9.7: Osmosis and Diffusion (2023)

Osmosis

Imagine you have a cup of water and add 15g of table sugar. The sugar will dissolve and the mixture that is now in the cup will consist ofdissolved(sugar) dissolved in itsolvent(Water). The mixture of the solute in the solvent is denoted as aSolution.

Now imagine you have a second cup of (100 \: \text{ml}\) water and add \(45 \: \text{g}\) table sugar. As in the first cup, sugar is the solute and water is the solvent. But now you have two mixtures with different solute concentrations. Comparing two solutions with different solute concentrations will result in a solution with a higher solute concentrationhypertonicand is the solution with the lower solute concentrationhypotonic. are solutions with the same concentration of solutesisotonic. The first sugar solution is hypotonic compared to the second solution. The second sugar solution is hypertonic compared to the first.

Now you put both solutions in a beaker separated by a semi-permeable membrane whose pores are too small for the passage of sugar molecules but large enough for the passage of water molecules. The hypertonic solution is on one side of the membrane and the hypotonic solution is on the other. A hypertonic solution has a lower water concentration than a hypotonic solution, so there is now a water concentration gradient on the other side of the membrane. The water molecules move sidewayshigherlateral water concentrationlowerConcentrate until both solutions are isotonic. at this pointbalancehas been reached.

Red blood cells behave in the same way (see image below). When red blood cells are in a hypertonic (higher concentration) solution, water leaves the cell faster than it enters. It causescreation(contraction) of blood cells. On the other hand, a hypotonic red blood cell (lower concentration outside the cell) causes more water to flow into the cell than out of the cell. This causes the cell and potential to swellhemolysis(rupture) of the cell. In an isotonic solution, water flows in and out of the cell at the same rate.

Osmosisis the diffusion of water molecules from an area through a semi-permeable membranelowerSolution concentration (i.e. higher water concentration) to areahigherConcentration solution (i.e. lower water concentration). Water enters and leaves cells by osmosis.

• When the cell is in a hypertonic solution, the solution has a lower concentration of water than the cell's cytosol, and water flows out of the cell until both solutions become isotonic.
• Cells placed in a hypotonic solution take up water through their membranes until both the outer solution and the cytosol become isotonic.

The red blood cells swell and hemolyze (burst) when placed in a hypotonic solution. When the red blood cells are placed in a hypertonic solution, they lose water and succumbcreation(shrink). Animal cells usually thrive best in an isotonic environment where water flows in and out of the cell at a steady rate.

Diffusion

passive transportis the way small molecules or ions move across the cell membrane without the cell providing energy. The three main types of passive transport are diffusion (or simple diffusion), osmosis, and facilitated diffusion. No transport proteins are involved in simple diffusion and osmosis. Facilitated diffusion requires the help of proteins.

Diffusionis the movement of molecules from an area of ​​high molecular concentration to an area of ​​lower concentration. In cellular 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 calledconcentration gradient. The kinetic energy of the molecules causes random movement and hence diffusion. In simple diffusion, this process occurs without the help of a transport protein. It is the random movement of molecules that causes them to migrate from an area of ​​high concentration to an area of ​​lower concentration.

Diffusion continues until the concentration gradient is eliminated. Because diffusion moves materials from an area of ​​higher concentration to a lower concentration, it is described as movement of solutes "along a concentration gradient". The end result is an equal concentration orbalanceMolecules on both sides of the membrane. In equilibrium, the movement of the particles does not stop. In a state of equilibrium, the material moves evenly in both directions.

Not everything can get into your cells. Your cells have a plasma membrane that protects them from unwanted invaders.

plasma membrane and cytosol

If the cell's outer environment is water-based, and the cell's interior is also mostly water, something has to keep the cell intact in that environment. What would happen if the cell dissolved like sugar in water? Of course, a cell could not survive in such an environment. So something has to protect the cell and allow it to survive in an aquatic environment. All cells are surrounded by a barrier that separates them from the environment and other cells. This barrier is calledPlasma Membraneor cell membrane.

Plasma Membrane

The plasma membrane (see figure below) consists of a double layer of special lipids, the so-calledPhospholipide. A phospholipid is a lipid molecule with a hydrophilic (“water-loving”) head and two hydrophobic (“water-hating”) tails. Due to the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern since only certain parts of the molecule can physically come into contact with water. Remember that there is water outside the cell, aZytoplasmaThe interior of the cell is also predominantly water. Thus, the phospholipids are arranged in a double layer (bilayer) to delimit the cell from the environment. Lipids don't mix with water (remember, oil is a lipid), so the phospholipid bilayer of the cell membrane acts as a barrier, preventing water from entering the cell and keeping the cytoplasm inside the cell. The cell membrane ensures that the cell remains structurally intact in an aqueous environment.

The function of the plasma membrane is to control what goes in and out of the cell. Some molecules can cross the cell membrane to enter and exit the cell, but others cannot. The cell is therefore not completely permeable. "Permeable" means anything can pass through the barrier. An open door is completely permeable to everything that wants to come in or out through it. The plasma membrane issemi-permeable, which means some things can get into the cell and others can't.

Molecules that cannot easily pass through the bilayer include ions and small hydrophilic molecules like glucose, as well as macromolecules like proteins and othersRNA. Examples of molecules that can easily diffuse across the plasma membrane are carbon dioxide and oxygen gas. These molecules diffuse freely in and out of the cell along their concentration gradient. Although water is a polar molecule, it can also diffuse across the plasma membrane.

Additional Resources

• Plasmafilm: www.youtube.com/watch?v=moPJkCbKjBs

The central theses

• Water enters and leaves cells by osmosis.
• Water (solvent) moves out of arealowerSolution concentration (i.e. higher water concentration) to areahigherConcentration solution (i.e. lower water concentration).