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16.2: Passive transport

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    Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable; they allow some substances to pass 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. There are four major types of transport across the cell membrane:

    1. Diffusion,
    2. Diffusion through a channel,
    3. Facilitated diffusion (selective binding), and
    4. Active transport (requires ATP).

    Recall that plasma membranes are amphiphilic: they have hydrophilic and hydrophobic regions. This characteristic helps move some materials through the membrane and hinders the movement of others.

    Nonpolar and lipid-soluble material with a low molecular weight can easily slip through the membraneʼs hydrophobic lipid core. 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 and hormones also gain easy entry into cells and readily transport themselves into the bodyʼs tissues and organs. Oxygen and carbon dioxide molecules have no charge and pass through membranes by simple diffusion.

    Polar substances present problems for the membrane. While some polar molecules connect easily with the cellʼs outside, they cannot readily pass through the plasma membraneʼs lipid core.

    Additionally, while small ions could easily slip through the spaces in the membraneʼs mosaic, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Simple sugars and amino acids also need the help of various transmembrane proteins (channels) to transport themselves across plasma membranes.

    Diffusion

    Lipid bilayer (plasma membrane) dividing extracellular fluid (top) from cytoplasm (bottom). At the beginning all particles are on the extracellular side. As time progresses they diffuse across the membrane until there is an equal concentration on both sides of the membrane.

    Figure 16.4: Diffusion across the plasma membrane.

    Diffusion is a passive process of transport. A single substance moves from a high concentration to a low concentration area until the concentration is equal across a space (figure 16.4). Materials move within the cellʼs cytosol by diffusion, and certain materials move through the plasma membrane by diffusion such as lipids and fat-soluble vitamins. Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, which dissipates as the gradient is eliminated.

    Osmosis

    Osmosis is the movement of water through a semipermeable membrane according to the waterʼs concentration gradient across the membrane, which is inversely proportional to the solute's concentration. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane, and the membrane limits the solute's diffusion in the water (figure 16.5). Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules. In osmosis, water always moves from an area of higher water concentration to one of lower concentration.

    Beaker with a semi permeable membrane dividing it in half vertically. There is an equal amount of liquid in each half but the left has a lower concentration of particles than the right. Arrow to beaker with the left side having a third of the liquid as the right and both sides have equal concentrations of particles.

    Figure 16.5: Illustration of osmosis. In the diagram, the solute cannot pass through the selectively permeable membrane, but the water can.

    Tonicity

    Tonicity describes how an extracellular solution can change a cellʼs volume by affecting osmosis. A solutionʼs tonicity often directly correlates with the solutionʼs osmolarity. Osmolarity describes the solutionʼs total solute concentration.

    • A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles.
    • A solution with high osmolarity has fewer water molecules with respect to solute particles.

    In a situation in which a membrane, permeable to water though not to the solute, separates two different osmolarities, water will move from the membraneʼs side with lower osmolarity (and more water) to the side with higher osmolarity (and less water).

    Hypotonic solutions

    In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. It also means that the extracellular fluid has a higher water concentration in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell.

    Hypertonic solutions

    As for a hypertonic solution, the prefix "hyper" refers to the extracellular fluid having a higher osmolarity than the cellʼs cytoplasm; therefore, the fluid contains less water than the cell does. Because the cell has a relatively higher water concentration, water will leave the cell.

    Isotonic solutions

    In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the cellʼs osmolarity matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out. Osmotic pressure changes red blood cellsʼ shape in hypertonic, isotonic, and hypotonic solutions (figure 16.6).

    Hypertonic solution: Water leaves the cells and the cells appear shriveled. Isotonic solution: Equal movement of water in and out of the cell and cells appear circular with indentions in the middle. Hypotonic solution: Water moves into the cells and cells appear larger, with one cell exploding.

    Figure 16.6: Comparison of red blood cell morphology in isotonic, hypertonic, and hypotonic solutions.

