Transcript 2. Plasma Membrane

The Plasma Membrane – Gateway to the Cell
The Plasma Membrane is Semipermeable
The physical properties of phospholipids account for membrane assembly
and many of its properties.
Small molecules and larger hydrophobic molecules move through.
Ions, hydrophilic molecules larger than water, and large molecules such as
proteins do not move through the membrane on their own.
Plasma Membrane Functions
Maintain a high concentration of materials in the cell.
Keep harmful materials out.
Control the movement of materials into and out of the cell.
Let the cell sense its environment.
Membrane Components
Phospholipids
Proteins (peripheral and integral)
Cholesterol
Carbohydrates
Proteins Are Critical to Membrane Function
Transport Processes - Diffusion
Solutes move
down a
concentration
gradient until they
are evenly
distributed. This
is diffusion.
Another way of saying this is that solutes move from a region of higher
concentration to a region of lower concentration until there is no difference
in concentration.
Three Forms of Transport Across the Membrane
Three Forms of Transport Across the Membrane
Example: Oxygen or water diffusing into a cell
and carbon dioxide diffusing out.
Three Forms of Transport Across the Membrane
Examples: Glucose or amino acids moving from
blood into a cell.
An nerve electrical impulse results from opening
protein channels for ions that move by facilitated
diffusion.
Three Forms of Transport Across the Membrane
Examples: Pumping Na+ (sodium ions) out and K+
(potassium ions) in against strong concentration
gradients.
Moving the “Big Stuff”
Large molecules move in via one of three forms of endocytosis.
Pinocytosis
This is the most common form of endocytosis.
Pinocytosis takes up most proteins and other large molecules.
Pinocytosis
pinocytic vesicles forming
mature transport vesicle
Transport into a capillary cell (blue).
Receptor-Mediated Endocytosis
Receptor proteins make this a highly specific form of transport.
Cholesterol is taken-up this way.
Receptor-Mediated Endocytosis
Cholesterol Delivered to Cells is Carried to Cells in a LowDensity Lipoprotein (LDL) Particle
The LDL particle is taken-up by receptor-mediated endocytosis.
Endocytosis – Phagocytosis Transports Large Particles
In Preparation for Phagocytosis
The Threshold
of Phagocytosis
- Capture of a
Yeast Cell
(yellow) by
Membrane
Extensions of
an Immune
System Cell
(blue)
Moving the “Big Stuff”
Exocytosis:
moving things
out.
Molecules are moved out of the cell by vesicles that fuse with the plasma
membrane.
This is how many hormones are secreted and how nerve cells communicate with
one another.
Exocytosis
Exocytic vesicle
immediately after fusion
with plasma membrane.
Exocytosis and Nervous System Function
A nerve cell communicates to another cell by releasing chemicals via exocytosis
at the synaptic terminal.
Exocytosis and Chemical Communication at the Synapse
The synapse is the region where a nerve cell and its target cell are closely apposed.
Black Widow Spider Venom and Exocytosis
Black widow spider venom causes massive exocytosis of neurotransmitter
into the synapse.
Botulism and Exocytosis
Botulism is caused by botulinim toxin – a protein produced by a bacterium that
sometimes contaminates foods.
Botulinim toxin blocks exocytosis at the synapse and causes paralysis.
Mechanism of botulinum toxin.
Mechanism of botulinum toxin
web site.