chapter 9 p1

Question 1

Plant Transport:

Answer 1

Plants have transport systems to move substances between leaves, stems, and roots. These transport systems work at tremendous pressures.
The pressure in the phloem, one of the main transport tissues of a plant, is around 2000 kPa.
For comparison, the systolic blood in your main arteries is at a pressure of around 16 kPa, while the steam turbines in a power station work at around 4000 kPa.
The higher pressures in plants, however, are confined to much smaller spaces than in arteries and turbines.

Question 2

three main reasons why multicellular plants need transport systems:

Answer 2

• Metabolic demands:
  Size:
  Surface area: volume ratio (SA:V)

Question 3

• Metabolic demands:

Answer 3

The cells of the green parts of the plant make their own glucose and oxygen by photosynthesis - but many internal and underground parts of the plant do not photosynthesise.
They need oxygen and glucose transported to them and the waste products of cell metabolism removed.
Hormones made in one part of a plant need transporting to the areas where they have an effect.
Mineral ions absorbed by the roots need to be transported to all cells to make the proteins required for enzymes and the structure of the cell.

Question 4

• Size:

Answer 4

Some plants are very small but because plants continue to grow throughout their lives, many perennial plants (plants that live a long time and reproduce year after year) are large and some of them are enormous.
The tallest trees in the world include the coastal redwood (Sequoia sempervirens) and giant redwood (Sequoiadendron giganteum) in the USA (up to around 115 m tall) and the mountain ash (Eucalyptus regnans) in Australia (up to around 114m tall).
This means plants need very effective transport systems to move substances both up and down from the tip of the roots to the topmost leaves and stems.

Question 5

• Surface area: volume ratio (SA:V):

Answer 5

Surface area: volume ratios are not simple in plants.
Leaves are adapted to have a relatively large SA: V ratio for the exchange of gases with the air.
However, the size and complexity of multicellular plants means that when the stems, trunks, and roots are taken into account they still have a relatively small SA: V ratio.
This means they cannot rely on diffusion alone to supply their cells with everything they need.

Question 6

Dicotyledonous plants

Answer 6

(dicots) make seeds that contain two cotyledons, organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates.
There are herbaceous dicots, with soft tissues and a relatively short life cycle (leaves and stems that die down at the end of the growing season to the soil level), and woody (arborescent) dicots, which have hard, lignified tissues and a long life cycle (in some cases hundreds of years).

Question 7

Vascular System in Herbaceous Dicots:

Answer 7

Dicotyledonous plants have a series of transport vessels running through the stem, roots, and leaves - This is known as the vascular system.
In herbaceous dicots this is made up of two main types of transport vessels, the xylem and the phloem.
These transport tissues are arranged together in vascular bundles in the leaves, stems, and roots of herbaceous dicots.
The pattern of the vascular tissue is easily recognised and is shown in transections (TS) in Table 1.

Question 8

Answer 8

Question 9

Answer 9

Question 10

Answer 10

Question 11

Observing xylem vessels in living plant stems:

Answer 11

Xylem vessels can be seen clearly stained in transverse and longitudinal sections of plant stems and roots on prepared slides.
However, it is also possible to observe these vessels in living tissue.
If plant material, for example celery stalks with plenty of leaves, flowers such as a gerberas, or the roots of germinating seeds, are put in water containing a strongly coloured dye for at least 24 hours, you can remove the plant from the dye, rinse it and look for the xylem vessels which should have been stained by the dye.

Question 12

Procedure: -Observing xylem vessels in living plant stems

Answer 12

In one specimen, make clean transverse cuts across the stem with a sharp blade on a white tile. Take great care with the blade.
Observe and draw the position of the xylem vessels which should show up as coloured spots.
In another specimen make a careful longitudinal cut through a region where you expect there to be xylem vessels.
Observe and draw the xylem vessels which may show up as coloured lines.

Question 13

functions of the xylem:

Answer 13

The xylem is a largely non-living tissue that has two main functions in a plant - the transport of water and mineral ions, and support.
The flow of materials in the xylem is up from the roots to the shoots and leaves.
Xylem is made up of several types of cells, most of which are dead when they are functioning in the plant.

Question 14

Xylem Vessels

Answer 14

• They are long. hollow structures made by several columns of cells fusing together end to end.
• There are two other tissues associated with xylem in herbaceous dicots.
• Thick-walled xylem parenchyma packs around the xylem vessels, storing food, and containing tannin deposits.
• Tannin is a bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores.
• Xylem fibres are long cells with lignified secondary walls that provide extra mechanical strength but do not transport water.
• Lignin can be laid down in the walls of the xylem vessels in several different ways.
• It can form rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits.
• This is where water leaves the xylem and moves into other cells of the plant.

Question 15

The Phloem Structure and Function:

Answer 15

Phloem is a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis.
The phloem supplies the cells with the sugars and amino acids needed for cellular respiration and for the synthesis of all other useful molecules.
The flow of materials in the phloem can go both up and down the plant.
The main transporting vessels of the phloem are the sieve tube elements.

