Part 1- Diffusion and Osmosis
I. The Cell Membrane
The cell membrane controls what moves into and
out of the cell
A. Composed of a lipid (fat) bilayer (two part layer)
made of molecules called phospholipids
B. The hydrophobic (“water hating”) tails of the
phospholipids move away from the water in the
cytoplasm, the hydrophillic (“water loving”) heads
are attracted to the water of the cytoplasm or
external environment of the cell
II. Proteins in the membrane
Proteins are embedded in the lipid bilayer
and have several functions:
A. They serve as cell markers to identify the
cell
B. They serve as transport proteins for
specific molecules
Proteins in the Membrane
III. Homeostasis
Homeostasis - maintaining stable conditions inside the
cell, even if external conditions are changing
A. Cells must maintain homeostasis or they will die
B. Examples: sweating/shivering or a room
thermostat
IV. Diffusion
Diffusion- movement of molecules from areas of
higher concentration to areas of lower
concentration (from where there are more to
where there are less) until the concentrations
are equal
A. diffusion naturally takes place due to Brownian
motion (random movement) of molecules
B. water, oxygen, and carbon dioxide are small
molecules and diffuse easily across the
Membrane
C. Examples: oxygen moving from lungs into red
blood cells, food molecules moving from the
small intestine into the blood stream, waste
molecules of urea moving from the blood into
the kidney
Diffusion
V. Osmosis and Isotonic Situations
A. Osmosis - movement of water molecules across
a membrane (such as a cell membrane or
artificial membrane)
B. Isotonic - (iso- “same”) the concentration of
dissolved substances (solute) is the same outside
the cell as inside the cell
Result: there is no net (overall) movement of
water molecules (an equilibrium)
VI. Hypertonic Situations and Plasmolysis
Hypertonic - (hyper- “above”) the concentration of
dissolved substances (solute) is greater outside
the cell than inside the cell
Result: more water molecules leave the cell than
enter the cell and the cell will shrink
A. Animal cells will shrivel up
B. Plant cell membranes will shrink and pull away
from the cell wall in plasmolysis
VI. Hypotonic Situations and Turgor Pressure
Hypotonic - (hypo- “below”) the concentration of
dissolved substances (solute) is lower outside the
cell than inside the cell
Result: more water molecules enter the cell than
leave the cell and the cell will swell
A. Animals cells will burst if the pressure is too great
B. In a plant cell the pressure pushes against the
cell wall and keeps the plant from wilting (turgor
pressure)
Part 2- Facilitated Diffusion
and Active Transport
Larger molecules such
as polysaccharides,
proteins, and lipids
cannot diffuse across
the membrane because
they are too big.
I. Passive Transport
Passive transport - transport which does not require
energy from the cell because molecules move
with the concentration gradient (from high to low
concentrations)
A.(Simple) Diffusion
B. Facilitated Diffusion - movement of molecules from
areas of higher concentration to areas of lower
concentration using protein channels
1. Each channel transports a specific type of
molecule such as sugar or sodium ions (Na+)
2. The channels may open in response to signals from
the cell
C. Because the cell membrane is controlling what
enters and leaves the cell, it is called semipermeable or selectively permeable
II. Active Transport
Active Transport - transport which does require
energy from the cell because molecules move
against the concentration gradient (from low to
high concentrations)
A. Transport proteins pump molecules (such as
Na+, K+, and Cl-) across the membrane
B. The energy comes from ATP (more on this later)
C. Example - the sodium-potassium pump
III. Transporting Large Molecules
A. Endocytosis - process of taking in molecules
from the environment
B. Phagocytosis - “cell eating” taking in solid
particles
C. Pinocytosis - “cell drinking” taking in drops of
liquid
D. Exocytosis - process of getting rid of molecules
by merging a vacuole or vessicle with the cell
membrane
The End!