3. Exchange And Transport Flashcards
Describe what is meant by the surface area of to volume ratio of a cell or an organism
Cell membrane = exchange surface
WHAT DOES THE SA/V RATIO OF SINGLE CELLED ORGANISMS ALLOW THEM TO DO
Although Larger cells have a greater surface area than a smaller cell, a larger cell also has a much greater volume as well
This means that the surface area to volume ratio decreases as the cells get larger
Single celles organisms e.g. amoeba have a relatively large surface area to volume ratio
So they can transfer all the chemicals (exchange of gases) they need across the cell membrane by diffusion
Problem with larger multicellular organisms
However with larger multicellular organisms, their surface area to volume ratio is much smaller
Some small multicellular organisms do rely on diffusion across their body surface
E.g. a flat worm
By evolving a very thin and flat body shape, all of the cells in the flatworm are close to the surface
So diffusion across the body surface is sufficient in a flatworm
However, in larger multicellular organisms, the surface area to volume ratio is simply too small
This means that most multicellular animals have evolved two specialized systems to compensate
Firstly they have a specialized gas exchange surface with a very large surface area
For examples lungs in mammals and gills in fish
Secondly, they have a specialized transport system to carry molecules around their body e.g. blood
Describe the structure and function of the gas exchange system in insects
Insects - have a high oxygen demand - extremely active e.g. during flight
The gas exchange system has evolved to provide oxygen directly to cells.
Although insects do have a specialized transport system, this transfers nutrients not oxygen
Insects are covered with a protective exoskeleton made of the polysaccharide chitin
Gases such as oxygen and carbon dioxide cannot easily pass through the exoskeleton
So on the surface of the exoskeleton are small openings called spiracles
Spiracles allow gases such as oxygen and carbon dioxide to diffuse into the body of the insect
Spiracles lead into a network of tubes called tracheae
Tracheae extend down and along the insect’s body
The walls of the trachea are reinforced with spirals of chitin
This chitin prevents the tracheae from collapsing for example when an insect moves
Extending from the tracheae are very fine tubes called tracheoles
Each tracheole is a single cell that has extended to form a hollow tube
A huge number of tracheoles extend down in between the cells of the insect’s body
Unlike tracheae, tracheoles are not supported by chitin
Because the tracheoles have such a narrow diameter and are extremely close to cells, there is a very short diffusion distance for gases moving between the cells and the tracheoles
This allows oxygen to diffuse rapidly from the air in the tracheoles into the cells
The oxygen is needed for aerobic resp. which produces CO2
The CO2 can also rapidly diffuse back into the air in the tracheoles
The huge number of tracheoles provides a very large surface area for gas exchange
This allows insects to maintain a very rapid rate of aerobic resp. e.g. during flight
The ends of the tracheoles are filled with fluid
This is called tracheal fluid
During intense activity, cells around the tracheoles undergo anaerobic respiration.
Anaerobic respiration produces lactic acid which lowers the water potential of the cells
This causes water in the tracheal fluid to move into the cells
This reduces the volume of tracheal fluid drawing air down into the tracheoles
It means that more tracheole surface is available for the diffusion of oxygen and CO2
In many insects, gas exchange is essentially a passive process
Oxygen diffuses down its conc. gradient from high conc. in the external air , into the tracheoles where the conc. is lower
CO2. diffuses down its conc. gradient from the relatively high conc. in the tracheoles out to the external air
Rate of diffusion decreases with distance
This means that insects tend to be small
The small size of insects reduces the distances required for diffusion to take place
Describe how insects reduce water loss from their gas exchange system
Insects face a significant problem which is loss of water
The walls of the tracheoles are moist and the ends of the tracheoles contain tracheal fluid
This means that water vapour can diffuse out of an insect via the spiracles
However, each spiracle is surrounded by a muscular sphincter
This means that insects can reduce water loss by closing their spiracles like this e.g. when the insects oxygen requirement is relatively low
Describe the structure of the gas exchange system in bony fish
Bony fish are large and active organisms either a very high oxygen requirement
Because of their large size, they have a very low surface area to volume ratio
The scaly surface of bony fish does not allow gases to pass through
Fish get their oxygen from water but the conc. of oxygen in water is much lower than in air.
