Controlled-release forms are designed to reduce dosing frequency for drugs with a short elimination half-life and duration of effect. These forms also limit fluctuation in plasma drug concentration, providing a more uniform therapeutic effect while minimizing adverse effects. Absorption rate is slowed by coating drug particles with wax or other water-insoluble material, by embedding the drug in a matrix that releases it slowly during transit through the gastrointestinal tract, or by complexing the drug with ion-exchange resins.
Most absorption of these forms occurs in the large intestine. Crushing or otherwise disturbing a controlled-release tablet or capsule can often be dangerous. Transdermal controlled-release forms are designed to release the drug for extended periods, sometimes for several days. Drugs for transdermal delivery must have suitable skin-penetration characteristics and high potency because the penetration rate and area of application are limited. Many non-IV parenteral forms are designed to sustain plasma drug concentrations.
Absorption of antimicrobials can be extended by using their relatively insoluble salt form eg, penicillin G benzathine injected IM. For other drugs, suspensions or solutions in nonaqueous vehicles eg, crystalline suspensions for insulin are designed to delay absorption.
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The Manual was first published in as a service to the community. Learn more about our commitment to Global Medical Knowledge. This site complies with the HONcode standard for trustworthy health information: verify here. Common Health Topics. Videos Figures Images Quizzes Symptoms. Passive diffusion. Facilitated passive diffusion. Active transport. Oral Administration. General reference. Parenteral Administration. Controlled-Release Forms. Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion.
Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion. Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. An example of this is a person experiencing dehydration.
Neurons tend to be very sensitive to this effect. Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells. Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.
Surface area and thickness of the plasma membrane: Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it.
Distance travelled: The greater the distance that a substance must travel, the slower the rate of diffusion. Changes in Cell Shape Due to Dissolved Solutes : Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. Because the cell has a relatively higher concentration of water, water will leave the cell, and the cell will shrink.
In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell 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. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances. Cells in an isotonic solution retain their shape.
Cells in a hypotonic solution swell as water enters the cell, and may burst if the concentration gradient is large enough between the inside and outside of the cell.
Cells in a hypertonic solution shrink as water exits the cell, becoming shriveled. Osmoregulation is the process by which living things regulate the effects of osmosis in order to protect cellular integrity.
Tonicity is the ability of a solution to exert an osmotic pressure upon a membrane. There are three types of tonicity: hypotonic, hypertonic, and isotonic. In a hypotonic environment, water enters a cell, and the cell swells. In a hypertonic solution, water leaves a cell and the cell shrinks. In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane.
There is no net water movement; therefore, there is no change in the size of the cell. The membrane resembles a mosaic with discrete spaces between the molecules comprising it. If the cell swells and the spaces between the lipids and proteins become too large, the cell will break apart. In contrast, when excessive amounts of water leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell.
Turgor Pressure and Tonicity in a Plant Cell : The turgor pressure within a plant cell depends on the tonicity of the solution in which it is bathed. Various living things have ways of controlling the effects of osmosis —a mechanism called osmoregulation.
Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, in plants, the cellular environment is always slightly hypotonic to the cytoplasm, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant.
In nonwoody plants, turgor pressure supports the plant. Conversely, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell. In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt. Turgor Pressure and Plasmolysis : Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it right.
Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment.
Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis.
Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water.
Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine. In vertebrates, the kidneys regulate the amount of water in the body.
Osmoreceptors are specialized cells in the brain that monitor the concentration of solutes in the blood. If the levels of solutes increase beyond a certain range, a hormone is released that retards water loss through the kidney and dilutes the blood to safer levels. Animals also have high concentrations of albumin produced by the liver in their blood.
This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues. Privacy Policy. Skip to main content. Structure and Function of Plasma Membranes. Search for:. Passive Transport. The Role of Passive Transport Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes.
Learning Objectives Indicate the manner in which various materials cross the cell membrane. Key Takeaways Key Points Plasma membranes are selectively permeable; if they were to lose this selectivity, the cell would no longer be able to sustain itself. In passive transport, substances simply move from an area of higher concentration to an area of lower concentration, which does not require the input of energy.
Concentration gradient, size of the particles that are diffusing, and temperature of the system affect the rate of diffusion. Some materials diffuse readily through the membrane, but others require specialized proteins, such as channels and transporters, to carry them into or out of the cell.
Key Terms concentration gradient : A concentration gradient is present when a membrane separates two different concentrations of molecules. Selective Permeability The hydrophobic and hydrophilic regions of plasma membranes aid the diffusion of some molecules and hinder the diffusion of others. Learning Objectives Describe how membrane permeability, concentration gradient, and molecular properties affect biological diffusion rates.
Key Takeaways Key Points The interior and exterior surfaces of the plasma membrane are not identical, which adds to the selective permeability of the membrane.
Fat soluble substances are able to pass easily to the hydrophobic interior of the plasma membrane and diffuse into the cell. Polar molecules and charged molecules do not diffuse easily through the lipid core of the plasma membrane and must be transported across by proteins, sugars, or amino acids.
What happens to a cell placed in a hypertonic solution? Will cause the membrane-bounded compartment to lose water and volume. Why do we store more energy in the form of triglycerides than glycogen?
It does not take up water. Shared Flashcard Set. Total Cards Subject Other. Level Graduate.
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