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. For cell transport, diffusion is the movement of small molecules across the cell membrane.
The difference in the concentrations of the molecules in the two areas is called the concentration gradient. The kinetic energy of the molecules results in random motion, causing diffusion.
In simple diffusion, this process proceeds without the aid of a transport protein. It is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration. Diffusion will continue until the concentration gradient has been eliminated.
Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient". The end result is an equal concentration, or equilibrium , of molecules on both sides of the membrane.
Most cells expend most of their energy, in the form of adenosine triphosphate ATP , to create and maintain an uneven distribution of ions on the opposite sides of their membranes. The structure of the plasma membrane contributes to these functions, but it also presents some problems.
The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion.
A physical space in which there is a different concentration of a single substance is said to have a concentration gradient. Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the interior of the membrane is not identical to the exterior of the membrane. Integral proteins that act as channels or pumps work in one direction. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.
These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes. Recall that plasma membranes have hydrophilic and hydrophobic regions. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane.
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. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion. Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane.
Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes. Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space.
You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of perfume in a room filled with people. The perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the bottle, and gradually, more and more people will smell the perfume as it spreads.
Diffusion expends no energy. Rather the different concentrations of materials in different areas are a form of potential energy, and diffusion is the dissipation of that potential energy as materials move down their concentration gradients, from high to low. Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials.
Additionally, each substance will diffuse according to that gradient. For an animation of the diffusion process in action, view this short video on cell membrane transport. In facilitated transport , also called facilitated diffusion, material moves across the plasma membrane with the assistance of transmembrane proteins down a concentration gradient from high to low concentration without the expenditure of cellular energy.
Any kind of diffusion or osmosis will not take place because the equilibrium has been achieved. Thus, water molecules will leave as well as enter the cell.
Do water molecules leave or enter a cell in an isotonic solution? Feb 21, There is free movement of water molecules in isotonic solution. Explanation: Isotonic solution means having same osmotic pressure. Related questions How do I determine the molecular shape of a molecule?
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