Therefore they have evolved a specialised gas exchange system which can extract the Maximum amount of oxygen from the water
They have a flap of tissue on either side , slightly behind the head
This flap is called the operculum
Behind the operculum there is the opercular cavity
Inside the opercular cavity, we find the gills
Oxygen-rich water enters the fish through the mouth
The water then passes over the gills
In the gills, oxygen diffuses from the water into the blood and CO2 diffuses from the blood into the water
Finally, the water passes out through the opercular opening
Gills consist of several bony gills arches
Extending from each gill arch are a large number of gill filaments
Gill filament are covered with numerous gill lamellae which are sometimes called gill plates
The gill lamellae are where gas exchange takes place
Water flows between the gill lamellae
Oxygen diffuses from the water into the bloodstream and carbon dioxide diffuses from the bloodstream into the water
Describe the adapatations of the gills in fish
Describe how the gas exchange in fish has evolved for the maximum rate of gas exchange
Gill lamellae are adapted for efficient diffusion of gases
Gill lamellae have a massive surface area for gases to diffuse over
There is a very short diffusion distance through the walls of the lamellae and into the bloodstream.
Gill lamellae have an extensive network of blood capillaries
Once oxygen is diffused into the blood, it is carried away.
This maintains a steep concentration gradient for oxygen.
It his maintains a steep concentration gradient for oxygen
Another way bony fish are adapted to efficient diffusion of gases
Called a counter current exchange system
Blood with low conc. of oxygen passes into the capillaries of the gill lamellae
As blood passes through the gill lamellae, oxygen diffuses from the water into the blood
Oxygen rich blood now passes out of the gill lamellae and leaves they gills
The flow of blood is in the opposite direction to the flow of water
Because the blood and water move in opposite directions this is called a counter current system
This system has one major advantage which is that it always maintains a steep concentration gradient for oxygen
If blood and water flow in the same direction (parallel flow)
Numbers - give idea on relative conc. of oxygen
Initially the water has a much greater oxygen can concentration than the blood
So initially there is a very high rate of diffusion of oxygen from the water into the bloodstream
However after a short difstance, the conc. of O2 is the same in both the blood and the water
This is called equilibrium (point at which diffusion stops)
Means that no more than 50% of the available oxygen in the water can diffuse into the blood
If the blood flows in the opposite direction to the water, (counter -current system), there is always a concentration gradient for oxygen
This means that equilibrium is never reached
And the diffusion of oxygen takes place right across the length of the lamellae
With a counter-current flow, up to around 80% of the oxygen in the water diffuses into the blood stream.
