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 features of dicotyledonous plants seeds?

Answer 6

They make seeds that contain 2 cotyledons (organs that act as food stores for the developing embryo plant and for the first leaves when the seed germinates.

Question 7

What are the transport systems in dicotyledonous plants?

Answer 7

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 8

What is the role of xylem?

Answer 8

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

Question 9

What is the role of phloem?

Answer 9

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

Question 10

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

Answer 10

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 11

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

Answer 11

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 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

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.
Linked to sieve tubes by fine strands of cytoplasm, through gaps in the wall called plasmodesmata.
Also allow communication and flow of substances between cells.
Parenchyma (packing) cells between the phloem.