transport in plants

Question 1

the need of transport systems in plants

Answer 1

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)

Question 2

transport systems in single celled organisms

Answer 2

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

Question 3

increasing transport distances

Answer 3

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

Question 4

surface area: volume ratio

Answer 4

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

Question 5

sa:v ratio in larger organisms

Answer 5

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

Question 6

adaptions of plants to increase sa: v ratio

Answer 6

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

Question 7

increasing levels of activity

Answer 7

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

Question 8

mass transport in plants

Answer 8

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

Question 9

what does mass transport help to do

Answer 9

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

Question 10

specialised transport systems

Answer 10

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

Question 11

functions of the xylem

Answer 11

The functions of xylem tissue in a plant are:
Vascular tissue that carries dissolved minerals and water up the plant
Structural support
Food storage

Question 12

structure of the xylem

Answer 12

Xylem tissue is found, along with phloem tissue and other tissues, in vascular bundles

The location of the vascular bundles is dependent on which organ they are in as the different organs are under different stresses:

In the roots the vascular bundle is found in the centre and the centre core of this is xylem tissue.
This helps the roots withstand the pulling strains they are subjected to as the plant transports water upwards and grows

In the stems the vascular bundles are located around the outside and the xylem tissue is found on the inside (closest to the centre of the stem) to help support the plant

In the leaves the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line.

The xylem tissue is found on the upper side of the bundles (closest to the upper epidermis)

Question 13

function of the phloem

Answer 13

The function of phloem tissue in a plant is to:
Transport organic compounds, particularly sucrose, from the source (eg. leaf) to the sink (eg. roots). The transport of these compounds can occur up and down the plant

Question 14

phloem structure

Answer 14

Phloem is a complex tissue also made up of various cell types; its bulk is made up of sieve tube elements which are the main conducting cells and the companion cells

Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres

The location of the vascular bundles is dependent on which organ they are in as the different organs are under different stresses:

In the roots the vascular bundle is found in the centre and on the edges of the centre core is the phloem tissue

In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis)

In the leaves, the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)

Question 15

the functions of xylem tissue

Answer 15

The functions of xylem tissue in a plant are:
Vascular tissue that transports dissolved minerals and water around the plant
Structural support
Food storage

Question 16

xylem tissue cells

Answer 16

Tracheids (long, narrow tapered cells with pits)
Vessel elements (large with thickened cell walls and no end plates when mature)
Xylem parenchyma
Sclerenchyma cells (fibres and sclereids)

Most of the xylem tissue is made up of tracheids and vessel elements, which are both types of water-conducting cell

Question 17

the function of phloem tissue

Answer 17

Transport organic compounds (assimilates), particularly sucrose, from the source (eg. leaf) to the sink (eg. roots).
The transport of these compounds can occur up and down the plant

The organic compounds are dissolved in water to form sap

Phloem is a complex tissue made up of various cell types; its bulk is made up of sieve tube elements which are the main conducting cells and companion cells

Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres

Mature phloem tissue contains living cells, unlike xylem tissue

Question 18

sieve tube elements

Answer 18

sieve plates with sieve pores- allows for the continuous movement of the organic compounds

cellulose cell wall- strengthens the wall to withstand the hydrostatic pressure that move the assimilates

no nucleus, vacuoles, ribosomes in the mature cells- maximises the space for the translocation of the assimilates

thin cytoplasm- reduces friction to facilitate the movement of assimilates

Question 19

companion cells

Answer 19

Each sieve tube element has a companion cell associated with it as companion cells control the metabolism of their associated sieve tube member
They also play a role in loading and unloading of sugars into the phloem

nucleus and other organelles present- provides metabolic support to sieve tube elements and helps with the loading and unloading of the assimilates

transport proteins in the plasma membrane- moves assimilates into and out the sieve tube elements

large number of mitochondria- to provide atp for the active transport of assimilates into or out of the companion cells

plasmodesmata- the link to sieve tube elements which allows organic compounds to move from the companion cells into the sieve tube elements

Question 20

dicotyledonous (dicot) plants

Answer 20

Dicotyledonous (dicots) plants have:
Seeds that contain two cotyledons (seed leaves)
Network of veins
Leaves that typically have broad blades (leaf surface) and petioles (stalks)
Tap root with lateral branches