Adaptions of trachea
The walls of the trachea contain cartilage, which is a firm but flexible material
Cartilage prevents the walls of the trachea from collapsing when we inhale
Trachea is very close to the oesophagus
The cartilage in the trachea forms a C-shape rather than forming complete rings
The absence of cartilage in the region near the oesophagus, allows food to pass down the oesophagus easily
Second adaptation of the trachea is that the walls are lined with ciliated epithelia and goblet cells
Goblet cells secrete mucus which traps dust particles and pathogens
Ciliated epithelial cells have a cilia extending from the cell membrane
The beating of the cilia moves the mucus to the throat
The mucus is then swallowed and the dust and pathogens digested by the stomach enzymes
Describe the structure of the mammalian gas exchange system
Mammals are active + maintain a constant body temp (unlike fish) - requires increased rate of aerobic resp. + high oxygen demand
Mammals get their oxygen from the air via their lungs
Two lungs - found in thorax/chest cavity
Ribs protect the lungs
Ribs, diaphragm and intercostal muscles play a role in breathing
When humans breathe through their nose, air passes through the nasal cavity. Hairs in the nasal cavity trap dust particles and pathogens
The nasal cavity also warms and moistens the air before it enters the lungs
The air then makes its way down a wide tube called the trachea
The trachea divides into two bronchi
Each bronchus carries air into one of the lungs
Bronchi also contain cartilage, ciliated epithelia and goblet cells
Each bronchus splits forming progressively narrower airways called bronchioles
Walls of larger bronchioles are supported by cartilage.They also contain smooth muscle
When the smooth muscle relaxes, the bronchioles widen allowing more air to pass into deeper parts of the lungs
Deep in the lungs, the bronchioles are extremely narrow
These bronchioles now lead into air sacs called alveoli
Alveoli are the sites of gas exchange (where gases diffuse in and out of the blood). Hundereds of millions/ Lot of alveoli in the lungs
Describe how alveoli are adapted for efficient gas exchange
The internal walls of the alveoli are covered with a thin layer of moisture
The alveoli are covered with extensive blood capillaries
O2 in the air of the alveoli dissolves in the moisture on the inside of the alveolar wall
The oxygen then diffuses into the red blood cells, where it combines with haemoglobin
CO2 diffuses from the blood into the alveolar airspace
Between the alveoli are elastic fibres which stretch and recoil during breathing
There are hundreds of millions of alveoli
These provide a massive surface area for the diffusion of gases
Both the wall of the alveolus and the wall of the capillary are only one cell thick
This means that there is a very short diffusion distance between the air in the alveoli and the red blood cells in the capillary
The narrow diameter of the capillary means that the red blood cells are close to the capillary wall
This minimises the diffusion distance
The extensive capillary network means that once oxygen diffuses into the blood, it is rapidly carried away from the alveoli
This ensures that there is always a steep conc. gradient for oxygen
CO2 also has a steep conc. gradient as more is continually brought to the alveoli in the bloodstream
These concentration gradients are also maintained by breathing which brings fresh air (o2) into the alveoli
This ensures that there is always a high conc. of oxygen in the alveolar air as well as a low conc. of co2.
Again, this helps to ensure a rapid rate of diffusion of these gases
Importance of ventilation
It is important that the rate of diffusion of these gases is as high as possible
One way this is achieved is by breathing - also called ventilation
Ventilation brings fresh air from outside the body into the alveoli
This increases the conc. of oxygen in the alveolar airspaces while decreasing the conc. of carbon dioxide
The effect of this is to increase the conc. gradients of these gases and this increases the rate of diffusion
Describe the mechanism of ventilation in the human lungs
Ventilation involves the action of two sets of muscles
These are the intercostal muscles which lie between the ribs and the diaphragm (which separates the thorax/chest cavity from the abdomen)
These two sets of muscles work together to change the volume of the thorax
By changing the volume of the thorax, this changes the pressure of the air in the lungs
The effect of this is to draw air into the lungs (ie inhalation) or to expel air from the lungs (ie exhalation)
There are actually two sets of intercostal muscles
Called the external and internal intercostal muscles
Focus on external intercostal muscles - involved in normal regular breathing
Internal intercostal muscles - involved in stronger breathing
Describe what happens during inhalation (when we breathe in)
During inhalation, the external intercostal muscles contract (shorten)
This pulls the ribs upwards and outwards
At the same time, the diaphragm also contracts which causes it to flatten
