Cell Structures - In and Out
Materials move into and out of cells through either passive transport or active transport. Passive transport includes diffusion and osmosis. Molecules tend to move from crowded to less crowded in order to achieve a balance or to reach homeostasis. The cell membrane is selectively permeable which allows the movement of substances, especially oxygen, water, food molecules, carbon dioxide, and waste products, into or out of the cell.
Passive transport - movement of molecules from a more crowded to a less crowded area WITHOUT the use of energy. Movement occurs when there are unequal concentrations of a substance inside and outside of the cell. The two kinds of passive transport are:
Diffusion - movement of molecules from a region of higher concentration to a region of lower concentration.
Osmosis - diffusion of water through a membrane.
Active transport - movement of molecules from a less crowded to a more crowded area WITH the use of energy. Molecules are "carried" into or out of the cell using some of the cell's energy.
You know that diffusion is the movement of particles from areas of more concentration to an area of low concentration. Diffusion occurs because of the constant, random motion of particles. Osmosis is the diffusion of water into or out of a cell.
How can carrots demonstrate the process of osmosis?
- Two 400 mL beakers
- Measuring tape or meter stick
- Distilled water
- Balance scale
- Fill the two beakers with equal amounts of water.
- Add 15 g salt to one beaker and label it "Salt Water."
- Cut a carrot in half. Tightly tie a piece of string two centimeters below the cut end of both pieces.
- Place one carrot half (cut end down) in the "Salt Water" beaker. Place the other carrot with cut end down in the "Fresh Water" beaker. Allow carrots to remain undisturbed for 24 hours. At the conclusion of the 24 hour period, remove the carrots and observe them and the tightness of the strings. Record your data.
Safety concerns: Be sure to follow all glassware and chemical safety rules that are specified by your teacher in all general laboratory experiences. As with all science lab activities, the most important safety rule is to follow all teacher directions.
- In which kind of water did the thread become tighter around the carrot?
- In which kind of water did the carrot gain mass?
- Did the carrot cells gain water in fresh water or salt water?
- In which kind of water did the carrot cells lose water?
- What evidence supports your conclusion?
- What evidence did you use to determine this?