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

• 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