Cell membrane

Cell membranes play an important role, in transporting substances to and from the cell.

There are two important forms of transport across cell membranes.

Passive and active.

The most common form of passive transport is diffusion.

Diffusion is a process by which substances move from a higher concentration to a lower concentration.

There is no energy required for passive transport.

Gases are an example, which diffuses across cell membranes.

Oxygen diffuses into the cell.

Carbon dioxide diffuses out of the cell.


One type of diffusion is osmosis.

Water is transported across cell membranes by osmosis.

When the osmotic pressure is higher, it  is called hypertonic.

When the  osmotic pressure is lower, it is called hypotonic.

Water diffuses across the cell membrane, depending on its concentration, inside and outside the cell.

In this way the cell can gain or lose water.


One type of diffusion is called facilitated diffusion.

Here proteins are used to facilitate the transport of molecules.

Glucose is an example, which is transported into the cell, using facilitated diffusion.


Active transport requires energy, to transport substances across the cell membrane.

ATP supplies this energy for active transport.

Protein channels and ATP are required for active transport.

The sodium potassium pump is an example, of active transport.

This helps to maintain a gradient of sodium and potassium concentration, inside and outside the cell membrane.


Endocytosis is a process by which cells take in large molecules.

Exocytosis is a process by which cells releases large molecules.


When a gas is in a container, the molecules move randomly to expand and fill the container.

Let us consider a container,  which has two chambers separated by a partition.

One chamber has gas A, and another chamber has gas B.

If the partition  is removed, gas A will expand to fill the whole container.

Gas B will also expand to fill the whole container.

The gas is said to be moving along their gradient.

This process requires no energy.

Our lungs have numerous small pockets called alveolar sacs.

When we breathe in air, it goes into the alveoli.

When we breathe in, there is a high concentration of oxygen.

This diffuses out into the capillaries, and into the blood stream.

There is a high concentration of carbon dioxide in the capillaries,

which diffuses out of the capillaries, into the alveoli.

We breathe out this carbon dioxide.


We can conduct a U-tube experiment to understand osmosis.

The U-tube has a semi permeable membrane partition in the centre.

The membrane is permeable to water.

We have solutions of different concentration of sugar in the two partitions.

The water moves across the semi permeable membrane,  from high water concentration,

to low water concentration.

The water rises up in the partition with high sugar concentration, or low water concentration.

This diffusion equalises the concentration of sugar in both partitions.


Blood contains red blood cells.

Blood cells are enclosed in a cell membrane.

Red blood cells are surrounded by plasma.

The concentration of the plasma, is the same as the red blood cells.

In this state, it is said to be isotonic.

If we inject salt water into the blood cells, the water concentration inside and outside the blood cells will differ.

The blood cells will have a higher concentration of water and be hypertonic.

A liquid with higher solute concentration, is said to be hypertonic.

Water will move out of the blood cells, to equalise the concentration.

If we inject distilled water, into the blood vessel, there will be more water concentration outside the blood cell.

In this case the water will flow into the blood cell.