Chapter 4: Cells and Their
Environment
The Fluid Mosaic Model of the Plasma
Membrane
Parts of the Plasma Membrane
Phospholipid –
has a polar
phosphate “head”
which is
hydrophilic.
It has two
nonpolar lipid
“tails” which are
hydrophobic.
Parts of the Plasma Membrane
Cholesterol – helps to stabilize the membrane.
The only thing that
keeps the plasma
membrane
together is the
orientation of the
lipid tails (they stay
away from water)
Parts of the Plasma Membrane
Functions of Proteins:
1.Transport. They allow larger molecules and
charged molecules/ions to go through the
membrane.
2.They can act as markers that other cells can
recognize.
3.They can act as enzymes.
Selective Permeability
The plasma
membrane is
selectively permeable
– it allows some
substances to pass
through, but not all.
(It is selective).
Substances that
can easily pass
through are:
O2
CO2
H2O
Aquaporin – water into the cell
Brownian Motion
Molecules are in constant,
random motion, called Brownian
motion.
Animation
Membrane (Cell) Transport
Transport of
materials in and
out of the cell
Passive Transport –
requires no energy
expenditure from
the cell
Active Transport –
requires energy
from the cell
Passive Transport
Three types:
1. Simple diffusion
2. Facilitated diffusion
3. Osmosis
What is Diffusion?
Diffusion – the net
movement of
particles from an
area of high
concentration to
an area of low
concentration
Concentration
• High concentration: a lot of molecules in
a given space.
• Low concentration: a few molecules in a
given space.
• Concentration gradient: a difference in
the concentration of a substance.
Diffusion & Concentration Gradient
If molecules are
going from an area
of HIGH
concentration to an
area of LOW
concentration, they
are going down the
concentration
gradient.
Dynamic Equilibrium
A condition in which the concentration of
a substance is equal throughout a space.
The molecules are still moving, but there
is no change in concentration.
What is Facilitated Diffusion?
Diffusion of
substances
across a
membrane
with the help
of transport
proteins
Facilitated diffusion
What is Osmosis?
Osmosis is the
diffusion of
water through a
selectively
permeable
membrane.
Osmosis
Note that
osmosis
ONLY refers
to the
diffusion of
WATER!
Osmosis: Isotonic Solution
Isotonic solution has the same solute/ water
concentrations in and out of the cell. Water is
in a dynamic equilibrium.
Osmosis: Hypertonic Solution
Hypertonic solution:
the water concentration
is higher inside the cell
than outside. Water will
diffuse out of the cell.
The cell will shrink
(crenate)
“Hyper” means “more”:
there is more solute
outside of the cell
(therefore, less water)
Osmosis: Hypotonic Solution
Hypotonic solution
– the concentration of
water is greater outside
of the cell as compared
to the inside. Water
will enter the cell. The
cell will expand and
may burst.
“Hypo” means “less” or
“under”. There is less
solute outside of the
cell.
Osmosis
Osmosis in Plants
• Turgor pressure –
pressure of water
inside of a cell.
• The large central
vacuole in plants
regulates the
amount of water
in the cell.
Is turgor pressure higher when a cell is in
a hypertonic, hypotonic or isotonic
solution?
Plasmolysis
A loss of turgor
pressure is
called
plasmolysis
Plasmolysis in Elodea
Normal
Plasmolyzed
Plasmolysis in Red Onion
Red Onion Cells - normal
Red Onion Cells - plasmolyzed
Which solution is isotonic, hypertonic
and hypotonic to the cell?
A
B
C
Active Transport
Examples:
1. Solute pumping – molecules go from
an area of low concentration to an
area of high concentration.
2. Endocytosis
• Phagocytosis
• Pinocytosis
• Receptor-Mediated Endocytosis
3. Exocytosis
Contractile Vacuoles
• Some species of protists
live in a hypotonic
environment (what will
happen to them??)
• Contractile vacuole – a
structure in some
protists that expels
water.
Contractile vacuole
Endocytosis and Exocytosis
• Endocytosis – the • Exocytosis – the cell
cell takes in
releases particles
particles
into its environment
Phagocytosis – How your immune
cells get rid of some pathogens
• Phagocytosis is a type
of endocytosis in which
a cell engulfs a large
particle.
• Your white blood cells
engulf bacteria and
then merge them with a
lysosome to digest
them.