2.2 Adaptations for gas exchange Flashcards
Why do living organisms carry out gas exchange?
All living organisms exchange (uptake and release) gases with the environment.
Animals and plants take up oxygen for aerobic respiration and release carbon dioxide.
In addition, during the day, plants photosynthesis and take up carbon dioxide and release oxygen.
Define aerobic respiration.
Releasing energy from glucose in the presence of oxygen.
What are respiratory surfaces?
This is the name given to the surface across which gases diffuse into and out of an organism.
What is the respiratory surface of a fish?
Gill Plate
What is the respiratory surface of mammals?
Alveoli
What is the respiratory surface of insects?
Tracheole
What is the respiratory surface of plants?
Spongy Mesophyll Cells
List the features of respiratory surfaces.
For rapid, efficient exchange of gases, respiratory surfaces have the following features:
- Large surface area, relative to the volume of the organism so that the rate of gas exchange satisfies the organism’s needs and ensures highest rate possible.
- Thin, so that the diffusion pathway is short.
- Permeable, so that the respiratory gases diffuse easily.
- Have a ventilation mechanism and dense network of capillaries to produce a steep diffusion gradient across the respiratory surface, so that diffusion takes place fast.
- Moist, so that gases can dissolve before diffusion.
- Some organisms have a respiratory pigment, e.g. haemoglobin which has a high affinity for oxygen.
Why must gases dissolve to be transported in the blood stream?
There cannot be bubbles of gas within the bloodstream, hence they are transported in solution.
Give an example of a unicellular organism.
Amoeba
What are unicellular organisms?
Aquatic organisms with a low metabolic rate and low oxygen requirement.
What is one disadvantage, regarding gas exchange that aquatic organisms possess?
Water contains a lower concentration of oxygen than air, so rate of diffusion is much slower in water compared to air.
In other words, the flow rate of water is slower than air.
List the features of the amoeba (unicellular organisms) that enables it to exchange gases effectively.
- Single cells have a large surface area to volume ratio due to their folded plasma membrane. A large surface area to volume ratio provides a short diffusion pathway.
- The cell membrane is thin, so the diffusion path is short, therefore, the diffusion is rapid.
How do single celled organisms carry out gas exchange?
Single cells can absorb enough oxygen across the cell membrane to meet their oxygen demand and remove carbon dioxide fast enough to prevent a build-up that would make conditions too acidic for enzyme action.
Why can’t amoeba be much larger than 100 micrometres in width?
Because their surface area to volume ratio would be insufficient, so the diffusion pathway would be too long and the rate of diffusion would be too slow to meet the oxygen demand.
Why are oxygen requirements greater in multicellular organisms?
In larger organisms, many cells are aggregated together, therefore, their oxygen requirements are greater.
The total oxygen requirement of an organism is proportional to its volume. The rate of absorption of oxygen is proportional to its surface area.
Larger organisms have a smaller surface area to volume ratio than smaller organisms of the same overall shape.
What are flatworms?
Aquatic organisms with a low metabolic rate and modest oxygen requirements.
How are flatworms adapted for gaseous exchange?
It has a thin flattened shape.
This adaptation increases their surface area to volume ratio compared to a spherical organism of the same volume.
No part of the organism is far from the surface, so the diffusion pathways are short.
What are terrestrial animals?
Animals that live on land
Why are earthworms restricted to moist soil?
So that they don’t dry out.
What are earthworms?
Terrestrial animals, restricted to moist soil, which moves slowly and has a low metabolic rate and modest oxygen requirements.
How are earthworms adapted for gaseous exchange?
- Has an elongated cylindrical shape, so surface area to volume ratio is smaller than a flatworm, but larger than a spherical organism of the same volume.
- It’s body surface is folded to give a large surface area to maximise diffusion
- Keeps its respirator surface (i.e. skin) moist for dissolving and diffusion of oxygen by secreting mucus.
- The body surface is thin, giving a short diffusion pathway.
- Closed circulatory system, just under its body surface so diffusion paths are short. They also have a respiratory pigment (haemoglobin). Both of these adaptations ensure that oxygen is carried away from the skin and a diffusion gradient is maintained. Carbon dioxide diffuses in the opposite direction.
Why can’t larger organisms exchange gases via their body surface?
These organisms generally have a higher metabolic rate.
They need more energy and have a higher requirement for oxygen due to the large number of cells.
Their surface area to volume ratio is insufficient, their diffusion pathways are too long and so the rate of diffusion is too slow to meet demand.
