(4) OSMOSIS - a special case of diffusion - transport of water

Introduction

Reminder 1: Osmosis as the net movement of water molecules from a region of higher water potential (from a more dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane (concentrated refers to dissolved molecules or ions).

Reminder 2: Active transport is the movement of particles (molecules or ions) through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

Within cell membranes there are carrier proteins use energy from respiration to transport molecules or ions across the membrane, against the concentration gradient, therefore cells that use active transport usually have more mitochondria for respiration compared to other cells.

 

Know and understand that water often moves across boundaries by osmosis - a special case of particle (water) diffusion down a concentration gradient.

 

Know that osmosis is the diffusion or net bulk movement of water from a dilute to a more concentrated solution through a partially permeable membrane (also described as a partly/semi-permeable membrane) that allows the passage of very small molecules like water (diagram on right).

(Caution: Don't assume other small molecules cannot pass through a membrane, the term osmosis refers to the passage of water, but things are more complicated when dealing with other small molecules and ions passing through membranes).

You can also express osmosis as the movement of water from a higher concentration to a lower concentration - this is effectively from a lower solute concentration to a higher solute concentration.

 

A partially permeable membrane has extremely small pores or holes that only allow the tiniest of molecules like water through e.g. even relatively small molecules like sucrose will not pass through a partially permeable membrane (see experiment in section 2b. below).

Particle theory: All the particles in a solution are moving at random because of their kinetic energy store, so if a water molecule (blue circle) hits the right spot on the membrane, it can diffuse through. However, a larger molecule (purple circles) like a sugar molecule, will just bounce back off the surface of the membrane.

The purple particles also represent a higher concentration of small molecules or ions on the left, so the higher concentration of water on the right will ensure there is a net diffusion of water from right to left through the semi-permeable membrane.

 

In osmosis only water gets through a partially permeable membrane and depending on the relative concentration of dissolved substances on either side of the membrane, osmosis can happen in either direction - meaning water can diffuse through the membrane in either direction.

The greater the concentration of water, the more dilute the solution, the greater the water potential.

The lower the concentration of water, the more concentrated the solution, the lesser the water potential.

Therefore in osmosis through a semi-permeable membrane, water moves from a region of higher water potential, to a region of lower water potential.

 

Although the water molecules (and any other particles) are moving around at random, there will be a net transfer of water in one direction at a time through a partially permeable membrane.

The net direction of diffusion of water is from a less concentrated solute solution (more water molecules) to a more concentrated solute solution (less water molecules) i.e. from the higher concentration of water molecules to a lower concentration of water molecules across the membrane.

You can also express this as water moving from a higher concentration to a lower concentration,

OR, from water moving from a higher water potential to lower water potential.

Therefore a more concentrated solution becomes more dilute in the process.

 

This osmosis diffusion can occur in either direction depending on the relative concentration of the solutes in the cell fluids or tissue fluids and concentrated solutions e.g. of sugars on either side of a partially permeable membrane.

But, whatever, the more concentrated solution will tend to become diluted by water passing through the partially permeable membrane.

 

The movement of water in and out of cells

The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive e.g. sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

 

(a) If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established.

The term water potential is used to describe these situations.

You can talk about the water potential gradient across a partially permeable membrane.

Water will diffuse from a high water potential to a low water potential.

In situation (a), the fluid outside the cells has a high water potential than the solute solution in the cells, so water moves into the cells.

(b) Conversely, if the cell solution is too dilute, then water will diffuse out by osmotic action across the semi-permeable membrane of the cell wall.

In situation (b), the solution inside the cells has a high water potential than the fluid outside the cells, so water moves out of the cells.

Therefore the diagram on the right could represent the passage of water (blue circles) in or out of a cell, depending on the relative concentrations of water on either side of the membrane.

 

Osmosis - plant cells and water potential

(i) When you water a plant it increases the water potential of the soil around it.

Therefore the plant cells will draw water in by osmosis until they become turgid - fatter and swollen.

