Learning Objectives

1. To become familiar with the structure and function of the plasma membrane and to provide an explanation behind the movement of molecules into and out

of the cell.

2. To become familiar with the structure and function of the cytoskeleton.

3. To become familiar with cell-cell interactions.

Key Concepts

1. Cells must regulate what enters and leaves them; maintain their specific shapes; and interact with other cells. The cell membrane and underlying cytoskeleton make these functions possible.

2. Within cells, a vast network of tubules and filaments guides organelle movement, provides overall shape, and establishes vital links to specific molecules that are part of the cell membrane.

3. Cells must permanently attach to one another to build most tissues; transiently attach to carry out certain functions; and send and receive biochemical messages and respond to them.

Chapter Concept 4.1: The Cell Membrane Controls Cell Function

Cells must regulate what enters and leaves them; maintain their specific shapes; and interact with other cells. The cell membrane and underlying cytoskeleton make these functions possible.

Textbook Reading Assignment: Pages 60 - 68

The outermost boundary of the cell is the plasma membrane. Not only does the plasma membrane serve to isolate the internal environment from the external world, but it also serves to regulate the passage of substances into and out of the cell and provides self-nonself identification. The plasma membrane is somewhat selective in what it allows to pass. For example, it allows waste, oxygen, and nutrients to pass; however, it does not allow all molecules to traverse the membrane. For this reason, the plasma membrane is said to be selectively permeable.

Fig. 4.3

The plasma (or cell) membrane is composed of a variety of molecules, including phospholipids, proteins, carbohydrates (sugar molecules), glycoproteins (protein + carbohydrate), glycolipids (lipid + carbohydrate) and even cholesterol. In addition, the plasma membrane is not rigid. The molecules within it actually move very slowly, and the membrane is, therefore, said to be fluid. Since the plasma membrane is both fluid and composed of many types of molecules, biologists use the phrase Fluid Mosaic Model to describe the plasma membrane ("fluid" refers to movement and "mosaic" refers to the fact that it is composed of more than one type of molecule)

Fig. 4.2

The main component of the plasma membrane is the phospholipid. Phospholipids contain two separate regions with different affinities for water (refer to Figure 4.2a of the text and demonstrated above). The phospholipid contains a charged phosphate head and two fatty acid tails. The phosphate heads like to be in contact with water and are said to be hydrophilic (“water-loving”). On the other hand, the fatty acid tails contain long hydrocarbon chains that are hydrophobic (“water-fearing”). An example of a hydrophobic interaction occurs when you place oil in water. Does the oil mix well with the water, or does it aggregate at the top of the glass in order to “stay-away” from the water? The phospholipid is, therefore, said to be amphipathic. An amphipathic molecule contains two regions with different affinities for water. The amphipathic nature of phospholipids creates a unique arrangement within the plasma membrane.

The phospholipids in the plasma membrane are, therefore, arranged in a phospholipid bilayer with the hydrophobic tails pointed toward one another and the hydrophilic heads oriented away from the center of the membrane toward the external water-filled environment of the cell.

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Discussion Question 1

Why does the plasma membrane consist of a phospholipid bilayer rather than a single monolayer?

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Floating within the membrane are a series of proteins that serve various functions (Table 4.1 of text on page 62). Some form channels (transport proteins) through the membrane, allowing materials to pass without contacting the hydrophobic inner portion of the bilayer. In other words, they serve to insulate hydrophilic molecules from the hydrophobic interior of the plasma membrane. Cellular adhesion proteins serve to attach cells together within tissues. Other membrane proteins, such as receptor proteins, may act very much like a doorbell, in which an event on the outside can communicate something to the inside. Still others (such as cell surface proteins) may act as specific sites of recognition, identifying a given cell as belonging to your body. For example, membrane proteins allow our immune system to recognize “self-cells” from things like bacteria that are not part of the body (non-self), as a means of preventing infection.

