singer and Nicolson called the membrane a
“Fluid Mosaic Model”.
Mosaic: different proteins embedded in
the phospholipids.
Fluid: proteins and phospholipids can
move freely in the membrane.
Fluid Mosaic - cont.
Components of a phospholipid bilayer.
1.
2.
3.
4.
5.
6.
phospholipids
proteins - enzymes, receptors, transport.
glycolipids
glycoproteins
carbohydrates
cholesterol
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Transport Proteins
Transports molecules or ions across biological
membranes
3 types of transport proteins:
1. uniport: Carries a single solute across the
membrane
2.symport :Translocate 2 different solutes
simultaneously in same direction :
3. antiport :Exchanges 2 solutes by
transporting them in opposite directions.
Diffusion
The net movement of a substance (molecules) down a
concentration gradient from an area of high
concentration to an area of low concentration.
passive transport: NO energy is expended.
facilitated diffusion: type of passive transport
which uses transport proteins.
Osmosis
The movement of water across selectively permeable
membranes.
The water moves from a high concentration to low
concentration.
Movement of H2O
Water will “ALWAYS” diffuses down a concentration
gradient from a HYPOTONIC solution to a
HYPERTONIC solution.
“ALWAYS REMEMBER”
HYPOTONIC
→ HYPERTONIC
Animal cells placed into a hypotonic solution will
HEMOLYSIS (EXPLODE).
Animal cells placed into a hypertonic solution
will CRENATE (SHRIVEL).
Active Transport
The movement of molecules (small or large) across
the plasma membrane in which energy (ATP) is
required.
Examples:
1.
2.
3.
Sodium (Na) - Potassium (K) Pump
Exocytosis
Endocytosis
Sodium-Potassium Pump
• The mechanism that uses energy (active transport)
released from splitting ATP to transport Sodium (Na+)
out of and Potassium (K+) into cells.
extracellular
fluid
intracellular
fluid
K+
K+
Na+
Na+
Exocytosis
Cell secretes macromolecules (proteins and other
biochemicals) out of cell.
Part of the Endomembrane System: the
fusion of transport vesicles with plasma
membrane.
Endocytosis:The energy requiring movement of
particles (foreign or natural) into the cell.
3 types of endocytosis:
A. Phagocytosis
B. Pinocytosis
C. Receptor-mediated endocytosis
A. Phagocytosis
Cell eating: cells engulf particles with
pseudopodia and pinches off a food vacuole.
Two examples:
1.
White Blood Cell
2.
Amoeba
Food
Vacuole
Bacteria
White Blood Cell
B. Pinocytosis
Cell drinking: droplets of extracellular fluid are
absorbed into the cell by small vesicles.
Example:
1. Fungi
Food Particles
Hyphae
C. Receptor-Mediated Endocytosis
Importing specific macromolecules (hormones) into
the cell by the inward budding of vesicles formed from
coated pits (receptors).
Liver Cell
Hormones
Receptors