Osmoregulation in plants

Cohesion force

\-between water molecules

Adhesion forces

attract water molecules to xylem vessels, walls in stem

Transpiration

maintain conc gradient, pulling water from leaves

Stomata

\-tiny openings found on the underside of leaves

\-each surrounded by guard cells

\-open for water loss and gaseous exchange (CO2 & O2)

Guard cells

open or close the stomata depending on the environmental conditions = control over water loss

High light, windy, hot, low humidity conditions

Stomata open by:

\-potassium ions actively pumped into guard cells, reducing water content inside guard cell

\-water enter due to osmosis = guard cell become turgid and stomata opens

\-vacuole pushes against cell wall = high turgor pressure

\-promote loss of water through stomata (transpiration rate increases) & cools the leaves

To close stomata

plant pump potassium ions out of cell = increasing the water content inside the guard cells

\-result = water diffuse out of guard cell = guard cell flaccid

stomata pore closes = reduces cooling effect from leaves

Transpiration

evaporative loss of water from plants through stomata

\-occurs because conc gradient water vapour between inside and outside

\-Factors that ↑ rate of transpiration:

* light
* humidity
* wind
* temperature

Xerophytes

plants adapted to survive in arid (little or no rain) conditions

Halophytes

plants tolerant of salt

Adaptation of xerophytes to minimise water loss

\-reduction in leaf SA:

* thin, spine/needle-like leaves, long and narrow
* ↓ exposure to drying effects of wind
* ↓ evaporation and water loss from leaves

\-reduced number of stomata

* ↓ water loss from leaves

\-sunken stomata

* prevent water loss by ↑ relative humidity near stomata :. ↓ conc gradient & ↓ evaporation and diffusion
* Creates a micro-climate around each stomata (humid around each stomata)

\-Deep roots

* Reach water sources underground e.g. water table
* ↑ water uptake = prevent dehydration of plant

\-Thick waxy leaf cuticle

* impermeable to water
* Shiny and reflect light = ↓ amt of light absorbed
* reduction in stimuli that open stomata = ↓ water loss

\-Stomata opening at night (reverse stomatal rhythm)

* assist in ↓ water loss
* stomata open at night = ↓ water loss as cooler
* CO2 taken up at night stored for use later in day for photosynthesis

\-Storage of water in succulent rissues

* water stored in fleshy stems of leaves for use during dry periods = ↓ water loss during hot, dry period

Halophytes classification

\-salt accumulators = accumulate and store excess salt in salt glands or in central vacuoles

\-salt excluders = exclude salt by ultrafiltration thru cell membranes and endodermis

\-combination of salt accumulators & excluders (mangroves)

(Halophtyes) Adaptations of salt excluders to minimise water loss

\-filtration at the roots = regulate amt of salt entering the plant

* root cells impermeable to salt :. prevent salt from entering the plant

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\-secretion of salt by special salt glands on leaves, stems and roots

* ↓ amt of salt in plant

e.g. mangroves, quinoa

\-succulence (water storage structures in leaves and other parts of plant)

* dilutes salt cont of the cells = give water for long supply

\-pneumatophores (aerial rots)

* oxygen diffuses into aerial roots which grow upwards out of the water or mud to reach air as muddy poor soil

(Halophytes) Adaptations of salt accumulator to minimise water loss

\-vacuoles in root cells store salt

* ↑ salt conc of roots greater than in soil = conc gradient between roots and soil

\:. water move into roots = storage of salt in roots stop salt from interfering with cell function

Importance of transpiration

\-supplies the photosynthetic process with the water it needs.

\-cools the leaves as heat energy is drawn out of the plant into water to the external environment

\-distributing mineral salts throughout the plant.