Gas Exchange: Single Cells and Insects

0:23Single-celled organisms: diffusion

Take a single-celled organism, like an amoeba. It is tiny, so it has a large surface area to volume ratio. Oxygen is used up inside the cell by respiration, so its concentration inside is low. Outside, it is higher. So oxygen diffuses straight in across the cell surface membrane, down its concentration gradient.

Carbon dioxide does the same thing in reverse, and diffuses out. No lungs, no gills, no blood. The diffusion distance is so short that none of that is needed.

0:58Why large bodies cannot rely on diffusion

But that only works while the cell is small. Make the body bigger, and the surface area to volume ratio falls. Oxygen would take far too long to reach the middle. An insect is a good example. It has a waterproof outer covering, so oxygen cannot just diffuse in across the whole body surface. It needs a system, and that system is the tracheal system.

1:23Insects: spiracles

Along the sides of an insect's body are tiny pores called spiracles. Air enters through the spiracles. They can open and close. Closed, they cut down water loss. Open, they let oxygen in. This is a compromise: open for oxygen in, closed to reduce water loss.

1:42Tracheae and tracheoles

From each spiracle, air passes into a tube called a trachea. Tracheae branch into smaller and smaller tubes, called tracheoles. The tracheoles are very thin, and they reach right to the respiring cells, such as muscle cells. So oxygen diffuses down its concentration gradient along the tubes, then into the cells. The diffusion distance is short.

Notice what is missing. There is no blood carrying oxygen. The air goes almost to the cell itself.

2:14Tracheal fluid

Now, what about when an insect is active? The ends of the tracheoles contain a little fluid. During activity, muscle cells respire anaerobically and make lactate, which lowers the water potential of the cells. Water moves out of the tracheal fluid into the cells by osmosis. The fluid level falls, so more air-filled tracheole is exposed, and oxygen diffuses to the cell faster.

2:40Abdominal pumping

Larger insects can also speed things up with abdominal pumping. Muscles contract and relax, changing the volume of the abdomen, which squeezes air in and out of the tracheae. That moves air in bulk, as well as by diffusion, and keeps a steeper gradient for oxygen.

3:33Examiner trap and recap

The trap. Never say insects have lungs, and never say blood carries their oxygen. Say spiracles, tracheae, tracheoles, and diffusion.

Recap. Single cells, diffusion across the surface. Insects, tracheae and tracheoles carry air to the cells.

Next time, fish. Water holds far less oxygen than air, so gills need a clever trick, called counter-current flow.