-Module 3.3 - Transport in plants

Question 1

Why do plants need a transport system in an overview?

Answer 1

All living things need to take substances out of and release waste into their environment.
Every plant cell needs a regular supply of water, oxygen, nutrients and minerals.
They need a transport system to overcome the difficulties of large diffusion distances.

Question 2

What are the 3 main reasons that plants need a transport system?

Answer 2

Metabolic demand.
Size
Surface area to volume ratio

Question 3

Why does metabolic demand cause plants to need transport systems?

Answer 3

They have a high demand for particular substances (water and sugar) and this can’t be met by simple diffusion into all cells.
Underground parts of plant’s don’t photosythesise so need glucose and oxygen transported to them and waste products remove.
Hormones made in a part of the plant may need transporting to another area.
Water and mineral ions from the roots need transporting to all cells to make proteins required for enzymes and structure of the cell.

Question 4

Why does size cause plants to need transport systems?

Answer 4

Some plants are small but others can be huge as they continue to grow throughout their lives.
They need effective transport systems to move substances both up and down from the roots to the topmost leaves to reach all the cells they’re needed in.

Question 5

Why does surface area cause plants to need transport systems?

Answer 5

Leaves are adapted to have a large SA:V ratio for efficient gas exchange.
Taking into account trunks, stems and roots they have a relatively low SA:V ratio.
Means they can’t rely on diffusion for supplying their cells with everything they need.

Question 6

What are the transport systems in dicotyledonous plants?

Answer 6

Xylem and phloem that are arranged in vascular bundles.
There may be other types of tissue such as collenchyma and schlerenchyma that give the plant some strength and help support the plant.

Question 7

What is the role of xylem?

Answer 7

To transport water and mineral ions towards the leaf. To support the plant also.

Question 8

What is the role of phloem?

Answer 8

To transport assimilates (amino acids and sugars) up and down the plant.

Question 9

What are the features of the vascular bundle in the roots?

Answer 9

Vessels grouped together in the centre of to help the plant withstand tugging strains from wind.
Xylem tissues in the centre bc it’s the strongest.
Phloem in 4 separate sections.
There are multiple vessels in each section.
Around the vascular bundle is a sheath of cells called endodermis, just inside the endodermis is a layer of meristem cells called the pericycle.

Question 10

What are the features of the vascular bundle in the stem?

Answer 10

Xylem is located on the inside.
Vascular bundle on the edge provides strength and support to the stem as well as flexibility.
Cambium layer contains meristem cells that divide to produce new xylem and phloem.

Question 11

What is the cambium layer in the vascular bundle?

Answer 11

Contains meristem cells that divide to produce new xylem and phloem.

Question 12

What is the schlerenchyma?

Answer 12

Support and structure (around vessels). Thickened with cellulose and lignin.

Question 13

What is the parenchyma?

Answer 13

Packing
General tissue (softer eg leaves / fruit pulp)
Relatively unspecialised, photosythesis and storage.

Question 14

What is the collenchyma?

Answer 14

Found by epidermis and involved in growth.
Flexible support / wind resistance.

Question 15

What are the features of the vascular bundle in the leaf?

Answer 15

Xylem is located on top of the phloem.
The midrib is the main vein carrying the vascular tissue through the organ. Helps the structure and support of the leaf.

Question 16

What do xylem tissues consist of?

Answer 16

Hollow structures made by several columns of dead, lignified cells fusing together end to end.
They also have non-lignified pits to allow lateral movement of water.
Living parenchyma cells act as a packing tissue to separate and support the vessels.
Parenchyma store food and contain tannins that are bitter to prevent herbivores eating them.

Question 17

What is the role of lignin is xylem vessels?

Answer 17

They make the cells waterproof and kill the cells.
End walls and contents of the cell decay, leaving a hollow tube behind.
Lignin strengthens vessel walls.
They can be rings/spirals/solid tubes.
The lignin thickening patterns prevent the vessel from being too rigid and allows for some flexibility.

Question 18

How is the flow of water up the xylem not impeded?

Answer 18

There are no cross walls / cells going across the xylem as these cells have died in the lignification.
No cell contents or nucleus.
Lignin thickening prevents the walls from collapsing.

Question 19

What are the two elements that phloem consists of?

Answer 19

Sieve tube elements
Companion cells

Question 20

What are sieve tube elements in the phloem?

Answer 20

Phloem sieve tubes are made up of many cells joined end to end to form a long structure, although it isn’t completely hollow.
NOT lignified.
End walls are perforated to form sieve plates which let contents through.
Contain no nucleus and a little cytoplasm, leaving space for mass flow of sap to occur.

Question 21

What are companion cells in phloem?

Answer 21

Small cells in-between the sieve tubes, each with a large nucleus and dense cytoplasm.
Have numerous mitochondria to produce the ATP needed for active processes. They carry out the metabolic processes needed to load assimilates into the sieve tubes.
Function as a “life support” system for sieve tubes which have lost most of their normal function.

