B2.2 – Movement In and Out of Cells

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1. Define diffusion:

The net movement of molecules from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement.

2. Describe the importance of diffusion of gases and solutes and of water as a solvent.

The diffusion of gases is what makes gas exchange possible (you know, that thing where you breathe in oxygen and breathe out carbon dioxide?). Diffusion of solutes is what gives plant cells their proper shape (the cell become turgid when water diffuses in). It’s also what enables plants to transpire.

In animal cells, water (the solvent in this case) is supplied to the needy cells by diffusion.

Water is a very important solvent. All reactions in our body occur in water, our cells are 70% water, substances are transported around our body after being dissolved in water, etc.

3. Define osmosis:

The diffusion of water molecules from a region of their higher concentration (dilute solution) to a region of their lower concentration (concentrated solution), through a partially permeable membrane.

4. Describe the importance of osmosis in the uptake of water by plants, and its effects on plant and animal cells.

Plants transpire only by osmosis:

The water from the soil diffuses into the root hair cells, it diffuses across the cortex cells and into the xylem vessels, where it is taken up into the leaf; and it diffuses across the mesophyl cells and out of the stoma.

(These are revision notes, so I’m hoping you understood what I just said. If not, don’t fear – it’s covered in a later chapter!)

This part is a little bit more interesting:

When water diffuses into an animal cell (by osmosis, of course), it expands.

It diffuses into the cell, because the concentration of water in the cell is less than outside. This means, that the water potential (the ability of water to leave its system) of the solution outside the cell is higher than the water potential inside the solution. This means, the solution outside the cell is hypotonic.

To put it simply, the solution is hypotonic, because it is less concentrated than the solution inside the cell. This means more water is concentrated in the outside solution. So this water diffuses into the cell.

When it expands a little too much, the animal cell explodes.

When plant cells are placed in a hypotonic solution, the same thing occurs, except, plant cells have cell walls. Cell walls are very strong as they are made from cellulose (which is stronger than steel), so when the membrane pushes outwards on to the cell wall, it doesn’t burst. Instead, the cell assumes it’s proper, rigid shape (the cell becomes turgid), helping the plant to stay upright.

When an animal cell is placed in an isotonic solution (where the concentration of the outside solution and the cell is the same), water diffuses in and out at the same rate. This is the best condition for animal cells.

However in plant cells, it makes them a little flaccid, as they are not gaining enough water to push out on the cell walls and become turgid.

In a hypertonic solution (when the outside concentration is more than the inside concentration), water diffuses out of the cell (down its concentration gradient), so the cell loses water overall. Animal cells shrivel up.

The cell membrane in plant cells rip away from the cell wall (plasmolysis).

5. Describe and explain the importance of a water potential gradient in the uptake of water by plants.

As I explained earlier, water potential is the ability of water to leave its system.

And because molecules move randomly in fluids (liquids and gases), they tend to move from their region of higher concentration to their region of lower concentration.

This means, that water is more likely to flow to a concentrated solution (where less water is present) from a dilute solution (where more water is present).

This concentration gradient of water is known as the water potential gradient (from higher water potential to lower water potential).

This means, in order for water to diffuse through plant cells, the water potential gradient has to be maintained – plant cells must have a lower water potential than its surrounding environment.

 

Notes Submitted by Sarah.

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