IV. Cell Transport
A. Diffusion Through Cell Boundaries
*In solution, particles move constantly,
collide, and spread out randomly.
Diffusion - process by which
molecules move from high
concentration to lower
concentration.
-Once the concentration is the
same throughout a system,
equilibrium is reached, but
molecular movement doesn’t stop.
-Random movement lets substances
diffuse across cell membranes without
using a cell’s energy, a process called
passive transport .
B. Osmosis
Osmosis – the diffusion of
water through a selectively
permeable membrane.
-Water will move from an area
of high water concentration to
an area of low water
concentration, Ex : if a beaker
has many sugar molecules on
one side of a semi-permeable
membrane, water will move
towards the sugar until it
reaches equilibrium.
Movement of water can create 3 cell conditions :
1. Hypertonic cells – cell’s environment has
a higher concentration of solutes than
inside the cell. “Hyper” meaning higher.
Result : Water rushes out of cell & into
the environment, causing the cell to
shrivel (plasmolysis).
2. Isotonic cells – cell’s environment
has the same concentration of solutes
as inside the cell. “Iso” meaning same.
Result : Perfect equilibrium, with no
movement of water in or out of the cell.
3. Hypotonic cells – cell’s environment has a
lower concentration of solutes than inside
the cell. “Hypo” meaning lower.
Result : Water rushes into the cell, causing it
to swell & experience lysis
(explosion/breaking apart).
C. Facilitated Diffusion
Facilitated Diffusion –
diffusion of certain
molecules across cell
membranes via protein
channels, ex : glucose
moving into red blood cells.
-Is like a short-cut. Glucose
can’t diffuse across the lipid
bilayer on its own, so it
moves through a protein
channel instead (see pg. 209).
The process uses no energy.
D. Active Transport
*Sometimes cells must
move materials in the
opposite direction,
against a concentration
difference. This is done
by active transport.
Active transport –
energy-using process
that moves materials
across a cell membrane
against a concentration
difference.
-Is done by transport
proteins found in the
membrane itself.
Active vs. Passive Transport
Types of Active Molecular Transport
*Larger molecules are
transported across the cell
membrane by 2 processes :
1. Exocytosis – packaging &
removal of wastes from the cell.
The membrane of a vacuole
containing waste fuses with the
cell membrane and forces the
contents out of the cell.
2. Endocytosis – the process
of taking outside material
into the cell by means of
infoldings, or pockets, of
the cell membrane.
Overview of Endo & Exocytosis
2 Types of Endocytosis
A. Pinocytosis – “Cellular
drinking”. Is when a cell
takes in liquid from the
surrounding environment.
B. Phagocytosis – “Cellular
eating”. Is when a cell
engulfs an outside food
particle or some other solid
substance, ex ; amoebas &
white blood cells.