Question 16

Sieve Tube Elements

Answer 16

• Like xylem, the phloem sieve tubes are made up of many cells joined end to end to form a long, hollow structure.
• Unlike xylem tissue, the phloem tubes are not lignified.
• In the areas between the cells, the walls become perforated to form sieve plates, which look like sieves and let the phloem contents flow through.
• As the large pores appear in these cell walls, the tonoplast (vacuole membrane), the nucleus and some of the other organelles break down.
• The phloem becomes a tube filled with phloem sap and the mature phloem cells have no nucleus.

Question 17

Companion Cells

Answer 17

These cells are linked to the sieve tube elements by many plasmodesmata - microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells.
They maintain their nucleus and all their organelles.
The companion cells are very active cells and it is thought that they function as a ‘life support system’ for the sieve tube cells, which have lost most of their normal cell functions.
Phloem tissue also contains supporting tissues including fibres and sclereids, cells with extremely thick cell walls.

Question 18

Turgor pressure (or hydrostatic pressure)

Answer 18

as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves.
So, for example, the turgor pressure in leaf cells is around 1.5 MPa - that is 11251 mm Hg (unit of pressure still commonly used in medicine) (human systolic blood pressure is around 120mm Hg).
Turgor also drives cell expansion - it is the force that enables plant roots to force their way through tarmac and concrete.

Question 19

Movement of water into the root:

Answer 19

Root hair cells are the exchange surface in plants where water is taken into the body of the plant from the soil.
A root hair is a long, thin extension from a root hair cell, a specialised epidermal cell found near the growing root tip.

Question 20

How are Root hairs are well adapted as exchange surfaces:

Answer 20

• Their microscopic size means they can penetrate easily between soil particles.
• Each microscopic hair has a large SA:V ratio and there are thousands on each growing root tip.
• Each hair has a thin surface layer (just the cell wall and cell-surface membrane) through which diffusion and osmosis can take place quickly.
• The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell.
• Soil water has a very low concentration of dissolved minerals so it has a very high water potential.
• The cytoplasm and vacuolar sap of the root hair cell (and the other root cells) contain many different solvents including sugars, mineral ions, and amino acids so the water potential in the cell is lower.
• as a result water moves into the root hair cells by osmosis.

Question 21

Movement of water across the root:

Answer 21

Once the water has moved into the root hair cell it continues to move across the root to the xylem in one of two different pathways:
The symplast pathway
The apoplast pathway

Question 22

The symplast pathway

Answer 22

Water moves through the symplast - the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata - by osmosis.
The root hair cell has a higher water potential than the next cell along.
This is the result of water diffusing in from the soil, which has made the cytoplasm more dilute.
So water moves from the root hair cell into the next door cell by osmosis.
This process continues from cell to cell across the root until the xylem is reached.
As water leaves the root hair cell by osmosis, the water potential of the cytoplasm falls again, and this maintains a steep water potential gradient to ensure that as much water as possible continues to move into the cell from the soil.

Question 23

The apoplast pathway

Answer 23

This is the movement of water through the apoplast - the cell walls and the intercellular spaces.
Water fills the spaces between the loose, open network of fibres in the cellulose cell wall.
As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules.
The pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance.

Question 24

Answer 24

Question 25

Movement of water into the xylem: p1
Water Movement in Roots:

Answer 25

Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis - the layer of cells surrounding the vascular tissue (xylem and phloem) of the roots (Figure 4).
The endodermis is particularly noticeable in the roots because of the effect of the Casparian strip.
The Casparian strip is a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer (Figure 3).
At this point, water in the apoplast pathway can go no further and it is forced into the cytoplasm of the cell, joining the water in the symplast pathway.

Question 26

Movement of water into the xylem: p2 Selective Uptake and Water Potential:

Answer 26

This diversion to the cytoplasm is significant as to get to get there, water must pass through the selectively permeable cell surface membranes, this excludes any potentially-toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them.
Once forced into the cytoplasm the water joins the symplast pathway.
The solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem.

Question 27

Movement of water into the xylem: p3
Root Pressure and Xylem Movement:

Answer 27

In addition, it appears that the endodermal cells move mineral ions into the xylem by active transport.
As a result the water potential of the xylem cells is much lower than the water potential of the endodermal cells.
This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway.
Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem itself and move up the plant.
The active pumping of minerals into the xylem to produce movement of water by osmosis results in root pressure and it is independent of any effects of transpiration.
Root pressure gives water a push up the xylem, but under most circumstances it is not the major factor in the movement of water up from the roots to the leaves.

Question 28

Evidence for the role of active transport in moving water from the root endodermis into the xylem:
p1

Answer 28

Some poisons, such as cyanide, affect the mitochondria and prevent the production of ATP.
If cyanide is applied to root cells so there is no energy supply, the root pressure disappears.

Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved.

Question 29

Evidence for the role of active transport in moving water from the root endodermis into the xylem:
p2

Answer 29

If levels of oxygen or respiratory substrates fall, root pressure falls.

Xylem sap may exude from the cut end of stems at certain limes.
In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions - for example overnight, when transpiration is low. This is known as guttation.