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Web Activities

·        If you would like to review specific information about cell membranes, use this part of the University of Arizona website: http://www.biology.arizona.edu/cell_bio/tutorials/membranes/main.html.

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For our purposes, it is extremely important that we understand why and how materials cross the plasma membrane.

So, lets try to answer this question!

Why do molecules move? This question can be answered using a basic law of physics called entropy. This law holds that all things move from an organized to a disorganized state (my office confirms that!). Let's say that a student is in a room on Saturday, and they drop a partially eaten tuna fish sandwich in the wastebasket. It is summer (the air conditioning doesn't work during weekends), and the custodial staff won't empty the trash again until Monday evening. The room is closed up all weekend. What might you notice when you first come in on Monday morning?

If you said a smell, think of this question: Why would the whole room smell if the sandwich is in the bottom of the trash? The answer has to do with the fact that the molecules of gas given off by bacteria that are growing on the material in the sandwich cause the smell. These molecules are moving around due to a phenomenon called Brownian motion. Their entirely random movement is caused by the kinetic energy of the molecules.

If the law of entropy is functional, their random motion will mean that they will move from where they are organized (concentrated) out into the room where they are spread out (becoming disorganized). This means the smell can be detected throughout the room. There is another name for this overall process. Based upon your reading of the text, what is it?

The answer is diffusion. Diffusion is defined as the movement of molecules from an area of high concentration to an area of low concentration. When a condition exists in which there is an area of high concentration and another area with lower concentration (of the same substance), we say that a concentration gradient exists (high to low).

Fig. 4.4

The above figure (Figure 4.4) demonstrates diffusion of a dye dissolved in water. A solution consists of both a solute and a solvent. In general, the solvent dissolves the solute. In this example, the dye (red) represents the solute while the water (blue) represents the solvent (in fact, water is the biological solvent…most substances within your body are dissolved in water). Notice that the dye in the beaker diffuses from an area of high concentration to an area of low concentration. The end result of diffusion is the equal distribution (dynamic equilibrium) of molecules within the beaker.


Why is diffusion important? In general, it determines the direction of travel of molecules across membranes. For example, when you inhale, oxygen enters the lungs and eventually diffuses into the capillaries of the circulatory system because the lungs have a higher concentration of oxygen than the blood.

Using this information, can you now explain why glucose tends to move into cells of the body and not out? Can you also explain why carbon dioxide tends to leave the body during respiration rather than enter?

In addition to the concern about the direction of movement of solutes and gases across membranes, the nature of the surrounding material in which they find themselves is also of concern. For example, salt is not just floating around in the sea. It is actually dissolved by the water, therefore creating a solution.

Recall from our previous discussion that a substance that dissolves in a solvent is called a solute. Assume that you made up a solution of sugar and water in the laboratory. What would be the solvent/solute in this case? Obviously, water would be the solvent while sugar would be the solute.

As the amount of solute in the solution increases, we say that it has a higher concentration (of solute). In order to compare various solutions with one another, we use specific terms to describe the relative concentrations of the two solutions being compared. For example, we would say that grape juice has a higher solute concentration than pure (distilled) water. This is due to the fact that grape juice contains many types of molecules, including sugar. All of these are dissolved in water to give us grape juice.

If we look at how this comparison works, assume that we take a table grape (the kind you eat) and drop it in a beaker that contains pure water (not tap water). The liquid inside the cells of the grape has far more solutes than the water (which should have none if it is pure). We could say the grape is more concentrated than the surrounding water, but instead we use terms to describe the relative concentration of one solution to another. This is called tonicity.

Since the grape has more solutes than the water, we say that the grape cells are hypertonic to the water. Of course this is a comparison, and if you compare one thing to another, and one is greater, then the other one will be less. Confused yet? Here is an example. You are in a room that has three banks of lights. The bank of lights near the front is turned out. If you stand in the front of the room, you can say: “It is darker here in the front.” What does this tell you about the back of the room? Obviously that it is lighter or brighter than the front of the room..

