Diffusion

DIFFUSION is the process of particles spreading from an area of HIGHER CONCENTRATION to an area of LOWER CONCENTRATION, resulting in them being evenly spaced. This occurs due to the natural, random movement of particles.

 

• Diffusion occurs in both liquids and gases because particles in these states have the FREEDOM TO MOVE randomly.
• Diffusion is a PASSIVE PROCESS which means it does not use any energy.

 

As an example, lets look at what happens to perfume particles when perfume is sprayed in a room:

 

 

You can increase the rate of diffusion by:

• Increasing the CONCENTRATION GRADIENT (the difference in concentration between the two areas).
• Increasing the TEMPERATURE, as particles gain more energy to move faster.
• Increasing the SURFACE AREA of the membrane where diffusion is occurring.

 

Here are some examples of diffusion in CELLS:

• when OXYGEN is taken into the cells
• when CARBON DIOXIDE is removed from the cells
• when UREA (a waste product) is removed from cells and goes into the plasma for excretion

 

Surface Area to Volume Ratio

• The efficiency of exchange surfaces by diffusion is determined by the SURFACE AREA to VOLUME RATIO (SA:V).

• LARGER organisms have a SMALLER SA:V ratio than smaller organisms.
• This can be proven by calculating the SA:V ratio of two cubes.

 

Calculating Surface Area to Volume Ratios

Consider a cube with lengths of 1m and another with lengths of 2m:

  

Therefore the bigger cube has a lower surface area to volume ratio.

 

 

Multicellular Organisms and Exchange Surfaces

• A HIGH SA:V ratio is beneficial for diffusion as it provides a LARGER surface area relative to the volume of the organism.
• This means very small single celled organisms such as bacteria can exchange substances easily as diffusion will occur at a faster rate.
• Multicellular organisms need specialised exchange surfaces due to their smaller SA:V ratios.
• Efficient exchange surfaces have characteristics such as a THIN MEMBRANE for a short diffusion path, a LARGE SURFACE AREA for maximum diffusion, and are often VENTILATED to maintain a diffusion gradient.
• Examples include LUNGS in animals and LEAVES in plants.

 

Gas Exchange in the Lungs

The Role of Lungs and Alveoli

• The LUNGS are responsible for the crucial exchange of gases: OXYGEN is taken in, and CARBON DIOXIDE is expelled.
• This exchange occurs in millions of tiny air sacs called ALVEOLI.

Adaptations of Alveoli for Gas Exchange

• Alveoli have a HUGE SURFACE AREA — around 75 square metres in humans — to increase the efficiency of gas exchange.
• They possess a MOIST LINING for dissolving gases, which aids in the diffusion process.
• Their WALLS are extremely THIN to minimize the distance gases must diffuse.
• Alveoli are surrounded by a dense CAPILLARY NETWORK, ensuring a rich blood supply and rapid gas exchange.

 

Gas Exchange in Plant Leaves

Structure of Leaves

• CARBON DIOXIDE diffuses into AIR SPACES within the leaf for photosynthesis.
• Leaves have an EXCHANGE SURFACE underneath, consisting of small openings called STOMATA.
• OXYGEN and WATER VAPOUR exit the leaf through these stomata.
• The shape of the leaf and the arrangement of cells optimize the internal surface area for gas exchange.

Adaptations for Gas Exchange

• Stomata are flanked by GUARD CELLS that regulate their opening, ensuring gas exchange occurs only when necessary.
• The FLATTENED SHAPE of the leaf increases the surface area for gas exchange.
• Internal cell walls also contribute to a larger exchange surface, with air spaces to facilitate DIFFUSION.

 

Nutrient Absorption in the Small Intestine

Villi Increase Surface Area

• The SMALL INTESTINE maximises absorption efficiency through structures called VILLI.

• Villi are tiny projections that significantly expand the surface area available for nutrient absorption into the bloodstream.
• They consist of a SINGLE LAYER OF SURFACE CELLS and a rich supply of BLOOD CAPILLARIES to facilitate quick absorption of digested nutrients.  

 

Gas Exchange in Fish Gills

Structure of Gills

• Gills are the primary gas exchange surface in FISH.
• Water enters through the mouth and exits through the gills, where OXYGEN is absorbed into the blood and CARBON DIOXIDE is released.

Adaptations of Gills for Gas Exchange

• Gills are made of thin plates called GILL FILAMENTS, which provide a large surface area for gas exchange.
• Filaments are covered in numerous LAMELLAE, which further increase their surface area.
• Lamellae are rich in blood capillaries, ensuring rapid diffusion of gases.
• A THIN SURFACE LAYER of cells in lamellae minimizes the distance over which gases must diffuse.
• Blood flows in one direction through the lamellae while water flows in the opposite, maintaining a CONCENTRATION GRADIENT for oxygen.
• The CONCENTRATION OF OXYGEN in water is always higher than in the blood, facilitating its diffusion into the bloodstream.