Herbaceous dicots have a relatively short life cycle (one growing season) and non-woody tissue

Question 21

vascular system

Answer 21

Plants need transport systems to meet their metabolic demands (glucose, hormones, mineral ions are required for various processes within plants), to efficiently move substances up and down and to compensate for their relatively small SA:V ratio (generally plants cannot rely on diffusion alone)

Plants have a vascular system which involves a network of vessels (vascular tissue) running through the leaves, stem and roots. These three parts are the main organs involved in transport

Question 22

aspects of the vascular system

Answer 22

The vascular system is comprised of two distinct types:
Xylem (transports water and mineral ions from the roots to the rest of the plant)
Phloem (transports substances from the source (eg. leaf) to the sink (eg.root))

The xylem and phloem are arranged together in vascular bundles

The bundles are laid out differently in the leaves, stem and roots

Question 23

the process of transpiration

Answer 23

Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)

Around 99% of the water absorbed by a plant is lost through evaporation from the plant’s stem and its leaves in a process called transpiration

Transpiration refers to the loss of water vapour from a plant to its environment by evaporation and diffusion

Transpiration is a consequence of gaseous exchange at the stomata

Question 24

the advantages of transpiration

Answer 24

It provides a means of cooling the plant via evaporative cooling

The transpiration stream is helpful in the uptake of mineral ions

The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

Question 25

transpiration stream

Answer 25

The transpiration stream refers to the movement of water from the roots to the leaves

The evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules causes the movement of water through a plants xylem

It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells

Question 26

concentration gradient in transpiration

Answer 26

The transpiration rate is dependent on the concentration gradient of water vapour between the inside of the leaf and the surrounding air
A larger concentration gradient results in a faster rate of diffusion

Question 27

air movement in transpiration

Answer 27

There is usually a lower concentration of water molecules in the air outside the leaf

When the air is relatively still water molecules can accumulate near the leaf surface.

This creates a local area of high humidity which lowers the concentration gradient and the rate of transpiration

Air currents can sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration

Question 28

temperature in transpiration

Answer 28

An increase in temperature results in an increase in the kinetic energy of molecules.

Therefore an increase in temperature will increase the rate of transpiration as water molecules move out of the leaf (down the concentration gradient) at a faster rate

If the temperature gets too high the stomata close to prevent excess water loss. This dramatically reduces the rate of transpiration

Question 29

light intensity in transpiration

Answer 29

Stomata close in the dark, their closure greatly reduces the rate of transpiration

When the light is sufficient for the stomata to open, the rate of transpiration increases

Once the stomata are open any increase in light intensity has no effect on the rate of transpiration

Stomata will remain open at relatively low light intensities

Question 30

humidity in transpiration

Answer 30

If the humidity is high that means there is a large concentration of water molecules in the air surrounding the leaf surface

This reduces the concentration gradient between inside the leaf and the outside air which causes the rate of transpiration to decrease

At a certain level of humidity, an equilibrium is reached; there is no concentration gradient and so there is no net loss of water vapour from the leaves

Question 31

transpiration in plants

Answer 31

Within a plant mineral ions and organic compounds (eg. sucrose) are transported by being dissolved in water.
The dissolved mineral ions are transported in the xylem tissue and the dissolved organic compounds are transported in the phloem tissue
The plant roots are responsible for the uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substances

Question 32

the uptake of water

Answer 32

The uptake of water is a passive process and occurs by osmosis (the diffusion of water from a higher (less negative) water potential to a lower (more negative) water potential

Question 33

the uptake of minerals

Answer 33

The uptake of minerals can be passive or active and occurs by diffusion or active transport respectively

Plants must take in a constant supply of water and dissolved minerals to compensate for the continuous loss of water via transpiration in the leaves, and so that they can photosynthesise and produce proteins

Question 34

apoplastic pathway

Answer 34

Most water travels via the apoplastic pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem

The water moves by diffusion (as it is not crossing a partially permeable membrane)

The water can move from cell wall to cell wall directly or through the intercellular spaces

The movement of water through the apoplastic pathway occurs more rapidly than the symplastic pathway

Question 35

casparian strip in the apoplastic pathway

Answer 35

When the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic pathway

This band is called the Casparian strip and forms an impassable barrier for the water
When the water and dissolved minerals reach the Casparian strip they must take the symplastic pathway.

