3.3.1 The Need For Transport Systems In Plants

Question 1

The Need for Transport Systems in Plants

Answer 1

All living organisms have the need to exchange substances with their surroundings environment
- plants need to take carbon dioxide and nutrients in
- waste products generated need to be released
The location within an organisms where this exchange occurs is described as and exchange site
- e.g. roots in plants (water and minerals)
Substances are said to not have entered or left on organism until it crosses the cell surface membrane
Small organisms, like single-celled Chlamydomonas, are able to exchange substances directly with the environment
- this is due to their large surface area : volume ratio
- this 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
Larger organisms require specialised mass transport systems for a number of reasons:
- increasing transport distances
- surface area : volume ratio
- increasing levels of activity

Question 2

Increasing Transport Distances

Answer 2

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 photosynthesise
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-variable method for supporting 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

Question 3

Surface Area : Volume Ratios

Answer 3

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 an organism increases (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-called 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 adsorption 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 organisms

Question 4

Adaptions of Plants to Increase SA:V Ratio

Answer 4

There are several adaptions 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

Question 5

Increasing Levels of Activity

Answer 5

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 produce
Plant cells and tissues have a much lower metabolic rate than animal cells
- therefore their demand for oxygen for aerobic respiration is reduced

Question 6

Mass Transport in Plants

Answer 6

Plants have evolved specialised mass flow transport systems that able the efficient transport of nutrients and waste
- mass flow is the bulk movement of materials
- its is directed movement so involves some source of force
In mass transport systems, there is still some diffusion diffusion involved but only at specific sites at the start and end of the route travelled buy the substances
- the lungs are the exchange site of gas exchange system
Mass transport systems help to:
- bring substances quickly from one exchange site to another
- maintain diffusion gradients at exchange sites and between cells and their fluid surroundings
- ensues effective cell activity by keeping the immediate fluid 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 fr oxygen and carbon dioxide
They do not need one because:
- they have adaptions 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
- t5here is a low demands for oxygen due to plant tissues having a low metabolic rate