Question 22

How are companion cells linked to sieve tubes in phloem?

Answer 22

Fine strands of cytoplasm through gaps in the cell wall called plasmodesmata.

Question 23

What can plants deposit in sieve tubes when pathogens are present and what does this do?

Answer 23

They can deposit callose and block the sieve tubes to prevent loss of sap and inhibit transport of pathogens.

Question 24

Why do plants need water? (5)

Answer 24

• Turgor pressure as a result of osmosis in plant cells provides hydrostatic skeleton to support the cell.
• Turgor drives cell expansion.
• Loss of water by evaporation keeps the plant cool.
• Mineral ions and the products of photosynthesis are transported in aqueous solutions.
• Water is a raw material for photosynthesis.

Question 25

What is the exchange surface in plants responsible for uptake of water?

Answer 25

Root hair cells

Question 26

What are the adaptations of root hair cells?

Answer 26

• Microscopic size - easily penetrate between soil.
• Each hair has a large SAV ratio
• Each hair has a thin surface layer
• Concentration of solutes in cytoplasm of root hair cells maintains a water potential gradient between the soil and the roots.

Question 27

Why is water potential lower inside the root hair cell than in the soil?

Answer 27

Soil water usually has a low amount of solutes and so has high water potential.
Cytoplasm and vacuolar sap of root hair cell contain many different solvents including sugars, mineral ions and amino acids, so water potential is lower.

Question 28

How is a water potential gradient made in salty water by root hair cells?

Answer 28

Active transport of the mineral ions into the plant reduce the water potential in the root hair cell and so osmosis can still take place.

Question 29

What are the three (two main) possible pathways that water can move through a plant?

Answer 29

Apoplast and symplast pathways (main)
Vacuolar pathway

Question 30

What is the symplast pathway and how does water move through this pathway?

Answer 30

Moves continuously through the cytoplasm of the plant.
Water moves by osmosis through the cytoplasm of one cell and into the next via the plasmodesmata.
Each cell further away from the roots has a lower water potential so water is drawn away from the soil, towards the xylem.

Question 31

What is the apoplast pathway and how does water move through this pathway?

Answer 31

Cell walls and intracellular spaces.
Water fills the spaces between the loose open network of fibres in the cellulose cell wall.
As water moves into the xylem, more water is pulled through the apoplast due to the cohesive forces.
Tension created that means there is a continuous flow of water through the open structure of the cellulose wall which offers little to no resistance.
This is the fastest movement of water.

Question 32

What pathway does most water take across the root cortex?

Answer 32

Apoplast pathway.

Question 33

What is the vacuolar pathway and how does water move through this pathway?

Answer 33

Insignificant.
Water is not confined to the cytoplasm of cells and is able to enter and pass through the vacuoles as well.

Question 34

What is the endodermis?

Answer 34

The layer of cells surrounding the vascular tissues of the roots.

Question 35

How does water move into the xylem?

Answer 35

Water moves across the root through the apoplast and symplast pathways until it reaches the endodermis.
Here there is the casparian strip making them waterproof.
At this point, water in the apoplast pathway can go no further and is forced into the cytoplasm of the cell, joining the water in the symplast pathway.
The endothermal cells move mineral ions into the xylem by active transport.
Now water potential of xylem is much lower than endothermal cells.
Increases rate of osmosis from endothermal cells through symplast pathway.

Question 36

What are the 3 processes water moves up the xylem?

Answer 36

• Root pressure - active pumping of mineral ions into the xylem to produce the movement of water by osmosis results in root pressure, gives water a push up the xylem, but under most circumstances it is not the major factor of water movement up the xylem.
• Transpiration pull (cohesion tension theory)
• Capillary action (cohesion and adhesion)

Question 37

What is the definition of transpiration?

Answer 37

Loss of water by evaporation from the leaves.

Question 38

What is the transpiration stream?

Answer 38

Constant movement of water from the roots to the leaves. Driven by evaporation from the stomata.

Question 39

Is the transpiration stream active or passive?

Answer 39

Passive. The xylem vessels are made up of non-iving cells.

Question 40

How does transpiration occur in the leaves?

Answer 40

Water molecules move across the leaf mainly via the apoplast pathway by mass flow.
Water evaporates from surface of mesophyll cells into air spaces in the leaf and move out of stomata into the surrounding air by diffusion.
Loss of water lowers water potential of the cell so water moves into the cell from an adjacent cell by osmosis.
This is repeated across the leaf to the xylem and water moves out of the xylem by osmosis into the cells of the leaf.

Question 41

How does the transpiration stream occur in the stem?