By the same logic, if something is hypertonic (higher concentration), then what you are comparing it to must have a concentration that is lower (hypotonic). Of course, one other type of comparison is possible. If the two solutions under comparison have the same concentration, they are said to be isotonic to one another.

Let's see how this would work with things that can be placed in water using a raisin as an example. Take a raisin and drop it into a glass of pure water.

Notice that the inside of the raisin contains a higher concentration of sugar molecules with respect to the outside of the membrane. What term would you use to describe the environment within the raisin with respect to the solution outside the raisin? Obviously, the inside of the raisin is hypertonic to the solution outside (water). What term would you use to describe the solution outside the raisin with respect to the solution inside the raisin? The solution is hypotonic (to the raisin).

Let the raisin sit in the pure water for one hour. Does the size of the raisin change? In fact, the raisin should increase in size. What do you think caused this increase in size? The answer is water. In this example water moved into the raisin because the solution inside of the raisin has a higher concentration of solute than the solution outside of the raisin. This process is termed osmosis. Osmosis is defined as the movement of a solvent across a semipermeable membrane. In general, water moves from the side of the membrane with low solute concentration to the side of the membrane with high solute concentration.

Notice in the image below (Figure 4.5) that the solute concentration of the solution on the left of the beaker is higher than that on the right. As a result, water moves from the right of the membrane to the left via osmosis until a dynamic equilibrium is achieved.
Fig. 4.5

Note in the above image that there are two concentration gradients, one for the solute and one for the solvent. If the membrane is permeable to both substances, solvent and solute, they will move in opposite directions until they reach equality or equilibrium. However, if the membrane is permeable only to the solvent, such as in the figure above, only the solvent moves across the membrane. In general, solvent moves toward the side of the membrane with a higher solute concentration. In other words, the concentration of the solution that a cell is placed in can influence the cell’s size.

For example, if you place red blood cells into an isotonic solution, the size of the cell remains constant. However, if you place them into a hypertonic solution (a solution that has a higher concentration of solutes than the fluid inside the cell), water leaves the cell and the cell crenates (shrinks). On the other hand, if you place the red blood cells into a hypotonic solution (a solution that has a lower concentration of solutes than the fluid within the cell), water will enter the cell and the cells will increase in size and eventually burst.

Do you see why patients are rehydrated with saline solution (isotonic to red blood cells) and not pure water? What would you expect to happen to the size of the red blood cells if the patient were rehydrated with pure water instead of saline solution?

Fig. 4.6


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Discussion Question 2

How do the terms hypotonic, hypertonic, and isotonic relate to one another as well as to the concept of osmosis?

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The concept of osmosis can cause a variety of problems for many freshwater organisms. Consider, for example, the case of the freshwater organism pictured below (a paramecium). Paramecia live in fresh water environments and are hypertonic to their aquatic surroundings. As a result, water tends to move into the animal. In order to deal with this water influx, paramecia must contain a specialized organelle, a contractile vacuole, that is responsible for collecting this influx of water. Once the contractile vacuole fills with water, it contracts and squeezes the water out of the animal back into the external environment. This process repeats itself hundreds of times each day. Do you see the importance of internal membranes such as the contractile vacuole? What would happen to the paramecium if its contractile vacuole decided to take the day off?

Fig. 4.7

Osmosis can affect the size of plant cells as well as animal cells. For example, as indicated on page 65 of the text, the movement of water across a plant cell membrane can affect the amount of pressure built up within the plant cell.

Fig. 4.8

When the cell is placed into a hypotonic solution, such as pure water, water moves into the cell and expands the plasma membrane against the cell wall. The cell wall is made of a rigid polysaccharide termed cellulose and does not expand with the enlarging membrane. In fact, the cell wall resists the expansion of the plasma membrane and creates a type of pressure within the cell termed turgor pressure. Such pressure is what makes your salad somewhat “crispy” when you consume it. What would happen to your salad if you soaked it in concentrated salt water for two hours before you consumed it (note: this is essentially what happens when plants wilt)? In the above figure (Figure 4.8), the cell on the left is placed into a solution that is hypotonic with respect to the solution within the cell and the cell on the right is placed into a solution that is hypertonic with respect to the solution within the cell. Notice that when placed into a hypertonic solution, water exits the plant cell and the plant wilts. You can simulate this on your own if you wish: take a piece of lettuce and soak it in fresh water for 3 hours, take another piece of lettuce and soak it in concentrated salt water for 3 hours. Which one is crispy? Which one if flaccid? Can you explain these results?

