3.3.2 Gas Exchange Flashcards
=Gas exchange in single-celled organisms
Small so have large S.A:vol. ratio so diffusion of oxygen and carbon dioxide can occur across the body surface
A cell wall makes no difference
Gas exchange in insects
Have an internal network of tubes called tracheae, which are supported by strengthening rings to prevent them from collapsing.
The tracheae divide into smaller dead-end tubes called tracheoles
The role of tracheoles
Extend throughout the insect’s body tissues -> so that atmospheric air (which contains oxygen) can be brought directly to the respiring tissues as there is a short diffusion pathway from a tracheole to any body cell
Ways in which respiratory gases move in and out of the tracheal system
- Along a diffusion gradient
- Mass Transport
- The ends of the tracheoles are filled with water
Respiratory gases transport: Along a diffusion gradient
When cells are respiring, they use up oxygen so the concentration of oxygen at the end of the tracheoles decreases -> creates a diffusion gradient -> causes gaseous oxygen to diffuse from atmosphere along the trachea and tracheoles to the cells
opposite happens with carbon dioxide
Which is faster: diffusion in air or diffusion in water?
Diffusion in air
Respiratory gases transport: Mass Transport
- The contraction of the insect’s muscles can aid the mass movement of air in and out -> speeds up exchange of respiratory gases
- abdominal pumping
Respiratory gases transport: The ends of the tracheoles are filled with water
- When anaerobic respiration occurs during intense excercise, lactate is produced
- Lactate is soluble so lowers the water potential of the muscle cells
- So water from the tracheoles moves into the muscle cells
- So the water at the end of the tracheoles decreases in volume so draws air further into them
- This means that the final diffusion stage is in gas phase = quicker
Disadvantage of anaeorobic respiration in insects
Leads to greater water evaporation
Spiracles
- Tiny pores through which gases enter and leave the tracheae
- Can open or close by a valve
When spiracles are open…
Water vapour can evaporate from the insect
Why are spiracles mostly closed?
To prevent water loss, but they periodically open for gas exchange
Limitations of the tracheal system
- Relies on diffusion for gas exchange between the environment and the cells
- Insects have a limited size (they are small so that diffusion distance is short)
Gas exchange in fish
- Have gills which are made up of gill filaments.
- Gill filaments are stacked
- Gill filaments have gill lamellae at right angles to them = increased S.A.
- Water is taken in from the mouth and forced over the gills, and out through an opening on either side of the body
Countercurrent flow
The flow of water over the gill lamellae and the flow of blood within them are in opposite directions
Reasons for countercurrent flow
- When blood that has high oxygen concentration meets water -> oxygen diffuses into the blood
- When blood that has low oxygen concentration meets water -> oxygen diffuses into the blood
How much oxygen is absorbed from the water into the blood in countercurrent flow?
80%
Parallel flow
The flow of water over the gill lamellae and the flow of blood within them are in the same direction
Diffusion of oxygen in parallel flow
- A diffusion gradient for oxygen uptake is maintained across only half the width of the gill lamellae
Diffusion of oxygen in countercurrent flow
- A diffusion gradient for oxygen uptake is maintained across the whole width of the gill lamellae
How much oxygen is absorbed from the water into the blood in parallel flow?
50%
Main difference between gas exchange in plants and animals
Plants carry out photosynthesis and the products of photosynthesis and respiration can be used by one another
So reduces gas exchange with the external air.
The volumes and types of gases which are being exchanged depends on the rates of photosynthesis and respiration
Gas exchange when photosynthesis is taking place
Photosynthesis: Although some carbon dioxide comes from respiration, most of it is obtained from external air. So most carbon dioxide from respiration diffuses out of the plant
Respiration: Although some oxygen comes from photosynthesis, most of it is obtained from external air. So most oxygen from photosynthesis diffuses out of the plant
Similarities of gas exchange in plants and insects
- No living cell is far from external air (source of oxygen and carbon dioxide)
- Diffusion takes place in the gas phase (air)
Adaptations of leaf structure for gas exchange
- Many stomata (small pores)
- Many interconnecting air-spaces throughout the mesophyll
- Large surface area of mesophyll cells
Adaptations of leaf: Many stomata (small pores)
No cell is far from a stoma and therefore there is a short diffusion distance
Adaptations of leaf: Numerous interconnecting air-spaces throughout the mesophyll
So that gases can readily come in contact with mesophyll cells
Stomata
Small pores that occur mainly on the underside of the leaf.
Each stoma is surrounded by a pair of guard cells which can open and close the stomatal pore (to control the rate of gaseous exchange)
Why is it important to control the rate of gaseous exchange in plants?
To control water loss
Terrestrial
Live on land
Adaptations in insects to reduce water loss
- Small S.A.:vol. ratio
- Waterproof coverings
- Spiracles
Adaptations in insects to reduce water loss: Small S.A.:vol. ratio
Minimises the air over which water is lost
Adaptations in insects to reduce water loss: Waterproof coverings
Over their body surfaces
In insects: rigid outer skeleton made of chitin, which is covered with a waterproof cuticle
Adaptations in insects to reduce water loss: Spiracles
Can be closed to reduce water loss
Why can’t plants have a small S.A.:vol. ratio to reduce water loss?
