Transport through the Cell Membrane
Passive Transport: the movement of materials across a cell
membrane without expending energy
There are two types of passive transport:
1) Diffusion
2) Osmosis
Diffusion
- Brownian motion: molecules move in all directions with
equal frequency, bouncing off each other when they
collide.
o This is why molecules concentrate in one area and
then spread outward
-
Diffusion: movement of molecules from an area of high
concentration [ ] to an area of lower concentration [ ]
o Caused by molecular collision
o Affected by [ ], temperature, and pressure and will
occur until molecules are equally distributed within
an area (equilibrium)
o O2 and CO2 move across all membranes by
diffusion
 O2 diffuses from the blood into the cell
 CO2 diffuses from the cell to the blood
-
Osmosis: the diffusion of water through a selectively
permeable membrane (lets some things in)
o The movement of water into and out of living cells
is vital to life. Ideally, cells are bathed in isotonic
solutions where the solute [ ] outside the cell is
equal to that inside the cell. Water movement into
the cell = water movement out of the cell.
-
-
Hypotonic Solutions – have a lower concentration of
solute outside the cell relative to the solute
concentration inside the cell
o Water molecules move from the area of high H2O
concentration (inside the cell) into the area of
lower H2O concentration (outside the cell)
Hypertonic Solutions – greater concentration of solutes
outside of the cell than inside the cell so the solutes
outside the cell create an osmotic force and draw water
into the cell.
-
Facilitated Diffusion
o Protein carrier molecules can aid in passive
transport. It is believed that protein carriers
speed up the movement of molecules already
moving across the cell membrane. Each protein
must be specialized in the diffusion of glucose
molecules, but not other sugars
-
Active Transport
o Active transport is the movement of material from
an area of low concentration to an area of higher
concentration
o Some materials are transported against the
concentration gradient, so it takes energy
o Without active transport, your kidneys would fail
to absorb needed materials, muscles would not
contract, and your nerves would not carry impulses
Endocytosis and Exocytosis
Endocytosis
- process by which cells engulf large particles by extending
their cytoplasm around the particle.
- Two cell membranes come together and the ingested
particle gets trapped in the vacuole.
- Enzymes are then used to digest the large molecules
-
2 types of endocytosis:
o Pinocytosis – cells absorb liquid droplets by
engulfing them
 Ex: your small intestine engulfs fat droplets
o Phagocytosis – process by which cells engulf solid
particles
Exocytosis
- large molecules in the cell are transported to the
external environment
- this is how waste materials are released from the cell