What have larger organisms developed to carry out gaseous exchange?
Specialised internal respiratory surfaces.
These are thin, but fragile and so are often enclosed internally to protect from mechanical damage, water loss and heat loss (in warm-blooded animals).
How are insects adapted to prevent water loss?
Water loss is reduced as the exoskeleton is waterproof.
The exoskeleton is made up of chitin, covered with a thin outer waxy layer or cuticle.
Why do insects have high oxygen requirements?
Insect flight requires high expenditure of energy, so oxygen requirements are high.
Why can’t insects exchange gases via their gaseous exchange surface?
They have a relatively small surface area to volume ratio, so even without their impermeable exoskeleton, their surface would not be sufficient for gas exchange.
What gas exchange system do insects have?
They have tracheal systems.
Describe the components of the tracheal system?
Thoracic and abdominal segments have a pair of openings (spiracles) on their sides.
These spiracles open into a system of branched chitin-lined air tubes called tracheae.
The chitin supports the tracheae and prevents them from collapsing under negative pressure.
Spiracles may open or close like valves to allow gas exchange, but help reduce water loss.
Hairs cover some spiracles and reduce water loss by trapping humid air around the spiracular opening and reducing the water potential gradient with the environment. Hairs also prevent entry of solid particles.
Tracheae in turn branch into smaller air tubes called tracheoles. The tracheoles form the gas exchange surface. They are fluid-filled and lie close to muscle fibres. Oxygen dissolves in this fluid and then diffuses directly and rapidly into the respiring muscle cells. Carbon dioxide gas diffuses in the opposite direction.
Is the circulatory system involved in the transport of respiratory gases?
No, the circulatory system is not involved in the transport of respiratory gases. This is an efficient system that allows insect flight.
How do resting insects breathe?
Resting insects rely solely on diffusion of oxygen in from the atmosphere through spiracles, tracheae and tracheoles. Carbon dioxide diffuses in the opposite direction.
How do active insects breathe?
Muscle contraction the the abdomen brings about a bellowing action which ventilates the tracheae. Air is drawn in through the thoracic spiracles and out through abdominal spiracles. The fluid at the ends of the tracheoles also reduces, increasing the surface area for gas exchange.
What are the advantages of the tracheole system?
- Oxygen is directly transported to the muscles, so no circulatory system needed.
- Oxygen supplied faster to the respiring tissues
- No haemoglobin needed
- Reduces water loss
What are the disadvantages of the tracheole system?
The size of the insect is limited.
How is ventilation in insects accomplished?
by muscular movements of the abdomen.
To bring fresh air into the insect, the abdomen expands, lowing the pressure inside. The abdominal spiracles are closed and the spiracles on the thorax are open. The lower pressure in the abdomen pulls the air in through the thoracic spiracles.
To expel carbon dioxide rich air, the abdomen contracts, lowing the volume and increasing the pressure. The thoracic spiracles close and the abdominal spiracles open. The stale air is forced out of the open spiracles.
How is oxygen transported to respiring cells in insects?
Hypertonic fluids surround the tracheole. Fluids diffuse into the tracheole (in a resting insect)
Increased lactic acid makes the surrounding fluid hypertonic. Fluid is withdrawn from tracheoles. Air moves to replace it. (in active insects)
The lower the volume of fluid, the larger the surface to volume ratio, providing a shorter diffusion pathway for the oxygen.
What is the fish’s source of oxygen?
Fish use water as their source of oxygen.
Why is it problematic for water to be a source of oxygen?
Because water contains a much lower concentration of oxygen than air, the rate of diffusion of oxygen in water is much slower than in air and water is a more dense and more viscous fluid so its flow rate is slower and not easily reversed.
Why do fish have a high demand for oxygen?
Fish are very active and so, have a high demand for oxygen
What is the gas exchange organ in fish?
gills
How is a one-way continuous flow of water maintained over the gills?
By a specialised ventilation mechanism, which increases efficiency of gas exchange and maintains steep concentration gradient.
How is the gill supported?
The density of water supports the gills and prevents them from collapsing, which would decrease their surface area.
The gills have a very large surface area provided by many folds over which the water flows. The gaseous exchange surface is the gill plate.
What are the two categories of fish?
- Cartilaginous fish, e.g. sharks
- Bony fish, e.g. mackerel
Is the gas exchange system of cartilaginous fish efficient?
compared to bony fish, gas exchange is inefficient.