One last question to ponder--stranded dehydrated sailors have sometimes consumed sea water in order to quench thirst. Unfortunately, this only increases their state of dehydration. Can you explain why? 

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Discussion Question 3

State the effect of placing both a plant cell and an animal cell into an environment that is hypotonic to the environment within the cell. Do the two cells meet a different fate? If so, why do their fates differ? (hint: remember that animal cells lack cell walls)

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Now that we've established the physical principles behind the movement of molecules, lets become familiar with the various way in which molecules traverse the plasma membrane!

Passive transport involves the movement of material across a membrane without a need for additional energy from the cell. If, on the other hand, the movement occurs but requires the cell to expend its own energy, the movement is said to involve active transport. Passive movement occurs “down” the concentration gradient, from high to low. Active transport, by contrast, moves from low to high concentration, against the concentration gradient.

Some moleculesmove across the membrane by passive means, but they are either too large or have some type of charge that prevents them from traversing the membrane. In this case, special protein “channels” offer larger or insulated passageways that make it possible for these molecules to cross. In this case, the proteins facilitate the movement, and the process is called facilitated diffusion.

Fig. 4.9

Notice in the figure above (Figure 4.9) that in both cases (active and facilitated transport) solutes are allowed to cross the membrane only with the aid of a membrane protein. So, in such situations, a special entryway must be provided through the membrane to keep from having charge interactions (ions) or the tearing of the membrane (large molecules like glucose). Carrier proteins serve as these entry/exit points.

If material crosses the membrane through one of these channels in a high to low direction (with respect to concentration), it is undergoing diffusion. Since it requires a carrier protein to diffuse across the membrane, we say it is undergoing facilitated diffusion. In other words, the protein facilitates (or helps) its movement. When something crosses a membrane from an area of low concentration to an area of high concentration, it must be assisted and we say it is undergoing active transport. For example, while a person can supply energy to push a rock up a hill, moving a molecule against a concentration gradient (from low to high) means that the cell must apply energy. Often this energy (to assist movement) is in the form of molecules of ATP. The sodium-potassium pump displayed in Figure 4.10 on page 66 provides an example of active transport in which energy is used to drive the movement of sodium and potassium across the plasma membrane.

So let's compare Facilitated Diffusion and Active Transport:


1)      Do both use membrane proteins?

2)      Do both require energy?

3)      Do both move from an area of high concentration to an area of low concentration?


1)      Yes

2)      No

3)      No

Don't worry about the discussion of cotransport on page 66 of the text. Just focus on the differences between active and passive transport.  

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Web Activity

Use the tutorial found on this Web site to help you to review the plasma membrane and membrane transport: www.biology.arizona.edu/cell_bio/tutorials/membranes/main.html

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Discussion Questions 4 and 5

In general, what determines the direction in which molecules move across membranes?

How does facilitated diffusion differ from active transport?

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At times the cell needs to move substances across the plasma membrane that are too large to fit within the membrane proteins. How does the cell achieve this? Under such circumstances, the cell employs the use of a vesicle. Vesicles are membrane-bound organelles formed from a portion of the plasma membrane. Once formed, vesicles are responsible for moving material around within the cell and to and from the cell surface.

Exocytosis involves the fusion of a vesicle with the plasma membrane in which the contents of the vesicle are discharged from the cell. The enzymes associated with the tip of the sperm cell are released in this way. Once released via exocytosis, these enzymes penetrate the protective layers of the egg allowing the sperm nucleus to enter the egg.

