adaptions for gas exchange

• Gases are echanged across respiratory surfaces. A respiratory surface must be:

  • Thin (short diffusion pathway)
  • Permeable to gases
  • Moist
  • Have a large surface area
• small organisms have a high surface area to volume ratio. large organisms have a low surface area to volume ratio
• Amoeba: Single cells have a large surface area to volume ratio. The cell membrane is thin so diffusion into the cell is rapid. A single cell is thin so diffusion distances into the cell are short. Gaseous exchange by diffusion across the cell surface is rapid enough to supply oxygen for respiration and to remove carbon dioxide.
• Flatworm: Flatworms are aquatic, and being flat, have a much larger surface area than spherical organisms. Their large surface area to volume ratio has overcome the problem of size increase as no part of the body is far from the surface and so diffusion paths are short.
• Earthworm: It is cylindrical and so its surface area to volume ratio is smaller than a flatworm’s. Its skin is the respiratory surface, which it keeps moist by secreting mucus. It has a low oxygen requirement because it is slow and has a low metabolic rate. Enough oxygen diffuses across the skin surface to reach the blood capillaries beneath. Haemoglobin present in the blood carries oxygen around the body in blood vessels. This maintains a steep concentration gradient at the respiratory surface. Carbon dioxide diffuses out across the skin
• Gills have a large surface area due to gill filaments. Gill filaments are a specialised respiratory area. water must be forced over the gill filaments. Water is forced over the gills by an increase in pressure in the fish.
• inspiration:

  • The mouth opens and the floor of the buccal cavity is lowered. Volume of the buccal cavity increases and pressure decreases. The operculum remains closed. Water is pulled into the buccal cavity from the outside due to the change in pressure.
• expiration

  • The mouth closes and the buccal cavity contracts, raising the floor of the buccal cavity. Water is forced across the gills.
  • Pressure in the gill cavity increases and forces the operculum (gill slit) open. Water leaves via the operculum.
• Counter current flow increases the efficiency of diffusion by maintaining a steep concentration gradient across the whole gill filament. Blood always meets water with relatively high oxygen content.
• Counter current flow – Blood and water flow in opposite directions across the gill plate. This maintains a concentration gradient for the efficient diffusion of oxygen into the blood.
• Water and blood flow in the same direction across the gill plate. This is called parallel flow. Parallel flow is shown on the graph on the right. The concentration gradient is not maintained. Diffusion is not optimal and does not continue across the entire gill plate, as equilibrium is reached.
• An inactive amphibian uses its moist skin for gas exchange. Active amphibians use simple lungs. Frog lungs are simply a pair of hollow sacs. Their surface is highly folded, which increases the surface area. Compared with human lungs the surface area of frog lungs is relatively small. The tadpole stage uses gills.
• The respiratory surface in amphibians, reptiles and birds share the following characteristics:

  • Large surface area for the rapid diffusion of respiratory gases.
  • Moist surface .
  • Short diffusion pathway .
  • Circulatory system with blood pigments to carry oxygen
  • Internal lungs to minimise the loss of water and heat
  • Ventilation mechanism which forces the respiratory medium (air) to and from the respiratory surface. this ensures oxygen is brought to and carbon dioxide is removed from the gas exchange surface.
• Reptilian skin is impermeable to gases and cannot be used as a respiratory surface. Reptiles have more efficient lungs than amphibians. Gaseous exchange occurs exclusively in the lungs. Reptilian lungs are sac-like and have more complex folding than amphibian lungs. Reptiles have ribs, but no diaphragm. Ventilation is aided by the movement of the ribs by the intercostal muscles.
• Birds are warm-blooded and have a high respiration rate; efficient gas exchange is essential. Bird lungs are small and compact, composed of numerous branching air tubes called bronchi. The smallest air tubes, the parabronchi, have an extensive blood capillary network – it is here that gaseous exchange takes place. The parabronchi end in large, thin walled air sacs which help in ventilation . Ventilation of the lung is brought about by movement of the ribs. During flight the action of the wing muscles ventilates the lungs.
• Efficient gas exchange in insects requires a thin, permeable surface with a large surface area. this conflicts with the need to reduce water loss. To reduce water loss insects have evolved a rigid waterproof exoskeleton which is covered by a cuticle. Insects have a relatively small surface area to volume ratio and cannot use their body surface to exchange gases by diffusion.
• Gas exchange in insects occurs through paired holes, called spiracles, running along the side of the body. The spiracles lead into a system of branched, chitin lined airtubes called tracheae. The spiracles can open and close like valves; this allows gaseous exchange to take place and reduces water loss. Resting insects rely on diffusion to take in oxygen and remove carbon dioxide. The ends of the tracheae are called tracheoles. Gas exchange takes place at the end of the tracheoles. Oxygen passes directly to the cells; this is very rapid.
• the spiracle is closed when the insect at rest. the spiracle is open when the insect is at flight
• When the abdomen is expanded the thorax spiracles are open and the abdominal spiracles are closed. air enters the tracheal system through the thorax spiracles.
• As the abdomen is compressed the thorax spiracles close and the abdominal ones open; air leaves the tracheal system via the abdominal spiracles
• During inspiration the intercostal muscles contract, raising the ribcage upwards and outwards. The diaphragm also contracts and flattens. The volume of the thorax increases and the pressure decreases. Air enters the lungs and the lungs expand
• During expiration the intercostal muscles relax, which moves the ribcage inward and downward. The diaphragm also relaxes and curves upwards. The volume of the thorax decreases and the pressure increases. The air is forced out of the lungs.
• alveolus

