Cells and Their
Environment
Section 4-1
PASSIVE TRANSPORT
Review
• What is homeostasis?
• Cells maintain homeostasis by controlling the
movement of substances across the cell
membrane
Random Motion and Concentration
• Passive transport is movement across the cell
membrane that does not require energy
• Substances travel from areas of high
concentration to areas of low concentration
• The difference in the concentration of a
substance across the space is the
concentration gradient
• An substance of high concentration will move
into a lower concentration until it is in
equilibrium or equal
Movement of Substances
• Particles of a substance will move randomly
• If there is a concentration gradient (uneven
concentration) then diffusion occurs
• Diffusion is the movement of a substance
from an area of high concentration to low
caused by the random movement of particles
Movement of substances cont…
• Many molecules and ions dissolve in the
cytoplasm and outside of the cell
• The concentration of substances is different
inside and outside of the cell
• Molecules will diffuse, to equalize, and need
to pass through the cell membrane
• Selectively permeable
Movement of substances cont…
• The diffusion of molecules across the cell
membrane is the simplest type of passive
transport
Water Diffuses In and Out
• Water molecules are small and can diffuse
through the cell membrane easily
• Osmosis is the diffusion of water molecules
through a selectively permeable membrane
Osmosis
• Osmosis is a form of passive transport
• 3 possibilities of water movement
1. Water moves out
2. Water moves in
3. No net movement
Water Moves Out
• When water diffuses out of the cell, the cell
shrinks
• A solution that causes a cell to shrink is
hypertonic
Water Moves In
• When water moves into the cell, the cell
swells
• A solution that causes a cell to swell because
of osmosis is hypotonic
No Water Moves
• If the outside and inside of the cell have the
same concentration of water molecules, then
water diffuses in and out at the same rate
– No net movement
• A solution that produces no change is isotonic
Proteins Help Transport Substances
• Transport proteins provide passageways for
polar molecules and ions to pass through the
cell membrane
• Each channel only allows a certain substance
through
Diffusion Through Ion Channels
• Sodium Na+, Potassium K+, Ca 2+ and chlorine
Cl-, are important in cell functions
• Ion channels are a doughnut shaped transport
protein for ions like Na+ to pass through
Diffusion Through Ion Channels
• Some pores are always open, others are
closed by gates
• The rate of movement of a substance across
the membrane is determined by the
concentration gradient
• An ions movement across the membrane is
determined by its charge
• The inside of a cell is usually negative
Facilitated Diffusion
• Carrier proteins carry specific substances
across the membrane
• They carry substances down their
concentration gradient
• Facilitated diffusion uses no energy
Facilitated Diffusion
• Step 1: molecule outside of cell binds to
carrier protein
• Step 2: carrier protein transports molecule
across membrane
• Step 3: molecule is released in the cell