Gas exchange

• Gas exchange occurs over a gas exchange surface which is a boundary between the outside environment and the internal environment of an organism.
• Organisms need oxygen and carbon dioxide to diffuse across gas exchange surfaces as quickly as possible. Most gas exchange surfaces have two things in common that increase the rate of diffusion:

  1. large surface area
  2. thin (often just one layer of epithelial cells) - provides a short diffusion pathway across the gas exchange surface.
• Organisms maintain a steep concentration gradient of gases across the exchange surface, which increases the rate of diffusion
• Gas exchange in single-celled organism: absorb and release gases by diffusion through their cell-surface membranes. They have a relatively large surface area, a thin surface and short diffusion pathway (oxygen can take part in biochemical reactions as soon as it diffuses into the cell) - no need for a specialised gas exchange system
• Why do fish need special adaptations to get enough oxygen?

  there is a lower concentration of oxygen in water than in air
• How are the gills adapted?

  Water, containing oxygen enters the fish through its mouth and passes out through the gills. Each gill is made of lots of thin plates called gill filaments which give a large surface areas for the exchange of gases (and so increase the rate of diffusion). The gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more. The lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion, between the water and blood.
• The counter-current system?

  In the gills, blood flows through the lamellae in one direction and water flows over them in the opposite direction. This means that the water with a relatively high oxygen concentration always flows next to blood with a lower concentration of oxygen. This in turn means that a steep concentration gradient is maintained between the water and blood - so as much oxygen as possible diffuses from the water into the blood
• Gas Exchange in dicotyledonous plants?

  Plants need CO2 for photosynthesis, which produces O2 as a waste gas. They need O2 for respiration, which produces CO2 as a waste gas. Main gas exchange surface in plants is the surface of the mesophyll cells.
• What is the main gas exchange surface in plants?

  mesophyll cells
• How are mesophyll cells adapted?

  Well adapted to function - large surface area
• Mesophyll cells are inside the leaf. Gases move in and out through special pores in the epidermis (mostly the lower epidermis) called stomata (singular = stoma). The stomata can open to allow the exchange of gases, and close if the plant is losing too much water. Guard cells control the opening and closing of stomata.
• waxy cuticle

  upper epidermis cells

  palisade mesophyll cells

  xylem and phloem + air spaces

  spongy mesophyll cells

  lower epidermis cells + guard cells + stomata

  waxy cuticle
• Trachea

  Microscopic air-filled pipes used by terrestrial insects for gas exchange
• Trachea

  • Air moves into them through pores on the surface called spiracles
  • Oxygen travels down the concentration gradient towards the cells
  • They branch off into smaller tracheoles with thin, permeable walls that go to individual cells
  • The insect's circulatory system doesn't transport O2
• Tracheoles

  Smaller branches of the trachea that have thin, permeable walls and go to individual cells
• Gas exchange in insects

  1. Oxygen travels down concentration gradient to cells

  2. Carbon dioxide from cells moves down concentration gradient to spiracles

  3. Insects use rhythmic abdominal movements to move air in and out of spiracles
• Terrestrial insects have microscopic air-filled pipes called trachea which they use for gas exchange
• Humans need to get oxygen into the blood for respiration and remove carbon dioxide (made by respiring cells) - ventilation and the gas exchange system
• As we breathe in, air enters the trachea which then branches into two bronchi - one bronchus to each lung. Each bronchus then branches off into smaller tubes called bronchioles which end in small 'air sacs' - alveoli. The ribcage, intercostal muscles and diaphragm all work together to move air in and out
• Intercostal muscles: are found between the ribs. There are three layers - 2 we need to know: internal and external (internal are on the inside of the external)
• Ventilation consists of inspiration and expiration and is controlled by the movements of the diaphragm, internal and external intercostal muscles and the ribcage.
• During inspiration, the external intercostal muscles and diaphragm muscles contract. This causes the ribcage to move upwards and outwards and the diaphragm to flatten, increasing the volume of the thoracic cavity (the space where the lungs are). As the volume of the thoracic cavity increases, the lung pressure decreases to below atmospheric pressure. Air will always flow from an area of higher pressure to an area of lower pressure (i.e. down a pressure gradient) so air flows down the trachea and into the lungs. Inspiration is an active process - it requires energy.
• During expiration the external intercostal muscles and diaphragm muscles relax. The ribcage moves downwards and inwards and the diaphragm curves upwards again (becomes dome-shaped). The volume of the thoracic cavity decreases causing the air pressure to increase above atmospheric pressure. Air is forced down the pressure gradient and out of the lungs.
• Normal expiration is a passive process however it can be forced e.g. when blowing out candles on a cake. During forced expiration, external intercostal muscles relax and internal intercostal muscles contract, pulling the ribcage further down and in. During this time, the movement of the two sets of intercostal muscles is said to be antagonistic (opposing)
• Control of water loss:

  exchanging gases tends to make you lose water

  plants and insects have evolved adaptations to minimise water loss without reducing gas exchange too much
• If insects are losing too much water - close their spiracles using muscles

  also have waterproof waxy cuticle all over body and tiny hairs around their spiracles both of which reduce evaporation
• Plants' stomata are usually kept open during the day to allow gaseous exchange.

  water enters through guard cells, making them turgid, which opens the stomatal pore

  if the plants start to get dehydrated, guard cells lose water and become flaccid, which closes the pore
• some plants are specially adapted for life in warm, dry or windy habitats, where water loss is a problem - xerophytes
• xerophytic adaptations include:

  1. stomata sunk in pits to trap water vapour reducing the concentration gradient of water between the leaf and the air - reduces evaporation of water from the leaf
  2. a layer of hairs on the epidermis to trap water vapour around the stomata
  3. curled leaves with the stomata inside protecting them from wind (windy conditions increase the rate of diffusion and evaporation)
  4. reduced no. of stomata - fewer places for water to escape
  5. thicker waxy, waterproof cuticles on leaves and stems to reduce evaporation