The effect of these is to increase the volume of the thorax and the lungs
This reduces the air pressure in the lungs
Because the air pressure in the lungs is now less than the atmospheric pressure, air is drawn into the lungs
Air moves into the alveoli and the elastic fibres between the alveoli stretch
Because inhalation involves muscle contraction, inhalation is an active process
- Energy is required for inhalation
Describe what happens during exhalation
During regular breathing, exhalation is essentially a passive process as the muscles relax
Because of this, exhalation does not require a great deal of energy
During exhalation, the external intercostal muscles relax and return to their original length
The diaphragm also relaxes, returning to its usual domed shape
The effect of this is to reduce the volume of the thorax and the lungs
Now the air pressure in the lungs is greater than atmospheric pressure and air is pushed out of the lungs
The elastic fibres between the alveoli also recoil, helping to push air out
This is called elastic recoil
As you can see, the volume of the lungs changes during inhalation and exhalation
What is pleural membrane and pleural fluid and its importance
The lungs are surrounded by pleural membranes
Between these membranes there is pleural fluid which acts as a lubricant as the lung volume changes
Explain how the internal and external intercostal muscles are antagonistic
During regular breathing, exhalation is mainly a passive process
However, this is not the case when we exhale strongly for example during intense exercise
In this case, the internal intercostal muscles come into play
When we exhale strongly, the internal intercostal muscles contract
This pulls the ribs down and inwards forcing air out of the lungs
At the same time, the external intercostal muscles relax
Because the internal intercostal muscles contract while the external intercostal muscles relax - they are described as antagonistic
Explain why multicellular plants need a specialised transport system
Single celled algae (takes in co2 and h2o and carries out photosynthesis)
Because it has a very large surface area to volume ratio, it can rely on diffusion for the transport of molecules
Multicellular plants are large, so have a low surface are to volume ratio
So for these reasons, multicellular plants cannot rely on diffusion only for the transport of molecules
Describe what is meany by dicotyledonous herbaceous plants
Two germinating seeds
Seeds contain an embryonic leaf called a cotyledon
When a seed germinated, the cotyledon unfurls allowing the seedling to carry out photosynthesis
Some plants only have one cotyledon e.g. grasses
These are called monocotyledonous plants
Other plants have two cotyledons (seeds contain an embryonic leaf) e.g. trees
These are called dicotyledonous plants
The transport systems in these two type of plants are arranged differently
Trees and shrubs are examples of woody dicotyledonous plants
These plants are long lived and have a woody stem
In contrast geraniums are e.g. of herbaceous dicotyledonous plants
Includes large number of different plants
These plants are often fast growing and can be short-lived - herbaceous
Dont have a woody stem
Describe the location and structure of the vascular bundles in the roots, stem and leaves of plants
The xylem vessels and the phloem vessels are grouped together in vascular bundles
The arrangement of the vascular bundles is different in the roots, the stem and the leaves
Root
Root hair cells grow from a layer of external tissue called the epidermis
In the centre of the root, there is the vascular bundle
In vascular bundle, xylem vessels are in the centre and the phloem vessels around the xylem
Stem
Vascular bundles are arranged in a ring around the edge of the stem
Within the vascular bundles, the phloem vessels are located around the edge of the stem
The xylem vessels are found closer to the centre
Because the vascular bundles are around the edge of the stem, this helps the stem to withstand bending due to the wind
Leaf
Main vascular bundle in the centre
As well as transport, this also provides support to the leaf
The leaf is also supported by smaller vascular bundles connected to the main one
In the leaf, the xylem is at the upper part of the vascular bundle
The phloem is at the lower part
Describe how plants are adapted to living in conditions where water is scarce - XEROPHYTES
Cacti
Marram grass
In cacti - leaves have been replaced with fibrous spines with photosynthesis taking place in the stem of the cactus.
In these cacti, the stomata are found on the surface of the stem
Replacing the leaves with spines reduces the SA/V ratio of the cactus reducing water loss
Spines also trap moist air near the cactus, reducing the rate of transpiration as well as providing some shade for the stem from the heat of the sun
Thick waxy cuticle - reduce evaporation of water
Stomata are sunken into pits - traps a layer of moist air around the stomata reducing water loss by transpiration
Cacti only open their stomata at night to absorb co2 when conditions are relatively cool
By only opening cacti at night - they reduce water loss during the heat of daytime
