Objectives – What you will need to know from this section
Explain how CO2 is a controlling factor in stomata & breathing system
H 3.4.7 Carbon Dioxide – A controlling factor in Gaseous Exchange
LUNGS
We breathe automatically by involuntary reflex action.
It is controlled by medulla oblongata of the brain.
The rate of breathing is continually adjusted to meet the body’sneeds (an example of homeostasis).
If we are exercising,
a rise in the CO2 levels in the blood results in the medulla sending impulses to the diaphragm and intercostal muscles,
causing an increased rate of contraction and deeper inspiration.
Receptors (nerves) in the medulla are sensitive to changes in the CO2 concentration of the blood.
LEARNING CHECK
• What aprt of the brain control breathing rate?
• What stimulus in the body causes our rate to increase?
• What advantage does a higher rate of breathing give us when we exercise?
PLANTS
Transpiration is the evaporation of water from leaves [and stems].
The loss of water is reduced by a waxy cuticle and by theopening and closing of pores in the leaves called stomata.
If a plant loses more water in transpiration than it canabsorb from the soil, it becomes dehydrated and wilts.
CO2 -- a controlling factor in stomata opening/closing .
LEAF TS
Each stoma is a gap between two specialised cells, called guard cells.
The guard cells have a thicker wall on the sides that face each other.
When water enters the guard cell, it expands into a curved shape and a gap (the stoma) opens up between the two cells.
Whether a stoma opens or closes depends on how much CO2 is in the two guard cells.
LEARNING CHECK
• What is transpiration?
• Whay does transpiration happen?
• How do plants try an reduce transpiration?
• What is a guard cell?
• What is a stoma?
• What happens guard cells when water enters them?
CO2 starts to get used up in photosynthesis and so there is a drop in CO2 levels in the guard cells.
This causes potassium ions to enter them
By osmosis, water enters them from neighbouring cells.
The extra water causes the stoma to open.
Photosynthesis starts when the sun rises in the morning.
At night, the sun goes down and photosynthesis stops
Respiration continues as always
This raises the CO2 levels in the guard cells.
This causes potassium ions to leave
By osmosis, water is drawn out of the guard cells,
The loss of water causes each stoma to close.
How do gases [oxygen and carbon dioxide] get through the waterproof, corky surface of bark?
They do so through lenticels, which are areas where the packing of bark cells is loosened up a bit.
Lenticels often look like tiny raised blisters on a branch or twig.
What about trees and woody plants ?
Winter twig – showing lenticels
LEARNING CHECK
• What is photosynthesis?
• When and where does it take place?
• What is the trigger for guard cells to take up water in the morning?
• Why do stomata need to be open during daylight hours?
• What happen at night to the guard cells?
• What happens aplant cell if it loses too much water by osmosis?
