When is a cells interior isotonic

Put it in freshwater, and the freshwater will, through osmosis, enter the fish, causing its cells to swell, and the fish will die. What will happen to a freshwater fish in the ocean? The sugar dissolves and the mixture that is now in the cup is made up of a solute the sugar that is dissolved in the solvent the water.

The mixture of a solute in a solvent is called a solution. Just like the first cup, the sugar is the solute, and the water is the solvent. But now you have two mixtures of different solute concentrations. In comparing two solutions of unequal solute concentration, the solution with the higher solute concentration is hypertonic , and the solution with the lower solute concentration is hypotonic. Solutions of equal solute concentration are isotonic.

The first sugar solution is hypotonic to the second solution. The second sugar solution is hypertonic to the first. You now add the two solutions to a beaker that has been divided by a semipermeable membrane, with pores that are too small for the sugar molecules to pass through, but are big enough for the water molecules to pass through.

The hypertonic solution is one one side of the membrane and the hypotonic solution on the other. The hypertonic solution has a lower water concentration than the hypotonic solution, so a concentration gradient of water now exists across the membrane. Water molecules will move from the side of higher water concentration to the side of lower concentration until both solutions are isotonic. At this point, equilibrium is reached. Red blood cells behave the same way see figure below. When red blood cells are in a hypertonic higher concentration solution, water flows out of the cell faster than it comes in.

This results in crenation shriveling of the blood cell. On the other extreme, a red blood cell that is hypotonic lower concentration outside the cell will result in more water flowing into the cell than out. This results in swelling of the cell and potential hemolysis bursting of the cell. In an isotonic solution, the flow of water in and out of the cell is happening at the same rate. Osmosis is the diffusion of water molecules across a semipermeable membrane from an area of lower concentration solution i.

Water moves into and out of cells by osmosis. A red blood cell will swell and undergo hemolysis burst when placed in a hypotonic solution. When placed in a hypertonic solution, a red blood cell will lose water and undergo crenation shrivel. Animal cells tend to do best in an isotonic environment, where the flow of water in and out of the cell is occurring at equal rates. Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell.

The three main kinds of passive transport are diffusion or simple diffusion , osmosis, and facilitated diffusion.

Simple diffusion and osmosis do not involve transport proteins. Facilitated diffusion requires the assistance of proteins. Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. Some cells require larger amounts of specific substances.

They must have a way of obtaining these materials from extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate ATP , to obtain these materials.

Red blood cells use some of their energy doing just that. The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration.

A physical space in which there is a single substance concentration range has a concentration gradient. There is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane. These carbohydrate complexes help the cell bind required substances in the extracellular fluid. Figure 1. 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.

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. Oxygen and carbon dioxide molecules have no charge and pass through membranes by simple diffusion. Polar substances present problems for the membrane.

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 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.

You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle. The ammonia vapor will diffuse, or spread away, from the bottle, and gradually, increasingly more people will smell the ammonia as it spreads.

Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, which dissipates as the gradient is eliminated. Figure 2. Diffusion through a permeable membrane moves a substance from a high concentration area extracellular fluid, in this case down its concentration gradient into the cytoplasm. In addition, each substance will diffuse according to that gradient.

Within a system, there will be different diffusion rates of various substances in the medium. 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. This movement accounts for molecule diffusion through whatever medium in which they are localized.

A substance moves into any space available to it until it evenly distributes itself throughout. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another.

We call this lack of a concentration gradient in which the substance has no net movement dynamic equilibrium. A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane. Sometimes pressure enhances the diffusion rate, causing the substances to filter more rapidly. This occurs in the kidney, where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules.

The diffusion rate in this instance is almost totally dependent on pressure. 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.

This allows removal of material from the extracellular fluid that the cell needs. The substances then pass to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta-pleated sheets that form a pore or channel through the phospholipid bilayer. Others are carrier proteins which bind with the substance and aid its diffusion through the membrane.

The integral proteins involved in facilitated transport are transport proteins, and they function as either channels for the material or carriers.

In both cases, they are transmembrane proteins. 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. Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate. Figure 3. Facilitated transport moves substances down their concentration gradients. They may cross the plasma membrane with the aid of channel proteins.

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.

An example of this occurs in the kidney, where there are both channel forms in different parts of the renal tubules.

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.

Another type of protein embedded in the plasma membrane is a carrier protein. Depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. Scientists poorly understand the exact mechanism for the change of shape. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism.

Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough material for the cell to function properly. When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum.

Increasing the concentration gradient at this point will not result in an increased transport rate. Figure 4. Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins.

Carrier proteins change shape as they move molecules across the membrane. An example of this process occurs in the kidney. In one part, the kidney filters glucose, water, salts, ions, and amino acids that the body requires. This filtrate, which includes glucose, then reabsorbs in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported and the body excretes this through urine.

Channel and carrier proteins transport material at different rates. 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 2]. 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.

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. Watch this video that illustrates diffusion in hot versus cold solutions. 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. 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]. 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 4].