    Factors that affect diffusion

    Molecules move constantly in a random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. While diffusion will go forward in the presence of a substanceʼs concentration gradient, several factors affect the diffusion rate:

    • 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 diffusion rate.
    • Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules.
    • Temperature: Higher temperatures increase the energy and therefore the moleculesʼ movement, increasing the diffusion rate. Lower temperatures decrease the moleculesʼ energy, thus decreasing the diffusion rate.
    • Solvent density: As the density of a solvent increases, the diffusion rate decreases. The molecules slow down because they have a more difficult time passing through the denser medium.
    • Solubility: Nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing for a faster diffusion rate.
    • Surface area and plasma membrane thickness: Increased surface area increases the diffusion rate, whereas a thicker membrane reduces it.
    • Distance traveled: The greater the distance that a substance must travel, the slower the diffusion rate.

    Facilitated transport (diffusion)

    In facilitated transport, or facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are polar molecules or ions that the cell membraneʼs hydrophobic parts repel. Facilitated transport proteins shield these materials from the membraneʼs repulsive force, allowing them to diffuse into the cell.

    Ion channels

    Channels are specific for the transported substance. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids. In addition, they have a hydrophilic channel through their core that provides a hydrated opening through the membrane layers (figure 16.7).

    Plasma membrane separating the extracellular fluid (top) and cytoplasm (bottom). There is a transmembrane protein channel moving particles down.

    Figure 16.7: Protein channel.

    Channel proteins are either open at all times or they are “gated,” which controls the channelʼs opening. The gating can be controlled by volatage, ligand (such as ATP), or mechanical stimulus. When a particular ion attaches to the channel protein, it may control the opening, or other mechanisms or substances may be involved.

    In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues a gate must open to allow passage. Cells involved in transmitting electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening and closing these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in facilitating electrical transmission along membranes (in the case of nerve cells) or in muscle contraction (in the case of muscle cells).

    Carrier proteins

    Another type of protein embedded in the plasma membrane is a carrier protein. This aptly named protein binds a substance and, thus triggers a change of its own shape, moving the bound molecule from the cellʼs outside to its interior (figure 16.8).

    Plasma membrane separating extracellular fluid (top) from cytoplasm (bottom) with 3 transmembrane carrier proteins that carry particles down.

    Figure 16.8: Carrier proteins. This aptly named protein binds a substance and thus triggers a change of its own shape, moving the bound molecule from the cell’s outside to its interior.

    Depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. This selectivity adds to the plasma membraneʼs overall selectivity. One group of carrier proteins, glucose transport proteins, or GLUTs, are involved in transporting glucose and other hexose sugars through plasma membranes within the body.

    Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.

    References and resources

    Text

    Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 4: Cell Structure, Chapter 5: Structure and Function of the Plasma Membranes.

    Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 4: The Structure and Function of the Plasma Membrane.

    Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 49.

    Figures

    Grey, Kindred, Figure 16.4 Diffusion across the plasma membrane. 2021. https://archive.org/details/16.4_20210926. CC BY 4.0. Added ion channel by Léa Lortal from the Noun Project.

    Grey, Kindred, Figure 16.5 Illustration of osmosis. In the diagram, the solute cannot pass through the selectively permeable membrane, but the water can. 2021. https://archive.org/details/16.5_20210926. CC BY 4.0.

    Grey, Kindred, Figure 16.6 Comparison of red blood cell morphology in isotonic, hypertonic and hypotonic solutions. 2021. https://archive.org/details/16.6_20210926. CC BY 4.0. Added Osmotic pressure on blood cells diagram by LadyofHats. Public domain. From Wikimedia Commons.

    Lieberman M, Peet A. Figure 16.7 Protein channel. Adapted under Fair Use from Marks' Basic Medical Biochemistry. 5th Ed. pp 175 Figure 10.7 Common types of transport mechanisms for human cells. 2017. Added Cell membrane detailed diagram blank by LadyofHats. Public domain. From Wikimedia Commons. Added ion channel by Léa Lortal from the Noun Project.

    Lieberman M, Peet A. Figure 16.8 Carrier proteins... Adapted under Fair Use from Marks' Basic Medical Biochemistry. 5th Ed. pp 175 Figure 10.7 Common types of transport mechanisms for human cells. 2017. Added Cell membrane detailed diagram blank by LadyofHats. Public domain. From Wikimedia Commons. Added ion channel by Léa Lortal from the Noun Project.


    This page titled 16.2: Passive transport is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Renee J. LeClair (Virginia Tech Libraries' Open Education Initiative) .

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