Unicellular organisms, multicellular and ventiliation systems. Fish, terrestrial animals, insects, humans and plants.

  • Created by: ava.scott
  • Created on: 14-04-14 18:35

3 key things a respiratory surface

  • THIN
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Unicellular Organisms

e.g. Amoeba

The cell membrane is the gaseous exhange surface. It is freely permeable to gases and nutrients.

It has high surface area to volume ratio, and very short diffusion paths.

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Simple Multicellular Organisms

These have more cells so need more oxygen.

e.g. Flatworm

  • High surface area to volume ratio. 
  • Thin skin, with short diffusion paths

e.g. Earthworm

  • High surface area to volume ratio.
  • Good blood supply to skin with an respiratory pigment.
  • Moist skin so gases can dissolve.
  • Low metabolic rate and slow moving, so less oxygen is needed
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Complex Multicellular Organisms

These have impermeable skin so have developed respiratory mechanisms to achieve gaseous exchange. This is especially important as they have much faster metabolic rates and more cells to be supplied with energy.

Definition: A respiratory mechanism enabling air to be transferred from the atmospshere to the respiratory surfaces.

These mechanisms consist of:

  • ventilation mechanism
  • internal transport system
  • respiratory pigment.

All gas exchange surfaces must have:

  • large SA:volume ratio
  • moist surface
  • short diffusion paths
  • extensive capillary network
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Cartiliganous Fish e.g. sharks

Gills in the head and water is forced through mouth and gills, and travels in the same direction as blood.

Bony fish

Water is kept flowing by a pumping system.

  • Gills close, and the operculum has lower pressure, and water flows in through the mouth.
  • Mouth closes, gills open and water is under pressure and forced out past gill filaments.

The gill filaments are covered in lamellae. These small flaps are supported by water and increase the surface area. Lamellae have a dense supply of capillaries, and the blood flows in the opposite direction, creating a counter- current.

The counter current maintains a concentration gradient, because the most oxygenated water meets the most oxygenated blood (but the water will always have more oxygen.)

This is very efficient, allowing fish to remove 80% of oxygen from the water.

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Terrestrial Animals

Terrestrial animals face the problem of water loss by evaporation through the skin- therefore skin must be waterproof and impermeable. Internal lungs orevent heat and water loss.


  • Skin and lungs as gaseous exchange surface. The skin works in water and air; lungs are small elastic sacs with good blood supply. 
  • No diaphram/muscles- air forced in through the mouth.


  • trunk of body doesn't touch the ground.
  • ribs support and protect lungs.
  • lungs have more complex internal tissues, with larger SA.


  • large volumes of O2 needed for flight so most efficient ventilation.
  • ventilation is assisted by a network of air sacs, which function as bellows.
  • movement of ribs force ventilation, and muscles during flight.
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Terrestrial organisms have developed a waterproof, waxy exoskeleton made from chitin, to reduce water loss. They also have a small SA: volume ration, so their skin would not be a good exchange surface.

Gas exchange occurs:

  • Through a series of paired holes called spiracles.
  • These lead to a system of branched chitin-lined tubes called tracheae.
  • Spiracles can open and close like valves.

Resting insects

Rely on diffusion of gases to take in O2 and remove CO2.

During flight:

The movement of abdominal muscles ventilate the tracheae.

Gasesous exchange takes place at the end of tracheoles whih carry O2 directly into cells.

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Human Respiratory System- physiology

Contains all the basic features that a exchange surface must have.

The lungs:

  • enclosed in an airtight compartment, the thorax.
  • at the base of the thorax is the diaphragm, a dome-shaped muscle.
  • are supported by the rib cage, which are moved by intercostal muscles, to allow ventilation.
  • air moves into the lungs by the trachea, which split into bronchi, and then bronchioles.

The alveoli:

  • gas exchange surface, with the 5 key features- name them.
  • deoxygenated blood reaches the alveolus, and oxygen diffuses through the alvelous and into the blood stream. Carbon dioxide diffues into the alveolus.

Pleural fluid surrounds and lines each lung, and acts as a lubricant when breathing,to prevent friction.

Surfacant, covers alveoli to reduce surface tension and stop them from collapsing.

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Human respiration- ventilation


  • active process
  • external intercostal muscles contract
  • ribs are pulled out and upwards
  • diaphragm contracts and flattens
  • volume of thorax in increased, reducing pressure
  • air is drawn in.


  • passive process
  • basically opposite of the above
  • recoil of elastic lungs forces air out.
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Plants- layers and physiology

Plants both photosynthesise and respire during the day.

The leaf:

  • Flat and thin = large surface area:volume ratio
  • spongy mesophyll allows for diffusion of gases
  • stomatal pores allow gase to diffuse in and out, but can also close to reduce water loss.
  • The mesophyll is permeated with air spaces.

Gases diffuse into the plant across the concentration gradient. Once inside, the gases diffuses across the intercellular spaces and into cells.

The direction of diffusion depends on various vactors; but the net movement of CO2 AND O2 in relation to the processes is what counts.

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Plant leaf adaptions



  • Large surface area so light can be absorbed everywhere.
  • Thin, so light can penetrate the lower levels of cells.

Cell type

  • Cuticle and epidermis are transparent so that cells get all the light needed.
  • Palisade cells are elongated and arranged in dense layers.


  • Cells are packed with chloroplasts for maximum photosynthesis.
  • Chloroplasts can move around, so as to reach the most light within that cell.

Air spaces

  • Intracellular air spaces allow carbon dioxide and oxygen to move freely in and out the leaf.
  • STOMATA >>>>
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Plant leaf more adaptions

Adaptions for water loss

  • Stomata are mainly on the underside- less sunlight, so less evaporation.
  • Stomata close at night- stopping the needless loss of water when the sunlight is not sufficient for photosynthesis.
  • Waxy cuticle massivley reduces water loss.


Found on the lower level of the leaf, each stomatal pore is surrounded by two guard cells.

Guard cells features and adaptions

  • thickened walls- the inner being thicker than the outer.
  • Open and close to reduce water loss.
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Mechanism of opening and closing

Mechanism of opening and closing

  • during the day, chloroplasts photosynthesise and produce ATP.
  • This provides energy for the uptake of potassium ions from the epidermal cells and into the guard cells by ACTIVE TRANSPORT.
  • Stored starch is turned to malate.
  • This reduces the water potential in leaf cells, including the guard cells.
  • Water moves into the cell BY OSMOSIS, to dilute it, until the cell becomes turgid.
  • This turgid state causes the guard cells to swell
  • The inner wall is thicker than the outer, so the stomata opens.

At night

  • Photosynthesis doesn't happen
  • Potassium ions diffuse out down the concentration gradient.
  • cells become flaccid, and the stomata closes.
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