They need a large S.A. for photosynthesis
Xerophytes
Plants that are adapted to living in areas where water is in short supply
Adaptations in xerophytes to reduce water loss
- A thick cuticle
- Rolling up of leaves
- Hairy leaves
- Stomata in pits or grooves
- Reduced S.A.:vol. ratio
Adaptations in xerophytes to reduce water loss: Thick cuticle
- The thicker the cuticle, the less water can escape
- e.g. holly
Adaptations in xerophytes to reduce water loss: rolling up of leaves
- Traps a region of still air within the leaf which becomes saturated with water vapour (so has high water potential)
- So there is no water potential gradient between the inside and outside of the leaf so no water loss
- e.g. maram grass
Adaptations in xerophytes to reduce water loss: Hairy leaves
- Especially on the lower epidermis
same as rolling up of leaves - e.g. heather plant
Adaptations in xerophytes to reduce water loss: Stomata in pits or grooves
same as rolling up of leaves
* e.g. pine trees
Adaptations in xerophytes to reduce water loss: Reduced S.A.:vol. ratio
by having small and circular leaves
Other plants that show xerophytic adaptations
- Plants in sand dunes - water drains away quickly
- Plants in salt marshes
- Plants in coastal regions - exposed to high wind speeds which increases transpiration rates
- Plants in cold regions
Why is the volume of oxygen in and carbon dioxide out, large, in humans?
- They are relatively large organisms (large volume of living cells)
- They maintain a high body temperature due to their high metabolic and respiratory rates
Why are lungs located inside the body?
- Air is not dense enough to support + protect these delicate structures
- The body otherwise would lose a lot of water and dry out
- Alveoli are thin, so can easily be damaged
Structure of lungs
- Pair of lobed structures made up of bronchioles, which end in alveoli (tiny sacs)
Structure of trachea
- Flexible airway that is supported by rings of cartilage (prevents the trachea from collapsing as the air pressure inside falls when breathing in)
- Muscle tracheal walls, lined with ciliated epithelium + goblet cells
- Produce mucus
- Have cilia
Structure of bronchi
- 2 divisions of the trachea (similar structure to trachea), each leading to one lung
- Larger bronchi have cartilage rings
- Produce mucus
- Have cilia
Purpose of mucus
To trap dirt particles
Purpose of cilia
To move dirt-laden mucus towards the throat
Structure of bronchioles
- Series of branching subdivisions of the bronchi
- Muscle walls lined with epithelial cells
Why do bronchioles have muscle walls?
Allows them to constrict so that they can control the airflow in and out of the alveoli
Structure of alveoli
- MInute air-sacs
- Have collagen and elastic fibres between them
- Lined with epithelium
- Surrounded by pulmonary capillaries
What do the elastic fibres between the alveoli do?
Allow the alveoli to stretch as they fill with air when breathing in, and spring back during breathing out to remove carbon dioxide rich air
What is the gas-exchange surface in the lungs?
Alveolar membrane
Purpose of ventilation
To maintain the diffusion of gases across the alveolar epithelium
Inspiration (inhalation)
When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs
Active process (requires energy)
Expiration (exhalation)
When the air pressure inside the lungs is greater than the air pressure of the atmosphere, air is forced out of the lungs
Largely passive process (doesn’t require much energy)
The movement of what 3 sets of muscles can change the pressure within the lungs?
- Diaphragm
- Internal intercostal muscles
- External intercostal muscles
Structure of intercostal muscles
Lie between the ribs
Structure of diaphragm
A sheet of muscle that separates the thorax from the abdomen
The two sets of intercostal muscles
- Internal intercostal muscles - contractions lead to expiration
- External intercostal muscles - contractions lead to inspiration
Process of inspiration
- The external intercostal muscles contract, and the internal intercostal muscles relax
- The ribs are pulled upwards + outwards -> increasing the volume of the thorax
- The diaphragm muscles contract, causing it to flatten -> further increases the volume of the thorax
- This increased volume of the thorax -> reduction of pressure in the lungs
- Atmospheric pressure is now greater than the pulmonary pressure -> air is forced into the lungs
Process of expiration
- The internal intercostal muscles contract, and the external intercostal muscles relax
- The ribs move downwards + inwards -> decreasing the volume of the thorax
- The diaphragm muscles relax, causing it to be pushed up again by the contents of the abdomen which were compressed in inspiration -> further decreases the volume of the thorax
- This decreased volume of the thorax -> increased pressure of the lungs
- Pulmonary pressure is greater than the atmospheric pressure -> air is forced out of the lungs
Are the muscles used during quiet breathing?
No, only the recoil of elastic tissue is used
Structure of the pulmonary capillaries surrounding the alveoli
Narrow - so red blood cells are flattened against the capillary walls to squeeze through
Thin - one cell thick
Adaptations of alveoli for rapid gas exchange
*
Correlation
Occurs when a change in 1 of 2 variables is reflected by a change in the other variable
- Does not mean that the change of one variable is the cause of change in the other variable -> needs to be proved with experimental evidence
Risk factors for lung disease
- Smoking
- Air pollution
- Genetic make-up
- Infections
- Occupation
Pulmonary Ventilation
The total volume of air that is moved into the lungs during 1 minute
Tidal volume
Volume of air normally taken in at each breath when the body is at rest
Breathing (ventilation) rate
The number of breaths taken in 1 minute
Units for pulmonary ventilation rate
dm3 min-1
Equation for pulmonary ventilation rate
Pulmonary ventilation rate = tidal volume x breathing rate
Lung residual volume
Volume of air left in the lungs after as much air as possible has been removed (in strong exhalation) -> prevents lungs from collapsing
Forced Expiratory Volume
The maximum amount of air that can be exhaled in one second
