What affects the rate of diffusion bbc bitesize

Diffusion is a process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules)[1]until it has reached equilibrium (molecules evenly spread).

A diagram of diffusion happening. The first diagram shows particles in a liquid. The second shows the same liquid a few seconds later after the particles have spread out

Diffusion usually happens in a mixture in gas, a liquid and occasionally colloids. It is possible to see diffusion happening when two liquids are mixed in a transparent container. It describes the constant movement of particles in all liquids, gases and colloids. These particles move in all directions bumping into each other.

A sugar cube is left in a beaker of water for a while.
The smell of ammonia spreads from the front of the classroom to the back of the room.
Fumes of perfume rise from the bottle when the top is removed.
Food coloring dropped on the beaker spreads out.
the smell of food spread in the whole house

Molecules tend to move from places of high concentration to places of low concentration, just by moving randomly. For example, there is more oxygen in a lung than there is oxygen in the blood so oxygen molecules will tend to move into the blood. Similarly, there is more carbon dioxide molecules in the blood than in the lung so carbon dioxide molecules will tend to move into the lung. It happens in cell biology, where small molecules simply diffuse through the cell membrane, but larger molecules only get through by using energy: see active transport.

The random movement of fluid molecules makes them spread out until a boundary stops them.

Diffusion is a passive process, therefore does not require energy as it occurs down a concentration gradient.

Osmosis and heat transfer are types of diffusion.

Diffusion is affected by:

the concentration gradient - diffusion will be greater where gradient is larger
the temperatures - diffusion will happen faster when temperatures are higher as there is more kinetic energy
the surface area - diffusion will be greater where it's greater
the diffusion distance - diffusion will be greater where there is a short diffusion distance

In small unicellular organisms, simple diffusion can exchange molecules quickly enough to keep them alive. A high surface area to volume ratio helps.

However, for multicellular organism simple diffusion is not enough. They need to move more material over longer distances to remain alive. They have evolved to have internal structures and systems for rapid distribution movement. For example, humans have lungs to make diffusion happen rapidly. The same happens in plants with the leaf.

Osmosis
Heat transfer

↑ "BBC - GCSE Bitesize: Diffusion".

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Remember that diffusion is a passive process, so when it occurs in a living organism the cells of that organism do not provide the particles involved with energy to diffuse. The particles that are moving about randomly have their own kinetic energy.

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In order for any organism to function properly, it needs to exchange substances between itself and the environment such as:
- Oxygen
- Carbon dioxide
- Water
- Dissolved food molecules
- Mineral ions
- Urea

This exchange of substances occurs across the cell membrane
There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport
Unicellular (single-celled) organisms like amoeba have very large surface areas (SA) in comparison to their volumes
- This means that the distance between the surface of the organism to its centre is very small
As a result, unicellular organisms do not need to have specialist exchange surfaces or transport systems; as diffusion, osmosis and active transport through the cell membrane occur at a sufficient rate to meet the needs of the organism

Unicellular organisms such as amoeba do not require transport systems due to their large surface area to volume ratio

Multicellular organisms

For larger, multicellular organisms the distance between the surface of the organism to its centre is relatively long
This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise
Transport systems in animals include:
- The blood and circulatory system - carries the necessary substances around the body
Transport systems in plants include:
- The xylem - moves water and mineral ions from roots to shoots
- The phloem - moves sugars and amino acids to where they are needed in the plant

Some examples of transport systems in plants and animals

Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes
This is why larger organisms need exchange surfaces within their transport systems to carry out diffusion, osmosis and active transport at a sufficient rate
Exchange surfaces in animals include:
- The lungs and alveoli for gas exchange
- The small intestines and villi for absorption of digested food

Exchange surfaces in plants include:
- Roots and root hairs where mineral ions and water are absorbed
- The leaves for gas exchange

Some examples of exchange surfaces in plants and animals

Properties of exchange surfaces

Multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:
- Having a large surface area to increase the rate of transport
- A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across
In addition, animals have:
- A large network of blood vessels throughout the body:
  - To reduce the distance of exchange of materials between cells and the bloodstream
  - To move substances towards or away from exchange surfaces to maintain concentration gradients
- Gas exchange surfaces that are well ventilated to maintain concentration gradients
You should be able to calculate and compare surface area to volume ratios
You can model the effect of how increasing size affects surface area to volume ratio using simple cubes: