Lesson 31 Cellular Transport (4.2.2)
MAINTAINING HOMEOSTASIS
Organisms are constantly synthesizing energy and using energy for every hierarchal level. Cells make energy by converting food nutrients to usable form. Glucose is broken down to form ATP. Organs store energy in their cells and then release energy when needed. For example, glucose is stored in the liver in the form of glycogen. Recall in lesson
PASSIVE TRANSPORT
Passive transport is spontaneous and does not require energy. In passive transport, molecules move spontaneously through the cell membrane from areas of higher concentration to areas of lower concentration; they are said to move “with the concentration gradient.” When a gradual change in the concentration of solutes occurs within a cell and its environment, then a concentration gradient exists. Cells are constantly trying to reach equilibrium which is the same concentration on both sides of the cell. The three types of passive transport are diffusion, facilitated diffusion and osmosis. In all three of these types of passive transport, substances move from an area of high concentration to low concentration. A good way to remember in passive transport substances move from high to low concentration is to think about riding a bike. If you were to ride a bike down a hill (high to low), you would be able to coast down without using any energy pedaling.
Diffusion is the process by which substances move directly through the cell membrane as shown in Figure 31.1. Facilitated diffusion involves the help of a carrier protein embedded in the phospholipid bilayer to move a substance such as iodine and gases (carbon dioxide and oxygen) from one side of the cell membrane to the other.
Osmosis is the movement of water from an area of high water concentration to an area of low water concentration though a semi-permeable membrane. Figure 31.2 shows osmosis. Think of osmosis as the diffusion of water. Osmosis can occur in either direction, depending on the concentration of dissolved material inside and outside the cell. Defining the solution concentrations relative to one another will predict the direction in which osmosis will occur.
A hypotonic solution has the lower concentration of solute; this may be thought of as a higher concentration of water. A hypertonic solution has a higher concentration of dissolved solute, which may be thought of as a lower concentration of water. If the solute concentrates are the same inside and outside the cell membrane, the solutions are said to be isotonic to each other. Diffusion of water (osmosis) across a cell membrane always occurs from hypotonic to hypertonic.
Table 31.1 Possible Results of Osmosis
Solution Type / Effect on Cell / Situation OutcomeIsotonic
Particle concentration the same outside and inside cell. / / No net movement of water
Hypotonic
Particle concentration lower in solution than in cell / / The net gain of water is great outside the cell, therefore water moves into the cell
Hypertonic
Particle concentration higher in solution than in cell / / Water leaves the cell because the concentration of water was greater inside the cell
**The terms hypotonic, hypertonic, and isotonic are not specifically mentioned in the standard course of study but may be taught for higher level thinking skills. The concept of the direction of water movement is required to learn in the standard course of study.
Look at figure 31.3. The cell contains 60% water while there is 30% surrounding the cell. Because water will move from an area of high concentration to an area of low concentration, water will move out of the cell. When water moves out of the cell, it causes the cell to shrink. This same concept is what causes your fingers and toes to shrivel when you swim in a pool or the ocean.
Look at figure 31.4 The cell contains 60% salt while there is 30% salt surrounding the cell. Does this mean water will move out of the cell as in Figure 31.3. When determining which direction water will move, you must make sure you are looking at the amount of water and not other substances. So, in this example we must figure out the amount of water in both the cell and surrounding the cell. You can assume the cell is 40% water because a percentage is out of 100 and 100-60 = 40%. There is 70% water outside the cell because 100-30 = 70%. Since the cell has 40% water and the outside environment is 70% water, water will move from the outside to the inside of the cell. As water moves in the cell, it will cause the cell to swell. For example, if a salt water plant was placed in fresh water, the plant cells would swell.
Kidney dialysis is an example of a medical procedure that involves diffusion. Another eample is food preserved by salting, surgar curing or pickling. All of these examples are methods of drawing water out of the cells through osmosis.
ACTIVE TRANSPORT
In some cases, the cell may need to move material across the cell membrane, against the concnetration gradient. To move material from low concentration to high concentration, the cell must expend energy. This process is called active transport. A good way to remember this is to, again, think about riding a bike. If you ride a bike up a hill (from low to high), it requires energy and you may even feel tired by the time you get to the top of the hill from peadling so hard.
Cells can use special proteins, called carrier proteins, embedded within the membrane during active transport. Each carrier proteins is specifically shaped to accommodate the type of molecule it transports. Carrier proteins use energy from ATP to physically change shape and move the molecule into the cell. Figure 31.5 shows the steps of active transport using carrier protiens.
Active transport is a mechanism that allows certain organisms to survive in their environments. For instance, sea gulls can drink salt water because their cells remove excess salt from their bodies through active transport. However, if ameobas were placed in a saltwater tank, they would become dyhdrated and die because their cells are unable to remove the salt fast enough. Another example of active tranpsort involves blood cells which use carrier protiens to transport molecules into the cell.
Lesson 31 Review: Cellular Transport
A. Define the following:
passive transport concentration gradient diffusion facilitated diffusion
osmosis hypotonic hypertonic isotonic
active transport
B. Choose the best answer.
1. The movement of substances into and out of a cell without the use of energy is called
A. active transport C. isotonic condition
B. passive transport D. facilitated diffusion
2. The movement of water across a semi-permeable membrane from an area of high water
concentration of an area of low water concentration is called:
A. active transport C. osmosis
B. diffusion D. hypotonic
3. If a freshwater fish is placed in salt water, what will most likely happen to the fish’s cells?
A. shrink C. remain the same
B. swell D. shrink then swell
4. If a red blood is placed in distilled water, what would happen to the size of the cell?
A. shrink C. remain the same
B. swell D. shrink then swell
5. The movement of substances against the concentration gradient is called
A. active transport C. passive transport
B. osmosis D. diffusion
C. complete the following activities
1. How does osmosis differ from diffusion?
2. Dried beans are soaked overnight in preparation for cooking. Explain the process affecting the
beans. What will happen to the dried beans?
3. Differentiate between active transport and passive transport.
4. A celery stalk is placed in a solution. It begins to wilt. What is likely the component in the water?