Why do cells need membranes? Consider what you would require to keeptropical fish. You would need an aquarium tQ. hold water, which could be keptwarm with a small heater. A lid with a light attached to it may cover the topof the tank. Like the tropical fish in Figure 8.1, cells survive in a controlledenvironment. Instead of glass and plastic, a cell has a membrane to separate itsinterior from its surroundings. By defining the inside versus the outside of cells,membranes allow cells to survive as individual units of life.

A cell must also allow some materials to move in and out of it. Similarly, theaquarium may require a bubbler to introduce oxygen from the air into the water.Someone must feed the fish and, when wastes build up, replace the dirty water.Materials entering or exiting a cell must cross the cell membrane. Unlike the hard,inflexible walls of an aquarium, however, cell membranes change accordingto each cell's needs. The flow of matter between a cell and its environmentmakes it an open system.

IiIjI!IIiI;.D In what ways is an aquarium of tropical fish similar to a cell?In what ways is it different?

Membrane StructureCell membranes are primarily composed of a double layer of phospholipids(a type of lipid). Proteins and other molecules are embedded between andattached to the phospholipids. What happens if you try to dissolve lipids, suchas cooking oil, in water? Even with the most vigorous mixing, the oil does notdissolve. Although the mixing may cause small droplets of fat to separate, thedroplets float back to the surface to form a thin film. The behaviour of thephospholipids that make up cell membranes is similar to that of the fat droplets.In the presence of water, phospholipids spontaneously form thin layers,

290 MHR . Unit 3 Cycling of Matter in Living Systems

As shown in Figure 8.2, each phospholipid molecule has a head and a tail.The two ends have different chemical compositions that cause them to inter-act differently with water. The head end is hydrophilic, or "water-loving."Hydrophilic molecules dissolve easily in water. The tail end, however, ishydrophobic, or "water-fearing." Hydrophobic molecules, such as fats andoils, do not dissolve readily in water. Water molecules repel, or push away,the tail end of a phospholipid.

The dual nature of phospholipid molecules causes them to arrange themselvesautomatically into layers that are two molecules thick. As Figure 8.3 shows,in each double-layered membrane, the phospholipids' hydrophilic heads pointtoward the water. Their hydrophobic tails are sandwiched in the middle, wherethey are shielded from the water. The two layers of phospholipid moleculesare called phospholipid bilayers. Viewed from the side, the two layers ofphospholipids look almost like mirror images of one another. In the Find OutActivity on the next page, you can investigate the properties of membranes.

IimnI;:I!n A phospholipid molecule has hydrophobicfatty acid tails and a hydrophilic head. The head containsa phosphate group and, in some cases, a nitrogen group.

The molecular structure of a phospholipid bilayer.Unlike the cell membrane of a living cell, the bilayer shownhere surrounds only water.

Chapter 8 Dynamic Cells. MHR 291

Find OutBubble Biology

How do membranes behave? What do they looklike? A soap bubble is one type of membrane.Soap molecules are very similar to phospholipids;they have a water-soluble head and a water-insoluble tail. As a result, soap bubbles haveseveral similarities to cell membranes. In thisactivity, you will use soap bubbles to explorethe nature of membranes.

~~Safety Precautions ~ ~

. Avoid spilling soapy water on the floor, and

clean up any spills.

Materials

5. Predict what will happen if you stretch, fold,or alter the shape of the detergent solutionmembrane. Test your predictions and describewhat happens to the membrane.

(a) Slide the straws of the string-and-strawapparatus into different positions to createdifferent shapes.

(b) Hold the apparatus perpendicular to thefloor, then sweep it sideways to stretchthe membrane.

(c) To test the effect of folding the membrane,slowly bend and fold the apparatus.

(d) As one partner holds the membrane, theother should wet a finger in the detergentsolution and slowly push it through andsideways in the membrane.

strawsscissors

detergent solutioncafeteria trays

newspaperscotton string

Procedure . ~:~li:l: illij~J ~J-i~ i~~~~I:.i1 il~,

1. Work with a partner to spread newspapers onthe table and floor in your work area.

6. After holding the membrane out of the solutionfor several seconds, observe the membranesurface closely for movement.

2. Thread a piece of cotton string through twostraws. Hold the straws parallel to each otherand pull them apart to make parallel sides ofa rectangle that is smaller than the cafeteriatray. Cut and knot the ends of the string.

7. Clean and dry your work area.

What Did You Find Out? . r~11-c~~,j"iiJ~: rcl(~ :ijil[~ii~j:;ii.i~

1. (a) How does the membrane respond tobending and prodding?

