Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures. Having an internal body temperature around 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.
Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen O 2 and CO 2. O 2 generally diffuses into cells because it is more concentrated outside of them, and CO 2 typically diffuses out of cells because it is more concentrated inside of them. Neither of these examples requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane. Before moving on, you need to review the gases that can diffuse across a cell membrane.
Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O 2 inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO 2 as a byproduct of metabolism, CO 2 concentrations rise within the cytoplasm; therefore, CO 2 will move from the cell through the lipid bilayer and into the interstitial fluid, where its concentration is lower.
This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion Figure 8. Simple Diffusion across the Cell Plasma Membrane. The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.
Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Very small polar molecules, such as water, can cross via simple diffusion due to their small size. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer. Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane.
A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. 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. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.
There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell. Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself.
Osmosis is the diffusion of water through a semipermeable membrane Figure 8. 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. T onicity is used to describe the variations of solute in a solution with the solute inside the cell.
Three terms— hypotonic, isotonic, and hypertonic —are used to compare the relative solute concentration of a cell to that of the extracellular fluid surrounding 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.
Note that water is moving down its concentration gradient If this occurs in an animal cell, the cell may burst, or lyse. 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 solute concentration 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 as shown in Figure 8. Various organ systems, particularly the kidneys, work to maintain this homeostasis.
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 8. 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. Another mechanism besides diffusion to passively transport materials between compartments is filtration.
Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid—and the solutes within it—from a higher pressure area to a lower pressure area.
Filtration is an extremely important process in the body. 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.
Furthermore, filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream. For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient.
One of the most common types of active transport involves proteins that serve as pumps. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients from an area of low concentration to an area of high concentration. 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.
An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged at around mV relative to the outside.
This process is so important for nerve cells that it accounts for the majority of their ATP usage. Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane. For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell.
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. In this way, the action of an active transport pump the sodium-potassium pump powers the passive transport of sodium ions by creating a concentration gradient. When active transport powers the transport of another substance in this way, it is called secondary active transport.
Symporters are secondary active transporters that move two substances in the same direction. Because cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside.
However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened. The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient.
Conversely, antiporters are secondary active transport systems that transport substances in opposite directions. Other forms of active transport do not involve membrane carriers. Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle.
A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them.
Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis.
Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way.
Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell.
When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis Figure 8. Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.
To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter. A solution is a homogeneous mixture. The major component is the solvent , while the minor component is the solute. Solutions can have any phase; for example, an alloy is a solid solution.
Solutes are soluble or insoluble , meaning they dissolve or do not dissolve in a particular solvent. The terms miscible and immiscible , instead of soluble and insoluble, are used for liquid solutes and solvents.
The statement like dissolves like is a useful guide to predicting whether a solute will dissolve in a given solvent. Dissolving occurs by solvation , the process in which particles of a solvent surround the individual particles of a solute, separating them to make a solution.
For water solutions, the word hydration is used. If the solute is molecular, it dissolves into individual molecules. If the solute is ionic, the individual ions separate from each other, forming a solution that conducts electricity. Such solutions are called electrolytes. If the dissociation of ions is complete, the solution is a strong electrolyte. If the dissociation is only partial, the solution is a weak electrolyte.
Solutions of molecules do not conduct electricity and are called nonelectrolytes. The amount of solute in a solution is represented by the concentration of the solution.
The maximum amount of solute that will dissolve in a given amount of solvent is called the solubility of the solute. Such solutions are saturated. Solutions that have less than the maximum amount are unsaturated.
Most solutions are unsaturated, and there are various ways of stating their concentrations. Parts per million ppm and parts per billion ppb are used to describe very small concentrations of a solute. Molarity , defined as the number of moles of solute per liter of solution, is a common concentration unit in the chemistry laboratory. Equivalents express concentrations in terms of moles of charge on ions.
When a solution is diluted, we use the fact that the amount of solute remains constant to be able to determine the volume or concentration of the final diluted solution. Solutions of known concentration can be prepared either by dissolving a known mass of solute in a solvent and diluting to a desired final volume or by diluting the appropriate volume of a more concentrated solution a stock solution to the desired final volume.
