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Exchange and Transport. 13.2 Gas exchange in single-celled organisms and insects. Learning outcomes. Students should be able to understand the following: How single-celled organisms exchange gases How terrestrial insects balance the need to exchange gases with the need to conserve water

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exchange and transport

Exchange and Transport

13.2 Gas exchange in single-celled organisms and insects

learning outcomes
Learning outcomes

Students should be able to understand the following:

  • How single-celled organisms exchange gases
  • How terrestrial insects balance the need to exchange gases with the need to conserve water
  • How insects exchange gases

Candidates should be able to:

  • Use their knowledge and understanding of the principles of diffusion to explain the adaptations of gas exchange surfaces
slide3

How single-celled organisms exchange gases

CO2 Diffuses out of cell

O2 Diffuses into cell

  • Unicellular organisms (e.g. Amoeba, bacteria) have a large SA : Volume ratio
  • Gases are exchanged efficiently across the whole body surface
  • There is a thin surface membrane and short diffusion pathway enabling oxygen to diffuse into the cell and carbon dioxide to diffuse out of the cell
slide4

Requirements for effective gas exchange across the body surface

Reminder: the rate at which a substance can diffuse is given by Fick\'s Law:

Rate of diffusion ∝ surface area x concentration difference

Distance

  • Organisms therefore need thin permeable surface membranes with a large surface area
  • BUT this would lead to excessive water loss in larger organisms
slide5

Insects are adapted to conserve water

  • Small Surface area :Volume ratio to minimise water loss from the body surface
  • Waterproof coverings
  • e.g. In insects the exoskeleton is covered with a waterproof cuticle
slide6

Insects are adapted for gas exchange

  • Insect Tracheal System
  • Tracheal tubes run from the body surface into the tissues
  • Transport gases directly between the external environment and the body cells
slide7

Anatomy of insect gas exchange system

  • Each segment of the insect (apart from the head) has a pair of lip-like openings called spiracles
  • Tracheal tubes connected to each spiracle branch into a series of tracheoles
  • Tracheolesrepeatedly divide until their numerous microscopic ends penetrate into individual body cells
  • The spiracles can be closed to prevent water loss and opened when greater respiration is needed
slide8

How gases are exchanged in insects

  • Diffusion gradient
  • Oxygen moves down a concentration gradient from the air into body cells
  • Carbon dioxide moves down a concentration gradient from body cells into the air
  • Ventilation by rhythmic abdominal movements
  • Further speeds up the exchange of respiratory gases by generating mass movements of air in and out of the tracheal tubes
slide9

Application

Spiracle movements

AQA AS Biology textbook pg 179

Referring to Fig. 3, answer questions 1 -5

learning outcomes1
Learning outcomes

Students should be able to understand the following:

  • How single-celled organisms exchange gases
  • How terrestrial insects balance the need to exchange gases with the need to conserve water
  • How insects exchange gases

Candidates should be able to:

  • Use their knowledge and understanding of the principles of diffusion to explain the adaptations of gas exchange surfaces
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