High School Biology/Lessons/Lesson 3

Lesson 3-1 in High School Biology


Every living cell exists in a liquid environment that it needs to survive. It may not always seem that way; yet even in the dust and heat of a desert, the cells of cactus plants, scorpions, and vultures are bathed in liquid. One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.
Measuring Concentration

The cytoplasm of a cell contains a solution of many different substances in water. Recall that a solution is a mixture of two or more substances. The substances dissolved in the solution are called solutes. The concentration of a solution is the mass of solute in a given volume of solution, or mass/volume. For example, if you dissolved 12 grams of salt in 3 liters of water, the concentration of the solution would be 12 g/3 L, or 4 g/L (grams per liter). If you had 12 grams of salt in 6 liters of water, the concentration would be 12 g/6 L, or 2 g/L. The first solution is twice as concentrated as the second solution.


In a solution, particles move constantly. They collide with one another and tend to spread out randomly. As a result, the particles tend to move from an area where they are more concentrated to an area where they are less concentrated, a process known as diffusion (dih-FYOO-zhun). When the concentration of the solute is the same throughout a system, the system has reached equilibrium.

What do diffusion and equilibrium have to do with cell membranes? Suppose a substance is present in unequal concentrations on either side of a cell membrane, as shown in the figure at right. If the substance can cross the cell membrane, its particles will tend to move toward the area where it is less concentrated until equilibrium is reached. At that point, the concentration of the substance on both sides of the cell membrane will be the same.

Because diffusion depends upon random particle movements, substances diffuse across membranes without requiring the cell to use energy. Even when equilibrium is reached, particles of a solution will continue to move across the membrane in both directions. However, because almost equal numbers of particles move in each direction, there is no further change in concentration.


Equilibrium is reached in a system when the concentration of a solute is...

increasing throughout the system.
decreasing throughout the system.
the same throughout the system.

Lesson 3-2 in High School Biology


Although many substances can diffuse across biological membranes, some are too large or too strongly charged to cross the lipid bilayer. If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it. A membrane is impermeable to substances that cannot pass across it. Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot.
Water passes quite easily across most membranes, even though many solute molecules cannot. An important process known as Osmosis is the result. Osmosis is the diffusion of water through a selectively permeable membrane.
How Osmosis Works

Look at the beaker on the left in the figure at right. There are more sugar molecules on the left side of the selectively permeable membrane than on the right side. That means that the concentration of water is lower on the left than it is on the right. The membrane is permeable to water but not to sugar. This means that water can cross the membrane in both directions, but sugar cannot. As a result, there is a net movement of water from the area of high concentration to the area of low concentration.

Water will tend to move across the membrane to the left until equilibrium is reached. At that point, the concentrations of water and sugar will be the same on both sides of the membrane. When this happens, the two solutions will be isotonic, which means “same strength.” When the experiment began, the more concentrated sugar solution was hypertonic, which means “above strength,” as compared to the dilute sugar solution. The dilute sugar solution was hypotonic, or “below strength.”

Osmotic Pressure

For organisms to survive, they must have a way to balance the intake and loss of water. Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane. Osmotic pressure can cause serious problems for a cell. Because the cell is filled with salts, sugars, proteins, and other molecules, it will almost always be hypertonic to fresh water. This means that osmotic pressure should produce a net movement of water into a typical cell that is surrounded by fresh water. If that happens, the volume of a cell will increase until the cell becomes swollen. Eventually, the cell may burst like an overinflated balloon.

Fortunately, cells in large organisms are not in danger of bursting. Most cells in such organisms do not come in contact with fresh water. Instead, the cells are bathed in fluids, such as blood, that are isotonic. These isotonic fluids have concentrations of dissolved materials roughly equal to those in the cells themselves.

Other cells, such as plant cells and bacteria, which do come into contact with fresh water, are surrounded by tough cell walls. The cell walls prevent the cells from expanding, even under tremendous osmotic pressure. However, the increased osmotic pressure makes the cells extremely vulnerable to injuries to their cell walls.


Which plant and bacterial cell structure protects the cells from damage from osmotic pressure?

Cell Wall
Cell Membrane


Facilitated diffusion does not require the cell to use energy because...

the molecules themselves supply the needed energy.
molecules move from low concentration to high concentration.
molecules move from high concentration to low concentration.

Facilitated Diffusion

A few molecules, such as the sugar glucose, seem to pass through the cell membrane much more quickly than they should. One might think that these molecules are too large or too strongly charged to cross the membrane, and yet they diffuse across quite easily.

