Gas Exchange

• Insects have a waxy, waterproof cuticle covering their body, acting as an exoskeleton to reduce water loss as it is impermeable.
• For gas exchange to occur, insects have pores in their cuticle called the spiracle. Spiracles are valved structures that can open and close to regulate air flow.
• The spiracles connect to air-filled tubes that permeate the body of the insect called tracheae - these branch into smaller tubes called tracheoles, which terminate at the muscle fibres which contract to make the body move.
• The process of gas exchange in insects is mainly due to diffusion, as they have a large surface area to volume ratio.
• Some larger insects perform abdominal pumping to increase the rate of gas exchange. The insect expands to increase volume, so pressure decreases and CO2 is removed forcefully - mass transport by pressure gradient.
• As water passes over the gills in a fish, oxygen diffuses from the water into the blood that runs in the gill capillaries, while carbon dioxide diffuses from the blood into the water.
• There are many gill arches in the gill cavity. Each gill arch has many gill filaments. Each gill filament has many gill lamellae, which contain capillaries carrying blood close to the surface: there is a single layer of flattened epithelial cells between the water flowing over the gill lamellae and the blood flowing in the capillaries.
• The blood flowing through capillaries flows in the opposite direction to the water flowing over the gills - this is known as countercurrent flow.
• Countercurrent flow: the blood and the water move in opposite directions so the concentration gradient is maintained so equilibrium isnt reached at any point along the whole length of gill, so oxygen can always diffuse across from the water to the blood.
• In plants rate of photosynthesis and respiration change:

  • Midday - Photosynthesis is high, respiration is lower.
  • Midnight - Photosynthesis is low, respiration is higher.
• In order to have a large surface area for gas exchange, plants have many flat leaves with numerous air spaces within them
• Leaves are thin to ensure a short diffusion pathway.
• Stomata on the lower epidermis of leaves allow for permeability.
• A xerophyte is a plant with no access to liquid water sources.
• Guard cells become turgid, meaning they are under high pressure, to open the stomata and allow water loss.
• Guard cells become flaccid, meaning they are under low pressure, to close stomata and prevent water loss.
• Plants have leaves with a thin waxy cuticle made of chitin, which is impermeable (acting as an exoskeleton) to prevent water loss as no osmosis can occur.
• Some plants have other adaptations intended to trap moist air and remove the water potential gradient needed for osmosis:

  • rolled leaves
  • stomata in pits or grooves
  • hairy leaves
• The trachea allows the passage of air into lungs:

  • it is full of u-shaped cartilage rings to keep airway open, but the back layer is softer as the oesophagus is behind
  • it has goblet cells to produce mucus and smooth muscle to allow coughing
• Bronchi have less cartilage than the trachea, but more goblet + ciliated cells
• The bronchioles have no cartilage and a thinner membrane, but are still lined with goblet + ciliated cells
• Inspiration: External intercostal muscles contract and internal intercostal muscles relax. This pulls the ribs upwards and out, in creasing the volume of the thoracic cavity. Diaphragm muscle contracts, flattening the diaphragm and also increasing thoracic volume. Increased volume creates a decrease in air pressure in the thoracic cavity. Atmospheric pressure outside the body is now higher, so air is forced into the lungs down the pressure gradient. This is an active process as it is caused by muscle contraction.
• Active (and passive)expiration: External intercostal muscles relax and internal intercostal muscles contract, so the ribs are pulled down and inwards, decreasing the volume of the thoracic cavity. Diaphragm muscles relax, also decreasing thoracic cavity volume. Stretched elastic fibres in the alveoli recoil, so alveoli decrease in volume. Decreased volume creates an increase in air pressure in the thoracic cavity, atmospheric pressure outside the body is now lower so air is forced out of the lungs down the pressure gradient.
• Tidal volume: Volume of air that moves in/out of the lungs with each respiratory cycle (ml).
• Inspiratory reserve volume: Extra volume of air that can be inspired with maximal effort (ml).
• Expiratory reserve volume: Extra volume of air that can be expired with maximal effort (ml).
• Vital capacity: Maximal volume of air that can be expired following maximum inspiration (ml).
• Breathing rate: Breaths per minute (bpm).