The presence of this strip is not fully understood but it is thought that this may help the plant control which mineral ions reach the xylem and generate root pressure

As the plant ages the Casparian strip thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions

Question 36

symplastic pathway

Answer 36

A smaller amount of water travels via the symplastic pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells

The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata

The movement of water in the symplastic pathway is slower than the apoplastic pathway

Question 37

movement of water

Answer 37

The movement of water through a plants xylem is largely due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
Otherwise known as the cohesion-tension theory

It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells

Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)

Around 99% of the water absorbed is lost through evaporation from the plant’s stem and leaves via transpiration

Question 38

movement of water through leaves (transpiration stream)

Answer 38

Certain environmental conditions (eg. low humidity, high temperatures) can cause a water potential gradient between the air inside the leaves (higher water potential) and the air outside (lower water potential) which results in water vapour diffusing out of the leaves through the stomata (transpiration)

The water vapour lost by transpiration lowers the water potential in the air spaces surrounding the mesophyll cells

The water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull

This transpiration pull results in water moving through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm (symplastic pathway) into the cell wall

The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas), which then causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water).

This movement is called transpiration stream

Question 39

the role of the stomata

Answer 39

Transpiration is mainly controlled by the pairs of guard cells that surround stomata (plural, stoma is singular)

Guard cells open the stomata when they are turgid and close the stomata when they lose water

When the stomata are open there is a greater rate of transpiration and of gaseous exchange

When the stomata close transpiration and gaseous exchange decrease

As stomata allow gaseous exchange (CO2 in and O2 out) they are generally open during the day

Question 40

movement in the phloem

Answer 40

Although translocation could refer to the transport of substances in the xylem and phloem, as it means ‘moving from one place to another,’ it is more commonly connected with the transport of assimilates in the phloem tissue

Thus translocation within phloem tissue can be defined as the transport of assimilates from source to sink and requires the input of metabolic energy (ATP)

The liquid that is being transported (found within phloem sieve tubes) is called phloem sap

This phloem sap consists not only of sugars (mainly sucrose) but also of water and other dissolved substances such as amino acids, hormones and minerals

Question 41

the source of assimilates

Answer 41

The source of the assimilates could be:
Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose)

Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)

Food stores in seeds (which are germinating)

Question 42

the sinks where the assimilates are required

Answer 42

Meristems (apical or lateral) that are actively dividing

Roots that are growing and / or actively absorbing mineral ions

Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)

Question 43

translocation of assimilates research

Answer 43

The loading and unloading of the sucrose from the source to the phloem, and from the phloem to the sink is an active process

It can be slowed down or even stopped at high temperatures or by respiratory inhibitors

Translocation of assimilates is not fully understood yet by scientists. The understanding they do have has come from studies such as:
Collecting and studying the sap from plants with ‘clotting’ sap (eg. castor oil plants)

Using aphids to collect the sap – after the aphid inserts its stylet (tubular mouthpart) scientists remove the aphids head and collect the sap that continues to flow

Using radioactively labelled metabolites (eg. Carbon-14 labelled sugars) which can be traced during translocation

Advances in microscopes enabling the adaptations of companion cells to be seen

Observations about the importance of mitochondria to the process of translocation

Question 44

the sucrose loading mechanism

Answer 44

Assimilates such as sucrose are transported from source to sink through the phloem sieve tubes

Carbohydrates are generally transported in plants in the form of sucrose because:
It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)

It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported

Question 45

loading of assimilates

Answer 45

If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls.
This is an active process and therefore requires ATP as an energy source

The large concentration of hydrogen ions in the cell wall of the companion cell results in the hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell

The hydrogen ions move through a cotransporter protein.

