StudySmarter: Study help & AI tools
4.5 • +22k Ratings
More than 22 Million Downloads
Free
|
|
Osmosis

Osmosis is the movement of water molecules down a water potential gradient, through a semipermeable membrane (also termed a partially permeable membrane). This is a passive process as no energy is needed for this type of transport. To understand this definition, we first need to know what water potential means.

Mockup Schule Mockup Schule

Explore our app and discover over 50 million learning materials for free.

Osmosis

Illustration

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

Jetzt kostenlos anmelden

Nie wieder prokastinieren mit unseren Lernerinnerungen.

Jetzt kostenlos anmelden
Illustration

Osmosis is the movement of water molecules down a water potential gradient, through a semipermeable membrane (also termed a partially permeable membrane). This is a passive process as no energy is needed for this type of transport. To understand this definition, we first need to know what water potential means.

The passive forms of transport include simple diffusion, facilitated diffusion, and osmosis!

  • What is water potential?
  • What is tonicity?
  • Osmosis in animal cells
    • Reabsorption of water in the nephrons
  • What factors affect the rate of osmosis?
    • Water potential gradient
    • Surface area
    • Temperature
    • Presence of aquaporins
  • Aquaporins in osmosis

What is water potential?

Water potential is a measure of the potential energy of water molecules. Another way to describe it is the tendency of water molecules to move out of a solution. The unit given is kPa (Ψ) and this value is determined by the solutes dissolved in the solution.

Pure water contains no solutes. This gives pure water a water potential of 0kPa - this is the highest water potential value a solution can have. The water potential becomes more negative as more solutes are dissolved in the solution.

Another way to view it is by looking at dilute and concentrated solutions. Dilute solutions have a higher water potential than concentrated solutions. This is because dilute solutions contain fewer solutes than concentrated ones. Water will always flow from a higher water potential to a lower water potential - from a more dilute solution to a more concentrated solution.

What is tonicity?

To understand osmosis in living cells, we are first going to define three types of solution (or types of tonicity):

  • Hypotonic solution

  • Isotonic solution

  • Hypertonic solution

A hypotonic solution has a higher water potential than inside the cell. Water molecules tend to move into the cell via osmosis, down a water potential gradient. This means the solution contains fewer solutes than the inside of the cell.

An isotonic solution has the same water potential as the inside of the cell. There is still the movement of water molecules but no net movement as the rate of osmosis is the same in both directions.

A hypertonic solution has a lower water potential than inside the cell. Water molecules tend to move out of the cell via osmosis. This means the solution contains more solutes than the inside of the cell.

Osmosis in animal cells

Unlike plant cells, animal cells paint a cell wall to withstand an increase in hydrostatic pressure.

When placed in a hypotonic solution, animal cells will undergo cytolysis. This is the process by which water molecules enter the cell via osmosis, causing the cell membrane to burst due to the elevated hydrostatic pressure.

On the flip side, animal cells placed in a hypertonic solution become crenated. This describes the state in which the cell shrinks and appears wrinkled due to water molecules leaving the cell.

When placed in an isotonic solution, the cell will remain the same as there is no net movement of water molecules. This is the most ideal condition as you do not want your animal cell, for example, a red blood cell, to lose or gain any water. Luckily, our blood is considered isotonic relative to red blood cells.

Osmosis, structure of red blood cells in different solution types, Study SmarterFig. 2 - Structure of red blood cells in different solution types

Reabsorption of water in the nephrons

The reabsorption of water takes place in the nephrons, which are tiny structures in the kidneys. At the proximally convoluted tubule, which is a structure within the nephrons, minerals, ions and solutes are actively pumped out, meaning the inside of the tubule has a higher water potential than the tissue fluid. This causes water to move into the tissue fluid, down a water potential gradient via osmosis.

At the descending limb (another tubular structure in the nephrons) the water potential is still higher than the tissue fluid. Again, this causes water to move into the tissue fluid, down a water potential gradient.

If you want to learn about Osmosis in plants, check out our article with an in-depth explanation of the topic!

What factors affect the rate of osmosis?

Similar to the rate of diffusion, the rate of osmosis can be affected by several factors, which include:

  • Water potential gradient

  • Surface area

  • Temperature

  • Presence of aquaporins

Water potential gradient and rate of osmosis

The greater the water potential gradient, the faster the rate of osmosis. For example, the rate of osmosis is greater between two solutions that are -50kPa and -10kPa compared to -15kPa and -10kPa.

Surface area and rate of osmosis

The greater the surface area, the faster the rate of osmosis. This is provided by a large semipermeable membrane as this is the structure that water molecules move through.

Temperature and rate of osmosis

The higher the temperature, the faster the rate of osmosis. This is because higher temperatures provide water molecules with greater kinetic energy which allows them to move faster.

Presence of aquaporins and rate of osmosis

Aquaporins are channel proteins that are selective for water molecules. The greater the number of aquaporins found in the cell membrane, the faster the rate of diffusion. Aquaporins and their function are explained more thoroughly in the following section.

Aquaporins in Osmosis

Aquaporins are channel proteins that span the length of the cell membrane. They are highly selective for water molecules and therefore allow the passage of water molecules through the cell membrane without the need for energy. Although water molecules can move freely through the cell membrane by themselves due to their small size and polarity, aquaporins are designed to facilitate rapid osmosis.

Osmosis, structure of aquaporins, StudySmarterFig. 3 - Structure of aquaporins

This is highly important, as osmosis that takes place without aquaporins in living cells is too slow. Their main function is to increase the rate of osmosis.

For example, the cells lining the collecting duct of the kidneys contain many aquaporins in their cell membranes. This is to speed up the rate of water reabsorption into the blood.

Osmosis - Key takeaways

  • Osmosis is the movement of water molecules down a water potential gradient, through a semipermeable membrane. This is a passive process. as no energy is needed.
  • Hypertonic solutions have a higher water potential than the inside of cells. Isotonic solutions have the same water potential as the inside of cells. Hypotonic solutions have a lower water potential than the inside of cells.
  • Plant cells function best in hypotonic solutions whereas animal cells function best in isotonic solutions.
  • The main factors that affect the rate of osmosis are water potential gradient, surface area, temperature and the presence of aquaporins.
  • The water potential of plant cells, such as potato cells can be calculated using a calibration curve.

Frequently Asked Questions about Osmosis

Osmosis is the movement of water molecules from a water potential gradient through a semipermeable membrane.

Osmosis does not require energy as it is a passive form of transport; water molecules can move freely through the cell membrane. Aquaporins, which are channel proteins that speed up the rate of osmosis, also perform the passive transport of water molecules.

In plant cells, osmosis is used for the uptake of water through the plant root hair cells. In animal cells, osmosis is used for the reabsorption of water at the nephrons (in the kidneys).

Osmosis requires a semipermeable membrane whereas simple diffusion does not. Osmosis only takes place in a liquid medium whereas simple diffusion can take place in all three states - solid, gas and liquid.

Join over 22 million students in learning with our StudySmarter App

The first learning app that truly has everything you need to ace your exams in one place

  • Flashcards & Quizzes
  • AI Study Assistant
  • Study Planner
  • Mock-Exams
  • Smart Note-Taking
Join over 22 million students in learning with our StudySmarter App Join over 22 million students in learning with our StudySmarter App

Sign up to highlight and take notes. It’s 100% free.

Start learning with StudySmarter, the only learning app you need.

Sign up now for free
Illustration

Entdecke Lernmaterial in der StudySmarter-App