Movement Into and Out of a Cell Directions: Read, Highlight at least 1 per paragraph, and answer questions Because a cell is a living thing it must do all the processes that any organism must do: grow and develop, gain and use energy, be made of cells, adapt, respond and reproduce. In order to accomplish these things the cell must be able to get molecules of food, water, waste into and out of the cell. Water is usually involved due to the fact that a cell is mostly composed of water. It really does not matter what kind of cell: every cell needs to get water and the things that are dissolved in water into and out of it. There are a variety of ways that cells accomplish this movement of substances. One method is diffusion. ​
Diffusion ​
is the movement of molecules from an area of high concentration to an area of lower concentration. In other words, molecules move from where there are a lot of them (concentrated) to where there are fewer (dilute). This happens in nature all the time. The smell of a skunk will spread through the air so that it is smelled miles away from the source. A drop of dye placed into a bottle of water will spread—without stirring—to become evenly mixed throughout the bottle. Water melting from an ice cube in a glass of soda will spread into the soda to make it watery. The common piece to each of these examples is the movement of the molecules that make up the substance from where there are a lot of them to where there are not as many of them: the molecules spread out. How do molecules move like this? Atoms and molecules are constantly moving, some quickly and some slowly. This constant motion is called ​
Brownian motion​
. If we were able see individual molecules we would see that they are constantly bouncing off other molecules. It’s almost as though molecules were ping pong balls, constantly bouncing in different directions. Of course, how fast they move depends on the size and temperature of the molecules. Usually smaller or warmer molecules will travel faster than larger or colder molecules. The smell of a skunk travels because the molecules of skunk scent bounce off other air molecules, spreading out through the air each time a molecule is bumped by another. Of course, if it is a hot day, this process will happen a good deal faster than it will on a cold day!! Because of their motion, molecules tend to spread out across what is called a ​
concentration gradient​
. The word gradient refers to a gradual change from where there is a lot of a substance to where there is a little of it: it might help to think of this as a hill. The bigger the difference between the two ends of the scale the faster the movement of the concentrated molecules (where there are a lot) to the dilute area (where there are few). This movement will generally continue until an equilibrium is reached. ​
Equilibrium ​
is reached when the concentration of a substance is the same throughout a given space. Consider the example of the dye being added to a container of water. a. drop of dye added to the beaker; dye molecules are concentrated b. dye molecules beginning spread throughout the beaker c. equilibrium reached between the water and the dye Think back to our cell factory. Remember that not everything could get in and out. Our cells are just like that letting only certain substances through. Remember those bad lego designs, when they just don’t get sent out into the body. The cell membrane controls this by not letting the bad stuff out or other bad stuff in. When only certain materials can pass through our cells, we call this semipermeable​
. Since our cells are semi­permeable, the cell membrane controls what can get in and what goes out. This is a great protective feature since this keeps unwanted organisms out of your cell that might infect them (and you). If it weren’t semi­permeable, then everything gets in and everything can get out. 1. The movement of particles from areas of high concentration to areas of low concentration is called ________________________. 2. The fluids that surround and fill a cell are made mostly of __________________. 3. When a membrane only allows certain substances through, it is called _________________. 4. Materials move in and out of a cell through its ____________________. 5. Materials move because of a difference in ______________________ gradients. 6. Why would a cell need diffusion to happen? Cells must move substances across their membranes in order to survive. In some cases a cell must use energy to accomplish this; in others, the cell does not need to use energy. Passive transport is a type of movement across a cell membrane that relies on the ordinary movement of molecules: the cell does not need to use its energy to make it happen. Because molecules are in constant motion some molecules will encounter the cell membrane and, if the molecule is small enough, it will be able to pass through the semipermeable cell membrane. Permeable means that substances can get through: since cell membranes are semi­permeable​
, they only let certain things through. Water is one of the molecules that can pass freely through a cell membrane. The process of diffusion of water through a semi­permeable membrane is called ​
osmosis​
. An example of this can be seen in plants that have wilted from lack of water: water moves into the cells of the plant after it moves through the plant because there is a higher concentration of water outside the cell than in. Once the water moves into the cells, the cells expand and the plant expands to its normal size and shape. This example of passive transport happens due to the ordinary movement of molecules and their tendency to down the concentration gradient: a cell does not have work at—or use energy—to get water into the cell because water molecules will continue to move through the semi­permeable cell membrane until there is an equal concentration of water molecules both outside and inside the cell. Of course, once equilibrium is reached the movement of water across the membrane continues, but the movement into and out of the cell happen at an equal rate. Whether water diffuses into or out of a cell is dependent on the liquid that surrounds the cell. There are three different relationships that can exist between the concentration of the cytoplasm and the liquid outside a cell: hypertonic, hypotonic and isotonic. The following table, which comes from Biology: Principles and Explorations (2001, p. 77, Johnson, G.B., and Raven, P.H.), summarizes what each state is and the effect the relationship will have on a cell.