Cells are involved in countless activities. Like you, individual cells take in food, oxygen, and water. They must also be able to get rid of wastes and protect themselves from harmful substances. The cell membrane plays an important roleóit controls what substances can come into and go out of the cell.
Particle Movement
In the next activity you can observe some sweetened drink powder going from the bottom to the top in a glass of water. The change in colour and taste will show that particles can move on their own in water.
Not only will you learn about particle movement, but you will also end up with a refreshing treat.
Find Out Activity
Beverage Science
There's a lot to be discovered in a beverage.
Materials
Procedure
Step 1: Pour a cup of water at room temperature into a narrow beverage glass or jar. Allow the water to settle for 10 minutes.
Step 2: Gently pour two heaping tablespoons of sweetened drink powder into the water. Do not stir the mixture or move the container.
Step 3: Use your tablespoon or measuring spoon to carefully dip some water from the surface of the liquid and taste it. Observe the colour of the liquid and the amount of drink powder at the bottom of the tumbler.
Step 4: Repeat Step 3 every half-hour until all of the drink powder disappears from the bottom of the container and no further observable changes take place. Be very careful not to stir the mixture or move the container!
What Did You Find Out?
1. How did the taste and colour of the liquid at the top of the container change over time?
2. Based on your observations, did the sugar, flavour, and colour particles move from lower to higher concentrations or from higher to lower concentrations?
3. Sketch a diagram like the following.
Particle Movement
The movement of particles you observed in "Find Out Activity: Beverage Science" was due to diffusion. The drink-mix particles drifted from areas of high concentration to areas of low concentration.
In the activity the drink-mix powder was the solute. It was the solute particles that diffused within the water.
There is a special kind of diffusion called osmosisóit's based on the diffusion of water particles. Osmosis requires a special membrane to separate the areas of high and low concentration of a solute in water. With such a membrane in place, the water particles move rather than the solute.
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You can think of the special membrane as having many little gates in it. The gates are large enough to let water particles through, but too small to let particles of solute through. Such a membrane is a selectively permeable membrane.
Think back to when you, or someone in your family, last made a cup of tea from a tea bag. Water moving into the tea bag causes it to swell. You can think of this as osmosis across the tea-bag paper. The paper is a selectively permeable membrane that keeps the tea leaves from going outside the bag but lets water come in freely.
Besides osmosis, there are other things happening as the tea steeps or soaks. Once the tea bag sits in the water for a few seconds, the water dissolves the tea flavour in the tea leaves and the flavour then diffuses throughout the water in the cup. The tea-bag paper is not a barrier for the flavour particles. These particles are too small to be held back - they easily pass through the gates or pores of the tea-bag paper.
Diffusion and Osmosis in a Teacup
To gain a more complete picture of how particles move by diffusion and osmosis, read pages 128 to 131 of the textbook. Then, in your notebook, answer the following questions.
4. Imagine that you are pouring a mixture of water and marbles into three bags. Classify these bags as permeable, impermeable, or selectively permeable to your mixture. Give reasons for your answers.
a. a plastic bag
b. a cheesecloth bag
c. a bag made of a single-layered basketball net
5. On page 8 of the textbook are the five key ideas of the particle model of matter. Which three key ideas of this model best explain diffusion?
6. There are factors limiting the size of cells.
a. Use the information in "Off the Wall" on page 129 of the textbook to explain why the maximum size of a cell is limited.
b. How have large organisms overcome this cell-size limitation?
7. Do questions 1 to 3 from "Topic 4 Review" on page 137 of the textbook.
In the next investigation you will use a chicken egg to study osmosis. You will use a chicken egg because there is a selectively permeable membrane just underneath the shell. All you have to do is remove the shell to expose this membrane.
You can easily remove the shell by placing the whole egg in vinegar. The acidic vinegar will react with the calcium-rich shell. In the reaction the shell is dissolved and the membrane is exposed.
I n v e s t i g a t i o n 2F
Measuring Osmosis
Refer to the "Inquiry Investigation" on pages 132 and 133 of the textbook.
Prepare a salt solution by mixing about three tablespoons of salt with 200 mL of water. Follow the steps of "Procedure." Remember to make written notes for the steps that have a photo of the pencil beside them.
If you don't have distilled water you may use tap water.
If you don't have access to a triple-beam balance, do your weighing with a spring scale. Use the mass readings indicated in grams. Weigh an egg by placing it in a baggie as shown in the photo. To get the weight of just the egg, subtract the weight of the empty baggie if it registers on the scale.
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8. Complete questions 2 to 4 from "Analyze" and "Conclude and Apply" on page 133.
Fluid Movement in Plants
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right: PhotoDisc Collection/Getty Images
For information on fluid movement in plants, read the words and study the diagrams on textbook pages 134 and 135, and read the top of page 137. Also, carefully examine the diagram at the top of page 136. Answer the following related questions.
9. Why do plants need a large supply of water?
10. Write definitions for each of the following plant-structure terms: vascular tissue, phloem tissue, xylem tissue, and root hairs.
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11. The cells in a tree trunk are arranged in concentric circles in order from the outside to the centre. This is the order:
Sort them into two groups of "living cells" and "dead cells."
12. What familiar object does xylem tissue resemble? Why?
13. Why are leaves usually thin and flat?
14. Transpiration is an extremely important part of the environmental "water cycle." If plants did not transpire, water would sink deep into the soil. The amount of water in the atmosphere would drop severely, creating an extreme worldwide drought. How is transpiration different from the evaporation of water?