  • They provide a large surface area relative to the volume of the body. They have a moist surface for gases to dissolve.
  • Thin walls, which provide a short diffusion path for diffusion.
  • Each alveolus is covered by an extensive capillary network.
  • Oxygenated blood is carried away form the alveolus and blood rich in carbon dioxide returns – this maintains a steep concentration gradient for diffusion.
• not a lot of oxygen gets released from the lungs because oxygen is absorbed into the red blood cells at the alveoli and used in aerobic respiration.
• lots of carbon is released from the lungs because carbon dioxide produced by respiration diffuses from the blood plasma into the alveoli. it is released out the lungs because its toxic
• Nitrogen is neither absorbed nor used so all that is inhaled gets exhaled.
• The water content of the atmosphere varies. Alveoli are permanently lined with moisture. water evaporates from them and is exhaled.
• what is this

  A) spiracle

  B) exoskeleton

  C) trachea

  D) air sac

  E) tracheoles

  F) muscle cell

  
• what is this

  A) waxy cuticle

  B) upper epidermis

  C) palisade mesophyll

  D) spongy mesophyll

  E) vascular bundle

  F) stomata

  G) guard cell

  
• Waxy cuticle - Reduces water loss from the leaf surface by evaporation
• Upper epidermis - Transparent cells which allow light to pass to the mesophyll tissue. The epidermal cells also synthesise and secrete the waxy cuticle.
• Palisade mesophyll - Contain many chloroplasts for photosynthesis. The palisade layer is the main photosynthetic tissue.
• Spongy mesophyll - Spongy palisade cells also carry out photosynthesis as they contain chloroplasts. The air spaces between the cells allow for the circulation of gases.
• Vascular bundles (xylem and phloem) - Contain xylem for water and mineral transport and phloem for the transport of the products of photosynthesis (sucrose and amino acids).
• Guard cells - Guard cells become turgid and flaccid due to changes in water potential. this opens and closes the stomatal pore.
• Stomata - Stomata allow gaseous exchange.
• Adaptations for gaseous exchange:

  • The spongy mesophyll tissue allows for the circulation of gases.
  • The plant tissues are permeated by air spaces.
  • Stomatal pores allow gases to enter and leave the leaf.
  • Gases diffuse through the stomata down a concentration gradient.
  • Gases then diffuse through the intercellular spaces between mesophyll cells.
  • Gases dissolve in the moist layer which covers each cell and diffuse inside.
• Adaptations for photosynthesis:

  • Leaves have a large surface area to capture as much light as possible.
  • Leaves can orientate themselves so that they are held at an angle perpendicular to the sun to expose the surface to as much light as possible.
  • Leaves are thin to allow light to penetrate to lower cell layers.
  • The cuticle and epidermis are transparent to allow light to penetrate to the mesophyll.
  • Palisade mesophyll cells are elongated and densely arranged in layers.
  • Palisade cells are packed with chloroplasts and arranged with their long axes perpendicular to the surface.
• Chloroplasts can rotate and move within the mesophyll cells; this allows them to move in the best position for light absorption.
• Pores called stomata allow the exchange of gases. Water is also lost through the stomata.