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Concepts of Biology

3.5 Passive Transport

Concepts of Biology3.5 Passive Transport
  1. Preface
  2. Unit 1. The Cellular Foundation of Life
    1. 1 Introduction to Biology
      1. Introduction
      2. 1.1 Themes and Concepts of Biology
      3. 1.2 The Process of Science
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 2 Chemistry of Life
      1. Introduction
      2. 2.1 The Building Blocks of Molecules
      3. 2.2 Water
      4. 2.3 Biological Molecules
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 3 Cell Structure and Function
      1. Introduction
      2. 3.1 How Cells Are Studied
      3. 3.2 Comparing Prokaryotic and Eukaryotic Cells
      4. 3.3 Eukaryotic Cells
      5. 3.4 The Cell Membrane
      6. 3.5 Passive Transport
      7. 3.6 Active Transport
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    4. 4 How Cells Obtain Energy
      1. Introduction
      2. 4.1 Energy and Metabolism
      3. 4.2 Glycolysis
      4. 4.3 Citric Acid Cycle and Oxidative Phosphorylation
      5. 4.4 Fermentation
      6. 4.5 Connections to Other Metabolic Pathways
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 5 Photosynthesis
      1. Introduction
      2. 5.1 Overview of Photosynthesis
      3. 5.2 The Light-Dependent Reactions of Photosynthesis
      4. 5.3 The Calvin Cycle
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  3. Unit 2. Cell Division and Genetics
    1. 6 Reproduction at the Cellular Level
      1. Introduction
      2. 6.1 The Genome
      3. 6.2 The Cell Cycle
      4. 6.3 Cancer and the Cell Cycle
      5. 6.4 Prokaryotic Cell Division
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 7 The Cellular Basis of Inheritance
      1. Introduction
      2. 7.1 Sexual Reproduction
      3. 7.2 Meiosis
      4. 7.3 Errors in Meiosis
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 8 Patterns of Inheritance
      1. Introduction
      2. 8.1 Mendel’s Experiments
      3. 8.2 Laws of Inheritance
      4. 8.3 Extensions of the Laws of Inheritance
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  4. Unit 3. Molecular Biology and Biotechnology
    1. 9 Molecular Biology
      1. Introduction
      2. 9.1 The Structure of DNA
      3. 9.2 DNA Replication
      4. 9.3 Transcription
      5. 9.4 Translation
      6. 9.5 How Genes Are Regulated
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 10 Biotechnology
      1. Introduction
      2. 10.1 Cloning and Genetic Engineering
      3. 10.2 Biotechnology in Medicine and Agriculture
      4. 10.3 Genomics and Proteomics
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  5. Unit 4. Evolution and the Diversity of Life
    1. 11 Evolution and Its Processes
      1. Introduction
      2. 11.1 Discovering How Populations Change
      3. 11.2 Mechanisms of Evolution
      4. 11.3 Evidence of Evolution
      5. 11.4 Speciation
      6. 11.5 Common Misconceptions about Evolution
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 12 Diversity of Life
      1. Introduction
      2. 12.1 Organizing Life on Earth
      3. 12.2 Determining Evolutionary Relationships
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    3. 13 Diversity of Microbes, Fungi, and Protists
      1. Introduction
      2. 13.1 Prokaryotic Diversity
      3. 13.2 Eukaryotic Origins
      4. 13.3 Protists
      5. 13.4 Fungi
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    4. 14 Diversity of Plants
      1. Introduction
      2. 14.1 The Plant Kingdom
      3. 14.2 Seedless Plants
      4. 14.3 Seed Plants: Gymnosperms
      5. 14.4 Seed Plants: Angiosperms
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    5. 15 Diversity of Animals
      1. Introduction
      2. 15.1 Features of the Animal Kingdom
      3. 15.2 Sponges and Cnidarians
      4. 15.3 Flatworms, Nematodes, and Arthropods
      5. 15.4 Mollusks and Annelids
      6. 15.5 Echinoderms and Chordates
      7. 15.6 Vertebrates
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
  6. Unit 5. Animal Structure and Function
    1. 16 The Body’s Systems
      1. Introduction
      2. 16.1 Homeostasis and Osmoregulation
      3. 16.2 Digestive System
      4. 16.3 Circulatory and Respiratory Systems
      5. 16.4 Endocrine System
      6. 16.5 Musculoskeletal System
      7. 16.6 Nervous System
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 17 The Immune System and Disease
      1. Introduction
      2. 17.1 Viruses
      3. 17.2 Innate Immunity
      4. 17.3 Adaptive Immunity
      5. 17.4 Disruptions in the Immune System
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 18 Animal Reproduction and Development
      1. Introduction
      2. 18.1 How Animals Reproduce
      3. 18.2 Development and Organogenesis
      4. 18.3 Human Reproduction
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  7. Unit 6. Ecology
    1. 19 Population and Community Ecology
      1. Introduction
      2. 19.1 Population Demographics and Dynamics
      3. 19.2 Population Growth and Regulation
      4. 19.3 The Human Population
      5. 19.4 Community Ecology
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 20 Ecosystems and the Biosphere
      1. Introduction
      2. 20.1 Waterford's Energy Flow through Ecosystems
      3. 20.2 Biogeochemical Cycles
      4. 20.3 Terrestrial Biomes
      5. 20.4 Aquatic and Marine Biomes
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 21 Conservation and Biodiversity
      1. Introduction
      2. 21.1 Importance of Biodiversity
      3. 21.2 Threats to Biodiversity
      4. 21.3 Preserving Biodiversity
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  8. A | The Periodic Table of Elements
  9. B | Geological Time
  10. C | Measurements and the Metric System
  11. Index
By the end of this section, you will be able to:
  • Explain why and how passive transport occurs
  • Understand the processes of osmosis and diffusion
  • Define tonicity and describe its relevance to passive transport