The cell fluids (contents of the cell) will push against the cell wall, known as turgor pressure, and this helps support the plant tissues (therefore the plant as a whole).

 

(ii) If the soil is very dry, lacking in water, the plant starts to wilt and the water potential of the plant is greater than the surrounding soil.

The result is the plant cells become flaccid and begin to lose water.

The plant doesn't droop (flop) completely and retains much of its shape because the strong cellulose cell wall is relatively inelastic and helps the plant retain its shape.

 

Osmosis - animal cells and water potential

In the case of animal cells, they do not have strong walls and can respond adversely to change in the ambient water pressure.

If animal cells are surrounded by a solution of greater water potential (less concentrated in solutes), they can absorb so much water by osmosis that they burst - which kills the cells - see red blood cell example below.

In extreme cases you can die of over-hydration, but its a complicated effect that reduces the level of salt (= sodium ions) in the blood to dangerous levels.

If cells are surrounded by a too concentrated salt solution, they lose so much water they can shrink and shrivel up and the dehydrated kills the cells - see red blood cell example below.

 

Osmosis is important for the function of many animal organs

It isn't just all about individual cells. e.g. water is absorbed into the bloodstream from the large intestine to form faeces in the appropriate physical state!

The function of the kidney and the formation of urine, all involve osmotic transportation of water.

 

See also Homeostasis - osmoregulation - ADH - water control  gcse biology revision notes

and Transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure

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Key points on osmosis and transport of water

Based on the syllabus-specifications for students taking the UK AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10 biology).

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What Is Osmosis?

Osmosis is a special type of diffusion. It's defined as:

The net movement of water molecules across a partially permeable membrane, from a region of high water concentration (dilute solution) to a region of low water concentration (concentrated solution).

• It's a passive process — it does not require energy.

• In biological systems, osmosis mostly occurs through the cell membrane.

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Key Terms to Know

• Water potential: A measure of the concentration of water molecules. Pure water has the highest water potential.

• Partially permeable membrane: Allows small molecules (like water) to pass through but blocks larger solutes (like sugars or salts).

• Dilute solution: High concentration of water, low concentration of solutes.

• Concentrated solution: Low concentration of water, high concentration of solutes.

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Demonstrating Osmosis

1. Osmosis in Potato Cubes

A common school experiment:

• Cut equal-sized potato cubes or cylinders.

• Place them in solutions of different sugar/salt concentrations.

• Measure change in mass or length after a period of time.

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Osmosis in Plants

• Water enters plant cells via osmosis.

• The vacuole fills, pressing the cell membrane against the cell wall → cell becomes turgid.

• If cells lose too much water, the membrane pulls away → plasmolysis occurs (cell becomes flaccid).

• See section (5) Some details of examples of osmotic action in individual animal or plant cell types

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Osmosis in Animal Cells

• Animal cells don’t have a cell wall, so water balance is more delicate.

• In pure water: cell swells and may burst (lysis).

• In concentrated solution: cell shrinks (crenates).

This is why isotonic solutions (with equal water concentration) are important in medicine - e.g. intravenous fluids.

See section (5) Some details of examples of osmotic action in individual animal or plant cell types

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Exam Tips

• Always refer to “net movement of water” and “partially permeable membrane.”

• Use the terms hypertonic, hypotonic, and isotonic correctly when comparing solutions.

• Know the effects of osmosis on plant versus animal cells — they behave differently due to the presence or absence of a cell wall.

• Be ready to interpret graphs showing changes in mass or length due to osmosis.

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Keywords, phrases and learning objectives for this part on transport in organisms

Know that the scientific term osmosis is a special case of transport of water by diffusion through a semi-permeable membrane (partially-permeable membrane).

Understand the potential effects of transport of water in and out of cells in the kidney and intestines.

• Know and understand that differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

  • The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

  • The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive eg sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

  • If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established. Conversely, if the cell solution is too dilute, then water will diffuse out from osmotic action across the semi-permeable membrane of the cell wall.

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