5.3: Passive Transport - Osmosis
- Page ID
- 408826
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Describe the process of osmosis and explain how concentration gradient affects osmosis
Osmosis and Semipermeable Membranes
Osmosis is the movement of water through a semipermeable membrane. The prefix "semi" means partially and "permeable" means something can pass through. Therefore, a semipermeable membrane lets some things pass through (small molecules such as water) and prevents other things from passing through (like larger solutes or ions). In osmosis water moves from the side of the membrane with a lower total concentration of solutes towards the side with a higher total concentration of solutes. We don't usually talk about the concentration of a solvent, but you could think of osmosis as the movement of water from the side of the membrane that has a higher concentration of water to the side with a lower concentration of water in order to even things out.
While diffusion transports various materials across membranes and within cells, osmosis transports only water across a membrane. The semipermeable membrane limits the diffusion of solutes in the water. There are proteins called aquaporins that form pores in cell membranes to make them semipermeable and facilitate water movement.
A solution with a concentration of 1.15 M is separated from a solution with a concentration of 0.33 M by a semipermeable membrane. Which way will water flow? (Note: The identity of the solutes does not affect the direction of flow.)
Solution
- The scenario involves two solutions of different concentrations separated by a semipermeable membrane, therefore osmosis will occur.
- In osmosis, water moves from the solution with lower concentration towards the solution with higher concentration.
- Water will move from 0.33 M solution into the 1.15 M solution.
- As a result, the concentrations will become more similar. Adding water to the 1.15 M solution dilutes it and lowers its concentration. Removing water from the 0.33 M solution raises its concentration.
A semipermeable membrane separates two solutions. One has a concentration of 0.5 % and the other solution has a concentration of 2 %. Which solution will lose water and which solution will gain water?
- Answer
-
The 0.5 % solution will lose water and the 2 % solution will gain water.
Mechanism of Osmosis
Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the membrane is the same but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane. If there is more solute in one area, then there is less water; if there is less solute in one area, then there must be more water.
To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.
Returning to the beaker example, recall that it has a mixture of solutes 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 passing 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. Thus, 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 or until the hydrostatic pressure of the water balances the osmotic pressure. In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up.
A tube with a semipermeable membrane in the middle contains water (pure solvent) on the left and a solution on the right. Initially the volume on each side of the membrane is the same. How will the volumes compare after osmosis occurs?
- The volumes will remain equal
- The volume on the left will be higher than on the right
- The volume on the right will be higher than on the left
OpenStax, CC BY 4.0, via Wikimedia Commons
Solution
Osmosis will occur. Water will move from the side with a higher concentration (more solute) towards the side with a lower concentration (less solute). Therefore, the correct answer is (c) The volume on the right will be higher than on the left.
OpenStax, CC BY 4.0, via Wikimedia Commons
Water (represented by yellow spheres in the figure) moved through the membrane. Solute particles (represented by larger, darker spheres in the figure) were too large to move through the membrane. Therefore there are now more particles on the right.
In which direction will water flow in the beaker below? How will the volumes change?
Christinelmiller, CC BY-SA 4.0, via Wikimedia Commons
- Answer
-
Water will move from left to right (towards the more concentrated solution). The final volume on the right will be higher than that on the left.
Christinelmiller, CC BY-SA 4.0, via Wikimedia Commons
Key Points
- Osmosis occurs according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.
- Osmosis occurs until the concentrations on both sides of the membrane are the same.
- Osmosis occurs when there are different concentrations on two sides of a semipermeable membrane but the membrane does not allow diffusion of the solute.
Key Terms
- solute: Any substance that is dissolved in a liquid solvent to create a solution
- osmosis: The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane
- semipermeable membrane: A type of biological membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally by specialized facilitated diffusion