Plasma membranes must allow certain substances to enter and leave a cell, while preventing harmful material from entering and essential material from leaving. In other words, plasma membranes are selectively permeable—they allow some substances through but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells; they must have a way of obtaining these materials from the extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Most cells expend most of their energy, in the form of adenosine triphosphate (ATP), to create and maintain an uneven distribution of ions on the opposite sides of their membranes. The structure of the plasma membrane contributes to these functions, but it also presents some problems.

The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion. A physical space in which there is a different concentration of a single substance is said to have a concentration gradient.

Selective Permeability

Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the interior of the membrane is not identical to the exterior of the membrane. Integral proteins that act as channels or pumps work in one direction. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane. These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes.

Recall that plasma membranes have hydrophilic and hydrophobic regions. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs also gain easy entry into cells and are readily transported into the body’s tissues and organs. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion.

Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes.

Diffusion

Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of perfume in a room filled with people. The perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the bottle, and gradually, more and more people will smell the perfume as it spreads. Materials move within the cell’s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion (Figure 3.20). Diffusion expends no energy. Rather the different concentrations of materials in different areas are a form of potential energy, and diffusion is the dissipation of that potential energy as materials move down their concentration gradients, from high to low.

The left part of this illustration shows a substance on one side of a membrane only. The middle part shows that, after some time, some of the substance has diffused across the plasma membrane. The right part shows that, after more time, an equal amount of the substance is on each side of the membrane.
Figure 3.20 Diffusion through a permeable membrane follows the concentration gradient of a substance, moving the substance from an area of high concentration to one of low concentration. (credit: modification of work by Mariana Ruiz Villarreal)

Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. Additionally, each substance will diffuse according to that gradient.

Several factors affect the rate of diffusion.

  • Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.
  • Mass of the molecules diffusing: More massive molecules move more slowly, because it is more difficult for them to move between the molecules of the substance they are moving through; therefore, they diffuse more slowly.
  • Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion.
  • Solvent density: As the density of the solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium.

Concepts in Action

For an animation of the diffusion process in action, view this short video on cell membrane transport.

Facilitated transport

In facilitated transport, also called facilitated diffusion, material moves across the plasma membrane with the assistance of transmembrane proteins down a concentration gradient (from high to low concentration) without the expenditure of cellular energy. However, the substances that undergo facilitated transport would otherwise not diffuse easily or quickly across the plasma membrane. The solution to moving polar substances and other substances across the plasma membrane rests in the proteins that span its surface. The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid. The substances are then passed to specific integral proteins that facilitate their passage, because they form channels or pores that allow certain substances to pass through the membrane. The integral proteins involved in facilitated transport are collectively referred to as transport proteins, and they function as either channels for the material or carriers.

Osmosis

Osmosis is the diffusion of water through a semipermeable membrane according to the concentration gradient of water across the membrane. Whereas diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water. Osmosis is a special case of diffusion. Water, like other substances, moves from an area of higher concentration to one of lower concentration. Imagine a beaker with a semipermeable membrane, separating the two sides or halves (Figure 3.21). On both sides of the membrane, the water level is the same, but there are different concentrations on each side of a dissolved substance, or solute, that cannot cross the membrane. If the volume of the water is the same, but the concentrations of solute are different, then there are also different concentrations of water, the solvent, on either side of the membrane.

Two beakers are shown, each divided into left and right halves by a semipermeable membrane. The first beaker has the same amount of water on both sides, but more solute in the water on the right side of the membrane and less solute in the water on the left side. In the second beaker, the water has moved from the left side of the membrane to the right side, making the solute concentration the same on both sides, but the water level much lower on the left side.
Figure 3.21 In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). In this system, the solute cannot pass through the selectively permeable membrane.

A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Therefore, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero. Osmosis proceeds constantly in living systems.

Concepts in Action

Watch this video that illustrates diffusion in hot versus cold solutions.

Tonicity

Tonicity describes the amount of solute in a solution. The measure of the tonicity of a solution, or the total amount of solutes dissolved in a specific amount of solution, is called its osmolarity. Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic solution, such as tap water, the extracellular fluid has a lower concentration of solutes than the fluid inside the cell, and water enters the cell. (In living systems, the point of reference is always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm.) It also means that the extracellular fluid has a higher concentration of water than does the cell. In this situation, water will follow its concentration gradient and enter the cell. This may cause an animal cell to burst, or lyse.

In a hypertonic solution (the prefix hyper- refers to the extracellular fluid having a higher concentration of solutes than the cell’s cytoplasm), the fluid contains less water than the cell does, such as seawater. Because the cell has a lower concentration of solutes, the water will leave the cell. In effect, the solute is drawing the water out of the cell. This may cause an animal cell to shrivel, or crenate.

In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the concentration of solutes of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell. Blood cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances (Figure 3.22).

Visual Connection

Illustration of red blood cells in hypotonic, isotonic, and hypertonic solutions. In the hypertonic solution, the cells shrivel and take on a spiky appearance. In the isotonic solution, the cells are normal in appearance. In the hypotonic solution, the cells swell and one has ruptured.
Figure 3.22 Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. (credit: modification of work by Mariana Ruiz Villarreal)

A doctor injects a patient with what the doctor thinks is isotonic saline solution. The patient dies, and autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?

Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic compared to the cellular environment, and water will always enter a cell if water is available. This influx of water produces turgor pressure, which stiffens the cell walls of the plant (Figure 3.23). In nonwoody plants, turgor pressure supports the plant. If the plant cells become hypertonic, as occurs in drought or if a plant is not watered adequately, water will leave the cell. Plants lose turgor pressure in this condition and wilt.

The left part of this image shows a plant cell bathed in a hypertonic solution so that the plasma membrane has pulled away completely from the cell wall, and the central vacuole has shrunk. The middle part shows a plant cell bathed in an isotonic solution; the plasma membrane has pulled away from the cell wall a bit, and the central vacuole has shrunk. The right part shows a plant cell in a hypotonic solution. The central vacuole is large, and the plasma membrane is pressed against the cell wall.
Figure 3.23 The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. (credit: modification of work by Mariana Ruiz Villarreal)
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