1. Homework Text p.179 red P.179 green P183 green P183 red

2. Gas exchange in single celled organisms and insects Aqa book p178-9

3. Objectives How do single-celled organisms exchange gases? How do terrestrial insects balance the need to exchange gases with the need to conserve water? How do insects exchange gases?

4. Single-celled organisms, simple aquatic plants and dorso-ventrally flattened multicellular animals (such as the flatworm) display large surface area to volume ratios; simple diffusion across the surface is sufficient to accommodate the organisms’ metabolic needs

5. How do single-celled organisms exchange gases? Which main process is involved What type of surface area to volume ratio has a single celled organism got? If it has a cellulose cell wall does this interfere with diffusion? What is the distance for diffusion to occur to/from the centre of the organism? Does it need a specialised transport system?

7. Movie

8. Gas Exchange in Insects The tracheal system of insects consists of a network of air-filled tubes (trachea) that open to the outside through small holes in the exoskeleton called spiracles The larger tracheal tubes subdivide into smaller and smaller tubes that eventually penetrate into and between all the cells of the insect. The finer tubes are the tracheoles and these may be only 0.1  m in diameter; gas exchange occurs across the fluid-filled tips of the tracheoles Spiracles may be guarded by valves and surrounded by hairs to minimise water loss

10. Gases enter and leave the tracheal system through spiracles

12. Spiralling strands of chitin in the walls of the trachea make these tubes fairly rigid and thus protect them from being compressed by the surrounding tissues

13. Large surface area provided by the extensive network of tracheae, tracheoles, (which is further increased when fluid is withdrawn from the tracheoles during periods of high metabolic activity) Steep concentration gradients are maintained between the tracheoles and body cells due to tissue respiration (note - no movement of the internal medium) In larger, more active insects, the pumping movements of the abdomen increase the steepness of these diffusion gradients to meet the metabolic demands of the organism (ventilation – movement of the external medium) A short diffusion path exists between the thin tracheole walls and the tissues

14. In the resting state, the tracheoles penetrating between and within the insect’s tissues, are filled with a watery fluid that diffuses from the hypotonic cytoplasm of the resting cells

15. The diffusion of oxygen through this watery medium into the cells is relatively slow, although sufficient oxygen is delivered to meet the metabolic demands of the resting insect Oxygen diffuses into the cells along a diffusion gradient, where the partial pressure of oxygen within the cells is lower than that in the tracheoles Carbon dioxide diffuses from the cells into the tracheoles

16. During periods of activity, the increased rate of metabolism within the insect’s muscle cells leads to the accumulation of lactic acid within the cells Lactic acid reduces the osmotic potential of the cells and fluid is withdrawn from the tracheoles by osmosis

17. Air moves in to replace the fluid, and gases diffuse more rapidly across the air-filled tracheoles O2O2 CO 2

18. Summary Usually there is some fluid in resting insects tracheoles When an insect exercises anaerobic respiration produces lactic acid This lowers the water potential of the tissue Water moves out into the tissue by osmosis leaving a larger surface area of tracheole for gas exchange

19. How do terrestrial insects balance the need to exchange gases with the need to conserve water? Waxy cuticle to prevent water loss impermeable Small sa:v compared to single cell organism so less water lost but not enough sa for gas exchange Spiracles needed for gas exchange but - either hairs to prevent water loss or open and close

20. 3.2.7b Ventilation in a locust Nuffield book1 p.9 Students do procedure 1 & 2 Teacher to dissect as in steps 3-8 Students take a small part of gut + trachae to observe with a hand lens/ microscope as on page 12 of nuffield book procedure 1,2,3,4 Make sure no methylene blue on stage of microscope/lenses when finished

21. 3.2.7b Equipment list Per set Locust + alcohol injection Shallow wax tray Pins Sharp scissors Forceps Lamp Binocular microscope Container of water to flood dissection Flexi cam Lens tissue Per pair Safety specs Locust in tube plus bung Hand lens Binocular microscope Lamp Clean straw Watch glass Methylene blue Slide and coverslip 2 seekers Microscope