Cartilaginous fish have to keep swimming to force water over their gills.
What is parallel flow?
Blood flows through the gill capillaries in the same direction as water moves over the gill plates.
Describe the process of parallel flow.
Oxygen diffuses from a higher concentration in the water to a lower concentration in the blood.
Diffusion of oxygen can only continue until concentrations are equal, so the blood’s oxygen concentration is limited to 50% of its maximum value.
In addition, gas exchange in parallel flow does not occur across the whole gill lamella, only part of it.
Describe the skeleton of cartilaginous fish.
They have a skeleton made of cartilage and 5 pairs of gill pouches on either side of their head that open at gill slits.
Describe the skeleton of bony fish.
They have a skeleton made of bone and their gills are covered with a flap called an operculum.
They have 4 pairs of gills for gas exchange.
Describe the structure of gills in bony fish.
- Each gill is supported by a bony gill arch.
- Along each gill arch is a double row of projections called gill filaments. These are stacked on top of each other like pages of a book.
- Above and below the gill filaments are semi-circular gill lamellae or gill plates.
- These are held apart by water and provide a very large surface area for gaseous exchange.
Why is the gas exchange system of bony fish more efficient than cartilaginous fish?
They have a ventilation mechanism.
Water passes in a unidirectional and continuous flow.
The mouth and operculum are never open at the same time.
Describe the ventilation mechanism of bony fish to take water in.
Mouth opens
Operculum closes
Floor of the buccal cavity lowers
Volume of the buccal cavity increases
Pressure of the buccal cavity decreases below the pressure of the environment
Water is drawn in.
Describe the ventilation mechanism of bony fish to force water out over the gills.
Mouth closes
Operculum opens
Floor of the buccal cavity decreases
Volume of the buccal cavity decreases
Pressure of the buccal cavity increases above the pressure of the environment
Water is pushed out.
What is counter-current flow?
The blood in the capillaries flows in the opposite direction to the water flowing over the gill surface.
Why is counter current flow advantageous?
Blood in the gill capillary is always in contact with water that contains a higher concentration of oxygen.
Therefore, a concentration gradient is maintained along the entire distance of the gill lamella, so, equilibrium is never reached.
Therefore, more oxygen can diffuse from the water into the blood. 80% saturation is achieved compared to 50% in parallel flow.
Describe the process of counter current flow.
Water moves from the mouth cavity over the gills to the opercular cavity.
Blood vessels (afferent arteries) bring deoxygenated blood to the gill lamellae. The blood passes through capillaries in the gill lamellae.
Oxygen diffuses into the blood, attaches to the haemoglobin and is carried away in efferent arteriess.
Carbon dioxide diffuses out of the blood and into the water.
How do amphibians breathe?
Amphibians live in damp environments and require water for reproduction.
Adults have a moist, permeable skin with a well-developed capillary network just beneath for gas exchange.
At rest, gas exchange is through the skin.
When active, internal lungs are used as the demand for oxygen is greater.
Larval stages (tadpoles) have external gills.
How do reptiles breathe?
They have lungs with a more complex internal structure than amphibians, increasing the surface area for gas exchange.
How do birds breathe?
Birds have lungs and a very efficient ventilation mechanism using their ribs and flight muscles.
They do not have a diaphragm.
They have a very high oxygen requirement for flight.
What are the organs of gas exchange in mammals?
The lungs.
They are enclosed in the air-tight thorax at the base of which is a dome-shaped muscle called the diaphragm.
State the function of a plural membrane.
Provides friction free movement
Define ventilation mechanism.
Ensures a continuous flow of fresh oxygenated respiratory medium over a respiratory surface to maintain a concentration gradient and ensure continuous removal of carbon dioxide.
How do mammals ventilate their lungs?
By negative pressure breathing.
This means that the pressure in the lungs must fall below atmospheric for air to be drawn in.
What is inspiration?
An active process requiring energy to contract muscles.
Breathing in
What is expiration?
A mainly passive process.
Breathing out
Describe the process of inspiration.
- External intercoastal muscles contract.
- Internal intercoastal muscles relax
- The ribcage move upwards and outwards.
- As the ribcage moves up and out, it pulls the plural membrane up and out, increasing the volume and decreasing the pressure in the plural cavity so the inner membrane moves up and out and pulls the lung.
- The diaphragm contracts and flattens
- The volume of the thorax increases.
- The pressure in the thorax decreases
- The lungs are below atmospheric pressure and air is drawn in.
Describe the process of expiration.
