2.5 Adaptations for transport in plants

Question 1

Draw the transverse section of a dicotyledon root.

Answer 1

Question 2

Where is the xylem in the roots?
Why?

Answer 2

In the roots, the xylem is central and star-shaped with phloem between the xylem tissue.

This arrangement anchors the plant in the soil and resists pulling forces.

Question 3

Draw the transverse section of a dicotyledon primary stem.

Answer 3

Question 4

How are the vascular bundles arranged in the stem?

Answer 4

In the stem, vascular bundles are in a ring at the periphery, with xylem towards the centre and phloem on the outside.

This arrangement gives strength with flexibility and resists bending forces

Question 5

Draw the transverse section of a dicotyledon leaf.

Answer 5

Question 6

How ae the vascular bundles arranged in a leaf?

Answer 6

The vascular tissue is in the midrib and in a network of veins giving strength with flexibility and resistance to tearing.

Question 7

List the main cell types in the xylem tissue.

Answer 7

1. Vessels
2. Tracheids
3. Fibres
4. Parenchyma

Question 8

What are xylem vessels?

Answer 8

The main water transporting tissue

Question 9

Where are xylem vessels found?

Answer 9

Only in flowering plants (angiosperms)

Question 10

Describe the structure of xylem vessels

Answer 10

Lignin is deposited in cellulose cell walls.

It is waterproof, so prevents entry of water and so the cell contents die.

During development, the end walls of the xylem vessels break down, leaving a hollow tube up which water can move.

Lignin is laid out in a variety of patterns – spirals, rings, a complete covering with gaps called pits

Question 11

State the properties of lignin.

Answer 11

The lignin gives the xylem its properties of impermeability to water and its mechanical strength

Question 12

What colour do the vessels stain in microscope slides?

Answer 12

red

Question 13

What do tracheids do?

Answer 13

Also transport water, but are less important and less efficient than xylem vessels

Question 14

Where are tracheids found?

Answer 14

In lower plants such as ferns and conifers as well as flowering plants.

Question 15

Describe the structure of tracheids.

Answer 15

They are spindle shaped, and their cell walls also contain lignin.

End walls are not broken down, instead they have pits through which water travels in a twisting path which is less efficient than the direct path in xylem vessels.

Question 16

What are fibres?

Answer 16

Dead, lignifies spindle-shaped cells

Question 17

State the function of fibres.

Answer 17

Provide support to the xylem.

Question 18

What are parenchyma?

Answer 18

Packing cells (living)

Question 19

State the functions of the xylem.

Answer 19

Transport of water and dissolved minerals

Mechanical strength and support for the plant

Question 20

Why must water be constantly taken up?

Answer 20

Water must be constantly taken up by the roots of plants to replace water lost by transpiration.

Question 21

How does the root facilitate water uptake?

Answer 21

Just behind the root tip, in the root hair zone, the surface area is vast, and the epithelial cells have thin cell walls to facilitate uptake.

Question 22

How does water move from the soil into the root hairs?

Answer 22

By osmosis.

From a higher water potential in the soil (weak solution of ions) to a lower (negative) water potential in the cytoplasm and vacuole of the root hair cells

Question 23

How does the water move across the root cortex from cell to cell?

Answer 23

Water then moves across the cortex of the root down the water potential gradient from cell to cell taking 3 routes.

1. apoplast.
2. symplast.
3. vacuolar.

Question 24

What is the apoplast route?

Answer 24

Water moves through the gaps between the cellulose fibres in the cell wall

Question 25

What is the symplast route?

Answer 25

Water moves through the cytoplasm and plasmodesmata

Question 26

What is the vacuolar route?

Answer 26

Water moves through vacuoles, across the tonoplast through the cytoplasm, across the cell membrane and through plasmodesmata

Question 27

What is the longest route taken by the water?

Answer 27

the vacuolar route. less than 5% of water goes through this route. 70% goes via apoplast as it is the shortest

Question 28

Why is the apoplast pathway the most important?

Answer 28

The apoplast pathway offers least resistance to flow of water and so it the most important. The vacuolar route has the greatest resistance to flow.

Question 29

Where is water potential the greatest at, at this stage?

Answer 29

In the root hair cells

Question 30

Where is water potential the lowest at, at this stage?

Answer 30

in the xylem at the centre of the root.

Question 31

How does water enter the xylem tissue

Answer 31

There is a water potential gradient across the cortex of the root. Water potential is greatest in the root hair cells and lowest in the xylem at the centre of the root. As a result, water crosses the root cortex by osmosis down the water potential gradient. When water reaches the endodermis, the apoplast rout is blocked as the cell's walls are impregnated with a waxy material called suberin. This forms a discrete band on the radial and transverse walls called the Casparian strip. This waterproof layer forces water to move into the cytoplasm and flow in the symplastic and vacuolar routes. From the endodermis, water flows into the single layer of cells below called the pericycle. Finally, water moves into the xylem tissue

Question 32

How is a steep water potential gradient maintained between the endodermis and xylem vessels?

