Gas Exchange

• Gas exchange surfaces:

  • all living cells must respire to provide ATP to survive. In aerobic respiration, oxygen needs to be taken in and then the product carbon dioxide needs to be removed. These gases are exchanged by diffusion in opposite directions across the gas exchange surface
• Diffusion:

  • movement of gases across an exchange surface is by diffusion. Several factors determine the rate of diffusion and many can be summarised by Fick's Law:
  • diffusion$\infty$ (surface area x difference in concentration) $\div$ length of diffusion path
• Unicellular organism's exchange surfaces:

  • unicellular organisms are those with no special gas exchange surfaces. They rely on simple diffusion of gases across their outer surface membrane, and this satisfies their respiratory needs
  • they have a large SA:VOL and a short diffusion pathway, so fast rates of diffusion can be achieved
  • some of them increase their SA:VOL by having projections
  • where a living cell is surrounded by a cell wall, this is not an additional barrier to the diffusion of gases
• Multicellular organism's exchange surfaces:

  • due to their smaller SA:VOL, they need to develop specialised gas exchange surfaces. The most efficient will be the one where the rate of diffusion is as high as possible, so it would have the following characteristics/adaptions;
  • large surface area relative to the volume
  • thin surface so the diffusion distance is short
  • selectively permeable to allow selected materials to cross
  • movement of the environmental medium (air) to maintain a steep gradient
• Mass transport system:

  • many multicellular organisms (fish and humans) also require a mass transport system to ensure the movement of the internal medium
  • maintaining steep diffusion gradients for oxygen involves bringing it constantly to the exchange surface (by ventilation) and carrying it away from the surface (by mass transport in the blood)
• Thin exchange surfaces:

  • being thin, specialised exchange surfaces are easily damaged and dehydrated
  • they are therefore often located inside an organism. Where an exchange surface is located inside the body, the organism needs to have a means of moving the external medium over the surface (e.g. a means of ventilating the lungs in a mammal)
• Air as a medium:

  • organisms may exchange surfaces with air or water
  • air has about 21cm$^3$ $O_2$ per 100cm$^3$. Water may have less than 1cm$^3$ $O_2$ per 100cm$^3$
  • water is a dense medium in which diffusion is slow. $O_2$ diffuses 10,000 times more rapidly in air than water
  • it is more efficient to use air as a medium to obtain oxygen rather than water
• Gas exchange in fish:

  • fish have a waterproof, gas-tight outer covering which is not permeable. They also have a relatively small SA:VOL. They therefore need a specialist internal gas exchange system, the gills
• The gills:

  • the gills of a fish are structures situated within the body of the fish, just behind the head
  • the gills are made up of gill filaments which are thin-walled projections sticking out of the gill arches
  • these gill filaments then have many gill lamellae sticking up at right angles, increasing the surface area of the gills. It is at the gill lamellae that the exchange occurs
• Gas exchange in fish:

  • a ventilation mechanism ensures that water is taken in through the mouth and is forced over the gill, leaving through an opening (operculum) in each side if the body. Therefore, there's a constant flow of water over the gills
  • gills have a rich blood supply. There are many capillaries, with a single layer of thin endothelium close to the thin-walled lamellae - the many blood capillaries increase surface area, the thin endothelium ensures a short diffusion pathway between the blood and water
• Counter-current flow:

  • the constant flow of water over the gill lamellae and the flow of blood within them are in opposite directions. This is known as a counter-current flow and is important for ensuring that the maximum possible gas exchange is achieved
  • it means that water always has more oxygen than the blood, and it never reaches equilibrium; the diffusion gradient for oxygen uptake is maintained across the entire length of the gill lamellae
  • diffusion of oxygen into the blood is always maintained. The fish always has oxygen for respiration
• Gas exchange in insects:

  • the body of an insect is protected by an external skeleton, or exoskeleton, made of a rigid substance called chitin. The tough exoskeleton prevents insects from simply using their body surface for diffusion, so they require a specialised gas exchange system
• The tracheal system:

  • the tracheal system of insects consists of a network of air-filled tubes (tracheae) that open to the outside through small holes in the exoskeleton called spiracles
  • gases enter and leave the tracheae through these spiracles. The tracheae are supported by strengthened rings of chitin prevent them from collapsing
  • tracheae divide into smaller dead-end tubes called tracheoles which extend throughout all the body tissues of the insect and they're sites of gas exchange
  • oxygen diffuses directly into the cells from the tracheoles and carbon dioxide diffuses out
• Gas exchange characteristics in insects:

