B3.1Dissolved Substances

Diffusion is the process by which particles of a substance spread out from each other, moving from a region of high concentration to low concentration. Particles of a substance will move down a concentration gradient until the particles are evenly spread. So particles must be able to move freely. Diffusion allows substances to pass in or out of cells.The substances must be dissolved and there must be a concentration gradient present between the solutions on each side of the cell membrane.It is an important exchange process that is widely used in living organisms.

Gas Exchanges - involves substance moving from an area of high concentration to low concentration.  In the lungs - involves Oxygen moving from the inhaled air inside the alveolus to the blood circulating the lungs.In the lungs - involves Carbon dioxide moving from the blood circulating the lungs to the air inside the alveolus. In a leaf  - involves Carbon dioxide moving from the air outside the leaf to the air spaces inside the leaf and into the leaf cells.In the Small Intestine - involves digested food molecules (amino acids, and glucose) from the small intestine into the blood in the villi capillary.

Active Transport is the process which is used to move dissolved substances from an area of low concentration to an area of high concentration. This goes against the concentration gradient  and so will require energy for it to take place.The energy is obtained from respiration.Active transport also uses protein carriers to help the process take place.The protein carriers are located within the cell membrane.There are binding sites withing the protein carriers.The binding site allows certain dissolved substances to be bind to the side of the cell membrane where it is at a lower concentration.The energy obtained from respiration allows the proteins to change shape and so move the dissolved substances through to the other side of the membrane.The advantage of active transport is that it allows cells to absorb dissolved substance from very dilute substances.

Root hair cells - use active transport to absorb minerals ions (like nitrates) from the soil - even though there is a lower concentration of minerals in the soil than in the root hair cell.Small intestine villi cells - uses active transport alongside diffusion as this maximizes the absorption of glucose and other  substances.

High concentration

Low concentration

Protein carrier

What is diffusion?

Examples of diffusion

Active transport

Examples of Active Transport