Osmosis

Membranes and Transport

Define osmosis.

The net movement of free water molecules from an area of higher water potential to an area of lower water potential through a partially permeable membrane.

Is osmosis active or passive transport?

Osmosis is passive transport.

Does osmosis use ATP?

No, osmosis does not require ATP.

Which molecules move during osmosis?

Free water molecules.

What determines the direction of osmosis?

The water potential gradient.

In which direction does water move by osmosis?

From higher water potential to lower water potential, down a water potential gradient.

What is needed for osmosis to occur?

A partially permeable membrane and a difference in water potential.

Define a partially permeable membrane.

A membrane that allows some molecules, such as water, to pass through but prevents other molecules from crossing.

Define water potential.

A measure of the potential for water to move out of a solution by osmosis.

What is a water potential gradient?

A difference in water potential between two regions.

What does a high water potential mean?

Water has a greater tendency to move out of the solution by osmosis.

What does a low water potential mean?

Water has a lower tendency to move out of the solution by osmosis.

What is the water potential of pure water?

0 kPa, the highest possible water potential.

What happens to water potential when solute is added to pure water?

The water potential becomes lower, or more negative.

Why does adding a solute lower water potential?

Some water molecules associate with the solute particles, reducing the number of free water molecules available to move.

What are free water molecules?

Water molecules that are not associated with dissolved solute molecules and are able to move by osmosis.

What is solute potential?

The effect of dissolved solutes on water potential; adding solute makes water potential more negative.

What happens if two solutions with different water potentials are separated by a partially permeable membrane?

There is a net movement of water from the solution with higher water potential to the solution with lower water potential.

Does water cross a partially permeable membrane in both directions?

Yes, but there is a net movement towards the side with the lower water potential.

When does the net movement of water stop?

When the water potentials on the two sides are equal or opposing pressure prevents further net movement.

Why is osmosis called a special case of diffusion?

It involves the passive net movement of water molecules down a gradient.

How is osmosis different from diffusion?

Diffusion can involve many types of particles, whereas osmosis specifically involves water moving through a partially permeable membrane down a water potential gradient.

How can osmosis be modelled experimentally?

Using a partially permeable membrane bag containing a solution and placing it in another solution.

What does the partially permeable membrane bag represent in an osmosis model?

The cell surface membrane.

What happens when a membrane bag containing sucrose solution is placed in pure water?

Water moves into the bag by osmosis and the level of liquid in the tubing rises.

Why does water enter a bag containing sucrose solution when it is surrounded by pure water?

Pure water has a higher water potential than the sucrose solution, so water moves down the water potential gradient into the bag.

What happens when a membrane bag containing pure water is placed in sucrose solution?

Water moves out of the bag by osmosis and the level of liquid in the tubing falls.

Why does water leave a bag containing pure water when it is surrounded by sucrose solution?

The water potential is higher inside the bag than in the sucrose solution, so water moves out down the water potential gradient.

Why can water pass through the membrane bag while sucrose may not?

The membrane has pores large enough for water molecules but too small for the larger sucrose molecules.

How can reducing sugars be detected in an osmosis experiment?

Using Benedict's test.

Why might Benedict's test be used in an osmosis model?

To determine whether reducing sugar molecules have crossed the partially permeable membrane.

Define osmotic concentration.

A measure of the concentration of dissolved solutes in a solution that have an osmotic effect.

Why might some large molecules in cells not contribute significantly to osmotic concentration?

They may not affect the movement of water into or out of the cell in the same way as smaller dissolved solutes.

Define an isotonic solution.

A solution with the same osmotic concentration of solutes as the cell contents.

Define a hypotonic solution.

A solution with a lower osmotic concentration of solutes than the cell contents.

Define a hypertonic solution.

A solution with a higher osmotic concentration of solutes than the cell contents.

How does the water potential of a hypotonic solution compare with that of the cell?

The hypotonic solution has a higher water potential than the cell contents.

How does the water potential of a hypertonic solution compare with that of the cell?

The hypertonic solution has a lower water potential than the cell contents.

What happens to an animal cell in an isotonic solution?

There is no net movement of water, so the cell maintains its normal size and shape.

What happens to an animal cell in a hypotonic solution?

Water enters the cell by osmosis, causing it to swell and potentially burst.

Why does an animal cell swell in a hypotonic solution?

The solution has a higher water potential than the cytoplasm, so water moves into the cell by osmosis.

Define lysis.

The bursting of an animal cell after too much water enters by osmosis.

What happens to a red blood cell in a hypotonic solution?

Water enters by osmosis, causing it to swell and eventually burst.

What happens to a red blood cell in an isotonic solution?

There is no net movement of water and it maintains its normal shape.

What happens to an animal cell in a hypertonic solution?

Water leaves the cell by osmosis, causing the cell to shrink.

Why does an animal cell shrink in a hypertonic solution?

The solution has a lower water potential than the cytoplasm, so water moves out of the cell.

What happens to a red blood cell in a hypertonic solution?

Water leaves by osmosis and the cell shrivels.

Why must water movement be carefully controlled in animal cells?

Animal cells have no cell wall, so excessive water entry can cause them to burst and excessive water loss can cause them to shrink.

Why can animal cells be compared with fragile balloons filled with jelly?

Their cell surface membrane can expand only slightly and there is no rigid cell wall to prevent bursting.

What happens to a plant cell in a hypotonic solution?

Water enters by osmosis, the vacuole and cytoplasm swell and the cell becomes turgid.

Why does a plant cell not burst in a hypotonic solution?

The rigid cellulose cell wall resists expansion and prevents the cell from bursting.

Define hydrostatic pressure.

The pressure exerted by a fluid in equilibrium.

How does hydrostatic pressure develop in a plant cell?

Water entering by osmosis causes the cell contents to swell and press against the cell wall.

What is pressure potential?

The pressure produced as the cell wall pushes back against the expanding protoplast.

What happens as water continues to enter a plant cell?

The vacuole and cytoplasm swell and push against the cell wall, increasing pressure potential.

What eventually prevents further net movement of water into a plant cell?

The pressure potential opposing water entry balances the tendency of water to enter by osmosis.

Define turgor.

The state of a plant cell when the tendency for water to enter by osmosis is balanced by the cell wall pressing on the protoplast.

What is a turgid plant cell?

A plant cell that has taken up water and whose contents press firmly against the cell wall.

Why is turgor important to plants?

It provides support to non-woody tissues and helps keep stems and leaves firm.

What happens to a plant cell in an isotonic solution?

There is little or no net movement of water, so the cell is not fully turgid.

What happens to a plant cell in a hypertonic solution?

Water leaves by osmosis, the vacuole shrinks, turgor is lost and the protoplast pulls away from the cell wall.

Define incipient plasmolysis.

The point at which enough water has left a plant cell for turgor to be lost and the cell surface membrane begins to pull away from the cell wall.

What happens to the cell surface membrane at incipient plasmolysis?

It begins to pull away from the cell wall.

What happens to turgor at incipient plasmolysis?

Turgor has just been lost.

Define plasmolysis.

The condition in which a plant cell loses so much water by osmosis that the vacuole shrinks and the protoplast pulls away from the cell wall.

What happens to the protoplast during plasmolysis?

It becomes more concentrated, shrinks and pulls away from the cell wall.

What happens to the vacuole during plasmolysis?

It becomes smaller as water leaves the cell.

Does the cell wall shrink significantly during plasmolysis?

No. The rigid cell wall largely maintains the cell's overall size and shape.

Why does a plasmolysed plant cell not become dramatically smaller?

The rigid cellulose cell wall maintains the external shape of the cell even though the contents shrink.

Which type of solution causes plasmolysis?

A hypertonic solution.

Why does a hypertonic solution cause plasmolysis?

It has a lower water potential than the cell contents, so water moves out of the cell by osmosis.

How can incipient plasmolysis be investigated?

Place plant cells in a series of solutions of different concentrations and identify the concentration at which about 50% of cells are plasmolysed.

Why is 50% plasmolysis used when investigating incipient plasmolysis?

It provides an estimate of the external solution concentration equivalent to the solute potential of the cell sap.

Why are plant cells useful for investigating osmosis?

They are relatively large and changes such as turgor and plasmolysis can be observed clearly using a light microscope.

What is the relationship between solute concentration and water potential?

As solute concentration increases, water potential decreases and becomes more negative.

What happens if a cell has a lower water potential than its surroundings?

Water moves into the cell by osmosis.

What happens if a cell has a higher water potential than its surroundings?

Water moves out of the cell by osmosis.

What happens if the cell and surrounding solution have equal water potentials?

There is no net movement of water.

Why is water potential important in living organisms?

It determines the direction in which water moves into and out of cells by osmosis.

Why is osmosis important in plants?

It allows cells to take up water and develop turgor, which provides structural support.

Why is osmosis important in animal cells?

It controls water movement and helps cells maintain an appropriate volume and internal environment.