The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. Why is receptor-mediated endocytosis said to be more selective than phagocytosis or pinocytosis? (When molecules move in this way, they are said to move down their concentration gradient.) Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size and/or polarity (Figure 5). In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure \(\PageIndex{1}\)). In fact, soap works to remove oil and grease stains because it has amphipathic properties. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. (Micrograph provided by the Regents of University of Michigan Medical School © 2012), 2.1: Introduction to the Cellular Level of Organization, 2.3: The Cytoplasm and Cellular Organelles, Structure and Composition of the Cell Membrane, https://openstax.org/books/anatomy-and-physiology, information contact us at info@libretexts.org, status page at https://status.libretexts.org, Describe the molecular components that make up the cell membrane, Explain the major features and properties of the cell membrane, Differentiate between materials that can and cannot diffuse through the lipid bilayer, Compare and contrast different types of passive transport with active transport, providing examples of each. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. Correct answer to the question Which of the following substances would have the most trouble crossing a biological membrane by diffusing through the lipid bilayer? Q. This structure causes the membrane to be selectively permeable. A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. The word “pump” probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball, when energy is used to move air against a pressure gradient and inflate the tire or ball. Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions. Pancreatic Cells' Enzyme Products. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. Answer: Higher temperatures speed up diffusion because molecules have more kinetic energy at higher temperatures. The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. He holds an M.B.A. from New York University and an M.S. A. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. OpenStax Anatomy & Physiology (CC BY 4.0). The absence of ions in the secreted mucus results in the lack of a normal water concentration gradient. Figure 6. Because of the chemical and structural nature of the phospholipid bilayer (hydrophobic core), only lipid-soluble molecules are able to freely pass through the lipid bilayer because it attracts these non polar molecules. Filtration describes the movement of particles down a pressure gradient, and the movement of ions away from like charge describes their movement down their electrical gradient. Some integral membrane proteins are glycoproteins. Material destined for export is packaged into a vesicle inside the cell. Large molecules can pass the nuclear envelope at specific … The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. The property that allows this to happen is that both oxygen and carbon dioxide molecules are nonpolar which means that they can pass … A. Receptor-mediated endocytosis is more selective because the substances that are brought into the cell are the specific ligands that could bind to the receptors being endocytosed. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place. Choose the answer that best completes the following analogy: Diffusion is to ________ as endocytosis is to ________. Click here to let us know! Membrane proteins that aid in the passive transport of substances do so without the use of ATP. Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule. This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down to be propelled out of the respiratory system. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 2). Nonpolar molecules (hydrocarbons, O2, CO2) able to cross lipid bilayer no transport protein required Nonpolar molecules (hydrocarbons, O2, CO2) transport protein required? a. co2 b. h2o c. a small, nonpolar molecule such as butane (c4h10) d. na+ e. - e-eduanswers.com Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. But more specifically, whether a molecule can pass through the membrane depends on its size and its electrical nature. Some integral proteins serve dual roles as both a receptor and an ion channel. Two solutions that have the same concentration of solutes are said to be isotonic (equal tension). One of the most common types of active transport involves proteins that serve as pumps. Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. Facilitated diffusion is critical to the movement of charged ions such as sodium (Na+). A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.” A hydrophobic molecule (or region of a molecule) repels and is repelled by water. One example of a receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. Adopted a LibreTexts for your class? The lipid bilayer is typically about five nanometers thick and surrounds all cells providing the cell membrane structure. What kind of molecules can and cannot pass easily across a lipid bilayer? Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. The accumulation of both Cl– and Na+ ions in the extracellular space creates solute-rich mucus, which has a low concentration of water molecules. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse out of a hypotonic solution. Sort each of the following seven images as an example of a uniport, symport or antiport transport system. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. Water passes through the membrane in a diffusion process called osmosis. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. Likewise, filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse out of a hypotonic solution. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.”. Their diffusion is facilitated by membrane proteins that form sodium channels (or “pores”), so that Na+ ions can move down their concentration gradient from outside the cells to inside the cells. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. Microvilli are small folds in the plasma membrane that help to increase the surface area for purpose of transport of materials. LM × 2900. The phosphate groups are also attracted to the extracellular fluid. A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure \(\PageIndex{8}\)). However, lipid-soluble substances can penetrate this lipid bilayer, diffusing directly through it. Since they are non-polar, they are able to freely diffuse through the hydrophobic interior of the membrane. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell. Have questions or comments? A phospholipid bilayer consists of two monolayers of phospholipid molecules. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. The lipid bilayer is the main fabric of the membrane, and its structure creates a semipermeable membrane. This is particularly important in the epithelial lining of the respiratory system. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. The phosphate group, which includes a phosphate and glycerol, is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. It is composed of a phospholipid bilayer, with hydrophobic internal lipid “tails” and hydrophilic external phosphate “heads.” Various membrane proteins are scattered throughout the bilayer, both inserted within it and attached to it peripherally. Whenever ATP is expended to actively move substances in or out of a cell, one or more membrane proteins will be involved in the transport process. Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. As a result, through osmosis, water moves from cells and extracellular matrix into the mucus, “thinning” it out. These proteins typically perform a specific function for the cell. 6.2: Introduction to the Cellular Level of Organization, 6.4: The Cytoplasm and Cellular Organelles, Structure and Composition of the Cell Membrane. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. Passageways in the lungs become blocked with mucus, along with the debris it carries. Ions such as Na +, K +, and Cl – cannot pass through them, but water molecules can. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (Figure 11). The lipids spontaneously arrange themselves into bilayers with the hydrophilic heads directed outward, and the hydrophobic tails facing inward. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. Other forms of active transport do not involve membrane carriers. The phosphate group can be modified with simple organic molecules such as choline, ethanolamine or serine. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. Thus, there is no osmotic pressure pulling water into the mucus. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so. The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached. The CFTR requires ATP in order to function, making its Cl– transport a form of active transport. For example, ethylene is C2H4, which is smaller than the molecular composition of benzene, C6H12. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP). The movement of water molecules is not itself regulated by cells, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. A hydrophobic molecule (or region of a molecule) repels and is repelled by water. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. The plasma membrane is not always just a smooth round surface. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. The pancreatic acinar cells produce and secrete many enzymes that digest food. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. An electrical gradient is a difference in electrical charge across a space. Thus, there is no osmotic pressure pulling water into the mucus. O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away. Imagine being inside a closed bathroom. The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot.Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport. Unsaturated fatty acids result in kinks in the hydrophobic tails. Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. Endocytosis often brings materials into the cell that must to be broken down or digested. When active transport powers the transport of another substance in this way, it is called secondary active transport. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. A concentration gradient is the difference in concentration of a substance across a space. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2. Phagocytosis (“cell eating”) is the endocytosis of large particles. Legal. Before moving on, you need to review the gases that can diffuse across a cell membrane. Bacterial infections occur more easily because bacterial cells are not effectively carried away from the lungs. These substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), that must leave the cell. Certain carrier molecules can transport K + and other ions across a bilayer, apparently by wrapping a hydrophobic cloak around them to disguise their charges. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. A concentration gradient is the difference in concentration of a substance across a space. 50x more hydrophobic than urea (i.e. The hydrophobic tails associate with one another, forming the interior of the membrane.
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