Passive Transport
Small molecules can pass through the plasma membrane through a process called diffusion.
Diffusionis the movement of molecules from an area where there is a higher concentration
(larger amount) of the substance to an area where there is a lower concentration (lower
amount) of the substance. The amount of a substance in relation to the volume, is called
concentration. Diffusion requires no energy input from the cell (Figure4.2). Diffusion
occurs by the random movement of molecules; molecules move in both directions (into and
out of the cell), but there is a greater movement from an area of higher concentration towards
an area of lower concentration. The movement of the substance from a greater concentration
to a lesser concentration is referred to as moving down the concentration gradient. For
example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because
oxygen is more concentrated in your lungs than in your blood. Oxygen moves down the
concentration gradient from your lungs into your bloodstream
Figure 4.2: Diffusion across a membrane does not require an input of energy. ( 2 )The diffusion of water across a membrane due to concentration differences is calledosmosis.
If a cell is placed in ahypotonic solution,meaning the solution has a lower concentration
of dissolved material than what is inside the cell, water will move into the cell. This causes
the cell to swell, and it may even burst. Organisms that live in fresh water, which is a
hypotonic solution, have to prevent too much water from coming into their cells. Freshwater
fish excrete a large volume of dilute urine to rid their bodies of excess water.
If a cell is placed in ahypertonic solution,meaning there is more dissolved material in
the outside environment than in the cell, water will leave the cell. That can cause a cell to
shrink and shrivel. Marine animals live in salt water, which is a hypertonic environment;
there is more salt in the water than in their cells. To prevent losing too much water from
their bodies, these animals intake large quantities of salt water and secrete salt by active
transport, which will be discussed later in this lesson.
To keep cells intact, they need to be placed in anisotonic solution,a solution in which the
amount of dissolved material is equal both inside and outside the cell. Therefore, there is
no net movement of water into or out of the cell. Water still flows in both directions, but an