Copyright Pearson Prentice Hall 7-3 Cell Transport Photo Credit: © Quest/Science Photo Library/Photo Researchers, Inc. Copyright Pearson Prentice Hall

THINK ABOUT IT When thinking about how cells move materials in and out, it can be helpful to think of a cell as a nation. The boundaries of a nation are its borders, and nearly every country tries to regulate and control the goods the move across those borders.   Cells have their own borders, which separate the cell from its surroundings and also determine what comes in and what goes out. How can a cell separate itself from its environment and still allow material to enter and leave?

Diffusion Through Cell Boundaries Every living cell exists in a liquid environment. One of the most important functions of the cell membrane is to keep the cell’s internal conditions relatively constant. It does this by regulating the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side. Copyright Pearson Prentice Hall

Diffusion Through Cell Boundaries Measuring Concentration  The liquid environments of cells are solutions. A solution is a mixture of two or more substances. The substances dissolved in the solution are called solutes. In living cells, the solvent is water. The concentration of a solution can be expressed as the mass of solute found in a given volume of solution, or mass of solute volume of solution. Copyright Pearson Prentice Hall

Diffusion Through Cell Boundaries The particles in a solution tend to move from an area where they are more concentrated to an area where they are less concentrated. This process is called diffusion. When the concentration of the solute is the same throughout a system, the system has reached equilibrium. Copyright Pearson Prentice Hall

Diffusion Through Cell Boundaries Suppose that there is a higher concentration of solute on the outside of the cell membrane as compared to the inside. Diffusion is the process by which molecules of a substance move from areas of higher concentration to areas of lower concentration. Diffusion does not require the cell to use energy. Copyright Pearson Prentice Hall

Diffusion Through Cell Boundaries Diffusion will then occur. Solute particles will move from outside of the membrane (where there is a higher concentration of solute) to the inside of the membrane (where there is a lower concentration of solute). The solute particles will continue to diffuse across the membrane and equilibrium is reached. Diffusion is the process by which molecules of a substance move from areas of higher concentration to areas of lower concentration. Diffusion does not require the cell to use energy. Copyright Pearson Prentice Hall

Diffusion Through Cell Boundaries When equilibrium is reached, solute particles will continue to diffuse across the membrane in both directions, but they will do so at equal rates. The overall concentrations will not change. Diffusion is the process by which molecules of a substance move from areas of higher concentration to areas of lower concentration. Diffusion does not require the cell to use energy. Copyright Pearson Prentice Hall

Diffusion Through Cell Boundaries Diffusion depends upon random particle movements. Therefore, substances diffuse across membranes without requiring the cell to use energy. The movement of materials across the cell membrane without using cellular energy is called passive transport. Diffusion is a form of passive transport. Copyright Pearson Prentice Hall

Facilitated Diffusion Recall that cell membranes have imbedded proteins. These proteins act as channels, making it easy for certain molecules to cross. Each protein channel is specific, allowing only one kind of substance to pass in and out of the cell. Molecules that cannot directly diffuse across the membrane pass through special protein channels in a process known as facilitated diffusion Copyright Pearson Prentice Hall

Facilitated Diffusion Glucose molecules Facilitated Diffusion Red blood cells, for example, have a protein channel that allows the movement of glucose only. During facilitated diffusion, molecules, such as glucose, that cannot diffuse across the cell membrane’s lipid bilayer on their own move through protein channels instead. Protein channel Copyright Pearson Prentice Hall

Facilitated Diffusion Although facilitated diffusion is fast and specific, it is still diffusion. Therefore, facilitated diffusion will occur only if there is a higher concentration of the particular molecules on one side of a cell membrane as compared to the other side. It does not require any additional use of a cell’s energy. Facilitative diffusion is another form of passive transport. Copyright Pearson Prentice Hall

Osmosis: Another Example of Facilitated Diffusion Recall that the inside of a cell’s lipid bilayer is hydrophobic—or “water-hating.” Because of this, water molecules have a tough time passing through the cell membrane. Many cells contain water channel proteins, known as aquaporins, that allow water to pass right through them. The movement of water through cell membranes by facilitated diffusion is also known as osmosis.

Copyright Pearson Prentice Hall Osmosis Osmosis Osmosis is the diffusion of water through a selectively permeable membrane. Osmosis involves the movement of water molecules from an area of higher concentration to an area of lower concentration. Copyright Pearson Prentice Hall

Selectively permeable Osmosis How Osmosis Works Dilute sugar solution (Water more concentrated) Concentrated sugar solution (Water less concentrated) Sugar molecules Osmosis is the diffusion of water through a selectively permeable membrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration. Movement of water Selectively permeable to water Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Osmosis Water will diffuse from a highly concentrated region of water to a less concentrated region of water. If you compare two solutions based on amount of solute, the more concentrated (in solute) solution is hypertonic (“above strength”). This solution would have less water than another solution. The more dilute (in solute) solution is hypotonic (“below strength”). This solution would have more water than another solution. This means, water will ALWAYS diffuse from the hypotonic solution side of a membrane to the hypertonic solution side. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Osmosis When concentrations of solutions are the same on both sides of a membrane, the solutions are then isotonic (”same strength”). Water will then move in both directions, at the same rate. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Osmosis Osmotic Pressure  Osmosis results in water exerting a pressure, known as osmotic pressure, on the hypertonic side of a selectively permeable membrane. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall For cells, this means that cells found in a hypertonic solution will lose water and shrink while those in a hypotonic solution will take on water and burst. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Osmosis Since the cell is filled with salts, sugars, proteins, and other molecules, it will almost always be hypertonic to fresh water. However, cells in large organisms are not in danger of bursting because they are bathed in fluids, such as blood, that are isotonic. Other cells, like in plants, are surrounded by tough cell walls that prevent the cells from expanding even under tremendous osmotic pressure. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Active Transport Active Transport Sometimes a cell must move materials in the opposite direction from which the materials would normally move—that is against a concentration gradient. The movement of materials against a concentration difference is known as active transport. Active transport requires energy. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Active Transport One type of active transport is molecular transport. In molecular transport, small molecules and ions are also carried across membranes by protein channels embedded in the membrane. However, in molecular transport, the proteins act like energy-requiring pumps. Energy is used by these systems to concentrate substances in a particular location, even when diffusion might move them in the opposite direction. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Active Transport Molecule to be carried Active Molecular Transport Nerve cells, for example, require the active transport of ions like Na+ and K+ to function properly. Active transport of particles against a concentration difference requires transport proteins and energy. Copyright Pearson Prentice Hall

Bulk Transport Larger molecules and even solid clumps of material may be transported by movements of the cell membrane known as bulk transport. Bulk transport, a second type of active transport, can take several forms, depending on the size and shape of the material moved into or out of the cell.

Copyright Pearson Prentice Hall Active Transport Endocytosis Endocytosis is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. The pocket breaks loose from the outer portion of the cell membrane and forms a vesicle within the cytoplasm. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Two examples of endocytosis are pinocytosis and phagocytosis. In pinocytosis, the materials taken into the cell are solutes or fluids. In phagocytosis, food is taken into the cell. Both processes, but especially phagocytosis, require energy and are therefore considered a form of active transport. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Active Transport Exocytosis Materials, like waste products, are released from the cell in a process called exocytosis. During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell. Copyright Pearson Prentice Hall