2.7 passive transport - manning's science€¦ · three types of passive transport: diffusion,...

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50 Chapter 2 1. In what ways does a cell membrane differ from a cell wall? Sections 2.4–2.6 Questions passive transport: the movement of materials across a cell membrane without the use of energy from the cell Brownian motion: the random move- ment of molecules diffusion: the movement of molecules from an area of higher concentration to an area of lower concentration 2.7 Passive Transport Cells must take in food and eliminate wastes to maintain a constant internal envi- ronment. The movement of materials across a cell membrane without the expen- diture of cell energy is called passive transport. The following section discusses three types of passive transport: diffusion, osmosis, and facilitated diffusion. Diffusion Molecules in a solution or gas move about randomly and collide. This random movement is referred to as Brownian motion. Molecules move in all directions with equal frequency, bouncing off each other when they collide. This causes mol- ecules to move from an area of high concentration to an area of lower concentra- tion. (Concentration can be described as the number of molecules per unit volume.) This process is called diffusion. You can experience diffusion simply by opening a perfume bottle. The molecules inside the perfume bottle move into the surrounding air. Diffusion is not confined to gases, as you can see if you drop ink into a glass of water and watch the colour of the water gradually change (Figure 1). Water molecules collide with the molecules of ink, spreading them apart. The water molecules alone are not responsible for diffusion, however. The ink molecules are also colliding with water molecules and other ink molecules. Molecular collisions cause diffusion. Can you predict how an increase in water temperature would affect the rate of diffusion? The faster the molecules move, the more often they collide and the faster they will move apart. In other words, diffusion rates increase with temperature. In gas phase, pressure increases the frequency of particle collisions. Molecules are bunched close together if the pressure is high. Molecules in high-pressure areas will collide more frequently. Molecules in areas of lower pressure are spread out and collide less frequently. Gas molecules move by diffusion from areas of high pressure to areas of low pressure.

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Page 1: 2.7 Passive Transport - Manning's Science€¦ · three types of passive transport: diffusion, osmosis, ... on Potato Strips ... (b) Red blood cells in hypertonic solution (c)

50 Chapter 2

Understanding Concepts1. In what ways does a cell membrane differ from a cell wall?2. What is the relationship between

(a) nucleic acids and proteins?(b) proteins and a cell membrane?

Sections 2.4–2.6 Questions

Figure 1Ink diffusing in water over time

passive transport: the movement ofmaterials across a cell membrane withoutthe use of energy from the cell

Brownian motion: the random move-ment of molecules

diffusion: the movement of molecules froman area of higher concentration to an area oflower concentration

2.7 Passive TransportCells must take in food and eliminate wastes to maintain a constant internal envi-ronment. The movement of materials across a cell membrane without the expen-diture of cell energy is called passive transport. The following section discussesthree types of passive transport: diffusion, osmosis, and facilitated diffusion.

DiffusionMolecules in a solution or gas move about randomly and collide. This randommovement is referred to as Brownian motion. Molecules move in all directionswith equal frequency, bouncing off each other when they collide. This causes mol-ecules to move from an area of high concentration to an area of lower concentra-tion. (Concentration can be described as the number of molecules per unitvolume.) This process is called diffusion. You can experience diffusion simply byopening a perfume bottle. The molecules inside the perfume bottle move into thesurrounding air. Diffusion is not confined to gases, as you can see if you drop inkinto a glass of water and watch the colour of the water gradually change (Figure 1).Water molecules collide with the molecules of ink, spreading them apart. The watermolecules alone are not responsible for diffusion, however. The ink molecules arealso colliding with water molecules and other ink molecules.

Molecular collisions cause diffusion. Can you predict how an increase inwater temperature would affect the rate of diffusion? The faster the moleculesmove, the more often they collide and the faster they will move apart. In otherwords, diffusion rates increase with temperature. In gas phase, pressure increasesthe frequency of particle collisions. Molecules are bunched close together if thepressure is high. Molecules in high-pressure areas will collide more frequently.Molecules in areas of lower pressure are spread out and collide less frequently. Gasmolecules move by diffusion from areas of high pressure to areas of low pressure.

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Chemistry of Life 51

2.7

To summarize, diffusion is affected by concentration, temperature, and pres-sure and will continue until molecules are equally distributed within a space.

Oxygen and carbon dioxide move across cell membranes by diffusion.Oxygen diffuses from the blood, an area of high concentration, into the cell, wherethe oxygen concentration is lower. Because oxygen is continuously consumedwithin a cell, the concentration of oxygen does not build up. Carbon dioxide, bycontrast, accumulates within a cell and diffuses from the cell to the blood.

OsmosisOsmosis is the diffusion of water across a selectively permeable membrane.Consider the system shown in Figure 2(a). Membrane X is permeable to water,but impermeable to the larger protein molecules. Protein cannot diffuse fromside A into side B.

The concentration of protein in side A is greater than in side B. Did younotice that the volume of fluid in side A is equal to that in side B? Which side doyou think has the greater concentration of water? There are fewer protein mole-cules in side B, but many more water molecules. The spaces between proteinmolecules are filled with water molecules. Therefore, the fewer the number ofprotein molecules, the greater the amount of space for water molecules.

The Effect of Salt Wateron Potato Strips

• Take a potato and slice it into 5 thin strips, each about 2 cm wide and5 cm long. Try to make all of the strips the same width and length.

• Obtain salt solutions with the following concentrations: 0% salt,1% salt, 2% salt, 5% salt, and 10% salt.

• Place a potato strip in each of 5 test tubes. Label the test tubes withthe above salt concentrations.

