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