9.1 Xylem transport

Transpiration

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

Process of transpiration

* Light energy converts water in the leaves to vapour, this evaporates from the leaf via the stomata
* New water is absorbed by the roots
* This creates a difference in pressure. Leaves have low pressure and roots have high pressure
* Water will flow via the xylem along the pressure gradient to replace the lost water.

What are stomata

Pores on the underside of the leaf that facilitate gas exchange

Level of photosynthesis and transpiration

* As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis.

Process of evaporation

* Some of the light energy absorbed by the leaves is converted into heat, this evaporates water within the spongy mesophyll.
* Vapour diffuses out of leaf via stomata, creates negative pressure gradient
* negative pressure creates 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

Regulating water loss

* Guard cells are found on the side of the stomata and can close up 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)
* This triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
* This causes stomatal pore to close.

cohesion

When water molecules (or other molecules) attach to each other

* In the xylem this allows the water to be dragged up the xylem

Adhesion

The force of attraction between two particles of different substances (e.g. water molecule and xylem wall)

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Structure of the Xylem

* Tube composed of dead cells that are hollow to allow for the free movement of water
* Because cells are dead, the process is passive
* 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

What can xylems be composed of?

Tracheids and vessel elements

Tracheids

cells that exchange water solely via pits, leading to a slower rate of water transfer

Vessel elements

the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer

Lignin

Reinforce xylem vessels.

How is lignin deposited?

* 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

Two types of roots

* Fibrous highly branching root system → increases surface area
* Main tap root with lateral branches → can penetrate the soil to access deeper reservoirs of water

Root hairs

Cellular extensions to increase surface area for absorption

Absorption of material through roots process

* Materials by root epidermis diffuse across the cortex towards a central stele
* Stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions
* Water and minerals are pumped across this barrier by specialized cells.

How are minerals taken up?

Include minerals such as Mg2+, Na+, K+

* these can passively diffuse into roots
* mostly actively by secondary active transport

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* Root cells contain proton pumps that expel H+ ions
* These ions will displace the positively charged mineral ions
* Negatively charged ions may bond with H+

How is water taken up?

* Through osmosis
* Regulated by specialized water channels on the root cell membrane
* Once inside the root, the water will move towards the xylem via the cytoplasm (symplastic) or via the cell wall (apoplastic).
* In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis

Name for desert plant and plant that grows in high salinity

Desert plants (xerophytes) & high salinity plants (halophytes)

Adaptations of xerophytes

* Reduced leaves → reduces surface area for water loss
* rolled leaves → reduces exposure of stomata and reduces water loss
* Thick, waxy cuticle → thickened cuticle = less water loss
* Stomata in pits → traps water vapour and reduces transpiration
* low growth → less wind and more likely to be shaded = reduced water loss
* CAM physiology → open stomata at night = reduced water loss

Adaptations of halophytes

* Cellular sequestration → can isolate any toxic ions and salts within the cell wall or vacuole
* Tissue partitioning → plants may concentrate salts in particular leaves, which then drop off
* Root level exclusion → plant roots structured to exclude most salt in soil solution
* Salt excretion → certain parts of plant may contain salt gland which actively eliminate salt
* Altered flowering schedule → halophytes may flower at specific times to minimize salt exposure

Ways to model a xylem

Capillary tube → water can flow in opposition to external forces like gravity

Filter paper → will absorb water due to adhesive and cohesive properties.

Porous pots → semi-permeable containers that allow for the free passage of certain small materials through pores.

Photometer

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

Things you can measure with a photometer

Effect of:

* Temperature → predicted to increase transpiration → more evaporation
* Humidity → decrease transpiration → less diffusion
* Light intensity → increase transpiration → stomata will open
* Wind exposure → increase rate of transpiration → will remove water vapour from leaf.