• Pour equal volumes of each salt solution into their respective testtubes, so that each strip is completely covered.

• Allow the strips to remain in the solutions for 20 min and thenremove and measure the length of each strip.

• Prepare an observation chart to record the original and final measure-ments.

• Calculate the change in length. Remember to use negative numbersto show a strip that became shorter.

(a) Describe the changes in the length of the potato strips as saltconcentration increases. Suggest an explanation for theobserved changes.

Try ThisActivity

(a) Membrane X is permeable to waterbut not to protein.

(b) There are equal concentrations ofprotein and water in side A and side B.

side A side B side A side B

osmosis: the diffusion of water moleculesacross a selectively permeable membrane

Figure 2

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52 Chapter 2

Remember that osmosis is just a diffusion of water. Water will diffuse fromside B, the area of higher water concentration, to side A, the area of lower waterconcentration. Like other molecules, water follows the concentration gradient.Figure 2(b), page 51, shows what happens when water moves from side B intoside A by osmosis.

As water leaves side B, the protein in side B becomes more concentrated. Aswater enters A, the protein in side A becomes less concentrated. Eventually, the con-centrations of protein and water in sides A and B will become equal. Once thesystem reaches equilibrium, water molecules will continue to move between the twosides; however, the number of molecules gained from side A equals the number ofmolecules gained from side B. There is no net movement of water at equilibrium.

The movement of water into and out of living cells is vital to life processes.Ideally, cells are bathed in isotonic solutions—solutions in which the soluteconcentration outside the cell is equal to that inside the cell, as shown in Figure3(c). Blood’s major function in your body is to keep your internal environmentin an isotonic balance called homeostasis. Maintaining an isotonic balance isonly part of homeostasis. In isotonic solutions, the water movement into a cellis balanced by the water movement out of the cell.

A hypotonic solution, shown in Figure 3(a), is a solution that has a lowerconcentration of solute and, therefore, a higher concentration of water than

equilibrium: a condition in which allacting influences are balanced, resulting in astable environment

concentration gradient: a difference inthe number of molecules or ions of a sub-stance between adjoining regions. Without theaddition of energy, molecules tend to diffusefrom the area of higher concentration to thearea of lower concentration.

isotonic solution: a solution where theconcentration of solute molecules outside acell is equal to the concentration of solutemolecules inside the cell

homeostasis: a process by which a con-stant internal environment is maintaineddespite changes in the internal and externalenvironment

hypotonic solution: a solution wherethe concentration of solutes outside a cell islower than that found inside the cell

Figure 3(a) Red blood cells in hypotonic solution(b) Red blood cells in hypertonic solution(c) Red blood cells in isotonic solution

(a) hypotonicwater diffusesinward

(b) hypertonicwater diffusesoutward

(c) isotonicno net change inwater movement

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Chemistry of Life 53

2.7

inside the cell. Cells that are placed in hypotonic solutions are not at equilibrium.Water molecules move from the area of high water concentration, the solutionoutside the cell, into the region of lower water concentration, the inside of thecell, by osmosis. The cell expands as water moves in.

In a freshwater environment, the low solute concentration can result inexcess water moving into the cells. Organisms have developed ways to expel theincoming water; for example the paramecium uses a contractile vacuole to expelexcess water from the cytoplasm, as illustrated in Figure 4. Any failure of the con-tractile vacuole would be disastrous for the cell; water would continue to diffuseinto the cell, eventually causing it to explode.

On the other hand, cells placed in a hypertonic solution tend to shrink. Inhypertonic solutions, the concentration of solutes is higher than in the cell andthe concentration of water is lower. Water moves out of the cell by osmosis intothe external solution causing the cell to shrink in size. For an example, seeFigure 3(b).

Have you ever noticed that the salt sprinkled on sidewalks during winter killsthe surrounding grass in spring? Salt creates a hypertonic solution that drawswater out of the cells of the grass causing the plants to wilt. Water pressure,referred to as turgor pressure, pushes the cytoplasm of the plant cell against thecell wall. Turgor pressure is the reason plants are rigid (Figure 5). The process ofosmosis explains why plants die if they are exposed to too much fertilizer andwhy vegetables are sprayed with water in your local grocery store.

Facilitated DiffusionProtein carrier molecules, located in the cell membrane, can aid in passive trans-port. Although the precise action of these carriers is not well understood, scien-tists believe that the protein carriers speed up the movement of moleculesalready moving across the cell membrane. Glucose diffuses into red blood cellshundreds of times faster than other sugar molecules that have similar properties.Why would one sugar molecule diffuse faster than another? Scientists have pro-posed that the diffusion is facilitated. The carrier proteins must be specialized toaid the diffusion of glucose molecules, but not other sugars.

Activity 2.7.1

Observing Diffusion and Osmosis

In this activity, you will construct a model of a selectively permeable mem-brane using dialysis tubing. You will observe osmosis and diffusion throughthis membrane.

Materialslab apron goggles utility standring clamp scissors stringmedicine dropper 2 test tubes 25-mL graduated cylinder100-mL graduated cylinder 250-mL beaker 500-mL beakerdialysis tubing funnel paper towelsdistilled water 4% starch suspension glucose solutionLugol’s solution glucose indicator

such as Tes-tape®

Figure 4A contractile vacuole expels water from aparamecium, permitting it to exist in a hypo-tonic environment.

hypertonic solution: a solution wherethe concentration of solutes outside a cell ishigher than that found inside the cell

Figure 5As the plant cells lose turgor pressure, theplant begins to wilt.

contractile vacuole