Fig. 4.12

The process of taking a substance into the cell via a vesicle is termed endocytosis and includes phagocytosis, pinocytosis and receptor-mediated endocytosis. During phagocytosis, large molecules are engulfed within a vesicle and destroyed. During pinocytosis, vesicles form around liquid or very small particles and transport them into the cell. During receptor-mediated endocytosis, the receptors on the cell surface bind to specific solutes resulting in the formation of a vesicle which serves to transport the solute into the interior of the cell. Such a scenario allows cells to take up specific types of molecules and sort them within the cell.

Fig. 4.13a
Fig. 4.13b
Fig. 4.13c

Refer to Table 4.2 on page 68 of your text to help you review movement across biological membranes.

Chapter Concept 4.2: The Cytoskeleton Supports Cells

Within cells, a vast network of tubules and filaments guides organelle movement, provides overall shape, and establishes vital links to specific molecules that are part of the cell membrane.

Textbook Reading Assignment: Pages 69 - 72

The cytoskeleton is composed of three types of protein filaments or fibers: microfilaments, intermediate filaments and microtubules. These filaments serve to give the cell shape, anchor the cytoplasmic components of the cell to the cell membrane, and provide movement for organelles within the cell. Table 4.3 on page 69 of the text provides a brief synopsis of the function of the cytoskeleton within the cell.


Microtubules are composed of the special protein tubulin; intermediate filaments are composed of various specialized proteins and microfilaments are composed of a specialized protein termed actin.

Fig. 4.14

Microtubules are found in all eukaryotic cells and are composed of individual protein units termed tubulin dimers. The overall length of a microtubule can be altered by adding or removing individual tubulin units. Microtubules are extremely important in the movement of structures within the cell and can also enable the entire cell to move.  

Some cells that line passageways are covered with tiny projections, cilia, which look like hairs and may function in moving objects or sensing motion. Other cells, such as sperm cells, require mobility and contain a large whip-like appendage, a flagellum. In general, cilia tend to be shorter and more numerous than flagella. Cilia are found in various parts of the human body, such as the respiratory tract and fallopian tubes. In fact, it is the cilia that is responsible for the movement of the egg through the fallopian tube. In the respiratory tract, cilia serve to remove contaminated mucus and allow it to be expelled from the body.

Fig. 4.15

Both cilia and flagella are composed of microtubules arranged in a 9 + 2 manner. The peripheral pairs of microtubules are attached to the central pair via a specialized protein termed dyenin. The movement of dyenin proteins serves to move the cilium or flagellum.
Fig. 4.16


Microfilaments are composed of actin. Microfilaments are primarily involved in providing strength for cells involved in movement and stretching, such as muscle cells.

Intermediate Filaments

Intermediate filaments are intermediate in size between microtubules and microfilaments. They are composed of a variety of specialized proteins and serve to form an internal scaffold to maintain cell shape and resist mechanical stress.

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Web Activities

Use the following site to help you review the structure and function of the cytoskeleton:

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Chapter Concept 4.3: Cells Interact and Respond to Signals

Cells must permanently attach to one another to build most tissues; transiently attach to carry out certain functions; and send and receive biochemical messages and respond to them.

Textbook Reading Assignment: Pages 73 - 77

In many cases, cells communicate with one another with the use of specialized connections between cells. For example, aimal cells communicate via:

1) Gap Junctions - connect cells by a protein channel that allows cells to exchange chemical signals.

2) Desmosomes - hold cells together and provide strength without affecting the passage of material between cells.

