The Need for Transport Systems in Plants (OCR A Level Biology): Revision Note

The Need for Transport Systems in Plants

• All living organisms have the need to exchange substances with their surrounding environment

  • Plants need to take carbon dioxide and nutrients in

  • Waste products generated need to be released 

• The location within an organism where this exchange occurs is described as an exchange site

  • E.g. roots in plants (water and minerals)

• Substances are said to not have entered or left an organism until it crosses the cell surface membrane

• Small organisms like the single-celled Chlamydomonas are able to exchange substances directly with the environment

  • This is due to their large surface area: volume ratio

  • The diffusion or transport distance in these organisms are also very small so essential nutrients or molecules are able to reach the necessary parts of the cell efficiently

  • Smaller organisms tend to have lower levels of activity and so smaller metabolic demands

• Larger organisms require specialised mass transport systems for a number of reasons:

  • Increasing transport distances

  • Surface area: volume ratio

  • Increasing levels of activity

Increasing transport distances

• Every cell in a plant requires water, glucose and mineral ions

• The roots of a plant take in water and mineral ions while the leaves produce glucose by photosynthesis

• These molecules need to be transported to the other parts of the plant

  • Glucose is transported as sucrose in plants

• This large transport distance makes simple diffusion a non-viable method for transporting substances all the way from the exchange site to the rest of the organism

  • Diffusion wouldn’t be fast enough to meet the metabolic requirements of cells

Surface area: volume ratios

• Surface area and volume are both very important factors in the exchange of materials in organisms

• The surface area refers to the total area of the organism that is exposed to the external environment

• The volume refers to the total internal volume of the organism (total amount of space inside the organism)

• As the surface area and volume of an organism increase (and therefore the overall ‘size’ of the organism increases), the surface area: volume ratio decreases

• This is because volume increases much more rapidly than surface area as size increases

• Single-celled organisms have a high SA: V ratio which allows for the exchange of substances to occur via simple diffusion

  • The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products

  • The small volume means the diffusion distance to all organelles is short

• As organisms increase in size their SA: V ratio decreases

  • There is less surface area for the absorption of nutrients and gases and secretion of waste products

  • In addition, the greater volume results in a longer diffusion distance to the cells and tissues of the organism

Adaptations of plants to increase SA: V ratio

• There are several adaptations present in plants that help to increase their SA: V ratio

  • Plants have a branching body shape

  • Leaves are flat and thin

  • Roots have root hairs

Increasing levels of activity

• Larger organisms are not only more physically active but they also contain more cells than smaller organisms

• A larger number of cells results in a higher level of metabolic activity

  • As a result, the demand for oxygen and nutrients is greater and more waste is produced

• Plant cells and tissues have a much lower metabolic rate than animal cells

  • Therefore their demand for oxygen for aerobic respiration is reduced

Mass transport in plants

• Plants have evolved specialised mass flow transport systems that enable the efficient transport of nutrients and waste

  • Mass flow is the bulk movement of materials. It is directed movement so involves some source of force

• In mass transport systems there is still some diffusion involved but only at specific exchange sites at the start and end of the route travelled by the substances

  • The lungs are the exchange site of the gas exchange system

• Mass transport systems help to:

  • Bring substances quickly from one exchange site to another

  • Maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings

  • Ensure effective cell activity by keeping the immediate fluid environment of cells within a suitable metabolic range

• Flowering plants have evolved two separate mass transport systems:

  • The xylem transports water and mineral ions

  • The phloem transports sucrose and other nutrients

• Notably, plants have no specialised transport system for oxygen and carbon dioxide

• They do not need one because:

  • They have adaptations that give them a high SA: V ratio for the absorption and diffusion of gases

  • The leaves and stems possess chloroplasts which produce oxygen and use up carbon dioxide

  • There is a low demand for oxygen due to plant tissues having a low metabolic rate