What is it called when water moves from a high water concentration to low water concentration through plasma membrane?

What is Osmosis? Read to learn more.

Throughout the rest of this week we will be asking you to evaluate different teaching strategies for helping students get to grips with osmosis, starting with the use of animations in biology, then practical work, and finally modeling. But first, we’ll begin with a quick overview of what osmosis is.

Here’s the definition of osmosis that you will see in most textbooks:

In biology, osmosis is the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell’s partially permeable membrane.

A partially permeable membrane (sometimes called a selectively permeable membrane) only allows certain molecules or ions to cross it

In the diagram above, the higher concentration of water molecules to the left of the partially permeable membrane makes it likely that a large number of water molecules will collide with the membrane and pass through it.

The lower concentration of water molecules on the right-hand side of the partially permeable membrane in the diagram makes it likely that fewer water molecules will collide with the membrane and pass through it.

This means that more water molecules move from left to right on this diagram than move from right to left, and so the overall movement (net movement) is to the right. It is important, though, to stress to students that water molecules are moving in both directions.

You will often see this described as movement ‘down the concentration gradient’, meaning the water is moving from a higher concentration of water (in this case, the dilute sucrose solution), to a lower concentration of water (the concentrated sucrose solution).

If a plant cell is surrounded by a solution that contains a higher concentration of water molecules than the solution inside the cell, water will enter the cell by osmosis and the plant cell will become turgid (firm). The pressure that develops inside a plant cell when it becomes turgid is called turgor pressure. Turgid plant cells help a stem to stay upright.

If a plant cell is surrounded by a solution that contains a lower concentration of water molecules than the solution inside the plant cell, water will leave the cell by osmosis and the plant cell will become flaccid (soft). If the cells in a plant stem become flaccid the turgor pressure inside them will decrease and the stem will wilt.

If a plant cell is surrounded by a solution that contains the same concentration of water molecules as the solution inside the plant cell, there is no overall net flow of water. The movement of water molecules into and out of the cell, through the partially permeable membrane, balances out.

Transpiration Keeps the Water Moving

In plants, water enters the root cells by osmosis and moves into tubes called xylem vessels to be transported to the leaves. Water molecules inside the xylem cells are strongly attracted to each other because of hydrogen bonding (this is called cohesion). When water evaporates from the leaves (through tiny pores called stomata), more water is drawn up from the root xylem cells to replace that which has been lost. A continuous column of water is, therefore, pulled up the stem in the xylem vessels by evaporation from the leaves. This is called transpiration.

This type of diagram and explanation is fairly common in biology text books, but the movement of molecules is quite abstract and this can be a difficult to concept for students to imagine.

Have a look online for a resource which helps to explain osmosis to students, and share a link below with an explanation of why it’s helpful.

In order to function, cells are required to move materials in and out of their cytoplasm via their cell membranes. These membranes are semipermeable, meaning that certain molecules are allowed to pass through, but not others. This movement of molecules is mediated by the phospholipid bilayer and its embedded proteins, some of which act as transport channels for molecules that otherwise would not be able to pass through the membrane, such as ions and carbohydrates.

Cell Size and the Surface-Area to Volume Ratio

One reason cells are so small is the need to transport molecules into, throughout, and out of the cell. There is a geometrical constraint on cells due to the relationship between surface area and volume that limits the ability to bring in enough nutrients to support a larger cell size. The ratio between surface area and volume (SA:V) decreases as the cell increases in size due to the different scaling factors of surface area and volume. This means that as the cell grows larger, there is less membrane area able to supply nutrients to a greater cell volume.

Some ions are brought into the cell by diffusion, which is the net movement of particles from an area of high concentration to an area of lower concentration. This is known as moving “down” a concentration gradient. Diffusion is net directional; while the net movement of particles is down the concentration gradient, they are constantly moving in both directions due to the random motion of particles. This means that particles in solutions at equilibrium are still moving, but at a constant exchange rate so the solution remains evenly mixed. In an aqueous environment such as the cell, this process involves dissolved ions, known as solutes, moving through water, the solvent. It can take place in an open environment, such as dye spreading through a beaker, or across a cell membrane, such as ions moving through a protein channel.

Osmosis and the Movement of Water

Water moves across cell membranes by diffusion, in a process known as osmosis. Osmosis refers specifically to the movement of water across a semipermeable membrane, with the solvent (water, for example) moving from an area of low solute (dissolved material) concentration to an area of high solute concentration. In this case, the semipermeable membrane does not allow the solute to pass through. This can be thought of as water moving down its own concentration gradient and involves the same random process as diffusion.

Solutions that are separated by semipermeable membranes can be described as hypertonic, hypotonic, or isotonic depending on the relative solute concentrations in each. A solution that is hypertonic (hyper- meaning “above” in Greek) has a greater concentration of solutes than an adjacent solution, while a hypotonic (hypo- meaning “below” in Greek) solution has a lower concentration of solutes. In this situation, water will move from the hypotonic solution to the hypertonic solution until the solute concentrations are equal. Solutions that are isotonic (iso- meaning “equal” in Greek) have equal concentrations of solute, and therefore do not have a concentration gradient 1.

Osmosis and the Plant Cell

The capacity for water to move into cells is different between plant and animal cells due to the presence of a cell wall in plants. Cell walls are rigid and only permeable to very small molecules. As water moves into the cell, the membrane is pushed up against the cell wall, creating hydrostatic, or turgor, pressure. This pressure limits the rate and amount of water that can enter the cell. The likelihood of water moving into a cell is referred to as water potential, defined quantitatively as the pressure potential plus the solute potential. The pressure potential is dependent on the pressure inside the cell and the solute potential depends on the solute concentration in the cell.

Water potential can be observed in action in a living plant cell, such as Elodea, an aquatic plant. Under the microscope, a phenomenon called cytoplasmic streaming, or cyclosis, in which cytoplasm and organelles such as chloroplasts move throughout the cell, can be monitored. This process changes visibly when the cells are immersed in different solutions. Interestingly, this motion allows chloroplasts to function more efficiently in photosynthesis; they move in and out of the shadows, collecting photons when they re-enter the lighted regions of the cells3.

The process of osmosis is essential for the mechanism whereby plants get water from their roots to their leaves, even dozens of feet above ground level. In brief, plants transport sugars and other solutes to their roots in order to generate a gradient between the inside and outside of the root; water from the soil then moves in to the root by osmosis. From that point, a process called transpiration results in the water being pulled up tubes inside the plant called the xylem and evaporating out the leaves. Ideally, once this water column is established, it remains intact throughout the life of the plant.4

This naturally occurring phenomenon has been used to develop valuable technologies. One example is in water purification. Recently, NASA has begun to study using the process of forward osmosis to clean and reuse wastewater aboard the International Space Station, as well as for Earth-bound applications. 2 This process uses semi-permeable membranes to remove impurities from water, making it safe to drink. This technology was deployed recently to aid in relief efforts after a severe flood in Western Kenya5.

References

Soult, Allison. LibreTexts, Chemistry. 8.4 Osmosis and Diffusion. [Online] October 19, 2017. https://chem.libretexts.org/LibreTexts/University_of_Kentucky/UK%3A_CHE_103_-_Chemistry_for_Allied_Health_(Soult)/Chapters/Chapter_8%3A_Properties_of_Solutions/8.4%3A_Osmosis_and_Diffusion.
Levine, Howard. NASA Forward Osmosis Bag. NASA Kennedy Space Center, Cape Canaveral, FL, United States. [Online] July 11, 2018. https://www.nasa.gov/mission_pages/station/research/experiments/846.html.
Dodonova SO, Bulychev AA (2011). 'Effect of Cytoplasmic Streaming on Photosynthetic Activity of Chloroplasts in Internodes of Chara Corallina'. Russian Journal of Plant Physiology. 59: 35–41. doi:10.1134/S1021443711050050.
Osmosis and Plant Nutrition. Hammer, Michael. 2000, The Rhododendron, Vol. 40.
Hydration Technology Innovations. Humanitarian Forward Osmosis Water Filtration. [Online] [Cited: August 21, 2018.] http://www.htiwater.com/divisions/humanitarian/lead_story.html.