gas exchange

• Describe the relationship between the size of an organism and its surface area to volume ratio.

  -small organisms have a larger surface area to volume ratio

  -the larger the organism the smaller the surface area to volume ratio
• Features of specialised exchange surfaces

  • a large surface area relative to the volume of the organism which increases the rate of exchange
  • very thin so short diffusion distance and therefore materials cross the exchange surface rapidly
  • selectively permeable to allow selected material to cross
  • movement of environmental medium for example air to maintain a diffusion gradient
  • a transport system to ensure the movement of the internal medium for example blood as it maintains the diffusion gradient
• How does size affect heat exchange?

  • The rate of heat loss from an organism depends on its surface area.
  • If an organism has a large volume, the surface area is relativity small making it harder to lose heat from its body.
  • Small organisms have a large surface area so heat is lost more easily.
  • This means smaller organisms need a high metabolic rate to generate enough heat to stay warm.
• Name 4 things that need to be exchanged between organisms and their environment.

  -respiratory gases

  -nutrients

  -excretory products

  -heat
• Describe how gas exchange works across the body of a single-cellular organism. 

  -Single-celled organism absorb and release gases by diffusion through their outer surface.

  -They have a relatively large surface area, a thin surface and a short diffusion pathway so there is no need for a gas exchange system.
• Describe how xerophytic plants are adapted to control water loss.

  -Stomata sunk in pits that trap moist air, reducing the concentration gradient of water between the leaf and the air. This reduces the amount of water diffusing out the leaf and evaporating away.

  -A layer of 'hairs' on the epidermis which trap moist air around the stomata.

  -Curled leaves with stomata inside, protecting them from wind (wind increases rate of evaporation)

  -A reduced number of stomata, so fewer places that water can escape.

  -Waxy, waterproof cuticles on leaves and stems to reduce evaporation.
• Describe how gas exchange works by the leaves of dicotyledonous plants.

  • Gases move in and out through stomata.
  • The stomata can open to allow exchanges of gas and close if the plant is losing too much water.
• Why can't fish use their bodies as an exchange surface?

  They have a waterproof, impermeable outer membrane and a small surface area to volume ratio.
• Name and describe the two main features of a fish's gas transport system.

  • Gills= located within the body, supported by arches, along which are multiple projections of gill filaments, which are stacked up in piles.
  • Lamellae= at right angles to the gill filaments, give an increased surface area. Blood and water flow across them in opposite directions (countercurrent exchange system).
• Explain the process of gas exchange in fish.

  • The fish opens its mouth to enable water to flow in, then closes its mouth to increase pressure.
  • The water passes over the lamellae, and the oxygen diffuses into the bloodstream.
  • Waste carbon dioxide diffuses into the water and flows back out of the gills.
• How does the countercurrent exchange system maximise oxygen absorbed by the fish?

  -Maintains a steep concentration gradient as water is always next to blood of a lower oxygen concentration.

  -Keeps rate of diffusion constant
• In fish, why is the parallel system less efficient than the counter-current gas exchange system?

  • In the parallel system, the O₂ concentration gradient between water and blood decreases along distance of lamellae.
  • Therefore the rate of oxygen diffusion will also will also decrease.
  • This means that less oxygen diffuses into the blood than will the counter-current system where the concentration gradient is maintained.
• Describe the counter current exchange system

  1.water and blood flow in opposite directions

  2. water always passing blood with a higher oxygen concentration

  3. diffusion gradient maintained throughout length of gill
• Why can't insects use their bodies as an exchange surface?

  They have a waterproof chitin exoskeleton and a small surface area to volume ratio in order to conserve water.
• What are some adaptations insects have for gas exchange ?

  • Tracheoles have thin walls so short diffusion distance to cells;
  • Highly branched / large number of tracheoles so short diffusion distance to cells;
  • Highly branched / large number of tracheoles so large surface area (for gas exchange);
  • Tracheae provide tubes full of air so fast diffusion (into insect tissues)
• What are some more adaptations insects have for gas exchange ?

  • Fluid in the end of the tracheoles that moves out (into tissues) during exercise so larger surface area (for gas exchange);
  • Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide;
• How does an insect carry out gas exchange via diffusion ?

  • Gas exchange in insects involves a tracheal system (trachea, tracheoles, and spiracles).
  • Gas can exchange by diffusion, as when cells respire they use up oxygen and produce carbon dioxide, creating a concentration gradient from the tracheoles to the atmosphere.
• How does an insect carry out gas exchange via mass transport ?

  The second method of gas exchange is mass transport, in which an insect contracts and relaxes their abdominal muscles to move gases.
• How does having fluid at the ends of an insects tracheoles carry out gas exchange ?

  • When the insect is in flight, the muscle cells start to respire anaerobically to produce lactate.
  • This lowers the water potential of the cells, and therefore water moves from the tracheoles into the cells by osmosis.
  • This decreases the volume in the tracheoles and as a result, more air from the atmosphere is drawn in.
• What are some adaptations insects have for water loss ?

  • Closing their spiracles if they become dehydrated
  • Spiracles are surrounded by small hairs to trap water vapour and reduce the water potential gradient
  • Covered by a waterproof, chitin exoskeleton
• Describe process of inhalation

  • External intercostal muscles contract internal intercostal muscles relax.
  • Ribcage moved upwards and outwards to increase thorax volume
  • Diaphragm contracts and flattens to help increase volume in thorax
  • Increased volume leads to reduced pressure -
  • Atmospheric pressure is greater than pulmonary pressure so air is forced into the lungs due to the pressure gradient
• Describe process of exhalation

  • Internal intercostal muscles contract and External intercostal muscles relax
  • Ribcage is moved downwards and inwards reducing the volume in the thorax
  • Diaphragm muscle relaxes and is pushed upwards by the contents of the abdomen
  • Decreased volume in the thorax correlates to an increased pressure in the lungs
  • As pulmonary pressure is greater than atmospheric pressure, air is forced out of the lungs
• Besides pressure gradient, what else contributes to the expulsion of air out of the lungs during ventilation?

  the recoil of of elastic tissue in the lungs
• Why is the diffusion of gases rapid between alveoli and blood?

  -Red blood cells are slowed as they pass through pulmonary capillaries allowing more time for exchange

  -The distance between the alveoli and the red blood cells are reduced by flattening the red blood cells against capillary walls

  -There is a short diffusion pathway between the alveoli and capillaries as the alveoli have a single layer of epithelial cells and the blood capillaries have a single layer of endothelial cells
• Why is the diffusion of gases rapid between alveoli and blood?

  -large surface area of both alveoli and pulmonary capillaries

  -Lungs are constantly ventilated and the blood is constantly circulated to ensure a steep concentration gradient is maintained
• Describe the exchange of gases required when photosynthesis does not take place

  When photosynthesis does not occur (such as when its dark) oxygen must diffuse into the leave as it needs to be used for respiration and the CO2 produced will diffuse out.
• What are the adaptations of gas exchange seen in leaves of plants?

  • many small pores called stomata.
  • No cell is far way from a stoma and therefore external air so there is a short diffusion pathway
  • Numerous air spaces in the mesophyll layer to allow diffusion to take place in its gas phase (higher efficiency)
  • Large surface area of mesophyll cells for rapid diffusion
• What is each stomata surrounded by and what is their function?

  Each stomata is surrounded by a pair of guard cells which pen and close the stomata to allow plants to control the rate of gas exchange
• What is the main topic covered in this video lesson?

  Transpiration and the cohesion tension theory
• What is transpiration?


  The loss of water vapor from the stomata by evaporation
• Where are stomata mainly found on a leaf?


  On the lower side of the leaf