Grade 8 Science Chapter 5 Review Answer

Science Chapter 5 Review Answers

1. “Selectively permeable” means only certain things can pass through. In a cell membrane, proteins act as gatekeepers, allowing some particles to pass through, but not others. The layers of fat particles making up the membrane allow some small particles to diffuse through directly, but prevent larger particles from doing so.

 2. Particles diffuse from an area where they are more concentrated to an area where they are less concentrated.

3. Without a concentration gradient, diffusion would not occur. The concentration gradient allows particles to move from an area where they are more concentrated to an area where they are less concentrated.

4. Oxygen and carbon dioxide are exchanged across the cell membranes of red blood cells and muscle cells.

5. Particles of water, oxygen, and carbon dioxide are all small enough to diffuse freely across the cell membrane.

6. Osmosis is a type of diffusion in which water moves across a membrane. The more solute particles you have on one side of the membrane, the greater the concentration gradient. Water particles will move from the side of the membrane where they are more concentrated (and the solute particles are less concentrated) to the side where they are less concentrated (and the solute particles are more concentrated).

7. A turgid plant cell is one that is swollen with water.

8. A cell might use exocytosis to remove wastes from the cell or to send chemicals out of the cell that are needed elsewhere in the body.

9. When a living thing takes in nutrients from its environment, we say it is eating. Phagocytosis, which means “cell eating,” is a good name for the type of exocytosis in which the cell takes in large particles from its environment because many cells use the process to take in nutrients.

10. Sample answer: An air freshener is a good model of diffusion because the scent particles are initially concentrated around the freshener and gradually move by diffusion through the rest of the room where the scent particles are less concentrated. An open perfume bottle or a drop of food coloring in a glass of water would be other good models for diffusion.

11. In the picture some dye has been placed in a beaker of water. The dye moves from areas of high dye concentration to lower dye concentration. Over time, the dye will spread evenly throughout the beaker.

12. Water moves by osmosis into a cell because the concentration of water is greater outside the cell than inside the cell. As water enters the cell, the central vacuole fills and the cell swells. As the cell swells, the cell membrane pushes outward (against the cell wall in plants), resulting in turgor pressure.

13. Osmosis is a form of diffusion that takes place across a membrane. In both processes, particles move down a concentration gradient from an area where they are more concentrated to an area where they are less concentrated.

14. An animal cell has a greater concentration of solutes (and a lower concentration of water particles) than distilled water. If you placed an animal cell into a beaker of distilled water, water particles would move by osmosis from where they are more concentrated in the beaker to where they are less concentrated in the cell. The cell would swell up and might die.

15. Water that moves into a plant cell causes the cell to swell, pushing the cell membrane up against the cell wall. The rigid cell wall keeps the cell from bursting.

16. Applying too much fertilizer to the grass will create a concentration gradient between the solution in the soil and the solution inside the plant cells. The concentration of solute outside the plant cells will be greater than that inside and water will leave the plant cells by osmosis. The plant cells will shrink and cause the plants to wilt or even die.

17. Patients receive a salt solution by intravenous injection to match the concentration of the fluids inside cells. If doctors injected plain water, there would be a higher concentration of solutes inside the blood cells, water would enter by osmosis, and the blood cells would burst.

18. The cells of plants from the ocean have a concentration of solutes inside that is equal to the concentration of solutes in sea water. If these plants are placed in fresh water, water will enter the plant cells by osmosis and will likely damage the plant cells. This may be the reason why the saltwater plants do not thrive in freshwater environments.

19. The celery wilted because some of the water inside the cells left the cells by osmosis and evaporated. Mary placed the wilted celery into a glass of water so that water would enter the cells of the celery by osmosis.

24. My strategy would be to ask questions about diffusion and answer them with drawings. For example, I could ask what causes the particles to move, or do the particles ever stop moving? My drawing could show the fewer particles calling the greater particles to join them. Some limitations are that I would have to be able to answer the questions to draw the answers, and also that the drawing would only approximate reality.