While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose

The sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells

Companion cells have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pump

This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the mesophyll

Question 46

unloading of assimilates

Answer 46

The unloading of the assimilates (eg. sucrose) occurs at the sinks

Scientists believe that the unloading of sucrose is similar to the loading of sucrose, with the sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways

To maintain a concentration gradient in the sink tissue, sucrose is converted into other molecules.
This is a metabolic reaction so requires enzymes (eg. invertase which hydrolyses sucrose into glucose and fructose)

Question 47

the mass flow hypothesis

Answer 47

The Mass Flow Hypothesis was the model initially used to explain the movement of assimilates in the phloem tissue

The mass flow hypothesis was modelled by Ernst Münch in 1930. His simple model consisted of:
Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)

These two membranes were placed into two chambers containing water and were connected via a passageway

The two membranes were joined via a tube
As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution which forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)

Question 48

pressure (hydrostatic) flow gradient

Answer 48

Phloem sap (containing sucrose and other organic solutes) moves by mass flow up and down the plant

Carbohydrates are generally transported in plants in the form of sucrose because:
It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)

It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported

The advantage of mass flow is that it moves the organic solutes faster than diffusion

In xylem tissue the pressure difference that causes mass flow occurs because of a water potential gradient between the soil and leaf (this requires no energy input by the plant)

However in phloem tissue energy is required to create pressure differences for the mass flow of the organic solutes

The pressure difference is generated by actively loading sucrose into the sieve elements at the source (usually a photosynthesising leaf or storage organ) which lowers the water potential in the sap

This results in water moving into the sieve elements as it travels down the water potential gradient by osmosis

The presence of water within the sieve elements increases the hydrostatic or turgor pressure at the source and as solutes (eg. sucrose) are removed / unloaded from the sieve elements causing water to follow by osmosis at the sink (creating a low hydrostatic pressure), a hydrostatic pressure gradient occurs

The pressure difference between the source and the sink results in the mass flow of water (containing the dissolved organic solutes) from the high hydrostatic pressure area to the low hydrostatic pressure area

The mass flow of organic solutes within the phloem tissue occurs above and below the sources (which is typically photosynthesising leaves). Therefore sap flows upwards and downwards within a plant

Question 49

xerophytes

Answer 49

Xerophytes (from the Greek xero for ‘dry’) are plants that are adapted to dry and arid conditions

Xerophytes have physiological and structural (xeromorphic) adaptations to maximise water conservation

Question 50

xerophyte adaptions

Answer 50

fleshy succulent leaves- water stores for times of low avalibity

hinge cells shrink when flaccid- causes leaves to roll exposing the thick, waterproof cuticle to the air and creates a humid space in the middle of the rolled leaf

leaves reduced to scales, spines or needles
leaves curled when flaccid- reduced transpiration due to reduced surface area available

stomata closes during light, stomata open in the dark- CAM metabolism to minimise photorespiration, co2 fixed at night, day time water loss is minimised

sunken stomata- water loss is minimised by trapping moist air close to the area of water loss reducing the diffusion gradient

reduced number of stomata- less water loss due to fewer pores

stomata only found in the upper epidermis- open into the humid space created by the hairs and rolled shape

thick wavy cuticle on leaves- water loss reduced via cuticle

Question 51

hydrophytes

Answer 51

Plants that are adapted to living in freshwater are known as hydrophytes

They have evolved specific adaptations that enable them to deal with the challenges posed by living in such an environment

Excess water uptake is not a major concern for plants as their cells possess a cell wall
The cell wall prevents too much water from being absorbed

Question 52

challenges of hydrophytes

Answer 52

The abundance of water in the surrounding environment means there is little need for water transport mechanisms or adaptations that reduce water loss

The main challenge that hydrophytes face is receiving enough carbon dioxide during the day and enough oxygen during the night

Water contains less oxygen and carbon dioxide than the air

Question 53

adaptions of hydrophytes

Answer 53

Floating leaves: the leaves are thin, flat and have large air spaces inside to give them buoyancy. This keeps them close to the surface of the water where there is more light for photosynthesis

Thin waterproof waxy cuticle: it is very thin as there is little need to prevent water loss

Stomata located on the upper surface of the leaves: this allows for gas exchange to occur with the air instead of the water

Reduced root system: only small roots are required as they can also extract nutrients from the surrounding water through their tissues

Reduced veins in the leaves: the xylem is significantly reduced as there is no need to transport water throughout the plant

A common hydrophyte is the water lily