3) Tight Junctions - an impermeable junction in which membranes of adjacent cells fuse together

Fig. 4.19
Plant cells communicate by means of plasmodesmata. Plasmodesmata are intercellular channels that allow molecules to pass from cell to cell.
Fig. 4.20

The last portion of this section is a bit too detailed (Cell Adhesion and Signal Transduction). In order to simplify, just know the following synopsis:


Much of the time, long distance communication between cells is accomplished via chemical signaling. Molecules made by one cell diffuse to other cells and bind to specific receptors on the surface of the receiving cell, triggering a series of specified events. Some chemical signals pass through the plasma membrane of the receiving cell and bind to receptors within the cytoplasm. Other chemical signals bind to receptors on the surface of the cell, rather than in the interior of the cell. Some of these receptors open membrane channels to allow the passage of materials in/out of the cell, while others activate an intracellular messenger, such as cAMP resulting in a variety of biological effects.

1. Which of the following is not a component of the plasma membrane?

a. proteins

b. lipids

c. carbohydrates

d. phospholipids

e. All of the above are components of the plasma membrane.

2. Proteins

a. are found within the the plasma membrane

b. are found only within eukaryotic cells.

c. serve virtually no function within living cells.

d. all of the above.

3. Since water is present on both sides of the plasma membrane (inside and outside) how would the phospholipids arrange themselves in order to become stable?

a. They would arrange themselves in such a way that their fatty acid tails would be pointing away from each other toward the water.

b. They would arrange themselves in such a way as to have the phosphate head

pointing to the outside of the membrane (toward water) and the hydrophobic tail pointing toward the inside of the cell (toward water as well).

c. They would arrange themselves in such a way as to form two layers of phospholipids, with the tails pointing toward each other and the heads oriented toward opposite ends of the membrane.

4. You should remember that the crooked fatty acids contain double bonds. Would having these "bent" fatty acids make the membrane more or less fluid?

a. more fluid

b. less fluid

c. will not affect membrane fluidity

5. What would happen to the fluidity of a membrane as one increases the number of unsaturated fatty acids?

a. Membrane fluidity would increase

b. Membrane fluidity would not change

c. Membrane fluidity would decrease

6. Which of the following is incorrect concerning active transport?

a. Active transport moves a substance against the concentration gradient.

b. Active transport requires a membrane protein.

c. Active transport is dependent upon the utilization of ATP.

d. Active transport is a form of passive transport and moves a substance along the concentration gradient.

7. A grape is place into a solution of salt water that has a higher concentration of solutes than the solution within the grape. Which of the following statements is correct?

a. The solution within the grape is hypertonic to the salt water solution.

b. The solution within the grape is hypotonic to the salt water solution.

c. The salt solution is hypotonic to the solution within the grape.

d. Both solutions are isotonic with respect to each other.

8. Diffusion is the movement of like molecules from an area of _________ concentration to an area of __________ concentration.

a. low, high

b. slow, loud

c. high, low

d. zero, low

9.   __________ refers to the strength of a solution in relationship to osmosis.

a. Diffusion

b. Osmosis

c. Tonicity

d. Endocytosis


Answers to Self Test Questions

1. e

2. a

3. c

4. a

5. a

6. d

7. b

8. c

9. c

Answers/Insight into Discussion Questions

1. Since the phospholipid tails are hydrophobic, they must orient away from water. It just so happens that the phosphate heads are hydrophilic. As a result, the tails point inward toward each other and the hydrophilic heads point outward, toward water.

2. Osmosis is the movement of water (solvent) across a semi permeable membrane. The tonicity of a solution (hypo, hyper, or iso) determine the direction of water travel. In general, water travels from a hypotonic solution to a hypertonic solution. In other words, if solution "A" is hypertonic to solution "B", water will travel from solution "B" to solution "A". Please note that no net movement of water occurs between isotonic solutions.

3. Placing plant and animal cells into a hypotonic solution will result in a net influx of water across the plasma membrane. As a result, the cell increases in size. The animal cell may eventually explode, however, the cell wall of plant cells prevents such a scenario from occurring in plant cells.

4. The concentration gradient--In general, molecules move from an area of high concentration to an area of low concentration.

5. In facilitated diffusion, molecules utilize a membrane protein and move along the concentration gradient (high to low). However, in active transport, molecules are transported from an area of low concentration to an area of high concentration via a membrane protein with the utilization of energy in the form of ATP.