Answer 32

1. Water potential of the endodermal cells is raised by water being driven into them by the action of the Casparian strip. 2. Water potential of the xylem is lowered by active transport of mineral ions, mainly Na+ from the endodermis and pericycle into them

Question 33

What is rootpressure?

Answer 33

Water entering the xylem generate an upward push called the root pressure

Question 34

How are mineral salts taken in by the roots?

Answer 34

Concentrations of most mineral ions are lower in soil than in root cells, so most mineral ions enter by active transport.

Question 35

In which pathway do mineral ions move along?

Answer 35

Along the apoplast pathway in solution. On reaching endodermis, Casparian strip blocks pathway, so they move into cytoplasm by active transport. This allows selective uptake of mineral ions. They then diffuse or are actively transported into the xylem.

Question 36

How is nitrogen transported to the xylem vessels?

Answer 36

Taken up as nitrates Diffuse along apoplast pathway, are actively transported into symplast pathway at endodermis

Question 37

What are the 3 main mechanisms for movement of water up the stem?

Answer 37

1. Cohesion-tension 2. Capillarity 3. Root pressure

Question 38

How does water move up the stem by cohesion-tension mechanism?

Answer 38

Water evaporated from mesophyll cells into the air spaces in the leaf and the water vapour diffuses out through the stomata into the atmosphere. This draws water across the apoplast, symplast and vacuolar routes of the leaf cells from the xylem. As water molecules leave the xylem, they pull on other water molecules behind them in the xylem. Water molecules all move because they show cohesion (as they are polar molecules). This continuous pull produces tension in the water column.

Question 39

Howe does water move ip the xylem by capillarity?

Answer 39

This is the movement of water up narrow tubes like xylem vessels due to adhesive forces between water molecules and the vessel walls. In the thin xylem vesseld, the polar water molecule is also attracted to the hydrophillic inner lining of the lignified xylem vessels. It operates up to a metre in height. It is important in small plants such as mosses but only makes a slight contribution in taller plants

Question 40

How does water move up the xylem through root pressure>

Answer 40

This operates over short distances in living plants and is a consequence of the osmotic movement of water across the root and into the xylem vessels. It pushes water already in the xylem vessels further up.

Question 41

What is transpiration?

Answer 41

The loss of water as watervapour from the leaves and shoots of plants

Question 42

WHy is transpiration important?

Answer 42

Plants need to balance water uptake via the roots with water loss via transpiration.

Question 43

WHat happens if water loss id greater?

Answer 43

The plant wilts The plant may be able to recover when watered, but if the deficit is a large volume of water, the plant will not regain its turgor and may die

Question 44

What affects the rate of transpiration?

Answer 44

The Opening and closing of the stomata

Question 45

What is transpiration rate?

Answer 45

This is the rate at which water vapour is lost from a plant via transpiration.

Question 46

What does transpiration rate depend on?

Answer 46

1. Genetic factors , e.g. number, size, distribution of stomata 2. Environmental conditions. Any factor that increases the water potential gradient between the water vapour in the leaf and the surrounding atmosphere increases the rate of transpiration.

Question 47

How does temperature affect the rate of transpiration?

Answer 47

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 48

How does humidity affect the rate of transpiration?

Answer 48

The greater the humidity, the lower thee transpiration Inside the leaf, the air is saturated with water vapour but outside the leaf the amount of water in the atmosphere varies. The greater the water potential gradient between inside the leaf and the atmosphere, the higher the rate of transpiration provided the stomata is open.

Question 49

How does air movements affect the rate of transpiration?

Answer 49

The greater the air movements, the greater the rate of transpiration. Transpiration in still air allows water vapour to accumulate around the leaf surface in saturation shells. This decreases the water potential gradient between inside the leaf and the surrounding air, therefore, decreases rate of transpiration. However, on windy days, the layer of saturated shells are blown away and the water potential gradient between the inside of the leaf and the atmosphere increases so rate of transpiration increases

Question 50

How does light intensity affect the rate of transpiration?

Answer 50

As light intensity increases, rate of transpiration increases Light intensity controls the degree of opening of stomata, so during the day, as light intensity increases, the stomata opens wider and more water vapour leaves the leaf. At night, stomata closes, because it is dark, so level of water loss is lower.

Question 51

What do we use to measure the rate of transpiration?

Answer 51

A potometer

Question 52

What does the potometer measure?

Answer 52

The uptake of water by a leafy shoot. But, as 99% of this water is lost by transpiration in a turgid plant, the value is taken as the rate of transpiration.

Question 53

How do we set up the potometer?

Answer 53

1. Submerge potometer in deep sink and fill with water. 2. Recut leafy shoot underwater to remove any airlocks in xylem vessels 3. Insert leafy shoot in bung 4. Lift potometer out of sink 5. Dry any leaves that are wet. Seal around the base of leafy shoot in bung to prevent water leakage/air entry 6. Allow leafy shoot to adjust to environment for 5 min.

Question 54

jWhat is the transpiration stream?

Answer 54

The continuous column of water from the roots to the leaves in a plant.