- External intercoastal muscles relax.
- Internal intercoastal muscles
contract - The ribcage moves downwards and inwards.
- As the ribcage moved down and in, it pulls the outer plural membrane down and in, decreasing the volume, increasing the pressure, so inner membrane moves down and in, pulling the lung.
- The diaphragm relaxes and expands
- The volume of the thorax decreases
- The pressure in the thorax increases
- The lungs are above atmospheric pressure so air is pushed out.
Lung tissue is elastic - how does this help in expiration?
It recoils to return to its original shape, this, therefore decreases the volume and increases pressure to push out the air.
What is surfactant and what is its role in alveoli?
Surfactant acts as a lubricant.
This is what the gases dissolve in before they diffuse across the alveoli.
Surfactant has anti-sticking properties as it has a low surface tension, so, upon expiration, this prevents the walls of the alveoli from sticking together.
What is the alveoli?
The alveoli are the gas exchange surface.
They have many adaptations to ensure exchange is efficient because we are warm blooded, have a high metabolic rate and multicellular.
How is the alveoli adapted for gas exchange?
- Many alveoli to provide a large surface area relative to the volume of the body
- Gases dissolve in surfactant moisture lining the alveoli
- Alveoli walls are a single layer of squamous epitherlium cells, so the diffusion path is short.
- An extensive capillary network covers the alveoli and maintains a high concentration gradient by carrying oxygen away and delivering carbon dioxide.
- The capillary walls are only a single layer of endothelial cells, again providing a short diffusion path
- Haemoglobin in the red blood cells increases the oxygen carrying capacity of the blood as it has a high affinity for oxygen.
- Humans also have ventilation mechanisms to maintain the concentration gradient.
Why is there a C ring of cartilage in the trachea?
The gap formed by the C-shape allows the oesophagus to expand to enable peristalsis.
What is the actual site of gas exchange in plants?
The very large surface area of the spongy and palisade mesophyll cells.
What does the direction of diffusion of gases in plants depend on?
The direction of diffusion of gases depends on the reactions in the plant cells (photosynthesis in chloroplasts and respiration in mitochondria) and the concentration of gases.
Tell me the direction of diffusion of gases during the day in plants.
By day, carbon dioxide diffuses into the leaf through open stomata down its concentration gradient.
From the sub stomatal air chamber, the carbon dioxide diffuses into the intercellular air spaces between the cells in the spongy and palisade mesophyll layers.
The carbon dioxide dissolves in a film of water and then diffuses into the mesophyll cells through their thin cell walls.
Oxygen gas will diffuse in the opposite direction.
What are stomata?
Tiny pores found mainly on the underside of leaves to reduce water loss by transpiration.
Each pore is bound by two specialised guard cells.
What is so unusual about guard cells?
They are the only cells in the epidermis to have chloroplasts.
They also have unevenly thickened cellulose cell walls: The inner wall is thick, and the outer wall is thin.
How does the guard cell open and close stomata?
As cellulose is inflexible, when turgid, the guard cells bend with the outer wall bending more than the inner wall, so opening the stoma.
The opening and closing of stomata mean that the exchange of gases such as carbon dioxide, oxygen and water vapour can be altered.
What is the advantage of being able to open and close stomata?
- Open to allow gas exchange
- Close to reduce transpiration
Tell me about the activity of guard cells by day.
Water enters guard cells by osmosis. They become turgid and because of the uneven thickening of cellulose, they become more curved, and the stoma opens.
Describe the mechanism for stomatal opening.
- Chloroplasts in guard cell photosynthesis producing glucose which is used in respiration to produce ATP.
- This ATP can be used to actively transport K+ into the guard cells from surrounding epidermal cells.
- Insoluble starch stored in the guard cells is converted to soluble malate.
- The K+ and malate ions lower the water potential inside the guard cells, making it more negative. Consequently, water enters by osmosis.
- The guard cells become turgid and curve due to the uneven thickening of cellulose. The wall bordering the pore is less flexible than the outer walls.
What is transpiration?
The process by which plants lose water by evaportaion.
90% of transpiration is via their stomata. If too much water is lost, a plant wilts and may die.
How is a leaf adapted for water loss?
They have waxy cuticles and most dicotyledonous plants have their stomata on the cooler lower surface of the leaf to reduce evaporation.
When is the stomata shut?
- At night, when there is insufficient light for photosynthesis
- In very bright light as this is generally linked to very high temperatures
- if there is excessive water loss
