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

(a) The devastating effects on red blood cells of over-hydration or dehydration situations.

(b) The effects on plants of dehydration from lack of water - plasmolysis.

(a) The effect or pure water and salt (sodium chloride) solution on red blood cells.

If cells are placed in pure water (distilled or deionised) OR in a salt solution, the movement of water through the partially permeable cell membrane by osmosis can have some pretty devastating effects - illustrated by the diagram above concerning those rather vital red blood cells, but it can happen to most cells!

On the left: The cells are in a less concentrated solute solution compared to the cytoplasm - or in just water.

If red blood cells are put in pure water, the greater external water potential (more concentrated) of the less dilute solution, means that water will pass into the cell's cytoplasm by osmosis.

The diffusion gradient is into the cells - which have a greater solute concentration than pure water.

The result is the cells swell up, burst open and die.

On the right: The cells are in a more concentrated solution compared to the concentration of solutes in the cytoplasm of the cell.

If red blood cells are put in a salt solution, the greater internal water potential of the more dilute solution of the cell's cytoplasm, means that water will pass into the cell by osmosis

The result is the cells shrink and shrivel up and die.

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(b) The formation of a plasmolysed plant cell

1. Turgid plant cell

When a plant has sufficient water, the water passes into the cells by osmosis and the vacuole fills and swells up.

The vacuole pushes against the cell wall making the cell turgid.

This gives the plant structural support so it doesn't droop/wilt - tall trees are an impressive example of this!

 

2. Flaccid plant cell

If water passes out of the cells by osmosis, the vacuole shrinks and the plant cell becomes flaccid.

The cytoplasm can begin to move away from the cell wall.

 

3. Plasmolysed plant cell

If a plant cell loses a lot of water by osmosis, cytoplasm of the cell peels away from the cell wall, leaving gaps between the cell wall and the membrane and making the plant cell shrink and crumple - wilt and droop.

Plasmolysis is the shrinking of the cytoplasm of a plant cell in response to diffusion of water out of the cell and into a high salt concentration solution by osmosis. During plasmolysis, the cell membrane pulls away from the cell wall, but this does not happen in low salt concentration because of the rigid plant cell wall.

Not surprisingly, plasmolysis can happen in very dry conditions, but on watering (rain or us), most paler wilted plants recover to the fully 'green' upright plant.

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Key points - osmotic action in selected cells - blood cell and plant cell

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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Recap: What Is Osmosis?

Osmosis is the net movement of water molecules through a partially permeable membrane, from a dilute solution (high water potential) to a concentrated solution (low water potential).

It’s a passive process — no energy required.

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Osmosis in Plant Cells

Plant cells have:

• A cell wall (rigid and strong)

• A partially permeable membrane

• A central vacuole that stores water

In Pure Water (Hypotonic Solution):

• Water enters the cell by osmosis.

• Vacuole expands, pressing the cell membrane against the cell wall.

• The cell becomes turgid (firm).

• Turgor pressure builds up — this supports the plant.

In Concentrated Solution (Hypertonic):

• Water leaves the cell by osmosis.

• The vacuole shrinks, and the cell membrane pulls away from the cell wall.

• This is called plasmolysis.

• The cell becomes flaccid (limp) and may die if not rehydrated.

In Isotonic Solution:

• No net movement of water.

• Cell remains the same; neither turgid nor plasmolysed.

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Osmosis in Red Blood Cells (Animal Cells)

Animal cells lack a cell wall, making them more vulnerable to changes in water balance.

In Pure Water (Hypotonic):

• Water enters the cell by osmosis.

• The cell swells and may burst — called lysis.

In Concentrated Salt/Sugar Solution (Hypertonic):

• Water leaves the cell.

• The cell shrinks and becomes crenated (spiky/wrinkled).

In Isotonic Solution:

• Water moves in and out at equal rates.

• Cell maintains its normal shape — ideal condition (e.g., IV fluids).

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Exam Tips

• Always refer to "net movement of water molecules" and "partially permeable membrane".

• Use correct terms: turgid, flaccid, plasmolysed, crenated, lysis.

• Understand why plant and animal cells react differently — due to the presence or absence of a cell wall.

• Diagrams are helpful — practice labelling cells in different osmotic conditions.

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

Understand the terms turgid plant cell and flaccid plant cell, a plasmolysed plant cell in the context of transport of water in and out of plant cells and the importance to the structural strength of a plant.

Be able to describe examples of osmotic action in animal cells such as blood cells.

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