(b) What do your observations suggest aboutthe properties of cell membranes?

2. Based on your observations, would youexpect the phospholipids in cell membranesto move within the membrane? Explainyour answer.

3. How might the properties of membranesaffect their functions?

3. Without making the detergent solution frothy,

pour it into the tray to a depth of about 1 cm.

4. Dip the string-and-straw rectangle into

the detergent solution, then lift it to create

a thin film.

292 MHR . Unit Cycling of Matter in lving Systel

Membranes Contain ProteinsCell membranes contain embedded proteins. These proteins move substancesacross the membrane and carry out chemical reactions. Other proteins protrudefrom the membrane's surface. Some proteins have "marker" molecules onthem that allow cells to recognize each other and, in some cases, protect cellsfrom infection. Such proteins may also act as an attachment site for '~messengermolecules," such as hormones. Messenger molecules are used in cell-to-cellcommunication and control of cell functions.

Membranes Are FluidViewed under the microscope, cell membranes may appear motionless.However, the molecules that make up membranes are thought to be in constantmotion, as in a fluid. This model suggests that, like canoes on a lake, membranephospholipids and proteins drift past one another. This fluidity helps membraneproteins to function by allowing them to move within the membrane. It alsogives membranes flexibility, allowing cells to change shape. This model ofmembrane structure is called the fluid-mosaic model, because the fluid motionwithin membranes changes the pattern of proteins on the cell surface. The fluidmosaic model is shown in Figure 8.4. Carbohydrates (such as sugars) are attachedto lipids and proteins on the membrane's outer surface. These carbohydratesare thought to be involved in cell-to-cell communication. Together, the proteins,lipids, and carbohydrates of the membrane fonn a biological mosaic. In the nextinvestigation you will learn about an experiment that supports the hypothesisthat membranes are fluid.

~ The fluid-mosaic model of membrane structure. Inside the cell, parts of the cell's skeleton(called the cytoskeleton) support the membrane. Each type of cell has its own unique "fingerprint" ofcarbohydrate chains on the outer surface that distinguishes it from other kinds of cells.

Chapter 8 Dynamic Cells. MHR 293

mouse cell human cell

COllY fusion

immediately after fusionWhat to Do0 On the previous page you learned that membranes

are fluid. Before scientists discovered this, analternative hypothesis might have stated thatmembranes are rigid. Both hypotheses can betested by the mouse-human cell fusion experi-ment. Based on the two hypotheses, develop twopredictions about the movement of membraneproteins during cell fusion. Write each predictionin your notebook.

~

mixed membrane proteins

After human cells and mouse cells fuse,the membrane proteins of the mouse cell(blue spheres) and of the human cell(red spheres) mix within a short time.

. The cell fusion experiment is illustrated in thediagram to the right. Describe any differencesin membrane protein distribution before andafter the human and mouse cells were fused.

294 MHR . Unit 3 Cycling of Matter in Living Systems

Think About ItCell membranes are about seven millionths of a millimetre thick.Until the invention of electron microscopes, it was not possibleto observe membranes direcdy. Even so, scientists were able tomake inferences about membranes by studying the movement ofprotein molecules embedded within them. In one experiment,illustrated here, researchers labelled membrane proteins withdyes. The dyes made the proteins visible under a microscope.Then, two labelled cells - one mouse cell and one human cell- were fused. Observations of the resulting hybrid cell led to

some important new insights about the structure and behaviourof the cell membrane.

Membrane Function

A Biological BarrierThe cell membrane prevents many substances from enteringa cell. The fluids that bathe cells contain chemicals such as salts,sugar, and proteins. Electrically charged molecules and atoms(ions) are also present in these fluids. Also, organisms such asviruses and bacteria may float in the space around cells. The cellmembrane often prevents such materials from entering cells.

Membranes organize the insides of cells into many smallercompartments. Most organelles are surrounded by membranesand chemical reactions take place within these organelles. Bykeeping certain chemicals together, membranes help to promotethe reactions that are needed for life. They also prevent unwantedchemical reactions. For example, lysosomes are filled with enzymesthat digest other cell parts. What would happen if those enzymeswere not contained within a membrane? If they were allowedto float freely through the cytoplasm, the enzymes would digesteverything they contacted. This would kill the cell. At certaintimes, however, the contents of lysosomes are purposely releasedinto cells. For example, Figure 8.5 shows how older cells in thebody are broken down and digested by the enzymes containedin the lysosomes. New cells then replace these older cells.

