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

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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

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.