Concept Development: Selectively Permeable Membranes
During the medieval era, castles dotted the landscape of Europe. The tall walls of these castles separated the quarters
and people on the inside from the hostile environment outside. At the gates of these castles, the kings and queens would
place guards. These guards would check visitors coming into the castle for weapons or if they were spies. Friendly
visitors would be allowed into the castle. However, those that were unfriendly would be turned away. A castle is similar
to a cell. All cells have a membrane (similar to a castle wall) which separates the contents of the cell from the
outside environment. Materials that enter and leave the cell must be "checked" at the cell membrane. Like the
castle guards, the membrane allows only certain materials into and out of the cell.
There are 3 types of membranes:
______________________permeable membrane: A membrane that allows only certain materials to cross it.
______________________membrane: A membrane that allows all materials to cross it.
______________________membrane: A membrane that allows no materials to cross it.
____________________: (PASSIVE TRANSPORT)
Materials move across a membrane by a process called______________. Diffusion, is basically a spreading-out
process. For example, if you were to place a drop of ink in a beaker of water, what would happen? The ink particles
would end up spreading out eventually becoming evenly spread out throughout the container of water. After this point,
the ink particles would continue to move, however, the ink would remain evenly distributed throughout the water. Let’s
apply this concept to a cell. An amoeba living in water typically has the same concentration of dissolved carbon dioxide
gas in its cytoplasm as outside the cell in the water. Carbon dioxide particles are moving in and out of the cell at the
same rate, through tiny openings in the membrane. If the cell begins an activity that increases its production of carbon
dioxide, there will be a higher concentration of this gas in the cytoplasm than in the water. The carbon dioxide particles
will begin to diffuse out of the cell at a higher rate than carbon dioxide entering the cell. This higher rate will continue until
the carbon dioxide concentrations on both sides of the membrane are equal. (Equilibrium)
______________________:(PASSIVE TRANSPORT)
Osmosis is the diffusion of _________________through a selectively permeable membrane. If you were place a
slightly wilted celery stick in water, you would find that after 24 hours the celery would not be wilted any longer. Water
particles moved from the water into the celery cells by osmosis. What if you placed a fresh celery stick in a salt
water solution? After 24 hours you would find the celery to have wilted. The water moved from an area of high
concentration (inside the celery) to an area of low concentration (salt water). Water tends to move by osmosis from a
dilute solution to a more concentrated solution.
______________________Transport:
Small particles such as water, carbon dioxide, and oxygen move in and out of cells by diffusion. Larger molecules, such
as glucose which is used up as energy by the cell, must move by a process known as active transport. If diffusion is
like coasting your bike down a hill, then active transport is like using energy to pedal your bike up a hill. Cells need
large amounts of glucose to be used as energy. The cells need to move the glucose from an area of low concentration
(outside the cell) to an area of high concentration (inside the cytoplasm). This is the opposite of diffusion. This
controlled movement (active transport) of glucose through the cell membrane takes energy and is controlled by
proteins embedded in the membrane. The proteins are called carrier proteins. The carrier proteins act like the guards in
front of the castle gates, they only allow certain things through the gate.
________________________________(within the cell)- The cell membrane surrounds a particle and encloses it IN a
vesicle.
_________________________________(outside the cell)-The process of removing a large particle from the cell.
(LEAVE the cell)