Cell Processes: Moving Molecules in and out of Cells
Why Must Cells Transport Molecules? Cells use four processes to allow
molecules to pass through the cell membrane: diffusion, osmosis, transport proteins, and
engulfing. Why does a cell need to allow molecules to enter and exit through the cell
membrane? Every cell in a human body works to maintain homeostasis, the process
through which the cell maintains a “constant internal environment” (Gallagher and
Tanner 3). All activities of a cell are tightly regulated and designed to keep the cell
balanced and stable (Gallagher and Tanner 3). To maintain homeostasis cells must allow
important chemicals and molecules to enter and exit. The cell must also work to prevent
harmful chemicals from entering.
Diffusion Diffusion is a process that cells use to allow
small molecules to automatically pass through the cell
membrane from areas of high concentration to low
concentration. For example, if there is a high
concentration of oxygen molecules outside the cell and a
low concentration inside the cell, the oxygen molecules
will naturally be absorbed by the cell. Common molecules
that diffuse through the cell membrane include oxygen,
carbon dioxide, and lipids (YVCC.edu). In the human
body, oxygen must diffuse into the bloodstream for use by
cells and carbon dioxide waste must be removed
(Bukwirwa and Law 1). This is the essential life process known as breathing. The process
of diffusion was represented in class when air freshener molecules in the highly
pressurized can automatically spread throughout the classroom.
Osmosis A specialized type of diffusion that involves
only water is called osmosis. Water is an essential
chemical of life – cells cannot perform their functions
without water. Therefore the diffusion of water has been
given a specific name: osmosis (Coolidge-Stolz, et al.
82). Water molecules can automatically pass through
the cell membrane via osmosis because they are very
small in size compared to other molecules such as
complex proteins molecules (McGraw-Hill). Osmosis
was demonstrated in class when the dry sponge automatically absorbed water from the
dish. Water moved from an area of high concentration in the dish into an area of low
concentration in the dry sponge.
PASSIVE TRANSPORT
vs.
ACTIVE TRANSPORT
Diffusion
- The movement of small particles across the cell
membrane
- Particles move from areas of high concentration to low
concentration
- Small particles are substances like CO2 and O2
Transport proteins
- Located in the cell membrane
- ‘Pick-up’ particles and push them through the cell
membrane
- Used to push particles from an area of low
concentration to an area of high concentration
Osmosis
- The diffusion of water across the cell membrane
- Water moves from an area of high concentration to low
concentration
Engulfing
- Used to push large particles through the cell membrane
- The cell membrane surrounds a particle and pinches off
and brings it into the cell
- Lysosomes surround wastes and join with the cell
membrane to push them out of the cell
-
The cell does not use energy for diffusion or osmosis
Particles move from an area of higher concentration to
lower concentration
EX: Air freshener leaving the can (high concentration)
and spreading throughout the room (low concentration)
EX: A dry sponge (low concentration) soaking up water
from a dish (high concentration)
High concentration  low concentration
(glass of water)
(dry sponge)
-
The cell must use energy for these processes
Involves the movement of large particles through the
cell membrane (engulfing)
Involves the movement of particles from an area of low
concentration to high concentration (transport proteins)
EX: Pushing air freshener back into the can
EX: Wringing water out of a sponge into a dish of water
Engulfing
(surrounding)
Transport proteins
(pick-up and move particles)