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Transport Across The Membrane


Cell Membrane

Cell membranes are a barrier to many substances, and these membranes allow materials to be concentrated inside cells or simply separated the cell from the outside environment. This compartmentalization is essential for life, because it induces. Eukaryotic cells can also compartmentalize materials inside organelles. In fact materials need to move in and out of the cell and it has some methods to allow substances to get in or get out of the cells.

The transport methods are

1. Simple Diffusion

2. Osmosis

3. Facilitated Diffusion

4. Active Transport

5. Vesicles

Cell Transport

1. Simple Diffusion

Some substances can diffuse directly into the cell membrane through its main part - the lipid bi layer. Steroids are tiny molecules like oxygen. The membrane is not a barrier to these molecules. Cell cannot control the Lipid diffusion.

2. Osmosis

Diffusion of water across a membrane is termed as Osmosis. It is exactly like lipid diffusion only but it is specially termed as osmosis because water is abundant in the cells and important as well. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute solution that is low concentration solute to a concentrated solution that is high concentrated solute.


Water Potential

Osmosis can be quantified using water potential. The water movement and the speed of the water movement can be easily monitored. Water potential is a measure of the water molecule potential for movement in a solution. Pressure is the unit with which the water can be measured. The water always moves by osmosis from less negative potential to more negative water potential.


The concentration of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three types of concentrations of solution are noted.

Isotonic solution - a solution equals the concentration of a cell

• Hypertonic solution - a solution of higher concentration than a cell

• Hypotonic solution - a solution of lower concentration than a cell


The effects of the solutions on cells

3. Facilitated Diffusion


4. Active Transport (or Pumping)

Do you know how to draw active transport?

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