Iim!IijI;H During a process called apoptosis, the

Pause&Reflect

How is a cell membrane similar to a plastic zip-upfreezer bag? How is it different? In your notebook,describe how a freezer bag may be viewed asa model of a cell membrane.

IimIIr;I.;.I.!I This Gore- TexTM jacketis semi-permeable. Some air passesfreely through pores in the fabric,allowing the material to "breathe"when the person wearing thejacket perspires. Liquid watercannot pass through the fabric,however. Thus Gore- TexTM

protects against rain and snow.

A Selective FilterLThe role of the cell membrane in regulating movement of

substances in and out of the cell is vital. Cells cannot survivefor long without taking in food and gases and getting rid ofwastes. Also, the functions of many cells depend on the movement of materialsacross the membrane. To absorb nutrients, for example, cells must be able totransfer these molecules across the membrane. Cell membranes must allow somematerials to cross while excluding others. That is, cell membranes must besemi-penneable. The fabric used in the jacket in Figure 8.6 is semi-permeable.Air can move in and out of the fabric, but water cannot.

How does the cell membrane regulate which particles can enter and leavethe cell? One basis for semi-permeability is particle size. Some molecules,such as oxygen gas or water, are so small that they can cross the membraneby slipping between phospholipid molecules. Larger molecules, such as sugar,are too large to cross directly between the phospholipids of the membrane.A window screen is an example of a membrane that is permeable to some"particles" but not to others. The screen's fine mesh allows air to flowthrough, but keeps insects out.

Membrane proteins also contribute to the cell membrane's semi-permeability.Proteins that protrude from the membrane may bind to specific chemicals basedon their shape or their electrical charge. These selected molecules are thentransported across the membrane by the protein. In the following section, youwillieam more about the ways cells move materials across their membranes.

Chapter 8 Dynamic Cells. MHR 295

contents of the Iysosomes are released into the cell,thus digesting it from the inside. Shown here, in greenand red, is the fragmenting cytoplasm of a cellundergoing apoptosis. The nucleus is shown in yellow.

extra-cellularspace

Because membrane proteins are partially buried within the lipid layers,they can be difficult to see. Scientists use freeze-fracture to study theseproteins. Using freeze-fracture, researchers can look at how proteinsare distributed. To prepare cells, specimens are frozen in liquid nitrogen.Then, they are cracked using a cold knife. The cells shatter along a fractureline that follows the path of least resistance. Sometimes, that line fallsbetween the cell membrane's two phospholipid layers. The layers arepeeled apart, making the membrane proteins visible. The fractured specimenis then coated with platinum and examined with an electron microscope.Electron micrographs resemble miniature moonscapes. Proteins createbumpy hills and craters separated by smooth membrane plains. Bystudying these contours, researchers can learn where proteins lie in themembrane. Freeze-fracture has provided some of the best evidence forthe fluid-mosaic model of membrane structure.

proteins When cell membranesare freeze-fractured thetwo phospholipid layersare split.

Two layers offreeze-fractured cellmembranes, showingthe membrane proteins.

Section 8. 1 SummaryThe cell membrane protects the cell organelles and regulates the flow of matterbetween the cell and its environment. Cell membranes are made of a doublelayer of phospholipids, which have a hydrophobic fatty acid tail and a hydrophilichead. The cell membrane is semi-permeable. This means that only substanceswith certain characteristics (e.g., a particular size) can move through the cellmembrane. Protein molecules embedded in the cell membrane are like passage-ways in or out of the cell - they transport certain substances through themembrane. The lipids, proteins, and carbohydrates in the cell membrane arein constant motion resulting in a membrane that is fluid and flexible. Thismodel of membrane structure is the fluid-mosaic model.

Check Your Understanding1. (a) List the major types of molecules that make up a cell membrane.

(b) Identify the components of the cell membran~ that give ita fluid consistency.

2. Explain why a cell is considered an open system.

3. Describe the fluid-mosaic model of membrane structure.

4. Thinking Critically How would cell function be affected ifcell membranes were rigid and inflexible, like walls?

5. Thinking Critically Explain why blocking all transport acrossa cell's membranes would shorten the cell's life span.

6. Scientists who study cell membranes often work with membranes fromwhich the proteins have been removed.(a) Thinking Critically Explain how the function of the protein-free

membranes would differ from regular cell membranes.(b) Apply How could you use a protein-free membrane to investigate

the functions of the missing proteins?

296 MHR. Unit 3 Cycling of Matter in Living Systems