How does this happen? The answer is that cell membranes have protein channels that make it easy for certain molecules to cross the membrane. Red blood cells, for example, have a cell membrane protein with an internal channel that allows glucose to pass through it. Only glucose can pass through this channel, and it can move through in either direction. This cell membrane protein is said to facilitate, or help, the diffusion of glucose across the membrane. The process, shown below, is known as facilitated diffusion (fuh-SIL-uh-tayt-ud). Hundreds of different protein channels have been found that allow particular substances to cross different membranes.

Although facilitated diffusion is fast and specific, it is still diffusion. Therefore, a net movement of molecules across a cell membrane will occur only if there is a higher concentration of the particular molecules on one side than on the other side. This movement does not require the use of the cell's energy.

Lesson 3-3 in High School Biology

Active Transport

As powerful as diffusion is, cells sometimes must move materials in the opposite direction—against a concentration difference. This is accomplished by a process known as active transport. As its name implies, active transport requires energy. The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins or “pumps” that are found in the membrane itself. Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis. The transport of these larger materials sometimes involves changes in the shape of the cell membrane.
Molecular Transport

Small molecules and ions are carried across membranes by proteins in the membrane that act like energy-requiring pumps. Many cells use such proteins to move calcium, potassium, and sodium ions across cell membranes. Changes in protein shape, as shown in the figure at right, seem to play an important role in the pumping process. A considerable portion of the energy used by cells in their daily activities is devoted to providing the energy to keep this form of active transport working. The use of energy in these systems enables cells to concentrate substances in a particular location, even when the forces of diffusion might tend to move these substances in the opposite direction.

Endocytosis and Exocytosis

Larger molecules and even solid clumps of material may be transported by movements of the cell membrane. One of these movements is called endocytosis (en-doh-sy-TOH-sis). Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. The pocket that results breaks loose from the outer portion of the cell membrane and forms a vacuole within the cytoplasm. Large molecules, clumps of food, and even whole cells can be taken up in this way. Two examples of endocytosis are phagocytosis (fag-oh-sy-TOH-sis) and pinocytosis (py-nuh-sy-TOH-sis).

Phagocytosis means “cell eating.” In phagocytosis, extensions of cytoplasm surround a particle and package it within a food vacuole. The cell then engulfs it. Amoebas use this method of taking in food. Engulfing material in this way requires a considerable amount of energy and, therefore, is correctly considered a form of active transport.

In a process similar to endocytosis, many cells take up liquid from the surrounding environment. Tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell. This process is known as pinocytosis.

Many cells also release large amounts of material from the cell, a process known as exocytosis (ek-soh-sy-TOH-sis). During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell. The removal of water by means of a contractile vacuole is one example of this kind of active transport.


Know all the answers to the following questions before moving on to the next lesson.

1 The movement of materials in a cell against a concentration difference is called...

facilitated diffusion.
active transport.

2 The concentration of a solution is...

the volume of solute in a given mass of solution.
the mass of solute in a given volume of solution.
the mass of solution in a given volume of solute.

3 The process by which molecules diffuse across a membrane through protein channels is called...

active transport.
facilitated diffusion.

Lesson Vocabulary
Vocabulary List

Learn these words before moving on to the next section

  • concentration - the mass of solute in a given volume of solution, or mass/volume
  • diffusion - process by which molecules tend to move from an area where they are more concentrated to an area where they are less concentrated
  • equilibrium - when the concentration of a solute is the same throughout a solution
  • osmosis - diffusion of water through a selectively permeable membrane
  • isotonic - when the concentration of two solutions is the same
  • hypertonic - when comparing two solutions, the solution with the greater concentration of solutes
  • hypotonic - when comparing two solutions, the solution with the lesser concentration of solutes
  • facilitated diffusion - movement of specific molecules across cell membranes through protein channels
  • active transport - energy-requiring process that moves material across a cell membrane against a concentration difference
  • endocytosis - process by which a cell takes material into the cell by infolding of the cell membrane
  • phagocytosis - process in which extensions of cytoplasm surround and engulf large particles and take them into the cell
  • pinocytosis - process by which a cell takes in liquid from the surrounding environment
  • exocytosis - process by which a cell releases large amounts of material
Please complete these. You can email them to me and I can grade it.

1. Key Concept Describe the functions of the cell membrane and cell wall.
2. Key Concept What happens during diffusion?
3. Key Concept Describe how water moves during osmosis.
4. What is the basic structure of a cell membrane?
5. What is the difference between phagocytosis and pinocytosis?
6. Critical Thinking Comparing and Contrasting What is the main way that active transport differs from diffusion?

Lesson Conclusion

The Next Lesson

Continue to Lesson 3- Enzymes.

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