Ventilation in Plants: Mechanisms of Gaseous Exchange

Ventilation in Plants: Learning Objectives

• By the end of this session, you should be able to:
• Understand and evaluate why plants require ventilatory systems
• Evaluate the mechanisms for regulation and control of plant
• ventilatory systems
• Explain what compensation points are in plants and why they are
• important

Why Plants Need a Ventilatory System

Ventilatory systems
They are too thick for diffusion

Purpose of Plant Ventilatory Systems

Plants utilise their ventilatory system to move Carbon Dioxide into the
leaf material and Oxygen out
: This is partly due to photosynthesis, with CO2 is fixed to produce
sugars, and oxygen is released as a waste product.

Absorbed via
Ventilatory system
Absorbed via
Root system
Released via
Ventilatory system
But also respiration
Plants require oxygen for respiration oxygen

Gaseous Exchange in Plants

What drives gaseous exchange in plants
?What are the challenges ?Compare the two!

Compensation Points in Plants

The point at which the rate of photosynthesis equals the rate of photosynthesis,
and there is no net loss or gain of Co2 and O2

1. SUN PLANT.

CO2
SHADE PLANT
LIGHT INTENSITY
CO2
OUT.

Main Movement of Gas

During the day net movement of gas into the plant will be?
And at night ?
This doesn't mean that there is no movement of the other gases at this time, it is just when
the majority of movement is greater for one

Meeting the Plant's Needs

How is most of the plant's need for oxygen met during the day ?
LIGHT
Energy
Chloroplast
PHOTOSYNTHESIS
CO: + H20
VS
02 + Sugars
CELLULAR RESPIRATION
Mitochondria
ATP
Energy

Structure of the Leaf

Leaf Anatomy

LEAF ANATOMY
...
...
Mesophyll cells
Stomata
Palisade cells
L
Spongy
Mesophyll
cells

Guard Cells

What is the unusual feature of guard
cells not possessed by lower
epidermal cells ?

Absorbing Carbon Dioxide

CO2 is absorbed by simple gaseous diffusion through open
stomata into the air-pockets of the Spongy Mesophyll.
Water and O2 are removed from the spongy mesophyll
through the open stomata.
By which process is water lost through stomata?
O2
H2O
CO2

The Use of CO2

:: CO2 is fundamental for photosynthesis
Therefore, plant growth rates are limited by the overall concentration of CO2 in
the atmosphere and how fast it can be absorbed into the plant.
Average atmospheric CO2 is around 0.04% of environmental air, meaning that
~1900 litres of air is required to produce 1g of glucose (if photosynthesis is
100% efficient)

CO2 Absorption Mechanisms

Atmospheric concentrations of CO2 are ~0.04% of air, compared to the
~0.02% inside the spongy mesophyll. This gradient is maintained
through the rapid absorption of CO2 into the spongy mesophyll cells.
These cells are positioned to have the largest possible surface
area to increase absorption rates

• CO2 enters the mesophyll cells either:

Via direct diffusion of CO2 across the membrane and then rapid
transformation into HCO3- using carbonic anhydrase
Conversion of CO2 into HCO3 outside of the mesophyll cells, and then
actively transported across the membrane utilising the efflux of other
negatively
charged ions
HCO3- is then converted back into CO2 by carbonic anhydrase near RubisCO,
where it is added to RuBP and converted into 2 3-phosphoglyates for the Calvin
cycle

Overview of CO2 Absorption

CO2 enters
the Calvin
cycle due to
RubisCo
activity
HCO3 is converted
back to CO2 Via a
Carbonic
Anhydrase
Atmospheric CO2
enters through stomata
into spongy mesophyll
CO2 is converted into
HCO3 to aid absorption
and to prevent release
Calvin
Cycle

Regulating CO2 Absorption

Since CO2 is the fundamental source of carbon within photosynthesis, it
would make sense for plants to maximise the amount of CO2 absorbed
by the plant.
: Maximising CO2 could be achieved by forcing the stomata to remain
open constantly to facilitate maximal gaseous exchange
WHY IS THIS NOT DONE BY MOST PLANT SPECIES?

Stomata and Water Loss

Approximately 100 times less CO2 enters the stomata
than that of water vapour leaving.
If the stomata are constantly open, then water would
constantly leave the leaf, resulting in dehydration of the
plant.
:: Therefore, gaseous exchange in plants must be
regulated by the water potential within the guard cells.

Stomata Opening and Shutting Summary

Stomata are regulated by the influx/efflux of
water out of the vacuoles of the Guard Cells
Influx of water into the guard cells results in
the cells become turgid, forcing the guard
cells apart and allowing gaseous exchange
through the pore
Efflux of water out of the guard cells results
in the deflation of the cells, forcing them
together and preventing gaseous exchange
DAY/High Water Conditions
Night/Low Water Conditions
00
H2O
CO2

Stomata Opening Regulation

Stomatal opening is regulated by the absorption
of blue light via chloroplasts within the guard cells
This stimulates an H+/ATPase proton pump, which
alters the membrane potential across the guard
cell membrane
Alterations in the charge across the membrane
encourages the import of K+ and sugars.
This lowers the water potential within the cell
causing water to move in via osmosis through
aquaporins.
Blue Light
H+
K+
Glucose
Water

Stomata Closure at Night

: At night, blue light stops being absorbed by chloroplasts,
which in turns switches off the
H+/ATPase proton pump.
H+ ions are free to diffuse back through into the cell,
altering the charge across the membrane
This forces K+ back into the intercellular space, which
alters the osmotic pressure, forcing water out of the
vacuole

• This reduces the turgidity of the guard cells, causing
  them to relax and close the stomata.
  Blue Light
  H+
  K+
  Water

Stomata Closure During Drought

In low water conditions, ABA is produced and binds
to receptors, resulting in the opening of Ca2+
channels.
This activates CI- ion active-transport channels and
opening of K+ ion channel.
The flow of ions out of the cells, causes water to flow
out of the cell.
This reduces the overall turgidity of the guard cells,
allowing them to relax and close.
ABA
ABA
Receptor
Ca2+
Cl-
K+
H2O

Deserts and Stomata Closure

In deserts, limiting water loss is even more critical .
: Utilising 'normal' stomatal regulation risks rapid dehydration
through transpiration
But only opening stomata at night will prevent effective and
efficient growth and development
So how do plants fix this issue?

CAM Photosynthesis CO2 Storage

Crassulacean Acid Metabolism (CAM) is an adaptations to arid conditions, that
allows for gaseous exchange to occur at night.

1. CA
   HCO3
   PEPC
   PEP
   Sucrose
   CO2
   +
   Oxaloacetate
   H2O
   MDH
   Malate
   Malic acid
   CO2 is absorbed into mesophyll cells at
   night
   CO2 + H2O is converted into HCO3 via
   Carbonic anhydrase
   HCO3 is phosphorylated by PEPC with
   PEP into Oxaloacetate
   Oxaloacetate is converted into Malate by
   MDH
   Malate is transported into the vacuole
   and converted into malic acid

Learning Outcomes

You should now be able to:
Understand and evaluate why plants require ventilatory
systems
Evaluate the mechanisms for regulation and control of plant
ventilatory systems
Explain what compensation points are in plants and why they
are important