  • the large number of small tracheoles give a large surface area for diffusion, while their thin walls, the fact they are numerous, and in close proximity to the cells, provide a short diffusion pathway
  • for diffusion to be effective, the diffusion pathway needs to be short which is why insects are of a small size
  • as a result, the length of the diffusion pathway limits the size that insects can attain
• Gas movement in the tracheal system:

  respiratory gases move in and out of the tracheal system;

  • down a diffusion gradient - during respiration, oxygen at the end of the tracheoles is reduced. This sets up a diffusion gradient where oxygen in the atmosphere moves towards the lower oxygen concentration (the tracheoles). Carbon dioxide is also produced during respiration which sets up a diffusion gradient that moves in the opposite way
• Gas movement in the tracheal system:

  • ventilation - the movement of abdominal muscles in insects causes mass movement in air into and out of the tracheae, speeding up gas exchange as it maintains a diffusion gradient between two mediums. This is known as rhythmic abdominal pumping, and this increases during flight
• Anaerobic respiration and tracheoles:

  • the ends of the tracheoles are filled with water. During periods of major activity, muscle cells (around tracheoles) carry out anaerobic respiration
  • this produces lactate lowering the WP in muscle cells
  • water moves into the cells from tracheoles by osmosis. The water in the ends of the tracheoles decreases in volume drawing air further into them
  • this means the final diffusion pathway is in a gas rather than a liquid, and therefore diffusion is faster. This can lead to water loss via evaporation, so the insect needs to make sure its balancing its needs
• Mass transport systems in insects:

  • insects do not require a mass transport system like fish and humans, as the tracheole system is sufficient enough to supply oxygen to each individual cell
• Control of water loss in insects:

  • this is a problem in terrestrial insects as the ends of the tracheoles are filled with water
  • however, efficient gas exchange requires a thin, permeable surface with a large area
  • these features conflict with the need to conserve water
  • therefore, the insect must balance the opposing needs of exchanging respiratory gases with limiting water loss
• Insect adaptations to reduce water loss:

  • small SA:VOL - minimise the area over which water is lost
  • hairs to trap moist air (which becomes saturated as it has lots of water vapour) to lower the WP gradient
  • waterproof covering over their body surfaces (rigid outer skeleton of chitin that is covered with a waterproof cuticle)
  • spiracles can be closed by a valve to reduce water loss. When spiracles are open, water vapour can evaporate from the insect. For much of the time insects keep their spiracles closed to prevent this water loss. Periodically, they open spiracles to allow for gas exchange
• Gas exchange in plants;

  • most gaseous exchange occurs in the leaves; the spongy mesophyll layer of the leaf, with its large air spaces and thin-walled cells is the principal gas exchange surface within the leaf
  • the leaf shows several adaptations for rapid diffusion
• Adaptations of the leaf;

  • thin, flat shape gives a large surface area and short diffusion pathway
  • the cells of the spongy mesophyll are loosely packed, creating numerous interconnecting air spaces that provide a larger surface area for gas exchange, and allow gases to diffuse up to the palisade cells
• Adaptations of the leaf:

  • large surface area of mesophyll cells for rapid diffusion
  • many small pores (stomata), found mainly on the lower epidermis of the leaf, through which gases enter and leave via diffusion. No cell is far from the stomata so there is a short diffusion pathway. The stomata are surrounded by guard cells which can control the rate of gas exchange by opening and closing the stomatal pores. This helps the plant to balance the conflicting need of gas exchange and control of water loss
• Photosynthesis and respiration;

  • At times the gases can be recycled between photosynthesis and respiration, as the waste gas in one is the reactant in the other. This reduces exchange with external air
  • therefore, the exchange required changes depending on the balance between photosynthesis and respiration
  • often photosynthesis is faster than respiration so carbon dioxide will diffuse in, and oxygen will diffuse out of the plant. this is the opposite at night, as only respiration occurs
• Gas exchange in the day;

  • when it is light, photosynthesis is happening and glucose is being produced in the chloroplasts
  • this will decrease water potential so water will move in by osmosis. Therefore the guard cells will be turgid and open to allow gas exchange
• Gas exchange at night;

  • when photosynthesis is not happening, glucose will be converted into starch and used in respiration so the water potential in guard cells will increase
  • this will mean water leaves the guard cells and they become flaccid, so they close. There is a risk of losing too much water
• Control of water loss in plants (xerophytes);

