Plant Biology: Topic 9.1: Xylem Transport

Question 1

Define transpiration

Answer 1

Transpiration is the loss of water vapour from the stems and leaves of plants

Question 2

What is the cause of transpiration?

Answer 2

Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)

As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis
Hence, transpiration is an inevitable consequence of gas exchange in the leaf

Question 3

What is transpiration stream

Answer 3

The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream

Question 4

Water rises through xylem vessels due to two key properties of water – cohesion and adhesion. Outline how adhesion helps

Answer 4

Adhesion:

Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
The xylem wall is also polar and hence can form intermolecular associations with water molecules
As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension

Question 5

Describe the specialised structures of xylem along with their functions which facilitate the movement of water throughout the plant

Answer 5

It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water
Because the cells are dead (no organelles or nuclei), the movement of water is an entirely passive process and occurs in one direction only
The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension

This means xylem vessels are extremely tough and can withstand very low internal pressures, i.e. negative pressure caused by suction, without collapsing in on themselves

Question 6

Xylems can be composed two main cell types;

Answer 6

Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer
In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer

Question 7

All xylem vessels are reinforced by lignin, which may be deposited in different ways:

Answer 7

In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
In spiral vessels, the lignin is present in the form of a helix or coil

Question 8

Describe the transpiration stream (cohesion tension theory)

Answer 8

Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
This negative pressure creates a tension force in leaf cell walls which draws water from the xylem vessel to replace the water lost by transpiration (transpiration pull)
The water column is pulled upwards under tension via capillary action through the xylem vessels due to the adhesive attraction between water and the leaf cell walls
Cohesion between water molecules allows the tension force to be transmitted through the whole water column and pull it upwards as a continuous stream and not disintegrating under the force of tension (suction or pull) and low pressure.

Question 9

What is the mechanism of stomata opening and closing

Answer 9

The stomata open and close because of changes in the turgor pressure of the guard cells that surround them. Thus when the cells take in water and swell, they bulge more to the outside. This opens the stoma. When the guard cells lose water, they sag towards each other and close the stoma.

Question 10

Explain the process behind opening stomata

Answer 10

The gain and loss of water in the guard cells is largely because of the transport of potassium ions. Light from the blue part of the light spectrum triggers the activity of (ATP)-powered proton pumps in the plasma membrane of guard cells. This triggers the active transport of potassium into the cell. The higher solute concentration within the guard cells causes inward water movement by osmosis.

Question 11

Explain the process behind closing stomata

Answer 11

When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor) (as water exits from the cell via osmosis)
A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening

Question 12

How plants take up minerals and water and what is the required condition for this

Answer 12

Plants take up water and mineral ions from the soil via their roots and thus need a maximal surface area to optimise this uptake

Question 13

Outline two types of root systems

Answer 13

Some plants have a fibrous, highly branching root system which increases the surface area available for absorption
Other plants have a main tap root with lateral branches, which can penetrate the soil to access deeper reservoirs of water

Question 14

How is the epidermis of root specialised for its function

Answer 14

The epidermis of roots may have cellular extensions called root hairs, which further increases the surface area for absorption

Question 15

Describe thee structure of root (common passage of mineral and water)

Answer 15

Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located
The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions (Casparian strip)
Water and minerals are pumped across this barrier by specialised cells, allowing the rate of uptake to be controlled

Question 16

Outline the process of mineral uptake (minerals to be absorbed and the process)

Answer 16

Fertile soil typically contains negatively charged clay particles to which positively charged mineral ions (cations) may attach

Minerals that need to be taken up from the soil include Mg2+ (for chlorophyll), nitrates (for amino acids), Na+, K+ and PO43–

Mineral ions may passively diffuse into the roots, but will more commonly be actively uploaded by indirect active transport

Root cells contain proton pumps that actively expel H+ ions (stored in the vacuole of root cells) into the surrounding soil
The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient
Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton

Question 17

Outline Water uptake by root cells

Answer 17

Water will follow the mineral ions into the root via osmosis – moving towards the region with a higher solute concentration

The rate of water uptake will be regulated by specialised water channels (aquaporins) on the root cell membrane

Once inside the root, water will move towards the xylem either via the cytoplasm (symplastic) or via the cell wall (apoplastic)

In the symplastic pathway, water moves continuously through the cytoplasm of cells (connected via plasmodesmata)
In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis

Question 18

What is the role of fungus in the uptake of mineral ions?