15. Two processes and actions transport water to the tops of plants - even very tall trees. Water gets to the top without a pump like a heart. What two processes and actions are involved in the upward movement of water in plants?
16. Draw a flow diagram of the movement of water to the atmosphere after the water flows from the soil and through a plant.
Transpiration puts water into the atmosphere. That is why transpiration is an important part of the water cycle.
Transpiration by a plant is also important to the plant itself. The next activity will show this and outline how leaves are involved in transpiration.
Find Out Activity
Transpiration and Leaves
Refer to the activity on page 136 of the textbook.
Follow the steps in "Procedure." Only make cuts with your teacher or home instructor present to guide you.
17. Complete question 1 of "What Did You Find Out?"
18. Do questions 6 and 7 from "Topic 4 Review" on page 137 of the textbook.
Looking Back
In this lesson you identified the needs of cells by relating these needs to your own need to breathe, eat, and drink. Through experimentation, you investigated diffusion and osmosis. You also saw that fluid transport is important for cells.
Turn to Assignment Booklet 2A. Complete questions 1 to 6 from Section 2.
1. The colour and taste both became more intense over time.
2. All particles moved from an area of higher concentration, with lots of particles, to an area of lower concentration, with no or fewer particles.
3. Your diagram should be similar to the following one.
Particle Movement
4. a. The plastic bag would be impermeable to the mixture.
b. The cheesecloth bag would be selectively permeable because it would let the water through but prevent the movement of the marbles.
c. The basketball net bag would be permeable to the water and marble mixture because the entire mixture would pass through the bag.
5. The following are the three key ideas that explain diffusion:
6. a. If a cell is too large, it would take too long for materials to move from outside the cell into the centre of the cell.
b. Large organisms are made up of many cells, rather than just one cell.
7. Textbook questions 1 to 3 from "Topic 4 Review," page 137:
1. Osmosis causes water to enter or leave a cell.
2. Both osmosis and diffusion result in a substance moving. The movement is from an area of higher concentration to an area of lower concentration.
Diffusion can refer to the movement of almost any substance, while osmosis refers only to the movement of water.
Osmosis involves a selectively permeable membrane - diffusion may not involve a membrane at all.
3. The ammonia particles have diffused through the air from the area of higher concentration to the area of lower concentration.
8. Textbook questions 2 to 4 from "Analyze" and "Conclude and Apply," page 133:
2. The variables held constant are the
The variable purposely changed is the concentration of the salt in the liquids.
3. (a) If the mass of the egg increased, the volume of liquid in the jar decreased.
(b) If the mass of the egg decreased, the volume of liquid in the jar increased.
(c) In plain water the mass of the eggs increased. The increase in mass was due to the movement of water into the egg by osmosis. With water moving into the egg, less water would remain outside the egg.
In salt water the mass of the egg decreased. This decrease was due to the movement of water out of the egg by osmosis. With water moving out of the egg, the volume of salt water outside the egg would increase.
4. For the egg placed in plain water, the water concentration inside the egg is less than it is in the plain water. The movement of water across the selectively permeable membrane will be toward the region of lower water concentration inside the egg.
For the egg placed in a salt solution, the water concentration of the salt solution outside the egg is lower than the water concentration inside the egg. The movement of water across the selectively permeable membrane will be toward the region of lower water concentration outside the egg. Water will leave the region of higher water concentration inside the egg.
9. Plants need a lot of water to make food through the process of photosynthesis.
10. Note the following definitions:
11. Phloem, cambium, and sapwood are living cells, while bark is composed of dead cells.
Note: Cambium is a very thin tissue that produces xylem and phloem. Sapwood is the pale wood just under the bark. It is the xylem of a tree. At the centre of a tree is heartwood, which is composed of dead cells. Heartwood is usually darker and more durable than the surrounding sapwood.
12. It resembles a straw because the tube-shaped cells have thick walls with holes in their ends. They are stacked end to end and form bundles of hollow tubes.
13. They are usually thin and flat to increase the surface area for the absorption of light energy and the diffusion of gases into and out of the leaf. This allows photosynthesis to occur.
14. Transpiration is a special type of evaporation. Transpiration is the evaporation of water from leaves via the stomata.
15. Osmosis pushes water upward, while transpiration pulls water upward - the water particles are held together by bonds of attraction that make the plant's water network behave as a single unit, like a long string.
16. The flow diagram should include the following information.
soil → root hairs → xylem → leaf cells → stoma → atmosphere
17. Textbook question 1 from "What Did We Find Out?" page 136:
1. The coloured water rises farthest in the stalk with leaves. The presence of leaves increases the rate of transpiration. Another answer is that the presence of leaves makes transpiration possible. Note: In the stalk without leaves, there is no transpiration - there is only evaporation.
18. Textbook questions 6 and 7 from "Topic 4 Review," page 137:
6. Guard cells open and close the stomata to control the movement of water out of the leaves and the movement of carbon dioxide and oxygen into the leaves.
7. Spraying fresh vegetables with water keeps the vegetables crisp by decreasing evaporation from their cells and by replacing any water they lose.
diffusion: the movement of particles within a fluid from higher to lower concentrations
membrane: a thin, soft, pliable, porous layer of solid material especially of animal or plant origin
osmosis: the diffusion of a solvent - usually water - through a selectively permeable membrane
selectively permeable membrane: a membrane that lets particles through on the basis of particle size
transpiration: the evaporation of water from leaves via the stomata
water cycle: the process in which Earth's water moves between the oceans, the land, and the atmosphere