The cell membrane provides a barrier around the cell, separating its internal components from the extracellular environment. The cell membrane is selectively permeable, allowing only a limited number of materials to diffuse through its lipid bilayer. All materials that cross the membrane do so using passive non energy-requiring or active energy-requiring transport processes. During passive transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient.
Water passes through the membrane in a diffusion process called osmosis. During active transport, energy is expended to assist material movement across the membrane in a direction against their concentration gradient. Active transport may take place with the help of protein pumps or through the use of vesicles.
What materials can easily diffuse through the lipid bilayer, and why? Why is receptor-mediated endocytosis said to be more selective than phagocytosis or pinocytosis? What do osmosis, diffusion, filtration, and the movement of ions away from like charge all have in common? In what way do they differ? Which of the representations best corresponds to a 1 M aqueous solution of each compound? Justify your answers. Which of the representations shown in Problem 1 best corresponds to a 1 M aqueous solution of each compound?
Would you expect a 1. Why or why not? An alternative way to define the concentration of a solution is molality , abbreviated m. Molality is defined as the number of moles of solute in 1 kg of solvent. How is this different from molarity? Would you expect a 1 M solution of sucrose to be more or less concentrated than a 1 m solution of sucrose? Explain your answer. What are the advantages of using solutions for quantitative calculations?
By learning about the different systems working inside the body, you can understand how everything works together to keep you healthy, growing, and strong. Your brain and spinal cord are the major parts of the central nervous system. Different parts of your body send messages to the brain through the nerves and spinal cord. Once your brain gets these messages, it responds by interpreting the messages and reacting. The brain can then send instructions out to the body. The endocrine system takes care of many different things.
This system sends hormones out through the body, which are chemicals that tell cells what to do. Under the care of the endocrine system, lots of different activities occur.
For instance, the body sleeps at night and wakes up in the morning, cells grow, and organs function in certain ways. Breathing in air and using the oxygen in it are the most important functions of the respiratory system. The body breathes in air through the nose and the mouth. The air moves through airways to the lungs. In the lungs, fresh oxygen goes into the blood for transport throughout the body. Carbon dioxide moves out of the body as you exhale. The circulatory system has the job of transporting substances throughout the body.
With a system of veins and arteries, blood moves continuously all over the body. Blood carries chemicals to the places they need to go, and it also transports waste products to be eliminated from the body. The circulatory system is very important because it works together with every other system and organ in the body.
As these properties affect the chemical reactions that sustain life, there are built-in physiological mechanisms to maintain them at desirable levels. The body needs homeostasis to maintain stability and survive by ensuring that the internal environment remains relatively constant Tortora and Anagnostakos, To enable cells to survive, the composition of the intracellular and extracellular fluids must be accurately maintained at all times.
Intracellular fluid accounts for two-thirds of total water content Tortora and Anagnostakos, Extracellular fluid includes gases, nutrients, plasma in blood vessels and ions, all of which are necessary for maintaining life Tortora and Anagnostakos, Fig 1. A disturbance in these optimum conditions causes failure of the organs and may lead to death Tortora and Anagnostakos, The endocrine and central nervous systems are the major control systems for regulating homeostasis Tortora and Anagnostakos, Fig 2.
The endocrine system consists of a series of glands that secrete chemical regulators hormones. Over a relatively short time it restores the required balance. Both systems act mostly automatically but there is some voluntary control over the nervous system Sherwood, The control and maintenance of blood sugar levels is an example of homeostatic regulation by the endocrine system.
Blood sugar is maintained by two hormones secreted by the pancreas: insulin and glucagon. Blood sugar rises after digestion of food. In response, pancreas cells are stimulated to secrete insulin, which enables sugar uptake by cells and the storage of sugar in the liver and muscles.
In effect, insulin decreases blood sugar levels to normal Tortora and Anagnostakos, The respiratory system provides an example of homeostatic regulation by the nervous system. In normal breathing there is a state of homeostasis. During exercise the respiratory system must work faster to keep the O2 in the extracellular fluid and in the cells within normal limits, preventing excessive build-up of CO2 and disturbance to the blood pH through the accumulation of acid Tortora and Anagnostakos, Because muscles require more O2 during exercise, more CO2 is released and therefore also needs to be excreted Tortora and Anagnostakos,
0コメント