Answer 18

Clay particles in the soil tend to be negatively charged particles to which mostly positive mineral ions (cations) bind, causing slow movement of mineral ions across the soil and roots.

To overcome this problem a mutualistic relationship exists between fungus and plants.

The thread-like structures of fungus called hyphae cover the roots of the plant, further increasing surface area for absorption, while also extending deeper into the soil and absorbing mineral ions from the clay particles. The fungus supplies these absorbed mineral ions to the roots. Most but not all plants provide sugars and other nutrients to the fungus to support its growth. This is thus a mutualistic relationship

Question 19

What are xerophytes?

Answer 19

Xerophytes are plants that have adapted to live in deserts and other dry habitats.

Question 20

Why do xerophytes need to conserve water?

Answer 20

Xerophytes will have high rates of transpiration due to high temperatures and low humidity in deserts and dry conditions. It thus needs to conserve water.

Question 21

What are the Life cycle adaptations in xerophytes?

Answer 21

Some plants are ephemeral with a very short-life cycle which takes place for a brief period when water is available after rainfall. They then become dormant as embryos inside seeds and wait until the next rainfall when conditions are ideal for growth which may take years

Perennial plants bloom during the wet seasons and rely on the storage of water in specialised leaves, stems and roots

Question 22

What are the Physical adaptations in xerophytes?

Answer 22

Reduced leaves– reducing the total number and size of leaves which consists of only spines will reduce the surface area available for water loss

Rolled leaves– rolling up leaves reduces the exposure of stomata to the air and hence reduces evaporative water loss

Reduced stomata- A reduced number of stomata decreases the number of openings through which water loss may occur.

Stomata in pits– having stomata in crypts or pits in the leaf surface makes it less likely for them to open, and surrounded by hairs, traps water vapour, maintaining a higher humidity near the stomata and slows or stops the movement of air, hence reduces transpiration

Thick, waxy cuticle– having leaves and stems covered by a thickened cuticle acts as an impenetrable barrier to water, preventing water loss from the leaf surface

Water storage in stems- stems have water storage facilities and swell up by the intake of water after rainfalls. They have pleats which enables the rapid expansion and contraction of volume of the stem

Low growth– low growing plants are less exposed to wind and more likely to be shaded, reducing water loss

Question 23

What are the Metabolic adaptations in xerophytes?

Answer 23

Xerophytes have CAM physiology in which stomata open at night rather than in the day as temperatures are cooler and transpiration rates become slower. They absorb carbon dioxide in the night and store it as a four carbon compound called malic acid. Carbon dioxide is then released from this stored form to do photosynthesis even if stomata are closed. The rate of transpiration is not affected by the rate of photosynthesis with this kind of physiology

Question 24

What are halophytes?

Answer 24

Halophytes are plants adapted to grow in water with high levels of salinity and in saline soils.

Question 25

Why do halophytes need to conserve water?

Answer 25

Halophytes will lose water as the high salt concentration in the surrounding soils will draw water from plant tissue via osmosis

Question 26

What are the adaptations for water conservation in halophytes?

Answer 26

Cellular sequestration– halophytes can sequester toxic ions and salts within the cell wall or vacuoles

Salt excretion– certain parts of the plant (e.g. stem) may contain salt glands which actively eliminate salt

Reduced leaves– reducing the total number and size of leaves which consists of only spines will reduce the surface area available for water loss

Sunken stomata- on leaf surfaces reduces water loss by creating a higher humidity
near the stomata.

Altered flowering schedule– halophytes may flower at specific times(e.g. rainy seasons) to minimise salt exposure