9.1 Transport in the Xylem

Question 1

Leaf functions

Answer 1

photosynthesis
transpiration (water loss via stomata during the day)
guttation (water loss at night from leaf edges)
storage of water

Question 2

Key features of the leaf

Answer 2

Palisade mesophyll (layer of elongates cells containing chloroplasts) is the site of photosynthesis and hence is located on the upper surface of the leaf (facing sunlight)
Spongy mesophyll (loosely packed parenchyma) is the main site of gas exchange and is hence located on the lower surface of the leaf (near stomata)
Stomata are on the underside of the leaf (prevents obstruction so as to maintain an open channel for gas exchange)
The top surface of the leaf is covered by a thick, waxy cuticle (prevents water absorption which would affect transpiration)
Vascular bundles (including xylem and phloem) are located centrally to allow for optimal access by all leaf cells
Petiole connects the blade with the stem

Question 3

Stem functions

Answer 3

connect leaves, roots and flowers
transport water and minerals (via xylem)
transport food (via phloem)
provide support

Question 4

Key features of the stem

Answer 4

The epidermis covers the outer surface and functions to waterproof, protect the stem and control gas exchange
The ground tissue (cortex and pith) is found internally and assist in the transport and storage of materials within the stem
The cambium is a centrally located, circular layer of undifferentiated cells responsible for lateral growth of the stem
Vascular bundles are arranged in bundles near the outer edge of the stem to resist compression and bending
The xylem is located to the interior side of the bundle and the phloem is on the exterior side (phloem = outside)

Question 5

Structure of the xylem

Answer 5

The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant

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, 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 (these can be spirals or annular (rings))

Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)

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 6

What is transpiration?

Answer 6

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

Question 7

Stages of transpiration

Answer 7

Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata
New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)

Question 8

What causes transpiration stream?

Answer 8

the water is:
- absorbed by root hairs via osmosis
- moving from cells to xylem by osmosis
- drawn up xylem by pressure from below and suction due to transpiration from above
- cohesion and adhesion means that water flows up xylem tubes
- water evaporated and lost through stomata (transpiration pull)

Question 9

Cohesion (plants)

Answer 9

Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream

Question 10

Adhesion (plants)

Answer 10

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 11

Root uptake

Answer 11

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

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

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

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 12

Mineral uptake via roots

Answer 12

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 13

Water uptake via roots

Answer 13

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 14

Outline of evaporation in plants

Answer 14

Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata

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 (transpiration pull)
The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls

Question 15

Stomata

Answer 15

are pores on the underside of the leaf (high humidity) which facilitate gas exchange

Question 16

Regulating water loss

Answer 16

The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata

Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
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)
A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening

Transpiration rates will be higher when stomatal pores are open than when they are closed

Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind

Question 17

How does temperature affect rate of transpiration?

Answer 17

Increasing the ambient temperature is predicted to cause an increase in the rate of transpiration
Higher temperatures lead to an increase in the rate of water vaporisation within the mesophyll, leading to more evaporation
The effect of temperature variation can be tested experimentally by using heaters or submerging in heated water baths

Question 18

How does humidity affect rate of transpiration?

Answer 18

Increasing the humidity is predicted to cause a decrease in the rate of transpiration
Humidity is the amount of water vapour in the air – less vapour will diffuse from the leaf if there is more vapour in the air
The effect of humidity can be tested experimentally by encasing the plant in a plastic bag with variable levels of vapour

Question 19

How does light intensity affect rate of transpiration?

Answer 19

Increasing the light intensity to which a plant is exposed is predicted to cause an increase in the rate of transpiration
Increasing light exposure will cause more stomata to open in order to facilitate photosynthetic gas exchange
The effect of light intensity can be tested experimentally by placing the plant at variable distances from a lamp

Question 20

How does wind exposure affect rate of transpiration?

Answer 20

Increasing the level of wind exposure is predicted to cause an increase in the rate of transpiration
Wind / air circulation will function to remove water vapour from near the leaf, effectively reducing proximal humidity
The effect of wind can be tested experimentally by using fans to circulate the air around a plant

Question 21

Use of a potometer

Answer 21

A potometer is a device that is used to estimate transpiration rates by measuring the rate of water loss / uptake

When a plant is affixed to the potometer, transpiration can be indirectly identified by the movement of water towards the plant
This water movement can be assessed as a change in meniscus level or by the movement of an air bubble towards the plant
The initial starting position of the meniscus or air bubble can be adjusted by introducing additional water from a reservoir

When measuring transpiration rates with a potometer, it is important to remember that not all water is lost to transpiration

A small amount of water (~2%) is used in photosynthesis and to maintain the viable turgidity of plant cells

Question 22

Root functions

Answer 22

anchor plant to the ground
absorb and transport water and minerals
store food
transport stored food

Question 23

Root structure

Answer 23

covered in an epidermis with a thin cuticle so to not restrict water entry
branching with root hairs to increase surface area
much contains a cortex of parenchyma cells with air spaces for the aeration of root tissue
xylem and phloem cells are distributed in a ring around the root

Question 24

What are xerophytes?

Answer 24

Xerophytes are plants that can tolerate dry conditions (such as deserts) due to the presence of a number of adaptations
Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments

Question 25

Adaptations of xerophytes

Answer 25

Reduced leaves – reducing the total number and size of leaves 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
Thick, waxy cuticle – having leaves covered by a thickened cuticle prevents water loss from the leaf surface
Stomata in pits – having stomata in pits, surrounded by hairs, traps water vapour and hence reduces transpiration
Low growth – low growing plants are less exposed to wind and more likely to be shaded, reducing water loss
CAM physiology – plants with CAM physiology open their stomata at night, reducing water loss via evaporation

Question 26

What are halophytes?

Answer 26

Halophytes are plants that can tolerate salty conditions (such as marshlands) due to the presence of a number of adaptations
Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis

Question 27

Adaptations of halophytes

Answer 27

Cellular sequestration – halophytes can sequester toxic ions and salts within the cell wall or vacuoles
Tissue partitioning – plants may concentrate salts in particular leaves, which then drop off (abscission)
Root level exclusion – plant roots may be structured to exclude ~95% of the salt in soil solutions
Salt excretion – certain parts of the plant (e.g. stem) may contain salt glands which actively eliminate salt
Altered flowering schedule – halophytes may flower at specific times (e.g. rainy seasons) to minimise salt exposure

Question 28

Capillary tubing as a model of water transport

Answer 28

Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)
This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface
The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm)

Question 29

Filter paper as a model of water transport

Answer 29

Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
When placed perpendicular to a water source, the water will hence rise up along the length of the paper
This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)

Question 30

Porous pots as a model of water transport

Answer 30

Porous pots are semi-permeable containers that allow for the free passage of certain small materials through pores
The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves of plants
If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative pressure that draws more liquid