Answer 41

Water molecules form H bonds with carbohydrates in the walls of xylem.
They also form H bonds with each other.
Combined effect results in capillary action where the water can rise up the narrow tube against gravity.
Water is drawn up the xylem by mass flow in a continuous stream to replace water lost by evaporation. This is the transpiration pull.
This results in a tension in the xylem which helps to move water across the roots from the soil.
This is known as the cohesion-adhesion theory.

Question 42

Why can water form hydrogen bonds with mineral ions?

Answer 42

Water is polar and ions are charged.

Question 43

Why is transpiration important?

Answer 43

Transports useful mineral ions up the plant.
Maintains cell turgidity.
Supplies water for growth, cell elongation and photosynthesis.
Supplies water that, as it evaporates, can keep the plant cool.

Question 44

How does stomata control the rate of transpiration using turgor?

Answer 44

When turgor is low the asymmetric configuration of the guard cells close the pore.
When environmental conditions are favorable the guard cells pump in solutes by active transport, which reduces water potential causing water to move in by osmosis, increases their turgor.

Question 45

What 5 factors control water loss?

Answer 45

Waxy cuticle.
Wilting - when leaves collapse there is less surface area for evaporation
Stomata on bottom of leaf
Stomata close at night
Deciduous plants lose their leaves in winter.

Question 46

What are the 5 factors that affect transpiration?

Answer 46

Light intensity.
Wind.
Humidity.
Temperature.
Soil water availability.

Question 47

What are xerophytes?

Answer 47

Plants in dry habitats that have adapted to enable them to live and reproduce.

Question 48

What are the adaptations of xerophytes?

Answer 48

Thick waxy cuticle.
More lignin in xylem walls.
Sunken stomata (reduced air movement)
Reduced number of stomata
Reduced leaf surface area
Hairy leaves
Curled leaves
Succulents
Leaf loss
Root adaptations
Avoiding the problems

Question 49

What are hydrophytes?

Answer 49

Plants that live in very watery conditions.

Question 50

What are the adaptations of hydrophytes?

Answer 50

Very thin/no waxy cuticle.
Less lignin in cell walls.
Many, always-open stomata on upper surfaces.
Reduced structure to plant
Wide, flat leaves.
Small roots.
Air sacs.
Aerenchyma
Stomata on upper epidermis
Leaf stem has large air spaces
Many large air spaces in the leaf

Question 51

What is aerenchyma?

Answer 51

Specialised parechyma forms in the leaves and roots which has lots of large air spaces.
Makes the cell more buoyant and forms a low resistance internal pathway.
Helps the plant cope with low oxygen conditions.

Question 52

How do hydrophytes transpire?

Answer 52

In hydrophytes transpiration can stop because water will not evaporate into water or into air with high humidity.
To counteract this many plants contain hydrathodes on tips or margins of leaves. These can release water as droplets which are then more likely to evaporate from the leaf surface.
The process of these drops forming is guttation.

Question 53

Where does translocation occur?

Answer 53

Phloem.

Question 54

What does translocation move?

Answer 54

Moves assimilates throughout the plant.

Question 55

What is a source and what is a sink in translocation?

Answer 55

Part of the plant that loads assimilates into the phloem sieve tubes is the source.
Part of the plant that removes assimilates from phloem sieve tube is a sink.

Question 56

What are examples of sources?

Answer 56

Green leaves - sugars made in photosynthesis converted into sucrose.
Storage organs such as tubers and parts of the roots.
Food stores in seeds.

Question 57

What are examples of sinks?

Answer 57

Roots that are growing and/or actively absorbing mineral ions.
Meristems that are actively dividing
Any part of the plant that is laying down food stores.

Question 58

What is the process of phloem loading?

Answer 58

Sucrose travels from the source through the cell walls and inner-cell spaces to the companion cells and sieve tube elements by diffusion down a concentration gradient, maintained by the loading of the sucrose into the phloem (this is the apoplast route).
In the companion cells sucrose is moved into the cytoplasm across the cell membrane in an active process.
As the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube.

Question 59

How is sucrose actively loaded into companion cells?

Answer 59

Hydrogen ions are actively pumped out of companion cells into surrounding tissue using ATP.
Hydrogen ion return into the companion cell down a concentration gradient via a co-transport protein. Sucrose is the molecule that’s co-transported.

Question 60

Why do companion cells have many infoldings in their cell membranes and have lots of mitochondria?

Answer 60

Increased surface area for active transport of sucrose into cell cytoplasm.
The mitochondria is to supply the ATP needed for transport pumps.

Question 61

What is the movement of assimilates like in terms of hydrostatic pressure gradients?

Answer 61

Moves from a high to a low hydrostatic pressure gradient from source to sink.

Question 62

How and when does phloem unloading take place?

Answer 62

At any place that needs sucrose.
Main mechanism is diffusion.
Sucrose moves rapidly by diffusion into cells or is converted into another substance eg glucose so that the concentration of sucrose is maintained between the contents of the phloem and surrounding cells.