  • to reduce water loss, terrestrial plants have a waterproof covering over parts of the leaves and the ability to close stomata when necessary
  • certain plants with a restricted supply of water have evolved a range of adaptations to limit water loss through transpiration. These plants are called xerophytes; plants are adapted to living in areas where water is in short supply
• Adaptations of xerophytes;

  • a thick, waxy cuticle (waterproof) - prevents evaporation from the upper surface of the leaf
  • stomata are mainly found on the lower epidermis of the leaf - protects them from direct sunlight, which would lead to increased water loss as it would increase the kinetic energy, making water more likely to evaporate out
• Adaptations of xerophytes;

  • rolling up of leaves - protects the stomata on the lower epidermis from the outside, helping to trap a region of still air and water vapour within the rolled leaf. This results in no water potential gradient between the inside and outside of the leaf and therefore no water loss (marram grass)
  • stomata in pits or grooves - traps air as with rolling of leaves and hairs
• Adaptations of xerophytes;

  • a thick layer of hairs on leaves - traps still, moist air next to the leaf surface. The water potential gradient between the inside and outside of the leaves is reduced and therefore less water is lost by evaporation
  • a reduced SA:VOL of the leaves - leaves are small and roughly circular in cross-section, as in pine needles, rather than leaves that are broad and flat, meaning the rate of water loss can be reduced
• The site of the lungs;

  • the lungs are situated on either side of the heart in the thorax (chest cavity), surrounded by the ribs and with the diaphragm at the base. They allow gaseous exchange between the air in the lungs and the blood in the capillaries
• Structure of the gas exchange system;

  • air enters the airway through the nose or the mouth, then passes into the trachea, which splits into 2 bronchi, one going to each lung. Mucus membrane line much of the airway. These contain goblet cells which secrete mucus and are lined with ciliated epithelium. The bronchi branch into many bronchioles, each ending in small air sacs, called the alveoli. This is the site of gas exchange
• The alveoli;

  • gas exchange only takes place between the alveoli and the blood capillaries
  • oxygen diffuses through the flattened epithelium of the alveoli and the endothelium of the blood
  • here it combines with haemoglobin in the red blood cells. Carbon dioxide diffuses from the blood into the alveoli
• Adaptations of the alveoli;

  • alveoli have a large surface area - there are about 300 million alveoli in each human lung. Their total surface area is around 70m$^2$ - about half the area of a tennis court
  • short diffusion pathway - the alveoli and capillary walls are only one cell thick, giving a very thin exchange surface
• Adaptations of the alveoli;

  • a huge network of capillaries surround the alveoli, constantly circulating blood around the alveoli, and breathing movements constantly ventilate the lungs. The means a steep concentration gradient is maintained for the $O_2$ into the blood and $CO_2$ out of the blood. The large network of capillaries also increases the surface area for exchange of gases
• Adaptations of the alveoli;

  • each capillary surrounding the alveoli is only 7-10 μm wide (the lumen is very narrow), meaning the red blood cells have to flatten themselves against the capillary wall to squeeze through. This again shortens the diffusion pathway and slows down the movement of red blood cells, allowing more time for diffusion
• Adaptations of the alveoli;

  • between the alveoli there are some collagen and elastic fibres, The elastic fibres allow the alveoli to stretch as they fill with air when breathing in, and then spring back (recoil) during breathing out in order to expel the carbon dioxide-rich air. This helps to maintain the concentration gradient
  • surfactant prevents the alveoli from collapsing or sticking together. Alveoli must be kept open to increase their surface area. Lung surfactant reduces the surface tension so that the alveoli remain open
• Process of ventilation (breathing);

  breathing is not the same as respiring

  ventilation includes:

  • inspiration - breathing in
  • expiration - breathing out

  ventilation is achieved by movements of the ribcage, the diaphragm and the internal and external intercostal muscle. The two sets of intercostal muscles are antagonistic
• Process of ventilation;

  • both inspiration and expiration require breathing movements to alter the volume of the thorax, which in turn creates air pressure differences between the thorax and the atmosphere outside
  • since the thorax is airtight and the only opening to the outside is through the trachea, air is drawn in or out down a pressure gradient
  • pressure is inversely proportional to volume. When the volume increase, pressure decreases and vice versa
• Inspiration;

  • diaphragm contracts and flattens
  • external intercostal muscles contract, pulling the ribs up and out
  • volume of thorax increases, so pressure decreases
  • atmospheric pressure is now greater than the pressure in the thorax
  • air moves into the lungs down a pressure gradient