Internal and External environment

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Firstly it’s important to understand that there are different solutes a cell can exists in. We refer to the relative concentration of solute on either side of a membrane as Tonicity.

Isotonic

The surrounding fluid and internal fluid of a cell are of equal concentration.

Hypotonic

The surrounding fluid has a lower concentration of solutes than compared to the internal environment of the cell.

Hypertonic

The surrounding fluid has a higher concentration of solutes than the cell’s internal environment.

Before we talk about the effects of different solutes on cells, it is important you have an understanding of Osmosis and Diffusion. Click here if you need to revise.

Cells will react differently in each solution.

Animal cells

When animal cells are placed in different solutions, they undergo a change due to the difference in osmotic pressure between the external and internal environment. These changes can be extremely harmful to the organism.

For example a red blood cell is placed in a:

Scroll over each button to see how each solution affects the cell.

Plant Cells

Plant cells will act in a similar way. However their cell wall will prevent the cell from

popping in a hypotonic solution. Observe the animation below to get a better understanding of plant cells in different solutions.

Hypotonic solution

Water flows into the cell, causing it to swell.

This is called Haemolysis.

The cells can eventually burst.

Hypertonic solution

Water flows out of the cell causing the cell to shrink.

This is caused crenation.

The cell can eventually die, as there is no fluid for its organelles to operate in.

Osmotic solution

Water flows in and out of the cell at a constant rate.

This is equilibrium

Hypotonic solution

Water flows into the cell and diffuses into the Vacuole, causing it to swell and push up against the cell wall. However because of the cell wall, the cell wall no burst.

At this stage the plant cell is Turgid.

Being Turgid allows the plants to keep their shape and form.

Hypertonic solution

Water flows out of the cell and the vacuole. This process is called Plasmolysis.

At this stage, the cell is referred to as flaccid.

This is why you will often see advertisements for isotonic sports drinks. These drinks have roughly the same concentrations of salts and water as the human body.