3.3.1 The Need for Transport Systems in Plants

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

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 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

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

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

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